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Transmission function:
MS, BTS and BSC are concerned with transmission HLR, VLR or EIR transmit their signaling needs.
RR is a group of functions concerned with administration of channels at Um. Main RR procedures:
Power control Channel quality monitoring Handover Frequency hopping Discontinuous transmission Timing advance.
Enables power adaptation of MS and BTS. Main reasons for power control:
Saving MS battery Power Improve 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.
MS Power (dBm)
Max. transmit power
Power increase step
Signal strength measured at BTS
Upper threshold
Lower threshold
Distance Power control increase command
Distance
Handover 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
Handover types: Inter cell handover
Involves change of carrier and BTS. Intra-cell handover Involves change of carrier in same cell.
BSC
BSC
MSC-A
BTS
BTS
BTS BTS
BSC
BTS
Inter-MSC HO
Inter-BSC HO Intra-MSC HO
Intra-BSC HO
Intra-cell HO
MSC-B
Handover types
Requirements for Handover
1. Execution speed 2. Reliability
3. Transparent to users (not aware) 4. Service-dependent (voice, data)
• No excessive HO
• No delayed HO
• No early HO
Criteria 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.
MSC-A
BTS
BTS
BSC
Handling of Handover
Reserved channel concept Some channels are reserved for HO
1
2
Nc
N
New calls
Handover
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.
Intra-BSC (Inter-cell) HO
Old BTS
1
2
BSC
5
2
6
5
4 3
New BTS
Old BTS 4
New BSC
7
6 5
New BTS
MSC
Old BSC
4
9
1
4
8
2 3
7 4
7
Inter-BSC (Intra-MSC) HO
Old BTS
New BSC
5
8
7 6
New BTS
MSC-A Old BSC
5
8
1
5
3 4
8
5
MSC-B
2
GSM PLMN PSTN
Inter-MSC HO
9 9
Mobility Management (MM)
o Main task of MM is to support mobility.
o MM activities include: 1. IMSI attach and detach. 2. Location update 3. Registration 4. Update HLR/VLR 5. TMSI re-allocation. 6. Authentication of MS.
Location 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
3
4
1
2
3
4
Location 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
3
2
Old VLR
4
Periodic Registration
o MS is forced to send registration periodically. o If MS miss registration, the network marks o it as detached. o This occurs if MS is out of area. o This avoids paging. o Periodic registration has Ack. o MS tries until it receives Ack.
IMSI Attach
o When MS is switched on, it performs attach.
BTS
BSC MSC/VLR
3 2
4
1
4
Mobile Terminated Call
8
GMSC
BSC TRAU
5
11
8
7
GSM PLMN PSTN
6
2 HLR
MSC VLR
4 5
11
9
8 8 9
11 10
BTS BTS
Local Exchange
Telephone
1
1
10
BTS
BSC MSC/VLR
2
1
4
3
1
2
3
4 4
Exchange
Telephone
5
GSM PLMN
PSTN
Mobile Originated Call
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