basic telecom questions and answers

7/30/2019 basic telecom Questions and Answers

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1) What is Time Division Multiple Access?

TDMA is a digital transmission technology, which works by dividing a radio

frequency into time slots and then allocating slots to each user within each

channel. In this way, a single frequency can support multiple, simultaneous

data channels.

2) What is FDMA?

Frequency Divison Multiple access is a scheme in which the entire frequency

band is divided into channels, each channel corresponding to a particular


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frequency range. With FDMA each channel can be assigned to one user at a

time.

3) Give details of the uplink and downlink band of GSM?

The uplink band of GSM is from 890MHz to 915MHz and the downlink band of

is from 935MHz to 960MHz

4) What is the bandgap between the uplink and downlink carrier in GSM?

45 MHz.

5) What is the bandgap between 2 consecutive carriers?

200KHz

6) How many carriers are present in the GSM band and how many channels

are there in each carrier?

124 carriers and 8 channels per carrier

7) What type of modulation is used in GSM?

Gaussian Minimum Shift Keying (GMSK)


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8) What is ARFCN?

In cellular mobile communications the radio channels are identified by their

Absolute Radio Frequency Channel Number. In GSM ARFCN 1 to 124 are used

GSM Network Architecture

9) What are the components of the GSM network? Draw a schematic diagram

of the GSM Network architecture?

The GSM network architecture consists of the following components

a. Mobile Station (MS)

b. Base Transceiver Station (BTS)

c. Base Station Controller (BSC)


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d. Mobile Switching Center (MSC)

e. Home Location Register (HLR)

f. Visitor Location Register (VLR)

g. Authentication Center (AuC)

h. Equipment Identity Register (EIR)

10) Describe in brief the functionalities of the Base Transceiver Station

(BTS)?

The BTS is a part of the Base Station Sub system and is in contact with the

MS through the radio interface. The BTS is in charge of management of

transmission and reception on the radio interface.

11) Describe in brief the functionalities of the Base Station Controller (BSC)?

The BSC is connected on one side to one or several BTSs and on the other

side to the MSC. The main function of the BSC is allocation and release of

radio channels and the handover management.

12) What is the function of the Mobile switching center (MSC)?


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The MSC performs the basic function of switching. The main function of the

MSC is to co-ordinate the setting up of calls to and from GSM users and the

external network The MSC has interface with the BSS on one side and the

external network on the other side.

13) What is the function of the Home Location Register (HLR)?

The Home Location Register (HLR) together with the MSC, provide the

callrouting and roaming capabilities of GSM. The HLR contains all the

administrative information of each subscriber registered in the

corresponding GSM network, along with the current location of the mobile.

14) What is the function of the Visitor Location Register (VLR)?

The Visitor Location Register contains roaming information. Once the visited

system detects the mobile, the VLR of that system enquires the HLR to make

sure you are a valid subscriber. It temporarily stores the last location area

visited by the MS, the power the mobile uses, the special services the MS is

subscribed to and so on.

15) What is the function of the Equipment Identity Register (EIR)?

The Equipment Identity Register (EIR) is a database that stores data related

to the mobile equipment. It contains a list of all valid mobile equipment on

the network,where each mobile station is identified by International Mobile

Equipment Identity (IMEI).This is useful when searching for stolen mobile

equipment or when monitoring misuse of mobile stations.

16) What is the function of the Authentication Center (AuC)?


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The Authentication Center is a database that stores a copy of the secret key

stored in each subscribers SIM card, which is used for authentication and

ciphering of the radio channel.

18) What is SIM? What are the functions of the SIM?

The Subscriber Identity Module is a card inside a mobile equipment which

contains subscriber related data. 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.

19) Describe how authentication is performed in GSM?

Authentication involves two functional entities, the SIM card in the mobile,

and the Authentication Center (AuC). Each subscriber is given a secret key

(Ki), one copy of which is stored in the SIM card and the other in the

Authentication Center. During authentication, the AuC generates a random

number (RAND) that it sends to the mobile. Both the mobile and the AuC

then use the random number, in conjunction with the subscribers secret key

and a ciphering algorithm called A3, to generate a number (SRES) that is

sent back to the AuC. If the number sent by the mobile is the same as the

one calculated by the AuC, the subscriber is authenticated.

19) What are the two types of logical channels?

The two types of logical channels are Traffic Channels (TCH) and Control

Channels

20) What are the various types of Control Channels?

There are three types of control channels mainly broadcast Control Channels,

Common Control Channels and Dedicated Control Channels.


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21) What are the various types of Broadcast Control Channels?

The three types of Broadcast Control Channels are Broadcast ControlChannel, Synchronization Channel and Frequency Correction Channel.

22) What are the various types of Common Control Channels?

The three types of Common Control Channels are Access Grant Channel,

Paging Channel and Random Access Channel.

23) What are the various types of Dedicated Control Channels?

The three types of dedicated control channels are Slow Associated Control

Channel, Fast Associated Control Channel and Stand Alone Dedicated Control

Channel

24) What is the function of the FCCH?

This downlink channel continuously transmits unmodulated carrier frequency

for frequency correction of the MS.

25) What is the function of the SCH?

The Synchronization Channel is used for frame synchronization of the Mobile

Station. It contains information about TDMA frame number and the BSIC.


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26) What is the function of the BCCH?

The Broadcast Control Channel broadcasts general information about the cell

viz. neighbor cell information, MS maximum transmit power on CCHs andlocal area identification.

27) What is the function of the FACCH?

The FACCH is used as a main signaling link for the transmission of data eg.

Handover commands. The FACCH is required for every call set up and

release. The FACCH is transmitted in the burst by pre-empting a portion ofthe speech/user data information bits during active call.

28) What is the function of the AGCH?

The AGCH is used for sending access grant messages, which assign the MS

to an SDCCH or directly to a TCH in response to a request placed on RACH by

MS.

29) What is the function of the RACH?

The RACH is used to transfer uplink messages in response to the call

initiation request placed by the MS or in response to a paging request or

automatically as part of a location update.

30) What is the function of the Paging Channel (PCH)?

The PCH is used to page the MS during mobile terminated call setup. Paging

messages are sent to the area where the recipient MS is located.


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31) What is the function of the SDCCH?

The SDCCH carries all signaling between the BTS and the MS when no TCH isallocated. It is used for service requests (eg. SMS), location updates,

subscriber authentication, ciphering, initiation, equipment validation and

assignment to a TCH.

32) Draw and explain the protocol layer between the Mobile Station and the

BTS?

33) What are the functions of the Radio resource Management Layer in GSM?

The main function of the Radio resource management layer is to establish

and release stable connections between mobile stations and the MSC for the

duration of the call and to maintain the connection despite user movements.

34) What are the functions of the Mobility Management sub layer in GSM?


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The mobility Management layer manages the location updating, handovers,

and registration procedures. The machines concerned with mobility

management are mainly the MS, the HLR and VLR. The security function is

performed by the AuC.

35) What are the functions of the Communications Management sub layer in

GSM?

Communications Management sub layer terminates at the MSC and contains

entities that currently consist of CC including call-related supplementary

services, SMS, and call independent supplementary services support (SS).

36) What is Idle Mode?

A mobile is said to be in idle mode when it is active (powered on) but is not

allocated any traffic channel. In the idle mode the MS listens to broadcast

channels in order to intercept paging messages, monitor the radio

environment in order to evaluate its quality and choose the most suitable

base station.

38) What is Base Station Identity Code?

The BSIC is a color code which the MSs use to be able to discriminate

between the cells transmitting their beacon channels on the same frequency.

Many cells bear the same BSIC and it is common for neighboring cells to

have the same BSIC.

39) What is IMSI/TMSI?


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Every Subscriber is assigned an IMSI associated with its SIM card. The IMSI is

known only to the subscriber and is kept internal and transmitted as rarely

as possible for security reasons keeping the identity of the subscriber

confidential (in case some one is listening on the air interface). The IMSI

consists of Mobile Country code (MCC) which identifies home country of the

subscriber, the Mobile Network Code (MNC) which identifies the PLMN of the

subscriber and the Mobile Station Identification Number (MSIN) which

identifies the subscriber within a PLMN.

`The MSC/VLR allocates a TMSI temporarily to a subscriber present in the

geographical area served by the MSC/VLR. Only the VLR stores the TMSI not

the HLR. The TMSI serves to identify the MS when it needs to communicate

with the network. It is used instead of the IMSI to avoid transmitting the IMSI.

Several MSC/VLRs can use the same TMSI

40) What is IMSI attach /Detach?

When a MS station is switched off(or when the SIM is removed by the user),

call towards that MS station cannot be completed. Hence important

resources are consumed for nothing. To alleviate this load, the IMSI attach

and IMSI detach procedure is used. The subscribers record in the MSC/VLRcontains a binary information indicating whether or not it is useful to try to

complete the call toward the subscriber. The IMSI detach procedure will set

the binary bit to Not Useful To Try whereas the IMSI attach procedure will

do the reverse.

41) What is the structure of IMSI?

3 Digits 2 Digits

MCC MNC MSIN


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Mobile Country Mobile Network Mobile Subscriber Identification

Code Code Number

42) What is MSISDN?

Mobile Station ISDN Number This is the subscriber phone number. It is the

identity of the subscriber known by the external world. It is the number

dialed when someone needs to call the mobile subscriber. The MSISDN

consists of the Country Code (CC), National Destination Code (NDC) and

Subscriber Number.

43) What is Mobile Station Roaming Number (MSRN)?

The MSRN is used to identify a subscriber when routing the call from the

GMSC to the visited MSC for mobile terminating calls.. It is allocated by the

MSC/VLR currently serving the subscriber on a call basis (temporary

assignment)

44) What is discontinuous Reception?

For the sake of battery consumption in MS, it is important to minimize the

amount of information the MS has to receive, demodulate and analyze when

it is in idle mode. Therefore the downlink common control channel is divided

into several paging sub channels and all messages pertaining to a given


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subscriber are sent on the same sub channel. Such a scheme allows the MS

to restrict the monitoring of paging messages to their own paging sub

channel, thereby increasing the life time of the battery at the expense of a

small increase in the delay for the setting up of incoming calls.

45) What is Discontinuous Transmission?

Discontinuous transmission (DTX) is a method that takes advantage of the

fact that a person speaks less that 40 percent of the time in normal

conversation by turning the transmitter off during silence periods. Power is

conserved at the mobile unit by appling Discontinuous Transmission.

Whether DTX should be applied or not is decide by the MSC while its

execution is controlled by the BSC.

46) What is Location Updating?

When a MS moves to a new location area or is switched on in a new location

area, it must register with the network to indicate its current location. A

location update message is sent to the new MSC/VLR, which records the

location area information, and then sends the location information to the

subscribers HLR

47) What are the various types of bursts in GSM?

Normal Burst

Synchronization Burst

Dummy Burst


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Frequency Synchronization Burst.

49) Draw and Explain the Protocol architecture of the GSM Network?

RR Management sub layer

Manages the Radio Interface

Terminates at BSS from MS

MM sub Layer

Terminated at MSC

Messages from or to the MSC are relayed transparently from BSS

Communications Management sub Layer

Terminates at MSC

Contains entities that consist of CC including call related supplementary

services, SMS and call independent Supplementary Services (SS) support.

TCAP and MAP

These are the interfaces between the MSC and HLR/VLR

BSSAP


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Used to implement all procedures between the MSC and the BSS that require

interpretation and processing of information related to single calls and

resource management.

SCCP and MTP

SCCP and MTP protocols are used to implement the data link layer and layer

3 transport functions for carrying the call control and mobility management

signaling message son the BSS_MSC link.

GSM Network Planning & Optimization

1) What is the basic procedure for RF planning using RF planning tool in

GSM?

Planning Tool

Loading of digital map data of the city on the planning tool, Model tuning for

proper prediction on the tool ,Nominal cell planning for the coverage,frequency planning and interference study.

Final system planning.

2) What are the different types of antennas used in a GSM network?

Antennas of different types based on antenna beamwidth and gain,

In GSM We have space diversity antenna and cross polorised antenna.Omni

antennas,


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3) How do the power, orientation, beamwidth, tilt and height of the

antenna affect the coverage?

Power increases the coverage, Proper Orientation, Beamwidth and tilt

improves the coverage of the planned area. Increase in antenna height

improves the coverage.

4) What is EIRP? How is it calculated?

Effective isotropic radiated power is the total o/p power of Base station

EIRP=BTS (O/P) power+Antenna gain-cablelosses-other (combiner)losses

5) What is frequency planning? Why is frequency reuse pattern used?

Frequency planning is to be done for the GSM network for a given operator

with A set of GSM frequency band.Frequency reuse pattern is used for

capacity planning. To minimize frequency interference problems.

6) What are all the patterns available? How is frequency planning done on

planning tool?

4/12 pattern.3/9 pattern .Automatic frequency planning can be done in

planning tool.

7) What is co-channel interference and adjacent channel interference?


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The interference caused by using same channels in a network is called

cochannel interference and interference caused by adjacent channels of is

called adjacent channel interference.

8) How do you minimize co-channel interference?

proper frequency planning and proper reuse pattern.

9) How do you minimize adjacent channel interference?

Proper frequency planning.

10) What is model tuning? Why is it used? Okarama Hatta model?

Model tuning is done for any planning tool to obtain proper prediction

expected from planning tool as there will be difference in actual and

predicted data for planning tool. One of the model is Okarama Hatta Hatta

Model is used which considers the signal propagation losses.

11) What is link Budget Analysis used for in GSM?

Link budget analysis is used for the path balance between both uplink

(Transmitter) Downlink (Receiver) part of the network.

12) How is path loss taken into calculation?


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This is the total path loss occurred due to multipath propagation of the signal

between transmitted signal and the received signal level.

13) What is capacity planning?

This is process of increasing traffic by adding Trxs traffic channels and

proper use of frequency reuse pattern. Addition of BTSs

14) How is site selection done for planning and site acquisition?

After nominal cell planning with the geographical coordinates identifying

best site candidate in a geographical area is the process of site selection.

15) What is optimization? What are the tools used for optimization?

Once GSM network is integrated in order to achieve proper planning

predictionRF parameters Optimization is done. Drive test tool,Planning tooland Post processing tools are Used for optimization.

16) How do we use the above antenna patterns for optimization?

Proper antenna orientation and tilts and antenna types can be used for

optimizing network.

17) How do we handle poor C/I, C/A, handover problems and blocking?


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Checking proper neighbor list, changing frequency planning and proper no of

radio channels availability.

18) What is system information?

System information is from BTS sent to Mobile for the idle mode and

dedicated mode of the call for call management.

19) What is bench marking in GSM?

Bench Marking used for comparing performance of different network for

quality and call performance parameters.

20) What is Erlang table?

This is the table for calculating Traffic in erlang for no of channels.

21) What is grade of service?

Grade of service is the blocking for the given traffic channels.

22) How do you optimize a network using OMCR performance data?

Proper increase in call success, and increase in handover success

performance and drop call reduction performance.


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23) What is daily report and traffic?

This is the report obtained in OMC for the performce of all BTS about traffic .

24) What is cell_reselect_hysterisis?

This is process of handover done by mobile in idle mode from selected cell to

another cell.

25) What is path loss criterion (C1)? How is it calculated?

This is the signal strength measured in idle mode for selecting best serving

channel.

basic telecom questions and answers

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