gsm technology - standards & implementation
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GSM900DCS1800
GSM TECHNOLOGY -Standards & Implementation
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Introduction to Cellular
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Omni-directional Cell
BTS
Sectorial Cell
BTS
Cell Configuration
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GSM uses concept of cells
One cell covers small part of network
Network has many cells Frequency used in one cell can be used
in another cells This is known as
F=1F=2
F=3
F=4,8F=5,9
F=6,10F=7
F=1F=2
F=3F=4,8
F=5,9F=6,10
F=7F=1
F=2F=3
F=4,8F=5,9
F=6,10
F=7
F= 1,2,3,4,5,6,7,8,9,10
Clusters
Co-Channel ( Re-use ) Cells
Frequency Reuse
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MS
BTS
BTS
BTS
BTSBTS
BTS
BTS
BTS
BSC
BSC
PSTN
VLR
TRAUHLR
EIR
OMC
SMSC
BC
AUC
VMSC
MSC
A
OML
GSM Network Architecture
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MSISDN : Human Identity used to call a Mobile Station
IMEI: Serial number unique to every Mobile Station
IMSI : Network Identity unique to a SIM
3 digits 2 digits 10 digits
TMSI : Identity unique in a LAI
MSRN : Mobile Station Roaming No
CC NDC SN
98 XXX 12345
MCC MNC MSIN
404 XX 12345
TAC FAC SNR S
6 digits 2 digits 6 digits 1 digit
Mobile Station Identities
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VLR
HLR/AUC
EIR
PSTN GMSC GSMSC
VLR
HLR/
AUC
Abis A
BC
F
E
G
D
SS7/R2
MSCBSC
BTSOML
OMC
A,B,C,D,E,F,G,H,I = SS7Abis = LAPDOML = X.25
SUPPI
H
GSM Signaling Interfaces
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Separate Bands for Uplink and Downlink Down link : 935 - 960 MHz ( E-GSM 925 - 960 MHz )
Uplink : 890 - 915 MHz ( E-GSM 880 - 915 MHz)
TDMA and FDMA Multiplex 124 Frequency Channels (ARFCN) for GSM900
1 to 124 for current band
975 to 1023 for E-GSM
200kHz Channels
8 Mobiles share ARFCN by TDMA
0.3 GMSK Modulation 270.833 kbits/sec. rate
Variable Tx Power and Timing
GSM Air Interface
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Time
Frequency
Amplitude
The GSM Burst
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20ms
Speech Coder
260 bits
50 132 78
1a 1b 11
Channel Coder
456 bits
13 kbps
22.8 Kbps
Tranceiver ( BTS )
22.8 Kbps
456 bits
260 bits13 kbps
Transcoder Handler
260 + 60 = 320 bits16 kbps
TRAU Frame
Abis
Speech coding Process
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UPLINK
ADJACENTCELL
BCH
DOWNLINK
RXLEV RXLEV RXLEV RXLEV RXLEV
Measuring Adjacent Cell BCH Power
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Mobile is commanded to change its Transmit Power
Change in Power is proportionate to the Path Loss Change is Power is done in steps of 2 dbs
Tx Level5
67.
14
15
Power dBm33
3129.
15
13
Mobile Power Control
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TDMA approach requires signals to arrive at BTS at the correct
time. They must not overlap
BTS
Timing Advance
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ARFCN
( 200Khz )
BCCH
FCH
SCH
Broadcast Channel
SDCCH
SACCH
FACCH
Dedicated Control Channel
PAGCH
AGCH
RACH
CBCH
NCH
Common Control Channel
Control Channel
Full Rate TCH Half Rate TCH
Traffic Channel
Logical Channels
Physical Channel
Logical Channels
TO
C T T T T T T T
T1 T2 T3 T4 T5 T6 T7
Physical and Logical Channel
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Traffic Channels
Traffic Channels carry either encoded speech or user data Two forms of Traffic Channels are defined :
-- Full rate Traffic Channel : Carries encoded information at gross rate of 22.8Kbps
-- Half rate Traffic Channel : Carries encoded information at gross rate of 11.4 Kbps
Speech Traffic Channels
Full rate Traffic Channel for speech : Speech out of encoded information is at 13 kbpsHalf rate Traffic Channel for speech : Speech out of encoded information is at 6.5 kbps
Data Traffic Channels
Full rate Traffic Channel for 9.6 kbps user data
Full rate Traffic Channel for 4.8 kbps user data
Full rate Traffic Channel for < 2.4 kbps user data
Traffic Channels Modes
Circuit Switching mode ( transparent connection to a service like telephony ) Packet Swithching mode ( as per recommendation X.25 or other standardised protocols)
Logical Channels
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Broadcast Channels ( BCH )
Frequency Correction Channels ( FCCH ) :
Carries Information for frequency correction of the mobile stations. ( Downlink )
Synchronisation Channels ( SCH ) :
Carries information for frame synchronization of the mobile stations and identification of BTS( Downlink )
Contains two pieces of information : BSIC & Reduced Frame Number *
Broadcast Control Channels ( BCCH ) :
Broadcasts various cell parameters and other information required by the mobile to access
the network. ( Downlink)
Logical Channels
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Broadcast Channels ( BCH )
0 1 2 3 4 5 6 7FCCH
SCHBCCHBCCHBCCHBCCHCCCHCCCHCCCHCCCHFCCHSCH
I
TN
FCCH & SCH occurs every 10th frame
BCCH is send in block of 4 frames
BCCH block occurs once in 51 frames
FN 0
FN 10
FN 11
FN 51
Logical Channels
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Common Control Channels
Paging Channel ( PCH ) :
Used to page Mobiles ( Downlink )
Access Grant Channel ( AGCH ) :
Used to for allocation of a dedicated channel ( SDCCH ) ( Downlink )
Random Access Channel ( RACH ) :
Used to request allocation of dedicated channel ( SDCCH ) ( Uplink )
Logical Channels
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CCCH is configured
to AGCH & PCH
either dynamically
or through O & M
AGCH & PCH also
occurs in block of 4
No of AGCH & PCH
in one Multiframe
depends on channelcombinations
FCCH
AGCH
AGCH
AGCH
AGCH
FCCH
SCH
PCH
PCH
PCH
PCH
PCH
PCH
PCH
PCH
FN 0
FN 10
FN 11
FN 51
FN 12
FN 19
0 1 2 3 4 5 6 7
RACHFN 0
FN 51
0 1 2 3 4 5 6 7
DownlinkUplink
RACH
RACH
RACH
RACH
RACH
RACH
RACH
RACH
RACH
RACH
RACH
RACH
RACH
RACH
RACH
Logical Channels
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Dedicated Control Channels
Standalone dedicated control channel ( SDCCH ) :
Used for conveying signaling information ( Downlink & Uplink )
Slow Associated Control Channel ( SACCH ) :
Used for conveying slow information associated with SDCCH
and TCH ( Downlink and Uplink )
Fast Associated Control Channel ( FACCH ) :
Associated with TCH for conveying fast signaling information
( D & U )
Cell Broadcast Control Channel ( CBCH ) :
Subset of SDCCH used for broadcasting cell broadcastmessages ( Downlink )
Logical Channels
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B + D
Mapping of Channel Combinations using Multiframe Concept
F S B C F S C C F S D0 D1 F S D2 D3 F S A0 A1 I
50Downlink0
TN=0F S B C F S C C F S D0 D1 F S D2 D3 F S A2 A3 I
D3 R R A2 A3 R R R R R R R R R R R R R R R R R R R R R R R D0 D1 R R D2
50
D3 R R A0 A1 R R R R R R R R R R R R R R R R R R R R R R R D0 D1 R R D2
0 Uplink
B
F S B C F S C C F S C C F S C C F S C C ITN=00 Downlink 50
R R R R R R R R R R R R R R R R R R R R R R R R R R R R R RR R R R R R R R R R R R R R R R R R R R
500
TN=0
Uplink
Mapping of Logical channels on PCHN
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Mapping of Channel Combinations using Multiframe Concept
B + C
I I B C I I C C I I C C I I C C I I C C ITN=0
0 Downlink 50
R R R R R R R R R R R R R R R R R R R R R R R R R R R R R RR R R R R R R R R R R R R R R R R R R R
500
TN=2,4,6
Uplink
D
I
50Downlink0
Uplink
D1 D2 D4 D5 D7 A0 A2 A3D0 D3 D6 A1 I I
ID1 D2 D4 D5 D7 A4 A6 A7D0 D3 D6 A5 I ITN=0
IA6 A7 D1 D2 D4 D5 D7 A0A5 D0 D3 D6I I
IA2 A3 D1 D2 D4 D5 D7 A4A1 D0 D3 D6I ITN=0
0 50
Mapping of Logical channels on PCHN
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Mapping of Channel Combinations using Multiframe Concept
T
T T T T T T T T T T T T A T T T T T T T T T T T T I
250
TN=0
Downlink
Uplink
T T T T T T T T T T T T A T T T T T T T T T T T T I
250
Mapping of Logical channels on PCHN
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1 Burst = 577usecs
0 1 2 3 4 5 6 71 TDMA Frame = 8 bursts = 4.616 ms
Multiframe
1 TCH Multiframe = 26 TF = 120ms 1 CCH Multiframe = 51 TF = 234.6ms
0 1 2 3 2044 2045 2046 2047
1 Hyperframe = 2048 Superframes = 2715648 TDMA Frames
= 3 hrs 28 min 53 sec 760 ms
0
01
1
2
2
4823
49
24
50
25
0 1 2 23 24 25 0 1 2 48 49 50
1 Superframe = 26 CCH MF = 1326 TF = 6.12 sec1 Superframe = 51 TCH MF = 1326 TF = 6.12 sec
FRAMES