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Page 1 of 84 1 GSM TECHNOLOGY 1.1 INTRODUCTION: Global System for Mobile Communication (GSM) is a set of ETSI standards specifying the infrastructure for a digital cellular service. The standard is used in approx. 85 countries in the world including such locations as Europe, Japan and Australia. GSM is worldwide standard that allows users of different operators to connect and to shares the services simultaneously. GSM has been the backbone of the phenomenal success in mobile telecommunication over the last decade. Now, at the dawn of the era of true broadband services, GSM continues to evolve to meet new demands. One of GSM's great strengths is its international roaming capability, giving consumers a seamless service in about 160 countries. This has been a vital driver in growth, with around 300 million GSM subscribers currently in Europe and Asia. In the Americas, today's 7 million subscribers are set to grow rapidly, with market potential of 500 million in population, due to the i ntroduction of GSM 800, which allows operators using the 800 MHz band to have access to GSM technology too. 1.1.1 The main points and strength of GSM technology is: GSM is globally accepted standard for DIGITAL CELLULAR COMMUNICATION. Provides recommendations and not requirements. Defines the functions and interface requirements in detail but do not address the hardware. It is an open interface. 1.1.2 Why GSM? The GSM study group aimed to provide the followings through the GSM: Improved spectrum efficiency. Internationa l roaming. Low-cost mobile sets and base stations (BSs) High-quality speech
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1 GSM TECHNOLOGY

1.1 INTRODUCTION:

Global System for Mobile Communication (GSM) is a set of  ETSI standards specifying

the infrastructure for a digital cellular service. The standard is used in approx. 85 countries in the

world including such locations as Europe, Japan and Australia.

GSM is worldwide standard that allows users of different operators to connect and to

shares the services simultaneously. GSM has been the backbone of the phenomenal success in

mobile telecommunication over the last decade. Now, at the dawn of the era of true broadband

services, GSM continues to evolve to meet new demands. One of GSM's great strengths is its

international roaming capability, giving consumers a seamless service in about 160 countries. This

has been a vital driver in growth, with around 300 million GSM subscribers currently in Europe

and Asia. In the Americas, today's 7 million subscribers are set to grow rapidly, with market

potential of 500 million in population, due to the introduction of GSM 800, which allows operators

using the 800 MHz band to have access to GSM technology too.

1.1.1  The main points and strength of GSM technology is:

GSM is globally accepted standard for DIGITAL CELLULAR COMMUNICATION.

Provides recommendations and not requirements.

Defines the functions and interface requirements in detail but do not address the hardware.

It is an open interface.

1.1.2  Why GSM?

The GSM study group aimed to provide the followings through the GSM:

Improved spectrum efficiency.

International roaming.

Low-cost mobile sets and base stations (BSs)

High-quality speech

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Compatibility with Integrated Services Digital Network (ISDN) and other telephone

company services.

Support for new services.

High transmission quality

1.2 GSM CELLULAR STRUCTURE

GSM uses a combination of FDMA (Frequency Division Multiple Access) and TDMA

(Time division Multiple Access).The GSM system has an allocation of 50 MHz band width in 900

MHz frequency band.

Now using FDMA this band is divided into 124 channels each with a carrier bandwidth of 

200 KHz .Using TDMA these channels are further divided into 8 time slots. Therefore,

combination of both FDMA and TDMA we can realize a maximum of 922 channels for transmit

and receive .Approximately 50 KHz of RF spectrum is reserved for each subscriber and if we have

a large number of subscribers(say 10000) the RF spectrum would then required will be 10000*50

KHz =5GHz. But this large Spectrum is not available for use. This limitation of RF spectrum and

in order to serve a hundreds of thousands of users‘ concept of frequency reuse was developed.  

The frequency reuse concept lead to development of cellular technology and it was

originally conceived by AT&T Bell labs in 1947.

1.2.1  CELL

Cell is the basic service area. The cell is the area given

radio coverage by one base transceiver station. The GSM

network identifies each cell via the cell global identity (CGI)

number assigned to each cell.

In a cellular system, the communication area of the

service provider is divided into small geographical areas called

cells. Each cell contains following components:

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  An antenna

  Solar or AC power network station.

  The solar or AC powered network station is called the Base Station (BS).

1.2.2  WHY HEXAGONAL SHAPED CELLS ARE BETTER?

Cells are drawn in hexagonal shape because the hexagonal shaped cells have no gaps or

overlaps between them. It causes no interruption to the communication of a mobile subscriber

moving from one cell to another. It is obvious from the figure that other shapes of the cells are

leaving gaps where no coverage is provided to the mobile users. On the other hand, there is no such

problem in hexagonal cells.

1.2.3  TYPES OF CELLS

Due to the uneven changes in the population density of different countries and regions in

the world, there are different types of cells used according to the best results in uninterruptible

communication. These are listed as:

Macro Cells

Micro Cells

Pico Cells

Umbrella Cells

Selective Cells

a) Macro Cells

A macro cell is a cell in a mobile phone network that provides radio coverage served by a

power cellular base station (tower). Generally, macro cells provide coverage larger than micro cell

such as rural areas or along highways. The antennas for macro cells are mounted on ground-based

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masts, rooftops and the other existing structures, at a height that provides a clear view over the

surrounding buildings and terrain. Macro cell base stations have power outputs of typically tens of 

watts.

b) Micro Cells

A micro cell is a cell in a mobile phone network served

by a low power cellular base station (tower), covering a limited

area such as a mall, a hotel, or a transportation hub. A micro cell

is usually larger than a Pico cell, though the distinction is not

always clear. Typically the range of a micro cell is less than a

mile wide.

The antennas for micro cells are mounted at street level.

Micro cell antennas are smaller than macro cell antennas and

when mounted on existing structures c an often be disguised as

building features. Micro cells provide radio coverage over

distances up to, typically, between 300m and 1000m. Micro cell

base stations have lower output powers than macro cells,

typically a few watts.

c) Pico Cells

Pico cells are small cells whose diameter is only few dozen meters; they are used mainly in

indoor applications. It can cover e.g. a floor of 

a building or an entire building, or for example in shopping centers or airports. [023] Pico cells

provide more localized coverage than micro cells, inside buildings where coverage is poor or there

are high numbers of users.

d) Umbrella Cells

A layer with micro cells is covered by at least one

macro cell, and a micro cell can in turn cover several Pico

cells, the covering cell is called an umbrella cell. If there

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are very small cells and a user is crossing the cells very quickly, a large number of handovers will

occur among the different neighboring cells. The power level inside an umbrella cell is increased

compared to the micro cells with which it is formed. This makes the mobile to stay in the same cell

(umbrella cell) causing the number of handovers to be decreased as well as the work to be done by

the network.

e) Selective Cells

The full coverage of the cells may not be required in

all sorts of applications, but cells with limited coverage are

used with a particular shape. These are named selective due

to the selection of their shape with respect to the coverage

areas. For example, the cells used at the entrance of the

tunnels are selective cells because coverage of 120 degrees

is used in them.

Clusters

The regular repetition of frequencies in cells results in a

clustering of cells. A cluster is a group of cells. No channels are

reused within a cluster. The generate in this way can consume

the whole frequency band. The size of cluster is defined by k,

the numbers of cells in a cluster and this also defines frequency

reuse distance.

Frequency Reuse Concept

The concept of cellular systems is the use of low power transmitters in order to enable‘s the

efficient reuse of the frequencies. If the transmitters of high power are used, there will be

interference between the users at the boundaries of the cells. However, the set of available

frequencies is limited and that is why there is a need for the reuse of the frequencies.

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A frequency reuse pattern is a configuration of N cells, N being the reuse factor, in which

each cell uses a unique set of frequencies. When the pattern is repeated, the frequencies can be

reused. There are several different patterns, but only two are shown below to clarify the idea.

1.3 MULTIPLE ACCESS TECHNIQUES

A limited amount of bandwidth is allocated for wireless services. A wireless system is

required to accommodate as many users as possible by effectively sharing the limited bandwidth.

Therefore, in the field of communications, the term multiple access could be defined as a means of 

allowing multiple users to simultaneously share the finite bandwidth with least possibledegradation in the performance of the system. There are several techniques how multiple accessing

can be achieved. There are four basic schemes

1. Frequency Division Multiple Access (FDMA)

2. Time Division Multiple Access (TDMA)

3. Code Division Multiple Access (CDMA)

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1.3.1  FDMA

FDMA is one of the earliest multiple-access techniques for cellular systems when

continuous transmission is required for analog services. In this technique the bandwidth is divided

into a number of channels and distributed among users with a finite portion of bandwidth for

permanent use. FDMA does not require synchronization or timing control, which makes it

algorithmically simple. Even though no two users use the same frequency band at the same time, guard

bands are introduced between frequency bands to minimize adjacent channel interference. Guard bands

are unused frequency slots that separate neighboring channels. This leads to a waste of bandwidth.

When continuous transmission is not required, bandwidth goes wasted since it is not being utilized for a

portion of the time. In wireless communications, FDMA achieves simultaneous transmission and

reception by using Frequency division duplexing (FDD). In order for both the transmitter and thereceiver to operate at the same time.

Figure 1-1 FDMA

1.3.2  Time Division Multiple Access (TDMA)

In digital systems, continuous transmission is not required because users do not use the

allotted bandwidth all the time. In such systems, TDMA is a complimentary access technique to

FDMA. Global Systems for Mobile communications (GSM) uses the TDMA technique. In

TDMA, the entire bandwidth is available to the user but only for a finite period of time. In most

cases the available bandwidth is divided into fewer channels compared to FDMA and the users are

allotted time slots during which they have the entire channel bandwidth at their disposal. TDMA

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requires careful time synchronization since users share the bandwidth in the frequency domain. In

GSM each carrier frequency is divided into the 8 time slots.

Figure 1-2 Time Division Multiple Access 

1.3.3  Code Division Multiple Access 

The code division multiple access (CDMA) scheme is based on spread spectrum. CDMA

accomplishes the communication in different code sequences. Special coding is adopted before

transmission, and then different information will lose nothing after being mixed and transmitted

together on the same frequency and at the same time. An example is the 3G cell phone system.

Figure 1-3 Code Division Multiple Access 

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1.4 GSM SPECTRUM

The spectrum is the range of electromagnetic frequencies. The spectrum allocated to GSM

is in the ultra high frequency (UHF) band, in one of three bands:

 

GSM 900

  GSM 850

  GSM 1800

  GSM 1900 

1.4.1  GSM 900

GSM systems use radio frequencies between 890-915 MHz for receive and between 935-

960MHz for transmit. RF carriers are spaced every 200 kHz, allowing a total of 124 carriers for

use. An RF carrier is a pair of radio frequencies, one used in each direction. Transmit and receive

frequencies are always separated by 45 MHz.

Figure 1-4 GSM 900 

1.4.2  GSM 850 (EGSM)

EGSM has 10MHz of bandwidth on both transmit and receive. Receive bandwidth is from

880 MHz to 890 MHz. Transmit bandwidth is from 925 MHz to 935 MHz. Total RF carriers in

EGSM is 50 more than GSM.

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Figure 1-5 GSM 850

1.4.3  DSC 1800

The DCS1800 systems use radio frequencies between 1710-1785 MHz for receive andbetween 1805-1880 MHz for transmit. RF carriers are spaced every 200 kHz, allowing a total of 

373 carriers. Transmit and receive frequencies are always separated by 95 MHz.

Figure 1-6 DSC 1800 

1.4.4  GSM 1900

The GSM 1900 band provides for a GSM uplink in the range 1850-1910 MHz, a a GSM

downlink in the range 1930-1990 MHz. The GSM 1900 band is used primarily in the United

States.

Figure 1-7 GSM 1900

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2 GSM ARCHITECTURE

A GSM system is basically designed as a combination of four major subsystems:

Radio subsystem (RSS)

Network subsystem (NSS)

Operation and maintenance subsystem (OMS)

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2.1 RADIO SUBSYSTEM (RSS)

Management of radio network and is controlled

by a MSC. One MSC controls many radio sub-system.

The Radio Subsystem (RSS) consists of:BSC: Base station controller.

BTS: Base transceiver station.

Radio subsystem mainly performs following

functions:

  Radio path control

  Synchronization

  Air and A interface signaling

  Connection between MS and NSS

  Mobility management

  Speech transcoding

  Handovers

2.1.1  Base Station Controller (BSC)

A BSC is a network component in the PLMN that function for control of one or more BTS.It is a functional entity that handles common control functions within a BTS. BSC within a mobile

network is a key component for handling and routing information. The BSC provides all the

control functions and physical links between the MSC and BTS. It is a high-capacity switch that

provides functions such as handover, cell configuration data, and control of radio frequency (RF)

power levels in base transceiver stations. A number of BSCs are served by an MSC.

The BSC is connected to the MSC on one side and to the BTS on the other. The BSC

performs the Radio Resource (RR) management for the cells under its control. It assigns and

releases frequencies and timeslots for all MSs in its own area. The BSC performs the inter-cell

handover for MSs moving between BTS in its control. It also reallocates frequencies to the BTSs

in its area to meet locally heavy demands during peak hours or on special events. The BSC

controls the power transmission of both BSSs and MSs in its area. The minimum power level for a

mobile unit is broadcast over the BCCH

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The BSC can direct the BTS to notify the MS to advance the timing such that proper

synchronization takes place. The BSC may also perform traffic concentration to reduce the number

of transmission lines from the BSC to its BTSs.

A BSC is often based on a distributed computing architecture, with redundancy applied to

critical functional units to ensure availability in the event of fault conditions. Redundancy often

extends beyond the BSC equipment itself and is commonly used in the power supplies and in the

transmission equipment providing the A-ter interface to PCU.

The databases for all the sites, including information such as carrier frequencies, 

frequency hopping lists, power reduction levels, receiving levels for cell border calculation, are

stored in the BSC. This data is obtained directly from radio planning engineering which involves

modeling of the signal propagation as well as traffic projections.

2.1.2  Base Terminal Station (BTS)The BTS handles the radio interface to the mobile station. The BTS is the radio equipment

(transceivers and antennas) needed to service each cell in the network. A group of BTSs are

controlled by a BSC.

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A BTS is a network component that serves one cell and is controlled by a BSC. BTS is

typically able to handle three to five radio carries, carrying between 24 and 40 simultaneous

communication. Reducing the BTS volume is important to keeping down the cost of the cell sites.

BTS with its antennae

A BTS compares radio transmission and reception devices, up to and including the

antennas, and also all the signal processing specific to the radio interface. A single transceiver

within BTS supports eight basic radio channels of the same TDM frame.

2.1.3  Functions of BTS

The primary responsibility of the BTS is to transmit and receive radio signals from a

mobile unit over an air interface. To perform this function completely, the signals are encoded,

encrypted, multiplexed, modulated, and then fed to the antenna system at the cell site. Transcoding

to bring 13-kbps speech to a standard data rate of 16 kbps and then combining four of these signals

to 64 kbps is essentially a part of BTS, though; it can be done at BSC or at MSC. The voice

communication can be either at a full or half 

rate over logical speech channel. In order to keep the mobile synchronized, BTS transmits

frequency and time synchronization signals over frequency correction channel (FCCH and BCCH

logical channels. The received signal from the mobile is decoded, decrypted, and equalized for

channel impairments.

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Random access detection is made by BTS, which then sends the message to BSC. The

channel subsequent assignment is made by BSC. Timing advance is determined by BTS.

BTS signals the mobile for proper timing adjustment. Uplink radio channel measurement

corresponding to the downlink measurements made by MS has to be made by BTS.

2.2 NETWORK SUB-SYSTEM (NSS)

Performs call processing and subscriber related functions. It includes:

MSC: Mobile Switching Centre

HLR: Home Location Register

VLR: Visitor Location Register

AuC: Authentication Centre

EIR: Equipment Identity Register

GMSC: Gateway MSC

The network and the switching subsystem together include the main switching functions of 

GSM as well as the databases needed for subscriber data and mobility management (VLR). The

main role of the MSC is to manage the communications between the GSM users and other

telecommunication network users. The basic switching function is performed by the MSC, whose

main function is to coordinate setting up calls to and from GSM users. The MSC has interface with

the BSS on one side (through which MSC VLR is in contact with GSM users) and the external

networks on the other (ISDN/PSTN/PSPDN). The main difference between a MSC and an

exchange in a fixed network is that the MSC has to take into account the impact of the allocation of 

RRs and the mobile nature of the subscribers and has to perform, in addition, at least, activities

required for the location registration and handover.

The Network Switching Subsystem, also referred to as the GSM core network, usually

refers to the circuit-switched core network, used for traditional GSM services such as voice calls,

SMS, and circuit switched data calls.

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There is also an overlay architecture on the GSM core network to provide packet-switched

data services and is known as the GPRS core network. This allows mobile phones to have access to

services such as WAP, MMS, and Internet access.

All mobile phones manufactured today have both circuit and packet based services, so most

operators have a GPRS network in addition to the standard GSM core network.

2.2.1  Mobile Switching Center (MSC)

An MSC is the point of connection to the network for mobile subscribers of a wireless

telephone network. It connects to the subscribers through base stations and radio transmission

equipment that control the air interface, and to the network of other

MSCs and wireless infrastructure through voice trunks and SS7. An MSC includes the

procedures for mobile registration and is generally co-sited with a visitor location register (VLR)

that is used to temporarily store information relating to the mobile subscribers temporarily

connected to that MSC. The MSC performs the telephony switching functions of the system. It

controls calls to and from other telephone and data systems. It also performs such functions as toll

ticketing, network interfacing, common channel signaling, and others.

Other network elements of MSC

a) Billing Center

Each MSC writes call accounting records to local disk memory. Billing Center periodically

polls the disk records of each MSC to collect the billing data for the PLMN.

b) Service CenterThe Service Center interfaces with the MSCs to provide special services, such as the Short

Message Service (SMS), to mobile subscribers in the PLMN. The Billing Center and Service

Center are not a basic part of the GSM system.

Gateway MSC (G-MSC)

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The gateway MSC (G-MSC) is the MSC that determines which visited MSC the subscriber

who is being called is currently located. It also interfaces with the PSTN. All mobile to mobile

calls and PSTN to mobile calls are routed through a G-MSC. The term is only valid in the context

of one call since any MSC may provide both the gateway function and the Visited MSC function;

however, some manufacturers design dedicated high capacity MSCs which do not have any BSSs

connected to them. These MSCs will then be the Gateway MSC for many of the calls they handle.

2.2.2  Home location register (HLR) 

The home location register (HLR) is a central database that contains details of each mobile

phone subscriber that is authorized to use the GSM core network. There can be several logical, and

physical, HLRs per public land mobile network  (PLMN), though one international mobilesubscriber identity (IMSI)/MSISDN pair can be associated with only one logical HLR (which can

span several physical nodes) at a time.

The HLR stores details of every SIM card issued by the mobile phone operator. Each SIM

has a unique identifier called an IMSI which is the primary key to each HLR record.

The next important items of data associated with the SIM are the MSISDNs, which are the

telephone numbers used by mobile phones to make and receive calls. The primary MSISDN is

the number used for making and receiving voice calls and SMS, but it is possible for a SIM to have

other secondary MSISDNs associated with it for fax  and data calls. Each MSISDN is also a

primary key to the HLR record. The HLR data is stored for as long as a subscriber remains with

the mobile phone operator.

2.2.3  Functions of HLR 

The main function of the HLR is to manage the fact that SIMs and phones move around a

lot. The following procedures are implemented to deal with this:Manage the mobility of subscribers by means of updating their position in administrative

areas called 'location areas', which are identified with a LAC. The action of a user of moving from

one LA to another is followed by the HLR with a

Location area update while retrieving information from BSS as base station identity code 

(BSIC). Send the subscriber data to a VLR or SGSN when a subscriber first roams there. Broker

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between the G-MSC or SMSC and the subscriber's current VLR in order to allow incoming calls

or text messages to be delivered. 

Remove subscriber data from the previous VLR when a subscriber has roamed away from

it.

2.2.4  Visitor locations register (VLR) 

The visitor location register is a temporary database of the subscribers who have roamed

into the particular area which it serves. Each base station in the network is served by exactly one

VLR, hence a subscriber cannot be present in more than one VLR at a time.

The data stored in the VLR has either been received from the HLR, or collected from the

MS. In practice, for performance reasons, most vendors integrate the VLR directly to the V-MSCand, where this is not done, the VLR is very tightly linked with the MSC via a proprietary

interface.

Data stored in VLR

IMSI (the subscriber's identity number).

Authentication data.

MSISDN (the subscriber's phone number).

GSM services that the subscriber is allowed to access.

access point (GPRS) subscribed.

The HLR address of the subscriber.

Functions of VLR

The primary functions of the VLR are:

To inform the HLR that a subscriber has arrived in the particular area covered by the VLR.

To track where the subscriber is within the VLR area (location area) when no call is

ongoing.

To allow or disallow which services the subscriber may use.

To allocate roaming numbers during the processing of incoming calls.

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To purge the subscriber record if a subscriber becomes inactive whilst in the area of a VLR.

The VLR deletes the subscriber's data after a fixed time period of inactivity and informs the HLR

(e.g., when the phone has been switched off and left off or when the subscriber has moved to an

area with no coverage for a long time).

To delete the subscriber record when a subscriber explicitly moves to another, as instructed

by the HLR.

2.2.5  Authentication centre (AUC) 

The authentication centre (AUC) is a function to authenticate each SIM card that attemptsto connect to the GSM core network (typically when the phone is powered on). Once the

authentication is successful, the HLR is allowed to manage the SIM and services described above.

An encryption key is also generated that is subsequently used to encrypt all wireless

communications (voice, SMS, etc.) between the mobile phone and the GSM core network.

If the authentication fails, then no services are possible from that particular combination of 

SIM card and mobile phone operator attempted. There is an additional form of identification check 

performed on the serial number of the mobile phone described in the EIR section below, but this is

not relevant to the AUC processing.

Proper implementation of security in and around the AUC is a key part of an operator's

strategy to avoid SIM cloning. 

2.2.6  Equipment Identity Register (EIR)

The EIR is a database that contains information about the identity of mobile equipment that

prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are

implemented as stand-alone nodes or as a combined AUC/EIR node.

EIR is a database that stores the IMEI numbers for all registered ME units. The IMEI

uniquely identifies all registered ME. There is generally one EIR per PLMN. It interfaces to the

various HLR in the PLMN. The EIR keeps track of all ME units in the PLMN. It maintains various

lists of message. The database stores the ME identification and has nothing do with subscriber who

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is receiving or originating call. There are three classes of ME that are stored in the database, and

each group has different characteristics:

White List: contains those IMEIs that are known to have been assigned to valid MS‘s. This

is the category of genuine equipment.

Black List: contains IMEIs of mobiles that have been reported stolen.

Gray List: contains IMEIs of mobiles that have problems (for example, faulty software,

and wrong make of the equipment). This list contains all MEs with faults not important enough for

barring. 

3 OPERATION AND MAINTENANCE SUBSYSTEM (OMS)

The Operations and Maintenance Center (OMC) is the centralized maintenance

and diagnostic heart of the Base Station System (BSS). It allows the network 

provider to operate, administer, and monitor the functioning of the BSS. An OMS consists of one

or more Operation & Maintenance Centre (OMC)

The operations and maintenance center (OMC) is connected to all equipment in the

switching system and to the BSC. The implementation of OMC is

called the operation and support system (OSS). The OSS is the functional entity from

which the network operator monitors and controls the system. The purpose of OSS is to offer the

customer cost-effective support for centralized, regional and local operational and maintenance

activities that are required for a GSM network. An important function of OSS is to provide a

network overview and support the maintenance activities of different operation and maintenance

organizations.

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The OMC provides alarm-handling functions to report and log alarms generated by the

other network entities. The maintenance personnel at the OMC can define that criticality of the

alarm. Maintenance covers both technical and administrative actions to maintain and correct the

system operation, or to restore normal operations after a breakdown, in the shortest possible time.

The fault management functions of the OMC allow network devices to be manually or

automatically removed from or restored to service. The status of network devices can be checked,

and tests and diagnostics on various devices can be invoked. For example, diagnostics may be

initiated remotely by the OMC. A mobile call trace facility can also be invoked. The performance

management functions included collecting traffic statistics from the GSM network entities and

archiving them in disk files or displaying them for analysis. Because a potential to collect large

amounts of data exists, maintenance personal can select which of the detailed statistics to be

collected based on personal interests and past experience. As a result of performance analysis, if 

necessary, an alarm can be set remotely.

The OMC provides system change control for the software revisions and configuration data

bases in the network entities or uploaded to the OMC. The OMC also keeps track of the different

software versions running on different subsystem of the GSM 

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3.1 MOBILE STATION

3.2 MOBILE SUBSCRIBER IDENTITIES IN GSM

It would be better to discuss some of the important subscriber identities in the GSM, which

make the use of this technology safer for every person whether he/she is a subscriber of GSM or

not.

1) International Mobile Subscriber Identity (IMSI)

An IMSI is assigned to each authorized GSM user. It consists of a mobile country code

(MCC), mobile network code (MNC) (to identify the PLMN), and a PLMN unique mobile

subscriber identification number (MSIN). The IMSI is the only absolute identity that a subscriber

has within the GSM system. The IMSI consists of the MCC followed by the MNC and MSIN and

shall not exceed 15 digits. It is used in the case of system-internal signaling transactions in order to

identify a subscriber. The first two digits of the MSIN identify the HLR where the mobile

subscriber is administrated.

2) Temporary Mobile Subscriber Identity (TMSI)

A TMSI is a MSC-VLR specific alias that is designed to maintain user confidentiality. It is

assigned only after successful subscriber authentication. The correlation of a TMSI to an IMSI

only occurs during a mobile subscriber‘s initial transaction with an MSC (for example, location

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updating). Under certain condition (such as traffic system disruption and malfunctioning of the

system), the MSC can direct individual TMSIs to provide the MSC with their IMSI.

3) Mobile Station ISDN Number

The MS international number must be dialed after the international prefix in order to obtain

a mobile subscriber in another country. The MSISDN numbers is composed of the country code

(CC) followed by the National Destination Code (NDC), Subscriber Number (SN), which

shall not exceed 15 digits. Here too the first two digits of the SN identify the HLR where the

mobile subscriber is administrated.

4) The Mobile Station Roaming Number (MSRN)

The MSRN is allocated on temporary basis when the MS roams into another numbering

area. The MSRN number is used by the HLR for rerouting calls to the MS. It is assigned upon

demand by the HLR on a per-call basis. The MSRN for PSTN/ISDN routing shall have the same

structure as international ISDN numbers in the area in which the MSRN is allocated. The HLR

knows in what MSC/VLR service area the subscriber is located. At the reception of the MSRN,

HLR sends it to the GMSC, which can now route the call to the MSC/VLR exchange where the

called subscriber is currently registered.

5) International Mobile Equipment Identity

The IMEI is the unique identity of the equipment used by a subscriber by each PLMN and

is used to determine authorized (white), unauthorized (black), and malfunctioning (gray) GSM

hardware. In conjunction with the IMSI, it is used to ensure that only authorized users are granted

access to the system.

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3.3 CALL ROUTING

3.3.1  OUTGOING CALL

  MS sends dialed number to BSS

  BSS sends dialed number to MSC

  3,4 MSC checks VLR if MS is allowed

the requested service. If so, MSC asks BSS

to allocate resources for call.

  MSC routes the call to GMSC

 

GMSC routes the call to local exchange of 

called user

  7, 8,

  9,10 Answer back(ring back) tone is routed from called user to MS via

GMSC,MSC,BSS.

3.3.2  INCOMING CALL

  Calling a GSM subscribers

  Forwarding call to GSMC

  Signal Setup to HLR

  5. Request MSRN from VLR

  Forward responsible MSC to GMSC

  Forward Call to current MSC

 

9. Get current status of MS

  11. Paging of MS

  13. MS answers

  15. Security checks

  17. Set up connection.

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4 GSM - USER SERVICES

GSM has much more to offer than voice telephony. Additional services allow you greaterflexibility in where and when you use your phone. You should contact your local GSM network 

operator for information on the specific services available to you.

But there are three basic types of services offered through GSM which you can ask for:

1.  Telephony Services

2.  Data Services.

3.  Supplementary Services

4.1 TELESERVICES OR TELEPHONY SERVICES:-

A Teleservice utilises the capabilities of a Bearer Service to transport data, defining which

capabilities are required and how they should be set up.

Voice Calls:-The most basic Teleservice supported by GSM is telephony. This includes

Full-rate speech at 13 Kbps and emergency calls, where the nearest emergency- service provider is

notified by dialing three digits. A very basic example of emergency service is 911 service available

in USA.

Videotext and Facsmile:-Another group of teleservices includes Videotext access, Teletextransmission, Facsimile alternate speech and facsimile Group 3, Automatic facsimile Group 3 etc.

Short Text Messages:-SMS (Short Messaging Service) service is a text messaging which

allow you to send and receive text messages on your GSM Mobile phone. Services available from

many of the world's GSM networks today - in addition to simple user generated text message

services - include news, sport, financial, language and location based services, as well as many

early examples of mobile commerce such as stocks and share prices, mobile banking facilities and

leisure booking services.

4.2  BEARER SERVICES OR DATA SERVICES:-

Using your GSM phone to receive and send data is the essential building block leading to

widespread mobile Internet access and mobile data transfer. GSM currently has a data transfer rate

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of 9.6k. New developments that will push up data transfer rates for GSM users are HSCSD (high

speed circuit switched data) and GPRS (general packet radio service) are now available.

4.3 SUPPLEMENTARY SERVICES:-

Supplementary services are provided on top of teleservices or bearer services, and include

features such as caller identification, call forwarding, call waiting, multi-party conversations, and

barring of outgoing (international) calls, among others. A brief description of supplementary

services is given here:

Multiparty Service or conferencing: The multiparty service allows a mobile subscriber to

establish a multiparty conversation.that is, a simultaneous conversation between three or more

subscribers to setup a conference call. This service is only applicable to normal telephony.

Call Waiting: This service allows a mobile subscriber to be notified of an incoming call

during a conversation. The subscriber can answer, reject, or ignore the incoming call. Call waiting

is applicable to all GSM telecommunications services using a circuit-switched connection.

Call Hold: This service allows a subscriber to put an incoming call on hold and then

resume this call. The call hold service is only applicable to normal telephony.

Call Forwarding: The Call Forwarding Supplementary Service is used to divert calls from

the original recipient to another number, and is normally set up by the subscriber himself. It can be

used by the subscriber to divert calls from the Mobile Station when the subscriber is not available,

and so to ensure that calls are not lost. A typical scenario would be a salesperson turns off his

mobile phone during a meeting with customers, but does not with to lose potential sales leads

while he is unavailable.

Call Barring: The concept of barring certain types of calls might seem to be a

supplementary disservice rather than service. However, there are times when the subscriber is not

the actual user of the Mobile Station, and as a consequence may wish to limit its functionality, so

as to limit the charges incurred. Alternatively, if the subscriber and user are one and the same, the

Call Barring may be useful to stop calls being routed to international destinations when they are

routed. The reason for this is because it is expected that the roaming subscriber will pay the

charges incurred for international re-routing of calls. So, GSM devised some flexible services that

enable the subscriber to conditionally bar calls.

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Number Identification: There are following supplementary services related to number

identification:

Calling Line Identification Presentation: This service deals with the presentation of the

calling party's telephone number. The concept is for this number to be presented, at the start of the

phone ringing, so that the called person can determine who is ringing prior to answering. The

person subscribing to the service receives the telephone number of the calling party.

Calling Line Identification Restriction: A person not wishing their number to be

presented to others subscribes to this service. In the normal course of event, the restriction service

overrides the presentation service.

Connected Line Identification Presentation: This service is provided to give the calling

party the telephone number of the person to whom they are connected. This may seem strange

since the person making the call should know the number they dialled, but there are situations

(such as forwardings) where the number connected is not the number dialled. The person

subscribing to the service is the calling party.

Connected Line Identification Restriction: There are times when the person called does

not wish to have their number presented and so they would subscribe to this person. Normally, this

overrides the presentation service.

Malicious Call Identification: The malicious call identification service was provided to

combat the spread of obscene or annoying calls. The victim should subscribe to this service, and

then they could cause known malicious calls to be identified in the GSM network, using a simple

command. This identified number could then be passed to the appropriate authority for action. The

definition for this service is not stable.

Advice of Charge (AoC): This service was designed to give the subscriber an indication of 

the cost of the services as they are used. Furthermore, those Service Providers who wish to offer

rental services to subscribers without their own Subscriber Identity Module (SIM) can also utilize

this service in a slightly different form. AoC for data calls is provided on the basis of time

measurements.

Closed User Groups (CUGs): This service is provided on GSM to enable groups of 

subscribers to only call each other. This type of services are being offered with special discount

and is limited only to those members who wish to talk to each other.

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5 UPGRADATION FROM 2G TO 3G

Wireless Generations:

1 G - First Generation - Analog - Only mobile voice services - AMPS, NMT-450,

TACS etc. (Cellular Revolution)

2 G - Second Generation - Digital - Mostly for voice services & data delivery possible –  

GSM, CDMA(IS-95), DAMPS(IS-136), ETDMA, PDC etc (Breaking Digital Barrier)

3 G - Third Generation - Voice & Data - Mainly for data services where voice services

will also be possible ( Breaking Data Barrier)

Limitations of 2G Systems

Multiple Standards - No Global Standards

No Common Frequency Band

Low Data Bit Rates

Low Voice QualityNo Support of Video

Various Network Systems to meet Specific Requirements

5.1 OVERVIEW OF GPRS

General packet radio service (GPRS) is a packet oriented mobile data service on the 2G

and 3G cellular communication system's global system for mobile communications (GSM). GPRS

was originally standardized by European Telecommunications Standards Institute (ETSI) in

response to the earlier CDPD and i-mode packet-switched cellular technologies. It is now

maintained by the 3rd Generation Partnership Project (3GPP).[1][2] 

GPRS usage charging is based on volume of data, either as part of a bundle or on a pay-as-

you-use basis. An example of a bundle is up to 5 GB per month for a fixed fee. Usage above the

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bundle cap is either charged for per megabyte or disallowed. The pay as you use charging is

typically per megabyte of traffic. This contrasts with circuit switching data, which is typically

billed per minute of connection time, regardless of whether or not the user transfers data during

that period.

GPRS is a best-effort service, implying variable throughput and latency that depend on the

number of other users sharing the service concurrently, as opposed to circuit switching, where a

certain quality of service (QoS) is guaranteed during the connection. In 2G systems, GPRS

provides data rates of 56-114 kbit/second.[3]  2G cellular technology combined with GPRS is

sometimes described as 2.5G, that is, a technology between the second (2G) and third (3G) 

generations of mobile telephony.[4]

 It provides moderate-speed data transfer, by using unused time

division multiple access (TDMA) channels in, for example, the GSM system. GPRS is integrated

into GSM Release 97 and newer releases.

5.1.1  GPRS standardization

The ETSI standardisation work on GPRS Phase 1 was officially finalised in Q1/1998. It

includes point-to-point (PTP) services and the complete basic GPRS infrastructure. Air interface,

mobility management, security, limited QoS, SMS service, GPRS support nodes, and the GPRS

backbone are all part of Phase 1.

The ETSI standardisation work on GPRS Phase 2 was frozen with GSM Release 99. Some

work items were included in the GSM Release 98. Phase 2 adds additional services like enhanced

QoS support and point-to-multipoint (PTM) connections. Some main point of GPRS phase 2 are

the support of:

IPv4 and IPv6

BSS co-ordination of radio resource allocation for class A GPRS services

Enhanced QoS support in GPRS

Charging and billing for GPRS – AoC

Charging and billing for GPRS – Pre-paidPoint-to-multipoint (PTM) services

Access to ISPs and intranets in GPRS Phase 2, separation of GPRS bearer establishment

and ISP service environment set-up

In GSM Release 4 (frozen March 2001) and GSM Release 5 (frozen June 2002), QoS

enhancements for the GPRS backbone were introduced to support packet switched real-time

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services (on the long run). This goes hand-in-hand with the introduction of the IP Multimedia

Subsystem (IMS). The Nokia IP Multimedia Subsystem can be combined with terminals

supporting downloadable applications, creating exciting opportunities for application developers

and operators to develop and offer new IP multimedia services in GPRS and 3G networks. Further

information on network details is available in the architecture module.

At the end of the year 2002, more that 120 operators are commercially offering GPRS and more

than 40 operators are testing GPRS or building up a GPRS

5.1.2  Services offered:- 

GPRS uses a packet-based switching technique, which will enhance GSM data services

significantly, especially for bursty Internet/intranet traffic.

Some application examples:

  Bus, train, airline real-time information

  Locating restaurants and other entertainment venues based on current Location

  Lottery

  E-commerce

 

Banking

  E-mail

  Web browsing

The main advantages of GPRS for users:

  Instant access to data as if connected to an office LAN

  Charging based on amount of data transferred (not the time connected)

  Higher transmission speeds

The main advantages for operators:

  Fast network roll-out with minimum investment

  Excess voice capacity used for GPRS data

  Smooth path to 3G services

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  In circuit switching, each time a connection is required between two points, a link between

the two points is established and the needed resources are reserved for the use of that single

call for the complete duration of the call.

  In packet switching, the data to be transferred is divided up into packets, which are then

sent through the network and re-assembled at the receiving end.

 

The GPRS network acts in parallel with the GSM network, providing packet

switched connections to the external networks. The requirements of a GPRS

network are the following:

  The GPRS network must use as much of the existing GSM infrastructure with the

smallest number of modifications to it.

  Since a GPRS user may be on more than one data session, GPRS should be able to

support one or more packet switched connections.

 

To support the budgets of various GPRS users, it must be able to support different

Quality of Service (QoS) subscriptions of the user.

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5.2 GPRS ARCHITECTURE

Figure: the architecture of a GPRS network 

. The GPRS system brings some new network elements to an existing GSM network. These

elements are:

Packet Control Unit (PCU)

Serving GPRS Support Node (SGSN): the MSC of the GPRS network 

Gateway GPRS Support Node (GGSN): gateway to external networks

Border Gateway (BG): a gateway to other PLMN Intra-PLMN backbone: an IP based

network inter-connecting all the GPRS elements

Charging Gateway (CG)

Legal Interception Gateway (LIG)

Domain Name System (DNS)

  GPRS extends the GSM Packet circuit switched data capabilities and makes the

following services possible:

  SMS messaging and broadcasting

  "Always on" internet access

  Multimedia messaging service (MMS)

  Push to talk over cellular (PoC)

  Instant messaging and presence — wireless village

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  Internet applications for smart devices through wireless application protocol (WAP)

  Point-to-point (P2P) service: inter-networking with the Internet (IP)

  Point-to-Multipoint (P2M) service: point-to-multipoint multicast and point-to-

multipoint group calls

  If SMS over GPRS is used, an SMS transmission speed of about 30 SMS messages

per minute may be achieved. This is much faster than using the ordinary SMS over

GSM, whose SMS transmission speed is about 6 to 10 SMS messages per minute.

5.3 EDGE

EDGE, or the Enhanced Data Rate for Global Evolution, is the new mantra in the Global

Internet Connectivity scene. EDGE is the new name for GSM 384. The technology was named

GSM 384 because of the fact that it provided Data Transmission at a rate of 384 Kbps. It consists

of the 8 pattern time slot, and the speed could be achieved when all the 8 time slots were used. The

idea behind EDGE is to obtain even higher data rates on the current 200 KHz GSM carrier, by

changing the type of the modulation used.

Now, this is the most striking feature. EDGE, as being once a GSM technology, works on

the existing GSM or the TDMA carriers, and enables them to many of the 3G services.

Although EDGE will have a little technical impact, since its fully based on GSM or theTDMA carriers, but it might just get an EDGE over the upcomming technologies, and of course,

the GPRS. With EDGE, the operators and service providers can offer more wireless data

application, including wireless multimedia-mail (Web Based), Web Infotainment, and above all,

the technology of Video Conferencing..

The current scenario clearly states that EDGE will definitely score higher than GPRS. The

former, allows its users to increase the data speed and throughput capacity, to around 3-4 times

higher than GPRS.

Secondly, it allows the existing GSM or the TDMA carriers to give the sophisticated 3G

services. And with 1600 Million subscribers of GSM in over 170 countries, offer the full Global

Roaming, anywhere between India to Japan and to San Fransisco.

Based on an 8 PSK modulation, it allows higher bit rate across the air Interface.

One Symbol for every 3 bits. Thus, EDGE Rate = 3x GPRS Rate.

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5.4 INTRODUCTION TO IMT-2000

International Mobile Telecommunications  – 2000 (IMT-2000) is an initiative of ITU that

seeks to integrate the various satellite, terrestrial, fixed and mobile systems currently being

deployed and developed under a single standard or family of standards to promote global servicecapabilities and interoperability after the year 2000.

These services are known as Third Generation or 3G services.

A future standard in which a single inexpensive mobile terminal can truly provide

communications any time, any where.

5.4.1  IMT-2000 Offers

The 3G networks must be capable of providing the following data rates

144 Kbps at mobile speeds

384 Kbps at pedestrian speeds Mbps in fixed locations

3G systems will be capable of providing data rates up to 2 Mbps, in addition to voice, fax

services.

3G networks will offer the high resolution video and multimedia services on the move such

as mobile service, virtual banking, online billing, video conferencing etc.

5.4.2  IMT-2000 Key features and objectives

Incorporation of a variety of systems

A high degree of commonality of design worldwide

Compatibility of services within IMT-2000 and with the fixed network 

High quality and integrity comparable to the fixed network 

Use of small pocket terminal world wide

Connection of mobile users to other mobile users or fixed users

Provisioning of these services over wide range of user densities and coverage areas.

Efficient use of radio spectrum consistent with providing service at acceptable cost.

A modular structure which will allow the system to grow in size and complexity

Provision of a framework for the continuing expansion of mobile network services and

access to services and facilities of the fixed network 

An open architecture which will permit easy introduction of advances in technology of 

different applications

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5.4.3  IMT-2000 will provide..

Enhanced voice quality, ubiquitous coverage and enable operators to provide service at

reasonable cost

Increased network efficiency and capacity

New voice and data services and capabilities

An orderly evolution path from 2G to 3G systems to protect investments.

5.4.4  Spectrum Allocation for IMT-2000

The following spectrum allocations are made for IMT-2000 by ITU till today:

1885-2025 MHz and 2110-2200 MHz (Core band for IMT-2000)

1710-1885 MHz and 2500-2690 MHz (Additional band).

806-960 MHz (Additional band)

5.5  3G CELLULAR SYSTEMS 

3G systems are planned with objective of integration of all kinds of wireless systems into

universal mobile telecommunication system. Work is continuing in European research consortium,

RACE, and in ETSI towards developing UMTS (Universal Mobile Telecommunication System)

on an joint European basis. At the same time ITU is working globally towards IMT-2000

(International Mobile Communication-2000) with mutual agreements and information exchange.

One of the main objective of 3G systems is that they will gather existing mobile services

(cellular, cordless, paging etc.) into one single network. The multiplicity of services and features of 

the system will make it possible for the users to choose among multiple terminals and service

provides. Terminals will become smarter and will be able to support several radio interface with

the help of software radio technology. Among the objectives that have been assigned to 3G system

designers are : voice quality as with fixed networks, satellite services for non covered areas, low

terminal and services costs, high bit rate mobile multi-media services (2 Mbps for indoor andreduced mobility users, 384 Kbps for urban outdoor , and 144 Kbps for rural outdoor), multiple

services per user (speech at 8 Kbps, data at 2,4 or 6 x 64=384 Kbps, video at 384 Kbps and

multimedia, security and antifraud features against access to data by non-authorized people or

entities.

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5.5.1  The architectural starting point 

The allocation in the early 1990s of spectrum for third generation mobile really spurred on

the technology solutions to deliver higher bit rate communications in the mobile arena which had

been restricted to low-rate voice and simple (<10 Kbit/s) circuit-switched data. A variety of drivers

and enabling technologies were tabled to provide the high-bandwidth network capabilities to

support the greater bandwidths expected from the advanced third generation radio technologies.

The protocol included the use of B-ISDN based techniques with ATM, use of pure N-ISDN

technology from both wired and wireless network, along with enhancements of the GSM

architecture.

The issue of incorporating new radio technology has been a significant one for operators

who have had to migrate from first generation (analogue) systems to second generation (digital)

systems such as GSM. The development of the W-CDMA and TDD radio technologies and their

associated radio access networks (RANs) would take a significant amount of time and effort. This

factor, combined with proposals to develop completely new core networks and architectures

would have severely delayed the availability of third generation communications and consequently

a pragmatic approach to the core network architecture was taken . The ensuing architecture grafted

the new UTRAN aspects on to the ‗front end‘ of an ‗evolved‘ GSM/GPRS phase 2 + core

network, comprising mobile switching centers (MSCs) and GPRS support nodes (GSNs). This

resulted in the concept architectures of Figure. This solution enables operator who have GSMnetworks, and also suppliers who have core network product lines offering GSM capability, to

minimize the technical changes from the contemporary GSM infrastructure. This approach also re-

applies the tried and tested GSM roaming, charging, signaling and service mechanisms to UMTS.

5.5.2  Network Launch Configurations

Those network operators who already have GSM/GPRS networks have two basic choices

for the UMTS launch architecture- an integrated solutions or an overlay solution (as shown infigure ). The integrated solution sees the current GSM/GPRS core network aspect upgraded and

reused with the same switching (MSC) and routing (GSN) elements used for both GSM and

UMTS radio. The new UTRAN is connected to this network using the re-application of common

O&M systems, service-delivery mechanisms, switch sites, and platforms; however, the capacity,

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performance and network growth impacts of connecting relatively new and unproven W-CDMA

access technology to a live, service-providing network need careful assessment.

The overlay solution relies upon operators using a different (overlay) network of switching

(MSC) and routing (GSN) elements to support the UMTS radio. The overlay solution enables a

parallel independent development of the UMTS access with lower risks to the live GSM/GPRS

network. The 3G MSCs need similar service delivery mechanisms to be developed (to enable users

to receive equipment services via 3G as well as 2G), as well as O&M and site/platform capacity to

support the new infrastructure. The benefits of the overlay is that operators can roll out and develop

theUMTS aspect of the network in relative isolation from the live, revenue-earning 2G network.

The open Iu(UTRAN core networks) interface enable operators to try an alternate supplier for 3G,

or to source a single supplier, turnkey network .

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WLL:WIRELESS LOCAL LOOP

WLL is a system that connects subscribers to the local telephone station wirelessly.

Systems WLL is based on:

Cellular

Satellite (specific and adjunct)

Microcellular

Other names

Radio In The Loop (RITL)

Fixed-Radio Access (FRA).

A GENERAL WLL SETUP

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6 HISTORY OF WLL

Wireless access first started to become a possibility in the 1950s and 1960s as simple radio

technology reduced in price. For some remote communities in isolated parts of the country, the

most effective manner of providing communication was to provide a radio, kept in a central part of 

the community. By the end of the 1970s, communities linked by radio often had dedicated radio

links to each house, the links connected into the switch such that they were used in the same

manner as normal twisted-pair links. The widespread deployment of the cellular base station into

switching sites helped with cost reduction. Similar access using point-to-point microwave links

still continues to be widely used today.

During the reunification of West and East Germany, much funding was put into increasing

the teledensity in East Germany. The installation of twisted-pair access throughout would have

been a slow process. In the interim, cellular radio was seen to offer a stop-gap measure to provide

rapid telecommunications capability. So in East Germany a number of cellular networks, based

upon the analog Nordic Mobile Telephone (NMT) standard, were deployed in the 800 MHz

frequency range. The key difference was that subscribers had fixed unit mounted to the sides of 

their houses to increase the signal strength and hence allow the networks to be constructed with

larger cells for lower costs. Thus, we see the first WLL network was born.

Early 1950s. Single-channel VHF subscriber equipment was purchased from Motorola, but

the maintenance costs were too high as a result of the valve technology used and the power

consumption too high. The trial was discontinued and the subscribers were connected by wire

Mid-1950s. Raytheon was given seed funds to develop 6 GHz band equipment, which

would have a better reliability and a lower power consumption. The designers failed to achieve

those goals and the system still proved too expensive

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Late1950s. Some equipment capable of providing mobile service to rural communities was

put on trial. Users were prepared to pay a premium for mobile use, but the system still proved to be

too expensive in a fixed application for which users were not prepared to pay a premium.

EarlY1960s. Systems able to operate on a number of radio channels were developed,

eliminating the need for each user to share a specific channel and thus increase capacity. The

general lack of channels and high cost, however, made these systems unattractive.

Early 1970s. A Canadian manufacturer developed equipment operating at 150 MHz that

proved successful in serving fixed subscribers on the island of Lake Superior. The lack of 

frequencies in the band, however, precluded its widespread use.

Late 1970s. The radio equipment from several US manufacturers was linked to provide

service to isolated Puerto Rican villages. The service was possible only because the geographical

location allowed the use of additional channels, providing greater capacity than would have been

possible elsewhere.

Early 1980s. Communication satellites were examined for rural applications but were

rejected as being too expensive.

1985. Trials of a point-to-multipoint radio system using digital modulation promised

sufficient capacity and reliability to make WLL look promising

6.1 WLL VERSUS WIRELINE

The cost of installing or maintain wireline systems broadly depends on the cost of labour

whereas cost of wireless depends on cost of subscriber unit, which tends to fall over time, with

increasing economics of scale.

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Cost of wireline is more due to use of copper a costly metal U/G cables whereas WLL is

independent of this factor.

Cost of wireline critically depends upon distances between houses and penetration levels

achieved. These factors are not there in the WLL.

If a subscriber moves to a different operator in case of wireline system, investment is lost

whereas in case of WLL the subscriber unit in such a case is simply removed and installed

elsewhere.

The cost of wireline system is incurred, even prior to marketing to the users whereas much

of cost of WLL is not incurred until the users subscribe the Network.

The widely used WLL systems are:

1. CorDECT WLL

2. CDMA WLL

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6.2 MOBILE CELLULAR SYSTEM

ANALOG CELLULAR RADIO TECNOLOGIES

There is significant momentum to use analog cellular for WLL because of its wide

availability resulting from serving high-mobility markets. There are three main analog cellular

system types operating in the world: advanced mobile phone system (AMPS), Nordic mobile

telephone (NMT), and total access communications systems (TACS). As a WLL platform, analog

cellular has some limitations in regards to capacity and functionality. In the late 1990's Analog

cellular systems were expected to be the major wireless platform for WLL.

DIGITAL CELLULAR TECNOLOGIES

Digital cellular has seen rapid growth and has outpaced analog cellular over the last few

years. Major worldwide digital cellular standards include global system for mobile

communications (GSM), time-division multiple access (TDMA), Hughes enhanced TDMA (E – 

TDMA), and code-division multiple access (CDMA). Although GSM is a dominant mobile digital

cellular standard, there has been little activity in using GSM as a WLL platform. It offers higher

capacity than the other digital standards (more than 10 to 15 times greater than analog cellular),

relatively high-quality voice, and a high level of privacy. Digital cellular is expected to play an

important role in providing WLL because it, like analog cellular, has the benefit of wide

availability. Digital cellular can support higher capacity subscribers than analog cellular, and it

offers functionality that is better suited to emulate capabilities of advanced wireline networks.

Approximately one-third of the installed WLLs were using digital cellular technology by year

2000.

DRAWBACKS OF MOBILE TELEPHONY:

Lower voice quality.

Higher call blocking rate.

Limited subs density.

Expensive.

CORDLESS TECNOLOGIES:

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CT-2(cordless telephony 2nd

generation)

DECT(digitally enhanced cordless telephony)

PHS(phony handy system)

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CorDECT WLL

corDECT Wireless in Local Loop System is based on Digital Enhanced Cordless

Telecommunications (DECT) standard of European Telecommunications standards Institute

(ETSI).

The CorDECT Wireless in Local Loop has been designed to be a modular system. The

basic unit provides service to 10o00 subscriber. Multiple CorDECT systems can be connected

together using a transit switch. The system has been designed in such a way that the initial

investment for fixed part is very low and most of the cost is incurred on the Subscriber Unit, which

needs to be obtained only when the operator signs up a subscriber. Further since CorDECT

Wireless in Local Loop does not require frequency planning the installation need not be

coordinated. Thus the low cost marks the system one of the most versatile in Local Loop System

available today.

6.3 SYSTEM ARCHITECTURE

The CorDECT system is designed to provide a cost effective wireless high quality voice

and DATA connection in dense Urban as well as sparse rural areas. The system enables wireless

subscriber to be connected to the PSTN in a cost effective manner.

DECT INTERFACE UNIT (DIU)

The DIU is a dect exchanges for Wireless subscriber and provides an interface to a public

Switched Telephone Network (PSTN). Functions such as call processing, CBS powering and

PCM/ADPCM transcoding. DECT Network Layer and link Layer functioned are handled by the

DIU. System operation and Maintenance (O&M) and remote fault monitoring can be performed

from the DIU or alternatively from a remote location using the Network Management System.

For 1000 wireless subscriber the DIU can be configured as –  

An exchange with R2-MF signaling on EI lines or

A Remote Switching Unit (RSU) to an exchange using V5.2 protocols on EI lines or,

An in dialing PBX connected to an exchange using two-wire junction lines or connected to

PSTN using R2-MF signaling on EI lines.

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An optional subscriber MUX (SMUC) unit in the DIU converts the EI interface to 30

 junctions lines, which can be connected to two-wire subscriber lines of an exchange. The SMUX

carries out polarity reversal detection and 16 KHz metering pulse detection. It allows pulse dialing

and DTMF dialing. The two-wire state is coded a transmitted on slot 16 of the EI line.

The DIU consists of between three to six standard 19‖ sub-racks in one or two cabinets

depending on the CorDECT system configuration. All critical cards have a hot standby so that

system availability is ensured in case of failure. The system is powered by 48V power supply.

6.4 COMPACT BASE STATION (CBS)

It provides wireless access in an area and supports twelve simultaneous full duplex

channels. The CBS is a small unobtrusive pole mounted or wall mounted unit. Each CBS serves

one cell providing upto 12 simultaneous speech channel gain of the handset/ wall set. Typically it

ranges from 150m-5kms. (10kms in rural areas).

The CBS has two antenna for diversity. A direction antenna with significant gain can be

used when coverage required is either confined to certain directions, or the coverage area is divided

into sector covered by different CBSs. Otherwise an omni-directional antenna could be used. Such

omni-directional antenna with 2 db, 4db, and 6db gain is available.

The CBS interfaced to the DIU using 3 standard subscriber pair from the existing loop

plant. Typically this would be from the reliable buried portion of the loop plant terminating at the

distribution points. The three pairs carry four ADPCM speech channel each in addition to signaling

data 2 B+D format of N-ISDN communications. The pairs also supply power to the CBS from the

DIU. The maximum distance between CBS and DIU is 4kms. With 0.4 mm dia copper twisted

pairs.

Alternatively the CBSs are interfaced the DIU through the base station distributor (BSD)

unit as shown in fug… In the case of BSD is connection to a DIU with an EI link using radio or 

fibre and CBS are connected to the BSD using three pairs of twisted pair copper able each of 

which carries both the power as well as signal to the CBS. The maximum distance between CBSW

and BSD is 1km when 0.6mm twisted pair copper cable is used.

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BASE STATION DISTRIBUTOR (BSD)

The base station distributor is an optional unit used when a cluster of the CBSS are to be

located some distance away from the DIU. The BSD is connected to the DIU on EI lines & each EI

line carrier signals for four CBSs. The BSD demultiplexes the signal on the EI line and fides it to

the four CBS. The four CBS are connected to the BSD each using 3 pairs of 0.6mm twisted pair

copper wires. The maximum distance supported is 1km. The copper wires carry both power and

signals from BSD to CBS

The health of the BSD as w4ellas the CBSs can be upgraded from the DIU.

WALLSET (SW)

The wall set is a small wall-mounted unit with an external antenna and powered from A/C

Mains. An internal battery provides backup in case of power failure. The external antenna provides

gain and extended the range of a CBS in areas where CBS densidity is low the wall set provides a

standard RJ-11 telephone socket so that any telephone FAX Machine modem or even a payphone

can be connected to it. The data rate supported on modem is typically 9600 kbph as the voice is

code (32 kbps ADPCM)_ before transmission on air.

The wall set software includes modem software DECT MAC layer, link layer Network 

layer and IWU layer software. ADPCM transcoding and DTMF tone detection are also

implemented

in software, in future a V.35 /RS232 interface will be provided at the wall set so that a PC

can be connected to it without a modem. 

7 SWITCHING

Switching in telecommunication is defined as the transfer of call from one user to another.

Circuit switching is the transmission technology that has been used since the

first communication networks in the nineteenth century. In circuit switching, a caller must first

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establish a connection to a called person before any communication is possible. During the

connection establishment, resources are allocated between the caller and the called. Generally,

resources are frequency intervals in a Frequency Division Multiplexing (FDM) scheme or more

recently time slots in a Time Division Multiplexing (TDM) scheme. The set of resources allocated

for a connection is called a circuit, as depicted. A path is a sequence of links located between nodes

called switches. The path taken by data between its source and destination is determined by the

circuit on which it is flowing, and does not change during the lifetime of the connection. The

circuit is terminated when the connection is closed.

7.1 BASIC PRINCIPLE OF ELECTRONIC EXCHANGE

The basic purpose of an exchange is to provide temporary path for simultaneous ,bi-

directional transmission of speech between.

Subscriber lines connected to same exchange (local exchange)

Subscriber lines and trunks to other exchange(outgoing trunk call)

Subscriber lines and trunks from other exchange(incoming trunk call)

Pairs of trunk towards different exchanges (transit switching)

These are also called switching functions of exchange and are implemented through

equipment called switching functions. These are also called the switching functions of an exchange

and are implemented through the equipment called the switching network. An exchange, which

can setup just the first three types of connections is called a Subscriber or Local Exchange. If an

exchange can setup only the fourth type of connections, it is called a Transit or Tandem Exchange.

The other distinguished functions of an exchange are:

Storage program control exchange (SPC )

In a SPC exchanges a processor similar to general purpose computer, is used to control

functions of exchange. All control functions are represented by series of various instructions is

stored in memory. Therefore processor memory holds all exchanges dependent data such as

subscriber date, translation tables, routing and charging information and call records.

In an SPC exchange, all control equipment can be replaced by a single processor. The

processor must, therefore, be quite powerful; typically, it must process hundreds of calls per

second, in addition to performing other administrative and maintenance tasks. However,

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totally centralized control has drawbacks. The software for such a central processor will be

voluminous, complex, and difficult to develop reliably. Moreover, it is not a good arrangement

from the point of view of system security, as the entire system will collapse with the failure of the

processor. These difficulties can be overcome by decentralizing the control. Some routine

functions, such as scanning, signal distributing, marking, which are independent of call processing,

can be delegated to auxiliary or peripheral processors. These peripheral units, each with specialized

function, are often themselves controlled by small stored programs processors, thus reducing the

size and complexity at central control level. Since, they have to handle only one function, their

programs are less voluminous and far less subjected to change than those at central. Therefore, the

associated program memory need not be modifiable (generally, semiconductors ROM's are used).

Basic schematic of SPC exchange

SPC exchanges consist of six main subsystems

Terminating equipment

Switching network 

Switching processor

Switching peripherals

Signaling interfaces

Data processing peripherals

Terminating equipment

In this equipment, line, trunk, and service circuits are terminated for detection, signaling,

speech transmission, and supervision of calls. In contrast to electromechanical circuits, the Trunk 

and Service circuits in SPC exchanges are considerably simpler because functions, like counting,

pulsing, timing charging, etc. are delegated to stored program.

Switching network

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In an electronic exchange, the switching network is one of the largest sub-system in terms

of size of the equipment. Its main functions are

It mainly performs two main functions:-

Switching i.e. setting up temporary connection between two or more exchange

terminations

for transmission of speech and signal between these terminations.

7.2 TYPES OF SWITCHING NETWORK

There are two types of electronic switching systems Viz. Space Division and Time

Division

a) Space division switching system

In it a continuous physical path is set up between i/p and o/p terminations and this path is

separate for each connection and is held for entire duration of call .path for different connections is

independent of each other. Once a continuous is established, signals are interchanged between two

terminations. They have advantage of compatibility with existing line and trunk signaling

conditions network.

b) Time division switching system

In it a number of connections share the same path on time division sharing basis. Path is

not separate for each call but it is shared sequentially for a fraction of time by different calls. This

process is repeated periodically at suitable high rate.

The repetition rate is 8 KHz i.e. once every 125 microseconds for transmitting speech on

network by many calls. The Time Division Switching was initially accomplished by Pulse

Amplitude Modulation (PAM). With the advent of Pulse Code Modulation (PCM), the PAM

signals were converted into a digital format overcoming the limitations of analog and PAM signals.

PCM signals are suitable for both transmission and switching. The PCM switching is popularly

called Digital Switching.

iii) Switching processor

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The Switching processor is a special purpose real time computer, designed and optimized

for dedicated applications of processing telephone calls. It has to perform some functions like

reception of dialed digits, sending of digits in case of transit exchange.

Central control processor: - It is a high speed data processing unit, which controls the

operation of switching network. It mainly controls the three sections as shown.

Program store: - In it set of instructions called programs are stored. These programs are

interpreted and executed by the central control.

Translation store: - It contains information regarding lines. E.g. category of calling and

called line, routing code, charging information, etc.

Data store: -  It provides space for temporary storage of transient data, required in

processing telephone calls, such as digits dialed by subscriber, busy/idle state of lines and trunks,

etc.

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iv) Switching peripheral equipment

The time interval in which switching processor operates is in order of microsecond where

as components in telephone operates in milliseconds. The interface used to connect them is known

as switching peripheral equipment

The various equipments used are:-

Scanner

Its purpose is to detect and inform CC of all significant events / signals on

subscriber lines and trunks connected to the exchange. These signals may either be continuous or

discrete. The equipments at which the events / signals must be detected are equally diverse.

i. Terminal equipment for subscriber lines and inter-exchange trunks and.

ii. Common equipment such as DTMF (Dual - Tone Multi Frequency) or MFC digit

receivers and inter-exchange signaling senders / receivers connected to the lines and trunks.

To detect new calls, while complying with the dial tone connection specifications,

each line must be scanned about every 300 milliseconds. It means that in a 40,000 lines exchange

(normal size electronic exchange) 5000 orders are to be issued every 300 milliseconds, assuming

that eight lines are scanned simultaneously.

Marker

Marker performs physical setup and release of paths through the switching network,

under the control of CC. A path is physically operated only when it has been reserved in the central

control memory. Similarly, paths are physically released before being cleared in memory, to keep

the memory information updated vis-à-vis switching network. Depending upon whether switching

is Time division or Space division, marker either writes information in the control memory (Time

Division Switching), or physical operates the cross - points (Space Division Switching).

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Distributor

It is a buffer between high - speed - low - power CC and relatively slow-speed-

high-power signaling terminal circuits. A signal distributor operates or releases electrically latching

relays in trunks and service circuits, under the direction of central control.

Bus System

Various switching peripherals are connected to the central processor by means of a

common system. A bus is a group of wires on which data and commands pulses are transmitted

between the various sub- units of a switching processor or between switching processor and

switching peripherals. The device to be activated is addressed by sending its address on the address

bus. The common bus system avoids the costly mesh type of interconnection among various

devices.

Line Interface Circuits

To enable an electronic exchange to function with the existing outdoor telephone

network, certain interfaces are required between the network and the electronic exchange.

Analogue Subscriber Line Interface

The functions of a Subscriber Line Interface, for each two-wire line, are often

known by the acronym: BORSCHT

B: Battery feed

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O: Overload protection

R: Ringing

S: Supervision of loop status

C: Codec

H: Hybrid

T: Connection to test equipment

All these functions cannot be performed directly by the electronic circuits and,

therefore, suitable interfaces are required.

v) Transmission Interface

Transmission interface between analogue trunks and digital trunks (individual or

multiplexed) such as, A/D and D/A converters, are known as CODEC, These may be provided on

a per-line and per-trunk basis or on the basis of one per 30 speech channels.

Signaling Interfaces

A typical telephone network may have various exchange systems (Manual,

Stronger, Cross bar, Electronic) each having different signaling schemes. In such an environment,

an exchange must accommodate several different signaling codes.

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8 NETWORKING

8.1  OSI MODEL 

8.1.1  INTRODUCTION

The Open systems Interconnection Reference Model (OSI Reference Model or OSI Model)

is an abstract description for layered communications and computer network protocol design. It

was developed as part of the Open Systems Interconnection (OSI) initiative. In its most basic form,

it divides network architecture into seven layers which, from top to bottom, are the Application,

Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often

referred to as the OSI Seven Layer Model. A layer is a collection of conceptually similar functions

that provide services to the layer above it and receives service from the layer below it.

Application

(Layer 7)

This layer supports application and end-user processes.

Communication partners are identified, quality of service is identified,

user authentication and privacy are considered, and any constraints on

data syntax are identified. Everything at this layer is application-

specific. This layer provides application services for file transfers, e-

mail, and other network  software services. Telnet and FTP are

applications that exist entirely in the application level. Tiered

application architectures are part of this layer.

Presentation(Layer 6)

This layer provides independence from differences in data

representation (e.g., encryption) by translating from application to

network format, and vice versa. The presentation layer works to

transform data into the form that the application layer can accept. This

layer formats and encrypts data to be sent across a network, providing

freedom from compatibility problems. It is sometimes called the

syntax layer.

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Session

(Layer 5)

This layer establishes, manages and terminates connections

between applications. The session layer sets up, coordinates, and

terminates conversations, exchanges, and dialogues between the

applications at each end. It deals with session and connection

coordination.

Transport

(Layer 4)

This layer provides transparent transfer of data between end

systems, or hosts, and is responsible for end-to-end error recovery and

flow control. It ensures complete data transfer.

Network 

(Layer 3)

This layer provides switching and routing technologies,

creating logical paths, known as virtual circuits, for transmitting data

from node to node. Routing and forwarding are functions of this layer,

as well as addressing,  internetworking, error handling, congestion

control and packet sequencing.

Data Link 

(Layer 2)

At this layer, data packets are encoded and decoded into bits. It

furnishes transmission protocol knowledge and management and

handles errors in the physical layer, flow control and frame

synchronization. The data link layer is divided into two sub layers:

The Media Access Control (MAC) layer and the Logical Link Control

(LLC) layer. The MAC sub layer controls how a computer on the

network gains access to the data and permission to transmit it. The

LLC layer controls frame synchronization, flow control and error

checking.

Physical

(Layer 1)

This layer conveys the bit stream - electrical impulse, light or

radio signal -- through the network at the electrical and mechanical

level. It provides the hardware means of sending and receiving data on

a carrier, including defining cables, cards and physical aspects. Fast

Ethernet,  RS232, and ATM are protocols with physical layer

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components.

How Data Flows

TCP/IP MODEL

TCP/IP is a suite of  protocols. The acronym TCP/IP means "Transmission Control

Protocol/Internet Protocol". It comes from the names of the two major protocols in the suite of 

protocols, i.e. the TCP and IP protocols).

In some ways, TCP/IP represents all communication rules for the internet and is based on

the IP addressing notion, i.e. the idea of providing an IP address for each machine on the network 

so as to be able to route data packets. Given that the TCP/IP protocol suite was originally created

with a military purpose, it is designed to respond to a certain number of criteria, including:

Splitting messages into packets. 

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Use of an address system. 

Routing data over the network. 

Error detection in data transmissions. 

Data encapsulation

During a transmission, data crosses each one of the layers at the level of the originator

machine. At each layer, a piece of information is added to the data packet, this is a header, a

collection of information which guarantees transmission. At the level of the recipient machine,

when passing through each layer, the header is read, and then deleted. So, upon its receipt, the

message is in its original state...

At each level, the data packet changes aspect, because a header is added to it, so the

designations change according to the layers:

The data packet is called a message at Application layer level

The message is then encapsulated in the form of a segment in the Transport layer

Once the segment is encapsulated in the Internet layer it takes the name of datagram

Finally, we talk about a frame at the Network Access layer level

The Data Encapsulation Process

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1. One computer requests to send data to another over a network.

2. The data message flows through the Application Layer by using a TCP or UDP port to

pass onto the internet layer.

3. The data segment obtains logical addressing at the Internet Layer via the IP protocol, and

the data is then encapsulated into a datagram.

4. The datagram enters the Network Access Layer, where software will interface with the

physical network. A data frame encapsulates the datagram for entry onto the physical network. At

the end of the process, the frame is converted to a stream of bits that is then transmitted to the

receiving computer.

5. The receiving computer removes the frame, and passes the packet onto the Internet

Layer. The Internet Layer will then remove the header information and send the data to the

Transport layer. Likewise, the Transport layer removes header information and passes data to the

final layer. At this final layer the data is whole again, and can be read by the receiving computer if 

no errors are present.

The TCP/IP model, inspired by the OSI model, also uses the modular approach (use of 

modules or layers) but only contains four:

The roles of the different layers are as follows:

Network Access layer: specifies the form in which data must be routed whichever type of 

network is used.

Internet layer: responsible for supplying the data packet (datagram)

Transport layer: provides the routing data, along with the mechanisms making it possible to

know the status of the transmission

Application layer: incorporates standard network applications (Telnet, SMTP, FTP,).

Network Access layerThe network access layer is the first layer of the TCP/IP stack; it offers the ability to access

whichever physical network, i.e. the resources to be implemented so as to transmit data via a

network.

So, the network access layer contains all specifications relating to the transmission of data over a

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physical network, when it is a local area network  (Token ring,  Ethernet,  FDDI), connected by

telephone line or any other type of link to a network. It deals with the following concepts:

Routing data over the connection

Coordination of the data transmission (synchronisation)

Data format

Signal conversion (analogue/digital)

Error detection on arrival.

The Internet layer

The Internet layer is the "most important" layer (they are all important in their way)

because it is this which defines the datagram and manages the IP addressing notions.

It enables the routing of datagram (data packets) to remote machines along with the management

of their division and assembly upon receipt.

The Transport layer

The protocols for the preceding layers make it possible to send information from one

machine to another. The transport layer enables applications running on remote machines to

communicate. The problem is identifying these applications.

In fact, depending on the machine and its operating system, the application may be a program, task,

process...

Furthermore, the name of the application may vary from system to system, that is why a

numbering system has been put in place so as to be able to associate an application type with a data

type, these identifiers are called ports. 

The transport layer contains two protocols enabling two applications to exchange data

independently of the type of network taken (i.e. independently of the lower layers), these are the

following two protocols:

TCP, a connection orientated protocol which provides error detection.

UDP, a connectionless orientated protocol where error detection is outdated.

The Application layer

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The application layer is located at the top of the TCP/IP protocol layers. This one contains

the network applications which make it possible to communicate using the lower layers. The

software in this layer therefore communicates using one of the two protocols of the layer below

(the transport layer), i.e. TCP or UDP. 

There are different types of applications for this layer, but the majorities are network 

services, or applications supplied to the user to provide the interface with the operating system.

They can be classed according to the services that they provide:

File and print management services (transfer)

Network connection services

Remote connection services

Various Internet utilities

IP protocol is part of the Internet layer of the TCP/IP protocol suite. It is one of the most

important Internet protocols because it allows the development and transport of IP datagram‘s

(data packets), without however ensuring their "delivery". In reality, IP protocol processes IP

datagram‘s independently from each other by defining their representation, routing and forwarding.

IP protocol determines the recipient of the message using 3 fields:

The IP address field: machine address

The subnet mask field: a subnet mask enables the IP protocol to establish the part of the IP

address which relates to the network 

The default gateway field: enables the Internet protocol to know which machine to deliver

a datagram to if ever the destination machine is not on the local area network.

The aim of TCP

Using the TCP protocol, applications can communicate securely (thanks to the TCP

protocol's acknowledgements system), independently from the lower layers. This means that

routers (which work in the internet layer) only have to route data in the form of datagram, without

being concerned with data monitoring because this is performed by the transport layer (or more

specifically by the TCP protocol).

During a communication using the TCP protocol, the two machines must establish a

connection. The originator machine (the one which requests the connection) is called the client, 

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while the recipient machine is called the server. So it is said that we are in a Client-Server 

environment.

The machines in such an environment communicate in online mode, i.e. the communication takes

place in both directions.

To enable the communication and all the controls which accompany it to operate well, the

data is encapsulated, i.e. a header is added to data packets which will enable the transmissions to be

synchronised and ensure their reception.

Another feature of TCP is the ability to control the data speed using its capability to issue

variably sized messages, these messages are called segments.

The multiplexing function

TCP makes it possible to carry out an important task: multiplexing/demultiplexing, i.e. to

convey data from various applications on the same line or in other words put information arriving

in parallel into order.

These operations are conducted using the concept of  ports  (or sockets), i.e. a number

linked to an application type which, when combined with an IP address, makes it possible to

uniquely determine an application which is running on a given machine.

8.2  USE OF AN ADDRESS SYSTEM

Computers communicate over the Internet using the IP protocol (Internet Protocol), which

uses numerical addresses, called IP addresses, made up of four whole numbers (4 bytes) between 0

and 255 and written in the format xxx.xxx.xxx.xxx. For example, 194.153.205.26 is an IP address

given in technical format.

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These addresses are used by networked computers to communicate, so each computer on a

network has a unique IP address on that network.

Every network interface on a TCP/IP device is identified by a globally unique IP address.

Host devices, for example, PCs, typically have a single IP address. Routers typically have two or

more IP addresses, depending on the number of interfaces they have. Each IP address is 32 bits

long and is composed of four 8-bit fields called octets. This address is normally represented in

―dotted decimal notation‖ by grouping the four octets and representing each octet in decimal form.

Each octet represents a decimal number in the range 0-255. For example, 11000001 10100000

00000001 00000101, is known as 193.160.1.5. Each IP address defines the network ID and host ID

of the device. The network ID part of the IP address is centrally administered by the Internet

Network Information Centre (Inter NIC) and is unique throughout the Internet. The host ID is

assigned by the authority which controls the network. The network ID identifies the systems that

are located on the same network or subnet. The network ID must be unique to the internetwork.

The host ID identifies a TCP/IP network device (or host) within a network. The address for each

host must be unique to the network ID. In the example above, the PC is connected to network 

―193.160.1.0‖ and has a unique host ID of ―.5‖. 

Special addresses.

When the host-id is cancelled, i.e. when the bits reserved for the machines on the network 

are replaced by zeros (for example 194.28.12.0), something called a network address is obtained.

This address cannot be allocated to any of the computers on the network.

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When the net ID is cancelled, i.e. when the bits reserved for the network are replaced by

zeros, a machine address is obtained. This address represents the machine specified by the host-ID

which is found on the current network.

When all the bits of the host-id are at 1, the address obtained is called the broadcast

address. This a specific address, enabling a message to be sent to all the machines on the network 

specified by the net ID

Conversely, when all the bits of the net ID are at 1, the address obtained is called the

multicast address.

Finally the address 127.0.0.1 is called the loopback address because it indicates the local

host.

Network classes

IP addresses are divided into classes, according to the number of bytes which represent the

network.

Class A

In a class A IP address, the first byte represents the network.

The most significant bit (the first bit, that to the left) is at zero which means that there are 27 

(00000000 to 01111111) network possibilities, which is 128 possibilities However, the 0 network 

(bits valuing 00000000) does not exist and number 127 is reserved to indicate your machine.

The networks available in class A are therefore networks going from 1.0.0.0 to 126.0.0.0

(the last bytes are zeros which indicate that this is indeed a network and not computers!)

The three bytes to the left represent the computers on the network, the network can

therefore contain a number of computers equal to:

224-2 = 16,777,214 computers.

A class A IP address, in binary looks like:

x

xxxxxx

x

xxxxxxx

x

xxxxxxx

x

xxxxxxx

Ne

twork Computers

Class B

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In a class B IP address, the first two bytes represent the network.

The first two bits are 1 and 0, which means that there are 214

(10 000000 00000000 to 10

111111 11111111) network possibilities, which is 16,384 possible networks. The networks

available in class B are therefore networks going from 128.0.0.0 to 191.255.0.0.

The two bytes to the left represent the computers on the network. The network can

therefore contain a number of computers equal to:

216-21 = 65,534 computers.

A class B IP address, in binary looks like:

0 xxxxx

x

xxxxxxx

x

xxxxxxx

x

xxxxxxx

Network Computers

Class C

In a class C IP address, the first three bytes represent the network. The first three bits are 11

and 0 which means that there are 221

network possibilities, i.e. 2,097,152. The networks available

in class C are therefore networks going from 192.0.0.0 to 223.255.255.0.

The byte to the left represents the computers on the network, the network can therefore

contain:

28-2

1= 254 computers.

In binary, a class C IP address looks like:

10 xxxx

x

xxxxxxx

x

xxxxxxx

xx

xxxxxx

Network Co

mputers

Allocation of IP addresses

The aim of dividing IP addresses into three classes A, B and C is to make the search for a

computer on the network easier. In fact, with this notation it is possible to firstly search for the

network that you want to reach, then search for a computer on this network. So, allocation of IP

address is done according to the size of the network.

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lass

Number of possible

networks

Maximum number of computers on

each one

126 16777214

16384 65534

2097152 254

Class A addresses are used for very large networks, while class C addresses are for

example allocated to small company networks.

8.2.1  Reserved IP addresses

It frequently happens that in a company or organization only one computer is linked to the

Internet and it is through this that other computers on the network access the Internet (generally we

talk of a proxy or gateway).

In such a case, only the computer linked to the network needs to reserve an IP address with

ICANN. However, the other computers still need an IP address to be able to communicate with

each other internally.

So, ICANN has reserved a handful of addresses in each class to enable an IP address to be

allocated to computers on a local network linked to the Internet without the risk of creating IPaddress conflicts on the network of networks. These are the following addresses:

Private class A IP addresses: 10.0.0.1 to 10.255.255.254, enabling the creation of large

private networks comprising of thousands of computers.

Private class B IP addresses: 172.16.0.1 to 172.31.255.254, making it possible to create

medium sized private networks.

Private class C IP addresses: 192.168.0.1 to 192.168.0.254, for putting in place small

private networks.

Subnet masks

In short, a mask is produced containing 1s with the location of bits that you want to keep

and 0s for those you want to cancel. Once this mask is created, you simply put a logical AND

between the values you want to mask and the mask in order to keep the part you wish to cancel

separate from the rest.

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So a net mask is presented in the form of 4 bytes separated by dots (like an IP address), it

comprises (in its binary notation) zeros at the level of the bits from the IP address that you wish to

cancel (and ones at the level of those you want to keep).

Importance of subnet masks

The primary importance of a subnet mask is to enable the simple identification of the

network associated to an IP address.

Indeed, the network is determined by a certain number of bytes in the IP address (1 byte for

class A addresses, 2 for class B and 3 bytes for class C). However, a network is written by taking

the number of bytes which characterize it, then completing it with zeros. For example, the network 

linked to the address 34.56.123.12 is 34.0.0.0, because it is a class A type IP address.

To find out the network address linked to the IP address 34.56.123.12, you simply need to

apply a mask where the first byte is only made up of 1s (which is 255 in decimal), then 0s in the

following bytes.

The mask is: 11111111.00000000.00000000.00000000

the mask associated with the IP address 34.208.123.12 is therefore 255.0.0.0.

The binary value of 34.208.123.12 is: 00100010.11010000.01111011.00001100

so an AND logic between the IP address and the mask gives the following result:

00100010.11010000.01111011.00001100

AND

11111111.00000000.00000000.00000000

=

00100010.00000000.00000000.00000000

Which is 34.0.0.0? It is the network linked to the address 34.208.123.12

By generalizing, it is possible to obtain masks relating to each class of address:

For a Class A address, only the first byte must be retained. The mask has the following

format 11111111.00000000.00000000.00000000, i.e. 255.0.0.0 in decimal;

For a Class B address, the first two bytes must be retained, which gives the following mask 

11111111.11111111.00000000.00000000, relating to 255.255.0.0 in decimal;

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For a Class C address, by the same reasoning, the mask will have the following format

11111111.11111111.11111111.00000000, i.e. 255.255.255.0 in decimal;

Creation of subnets

Let us re-examine the example of the network 34.0.0.0, and assume that we want the first

two bits of the second byte to make it possible to indicate the network.

The mask to be applied will then be:

11111111.11000000.00000000.00000000

That is 255.192.0.0

If we apply this mask to the address 34.208.123.12 we get:

34.192.0.0

In reality there are 4 possible scenarios for the result of the masking of an IP address of a

computer on the network 34.0.0.0

When the first two bits of the second byte are 00, in which case the result of the masking is

34.0.0.0

When the first two bits of the second byte are 01, in which case the result of the masking is

34.64.0.0

When the first two bits of the second byte are 10, in which case the result of the masking is

34.128.0.0

When the first two bits of the second byte are 11, in which case the result of the masking is

34.192.0.0

Therefore, this masking divides a class A network (able to allow 16,777,214 computers)

into 4 subnets - from where the name of subnet mask - can allow 222 computers or 4,194,304

computers.

It may be interesting to note that in these two cases, the total number of computers is the

same, which is 16,777,214 computers (4 x 4,194,304 - 2 = 16,777,214).

The number of subnets depends on the number of additional bits allocated to the network 

(here 2). The number of subnets is therefore:

Routing data over the network. 

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Routers

Routers are devices which make it possible to "choose" the path that datagrams will take to

arrive at the destination.

They are machines with several network interface cards each one of which is linked to a different

network. So, in the simplest configuration, the router only has to "look at" what network a

computer is located on to send datagrams to it from the originator.

However, on the Internet the schema is much more complicated for the following reasons:

The number of networks to which a router is connected is generally large

The networks to which the router is linked can be linked to other networks that the router

cannot see directly

So, routers work using routing tables and protocols, according to the following model:

The router receives a frame from a machine connected to one of the networks it is attached

to

Datagrams are sent on the IP layer.

The router looks at the datagram's header

If the destination IP address belongs to one of the networks to which one of the router

interfaces is attached, the information must be sent at layer 4 after the IP header has been un

encapsulated (removed)

If the destination IP address is part of a different network, the router consults its routing

table, a table which establishes the path to take for a given address.

The router sends the datagram using the network interface card linked to the network on

which the router decides to send the packet.

So, there are two scenarios, either the originator and recipient belong to the same network 

in which case we talk about direct delivery, or there is at least one router between the originator

and recipient, in which case we talk about indirect delivery. In the case of indirect delivery, the role

of the router and in particular that of the routing table is very important. So, the operation of a

router is determined by the way in which this routing table is created.

If the routing table is entered manually by the administrator, it is a static routing (suitable

for small networks)

If the router builds its own routing tables using information that it receives (via the routing

protocols), it is a dynamic routing.

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8.3 STATIC ROUTING

Routes to destinations are set up manually.

Network reachability is not dependent on existence and state of network.

Route may be up or down but static routes will remain in routing tables and traffic would

still be sent towards route.

Not suitable for large number of network.

It is also known as non-adaptive routing.

DYNAMIC ROUTING

Routes are made via internal and external routing protocols.

Network reachability is dependent on the existence and State of network.

It is also known as adaptive routing.

The routing protocols are:

- RIP (routing information protocols)

- OSPF (open shortest path first)

8.4 SUBNETTING

A sub network, or subnet, describes networked computers and devices that have a common,

designated IP address routing prefix.

Subnetting is used to break the network into smaller more efficient subnets to prevent

excessive rates of  Ethernet  packet collision in a large network. Such subnets can be arranged

hierarchically, with the organization's network address space (see also Autonomous System) 

partitioned into a tree-like structure. Routers  are used to manage traffic and constitute borders

between subnets.

A routing prefix is the sequence of leading bits of an IP address that precede the portion of 

the address used as host identifier. In IPv4  networks, the routing prefix is often expressed as a

"subnet mask", which is a bit mask covering the number of bits used in the prefix. An IPv4 subnet

mask is frequently expressed in quad-dotted decimal representation, e.g., 255.255.255.0 is the

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subnet mask for the 192.168.1.0 network with a 24-bit routing prefix (192.168.1.0/24). All hosts

within a subnet can be reached in one "hop" (time to live = 1), implying that all hosts in a subnet

are connected to the same link. 

A typical subnet is a physical network served by one router, for instance an Ethernet 

network (consisting of one or several Ethernet segments or local area networks, interconnected by

network switches and network bridges) or a Virtual Local Area Network (VLAN). However,

subnetting allows the network to be logically divided regardless of the physical layout of a

network, since it is possible to divide a physical network into several subnets by configuring

different host computers to use different routers.

While improving network performance, subnetting increases routing complexity, since

each locally connected subnet is typically represented by one row in the routing tables  in each

connected router. However, with a clever design of the network, routes to collections of more

distant subnets within the branches of a tree-hierarchy can be aggregated by single routes. Existing

subnetting functionality in routers made the introduction of  Classless Inter-Domain Routing 

seamless.

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•Class A natural mask 255.0.0.0 

•Class B natural mask 255.255.0.0 

•Class C natural mask 255.255.255.0 

By separating the network and host IDs of an IP address, masks facilitate the creation of 

subnets. With the use of masks, networks can be divided into sub networks by extending the

network IDs of the address into the host ID. Subnetting increases the number of sub networks and

reduces the number of hosts.

Defining a subnet mask based on the number of subnets required

Add two to the number of subnets required and convert to binary

Count the number of bits required

Convert the required number of bits to decimal in high order

Example: Class C address, 5 subnets required

7 converted to binary is 110 ( 3 bits)

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Three bits are required so configure the first three bits of the host ID as the subnet ID

The decimal value for 1110 0000 is 224

The subnet mask is 255.255.255.224 for this class C address

If you are dividing your network into subnets, you need to define a subnet mask. Follow

these steps:

1. Determine the number of subnets you require. Add two to the number of subnets

required and convert to binary.

2. Count the number of bits required to represent the number of physical segments in

binary. For example, if you need five subnets, the binary value of seven is 110. Representing seven

in binary requires three bits.

3. Convert the required number of bits to decimal format in high order (from left to right).

For example, if three bits are required, configure the first three bits of the host ID as the subnet ID.

The decimal value for binary 11100000 is 224. The subnet mask is 255.255.225.224 (for a Class C

address).

Defining a subnet mask based on the number of hosts

Add two to the number of hosts required and convert the sum to binary

Count the number of bits required for the host portion

Subtract this number from the total number of bits in the host ID

Convert the required number of bits to decimal in high order

Example:

Class B address, 2000 devices per subnet required

2002 converted to binary is 11111010010 ( 11 bits)

Eleven bits are required for the host so configure the first five bits of the host

ID as the subnet ID (16 - 11 = 5)

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The decimal value for 1111 1000 is 248

The subnet mask is 255.255.248.0 for a class B address

If you do not want all your hosts to be on the same subnet, you need to define a subnet

mask, assuming that you have been allocated a single network address. Follow these steps:

1. Decide on the number of hosts you want to have on each subnet. Convert this number to

binary format.

2. Count the number of bits required to represent the number of hosts in binary. For

example, if you want up to 2,000 hosts per subnet, the binary value for 2002 is 11111010010.

Representing 2,002 in binary requires 11 bits. To calculate the number of bits required for the

mask, subtract the number of bits required for the host from the total number of bits in the host. In

this example the result is five (16 - 11).

4. Convert the required number of bits to decimal format in high order (from left to right).

In this example, five bits are required. Configure the first five bits of the host ID as the subnet ID.

The decimal value for 11111000 is 248. The subnet mask is 255.255.248.0 (for a class B address).

The subnet conversion table above shows all the possible combinations of subnets and

hosts for a Class C network address. For example, if we want to implement five subnets, we would

use a subnet mask of 255.255.255.224. This would allow up to a maximum of six subnets with 30

devices per subnet. If there are zero bits in the subnet

mask we are not using subnetting and are left with the default of one network with 254

hosts. We cannot just use one bit in the subnet mask because the only subnet IDs would be 0 and 1

neither of which are valid. Similarly we cannot use 7 bits in the subnet ID because the only host

IDs would again be 0.

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The subnet conversion table above shows all the possible combinations of subnets and

hosts for a Class C network address. For example, if we want to implement five subnets, we would

use a subnet mask of 255.255.255.224. This would allow up to a maximum of six subnets with 30

devices per subnet. If there are zero bits in the subnet mask we are not using subnetting and are left

with the default of one network with 254 hosts. We cannot just use one bit in the subnet mask 

because the only subnet IDs would be 0 and 1 neither of which are valid. Similarly we cannot use 7

bits in the subnet ID because the only host IDs would again be 0.

When a portion of the address, blocked out by the subnet mask changes, the network 

devices know that these addresses are in different subnets. For example, for all addresses between

16 and 31 in the diagram above, the 4 bits blocked by the mask are 0001. These are on the same

subnet. Therefore, for address 32 which is binary 0010 0000, we can see that the four bits blocked

by the mask portion have changed. Therefore this must be a different subnet. Note: in the example

above, 16 is the subnet ID but it is not a valid host ID since 16 = 0001 0000 and we cannot have all

zeros in the host portion. Similarly 31 is not a valid host ID since 31 = 0001 1111 which is the

broadcast address for this subnet. Subnet IDs comprised of all 0s or all 1s are called special case

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subnet addresses. A subnet ID of all 1s indicates a subnet broadcast while a subnet ID of all 0s

indicates ―this subnet‖. When subnetting it is strongly

recommended not to use these subnet IDs. However, it is possible to use these special case

subnet addresses if they are supported by all routers and hardware on the network. Request For

Comment (RFC) 950 details the limitations imposed when using special case addresses.

Shortcut method for defining Subnet ID‘s using the Subnet Conversion Table 

From the maximum number of hosts

Add 2 to the maximum number of hosts and this gives the first valid subnet ID. All

subsequent IDs are multiples of the first valid subnet ID.

Example: maximum number of hosts = 14 14+2=16

Sub net IDs = 16, 32, 48, 64,…….. 

From the maximum number of subnets

Add 2 to the maximum number of subnets. Divide 256 by this number and the result is the

first valid subnet ID. All subsequent ID‘s are multiples of the first valid subnet ID.

Example: maximum number of subnets = 14 14+2=16 256/16 = 16

Sub net Ids = 16, 32, 48, 64.

8.5 SUBNET IDS

There are two shortcut methods to define the subnet ID

1. Based on the subnet conversion table. This is described in the overhead above.

2. Based on the number of bits in the host portion. This is described in the following text.

Shortcut method for defining subnet IDs from the number of bits in the host portion. Count

the number of bits in the host ID portion. Multiply this number by a power of two and this is

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the first valid subnet ID. All subsequent subnet IDs are multiples of the first valid subnet ID.

Mask = 255.255.255.192

192 = 1100 0000

Six bits in host portion 2^6=64

Subnet IDs 0, 64, 128, 192

Mask = 255.255.255.224

224 = 1110 0000

Five bits in host portion 2^5=32

Subnet IDs 0, 32, 64, 96, 128, 160, 192, 224

Mask = 255.255.255.240

240 = 1111 0000

Four bits in host portion 2^4=16

Subnet IDs = 0,16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240

Mask = 255.255.255.248.

248 = 1111 1000

Three bits in host portion 2^3=8

Subnet IDs = 0, 8, 16, 24, 32, 40,……………., 224, 232, 240, 248  

Mask = 255.255.255.252

252 = 1111 1100

Two bits in host portion 2^2=4

Subnet IDs = 0, 4, 8, 12, 61, 20,…………., 240, 244, 248, 252 

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Note; in the last example there are only two valid host IDs on each subnet. For example; in

subnet ID = 4 address 5 and 6 are the only two valid source addresses

the example above, a small company has been assigned a single Class C network. Without

subnetting, up to a maximum of 254 hosts can share this network. In this configuration, if one

device sends out an IP broadcast (e.g. DHCP Discover message) it will be received by every

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device on the network. To improve performance, the network administrator may reduce the

number of devices that receive the broadcast by splitting the network into smaller subnets

separated by a router. In the example above, the network has been split into six smaller subnets

with a maximum of 30 hosts on each subnet. Note: the total maximum number of hosts on the

network has been reduced from 254 to 180 hosts. Consult the subnet conversion table for all

possible combinations of hosts and subnets.

The subnet conversion table above shows all the possible combinations of subnets and

hosts on a class B network address. For example, if we want to implement subnets with

approximately 100 devices on each we would use a subnet mask of 255.255.255.128. This would

allow up to a maximum of 510 subnets with 126 devices on each. A commonly used subnet mask 

in class B networks is 255.255.255.0. This allows for 254 subnets with 254 devices each.

Static Subnetting vs. Variable Length Subnetting

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Static subnetting means that all subnets in the subnetted network use the same subnet mask 

Simple to implement and easy to maintain, but results in wasted address space for small networks.

For example, a network of four hosts that uses a subnet mask of 255.255.255.0 wastes 250

IP addresses

Variable Length subnetting implies that the sub networks that make up the network may

use different subnet masks

A small subnet with only a few hosts needs a subnet mask that accommodates only these

few hosts

Each host on a TCP/IP network requires a subnet mask. A default subnet mask is used

when a network is not divided into subnets. A customized subnet mask is used when a network is

divided into subnets. In a default subnet mask, all bits that correspond to the network ID are set to

1. The decimal value in each of these octets is 255. All bits that correspond to the host ID are set to

0. For example, the class B address 160.30.100.10 has a network ID of 160.30.0.0 and a host ID

100.10. The default mask is therefore 255.255.0.0. There are two types of subnetting: static and

variable length.

8.6 VARIABLE LENGTH SUBNET MASK (VLSM)

Variable Length Subnet Mask (VLSM) refers to the fact that one subnet network can be

configured with different masks

252 (1111 1100) - 62 subnets with 2 hosts each

248 (1111 1000) - 30 subnets with 6 hosts each.

240 (1111 0000) - 14 subnets with 14 hosts each.

224 (1110 0000) - 6 subnets with 30 hosts each.

192 (1100 0000) - 2 subnets with 62 hosts each.

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topology

mesh

star

hybrid

tree

bus

ring

Variable Length Subnet Mask (VLSM) refers to the fact that one network can be

configured with different masks. The idea behind Variable Length Subnet Masks is to offer more

flexibility in dividing a network into multiple subnets while still maintaining an adequate number

of hosts in each subnet. Without VLSM, one subnet mask only can be applied to a network. This

restricts the number of hosts given the number of subnets required. If you pick the mask so that

you have enough subnets, you might not be able to allocate enough hosts in each subnet. The same

is true for the hosts; a mask that allows enough hosts might not provide enough subnet space.

Suppose for example, you were assigned a Class C network 192.214.11.0 and you need to

divide that network into three subnets, with 50 hosts in one subnet and 25 hosts for each of the

remaining subnets. Without subnetting you have 254 addressees available, 192.214.11.1 to

192.214.11.2. The desired subdivision cannot be done without VLSM, as we shall see.

There are a handful of subnet masks of the form 255.255.255.X that can be used to divide

the class C network 192.214.11.0 into more subnets. Remember that a mask should have a

contiguous number of one starting from the left (network portion) and the rest of the bits should be

zeros. The masks shown in the diagram above could be used to segment the 254 addresses

available to you into more subnets.

NETWORK TOPOLOGY

The physical topology of a

network 

refers to the configuration of 

cables, computers, and other peripherals.

The way in which the network is

laid physically or logically.

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Mesh topology

Every computer is connected to each other.

Advantages:

Dedicated link eliminates traffic problem.

Failure of link does not affect other link.

Privacy of link is maintained.

Fault identification and fault isolation is easy.

Disadvantages:

Hardware requirements and i/o ports increases.

More space is required.

Star topology

Each computer has a dedicated link to a central

Controller called HUB or SWITCH. It acts as an exchange.

Advantages:

Less expensive.

Fault identification and isolation is easy.

Failure of one link does not affect other.

Disadvantages:

If HUB or SWITCH fails, then whole network will be

down.

Tree topology

There is one main backbone (central) HUB and

secondary HUB emerges out from it. Central HUB is

active while secondary can be active or passive. Active

HUB contains Repeater.

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Advantages:

More and more devices can be attached.

Allows network to identify and isolate faults.

Point to point wiring for individual segments.

Disadvantages:

If backbone gets down then whole system fails.

Difficult to configure.

Bus topology

It consists of a main run of cable with a terminator at each end.

Advantages:

Ease in installation.

Uses less cable.

Disadvantages:

If backbone fails, whole network ceases.

Difficult to add new devices.

Ring topology

Each device is connected to only two

devices.

Advantages:

Easy in installation.

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To add and delete only 2 connections are altered.

Fault isolation is simplified.

Disadvantages:

Break in ring ceases whole network.

Difficult to add new devices.

Hybrid topology

Combination of all the

topologies.

Advantages:

Increased number of devices.

Contains all topologies

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