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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
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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