03.133GPPDiscontinuous Reception (DRX) in the GSM systemTS
DRX is a technique that allows the mobile station to power down significant amounts of its internal circuitry for a high percentage of the time when it is in the idle mode.
It also ensures that the MS is aware of exactly when page requests for it may be transmitted and it can then therefore schedule other tasks such that it avoids the problem of not decoding valid page requests transmitted by the network in the idle mode periods.
The technique works by dividing the MSs within a cell into a set of groups. The group in which an MS resides is then known locally at both the MS and the BSS. All paging requests to each group are then scheduled and sent at a particular time which is derived from the TDMA frame number in conjunction with the IMSI of the MS and some BCCH transmitted data.
Thus both the BSS and the MS know when relevant page requests will be sent and the MS can power down for the period when it knows that page requests will not occur.
The page request can contain the IMSI and may contain the TMSI in order to identify the MS concerned. The IMSI is however always used to identify the paging population. Paging by means of the IMEI is for further study.
Mechanisms for the application of DRX for the voice group call and voice broadcast services are defined in stage 2 descriptions in GSM 03.68  and GSM 03.69 , respectively.