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Suppose a developer is interested to study the GSM BSS that consists of BTSs and BSC network elements. Further, suppose that developer is interested in the air interface Layer 3 protocol, between an MS and the BSC, of a GSM system. Figure 3.21 below shows the GSM air interface Layer 3 protocols along with its sublayer protocol, as follows:

 Call Control Management (CM),

 MM, and

 Radio Resource Management (RR).

Figure 3.21 Illustration: GSM air interface Layer 3 protocol stack.

1 As shown in Figure 3.21, the GSM air interface Layer 3 consists of the CM, MM, and RR layers. The developer may be further interested in the Radio Resource Management, (RR) sublayer of the GSM Layer 3 protocol stack. The RR layer of a BSC deals with the signaling functions/messages of the GSM Layer 3 protocol stack. Now refer to the 3GPP site [2].

2 For the GSM signaling protocols/messages that are exchanged between an MS and the BSC and vice versa, the corresponding 3GPP TS series number will be either 44 Series (After Release 4, including, GSM) or 4 Series (Before Release 4 GSM). Assume that you have considered the 3GPP TS Series 44. Within this series, one will find all the technical specifications, listed in ascending order, related to signaling messages between MS and the BSC. Now, look for the technical specification having the title Radio Resource Management protocol. You got the desired TS. In this case, it is the 3GPP TS 44.018 [130]. For Series 4, the corresponding TS will be the 3GPP TS 04.18.

Similarly, try to find the technical specification number for the GSM CM and MM sublayer, which is TS 24.008[45]. 3GPP TS 24.008 [45] covers the entire mobile radio air interface Layer 3 specification for GSM/GPRS Edge Radio Access Network (GERAN), UMTS system. By following the same way as described above, one can find the corresponding technical specification number for the RRC protocol in the case of UMTS or LTE or 5G system. The technical specification series number for the LTE system is 36; for UMTS, it is 25 series; for 5G, it is 38 series.

In Figure 3.21, the GSM BSC and its BTSs are being shown as the combined BSS. However, a BTS and BSC of a BSS are connected by the logical A‐bis interface that is not shown in Figure 3.21. A‐bis interface is proprietary with its protocol stack. On the CN side, a BSC and the MSC are connected by the logical A‐interface which is an open standard defined by 3GPP. Look at the protocol stack of the A‐interface. At the top of the stack are the Base Station Subsystem Mobile Application Part (BSSMAP), Direct Transfer Application Part (DTAP), and Signaling Connection Control Part (SCCP) layer, which is the Layer 3 protocol. The BSSMAP and DTAP protocols are defined in the 3GP TS 48.008 [134]. The Layer 2 is the Message Transfer Part (MTP). The physical layer used for both the A‐bis and A‐interface is the E1 interface, as described earlier in Section 3.1.1. The SCCP, MTP is part of the standard Signaling System #7 (SS#7). For more information on the SCCP and MTP layers, refer to TS 48.006 [133].