GPRS User Devices

GPRS is effectively a packet-switching data service overlaid on the GSM
infrastructure, which is primarily designed for voice. Furthermore,
although certainly a demand exists for data services, voice is still the big
revenue generator—at least for now. Therefore, it is reasonable to assume
that users will require both voice and data services, and that operators will
want to offer such services either separately or in combination. Consequently,
GPRS users can be grouped into three classes:
■ Class A Supports the simultaneous use of voice and data services.
Thus, a Class-A user can hold a voice conversation and transfer GPRS
data at the same time.
■ Class B Supports simultaneous GPRS attach and GSM attach, but
not the simultaneous use of both services. A Class-B user can be
“registered” on GSM and GPRS at the same time, but cannot hold a
voice conversation and transfer data simultaneously. If a Class-B user
has an active GPRS data session and wants to establish a voice call,
then the data session is not cleared down. Rather it is placed on hold
until such time as the voice call is finished.
■ Class C Can attach to either GSM or GPRS, but cannot attach to
both simultaneously. Thus, at a given instant, a Class-C device is
either a GSM device or a GPRS device. If attached to one service, then
the device is considered detached from the other.
In addition to the three classes described, other aspects of the MS are
important. Most notable is the multi-slot capability of the device, which
directly affects the supported data rate. For example, one device might support
three timeslots, whereas another might only support two. Note also
that GPRS is asymmetric—it is possible for a single MS to have different
numbers of timeslots in the downlink and uplink. Normal usage patterns
(such as Web browsing) generally require more data transfer in the downlink
direction. Consequently, it is common for a user device to have different
multi-slot capabilities between the uplink and downlink. For example,
many of today’s handsets support just a single timeslot in the uplink direction,
while supporting three or four timeslots in the downlink direction.