2.6 GPRS Radio Resource Management
As described earlier, a GPRS timeslot can be assigned to several users at the same time. It is also possible to assign several timeslots to a single subscriber to increase his data transmission speed. In any case, the smallest transmission unit that can be assigned to a user is one block, which consists of four bursts on one timeslot on the air interface for GPRS and two bursts for EDGE MCS 7-9. A block is also called a GPRS RLC/MAC (radio link control/Medium Access Control) frame.
Temporary Block Flows (TBF) in the Uplink Direction
Every RLC/MAC frame on the PDTCH or PACCH consists of an RLC/MAC header and a user data field. When a user wants to send data on the uplink, the mobile device has to request for resources from the network by sending a packet channel request message via the RACH or the PRACH as previously shown in Figure 2.13.
The PCU then answers with an immediate packet assignment message on the AGCH. The message contains information as to the timeslots in which the mobile device is allowed to send data. As a timeslot in GPRS may not only be used exclusively by a single subscriber, a mechanism is necessary to indicate to a mobile device when it is allowed to send on the timeslot. Therefore, the uplink assignment message contains a parameter called the uplink state flag (USF). A different USF value is assigned to every subscriber that is allowed to send on the timeslot. The USF is linked to the so-called temporary flow identity (TFI) ...