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.
Table 4.1(Available Data Rates for HSCSD)
(Adapted from Ericsson Document EN/LZT 123 5374 R1B)
Although HSCSD delivers major advantages in data transmission over GSM CS it does have several major disadvantages, it still uses a connection-orientated mechanisms of GSM, these mechanisms are not very efficient when it comes to computer data traffic, which typically uses bursts of data. If a large file is being downloaded HSCSD may require all channels to be reserved, where as typical web browsing would leave the channels idle most of the time. The allocation of channels is reflected directly in the service cost, as once the channels have been reserved by one HSCSD user other users can not use them, even if they are idle.
HSCSD was not used by any of the UK operators, this was because of the disadvantages stated above and the fact that GPRS came along so fast.
There are two main ways of transmitting data, Circuit-Switched (CS) and Packet-Switched (PS) communication, this is sometimes referred to packet data communication).
Figure 19 illustrates areas of bursty and/or bandwidth consuming communications. Burstiness and bandwidth requirements affect the type of communication chosen – circuit-switched, packet-switched, or e.g. SMS (Short Message Service) communication. However it should be noted that when choosing the manner of communication for an application the cost should be considered.