LAC design

• LAC design = optimised size + location of borders
• Compromise between paging load <-> location update load PCH <-> SDCCH • literature gives a theoretical approach to LAC design : Graph Theory sets start for a more "visual" approach
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Insert Subscriber
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Normal Location update Periodical location update
– relevant info transfer from HLR – TMSI calculated
LA1 LA2 LA3
– timer [off, 25.5h] in 6 min interval PS : during HO there is no locup performed, this happens after call termination (idle mode required)
(cfr. C3000 : PAGING MASSAGES SEND TO BTS) (64kbit/s 400 000 pages/h © NOKIA CCCH-SDCCH:WS Proximus / 05.04.00 / PIPE / SDE ) page: 8
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Typical 256 FR-TRX BSC (from Nokia BSC product description : 60% CPU load) • Total traffic handling capacity: 1520 Erlangs A • Average Subscriber's traffic: 25 mErl B • # subscribers: 60 000 A/B • Busy hour call attempts (BHCA): 45 000 C • Mean hold time: 120 seconds A*3600/C • Mobile originated calls (MO): 70 % 1-D • Mobile terminated calls (MT): 30 % D • Handover (HO) per calls: 1,5 E • Average location updates (LU) per call: 2 F • IMSI detach per call: 0,1 G • No paging response: 63% H • SMS call rate subs/hour: 1 I • terminated SMS 80% J
© NOKIA CCCH-SDCCH WS Proximus / 05.04.00 / PIPE / SDE page: 6
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MSC Paging settings
• MSC paging parameter to "LA" paging method : prevent unnecessary cell level CI information to be send to all cells on A & A-bis interfaces !
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Graph Theory (2)
• Convergent process of grouping the nodes is started, while cost is optimised and limitations are respected • Example :
• Signalling traffic load should not exceed 8 kbit/s (1000 bytes/s)
Risk of overflow if load > 1200 bytes/s
• Paging message is most common message sent on BCCH TRXSig link
© NOKIA CCCH-SDCCH WS Proximus / 05.04.00 / PIPE / SDE page: 5
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Paging load per BSC
• Call mix : capacity related & long term nework observation
Location Area dimensioning
• Graph Theory
• Paging load and capacity (A-bis and air) • Design process
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CCCH-SDCCH WS Proximus / 05.04.00 / PIPE / SDE page: 1
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max 2 signalling messages unacknowledged at any time, all subsequent messages must wait for ack of one of these 2 (GSM Recs 08.56) ack delay is few ms to tens of ms dependant on "other direction" traffic (meas. reports), thus reduces max capacity below 16 kbit/s disturbances in 2Mbit/s may further cause delays average AS7 transmit buffer occupancy should be close to 0 (LAPD-SS7 unit in BCSU)
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Players in Cost Function formula
• Paging - PCH
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Outside World
A-bis signalling capacity air interface capacity
MSC VLR
• Location update - SDCCH
• BSC paging load = (C*D + A/B*I*J) * (1+H) = (45000*0.3+60000*1*0.8)*(1.63) = 100 245 this number of users has to be paged per hour.
PS : Specific call mix could be used too -> specific settings !
• Which are the main players in a Cost Function formula ?
• How to implement a "low cost" LAC design in an existing operational network ?
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CCCH-SDCCH WS Proximus / 05.04.00 / PIPE / SDE page: 4
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A-bis Paging Capacity
• A-bis LAPD signalling link characteristics (16 kbit/s)
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CCCH-SDCCH WS Proximus / 05.04.00 / PIPE / SDE page: 2
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Graph Theory (1)
• Network is made of
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nodes lines
• Node could be cell or goup of cells (e.g. BSC) • Weight is allocated to each node and line representing :
length = 21 (including FCS and flags)
on BSC2E : 60% of nominal load and call mix can be given to paging messages => 0.6 * 1000/21 = 29 paging messages/sec 100 000 pages/h
• Timer for paging repetitions in MSC :
time between initial paging attempt and re-page shall be set so that the subscriber is able to respond in the mean time (risk of effectively doubling the paging load !) (cfr. BSC parameter NumberOfMultiframesBetweenPaging !)
• Objective is to two-fold : • respect paging capacity (location area size if # of subs ) • limit # of location update (location area size by reducing borders) [PS : 'unpaid' resources ]
Air interface Paging Capacity (1)
• Example
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BTS MS
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Combined BCCH / SDCCH Configuration : 3 CCCH blocks avail One Block Reserved for AGCH => 2 Blocks for PCH Paged Mobile Stations per Paging_Request Message : from 2 to 4, in average 3 3 Pages/Blocks * 2 Blocks = 6 Pages every 51-frames Multiframe ( 235 ms. ) In Average 2 Pages per Mobile Station due to repaging 2 Pages / Paged MS => 3 Paged MS every 235 ms. ( ( 3600 * 1000 ) / 235 ) * 3 = 45.957 Paged MS per Hour.
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for a node : total traffic in cell for a line : # MS transiting from node to node
thickness or filling represents relative weight
• Cost Function represents the SUM of different effects linked to the existence of the nodes and lines.
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Problem definition
• Location area = group of cells to locate a specific user in the GSM system, in this group of cells the specific mobile will be PAGED if the system wants to reach it.