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