High-Speed Circuit Switched Data (HSCSD)

Prior to the arrival of GPRS or EDGE, the need for higher speeds of data
service was well recognized. At the time, GSM supported only data services
of up to 9.6 Kbps—the maximum that could be provided on a single timeslot.
In order to support higher data rates, the most obvious approach was a
solution whereby a given MS could use more than one timeslot, which is
basically what HSCSD offers.

Like GPRS, HSCSD enables the asymmetric allocation of resources on
the air interface. Unlike HSCSD, however, those resources cannot be shared
among multiple users. After all, the connection is circuit-switched. Consequently,
HSCSD is a rather inefficient use of valuable RF bandwidth, particularly
if the data session is bursty in nature. HSCSD provides for the
modification of allocated resources during a call, which can be useful if the
network needs to reclaim some of the resources that are being consumed by
HSCSD. This flexibility, however, does not approach the efficient use
enabled by GPRS.

The initial versions of HSCSD allowed for multiple timeslots, each offering
up to 9.6 Kbps of user data. Thus, four timeslots, for example, could offer
up to 38.4 Kbps. Subsequently, a change in the channel coding scheme was
proposed to allow a single timeslot to carry 14.4 Kbps of user data. One of
the main reasons for this change was to enable the mobile fax service to
support a fax transmission at 14.4 Kbps over just a single timeslot. Concatenation
of four such timeslots could therefore offer speeds up to
57.6 Kbps.
With the advent of the 8-PSK modulation that EDGE can provide, it is
possible for HSCSD to achieve high throughput levels with fewer timeslots.
For example, depending on the coding scheme chosen, a given timeslot
can support 28.8 Kbps, 32.0 Kbps, or 43.2 Kbps, as well as 14.4 Kbps,
and it is still possible to concatenate timeslots. An upper limit of 64 kpbs is imposed, however, not because of air interface limitations, but because of
limitations within the network. Specifically, the A interface (between the
MSC and BSC) is not designed for a given call to occupy more than one 64
Kbps channel.

Although HSCSD has seen some deployment in GSM networks, it cannot
be considered widely used. That situation is unlikely to change. With the
arrival of 8-PSK modulation, HSCSD will become somewhat more efficient.
Nonetheless, the packet-switching technologies of GPRS and EGPRS are
still vastly more efficient. Given a choice between HSCSD and the efficiencies
of GPRS and EGPRS, it makes sense for a network operator to opt for
a packet-switched solution and not HSCSD.