SBS: Paging Procedure in Siemens MSC, BSC, BTS and MS
Version 01.08.2009
Detailed Description: Management of the PAGING procedure in the Siemens MSC, BSC, BTS and MS The paging procedure is performed for Mobile Terminating Calls (MTC) as well as for SMS mobile terminated. The following message flow illustrates the process:
MS
BTS
BSC
UDT (PAGIN)
MSC
PGCMD PCH: PAGING REQUEST TYPE x
RACH: CHAN REQ
CHNRD CHNAV
CHNAK IACMD (IMASS) AGCH: IMASS SDCCH: SABM (PAGRES) SDCCH: UA
ESTIN (PAGRES)
MRPCI CR (CL3I;PAGRES) call continuation . . . . . .
MSC The MSC sends a PAGING message in SCCP connection-less mode. This message contains • the IMSI and optionally the TMSI of the paged MS If TMSI reallocation is activated the MSC will always transmit both the IMSI and the TMSI in the first paging attempt. If no PAGING RESPONSE has been received within an defined time period time the MSC repeats the paging procedure; in the second attempt, however, only the IMSI is transmitted in the PAGING message. The timer for the supervision of the PAGING RESPONSE is call T_PAG and can be administered with the SSS command MOD NETTIMER:BSSAPT=TPAG-. The paging procedure is repeated only once. • the area in which the MS has to be paged. This information can be contained in different formats: either the MSC only transmits the Location Area Identity (LAI) - which means that the BSC has to determine the associated cells from its database itself - or the MSC may transmit all Cell Identities. The most common format is to only transmit the Location Area Identity and to leave the determination of the associated cells to the BSC as this variant causes less load on the CCS7-link.
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SBS: Paging Procedure in Siemens MSC, BSC, BTS and MS
Version 01.08.2009
BSC The BSC, having received the PAGING message from the MSC, determines the cells associated to the Location Area Identity received and sends a PAGING COMMAND message to all affected BTSEs via the LPDLR. This message contains. • the IMSI or the TMSI of the paged MS Whether the BSC uses the TMSI or the IMSI for the paging procedure depends on the contents of the received PAGING message: if the PAGING message from the MSC contains the TMSI in addition to the IMSI the BSC will always transmit the TMSI in the PAGING COMMAND, otherwise it will transmit the IMSI. • the paging group The paging group is used for ‘Discontinuous Reception (DRX)’ and is calculated from the IMSI and the used CCCH configuration. The paging group determines which CCCH blocks the MS shall monitor for incoming paging messages (by just monitoring a subset of the CCCH blocks for paging the MS can save battery capacity). Note: Another radio parameter relevant at this point is 'BS-PA-MFRMS' (database name NFRAMEPG = number of multiframes between paging). This parameter determines the distance (in multiframes) between two pagings of the same paging group. It is broadcast in the SYS_INFO_TYPE3 in the IE 'Control Channel Description'. With this parameter the time needed for the paging procedure might be extended (in addition to the repetition time determined by the MSC). • the channel number The channel number indicates the type of channel via which the information is to be transmitted (in this case the vale is 'Downlink CCCH (PCH + AGCH)'). Overload handling in case of PCH overload in the BTS
BTS The BTS, having received a number of PAGING COMMAND messages from the BSC assembles these pagings to the Um-message PAGING REQUEST TYPE 1, TYPE 2 or TYPE 3 with which several MSs can be paged simultaneously. Which type of PAGING REQUEST is used depends on the number and type of mobile identities that are to be transmitted within the same PAGING REQUEST (see table below). PAGING REQUEST TYPE PAGING REQUEST TYPE 1 PAGING REQUEST TYPE 2 PAGING REQUEST TYPE 3
2
Transmittable Identities 2 IMSI or 2 TMSI 2 TMSI + 1 IMSI or TMSI 4 TMSI
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SBS: Paging Procedure in Siemens MSC, BSC, BTS and MS
Version 01.08.2009
The BTS can buffer twice the number of CCCH blocks (1 CCCH block consists of 4 bursts) that are contained in one cycle of the selected CCCH configuration for transmission. Examples: a) MBCCHC: 1.
F S 0 1
2
BCCH 6
2.
F S BCCH 3 0 1 2
7 6
4 3
CCCH CCCH
F S CCCH 10 11 12
16
F S CCCH 13 10 11 12
17 16
8 7
14 18 13 17 ‘downlink’
CCCH
F S SDCCH0 20 21 22
SDCCH1 26
F S SDCCH2 30 31 32
SDCCH3 36
F S SACCH0 40 41 42
SACCH1 46
I 50
CCCH
F S SDCCH0 23 20 21 22
SDCCH1 27 26
F S SDCCH2 33 30 31 32
SDCCH3 37 36
F S SACCH2 43 40 41 42
SACCH3 47 46
I 50
24
28
34
38
23 + 4 SDCCH 27 BCCH + CCCH / 4, F = 33 FCCH, S37= SCH
44 43
48 47
3 CCCH blocks available → the BTS can buffer 6 CCCH blocks b) MAINBCCH: 9 blocks available → the BTS can buffer 18 CCCH blocks How many bursts resp. blocks are needed for a PAGING request message depends on the actual length of the layer 3 message: A layer 3 message of 20 octets length can be transmitted within one layer 2 message and thus within 1 CCCH block. If the layer 3 message is longer, another complete block has to be used even if not all the bursts of the block have to be used. The remaining part of the block is filled with filling bits. For the possible lengths of the PAGING REQUEST TYPE x please refer to GSM 04.08. Conclusion: The number of CCCH blocks required for the paging procedure dynamically depends on the type and length of the layer 3 message (i.e. the PAGING REQUEST TYPE x) which in turn depends on the number of mobile identities that have to be paged at the same time. Paging queues The BTS buffers the PAGING REQUESTs in so-called 'paging queues’. For every paging group (see explanation under ‘Mobile Station’) an own paging queue with two queue places is available. The number of paging groups depends on the created CCCH configuration: the higher the CCCH capacity, the more paging groups (and thus paging queues) are available. For more details please refer to the document ’Overload Handling in BTS Paging queues’.
Mobile Station The MS monitors only a subset of the paging channels on the basis of the following principle: The radio network parameter ‚Number of multiframes between paging‘ (BSC database parameter SET BTS PKGBTSC:NFRAMEPG, range 2..9) is broadcast on the BCCH (SYS_INFO_Type3) in the IE ‘Control Channel Description’. It defines the number of multiframes (51 TDMA frames) between two transmissions of the same paging message to mobiles of the same ‘paging group’. The paging group determines which CCCH blocks the MS shall monitor for incoming paging messages. By just monitoring a subset of the CCCH blocks for paging the MS can save battery capacity (Discontinuous Reception (DRX)). The BSC and the MS calculate the number of the paging group from the IMSI and the used CCCH configuration. As a basic rule, the higher the CCCH capacity in the cell, the more paging groups are available (i.e. a BCCH of the type „MAINBCCH“ includes more paging groups than a BCCH of the type „MBCCHC“). In addition, the database parameters “NBLKACGR“ (number of blocks reserved for access grant) and NFRAMEPG (number of multiframes between paging) have an influence on the number of paging groups: a) NBLKACGR: the higher the value of NBLKACGR (i.e. the more CCCH blocks are TAC3 SBS
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SBS: Paging Procedure in Siemens MSC, BSC, BTS and MS
Version 01.08.2009
reserved for AGCH), the lower the number of paging groups (and thus paging queues in the BTS). b) NFRAMEPG: the higher the value of NFRAMEPG, the more paging groups (and thus paging queues in the BTS) are available. In case of paging channel overload the BTS can instruct the MS to also monitor paging channels of other paging CCCH blocks (page mode ‚extended paging’ and ‚paging reorganization’). For more details please refer to the document ’Overload Handling in BTS Paging queues’. The more paging groups are available, the higher the paging buffer space in the BTS and the better the BTS can manage paging ‘peaks’ without starting ‚extended paging’ or/and ‚paging reorganization’. Paging response If the MS has recognized its identity (IMSI or TMSI) in one of the PAGING REQUEST messages transmitted in the cell it performs an access via the RACH (using the establishment cause 'answer to paging'), receives the IMMEDIATE ASSIGNMENT for the SDCCH via the AGCH and and transmits the PAGING RESPONSE together with the SABM (setup of the layer-2 connection on the Um). This leads to the setup of a dedicated SCCP connection between MSC and BSC and to completion of the transaction (MTC or SMS-MT).
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