PAGING AND LAC SPLITTING

By: S.M.KAMRAN

Outline:• Basic Call Flow• Paging• Paging Related Channels• Paging Request Types• Paging Related Parameters• Paging Statistics• Paging Problems in MSC, BSC and BTS• Location Update• Location Update Types• Splitting LAC • Calculation of LA Capicity

Basic Call Flow

Basic Call Flow cont…

1- MS after dialing a number & pressing SEND key, sends Channel Request(Chan_Req) message on RACH to ask for a signaling channel (Radio Resources). [RACH - Random Access channel]2- The BSC allocates a Traffic Channel(TCH) using AGCH. TCH allocation assigns a specific Frequency & a Timeslot on that frequency. [AGCH - Access Grant Channel]3- The MS sends a call setup request through SDCCH, to the MSC/VLR. [SDCCH - slow dedicated control channel]. Over SDCCH, all signaling takes place. This includes: marking the MS status as active in the VLR 4 -Then comes Authentication Procedure which includes Ciphering (The channel is ciphered so as to protect the call), Equipment Identification, etc.5 -The MSC/VLR instructs the BSC to allocate an Idle TCH (this message contains the dialed digits and other information needed for call establishment). The BTS and MS are told to tune to the TCH. 6- The MSC allocates a voice circuit on one the digital trunks between the MSC and the BSS.7- MSC informs the BSS about the allocated voice circuit. The call is also switched from signaling to voice.8- The BSS notifies the Mobile about the changeover to voice mode.9- The MSC routes the call and sends the call towards the called subscriber.10- The PSTN indicates to the MSC that it has received all the digits and the called subscriber is being rung.11- The MSC informs the mobile that the called subscriber is being alerted via a ring.12- The called subscriber answers the call.

Paging

Paging cont…• MSC will send paging command to its all BSC knowing its LAC (Location Area Code)

information, BSC’s will send command to all cells in this LA. After BTS receives this command it will send out on paging sub-channel of paging group containing IMSI or TMSI number of paged subscriber. After receiving the Paging Request, MS will request through RACH for SDCCH allocation.

• BSC will assign this SDCCH to MS through Immediate Assignment message in AGCH after it confirmed the activation of the needed SDCCH by BTS. MS will use this SDCCH to send Paging Response. BSC will then forward this Paging Response to MSC, finishing a 1st radio paging successfully.

• The switch in GSM network usually adopts second paging with a paging interval of 5 seconds. After MSC obtains the current LAC information of the MS from VLR, it will send out paging message for the first time to all BSCs under the LAC the MS is in, finishing 2nd radio paging successfully.

• If no paging response is received within 5 seconds, MSC will send out paging message again to all BSCs under the LAC the MS is in. If there is still no paging response within 5 seconds, this radio paging fails, and MSC will send a voice notification “the subscriber you dialed cannot be connected for the moment” to the MOC subscriber.

Paging Flow Chart

Paging Related Channels

Five channels play vital role in making paging successful. i.e, PCH,AGCH,SDCCH,BCCH and RACH. Major contribution of every channel given as,

Broad Cast Control Channel (BCCH)Downlink Channels , it contains the detailed Network and cell specific information such as :

• Frequency used by Cell and its Neighboring cells.• Frequency HSN• Paging Groups• LAI• Max output power allowed in the cell

Paging Channel (PCH)• Downlink Channels, • BTS: Broadcast the paging message to indicate the Incoming Calls or Incoming SMS. Paging

message also includes the MS’s identity number IMSI/TMSI• MS: MS listens to the PCH. If it identifies its own mobile subscriber identity number on the

PCH, it will respond.

Random Access Channel (RACH)• RACH is transmitted Uplink only, When mobile is paged , it replies on RACH

requesting a signaling channel. RACH can also used if the MS wants to make a contact the NW/Originating calls

Paging Related Channels cont…Access Grant Channel (AGCH)Downlink channel, AGCH is answer to the RACH• NW assigns a signaling channel (SDCCH) to the MS. This assignment is performed on the

AGCH

Stand Alone Dedicated Control Channel (SDCCH)• SDCCH is bi-directional Channel• System Signaling• Call Setup• Authentication• Location Update• Assignment of Traffic channels and• Transmission of Short messages

Paging Request Types

• The Paging request Type setting determines whether the mobile station is paged using its IMSI or TMSI. This parameter can only be set while the Call Status is Idle and the Data Connection Status is Idle. • When there is a service request for a MT Point-to-Point SMS (GPRS transport type), the identifier type used is P-TMSI (Packet TMSI).• If to page the mobile station using a TMSI, it is strongly recommended that the TMSI value be allocated to the mobile station when it registers with the test set to ensure that the MS recognizes the TMSI value used in the CS page request. The TMSI is allocated to the mobile station when it registers to the test set if TMSI Assignment is set to On . • Sets/queries the paging IMSI (International Mobile Subscriber Identity) field, used for paging the MS. The test set will stay in Active Cell Status (Setup Request), The paging IMSI is automatically updated by the test set during an MS originated call using the IMSI reported by the MS. If the MS has no SIM the paging IMSI is left unchanged.

There are three types of paging request messages.• Paging Request Type 1: We can Page max 2 MS with IMSI Or TMSI in 1 paging messages.• Paging Request Type 2:

We can Page max 3 MS in which we have to page at least 2 MS with TMSI.• Paging Request Type 3:

We can Page max 4 MS in which we have to page all MS with TMSI.

Paging Related Parameters

Paging Group• After MS succeeds in decoding BCCH and system information, it will wait for the paging

channel it should monitor. The size of a paging channel is defined by operator. It defines that how many paging sub-channels in a cell could be divided into, MS monitors only the paging sub-channel it belongs to and ignores the contents in other paging sub-channels.

• The larger the paging group, i.e. the more the paging sub-channels in a cell, the fewer the subscribers in each paging sub-channel. Thus, the average service time of the MS battery could be extended. By large paging groups, the paging capacity in system could be increased; but the average time delay of paging messages on radio channel will be extended, lowering the paging success rate.

• By small paging groups, the paging response could be speeded up, but the MS battery will be used up quickly and the system paging capacity will also be small.

BS-AG-BLKS-RES• BS-AG-BLKS-RES indicates the number of message blocks in CCCH reserved for Access

Granted Channel. This parameter actually shows the proportion of AGCH and PCH allocated on CCCH. The setting of this parameter will affect both the time for MS to respond to paging and the system service performance. If PCH is idle when all AGCHs are fully occupied, it could be used for sending Immediate Assignment Command. If BS-AG-BLKS-RES is set as 0, Immediate Assignment will have to be sent on idle paging channels.

Paging Related Parameters cont…BS-PA-MFRMS• Refers to the number of multi-frames that could form a period of paging sub-channel. This

parameter actually defines how many sub-channels the paging channels in a cell could be divided into. The larger the BS-PA-MFRMS, the more the paging sub-channels in a cell; this parameter should be set to the minimum value, provided no paging channel overload occurs. If BS-PA-MFRMS = 9, the MS belonging to a paging group will be paged every nine multi-frames, with an interval of 2.1 seconds (235.4ms*9).

Relation between Paging Group, BS-PA-MFRMS, and BS-AG-BLKS-RES (AGB）• Number of paging groups for combined BCCH/SDCCH = (3-AGB)*BS-PA-MFRMS• Number of paging groups for Non-combined BCCH/SDCCH = (9-AGB)*BS-

PAMFRMS

BS-PA-MFRMS

Time between Transmission of each

paging group

Number of paging groupsCombined BCCH/SDCCH

Number of paging groupsNon-Combined BCCH/SDCCH

3 paging blocksper multi-frameAGB=0

2 paging blocksper multi-frameAGB=1

9 paging blocksper multi-frameAGB=0

8 paging blocksper multi-frameAGB=1

2 0.47 6 4 18 16

3 0.71 9 6 27 24

4 0.94 12 8 36 32

5 1.18 15 10 45 40

6 1.41 18 12 54 48

7 1.65 21 14 63 56

8 1.89 24 16 72 64

9 2.12 27 18 81 72

Paging Related Parameters cont…

• MFRMS (multi-frame period and defines the transmission interval of paging messages to the same paging group)

• AGBLK (sets the number of CCCH blocks in each multi-frame that will be reserved for access grants)

• BCCHTYPE (Combine with CCCH and SDCCH or not)• T3212 (time between the periodic registration)• ATT (determines if attach/detach is allowed)• MAXRET (maximum number of retransmission of MS may do when

accessing the system on RACH)• CRH (hysteresis value used when the MS in idle mode crosses an LA

border)• PAGLIMIT (max number of paging orders allowed to be sent to the TRH

per second)

PAGING STATISTIC (BTS & BSC)Object Type Counter Definition

PAGTOOOLD # of discarded paging messages sent out on the PCH due to old paging messagesPAGPCHCONG # of discarded paging messages sent out on the PCH due to full paging queue.

BSC TOTPAG # of paging messages received from the MSC.PAGCSBSC Cumulative # of PAGING CS messages received from SGSN with paging area set to the BSC or LA.PAGPSBSC Cumulative # of PAGING PS messages received from SGSN with paging area set to the BSC or LA.

RANDOMACC RAANPAG # of random accesses, answer to paging.RAAPAG1 # of random accesses, answer to paging and the channel required is a TCH/F with dual rate mobile capability.RAAPAG2 # of random accesses, answer to paging and the channel required is a TCH/F or TCH/H with dual rate mobile capability.

CELLPAG

BSCGPRS

RNDACCEXT

If the BSC have more than 2 LAI, then :

Network Environment Paging Success (%)Urban With good RF 98Urban 95Suburban 93Rural 88

Paging Success Rate depend also on NetworkCoverage, the better the coverage the better The Paging Success Rate.

PAGING PROBLEM in MSC, BSC and BTSMSC• Poor Paging Performance in MSC can happen with reason :

– Congestion (SAE)– Poor Parameter Setting– Poor Paging Strategy– Signal between BSC and MSC not stable– Poor LU performance

BSC and BTS• Poor Paging Performance in BSC can happen with reason :

– Poor Coverage– Too Much of “Ping Pong” Location Updating– Congestion in BTS or BSC– SDCCH Congestion– High interference in network– Poor Parameter Setting– Signal between BTS and BSC not stable– BTS HW fault– MS Equipment fault– TRH load check– Check Signalling– Optimize LAC Border (Splitting and Reboundary)– Optimize Parameter, such as : paging timer, paging groups, paging method selection, paging

limit,

Location Update• Bandwidth is a very insufficient resource in radio networks, it is necessary to subdivide the

service area into many cells called as Location Area (LA) in order to allow the reuse of frequencies.

• If a call has to be established, it is need to know the current cell of the subscriber and transmit

the call setup message (called paging operation and paging message) only in this cell.

• Due to the mobility of the user, it is obvious that the mobile station will enter and leave cells and

this information must be updated/registered in the network management to establish mobile

terminated calls, this type of registration to network called Location Update.

Location Update TypesThree types of LOCATION UPDATE used as,

1. Generic Location Update

i. Intra VlR Location Update

ii. Inter-VLR Location Updating, Sending TMSI

iii. Inter-VLR Location Updating, Sending IMSI

2. Periodic Location updating

3. IMSI Attach and Detach

Location Update Types cont…1. Generic Location UpdateWhen the MS moves from one LA to another LA, registration is required. If the LAI stored in the MS is different from the LAI of the current cell, the MS informs the network to change the location information it stores. This procedure is called generic location update. The MS will reselect the procedure and will not inform the network if LA is same, whilst if LA change, it will inform network called forced Location Update.i. Intra VLR Location UpdateThis type of “LU” requires no IMSI updation, It happens in the current VLR without informingthe HLR. Only VLR behaves as Master device, it allocates a new TMSI if required otherwise former TMSI used.ii. Inter-VLR Location Updating, Sending TMSIAfter the MS enters a cell, if the current LAI is different from the LAI it stores, it sends its LAI and TMSI to VLR through MSC in location updating request. VLR identifies its former VLR, current LAI and TMSI. If current LAI differs from last stored then it sends a request to former VLR for its IMSI and AC parameters. After receiving VLR sends information to HLR for authentication and LU. HLR stores current VLR number and send a MAP/D_CANCEL_LOCATION request to former VLR and delete all information of MS and confirms. HLR will send MAP_INSERT_SUBSCRIBER_DATA command to current VLR with AC parameters, after confirmation from VLR to HLR, “LU” procedure completes.iii. Inter-VLR Location Updating, Sending IMSIThe rest procedure is same as described above, but it requests for authentication parameter from the HLR through IMSI directly not TMSI.

Location Update Types cont…2. Periodic Location updating

The network and the MS lose contact when:

MS is switched on but moves out of the network coverage area (dead zone). The network lost

contact with the MS and regards it still in attach status

MS sends IMSI detach message and the uplink quality is bad due to interference, the network

may not be able to decode this message correctly. The MS is still regarded in attach status.

MS is power off. It cannot inform the network of its status and the contact is lost.

So, Location update cannot be executed but waste of radio resources happens. A T3212 an

implicit detach timer works in VLR, the MS reports its location periodically by force at timer’s

expiry. Network sends T3212 value to all MS’s through BCCH.

If this timer value be kept shorter, network performance will be more proficient, but will

increase the signaling flow, power consumption and reduce the utilization of the radio resources,

The T3212 setting should be based on comprehensive consideration.

Location Update Types cont…3. IMSI Attach and DetachIMSI attach and detach means to attach a binary mark (1 or 0) to the subscriber record in MSC/VLR. An IMSI ATTATCH request sent to network when MS is switched on, and starts “LU”. If current LAI and former stored LAI resulted as same, IMSI attach flag is enabled, process same as Intra-VLR LU.If LAI differs from former LAI stored, generic LU process repeated.An IMSI DETACH triggered in MSC/VLR when an MS switched off, MSC sends request to VLR to update accordingly. When the paging for this user occurs, HLR requests for the MSRN (Mobile subscriber roaming number) from the VLR and is informed of the no-radio of this user by this time. Therefore, no paging program is implemented. The paging message is handled directly, such as playing the record: "The subscriber is powered off."

Location Area Code

SPLITTING LAC (1)• To avoid Paging Congestion, splitting LAC is one major Solution. Step to split LAC :

– Define Call (Attempt) Concentration– Check Outgoing HO (Handover) per Cell– Define New LAC Sheme based on Geographical Characteristic (island, mountain, river,

main road, etc)

• Don’t Split LAC on the Area that have high Call (attempt) and high Outgoing HO.• Look for Field Information about Geographical Characteristic, don’t split the LAC on Main

Road, island grouping, River path etc.

SPLITTING LAC (2)Splitting LAC based on call attempt (with parameter TASSAL)

Call Concentration

LAC A LAC B

SPLITTING LAC (3) Splitting LAC based on Outgoing HO (with parameter SUMMOHOAT)

LAC A LAC B

SPLITTING LAC (4)Splitting LAC based on Geographical Characteristic

LAC A LAC B

LAC LIMITATION

• LAC Limitation :– The threshold will be define based on BTS/BSC Paging Capacity (Depends on HW

and SW installed).

– The LA dimensioning is therefore a tradeoff between the signaling load caused by paging and location update procedures

BSC PROCESSORAPZ 212 11APZ 212 25APZ 212 30*

# AGBLK PAGING CAPACITY0 21.2 Paging commands / second1 14.4 Paging commands / second0 63.8 Paging commands / second1 57.5 Paging commands / second

Combined BCCH/SDCCH

Non Combined BCCH/SDCCH

CELL CCH USED

PAGING CAPACITY#Cells/LA * #Paging Commands/cell and sec < 8000 Paging Commands/sec#Cells/LA * #Paging Commands/cell and sec < 18000 Paging Commands/sec#Cells/LA * #Paging Commands/cell and sec < 50000 Paging Commands/sec

* APZ 212 30 onwards equipped BSC other limitation will be reached before the paging capacity in the BSC (Mostly on TRH or abis/LAPD link).

Calculation of LA Capacity• Number of paging blocks per second X Number of paging messages per paging block =

Maximum number of sending paging messages per second ( Each hour support paging number of times) Maximum number of sending paging messages allowed in an hour Traffic allowed in an LA.

• Number of paging blocks per second, 1 multi-frame = 51 frames = 0.2354s. If the number of BS_AG_BLK_RES is AGB, the formula to calculate the number of paging blocks per second is given as ：

• Non-combined BCCH: the number of paging blocks/s = （ 9- AGB ） /0.2354 (paging blocks/s)

• Non-combined BCCH: when AGB=2, the number of paging blocks/s = 27.9 paging blocks/s. When AGB = 0, the number of paging blocks/s= 38.2 paging blocks/s.

• Number of Sending Paging Messages per Paging Block: There are 23 octets in each paging block, which could send two IMSI paging messages (a); or two TMSI paging and one IMSI paging messages (b); or four TMSI paging messages (c).

• If IMSI paging mechanism is adopted, each paging block could send paging messages for X times in average, X ＝ 2 Number of Sending Paging Messages/paging blocks If TMSI paging mechanism is adopted, then, X = 4 Number of Sending Paging Messages/paging blocks

• Maximum Number of Sending Paging Messages per Second (P): This could be calculated through the following formula:: Non-combined BCCH: P = （ 9-AGB ） /0.2354 (paging blocks/s) × X (number of sending paging messages/paging blocks)

• IMSI paging mechanism: to non-combined BCCH, when AGB=2, P=59.47 times of sending paging messages/s; TMSI paging mechanism: to non-combined BCCH, if AGB=2, P=118.95 times of sending paging messages/s Therefore, the paging capacity in IMSI paging is half that in TMSI paging.

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