High Speed Circuit Switched Data (HSCSD)
HSCSD is basically an upgrade of the original GSM CS data transmission system, by using HSCSD the speed at which data is transmitted is greatly improved. The higher data transmission rates are achieved by making use of bundled Traffic Channels (TCH). The way that this works is the MS requests one or more TCHs from the GSM network, in other words the MSC will allocate TDMA slots within a TDMA frame. This allocations do not need to be asymmetrical i.e. more slots can allocated downlink than the uplink, this fit the behavior of most users, typically the user will download more than they will upload. HSCSD requires software upgrades in an MS and MSC, this is because both have to be able to split a single traffic stream into several traffic streams, each using a TCH, and then to combine the streams again.
In theory a single MS could use
all eight time slots within a TDMA frame to achieve an Air Interface
User Rate (AIUR), for example 8 TCH/F14.4 channels or 115.2 kbit/s
(ETSI 1998) TR 101 186. One major problem with this configuration
is that the MS is required to send and receive at the same time.
However standard GSM does not support this, uplinks and down links
are always shifted for three slots. ESTI, (1997) EN 301 344, specifies
that the AIUR available at 57.6 kbit/s (duplex) using four time
slots and four time slots for the downlink, the table on the next
page shows the allowable combinations of TCHs and allocated slots
for non-transparent services.
| AIUR |
|
|
|
| 4.8 kbit/s |
|
|
|
| 9.6 kbit/s |
|
|
|
| 14.4 kbit/s |
|
|
|
| 19.2 kbit/s |
|
|
|
| 28.8 kbit/s |
|
|
|
| 38.4 kbit/s |
|
|
|
| 43.2 kbit/s |
|
|
|
| 57.6 kbit/s |
|
|
|