Telecom Interview Questions & Answers "IMPORTANT"

Telecom Interview Questions & Answers

What is ciphering

Ciphering modifies the contents of the eight interleaved blocks through the use of encryption techniques to the particular mobile station and base transceiver station.

Two types of ciphering algorithms, called A3 and A5, are used in GSM to prevent unauthorized network access and privacy for the radio transmission respectively.

How many carrier frequencies are there in GSM-900 and DCS-1800? How much is the separation between the carrier frequencies?

124 carriers.

There are 8 channels in each carrier.

Channel separation is 200 KHz.

What is the Downlink and uplink spectrum is allocated to DCS-1800?

1800 MHz. Up link Frequency 1725-1780 MHZ.

Down link Frequency 1820-1870 MHz

What is the Downlink and uplink spectrum is allocated to GSM-900?

900 MHz.  Up link Frequency 890-915 MHZ.

Down link Frequency 935-960 MHz

Three services offered by the GSM

Teleservices	Speech SMS (Short Message	Full Rate 13Kbps Point to Point & Cell Broadcast
Bearer Services	Asynchronous Data Synchronous Data	300 – 9600bps 300 – 9600bps
Supplementary Services	Call Forward Call Barring	Subscriber Busy, Not Reachable International & Incoming Calls

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.

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 frequency range. With FDMA each channel can be assigned to one user at a time.
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
What is the bandgap between the uplink and downlink carrier in GSM?
45 MHz
What is the bandgap between 2 consecutive carriers?
200KHz

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

What type of modulation is used in GSM?

Gaussian Minimum Shift Keying (GMSK)

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.

The GSM network architecture consists of the following components?

a. Mobile Station (MS)

b. Base Transceiver Station (BTS)

c. Base Station Controller (BSC)

d. Mobile Switching Center (MSC)

e. Home Location Register (HLR)

f. Visitor Location Register (VLR)

g. Authentication Center (AuC)

h. Equipment Identity Register (EIR)

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.

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.

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

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.

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.

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.

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.

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

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

stored in each subscriber's SIM card, which is used for authentication and ciphering

of the radio channel.

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.

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 subscriber's 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.

What are the two types of logical channels?

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

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.

What are the various types of Broadcast Control Channels?

The three types of Broadcast Control Channels are Broadcast Control Channel,

Synchronization Channel and Frequency Correction Channel.

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.

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

What is the function of the FCCH?

This downlink channel continuously transmits unmodulated carrier frequency for

frequency correction of the MS.

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.

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 and local area identification.

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 of the speech/user data information bits during active call.

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.

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.

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.

What is the function of the SDCCH?

The SDCCH carries all signaling between the BTS and the MS when no TCH is allocated. It is used for service requests (eg. SMS), location updates, subscriber authentication, ciphering, initiation, equipment validation and assignment to a TCH.

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.

Telecom Interview Questions & Answers

IMPORTANT

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

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.

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).

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.

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.

What is IMSI/TMSI?

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/VLR’s can use the same TMS

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/VLR contains 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.

What is the structure of IMSI?

3 Digits 2 Digits

Mobile Country Mobile Network Mobile Subscriber Identification

Code Code Number

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.

Misc.

The network structure is defined within the GSM standards. Additionally each interface

between the different elements of the GSM network is also defined. This facilitates the

information interchanges can take place. It also enables to a large degree that network

elements from different manufacturers can be used. However, as many of these interfaces were not fully defined until after many networks had been deployed.

1. Um interface The "air" or radio interface standard that is used for exchanges

between a mobile (ME) and a base station (BTS / BSC). For signaling, a modified

version of the ISDN LAPD, known as LAPDm is used.

2. Abis interface This is a BSS internal interface linking the BSC and a BTS, and it

has not been totally standardized. The Abis interface allows control of the radio

equipment and radio frequency allocation in the BTS.

3. A interface The A interface is used to provide communication between the BSS

and the MSC. The interface carries information to enable the channels, timeslots

and the like to be allocated to the mobile equipments being serviced by the BSSs.

The messaging required within the network to enable handover etc to be

undertaken is carried over the interface.

4. B interface The B interface exists between the MSC and the VLR . It uses a

protocol known as the MAP/B protocol. As most VLRs are collocated with an

MSC, this makes the interface purely an "internal" interface. The interface is used

whenever the MSC needs access to data regarding a MS located in its area.

5. C interface The C interface is located between the HLR and a GMSC or a SMSG.

When a call originates from outside the network, i.e. from the PSTN or another

mobile network it ahs to pass through the gateway so that routing information

required to complete the call may be gained. The protocol used for

communication is MAP/C, the letter "C" indicating that the protocol is used for

the "C" interface. In addition to this, the MSC may optionally forward billing

information to the HLR after the call is completed and cleared down.

6. D interface The D interface is situated between the VLR and HLR. It uses the

MAP/D protocol to exchange the data related to the location of the ME and to the

management of the subscriber.

7. E interface The E interface provides communication between two MSCs. The E

interface exchanges data related to handover between the anchor and relay MSCs

using the MAP/E protocol.

8. F interface The F interface is used between an MSC and EIR. It uses the MAP/F

protocol. The communications along this interface are used to confirm the status

of the IMEI of the ME gaining access to the network.

9. G interface The G interface interconnects two VLRs of different MSCs and uses

the MAP/G protocol to transfer subscriber information, during e.g. a location

update procedure.

10. H interface The H interface exists between the MSC the SMS-G. It transfers

short messages and uses the MAP/H protocol.

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) 

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 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.
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.
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 subscriber's HLR
What are the various types of bursts in GSM?
Normal Burst
Synchronization Burst
Dummy Burst

Frequency Synchronization Burst.

Draw and Explain the Protocol architecture of the GSM Network?
a. RR Management sub layer
Manages the Radio Interface
Terminates at BSS from MS
b. MM sub Layer
Terminated at MSC
Messages from or to the MSC are relayed transparently from BSS
c. 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.
d. TCAP and MAP
These are the interfaces between the MSC and HLR/VLR
e. BSSAP
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.
f. 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.
What is the basic procedure for RF planning using RF planning tool in GSM?
Planning Toolloading 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.
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,
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.
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

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.
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.
What is co-channel interference and adjacent channel interference?
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.
How do you minimize co-channel interference?
proper frequency planning and proper reuse pattern.
How do you minimize adjacent channel interference?
Proper frequency planning.
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 toolas 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.
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.
How is path loss taken into calculation?
This is the total path loss occurred due to multipath propagation of the signal between transmitted signal and the received signal level.
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
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.
What is optimization? What are the tools used for optimization?
Once GSM network is integrated in order to achieve proper planning prediction
RF parameters Optimization is done. Drive test tool,Planning tool and Post processing tools are Used for optimization.
How do we use the above antenna patterns for optimization?
Proper antenna orientation and tilts and antenna types can be used for optimizing network.
How do we handle poor C/I, C/A, handover problems and blocking?
Checking proper neighbor list, changing frequency planning and proper no of radio channels availability.
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.
What is bench marking in GSM?
Bench Marking used for comparing performance of different network for quality and call performance parameters.
What is Erlang table?
This is the table for calculating Traffic in erlang for no of channels.