15 GSM interview questions and answers

This page covers GSM questionnaire set prepared by experienced specialists in GSM/GPRS domain. GSM jobs are available at various levels such as testing,maintenance,installation,development and so on. These top 15 GSM interview questions and answers help engineers seeking GSM technology related job to crack the interview with ease.These questions are very useful as viva questions also.

Question -1.Explain the call flow of mobile originated call and mobile terminated call flow in GSM.
Answer -1.Call originated from mobile to the other subscriber through base station is referred as mobile originated call or MO Call. Call originated from base station and getting terminated on mobile subscriber is referred as mobile terminated call or MT Call. Suppose mobile subscriber-A is calling mobile subscriber-B via base station then call from A to base station is referred as MO call and from base station to B is referred as MT call. There are two main categories of calls in GSM i.e. CS call (Circuit Switched Call) and PS Call (Packet Switched Call). Following are the links which describes all the call flow messages between base station and mobile subscriber for both the MO/MT CS call and MO/MT PS call.
CS MO Call
CS MT Call
PS downlink MT call
PS uplink MO call

Question -2.Explain the GSM frame structure..
Answer -2. There are two frame structures in GSM viz. 51 frame multiframe and 26 frame multiframe. Interviewer expects both of them to be answered with frame duration including divisions(slot,frame,multiframe,superframe and hyperframe). As we know GSM frame will have 8 time slots having duration of 4.615ms and each slot of 0.577 ms duration. 26 frame multiframe is used for traffic channel and SACCH channel and is of duration of about 120ms. 51 frame multiframe is used for control or signalling purpose and is of duration of about 235ms. Refer GSM frame structure to know more in depth of these frames.

Question -3. Explain the GSM channel types.
Answer -3. There are two main types in GSM logical channels and physical channels. Physical channel means allocated one frequency and one time slot on which logical channel data is carried into the air. Logical channel is divided into two main types based on the information is carried on it viz. traffic channel and signalling channel. Traffic channels are TCH Half rate and TCH full rate. COntrol or signalling channels are divided into three main categories based on their function i.e. broadcast control channel(BCH),common control channel(CCCH) and dedicated control channel(DCH). Broadcast channels are FCCH,SCH and BCCH. Common control channels are PCH,AGCH,RACH and AGCH. Dedicate control channels are SDCCH,SACCH and FACCH. Refer GSM Channel types for more.

Question -4. What are the difference frequency bands supported in GSM?
Answer -4. There are four categories of frequency bands supported by GSM standard. It includes GSM850,GSM900,DCS1800 and PCS1900 bands. Refer GSM frequency bands page to know all the supported bands in GSM technology.

Question -5. What is the difference between GSM and GPRS?
Answer -5. GSM is mainly developed for carrying voice traffic while GPRS is developed for data traffic. In GSM 51 frame and 26 frame multiframe structure is used. In GPRS 52 frame multiframe is used. GPRS supports CS1,CS2,CS3 and CS4 modulation and coding schemes.EGPRS supports MCS1 to MCS9. Refer GSM versus GPRS for more.

Question -6. Explain GMSK modulation scheme?
Answer -6. GMSK stands for Gaussian Minimum Shift Keying. GMSK modulation

Question -7. Explain the difference between TDMA, FDMA and CDMA?
Answer -7. TDMA,FDMA and CDMA are access techniques to utilize time,frequency and code respectively. TDMA means Time Division Multiple Access,FDMA means Frequency Division Multiple Access and CDMA means Code Division Multiple Access. GSM uses both TDMA and FDMA. FDMA comes by dividing the frequency band say 890 to 915 and 935 to 960 MHz bands into sub-carriers of 200 KHz bandwidth each. Hence by FDMA we get 124 subcarriers. TDMA comes by allocating eight time slots TS0 to TS7 to each subcarrier. Refer TDMA,FDMA and CDMA for more information.

Question -8. What is RXQUAL? What is the relation between RXQUAL and BER in GSM?
Answer -8. GPRS RXQUAL vs BER

Question -9. What is the function of FCCH and SCH in GSM?
Answer -9. FCCH channel SCH channel

Question -10. What is BSIC? Why it is needed in GSM system?
Answer -10. BSIC means Base Station Identity Code. It is 6 bits in size and carried in GSM SCH channel. This BSIC will have 3 bits of BCC and 3 bits of NCC. BCC stands for Base Station color code and NCC stands for Network Color Code. BSIC helps in differentiating and identifying the correct cell when two similar cells with same broadcast frequencies are transmitting. This is unique to all the GSM BTS.

Question -11. Explain types of handover.
Answer -11. Handover or Handoff refers to connection of mobile subscriber with base station when mobile moves at higher speed. There are various ways handover happens in GSM viz. within the same BTS region, between BTS and another BTS, between BSC and another BSC. The main types are hard handover and soft handover. Hard handover means connection is broken with Active BTS cell before new connection with neighbour cell is made. Soft handover means connection with new BTS is first made before the connection with the active BTS cell is broken. Refer GSM handoff to know more. Now-a-days handover between two different RATs have become common due to demand and availability of multi RAT features in single mobile terminals. Inter-RAT handover refers to handover betwen GSM to UMTS,UMTS to GSM,GSM to LTE,LTE to GSM, CDMA to LTE , LTE to CDMA and so on. Refer Inter RAT handover to know more.

Question -12. Explain different elements of GSM network architecture.
Answer -12. In the interview from this equation interviewer trying to judge the understanding of GSM system as a whole. So draw various elements of GSM architecture such as BTS,BSC,MS,MSC,HLR,VLR,AUC,EIR etc. and explain the functions of each. If you remember various interfaces between these elements it will be good. Refer GSM Tutorial for GSM architecture block diagram with interfaces.

Question -13. Explain FDM, TDM and OFDM.
Answer -13. FDM stands for Frequency Division Multiplexing. TDM stands for Time Division Multiplexing and OFDM means Orthogonal Frequency Division Multiplexing. This all are resource multiplexing techniques for utilizing the scared resource(time and frequency) efficiently. FDM makes use of frequency,TDM time and OFDM also frequency resource. The limitations of FDM is taken care in OFDM by allocated subcarriers very close without introducing ICI(Inter Carrier Interference). ISI is also avoided in OFDM by inserting guard interval. Refer to of our popular pages to know the difference between FDM and OFDM as well as FDM and TDM.
Refer FDM vs OFDM and FDM vs TDM for more information.

Question -14. Explain Min. BEP with respect to GPRS.
Answer -14. BEP stands for Bit Error Probability in GSM. It helps in finding the radio condition and choose the best coding scheme for the mobile subscriber. There are two main types of BEP in GSM i.e. Mean BEP and Actual BEP. Refer following pages to know more.
What is BEP
Mapping between Reported and Actual CV BEP Mean BEP and Actual BEP

Question -15. Explain AMR.
Answer -15. This topic along with VAMOS,SAIC are usually asked by interviewer in the interview as part of judging the interviewee's knowledge on advanced GSM topics. AMR stands for Adaptive Multi rate and it is the method by which mobile subscriber changes the modulation-coding scheme based on varying C/I conditions at any moment of time.
Refer GSM AMR basics and GSM AMR conformance testing to know more about AMR basics and conformance requirements of GSM Mobile Subscriber.

Question -16. Explain concept of GPRS multi-slot class.
Answer -16. In the Multislot mode, one mobile subscriber will be allocated more than one slot in the downlink and uplink to increase the data rate requirements of the user/customer. There are various classes which mention limit for maximum time slots supported in the uplink and downlink. Refer GPRS multislot classes for more information.

These GSM interview questions and answers will help pass the GSM technology related interview with comfort and confidence.

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