55594350 General Questions About GSM

7/31/2019 55594350 General Questions About GSM

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General Questions about GSM

1. What are the three services offered by GSM? Explain each of them briefly.

These objectives have been gradually achieved and a broad collection of services are provided. The GSM services are

grouped into three categories:

y 1. Teleservices (TS)y 2. Bearer services (BS)

y 3. Supplementary services (SS) TS cover, in essence, telephony, BS encompass basically data transmission, andSS are the value-added features.

Teleservices

Regular telephony, emergency calls, and voice messaging are within TS. Telephony, the old bidirectional speech calls, is

certainly the most popular of all services. An emergency call is a feature that allows the mobile subscriber to contact a

nearby emergency service, such as police, by dialing a unique number. Voice messaging permits a message to be stored

within the voice mailbox of the called party either because the called party is not reachable or because the calling party

chooses to do so.

Bearer Services

Data services, short message service (SMS), cell broadcast, and local features are within BS. Rates up to 9.6 kbit/s are

supported. With a suitable data terminal or computer connected directly to the mobile apparatus, data may be sent

through circuit-switched or packet-switched networks. Short messages containing as many as 160 alphanumeric

characters can be transmitted to or from a mobile phone. In this case, a message center is necessary. The broadcast

mode (to all subscribers) in a given geographic area may also be used for short messages of up to 9 3 alphanumeric

characters. Some local features of the mobile terminal may be used. These may include, for example, abbreviated

dialing, edition of short messages, repetition of failed calls, and others.

Supplementary Services

1. Some of the SS are as follows:2. Advice of charge. This SS details the cost of a call in progress.3. Barring of all outgoing calls. This SS blocks outgoing calls.4. Barring of international calls. This SS blocks incoming or outgoing international calls as a whole or only those

associated with a specific basic service, as desired.5. Barring of roaming calls. This SS blocks all the incoming roaming calls or only those associated with a specific

service.6. Call forwarding. This SS forwards all incoming calls, or only those associated with a specific basic service, to

another directory number. The forwarding may be unconditional or may be performed when the mobilesubscriber is busy, when there is no reply, when the mobile subscriber is not reachable, or when there is radiocongestion.

7. Call hold. This SS allows interruption of a communication on an existing call. Subsequent reestablishment of the

call is permitted.8. Call waiting. This SS permits the notification of an incoming call when the mobile subscriber is busy.9. Call transfer. This SS permits the transference of an established incoming or outgoing call to a third party.10. Completion of calls to busy subscribers. This SS allows notification of when a busy called subscriber becomes

free. At this time, if desired, the call is reinitiated.11. Closed user group. This SS allows a group of subscribers to communicate only among themselves.12. Calling number identification presentation/restriction. This SS permits the presentation or restricts the

presentation of the calling partys identification number (or additional address information).13. Connected number identification presentation. This SS indicates the phone number that has been reached.14. Freephone service. This SS allocates a number to a mobile subscriber, and all calls to that number are free of

charge for the calling party.


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15. Malicious call identification. This SS permits the registration of malicious, nuisance, and obscene incoming calls.16. Three-party service. This SS permits the establishment of conference calls.

2. Which uplink/downlink spectrum is allocated to GSM-900?

a.) GSM 900: Uplink spectrum is 890.2914.8 Mhz and downlink spectrum is 935.2959.8 Mhz

3. Which uplink/downlink spectrum is allocated to DCS-1800?

b.) DCS 1800: Uplink spectrum is 1710.21784.8, and downlink spectrum is 1805.21879.8

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

the carrier frequencies?

900

y To allow maximum number of users access, each band is subdivided into124 carrierfrequencies spaced 200 kHz apart, using FDMA techniques.

y Each of these carrier frequencies is further subdivided into time slots using TDMA.y TDMA (Time Division Multiple Access) has 8 time slots (i.e. transmitting for one

eighth of the time).y Hence, one radio channel can support 8 'full rate' traffic.y A more economical 'half rate' scheme with 16 traffic channels is being introduced.y TDMA provides each user with the carrier frequency for approximately0.577ms.

1800

GSM 1800 the number of carriers is 374.

In GSM 900 the duplex frequency (the difference between uplink and downlink frequencies) is 45 MHz. In GSM 1800 this

duplex frequency is 95 MHz. Both in GSM 900 and GSM 1800, the lowest and highest channels are never used, in orderto avoid interference with services using neighbouring frequencies.

However, no base station is allowed to use this complete set of carrier frequencies. In order to reduce the interferencelevel, adjacent cells must use different sets of frequencies. Only widely separated cells are allowed to use the samecarrier frequencies. This concept, known as frequency reuse, greatly increases the capacity of a cellular network.

Channel Uplink signal (MHz) Downlink signal (MHz)

1 890.1 - 890.3

(890.2 -centre freq.) 935.1 - 935.3

(935.2 -centre freq.)

2 890.4 (centre freq.) 935.4 (centre freq.)

3 890.6 (centre freq.) 935.6 (centre freq.)... ... ...

124 914.8 (centre freq.) 959.8 (centre freq.

5. What is Ciphering? Why do we need it? Name the algorithm(s) used in it?

The data being transmitted from the BTS to the Phone and back is

enciphered. This is to prevent 'snoopers' from capturing the ID. In

the days of AMPS phones, it was aquite easy to acquire IMEI numbers


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right off the air, clone them, and start making calls which are

charged to the original owner. This lead to authentication, and in

GSM, the ID details from the SIM, not being transmitted over the air.

It also prevents people from simplyh listening to cell phone

converations, as could easily be done with AMPS phones.

The ciphering is nothing to get overly excited about. I figure the NSA

could break the codes in a second or two, but it will certain

discourage the average listener from getting anywhere!

The algorithm 129-EEA3 and 128-EIA3, 3GPP Confidentiality and Integrity Algorithm is used for ciphering in GSM

6. What is Authentication? Why do we need it? Name the algorithm(s) used in it?

The authentication algorithm used in the GSM system is known as the A3 algorithm.

Most GSM network operators utilize a version of the COMP128 algorithm as the implementation of the A3 algorithm.

A3s task is to generate the 32-bit Signed Response (SRES) utilizing the 128-bit random challenge (RAND)

generated by the Home Location Register (HLR) and the 128-bit Individual Subscriber Authentication Key (Ki) from

the Mobile Stations Subscriber Identity Module (SIM) or the Home Location Register (HLR).

A3 actually generates 128 bits of output. The first 32 bits of those 128 bits form the Signed Response.

The A3 algorithm is implemented in the Subscriber Identity Module (SIM).

7. What is equalisation? Why do we need it?

8. What is Interleaving? Why do we need it?

9. Why do we need digitisation?

10. Explain Speech Coding.

11. What is channel coding?

12. What do you mean by Frequency re-use?

13. What is Cell Splitting?

14. Name the interfaces between a) BTS and MS b) BTS and BSC c) BSS and MSC d) TRAU and BSC e)BSC

and PCU

15. What are LAPD and LAPDm?

16. What is WPS?

17. What is MA?

18. What is MAIO?

Created by: Faisal Adnan Siddiqui

19. What is the difference between Synthesised Frequency Hopping and Base Band Frequency Hopping?

20. What is Cycling Frequency Hopping?

21. What is HSN? How do we apply it?


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22. What is DTX? Why is it used?

23. What is DRX? Why do we need it?

24. What is the gross data rate of GSM?

25. What is Erlangs? What is meant by GoS?

26. We use two different bands for GSM/DCS communications; GSM900 and DCS-1800. Which one is the

better of the two in terms of quality and coverage?

27. What is TA? Why do we need TA?

28. What is meant by Location Area?

29. What is location update? Why do we need location update?

30. What is meant by IMSI, TMSI, IMEI and MS-ISDN? Why they are needed?

31. What is ARFCN? Which ARFCNs are allocated to Ufone?

32. Explain Power Control.

33. What is the difference between FDD and TDD?

34. What is an extended cell? How does it impact the system?

Channels and TDMA structure

35. Why do we use Multiple Access Schemes? What is the difference between FDMA, TDMA and CDMA?

36. Which channel(s) is used for SMS?


37. Which channel is used by MS to request access to the network?

38. What is AGCH?

39. Why do we need SDCCH?

40. What is a physical channel? How do we differentiate between physical and logical channels?

41. What are TDMA frames, multiframes, superframes and hyperframes?

42. Why do we need FCCH, SCH and BCCH?

43. Why do we need SACCH?

44. What is the purpose of PCH and CBCH?

45. Do we keep BCCH on a hopping radio? Give the reason to support your answer.

46. How much delay is present between downlink and uplink frames? Why do we need this delay?

47. Explain the structure of a Traffic Multiframe. Why do we need SACCH and Idle bursts in a traffic

multiframe?

48. How is a FACCH formed? When is a FACCH used?

49. What are bursts? Explain various types of bursts.

Radio Propagation and Antennas

50. What is VSWR? Why do we need it?

51. What do you mean by EIRP?

52. What is Polarisation? What are the types of polarisation?

53. What is fading? What are its different types: a) Based on Multipath time delay spread b) Based on

Doppler Spread?

54. What is Rayleigh Fading?

55. What is multipath fading?


56. How can we minimise multipath fading?


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94. What is SQI? Why do we prefer it over RxQual?

95. What is BSIC? Why do we need it?

96. What is AMR?


97. What can be the reasons of a Call drop?

98. What are counters? Why do we need them?

99. When do we need drive test?

100. What is cell-reselection?

101. What are C1 & C2?

102. What is call re-establishment?

103. Why do we make short calls and long calls during drive test?

104. What do you mean by CEFR and CSSR?

105. What is RSSI?

106. What is the difference between RxLev and RxQual?

107. What is the difference between FER and BER?

Procedures

108. What is cell selection? How does MS select a cell?

109. Explain the call flow for MOC and MTC.

110. Handover procedures.

111. How does a MS get registered with the network? (Explain IMSI attach procedure)

GPRS and EDGE

112. What is GPRS?

113. What is the basic difference between GSM and GPRS architecture?

114. What makes GPRS technology different from traditional GSM?

115. What are the functions of GGSN and SGSN?

116. How many coding schemes are used in GPRS? Why are they important?

117. What is the gross data rate offered by GPRS and EDGE?

118. What is EDGE? How is it different from normal GSM/GPRS?

119. How do we classify GPRS terminals?

GSM System Architecture

120. What are the main components of BSS?

121. What are the main components of NSS?

122. Why do we need HLR and VLR?

123. Why do we need EIR and AuC?

124. What is RBS?

125. What are the paging limitations of a BSC?

126. What is a coupling system?

127. What do we mean by E1 and T1?


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1)What is a typical NodeB sensitivity level?

2)What are the processing gains for CS and PS services?

3)How to calculate maximum number of users on a cell?

4)Brief describe the advantages and disadvantages of soft handover?

5)How many channelization codes are available?

55594350 General Questions About GSM

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