Traffic Channel Data Services

The basic idea in providing TCH data service is to provide a CS conduit for carrying data from the user through the GSM PLMN and vice versa. The conduits are composed of CS bearers in the network and the TCH over the air interface.The data path establishment procedure is very similar to the voice path establishment. There is also an IWF with the MSC, which serves as a gateway between data signals in the GSM PLMN and other networks, such as PSTN (public switched telephone network) and PSPDN (public switched packet data network). With a TCH-Full rate (TCH/F), the maximum effective data rate possible is 9.6 Kbps.

With a TCH-Half rate (TCH/H), the maximum data rate is 4.8 Kbps. The GSM PLMN also provides high-speed circuit switched data (HSCSD), which combines multiple TCH/Fs and can provide a maximum data rate of 56 Kbps.GSM provides transparent and nontransparent connection for the TCH data services. In the transparent connection, the data services see the path between the IWF and terminal as a constant rate conduit. The GSM PLMN doesn't intercept the data protocol and the data traffic. The radio interface doesn't do any extra processing to safeguard the data from errors. The transparent connection is usually suited for constant rate and fixed delay applications.
In the nontransparent connection, the data protocol and the traffic are intercepted in the GSM PLMN. The data are intercepted to provide reliable transport over the low-bandwidth and error-prone radio link. The GSM specifications have defined link-layer protocols, L2R and RLP, for this purpose. Example applications for nontransparent connections are X.25 LAP-B and character-mode protocols like X.28.
For carrying data through a GSM PLMN, a set of services is defined. These services, called bearer services and teleservices, allow different types of data applications to carry data through the GSM PLMN. We will discuss these services in the next section. The GSM PLMN also interconnects with ISDN, CSPDN, and PSPDN for transparent/nontransparent data services using an IWF. The GSM PLMN performs a set of functions so the data can be carried between the IWF and mobile terminal. These functions are mainly to adapt the data to the circuit switch nature of the transport and the radio nature of the air interface. In this section we will also review those functions.