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GSM-GPRS Operation Signalling Dimensioning Module 4
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GSM-GPRS Operation
Signalling Dimensioning
Module 4
GSM-GPRS Operation
Channel Configuration
5
MS Mode
Search for Frequency Correction Burst
Search for Synchronisation sequence
Read System Informations
listen for Paging
send Access burst
wait for signalling channel allocation
Call setup
traffic channel is assigned
Conversation
Call release
FCCH
SCH
BCCH
PCH
RACH
AGCH
SDCCH
FACCH
TCH
FACCH
idle mode
“off” state
dedicated mode
idle mode
6
0 7
ts0=bcch/pch/agchts1=sdcch/8
0 7
ts0=bch/sdcch/4/pch/agch
Combined Configuration Time slot 0 contains 1 BCCH, 3 CCCH and 4 SDCCH
0 7
ts0=bcch/pch/agch/sdcch/4
ts1=sdcch/8
Non-Combined Configuration Time slot 0 contains 1 BCCH, 9 CCCH Time Slot x (<>0 of BCCH TRX) contains 8 SDCCH
Hybrid Configuration Time slot 0 contains 1 BCCH, 3 CCCH, and 4 SDCCHTime Slot x (<>0 of BCCH TRX) contains additional 8 SDCCH
Configuration of Signalling Channels
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r r rr r r r fr rr r r r r r rr r r rr fr r r r r rr r r r fr r r r rr r r rr fr r r r rr r rr
Uplink
f s bb bbc fc fc scccc cc cc fc fscccc cc cc f fcccc cc cc fs fcccc cc ccs
Downlink 51 TDMA frames = 235ms
r
Time Slot 0
CHANNELS f = FCCH b = BCCH r = RACH s = SCH/SACCH c = CCCH t = SDCCH/4
Non-Combined ConfigurationBCCH/CCCH Multiframe
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CHANNELS t = SDCCH/8 s = SACCH/8
t t tt t t t ft tt t t t t t tt t t tt ft t t t t tt t t t fssss ss s ss fssssss
ss ss sss fs ss s t tt t t ts ft t t t t tt t t t ft t t t tt t t tt ft t t t tt t tt
Downlink
Uplink
t
t
51 TDMA frames = 235 ms
s
1. 2. 3. 5. 6. 7. 8.4.
Time Slot 1
Non-Combined SDCCH/8 Multiframe
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Channel Combination
The following combinations of logical channel types are allowed for the radio timeslots and specified by GSM (Rec. 05.02):
TCH/F + FACCH/F + SACCH/TF TCH/H + FACCH/H + SACCH/TH SDCCH/8 + SACCH/C8 FCCH + SCH + BCCH + CCCH (TS 0) FCCH + SCH + BCCH + CCCH + SDCCH/4 + SACCH/C4 (TS 0) BCCH + CCCH SDCCH/8 + SACCH/8 + CBCH FCCH + SCH + BCCH + CCCH + SDCCH/4 + SACCH/C4 + CBCH (TS
0)
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SDCCH Dimensioning
SDCCH used for: Call setup signalling Short message service (SMS) Location update (LU) IMSI attached/detached
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SDCCH Dimensoning (2) TCH load per subscriber = [ # of call attempts per sub per hour*
%MOC + # of call attempts per sub per call*%MTC*(1-%no response)*(1-% no answer)*%MTC success] * mean call duration / 3600 = [ 1.1 * 60% + 1.1 * 40% * (1-32.5%) * (1-13.5%) * 54%] * 115/3600 = 0.026 Erlang
SDCCH load per subscriber = [number of call attempts*setup signalling + #LU per sub per hour*duration for loc update] / 3600 = [1.1 * 3 + 2.2 * 5] / 3600 = 0.004 Erlang
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SDCCH Dimensioning (3)
A cell with 2 TRXs Combined SDCCH 1 comb. CCCH/SDCCH →4 SDCCH 15 TCH
Offered TCH load at 2% blocking 9.01 Erlangs 9.01/0.026=346 subscribers
Offered SDCCH load at 1% blocking 4 SDCCH blocks for combined configuration Thus from Erlang B table, lookup 4 channels at 1%
blocking 0.87 Erlangs 0.87/0.004=217 subs
SDCCH limited: 217 subscribers
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SDCCH Dimensioning (4)
Non-combined SDCCH 1 SDCCH/8 →8 SDCCH 14 TCH
Offered TCH load at 2% blocking 8.20 Erlangs 8.20/0.026=315 subscribers
Offered SDCCH load at 1% blocking 8 SDCCH blocks for non-combined configuration Thus from Erlang B table, lookup 8 channels at 1%
blocking 3.13 Erlangs 3.13/0.004=782 subs TCH limited: 315 subscribers
This configuration is preferred as it is TCH limited
GSM-GPRS Operation
Parameters Related To Channel
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Common Control Channel Load (3)
CCCH Capacity Calculation (non-combined):
Uplink : 51*3600/(51*0.004615) = 780000 RACH slots per hour
downlink : 36*3600/(51*0.004615) = 137658 CCH blocks per hour
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Common Control Channel Load (4)
CCCH Capacity Calculation (combined):
Uplink : 27*3600/(51*0.004615) = 413000 RACH slots per hour
downlink : 12*3600/(51*0.004615) = 45886 CCH blocks per hour
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Common Control Channel Load (5)
CCCH Capacity 1 RACH slot : a channel message for 1
subscriber 1 CCCH block (4 slots): 1 paging message for 1..4
subscribers* or 1 access grant message for 1..2 subscribers
* Depends on IMSI(2) or TMSI(4) paging
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Common Control Channel Load (6) - RACH
Used by MS to request a dedicated channel (SDCCH)
The causes for the channel request can be: A paging response in MTC An emergency call A MOC LU or IMSI attach/detach
Parameters related to RACH maxNumberOfRetransmission (1, 2, 4 or 7) numberOfSlotsSpreadTrans (3 ... 12, 14, 16, 20, 25, 32,
50)
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Common Control Channel Load (7) - RACH
The combination of maxNumberOfRetransmission and numberOfSlotsSpreadTrans values determine the time period between sending of two channels requests.
This period is measured in RACH slots and is the sum of a deterministic part S and a random part tr. (refer to GSM 04.08)
RACH can be configured in combined and non-combined case Combined: all 27 timeslot0 out of 51 timeslots Non-combined: all timeslot0,2,4,6
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Common Control Channel Load (8) - RACH
values for 50000 RACH activities per hour
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Common Control Channel Load (9) - RACH
The minimum blocking is achieved by the following setting of parameters: maxNumberOfRetransmission = 7, numberOfSlotsSpreadTrans = 50
The configuration of CCCH is mainly determined by the capacity needed by the downlink channels, the RACH configuration is uncritical
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Common Control Channel Load (10) - PCH
PCH may be used as AGCH but not vice-versa MOC requires AGCH and MTC requires PCH Typical network will have MOC higher than MTC Strategy: to dimension the AGCH to a smaller value
and let the system organise the use of channels Parameter related to PCH:
noOfMultiframesBetweenPaging (2 ... 9) It indicates the number of TDMA multiframes between
transmission of paging message to the same paging sub-group
It impacts on the MS battery life and MTC setup time Higher value will save battery life but longer call setup time
and vice versa Recommended value is 5
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Common Control Channel Load (11) - AGCH
Parameter related to AGCH: NumberOfBlocksForAccessGrant (0 ... 7) It defines the number of blocks reserved for
access grant messages from the CCCH during the 51 TDMA frame (a multiframe)
Recommended value is 1 for combined and 2 for non-combined configuration
Note that if the AGCH is insufficient, PCH can be used as AGCH
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Common Control Channel Load (12) – Example Cont..
Analysis: DL CCCH (PCH and AGCH) is the limiting factor The usage for PCH and AGCH is almost equal
For MOC, response to paging in MTC, LU and IMSI attached/detached, thus 4 RACHactivities per sub per hour
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Common Control Channel Load (12) – Paging Capacity
Paging demand is a function of: Number of MTC Number of subscribers in the LAC Paging repetition
PCH can be configured as combined or non-combined : Combined: both BCCH and SDCCH occupy the TS0 12 out of 51 slots per multiframe form 3 PCH/AGCH,
each block consists of 4 slots
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Common Control Channel Load (12) – Paging Capacity
PCH can be configured as combined or non-combined: Non-combined: BCCH occupies TS0 and SDCCH occupies TS1 36 out of 51 slots per multiframe form 9 PCH/AGCH, each block
consists of 4 slots
Split of blocks between PCH and AGCH: the available blocks in 51 multiframe is splitted between PCH and AGCH and it is set by parameter NumberOfBlocksForAccessGrant
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Common Control Channel Load (13) – Paging Capacity
IMSI/TMSI paging: IMSI: 2 mobiles can be paged with each page
message occupying 4 slots TMSI: 4 mobiles can be paged with each page
message occupying 4 slots
Paging capacity calculation: PCH per second
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Common Control Channel Load (14) – Paging Capacity
What does the table mean? CCCH can be used for both AGCH and PCH Example for combined configuration, 3 CCCH blocks are
available If u reserve 0 blocks for AGCH, all 3 CCCH blocks will be used
for PCH If u reserve 2 blocks for AGCH, only 1 CCCH blocks will be used
for PCH
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Common Control Channel Load (15) – Paging Capacity
For combined BCCH:
Making sense out of the table: Number of MS that can be paged in a
second: Depends on IMSI or TMSI paging and Number of blocks reserved for AG
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Common Control Channel Load (16) – Paging Capacity
For Combined BCCH Example: if u reserved 1 block for AG, then u
have 2 blocks for PCH If u decide TMSI paging, I.e. 4 MS can be paged
with 1 block of PCH Thus; (number of PCH blocks*number of pages
per block)/(51*one TDMA frame period); =(2*4) / (51*0.004615) = 34 34 MS can be paged in a second (using TMSI
paging)
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Common Control Channel Load (18) – Paging Capacity
For Non Combined BCCH
Making sense out of the table: 3 blocks reserved for AG, remaining 6 blocks used for PCH Number of MS can be paged in a second (TMSI paging) = (6*3)/(51*0.004615) = 76 MS/second
GSM-GPRS Operation
Location Area Design
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Location Area Design
What is Location Area? A location area is an area in which MSs may
roam without updating the location registers A location area consists of one or more cells
What information you need? Traffic model
• What is next? Calculate SDCCH traffic Determine paging capacity Calculate LA size (in term of number of cells)
GSM-GPRS Operation
End of Section 4
Signalling Dimensioning
