The GPRS Air Interface

The GPRS air interface is built upon the same foundations as the GSM air
interface—the same 200-kHz RF carrier and the same eight timeslots per
carrier. This allows GSM and GPRS to share the same RF resources. In fact,
if one considers a given RF carrier, then at a given instant, some of the
timeslots may be carrying GSM traffic, while some are carrying GPRS data.
Moreover, GPRS enables the dynamic allocation of resources, such that a
given timeslot may be used for standard voice traffic and subsequently for
GPRS data traffic, depending on the relative traffic demands. Therefore, no
special RF design or frequency planning is required by GPRS above that
required for GSM. Of course, GPRS demand may require the addition of
additional carriers in a cell. In such a situation, additional frequency planning
effort may be required, but this is no different from the frequency planning
that is required with the addition of an RF carrier to support
additional GSM voice traffic.
Although GPRS uses the same basic structure as GSM, the introduction
of GPRS means the introduction of a number of new logical channel types
and new channel coding schemes to be applied to those logical channels.
When a given timeslot is used to carry GPRS-related data traffic or control
signaling, then it is known as a Packet Data Channel (PDCH). As shown in
Figure 5-1, such channels use a 52-multiframe structure as opposed to a 26-
multiframe structure for GSM channels. In other words, for a given timeslot
(that is, PDCH), the information that is being carried at a given instant is
dependent upon the position of the frame within an overall 52-frame structure.
Of the 52 frames in a multiframe, 12 radio blocks carry user data and
signaling, 2 idle frames are used, and 2 Packet Timing Control Channels
(PTCCHs) as described in the following section. Each radio block occupies
four TDMA frames, such that 12 radio blocks are used in a multiframe. In
other words, a radio block is equivalent to four consecutive instances of a
given timeslot. The idle frames in the multiframe can be used by the MS for
signal measurements.