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GSM Network Protocols
BY: ENG. Waleed Elsafoury
Copyright © 2013 Waleed Elsafoury
GSM Network Protocols
Main RR procedures:Power controlChannel quality monitoringHandoverFrequency hoppingDiscontinuous transmissionTiming advance.
Transmission function:
MS, BTS and BSC are concerned with transmission
GSM Network ProtocolsEnables power adaptation of MS and BTS.
Main reasons for power control:Saving MS battery PowerImprove CIR.
Can be enabled or disabled.Performed separately for uplink and DL.Measurements on UL and DL are sent to BSC every 480 ms.
Measurements in terms of:Signal strength in dB.Signal quality in BER.
GSM Network ProtocolsMS Power (dBm)Max. transmit
power
Power increase step
Signal strength measured at BTSUpper threshold
Lower threshold
DistancePower control
increase command
Distance
GSM Network ProtocolsHandover Control
Automatic switching of a call from one TCH to another.Can be within the cell or between cells.
Handover occurs on TCH when call is in speech stage.Only started if power control is not helpful.Unique feature of mobile network.
Reasons for intra-cell handover 1. reduction of co-channel interference. 2. Maintenance activity 3. Equipment failure
GSM Network ProtocolsHandover types:Inter cell handover
Involves change of carrier and BTS.Intra-cell handoverInvolves change of carrier in same cell.
GSM Network Protocols
BSC
BSC
MSC-A
BTS
BTS
BTSBTS
BSC
BTS
Inter-MSC HO
Inter-BSC HOIntra-MSC HO
Intra-BSC HO
Intra-cell HO
MSC-B
Handover types
GSM Network ProtocolsRequirements for Handover
1. Execution speed2. Reliability
3. Transparent to users (not aware)4. Service-dependent (voice, data)
• No excessive HO• No delayed HO• No early HO
GSM Network ProtocolsCriteria for Handover
BSC receives measurement data from BTS and MS.
The MS measures downlink:Signal strength (dBm)Signal quality (BER)Signal strength of 6 neighboring BTSs downlink
The BTS measures UL:Signal strength received from MS.Signal quality received from MS.Distance between BTS and MS.
GSM Network Protocols
MSC-A
BTSBTS
BSC
GSM Network ProtocolsHandling of Handover
Reserved channel conceptSome channels are reserved for HO
1
2
Nc
N
New calls
Handover
GSM Network Protocols
N-times retry Multiple request for Handover
Handover queue HO wait for a free channel.
Sub-rating algorithm Full rate TCH is split into 2 half rate TCHs.
GSM Network ProtocolsIntra-BSC (Inter-cell) HO
Old BTS
1
2
BSC
5
2
6
5
4 3
New BTS
GSM Network Protocols
Old BTS4
New
BSC
7
6 5
New BTS
MSC
OldBSC
49
1
48
23
7 4
7
Inter-BSC (Intra-MSC) HO
GSM Network Protocols
Old BTS
NewBSC
5
8
7 6
New BTS
MSC-A
OldBSC5
8
1
5
34
85
MSC-B
2
GSM PLMN PSTN
Inter-MSC HO
99
GSM Network ProtocolsMobility Management (MM)
o Main task of MM is to support mobility.
o MM activities include:1. Location update2. Update HLR/VLR 3. TMSI re-allocation.4. Authentication of MS.
GSM Network ProtocolsLocation Update in same MSC/VLR
If MS detects a change in LAI, it informs network MSC/VLR check if it is a visitor
BTS
BSC
VLR
HLR
MSC
2
34
12
3
4
GSM Network ProtocolsLocation Update with New MSC/VLR
When MS enters into new LA, location update. LA may belong to new MSC/VLR.
BTS
BSC
New
VLR
HLR
MSC
4
1
5
32
OldVLR
4
GSM Network ProtocolsPeriodic Registration
o MS is forced to send registration periodically.
o If MS miss registration, the network marks
it as detached.
o This occurs if MS is out of area.
o This avoids paging.
GSM Network ProtocolsIMSI Attach
o When MS is switched on, it performs attach.
BTS
BSC MSC/VLR
32
4
1
4
GSM Network Protocols
BTS
BSC MSC/VLR
2
1
4
3123
44
Exchange
Telephone
5
GSM PLMN
PSTN
Mobile Originated Call
GSM Network ProtocolsMobile Terminated Call
8
GMSC
BSC
5
11
8
7
GSM PLMN PSTN
6
2HLR
MSCVLR
4 5
11
9
8
89
1110
BTSBTS
Local Exchange
Telephone
1
1
10
MSISDN
MSISDNMSISDN
IMSI
MSRN
MSRN
MSRN
TMSI
TMSI TMSI
PCH
RACH
AGCH
SDCCH
STOP Paging
SDCCH INFO