Paging Procedures

GSM Paging Procedures Prepared By, Syed Masroor Ali Pakistan www.powerpointpresentationon.blogspot.com

Paging Procedures Paging is the procedure used for identifying the current cell location of an MS in order to route an incoming (mobile terminated) call.

PAGING MESSAGE TYPES Three paging message types have been defined:  Type 1: can address up to 2 mobiles using either IMSI or TMSI.  Type 2: can address up to 3 mobiles, one by IMSI and the other 2 by TMSI.  Type 3: can address up to 4 mobiles using the TMSI only.

PAGING MESSAGE TRANSMISSION The reason different numbers of MS identities can be paged in a single message type is that the IMSI and TMSI are of different lengths.

The IMSI comprises 8 octets containing MCC, MNC & MSIN, however the TMSI is operator-assigned and therefore does not require the MCC or MNC. Hence it is only 4 octets in size. As a result, a paging message can store twice as many TMSIs as IMSIs. To reduce the signaling channel load, the BSS stores these requests temporarily until there are enough to make up a full type 1,2 or 3 message or until a configurable timer (set by the operator) expires. The contents of the paging message are then broadcast on the paging channel (PCH).

PAGING AND DRX To save battery power, Discontinuous Reception (DRX) functionality can be implemented. In this case, the MS is assigned to a particular CCCH paging block (paging group) and therefore is only required to listen for paging messages on that block rather than all CCCH blocks in the control channel multiframe. The designated paging group is defined in the NFRAMEPG parameter. This parameter informs the mobile of the number of multiframes (ranging from 2 to 9) after which the same paging group is repeated

The total number N of paging groups is determined for a given Location Area according to following criteria:  number of subscribers,  average number of paging requests per subscriber per unit of time,  maximum number of MS receiving a paging request at the same time in a group. Moreover, N can be determined by the following rules.

For instance, if the number of CCCH blocks available is 9, the number of multi frame between paging is 2 and the number of CCCH reserved for access grant is 0: There will be 18 paging groups. Each mobile will only listen to its relevant paging group.

The Radio Capacity Nb paging per sec = (Paging per CCCH block) X (PCH Available/Sec) The BTS can receive simultaneously a paging request message with:  2 IMSI (type request 1)  1IMSI + 2 TMSI (type request2)  4 TMSI (type request 3) To compute the theoretical radio capacity, let’s consider that N % of MS are paged on TMSI, the average number of paging per CCCH block will be: Paging per CCCH block = 4 x N % + 2 x (1- N %) Example: If It is assumed that 80% of MS are paged on TMSI Paging per CCCH block = 80% x 4+ 20% x 2 =3.6 paging Also, If It is assumed that 100% of MS are paged on TMSI Paging per CCCH block = 100% x 4+ 0% x 2 =4 paging

PCH Available/Sec = 4.255 X number of paging available per 51 multi frame Where:

4.255 is the number of 51- multi frames per second (1/235ms) & Number of paging available per 51 multi frame = (9 – NBLKACGR) for BCCH not combined Example: CCCH configuration: BCCH not combined; NBLKACGR = 1 It is assumed that 80% of MS are paged on TMSI Paging per CCCH block = 80% x 4+ 20% x 2 =3.6 paging

Number of paging available per 51 multi frame is (9-1) = 8 PCH Available/Sec = 8 x 4.255 = 33.8 PCH/Sec Nb paging per sec = (33.8 X 3.6) = 121.68 paging/s

LAC DIMENSIONING The dimensioning of the location area has a high impact on the number of paging and the location update messages. With a larger location area, the number of location message decreases, but the number of paging increases because there are more subscribers in the location area. The LAC size should therefore be limited either by the product constraints or the radio capacity; the maximum constraint has to be taken.

BSC Paging Queue Overflow When the BSC receives PAGING messages from the MSC, it buffers them in a queue first, before it determines which BTSs belong to the affected LAC and delivers them towards the BTSs as PAGING COMMAND via the Abis Paging messages are discarded due to lack of space in the BSC paging queues available for one LAC When the percentage of discarded paging messages has exceeded the threshold defined by PAGQOVLIND in the last second • An alarm BSC paging Queue overload is sent to RC/LMT/event log file • BSSMAP OVERLOAD message is sent to MSC