The Protocol model of GPRS

The UM interface is a GSM air interface. The communication protocol on the UM interface has 5 tiers, from the bottom up to the physical layer, the MAC (Media Access control) layer, the LLC (Logical Link control) layer, the SNDC (subnetwork dependant convergence) layer and network layer.

The physical layer of the UM interface is the RF interface section, while the physical link layer provides the various logical channels for the air interface. The carrier bandwidth of GSM air interface is 200KHZ, and one carrier frequency is divided into 8 physical channels.

If all 8 physical channels are allocated to transmit GPRS data, the raw data rate is up to 200kb/s. Considering the overhead of forward error-correcting codes, the final data rate can reach about 164kb/s.

The MAC is the media access control layer. The main function of Mac is to define and distribute the GPRS logic channel of air interface, so that these channels can be shared by different mobile terminals. There are 3 kinds of GPRS logic channel, namely public control channel, packet service channel and GPRS broadcast channel. The public control channel is used to transmit the control signaling of data communication, which is divided into paging and answering channels. Packet traffic channels are used to transfer grouped data. The broadcast channel is used to send network information to the mobile terminal.

The LLC layer is the logical link control layer. It is a wireless Link protocol based on high speed data link procedure HDLC. The LLC layer is responsible for forming the LLC address and Frame field on the SNDC data unit of the high SNDC layer to generate the full LLC frame. In addition, the LLC can implement a point-to-point addressing and data-frame-reset control.

The LLR layer in BSS is the logical link transfer layer. This layer is responsible for transferring the LLC frames between MS and SGSN. The LLR layer is transparent to the SNDC data unit, which is not responsible for processing the SNDC data. (Computer science)

SNDC is called a subnet-dependent binding layer. Its main function is to complete the packet transfer of data, packaging, determine the TCP/IP address and encryption mode. In the SNDC layer, the data transmitted between the mobile terminal and the SGSN is segmented into one or more SNDC packet units. The SNDC packet unit is generated and placed inside the LLC frame.

The protocols at the network layer are currently mainly the TCP/IP and L25 protocols provided by the phase L phase. TCP/IP and X.25 protocols are transparent to traditional GSM network devices such as BSS and NSS.