In order to use its maximum capability, it will be necessary to establish a TBF in two phases.
On the RACH, the MS is unable to indicate its multislot capability in the CHANNEL REQUEST message. Thus, access through the PRACH allows the mobile to send more precise information on its capability and requirements in order to establish as quickly as possible an uplink TBF. All the bit combinations of 8 or 11 bits of information within the access message are reserved for packet access only. On PRACH, no circuit-switched access can occur. As some of the combinations are reserved for GSM access, fewer combinations are available for packet access. This message contains 8 bits of information, allowing a limited number of uplink combinations. The CHANNEL REQUEST message is used for both circuit-switched access and packet access. The CHANNEL REQUEST message carries 8 bits of information. One format allows the transport of 8 bits information and the other one 11 bits information. There are two codings for the PACKET CHANNEL REQUEST message. The mobile may also request the establishment of an uplink TBF during a downlink TBF on PACCH. The mobile triggers the establishment of an uplink TBF:īy sending a CHANNEL REQUEST message on the RACH when there is no PBCCH in the cell īy sending a PACKET CHANNEL REQUEST message on the PRACH if PBCCH is present in the cell.