as an address. GSM does not allow paging by IMEI (GSM 04.08 Section 9.1.22.3 as exception to the definition in 10.5.1.4). This paging step occurs only for a transaction initiated by the network. Random Access. The mobile station sends a burst on the RACH. This burst encodes an 8-bit transaction tag and the BSIC of the serving BTS. A variable number of most-significant bits in the tag encode the reason for the access request, with the remaining bits chosen randomly. In L3, this tag is presented as the RR Channel Request message (GSM 04.08 9.1.8). The mobile also records the TDMA clock state at the time the RACH burst is transmitted. In cases where the transaction is initiated by the MS, this is first step. Assignment. On the AGCH, the network sends the RR Immediate Assignment message (GSM 04.08 Section 9.1.18) for a dedicated channel, usually an SDCCH. This message is addressed to the MS by inclusion of the 8-bit tag from the corresponding RACH burst and a time-stamp indicating the TDMA clock state when the RACH burst was received. If no dedicated channel is available for assignment, the BTS can instead respond with the RR Immediate Assignment Reject message, which is similarly addressed and contains a hold-off time for the next access attempt. Emergency callers receiving the reject message are not subject to the hold-off and may retry immediately. Retry. If the RACH burst of step 2 is not answered with an assignment or assignment reject in step 3 within a given timeout period (usually on the order of 0.5 second), the handset will repeat step 2 after a small random delay. This cycle may be repeated 6-8 times before the MS aborts the access attempt. Note that there is a small but non-zero probability that two MSs send identical RACH bursts at the same time in step 2. If these RACH bursts arrive at the BTS with comparable power, the resulting sum of radio signals will not be demodulable and both MSs will move to step 4. However, if there is a sufficient difference in power, the BTS will see and answer the more powerful RACH burst. Both MSs will receive and respond to the resulting channel assignment in step 3. To ensure recovery from this condition, Um uses a "contention resolution procedure" in L2, described in GSM 04.06 5.4.1.4 in which the first L3 message frame from