GSM in a nutshell

Qn. MSC stands for?
A) Main Switching Centre
B) Master Switching Centre
C) Mobile Switching Centre ...

Qn. What is the function of MSC?

When a user turns on his mobile, it is registered by MSC. MSC contacts HLR to get the details about the user. If the user is not connected to that MSC, then MSC contacts the HLR of the User through SS7 links.

Qn. What is the function of HLR and VLR?
When a user moves to a new location, VLR of that location contacts the HLR and collects information about the user. HLR informs the previous VLR to drop the details about this user.

 
Qn. Draw the block schematic of GSM system?

Qn.MSC is connected to HLR by SS7. What is SS7?


SS7 stands for Signalling system 7 (7 is the version number of this protocol).
TCP/IP is used for interconnecting computers. SS7 is used for interconnecting SSPs.
SSP(service switching points are telephone switches which are interconnected by ss7  signalling.
STP(service transfer points) are like routers used for routing the messages.
SCP(service control point): If an information is needed by ssp, it contacts scp which intern contacts stp. stp is connected to the database. stp gets the data and transfers it.

Qn. What is the function of MSC?
MSC stands for Mobile Switching Centre




Qn. What is the difference between GSM , WCDMA and LTE?

GSM 2G is just GSM
GSM 3G  is called as WCDMA
GSM 4G is called as LTE (Long Term Evaluation)


Qn: What MODULATION method is used by GSM?
A)ASK
B)FSK
C)PSK
D)GMSK.  correct

Qn. What is the frequency used by GSM 900?

933 - 960 MHz is downlink,  890 - 915 MHz is uplink.
FDMA divides 25MHz into 124 channels of 200 Khz using FDM.
(25mhz divided by 200khz comes to 125 channels. But you say it is only 124 channels.why? 100khz at each end of the spectrum is left as guard band.)
This 200Khz channel is against split into 8 Timeslots using TDM. So, for every channel of 200Khz, there are 8 speech circuits.

Qn. How the 124 channels are shared among cells?
Each cell is divided into 3 sectors (by 120 degree). Four such cells form one cell set. So a total of 12 sectors share the 124 channels.




Qn: What ACCESS method is used by GSM?
A) TDMA
B) FDMA
C) Combination of FDMA & TDMA   <<correct 




Qn. How many  200Khz channels are allotted to each Base Station?

one or more. BSS converts each 200Khz into 8 timeslots.

Why transmitting timeslot and receiving timeslot are not at the same time?
A mobile phone will  NOT  transmit and receive at the same time. It will either transmit or receive at a time. This is to avoid spending more energy at the same time.


Normally 64kbps is needed for speech. But  GsM uses only 13kbps. Which coding technique is used to achive this?

Linear Predictive Coding (LPC) is used.
More details about LPC is here


What are the components in BTS?
DUPLEXER : It is  where TX and RX signals are connected to same antenna for simultaneous transmission and reception.

MULTICOUPLER: It is where received signal is split to feed a number of receivers say 4,8,16

COMBINER is  where two or more transmitters are combined to feed to a single antenna.

In 2G we call it as BTS, in 3G we can such equipment as....?

Qn: What is the function of AuC? How security works in GSM? How speech is encrypted in GSM? What is A3,A8,A5 algorithms?




Step 1: AuC(Authentication centre) which is connected to your MSC sends a CHALENGE (A 128 bit random number RAND)  to your mobile

Step 2: A secret key  Ki(128 bit) which is stored in your SIM while you purchase your SIM, is combined with the RAND received from AuC and  a response(SRES:signed response) is generated using A3 algorithm.And send back to AuC. If the received SRES is correct, AuC successfully authenticates your mobile.

Step 3: For transmission of speech and data, your mobile needs an encryption key (called as session key). Your mobile generates this key(Kc) using A8 algorithm using the same  Ki and RAND.

Step 4. Using this Session key Kc, the voice and data is encrypted using A5 algorithm.