Gsm interview question

GSM Interview Question -Answer
1. What is the function of SDCCH & SACCH?
Ans. (a)SDCCH---- SlowDedicated Control Channel.
Function----
a) Location updates
b) SMS
c) Ciphering Initiation
d) Equipment Validation
e) Subscriber authentation
f) Call set up signaling
(b) SACCH---SlowAssociated Control Channel.
Function:
(a) Timing advance data
(b) Transmit power control
(c) transmission of signaling data
(d) radiolink supervision measurements
2. What arethe reasons for Hand Overs?
(Ans). (a) Signal Strength (RX LEVEL)
(b) Signal Quality (RX Qual)
(c) Power Budget
(d)Timing Advance.
(e) Interference
2. What are the shortcomings in handovers?
a) Call Dropping.
b) Ping- Pong handover
c) Far- Away cell effect
4. What is intelligent hand over?
Ans)Fuzzy logic
a) Neutral networks
5. What are internal & external hand over?
Ans
(1)INTERNALHANDOVER
a) ( INTER BTS)---- Transfer betweentwo channels (time slot)in same
cell.
b) ( INTRA BSC ( BTS –BTS)-----1 . transfer between BTS undercontrol of same Bsc
2 Measuring the quality of radioconnection
3 Power levels
2. EXTERNALHANDOVER.
( a) INTER BSC (BSC_ BSC)---- 1. TransferbetweenBTS under
the control of diff
BSC
2. BSC TO BSC
3 . NSS to attends the hand over
4. MSC controls.
( b) INTER MSC ( MSC- MSC) ----1. transferbetweencell underthe control
of diff MSC
.What is the frequency Hopping its imp?
(Ans)

 It is defined as sequential change of carrierfrequency on the radiolink between
mobile & base station.
 Two types of freq hopping----- 1. Base band freq hopping.
2. synthesized frequency hopping.
7. Explainthe major diff between BBH & SFH?
Ans
 In BBH the noof hopping freq is same as noof TRX.
 In SFH the noof Hoping freq can be in the range of 1to63.
8. what are the advantages of Frequency Hopping?
1. Frequency Diversity
2. Interference Averaging
3. capacity
9. How in frequency hoppingthereis enhancement of network capacity?
 Freq hopping implement will enable more aggressive freq reuse pattern, that leads
tobetter spectrum efficiency.
 It can add more transceiverin the existing sites , while maintaing the net work
quality/
 Freq hopping compressing the available spectrum tomake room for extra capacity
.
10. Define the freq. hopping parameters?
Frequency Hopping Parameters
GSM defines the following set of parameters:
Mobile Allocation (MA): Set of frequencies themobile is allowed to hop over. Maximum of
63 frequencies can be defined in the MA list.
Hopping Sequence Number (HSN): Determines the hopping order used in the cell. It is
possible toassign 64 different HSNs. Setting HSN = 0 provides cyclichopping sequence and HSN = 1
to 63 provide various pseudo-random hopping sequences.
Mobile Allocation Index Offset (MAIO): Determines inside the hopping sequence, which
frequency the mobile starts dotransmit on. The value of MAIO ranges between 0 to (N-1) where N is
the number of frequencies defined in the MA list. Presently MAIO is set on per carrier basis.
Motorola has defined an additional parameter, FHI.
Hopping Indicator (FHI): Defines a hopping system, made up by an associated set of
frequencies (MA)tohop over and sequence of hopping (HSN). The value of FHI varies between 0 to
3. It is possible to define all 4 FHIs in a single cell.
Motorola system allows to define the hopping system on a per timeslot basis. So different
hopping configurations are allowed for different timeslots. This is very useful for interference
averaging and to randomize the distribution of errors.
11. What are the effects of freq hopping?
(Ans)
1 Handovers:
2 Call setup:
3 Frame Erasure Rate (FER):

12 .Explaininbriefwhat is FER.
 Ans FER= Number of erased blocks total noof blocks *100
 It is the right measure of voice quality.
 FER is performed on speech& signaling frames
 FER------- 0 to4%, GOOD.
4 to15% , slightly degraded
Greaterthan15%, useless
13. What happens when speech frames discarded inFER?
 System will interpolate.
14. What happens whensignaling frames discarded in FER?
 MS is instructed toresend.
15. What is TCH_TCH Interference? Howit is measured.
 When TCH carries are reused that leads toco-channel interference.
 When TCH carrierhave call activity.
 This is measured by delta measurement. --- 1. BCCH carries are diff
2. TCH carriers in both cell 1& cell2are same AFRCN TCH
16. Define the terms?
 BER--- The number of erroneous bits received
Total noof bits received.
 RBER---1 Residual bit error rate
2 It is performed on demodulated speech frames that are not mark corrupt
.BFI -- Bad frame indication.
17. Explainthe parameters in TEMS POCKET mobile.
1.
2. Llcell BCCH ARFCN
3.
4.
L BC BS RXLEV
L TC TS TX
C/I RQ FE
TA DSC CHM
RH CiMd
MCC MNC LAC
RA CI

5.
 L1. Logical channel.----- BCCH
 L2. Logical channel ----- TCH
 BC-- serving cell BCCH AFRCN.
 BS-- base station identity code.
 RXLEV- recieved signal strength
 TC-- trafficchannel
 TS - time slot number.
 TX - transmit power
 C/I -- Carriertointerference ratioin db
 RQ -- Receive bit error rate
 FE –frame erasure rate.
 TA -- Timing advance
 CHM --C hannel Mode
 RH -- cell reselction Hystresis
 CiMd—Ciphering mode
 RAC – Routing area code.
 LAC—Location area code.
18. Explainthe analysis behind RX Qual.?
 RX Qual is thebasicmeasure. It reflects the average BER over the certainperiod of
time(0.5s)
 RX QUALdone over 104 TDMA frames.
 Limitation of RXQUAL----1. The distributions of bit error overtime.
2. Frame erasure
3. Hand over.
19. What are type of interference occur?
1. Co- channel interference.
2. Adj-channel interference.
3. Near end- Far end interference.
20. What is ERLANG?
 Unit of telephone trafficintensity is called Erlang.
 One ERLANG is one channel occupied continuously for one hour.

 1E = 64Kbps.
21. what do you mean by GOS?
 It is the probabity of having a call blocked duringbusiest hour.
 Ex GOS=0.05 means one call in 20 will be blocked call during busiest hourbecause of
insufficient capacity.
22. What are the technique GSMoffers which combat Multipath fading?
 Equalization
 Diversity
 Freq Hopping
 Interleaving
 Channel coding
23. What are control &trafficchannels?
 CONTROLCHANNEL.----1 BCH
2. CCCH
3. DCCH.
 TRAFFIC CHANNEL-- Half rate
Full rate
EFR == Enhanced full rate.
24. What are BCH, CCH, DCCH channels?
 BCH-- 1. BCCH
2 .FCCH
3. SCH
 CCCH. --- 1.PCH
2. AGCH
3. RACH
 DCCH---- 1.SDCCH.
2. SACCH
3. FACCH
25. What are types of bursts?
 Normal Burst
 Frequency Correction Burst
 Synchronization Burst.
 Dummy Burst
 Access Burst.
26. What is adjacent channel separation in GSM?
 Urban Environment--------200khz
 Sub Urban Environment ---- 400khz
 Open environment ----- 800khz
27. What is the watt todBm conversions?
 Power in dBm = 10 log( watts *100)

 0 dBm= 1miliwatt
 1watt = 30dbm
 28. What are the optimizations you have done during Drive Test?
 What are samples in gsm?
 Which modulation take place in GSM
 In one TRU how many frames are there?
 What is thevalue RXLEV of neighboring cell?
 What doyou mean by VAD?
 What is BFI, where it is use?
2929. Define thehopping parameters in detail?
The MA is a list of hopping frequencies transmitted toa mobile every timeit is assigned toa
hopping physical channel. The MA-list is a subset of the CA list. The MA-list is automatically
generated if the baseband hopping is used. If the network utilises the RF hopping, theMA-lists have
tobe generated for each cell by the network planner. The MA-list is able topoint to64 of the
frequencies defined inthe CA list. However, the BCCH frequency is alsoincluded in the CA list, sothe
practical maximum number of frequencies in the MA-list is 63. The frequencies in the MA-list are
required tobe in increasing order because of the type of signaling used totransfer the MA -list.
1.1 Hopping Sequence Number
The Hopping Sequence Number (HSN) indicates which hopping sequence of the 64 available
is selected. The hopping sequence determines the order inwhich the frequencies inthe MA-list are to
be used. The HSNs 1 - 63 are pseudorandom sequences used in the random hopping while the HSN 0
is reserved for a sequential sequence used in the cyclichopping. The hopping sequence algorithm
takes HSN and FN as an input and the output of the hopping sequence generation is a Mobile
AllocationIndex (MAI) which is a numberrangingfrom 0 tothe number of frequencies in the MA -
list subtracted by one. The HSN is a cell specificparameter. For the baseband hopping twoHSNs
exists. The zerotime slots in a BB hopping cell use the HSN1 and the rest of the time slots followthe
HSN2 as presented inError! Reference source not found.. All the time slots in RF hopping cell follow
the HSN1 as presented inError!Reference source not found..
1.2 Mobile Allocation Index Offset
When there is more than one TRX in theBTS using the same MA-list the Mobile Allocation
Index Offset (MAIO) is used to ensure that each TRXuses always an uniquefrequency. Each hopping
TRX is allocated a different MAIO. MAIO is added toMAI when the frequency tobe used is
determined from the MA-list. Example of the hopping sequence generation is presented inError!
Reference source not found.. MAIO and HSN are transmitted toa mobile togetherwith theMA -list.
In Nokia solution the MAIOoffset is a cell specificparameter defining the MAIOTRX for the first hopping
TRX in a cell. The MAIOs for the otherhopping TRXs are automatically allocated according tothe
MAIOstep -parameter introduced in the following section.
30. What is the effect of frequency hopping inRXQual?
Frequency hoppingcauses some changes in the RXQUALdistribution. Also, there are some
differences in a way the RXQUALdistribution should be interpreted.
The Frame Erasure Ratio (FER) is a ratioof discarded speech frames compared toall the
received speech frames. A speech frame is generally discarded if afterthe decoding and error
correction process any of the category 1a bits is found tobe changed based on the three parity bits
following them in a speech frame.
FER is a measure of how successfully thespeech frame was received afterthe error
correction process and it is thus a betterindication of the subjectivespeech quality compared tothe
RXQUALwhich gives anestimateof the link quality in terms of BER. The RXQUALdoesn’t indicate
how the bit errors were distributed ina speech frame. The bit error distribution affects the ability of
the channel decoding tocorrect the errors.
The following table gives an idea of the correlation between RXQUALand FER and between
subjective speech quality and different FER classes.

31. What is the relation link between RXQUAL& FER?
Table 1. RXQUALvs. FER comparison according tothe laboratory tests.
The relation of downlink FER and RXQUAL was measured during a FH trial. The relation is
clearly different in the hopping case compared tothe non-hopping case. The distributions of FER in
each RXQUALclass are presented in Error! Reference source not found. and Error!Reference source
not found.. One clear observation can be made;in the non-hopping case there are significant amount
of samples indicating deteriorated quality (FER>10%)in RXQUALclass 5 while inthe hopping case
the significant qualitydeterioration (FER>10%)happens in RXQUALclass 6. Thus, it may be
concluded that inthe frequency hopping networks significant quality deterioration starts at RXQUAL
class 6 while in non-hopping network this happens at RXQUALclass 5.
This improvement of FER means that the higher RXQUALvalues may be allowed in a
frequency hopping network. RXQUAL thresholds are used in the handover and power control
decisions. Because of the improvement inthe relativereception performance on the RXQUALclasses
4-6, the RXQUALthresholds affecting handoverand power control decisions should be set higherin
a network using frequency hopping network. In a frequency hopping networkRXQUALclasses 0 -5
are indicating good quality.
Typically, theshare of the RXQUALclasses 6 and 7 may increase afterFH is switched on, even
if no other changes have been made. This may seem tobe surprising since it is expected that
frequency hopping improves the network quality. However, inmost cases the quality is actually
improved, but the improvement is more visible in the call success ratio. The improved tolerance
against interference and lowfield strength in FH network means that it is less likely that the
decoding of SACCH frames fails causing increment in the radiolink timeout counter. Thus, it is less
likely that a call is dropped because of the radiolink timeout. Instead, the calls generating high
RXQUALsamples tend tostay on. This may lead toincrease in the share of RXQUAL6-7. However, at
the same time the call success rate is significantly improved.
In the Error!Reference source not found., there are presented some trial results of a DL
RXQUALdistribution with different frequency allocation reuse patterns. As can be seen from the
figures, the tighterthe reuse becomes, the less samples fall in quality class 0 and more samples fall in
quality classes 1-6. There’s biggerdifference in downlinkthan inuplink direction.
This difference is a consequence of interference and frequency diversities that affect the
frequency hopping network. Because of these effects, the interference or lowsignal strength tend to
occur randomly, while in a non-hopping network it is probable that interference or lowfield strength
will affect several consecutive bursts making it harderfor the error correction toactually correct
errors. The successful error correction leads toless erased frames and thus improves the FER.
32. What doyou understand by idle channel measurement?

 When a newcall is established or a handoveris performed, the BSC selects the TRX
and the time slot for the trafficchannel based on the idle channel interference measurements. The
frequency hopping has a significant effect on theidle channel interference measurement results.
When the frequency hopping is used, the frequency of a hopping logical channel is changed
about 217 times ina second. The frequency of the idle time slots changes according tothe same
sequence.
In a case of the random hopping, this means that the measured idle channel interference is
likely tobe the same for all the TRXs that use the same MA-list. If the interference is averaged over
more thanone SACCH frame, the averaging effect is even stronger. However, normally the
interferers are mobiles located ininterfering cells. In this case, there are probably differences in the
measured idle channel interferences between different time slots in the cell. This happens, because
the interfering mobiles are only transmitting during the time slot that has been allocated tothem.
This is illustrated inFigure Error! Notext of specified style in document.-1.
If the cyclichopping sequence is used, there might occur differences on the measured idle
channel interference levels between the TRXs on the same time slot as explained in the following
section.
Figure Error!Notext of specified style indocument.-1. Idle channel interference in a case of
the random RF hopping
33 .what are types of handover?
There are fourdifferent types of handoverin the GSMsystem, which involve transferring a
call between:
 Channels (time slots)in the samecell
 Cells (Base TransceiverStations)underthe control of the same Base Station
Controller (BSC),
 Cells under thecontrol of different BSCs, but belonging tothe sameMobile services
Switching Center(MSC), and
 Cells under thecontrol of different MSCs.
34. what are important parameterof power saving in GSM
Discontinuous transmission
Minimizing co-channel interference is a goal in any cellularsystem, since it allows better
service for a given cell size, or the use of smallercells, thus increasing the overall capacityof the
system. Discontinuous transmission (DTX)is a method that takes advantage of the fact that a person
speaks less that 40 percent of the time in normal conversation [22], by turning the transmitter off
during silence periods. An added benefit of DTX is that power is conserved at the mobile unit.
The most important component of DTX is, of course, Voice ActivityDetection. It must
distinguish betweenvoice and noise inputs, a task that is not as trivial as it appears, considering
background noise. If a voice signal is misinterpreted as noise, the transmitter is turned off and a very
annoying effect called clipping is heard at the receiving end. If, on the other hand, noise is
misinterpreted as a voice signal toooften, the efficiency of DTX is dramatically decreased. Another
factor toconsider is that whenthe transmitteris turned off, there is total silence heard at the
receiving end, due tothe digital nature of GSM. To assure the receiverthat the connection is not
dead, comfort noise is created at thereceiving end by trying tomatch the characteristics of the
transmitting end's background noise.
Discontinuous reception
Anothermethod used toconserve power at themobile station is discontinuous reception.
The paging channel, used by the base station tosignal anincoming call, is structured intosub-
channels. Each mobile station needs tolistenonly toits own sub-channel. In the time between
successive paging sub-channels, themobile can gointosleep mode, when almost nopower is used.
All of this increases battery life considerably whencompared toanalog
: What is Tri-band and Dual-band?
A: A tri-band phone operates at three supported frequencies, such as 900/1800/1900 MHz or 850/1800/1900 MHz. A dual-
band phone operates at two frequencies, such as 850/1900 MHz or 900/1800

Telecom Basic Questions
1. The OSI model was developed by ISO in the
year___________
a. 1985
b. 1988
c. 1965
d. 1984
2. The other name of directional antenna is..
a. Omni-directional antenna
b. Sectored antenna
c. Isotropic antenna
d. Yagi-Uda antenna
3. The main purpose of multi plexer is..
a. Combine voice and data traffic
b. Perform error detection and correction
c. Reduce network costs
d. Perform analog to digital conversion
4. Walkie – Talkie is the best example for..
a. Simplex
b. Half-Duplex
c. Full Duplex
d. None
5. ATM Packet Size is
a. 50 octets
b. 53 octets
c. 56 octets
d. 64 octets
6. In phase modulation the signal has________
phase and___________ phase.
a. 180,180

b. 90, 90
c. 0,180
d. 0, 0
7. What is Buffering?
a. The process of temporarily storing the data to allow
for small variation in device speeds
b. A method to reduce cross talks
c. A method to reduce routing overhead
d. Storage of data within transmitting medium until the
receiver is ready to receive
8. Line of sight is...
a. Straight Line
b. Parabolic
c. Tx and Rx should be visible to each other
d. None of the above
9. The layers of the OSI model, from the top -down,
are
a. Application, Presentation, Session, Transport,
Network, Data link, Physical
b. Session, Presentation, Data transport, MAC,
Network, Physical
c. Presentation, Application, Session, Network,
Transport, Data link, Physical
d. Application, Encryption, Network, Transport,
Logical link control, Physical
10. Quadrature Amplitude Modulation (QAM) is a
combination of ___________.
a. ASK & FSK

b. FSK & PSK
c. PSK & ASK
d. None
11. Transmission media are usually categorized as
_______.
a. Fixed or Unfixed
b. Guided or Unguided
c. Determinate or Indeterminate
d. Metallic or Nonmetallic
12. When a beam of light travels through media of two
different densities, if the angle of incidence is greater
than the critical angle, _______ occurs.
a. Reflection
b. Refraction
c. Incidence
d. Criticism
13. Which of the following is a type of fiber connector:
a. SC
b. ST
c. MTRJ
d. All of the above
14. GSM allows a reuse factor of _______.
a. 1
b. 3
c. 5
d. 7
15) What is the modulation method used in GSM at
Air interface ?
a)Pulse Code Modulation
b)Quadrature Amplitude Modulation
c) GMSK Modulation

d )PSK Modulation
16) Multipath Fading is also called as
a) Lognormal Fading
b) Shadow fading
c ) Rayleigh Fading
d ) Rician Fading
17) GSM is the digital standard for Europe; what
do the letters GSM currently mean?
a) Global Special Mobile
b)Greater System's Mobile
c) Global Systems for Mobile Communications
d) None of the above!
18) Full Rate Speech Coding is at
a) 9.6 Kbps
b) 13 Kbps
c) 6.5 Kbps
d) 4.8 Kbps
19) _______ is a first-generation cellular phone
system.
a) AMPS
b) D-AMPS
c) GSM
d) None
20) MEO satellites are located at altitudes between
km.
a) 3000 and 5000
b) 5000 and 10,000
c) 5000 and 15,000
d) None

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