Telecom Interview Question & Answer.

(A)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

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 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)

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.

9) 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)

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)?

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)?

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.

17) 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.

18) 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.
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.
21) 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.
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.
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 and local 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 of the 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.
31) 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.
32) 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?
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.
37) 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.
38) 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
39) 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.
40) What is the structure of IMSI?
3 Digits 2 Digits
Mobile Country Mobile Network Mobile Subscriber Identification
Code Code Number
41) 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.