15 GSM Interview Questions and Answers

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

Question 1: Explain the call flow of a mobile originated call and a mobile terminated call flow in GSM

Answer: A call originated from a mobile to another subscriber through a base station is referred to as a mobile originated call or MO Call. A call originated from a base station and terminated on a mobile subscriber is referred to as a mobile terminated call or MT Call.

Suppose mobile subscriber A is calling mobile subscriber B via a base station, then the call from A to the base station is referred to as an MO call, and from the base station to B is referred to as an MT call.

There are two main categories of calls in GSM:

• CS Call (Circuit Switched Call)
• PS Call (Packet Switched Call)

Question 2: Explain the GSM frame structure

Answer: There are two frame structures in GSM: a 51-frame multiframe and a 26-frame multiframe. The interviewer expects both of them to be answered with frame duration including divisions (slot, frame, multiframe, superframe, and hyperframe).

As we know, a GSM frame will have 8 time slots having a duration of 4.615ms, and each slot has a duration of 0.577 ms.

• The 26-frame multiframe is used for traffic channels and SACCH channels and has a duration of about 120ms.
• The 51-frame multiframe is used for control or signalling purposes and has a duration of about 235ms.

Question 3: Explain the GSM channel types

Answer: There are two main types in GSM: logical channels and physical channels.

A physical channel means allocating one frequency and one time slot on which logical channel data is carried into the air.

A logical channel is divided into two main types based on the information carried on it: a traffic channel and a 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:
  • Broadcast control channel (BCH)
  • Common control channel (CCCH)
  • Dedicated control channel (DCH)

Broadcast channels are FCCH, SCH, and BCCH. Common control channels are PCH, AGCH, and RACH. Dedicated control channels are SDCCH, SACCH, and FACCH.

Question 4: What are the different frequency bands supported in GSM?

Answer: There are four categories of frequency bands supported by the GSM standard. It includes GSM850, GSM900, DCS1800, and PCS1900 bands.

Question 5: What is the difference between GSM and GPRS?

Answer: GSM is mainly developed for carrying voice traffic, while GPRS is developed for data traffic.

• In GSM, a 51-frame and 26-frame multiframe structure is used. In GPRS, a 52-frame multiframe is used.
• GPRS supports CS1, CS2, CS3, and CS4 modulation and coding schemes. EGPRS supports MCS1 to MCS9.

Question 6: Explain the GMSK modulation scheme

Answer: GMSK stands for Gaussian Minimum Shift Keying. Gaussian Minimum Shift Keying (GMSK) is a continuous-phase frequency modulation technique used in digital communication systems, notably in GSM (Global System for Mobile Communications). It is a variant of Minimum Shift Keying (MSK) but applies a Gaussian filter to smooth the phase transitions, reducing spectral bandwidth and minimizing interference. GMSK maintains a constant envelope, making it power-efficient and suitable for non-linear amplifiers. Its smooth phase shifts help reduce adjacent channel interference, enhancing spectral efficiency. Despite its advantages, GMSK has a slightly higher bit error rate (BER) compared to some other modulation schemes due to inter-symbol interference (ISI) introduced by the Gaussian filter.

Question 7: Explain the difference between TDMA, FDMA, and CDMA

Answer: 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.
• 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.

Question 8: What is RXQUAL? What is the relation between RXQUAL and BER in GSM?

Answer:

1. RXQUAL (Receiver Quality) is a parameter in GSM networks that measures the quality of the received signal based on Bit Error Rate (BER). It is expressed on a scale from 0 to 7, where 0 represents the best signal quality (low BER) and 7 indicates the worst quality (high BER). RXQUAL is crucial for assessing voice call clarity and overall network performance, as high RXQUAL values suggest interference, weak signals, or poor coverage. Mobile networks use RXQUAL metrics to optimize handovers and improve service quality.

2. Relation between RXQUAL and BER – RXQUAL is directly derived from BER, as it categorizes the percentage of erroneous bits in the received signal. A lower BER corresponds to a better RXQUAL value, ensuring higher communication reliability. The mapping follows a predefined GSM standard: for example, RXQUAL 0 corresponds to BER < 0.2%, while RXQUAL 7 corresponds to BER > 12.8%. As BER increases, more errors occur in the received signal, leading to degraded call quality, dropped calls, and reduced data throughput. Thus, minimizing BER improves RXQUAL, enhancing network performance and user experience.

Question 9: What is the function of FCCH and SCH in GSM?

Answer:

• FCCH (Frequency Correction Channel) : Helps mobile devices synchronize with the GSM network by providing frequency correction bursts.
• SCH (Synchronization Channel) : Provides frame synchronization and BSIC (Base Station Identity Code) to help mobiles identify and lock onto the correct cell.

Question 10: What is BSIC? Why is it needed in the GSM system?

Answer: BSIC means Base Station Identity Code. It is 6 bits in size and carried in the GSM SCH channel. This BSIC will have 3 bits of BCC and 3 bits of NCC.

• BCC stands for Base Station Color Code.
• NCC stands for Network Color Code.

BSIC helps in differentiating and identifying the correct cell when two similar cells with the same broadcast frequencies are transmitting. This is unique to all the GSM BTS.

Question 11: Explain types of handover

Answer: Handover or Handoff refers to the connection of a mobile subscriber with a base station when the mobile moves at a higher speed.

There are various ways handover happens in GSM: within the same BTS region, between BTS and another BTS, and between BSC and another BSC.

The main types are hard handover and soft handover.

• Hard handover means the connection is broken with the Active BTS cell before a new connection with the neighbor cell is made.
• Soft handover means the connection with the new BTS is first made before the connection with the active BTS cell is broken.

Nowadays, handover between two different RATs has become common due to the demand and availability of multi-RAT features in single mobile terminals.

Inter-RAT handover refers to handover between GSM to UMTS, UMTS to GSM, GSM to LTE, LTE to GSM, CDMA to LTE, LTE to CDMA, and so on.

Question 12: Explain different elements of GSM network architecture

Answer: In this question, the interviewer is trying to judge the understanding of the 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.

Question 13: Explain FDM, TDM, and OFDM

Answer:

• FDM stands for Frequency Division Multiplexing.
• TDM stands for Time Division Multiplexing.
• OFDM means Orthogonal Frequency Division Multiplexing.

These are all resource multiplexing techniques for utilizing the scarce resource (time and frequency) efficiently. FDM makes use of frequency, TDM time, and OFDM also frequency resource.

The limitations of FDM are taken care of in OFDM by allocating subcarriers very close without introducing ICI (Inter-Carrier Interference). ISI is also avoided in OFDM by inserting a guard interval.

Question 14: Explain Min. BEP with respect to GPRS

Answer: BEP stands for Bit Error Probability in GSM. It helps in finding the radio condition and choosing the best coding scheme for the mobile subscriber. There are two main types of BEP in GSM: Mean BEP and Actual BEP.

Question 15: Explain AMR

Answer: This topic, along with VAMOS and SAIC, are usually asked by the 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 a mobile subscriber changes the modulation-coding scheme based on varying C/I conditions at any moment of time.

Question 16: Explain the concept of GPRS multi-slot class

Answer: In 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 the limit for maximum time slots supported in the uplink and downlink.

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