Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 80

A particular protocol layer communicates and exchanges information in its defined format only between two network elements. A network element could be the mobile station, BTS, BSC, MSC, SGSN, eNodeB, MME, S‐GW, 5G core UPF, and so on. The communications between the protocol layers of two network elements may be direct and point to point, or it may be routed through another network element that may work on different protocol stacks. If the communication is not through a direct path, then the original message sent by the sender needs to be forwarded by an intermediate network element to the destination network element using protocol conversion. This is illustrated, in general, in Figure 3.16.

Figure 3.15 Illustration: air interface sublayers: GSM, GPRS, UMTS, LTE, and 5G.

Figure 3.16 Illustration: protocol information conversions in a cellular system.

In Figure 3.16, consider that a user wants to access the Internet (e.g. www, FTP, ping, and so on) through the GPRS or LTE/EPS network. The UE will send the user's request to the RAN using RLC/MAC protocol across the air interface. The RAN will collect the RLC/MAC layer block, in the case of GPRS, or RLC layer PDU, in the case of LTE. The RAN will format the RLC/MAC layer information into an appropriate protocol layer format of the concerned CN logical interface, for example, GPRS Gb interface Frame Relay format for SGSN, or LTE/EPS S1‐U format, and forward it to the SGSN or S‐GW. As an analogy with a traditional IP network/Internet, in a mobile communication network also, the user data or signaling data packets pass through different protocol layers and intermediate devices between a source and destination.