MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
__________________________________________________________________________________________________
1
Important Instructions to examiners:
1. The answers should be examined by key words and not as word-to-word as given in the
model answer scheme.
2. The model answer and the answer written by candidate may vary but the examiner may try
to assess the understanding level of the candidate.
3. The language errors such as grammatical, spelling errors should not be given more
Importance (Not applicable for subject English and Communication Skills.
4. While assessing figures, examiner may give credit for principal components indicated in the figure.
The figures drawn by candidate and model answer may vary. The examiner may give credit for
any equivalent figure drawn.
5. Credits may be given step wise for numerical problems. In some cases, the assumed constant
values may vary and there may be some difference in the candidate‟s answers and model answer.
6. In case of some questions credit may be given by judgement on part of examiner of relevant
answer based on candidate‟s understanding.
7. For programming language papers, credit may be given to any other program based on equivalent
concept.
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Q.1A.) Solve any three of the following
a) Describe the basic cellular system.
Answer:- (2M Diagram & 2 M Explanation)
OR
Fig. shows a cellular telephone system that includes all the basic components necessary for cellular
telephone communication.
The radio network is defined by a set of radio frequency transreceiver located within each of the
cells. The location of these radio frequency transreceiver are called base station
Base station: base station serves as central control for all users within that cell.
Mobile unit communicate directly with the base stations & the base stations communicate directly
with a mobile
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
__________________________________________________________________________________________________
2
Telephone switching office (MTSO): The Switching center is one component which co-ordinates
the routing of calls in a large service area. An MTSO controls channel assignment, call processing,
call setup & call termination which includes signaling switching, supervision & allocating radio-
frequency channels. The MTSO provides a centralized administration & maintenance point for the
entire network & interfaces with the public telephone network over wire line voice trunks & data
links.
OR
{Note: Student can interpret block diagram in different way but it should consist basic blocks
marks should be given for that also.}
b) Draw the block diagram of mobile unit & explain each block
Answer:- . (2 M Diagram & 2 M Explanation)
Mobile unit:-
The RF selection contents the transmitter and receiver CKT including mixers local oscillators
or frequency. Synthesizers for channel selection. The receiver LNA and the transmitter power
ampere.
The base band selection contains the Vocoder with its A / D and D/ A converts plus a DSP
chip that handles many processing functions typically performed by analog ckt in older
synthesizers.
An embedded control handles all the digital control and signaling , hand offs and connection
and identification, operation that take place transparent to the subscriber.
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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3
It also takes care of running the display and keyboard and all user functions such as number
storage, auto dialing and caller ID.
This has led to some interesting architectures especially, in the receiver section. Although
super heterodyne design is still used, several variations have emerged as dominant. These are
the down conversion and very low - IF designs.
c) Describe the functional Architecture of IMT-2000
Answer. (2M Diagram & 2M Explanation)
OR
Functional architecture of IMT – 2000 :-
IMT 2000 expected to support a number of different radio operating environments covering
indoor picocells with very high overall system capacity all the way through large outdoor
terrestrial cells and satellite.
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
__________________________________________________________________________________________________
4
The broad classes of functions that need to be supported by an IMT -2000 system are shown in
the fig. above. It is seen that separation of call and collection controls functions and is
desirable for supporting multimedia and advanced services in IMT 2000 .
In this architecture the connection and call control function are integrated rather than separated.
It illustrates one possible assignment of functional entities to physical entities and identifies
relationship between these collection and function that need to be supported by the physical
interface specification.
d) Describe the GSM system architecture with neat block diagram.
Answer : (2 M Diagram & 2M Explanation)
z
Mobile Station
The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called
the Subscriber Identity Module (SIM).
The mobile equipment is uniquely identified by the International Mobile Equipment Identity
(IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to
identify the subscriber to the system, a secret key for authentication, and other information.
Base Station Subsystem:
The Base Station Subsystem is composed of two parts:
1. The Base Transreceiver Station (BTS) and
2. The Base Station Controller (BSC).
These communicate across the standardized Ab is interface, allowing (as in the rest of the system)
operation between components made by different suppliers.
The Base Transreceiver Station houses the radio transreceivers that define a cell and handles the
radio-link protocols with the Mobile Station.
BTS
BTS
BTS
BTS
BTS
BTS
BSC
BSC HLR VLR AUC
OMC
MSC
PSTN
ISDN
DATA
NETWORKS
PUBLIC
NETWORK
NETWORK SWITCHING
SYSTEM BASE STATION
SUBSYSTEM
MS: MOBILE STATION BTS: BASE TRANSWERCIVER STATION
BSC: BASE STATION CONTROLLER MSC: MOBILE SWITCHING CENTER
VLR: VISITOR LOCATION REGISTER HLR: HOME LOCATION REGISTER
AUC: AUTHENTICATION CENTER OMC: OPERATIONAL & MAINTAINANCE
CENTER
PSTN: PUBLIC SWITCH TELEPHONE ISDN: INTEGRATED SERVICES DIGITAL
NETWORK NETWORK
MS
MS
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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5
The Base Station Controller manages the radio resources for one or more BTSs. It handles radio-
channel setup, frequency hopping, and handovers, as described below. The BSC is the connection
between the mobile station and the Mobile service Switching Center (MSC).
Network Subsystem:
The central component of the Network Subsystem is the Mobile services Switching Center (MSC).
It acts like a normal switching node of the PSTN or ISDN, and additionally provides all the
functionality needed to handle a mobile subscriber, such as registration, authentication, location
updating, handovers, and call routing to a roaming subscriber.
These services are provided in conjunction with several functional entities, which together form
the Network Subsystem. The MSC provides the connection to the fixed networks (such as the
PSTN or ISDN). Signaling between functional entities in the Network Subsystem uses Signaling
System Number 7 (SS7), used for trunk signaling in ISDN and widely used in current public
networks.
The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC,
provide the call-routing and roaming capabilities of GSM.
Q.1B.) Solve any one of the following. (6 Marks)
a) With the help of neat sketch explain the forward & reverse link channel structure used in IS-
95.
Answer : (1M Diagram & 2 M Explanation for forward & reverse link structure)
Forward channel is a communication channel used for transmission of information from BS to mobile
station.
Pilot channel: This channel allows a MS to acquire timing for forward CDMA channel. It provides a phase
reference for coherence demodulation. It provides each mobile with an answer for signal strength
comparison between BS for determining when to handoff.
Synchronization channel: The synchronization channel broadcasts synchronization message to the MS.
The sync channel message parameters are System Identification (SID) and Network Identification (NID)
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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6
Paging Channel: The paging channel is used to send control information and paging messages from BS to
the mobile and paging messages from BS to the mobile.
Forward Traffic channel: Information on the forward traffic channel includes the primary traffic (voice
and data) secondary traffic (data) and signaling.
Reverse channel is a communication channel used for transmission of information from MS to base
station
.
Access channel: On access channel mobile users communicate short messages such as information
registration, call origination, and responses to pages, or providing other data to cell site. When mobile user
places a call, it uses the access channel to inform the serving base station.
Reverse Traffic channel: This traffic channel carries the user speech or data. It is similar to the forward
traffic channel and is intended to transfer dedicated user data. The traffic channels in reverse channel are
unique to each mobile user.
b) Draw & describe the transmitter system in cellular communication with block diagram.
Answer:- (3 M Diagram & 3 M Explanation).
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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7
It is a Low-Power FM unit operating in Frequency range of 825 MHz. Channel 1 is 825.03 MHz,
Channel 2 is 825.06 MHz & So on.
The carrier furnished by frequency synthesizer is phase-modulated by the voice signal.
The modulator output is translated up to the final transmitter frequency by a mixer whose second
input also comes from frequency synthesizer.
The mixer output is fed to class c & or class E power ampere stages where output signal is
developed. The final ampere stage is designed to supply about 600 mw to the antenna.
The APC ckt can introduce power attenuation in steps of 4db to 28db.
This s/g is used in APC ckt & is Transmitted back to the cell site, permitting the MTSO to know the
current power level.
The transmitter output is fed to a duplexer ckt or isolator that allows the transmitter & receiver to
share the same antenna. Since cellular telephone units use full duplex operation, the transmitter &
receiver operates simultaneously.
The duplexer consists of two very sharp BPF, one for the transmitter & one for the receiver. The
transmitter output passes through this filter to the antenna.
Q.2) Solve any four of the following.
a) Draw frequency reuse pattern with cluster size 7 & 12.
Answer: - (2 marks for each pattern)
For cluster size 7:
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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8
For cluster size 12:
b) List various features & services of IS-95.
Answer:- Features & services of IS-95 – (Any Eight 4M)
1. Diversity
2. Power control
3. Soft handoff
4. IS-95 system capacity
5. Soft capacity
6. Quality of service
7. Economics
8. Variable Rate Vocoder Offering high speed coding and reducing background noise and system
9. Interference based on the detection of the voice activity.
10. Coding Technique for Enhancing the privacy and security.
11. Roaming
c) Describe the mobile call Terminations
Answer:- . ( 2M Diagram & 2M Explanation)
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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9
Mobile call termination:
1. The PSTN user dials the MSISDN of the called user in GSM.
2. The LE routes the call to the GMSC of the called GSM user.
3. The GMSC uses the dialed MSISDN to determine the serving HLR for the GSM user and
interrogates it to obtain the required routing number.
4. The HLR requests the current serving VLR for the called MS for a MSRN(MS roaming number)
so that the call can be routed to the correct MSC.
5. The HLR passes the MSRN to the GMSC.
6. Using the MSRN, the GMSC routes the call to the serving MSC.
7. The MSC interrogates the VLR for the current location area identity (LAI) for the MS.
8. The VLR provides the current location for the MS.
9. The MSC pages MS via the appropriate BSS. The MS responds to the page and sets up the
necessary signaling links.
10. When the BSS has established the necessary radio links, the MSC is informed and the call is
delivered to the MS.
12. When the MS Answer the call, the connection is completed to the calling PSTN user. The
VLR passes the MSRN to the HLR
d) Draw & describe the network architecture of UMTS
Answer:- (2 marks Diagram & 2 marks Explanation)
The UMTS is a system that is capable of providing a variety of mobile services to a wide range of
global mobile communication standards.
To handle a mixed range of traffic, a mixed cell layout, which would consist of macrocells
overlaid on microcells & picocells is one of the plans being considered.
This type of network distributes the traffic with the local traffic operating on the microcells &
picocell, while the highly mobile traffic is operated on macrocell, thus reducing the number of
handoffs required for fast moving traffic.
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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10
It is observed from the fig. that the macro cells cover the spots not covered by other cells & also
provide redundancy in certain areas.
Thus macro cells will also be able to avoid the failures of the overlapped cells.
e) Define the following terms w.r.t. mobile communication.
Base station
Forward Channel
Control Channel
Voice Channel
Answer:- (1M each)
Base station: - A fixed station in a mobile radio system used for radio communication with
stations. Base stations are located at the center or on the edge of a coverage region & consist of
radio channel & Transmitter and Receiver antennas mounted on a tower.
Forward Channel: - Radio channels used for Transmission of information from the BASE
STATION to the MOBILE.
Control Channel :- Radio channels used for Transmission of call setup, call request, call
Initiation & other beacon or control purpose.
Voice Channel: Radio channels used for Transmission & reception of voice messages.
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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11
Q3 Attempt any four. ( 16 Marks)
a) Describe the near end far end interference in cellular system
Answer:
If motor vehicle in a given cell is moving, then some mobile phone units are close to the
cell site and some are not. The close in mobile phone unit has a strong signal which causes
adjacent-channel interference
In this situation, near end – far end interference can occur only at the reception point in the
cell site
If a separation of five channel bandwidth 95B) is needed for two adjacent channels in a
cell in order to avoid the near end – far end interference, it is then implied that a minimum
separation of 5B is required between each adjacent channel used with one cell
The adjacent channel interference can also occur between two systems in a duopoly market
system
In this situation, adjacent channel interference can occur at both the cell site and the mobile
phone unit
For instance, mobile phone unit A can be located at the boundary of its own home cell A in
the system A but very close to cell B of system B
The other situation would occur if mobile phone unit B was at the boundary of cell B of
system B but very close to cell A of system A.
The same interference will be introduced at cell B and mobile unit B
The two causes of near end – far end interference are as below
Interference caused on the set up channels
Interference caused on the voice channels
The adjacent channel interference can be minimized through careful filtering and channel
assignments
Since each cell is given only a fraction of the available channels a cell need not be assigned
channels which are all adjacent in frequency
By keeping the frequency separation between each channel in a given cell as large as
possible the adjacent channel interference may be reduced considerably
Some channel of allocation schemes also prevent a secondary source of adjacent channel
interference by avoiding the use of adjacent channels in neighboring cell sites
In practice, the base station receivers are preceded by high Q cavity filter in order to reject
adjacent channel interference
The adjacent channel interference can be reduced by the frequency assignment
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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12
b) Describe the WLL technology and also write its application
Answer: (3M diagram , 3 M explanation)
WLL stands for Wireless Local Loop. Microwave wireless links can be used to create a wireless
local loop such as shown in figure below.
Local Loop is a network that resides between the central office (CO) and the individual
homes and business in close proximity to the central office (CO) as shown in figure above.
In most developed countries, copper or optical fiber cable already has been installed to
residence and business.
One more advantage of WLL is that we have to pay only once for that wireless equipment,
after there is no additional costs involved.
System WLL is based on Cellular, satellite, microcellular
The WLL can greatly improve the telecommunication facilities and services in an
inexpensive way.
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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13
OR 3 marks Wi Wireless Access Network Unit (WANU): Interface between underlying telephone network and
wireless link. It consists of Base Transceiver Station (BTS), Radio Controller (RPCU), Access
Manager (AM), Home Location Register (HLR)
Wireless Access Subscriber Unit (WASU): Located at subscriber side. Translates wireless link into
a traditional telephone connection
c) Describe the microcell zone concept
Answer: 2 Marks--- Diagram
2 Marks--- Explanation
The problem associated with the sectoring is the number of handoffs. This puts additional
load on the switching and control link elements of the mobile system.
A solution to this problem is based on microcell concept for seven cell reuse.
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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14
In this scheme, all the three or more zone sites represented as Tx/Rx are connected to the
same base station and share the same radio equipment.
The transmission media used for connecting the zones to the base station are coaxial cable,
fiber optics cable or a microwave link.
So each cell consists of a base station and multiple zones.
A mobile travelling within a cell is served by the zone that has the strongest signal of all.
As shown in figure, the antennas in zones are placed at the outer edge of the cell and any
base station channel can be assigned to any zone by the base station.
As a mobile travels from one zone to the other within a cell, it uses the same channel. This
will avoid handoff. The base station will just switch the channel to the appropriate zone
site.
Thus a given channel is being used only in a particular zone in which the mobile is
travelling. So the base station radiation is localized. This will reduce interference.
The channels are distributed in space and time by all zones and are reused in the co channel
cells.
The microcell zone concept is very useful along highways or in the busy areas
d) List the basic features and coding methods in CDMA
Answer:- The various types of codes and coding methods used in CDMA system are as under:
Auto correlation codes- a) Barker code b) Pseudo- noise codes
Orthogonal codes
Walsh codes
Scrambling codes
Channelization codes
Carrier modulation codes
Features: (any 3 feature) 3 marks
It is wide band DS-CDMA system
W-CDMA assures backward compatibility with the 2G GSM.
Packet data rate-2.048 Mbps
Channel Bandwidth- 5MHz
Provides high capacity upgrade path for GSM
Frame structure- 16 slots per frame
Network structure and bit level packaging of GSM data is retained
Computers, entertainment devices, and telephone may all share same wireless network and
be connected to internet anytime anywhere.
Each channel will be able to support between 100 to 350 voice channels.
Uses Direct Sequence Spread Spectrum (DSSS) with chip rate of 16Mcps.
Provides Videoconferencing and Virtual Home Entertainment (VHE)
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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15
e) Write a short note on the evolution of radio mobile communications
Answer:-
Mobile wireless (or radio) communication has started after of electromagnetic (or radio)
waves by Hertz
Mobile radio communication systems using simplex channel introduced in 1920 for
emergency services in USA
First public mobile radio system in USA introduced in 1946
The cellular concept was developed by ATT in 1947 and commercial tests were done in
1962
In 1970, ATT proposed AMPS.
In 1979 worlds first cellular system was implemented by Nippon Telephone. The analog
cellular mobile systems fall in the category of 1st generation mobile system
Besides frequency reuse, digital signal processing, integrated circuits, increased battery life
lead to exponential growth in mobile communication.
The first digital system cellular system was introduced in 1990 by ETSI for GSM. This was
considered to be second generation mobile comms
The 3rd generation digital cellular system called PDC was developed by Japan for
commercial uses
In late 1991, DSDC system hardware was installed in USA, it improved the capacity by 3
times of that provide by AMPS
A cellular system based on CDMA was developed by Qualcomm and standardized by TIA.
With the rapidly increasing penetration of laptop computers which are primarily used by
mobile users to access internet services. Mobile IP allows IP nodes to change physical
location without having to change IP address there by offering nomadicity to internet users
IMT-2000 standard is being developed to set stage for 3rd generation mobile
communication system. It will consolidate different wireless environment and will also
ensure global mobility in terms of global seamless roaming and delivery of services
ETSI has also developed 3rd generation mobile communication system call UMTS which
belongs to IMT-2000 system
Q-4 Attempt any three
a) Describe the messages and call processing in GSM system
Answer: 1 mark—Diagram
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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16
1 mark explanation
Mobile call origination in GSM
The MS sends the dialed number indicating service requested to the MSC(via BSS)
The MSC checks from the VLR if the MS is allowed the requested service. If so, MSC asks
BSS to allocate necessary resources for the call.
If the call is allowed, the MSC routes the call to GMSC.
The GMSC routes the call to the local exchange of called user.
The LE alerts (applies ringing) the called terminal.
Answer back (ring back tone) from the called terminal to LE.
Answer back signal is routed back to the MS through the serving MSC which also
completes the speech path to the MS.
OR
1 mark—Diagram
1 mark—Explanation
Mobile call termination:
The PSTN user dials the MSISDN of the called user in GSM.
The LE routes the call to the GMSC of the called GSM user.
The GMSC uses the dialed MSISDN to determine the serving HLR for the GSM user and
interrogates it to obtain the required routing number.
The HLR requests the current serving VLR for the called MS for a MSRN (MS roaming
number) so that the call can be routed to the correct MSC.
The VLR passes the MSRN to the HLR.
The HLR passes the MSRN to the GMSC.
Using the MSRN, the GMSC routes the call to the serving MSC.
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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17
The MSC interrogates the VLR for the current location area identity (LAI) for the MS.
The VLR provides the current location for the MS.
The MSC pages MS via the appropriate BSS. The MS responds to the page and sets up the
necessary signaling links.
When the BSS has established the necessary radio links, the MSC is informed an the call is
delivered to the MS.
When the MS answers the call, the connection is completed to the calling PSTN user.
Message Processing 2M OR any other correct explanation should be considered
1. Types: The types of messages to be transmitted over the reverse control channel
(RCCH) are:
Page response message: When the mobile station receives a page from the land station, the
mobile station responds back.
Origination message: The mobile station originates the call.
Order confirmation message: The mobile station responds to the order from the land
station.
Order message: The mobile station orders the tasks which should be performed by the land
station and the mobile transmission switching office (MTSO).
2. Structure: All messages contain the following:
An application message header
Mandatory fixed parameters
Mandatory variable parameters
Remaining length
Optical variable parameters
b) Describe the architecture of SS
Answer:-
The message transfer part (MTP) provides OSI level protocol model as level-1 data service,
level-2 link service and level-3 network service
The full level-3 service is provided by the signaling connection control part (SCCP)
The SCCP provides an enhanced addressing capability that may be considered as level 3+
or close to level-4
The layer 4-6 in the OSI model don‟t exist in SS7 protocol model
The transaction capability application part (TCAP) level and operations maintenance and
administration (OMAP) part level are considered the same as the Application level-7 in the
OSI
The application service element ASE is at the same level as the OMAP
The TCAP includes protocol and services to perform remote operations
The primary use of TCAP in these networks is for invoicing remote procedures in
supporting IN service like, 800 services
The OMAP provides the application protocols and procedures to monitor coordinate and
control all the network resources which make communication based on SS7 possible
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(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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18
The ASE is for the MTP routing verification test (MRVT) which uses the connectionless
services of TCAP
The MRVT is an important function on OMAP
c) How and why is the variable spread factor used in CDMA 2000
Answer:-
In case of CDMA 2000 the spread factor can vary from 4 to 256 depending upon the data
rates.
The use of interleaving and variable spread factor enables support of the data rates up to
386kbps
When high data rate signals are to be transmitted then multi carrier transmission can be
used
We can use N carriers where N=1, 3, 6, 9 or 12 in CDMA 2000. Similarly chipping rates of
Nx1.2288 M-chips-per-sec can be used
In CDMA 2000 there are three carriers of 1.25 MHz each for N=3 and all carriers have a
separate code for each channel
Any relevant answer should be given full marks
d) What is the necessity of logical channel in GSM system
Answer:-
In practice, a multimode terminal used by a third generation (3G) mobile communication
system network will have to scan for suitable frequency band or channel, identify
application radio and standard and select from among the set or available services
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Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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19
If it develops at a very large number of frequency bands need to be scanned and the many
standards need to be searched registering such a roaming multimode terminal by means of
systematic scanning procedure will become very inefficient, tending to degrade quality of
service for the users perspective
This problem can be alleviated by using common physical or logical broadcast channel
called the global radio control channel
Therefore there is a necessity of logical channel in GSM system to scan a single frequency
or a small range of frequency and thereby find the required information on available
network or standards and services
Q.5. Attempt any two: 16 Marks
a) Describe how the improving coverage and capacity in cellular system and also list the
techniques for improving the capacity of cellular system.
Answer:
(Note: If student draw the diagram of Cell splitting, Cell sectoring, microcell zone concept,
frequency reuse then 2 M for diagram 2 M for explanation should be given)
Description:
Cell splitting is the process of subdividing congested cell into smaller cells, each with its own
base station and a corresponding reduction in antenna height and transmitter power.
Cell splitting increases capacity of a cellular system since its increases the number of times
that channels are reused.
Cell sectoring increases SIR using directional antennas, then capacity improvement is
achieved by reducing the number of cells in a cluster, thus increasing frequency reuse.
Often wireless operator needs to provide dedicated coverage for hard-to-reach areas. Such as
within buildings, or in valleys or tunnels. Radio transmitters, known as ―repeaters‖ are often
used to provide such range (coverage) extension capabilities.
In microcell zone concept each of three (or possibly more) zone sites are connected to a single
base station and share the same radio equipment. Decreased co-channel interference improves
the signal quality and also leads to increase in capacity without the degradation in trunking
efficiency caused by sectoring.
By limiting the coverage area to within the boundaries of a cell, the same group of channels
may be used to cover different cells that are separated from one another by distance large
enough to keep interference levels within tolerable limits and capacity of system can be
increases is called a frequency reuse or frequency planning.
Techniques for improving capacity:
Cell splitting
Cell sectoring
Microcell-zone concept
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)
(ISO/IEC - 27001 - 2005 Certified)
Summer – 14 EXAMINATION
Subject Code: 12272 Model Answer
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20
Frequency Reuse
b) Write a note on following components:
a) Mobile station
b) Forward channel
c) Base Station controller
And also state the advantages of cellular mobile services.
Answer :
Mobile Station: (2 Marks)
A Station in cellular radio service intended for use while in motion at unspecified locations.
Mobile stations may be handheld personal units (portable) or installed in vehicles (mobiles). A MS
comprises of single antenna, transceiver, and microprocessor- based control circuit. Because
cellular system is full duplex, the transmitter and receiver must operate with single antenna.
Forward channel: (2 Marks)
A radio channel used for transmission of information from the base station to mobile is called
forward channel. The forward voice channel can be further divided in to Forward Voice Channel
(FVC) which is used for voice transmission from base station to mobile, and Forward Control
Channel (FCC) which is responsible for initiating mobile calls, and service requests, and are
monitored by mobile when they do not have a call in progress.
Base Station Controller (BSC): (2 Marks)
The base station controller manages each of the radio channels at each cell-site, turns the radio
transmitter and receiver ON and OFF, transfers‟ data on to the control and voice channels,
monitors calls and performs built-in diagnostic tests on the cell-site equipment. The BSC operates
under the control of the central switching center MSC or MTSO.
Advantages of cellular mobile services: (2 Marks)
Instant Communications- “Coast to Coast in Under a Second”.
Solves many interoperability problems with first responders.
Bypasses the PSTN.
Higher voice compression= less blocking.
Priority Connect available.
High and variable data rate.
On the air privacy
Subscriber Identity Module (SIM)
c) Draw the block diagram and explain each block and also write the list of various types of SS7
services.
Note: Any of the block diagram or Protocol architecture should be given marks
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Answer:
The SS7 signaling protocol is widely used for Common Channel Signaling (CCS) between
interconnected networks.
Block Diagram: (3 Marks)
Explanation of each block: (3 Marks)
An SS7 network is composed of service switching points (SSPs), signaling transfer points
(STPs), and service control points (SCPs).
Service Switching Point (SSP): The SSP gathers the analog signaling information from the local
line in the network (end point) and converts the information into an SS7 message. These messages
are transferred into the SS7 network to STPs.
Signaling Transfer Points (STPs): The STP transfers the packet closer to its destination. When
special processing of the message is required (such as rerouting a call to a call forwarding
number), the STP routes the message to a SCP.
Service Control Points (SCPs): The SCP is a database that can use the incoming message to
determine other numbers and features that are associated with this particular call.
Signaling Connection Control Part (SCCP) translation function: This is a process in the SS7
system that uses a routing tables to convert an address (usually a telephone number) into the actual
destination address (forwarding telephone number) or into the address of a service control point
(database) that contains the customer data needed to process a call.
OR
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Protocol Architecture: (3 Marks)
Explanation of protocol architecture: (3 Marks)
Network Service Part of SS7: The NSP provides ISDN with a highly reliable and efficient means
of exchanging traffic using connectionless services.
a) Signaling data link functions (MTP Level 1): This level provide an interface to the actual
physical channel over which communication takes place.
b) Signaling link function (MTP Level 2): It provides a reliable link for the transfer of traffic
between two directly connected signaling points.
c) Signaling Network Function (MTP Level 3): Provides procedure that transfer message
between signaling nodes.
d) Signaling Connection Control Part (SCCP): The SCCP provides enhancement to the
addressing capabilities provided by the MTP. SCCP also provides the ability to address global
title messages, such as 800 numbers or non-billed numbers.
SS7 User Part: SS7 user part provides call control and management functions and call setup
capabilities to the network.
a) Integrated Service Digital Network User Part (ISUP): The ISUP provides the signaling
functions for carrier and supplementary services for voice, data and video in ISDN
environment. ISUP uses the MTP for transfer of messages between different exchanges.
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b) Transaction Capabilities Application Part (TCAP): The TCAP part in SS7 refers to the
application layer which invokes the services of the SCCP and the MTP in a hierarchical
format.
c) Operation Maintenance and Administration Part (OMAP): The OMAP functions include
monitoring, co-ordination and control function to ensure that trouble free communications are
possible.
Various types of SS7 services: (any two) (2 Marks)
Touch star
800 services
Alternate billing services and Line Information Database (LIDB)
Q.6 Attempt any two: 16 Marks
a) Draw the schematic diagram of a basic cellular telephone system and describe it.
Answer:
A cellular telephone system provides a wire-less connection to the PSTN user for any user location
within the radio range of the system.
Schematic Diagram: (4 Marks)
Description: (4 Marks)
Figure above shows a basic cellular system which consists of
Mobile Station (MS)
Base Station (BS), and
Mobile Switching Centre (MSC) OR Mobile Telephone Switching Office (MTSO)
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1) Mobile Station (MS): The mobile station contains a transceiver, an antenna, and control circuitry
and may be mounted in a vehicle or used as a portable hand-held unit.
2) Base Station (BS):
The base stations consist of several transmitter and receiver which simultaneously handle
full duplex communication and generally have towers which support several transmitting
frequency and receiving antennas.
The BS serves as a bridge between all mobile users and connects simultaneous mobile calls
via telephone lines or microwave links to the MSC.
3) Mobile Switching Centre (MSC):
The MSC co-ordinates the activities of all the base stations and connects the entire cellular
system to the PSTN.
A typical MSC handles 100,000 cellular subscribers and 5,000 simultaneous conversations
at a time, and accommodates all billing and system maintenance functions as well.
Communication between the BS and mobiles is defined by a standard Common Air
Interface (CAI) that specifies four different channels.
4) Forward Voice Channel (FVC): This channel is used for voice transmission from the BS to
mobile.
5) Reverse Voice Channel (RVC): This channel is used for voice transmission from mobile to BS.
6) Forward Control Channel (FCC) and Reverse Control Channel (RCC):
These channels are responsible for initiating mobile calls. Control channels are often called
“setup channels” because they are only involved in setting up a call and moving it to an
unused voice channel.
Control channels transmits and receives data messages that carry call initiation and service
requests, and are monitored by mobiles when they do not have a call in progress.
b) Describe the Co-channel interference and system capacity in mobile communication.
Answer:
Co-channel Interference:
Diagram (any 1):
(2 marks)
OR
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Explanation: (3 Marks)
Frequency reuse implies that in a given coverage areas there are several cells that use the same set
of frequency.
These cells are called co-channel cells, and the interference between signals from these cells is
called co-channel interference.
Unlike thermal noise which can be overcome by increasing the signal-to-noise ratio (SNR), co-
channel interference cannot be combated by simply increasing the carrier power of transmitter.
This is because an increase in carrier transmits power increase the interference from neighboring
co-channel cells.
To reduce co-channel interference, co-channel cells must be physically separated by a minimum
distance to provide sufficient isolation due to propagation.
When the size of each cell is approximately the same and the base stations transmit the same
power, the co-channel interference ratio is independent of transmitted power and becomes a
function of the radius of the cell (R) and the distance between centers of the nearest co-channel
cells (D).
By increasing the ratio D/R, the spatial separation between co-channel cells relative to the
coverage distance of a cell is increased.
Thus interference is reduced from improved isolation of RF energy from the co-channel cell.
Capacity of system: (3 Marks)
The parameter Q is called the co-channel reuse ratio, is related with cluster size (N).
For hexagonal geometry;
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A small value of „Q‟ provides larger capacity since the cluster size „N‟ is small.
Whereas a large value of „Q‟ improves the transmission quality, due to a smaller level of co-
channel interference.
A trade-off must be made between these two objectives in actual cellular design.
c)List various features and services of IS-95 and also write the various services offered by a GSM
system.
Answer:
Features of IS-95: (any three) (3 Marks)
Diversity
Power control
Soft hand-off
A 3-finger RAKE receiver is used in IS-95 to improve reception at the BS and MS.
Many users of a CDMA system share the same frequency. Either TDD or FDD is used.
Channel data rate is very high in IS-96 CDMA system.
Services of IS-95: (any two) (2 Marks)
Short Message Services (SMS)
Slotted paging
Over the air activation
Call services
Data services
Services offered by GSM system: (any three) (3 Marks)
1) Telephone Services:
Standard mobile telephone
Mobile-originated
Base-originated traffic.
emergency calling
Fax
Videotext
Tele text,
SMS
MMS.
2) Supplementary ISDN services:
Call diversion
Caller line ID
Closed user group
Call barring
Call waiting
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Call hold
Connected line ID
Multiparty (Teleconferencing)
Call charge advice
This service also include the Short Messaging Service (SMS) which allow GSM subscriber and
BS to transmit alphanumeric pages of limited length (160 -7 ASCII characters) while
simultaneously carrying normal voice traffic.