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Section 3.1.2 described the different logical interfaces used among the LTE/EPC or 5G network elements where user data or signaling information is transported over the same underlying IP transport network. However, in a GPRS and GSM network, different transport networks may be used apart from the IP network as illustrated below:

 GPRS Gb‐Interface Protocol Stack and Layers

In the case of the GPRS system, the Gb‐interface between the BSC and the SGSN is used for transferring the signaling and user data exchanged between the MS and the SGSN. The GPRS Gb interface consists of the following applications protocol layers:

 BSSSGP – Base Station Subsystem GPRS Protocol

 NS – Network Service

The Network Service (NS) layer is further divided into the following components:

 Network Service Control (NSC) Protocol

 Sub‐Network Service (SNS) Protocol

The Gb interface stack with the above protocol layers follows the protocol structure shown in Figure 3.5. Below the NS layer, there are two transmission network protocols, having its physical layer and media that can be used to exchange upper‐layer information between the Base Station Subsystem (BSS) and the SGSN. Those transport networks are:

 Frame Relay (FR) Protocol Transmission Network and

 IP Transmission Network.

Based on the transmission network protocol, Frame Relay, or IP, shown in Figure 3.5, the concerned physical layer shall be used accordingly. Note that the Gb‐interface cannot use both the Frame Relay and the IP transmission network simultaneously but shall use only one transmission network, either Frame Relay or IP, at a time. The particular transmission network to be used is configured by the operator and maintenance personnel, and it is taken care of by the SNS Protocol at the NS layer level.

Figure 3.5 GPRS: Gb‐interface protocol stack with IP and frame relay transport network.

The NS Control Protocol deals with transferring of the information of the upper layer in the form of NS Data Units (NS SDU), whereas the SNS Protocol deals with the management of the underlying transmission network protocol (Frame Relay or IP) to be used at a time to transfer upper‐layer information. For further details on the GPRS NS layer, refer to TS 48.016 [135].

Figure 3.6 Protocol layers classifications in GSM, GPRS, UMTS, LTE, and 5G systems.