Paging Success by far is the most complex KPI to deal with as the process of paging touches almost all the nodes in GSM system and is influenced by performance of each of them. That’s the reason why this write up on paging looks too interwoven and cross refers to too many things. But the plus point with paging is by the time paging success rate in a network gets improved; almost all the other KPIs too stand improved. In response to an incoming call, the MSC initiates the paging process by broadcasting a "paging request" message on the paging sub channel (IMSI or TMSI of the MS and its Paging Group) and starts timer T3113. A "paging message" consists of the mobile identity (IMSI or TMSI) of the MS being paged and its "paging group number". A Paging Request Message may include more than one MS identification. The maximum number of paged MS per message is 4 when using "TMSI" for identification of the MS (maximum number of paged MS per message is 2 when using IMSI). The BSC receives this page and processes the paging request and schedules it for transmission on the PCH at appropriate time. The MS on its part will analyse the paging messages (and immediate assignment messages) sent on the paging sub channel corresponding to its paging group. Upon receipt of a "paging request" message, MS will initiate within 0.7s an immediate assignment procedure. Upon receipt of a page at the MS, the MS responds by transmitting a channel request on the RACH. BSS in response to the received "channel request", will process it and immediately assign the MS a SDCCH (immediate assignment / assignment reject; done over AGCH). MS Paging response- After receiving the immediate assignment command, MS switches to the assigned "SDCCH" and transmits a "Paging Response". The establishment of the main signalling link is then initiated with information field containing the "PAGING RESPONSE" message and the "paging response" is sent to the MSC. Upon receipt of the "Paging Response" MSC stops the timer T3113. If the timer T3113 expires and a "Paging Response"message has not been received, the MSC may repeat the "Paging Request" message and start T3113 all over again. The number of successive paging attempt is a network dependent choice.
One control channel Multi Frame is made of 51 TDMA frames with a time duration of 235 ms. Each 51 TDMA frame Multi Frame will have 9 Common Control Channel (CCCH) blocks. Each of these 9 CCCH block is made of 4 TDMA frames. Each CCCH block can carry Paging Messages for 2 MS if IMSI based paging is used or 4 MS if TMSI based paging is used. Thus the paging capacity for one 51 TDMA frame Multi Frame will be 9(number of CCCH blocks available per Multi Frame) * 4 (when TMSI based paging is used) = 36 mobiles per 235 ms or 9*2 = 18 mobiles per 235 ms when IMSI based paging is used. Thus the paging capacity of a cell is 153 mobiles per second when TMSI based paging are used and 68 mobiles per second when IMSI based paging are used. This means we can improve the "paging bandwidth" for a cell (if there are too many "paging discards at the cell level") by using TMSI based paging rather than IMSI based (at the expense of increased processor load at the BSC and MSC). When the rate of "paging load" at the BTS becomes higher than what the BTS is able to handle (paging capacity of BTS), BTS will start discarding pages (check for high "page discard" stats at the cell level). Once an MS deciphers its paging group, in an idle mode, it will tune in and check for an incoming page only during broadcast time for its paging group (so further the paging groups are places across multiple 51 frame multi frames, less frequently it will tune in to check for an incoming page and longer will be its battery life.
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