, and transmission error detection and correction. The physical link layer can support multiple mobile stations that share the same physical channel. The RLC/MAC layer is present over the physical RF layer. The RLC (radio link control) is responsible for data transmission over the physical layer of the GPRS radio interface. Also, it provides a link to the upper layers. The MAC (medium-access control), as the name suggests, is responsible for control of the access functions of the mobile over the air interface. These functions include the allocation of channels and multiplexing of resources. The LLC (logical link control) layer is present over the RLC layer and, as the name suggests, it is responsible for the creation of a logical link between the mobile and the SGSN. Thus, is responsible for the transfer of the signalling and data transfer. It is independent of the lower layers. GPRS: NETWORK PLANNING AND OPTIMISATION The SNDCP (sub-network dependent convergence protocol) layer performs mapping and compression functionalities between the network layers and lower layers. Compression of both the control and data information is done by this layer. It also performs the segmentation/de-segmentation of the information to/from the lower LLC layer. The above-mentioned are responsible for the information flow between the MS and SGSN. The IP layer is responsible for the formation of the backbone of the GPRS network and a direct interface between the MS and GGSN. 5.4.2 BSS Protocols The BSSGP (base station subsystem GPRS protocol) layer is responsible for information transfer between the SGSN and the RLC/MAC layer. Thus, its primary function is to create the environment for data flow between these two entities. 5.4.3 SGSN protocols The GTP (GPRS tunnelling protocol) layer runs over the UDP layer and is responsible for the data and signalling information between the BSC and the GSN nodes. This protocol forms a tunnel for each subscriber and each tunnel is identified by a tunnel endpoint identifier. The UDP/TCP (user data pro