GPRS Traffic Scenarios

The following sections provide some straightforward examples of GPRS
traffic. This allows for an understanding of the differences between GSM
and GPRS, and later, the differences between GPRS and UMTS. Prior to
describing these, we need to familiarize ourselves with some terms.
Temporary Block Flow (TBF) is the physical connection between the MS
and the network for the duration of the data transmission. The TBF can be
considered to use a number of radio blocks over the air interface.
Temporary Flow Identity (TFI) is an identifier assigned to a given TBF
and is used for distinguishing one TBF from another. A TFI is used in control messages (such as acknowledgements) related to a given TBF, so that
the entity receiving the control message can correlate the message with the
appropriate TBF.

Temporary Logical Link Identity (TLLI) is an identifier that uniquely
identifies an MS within a routing area. The TLLI is sent in all packet transfers
over the air interface. The TLLI is derived from the Packet Temporary
Mobile Station Identity (P-TMSI) assigned by an SGSN, provided that the
MS has been assigned a P-TMSI. In case the MS has never been assigned
a P-TMSI, then the MS may generate a random TLLI.
An Uplink State Flag (USF) is an indicator used by the network to specify
when a given MS is entitled to use a given uplink resource. In GPRS,
resources are shared in both the downlink and the uplink. The downlink is
under the control of the network, which can schedule transmissions for a
given user on a given downlink PDTCH as appropriate. On the uplink, however,
a mechanism is necessary to ensure that only a given MS transmits on
a given uplink resource at a given time. This can be done in two ways,
through fixed allocation or dynamic allocation.

With fixed allocation, the network allocates some number of uplink
timeslots to a user, some number of radio blocks that the MS may transmit,
and specifies the TDMA frame when the user may begin transmission.
Thus, the MS is provided with exclusive access to the timeslot for a particular
period of time.With dynamic allocation, the network does not allocate
a specific time upfront for the user to transmit. Rather, it allocates the user
a particular value of USF for each timeslot that the user may access. Then
on the downlink, the network transmits a USF value on each radio block.
This value indicates which MS has access to the next radio block on the corresponding
timeslot in the uplink. Thus, by examining the value of USF
received on the downlink, the MS can schedule its uplink transmissions.
The USF is a three-bit field and thus has eight possible values. Thus, with
dynamic allocation, up to eight MSs can share a given uplink timeslot.