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].
3.11.2 Identification of Technical Requirements Using Interface Name
A 3GPP technical specification defines its scope and describes the protocols, functions, and procedures that may be applied to more than one mobile communications systems, from GSM to the 5G system. From the descriptions available in a given 3GPP TS, it is important to identify such requirements that are specific to a particular mobile communications system, i.e. GSM, GPRS, UMTS, LTE, and 5G. A 3GPP TS mentions the corresponding CN logical interface name while describing the detailed requirements of functions and procedures. Generally, a 3GPP technical specification mentions the applicability of a specific requirement/section being described using the term: A/Gb mode to refer to GSM/GPRS system; Iu mode to refer to UMTS system; S1 mode to refer to LTE/EPS system; and N1 mode to refer to 5G system. A logical interface name also identifies a particular mobile communications system that is being referred to and the requirements that belong to it.
3.12 Protocol Layer Procedures over CN Interfaces
In the preceding sections, we have discussed the CN interfaces of GSM, GPRS, UMTS, LTE/EPS, and 5G networks. The logical interfaces between the respective RAN and its CN element are as follows:
S1, between LTE eNodeB and MME; NG between 5G NR NG‐RAN and AMF
Iu, between UMTS/UTRAN RNC and MSC; RNC and SGSN
A, between GSM BSC and MSC
Gb, between GSM BSC and SGSN
Over a particular logical interface, as mentioned above, the following types of signaling messages are exchanged in the forms of an AP between the RAN and CN for the execution of
Interface‐specific protocol functions and procedures
Session Management, Call Management, and MM air interface Layer 3 or NAS layer messages between a UE/MS and the CN, transparently through the RAN.
The AP functions and signaling procedures executed over the respective logical interfaces are specific to a particular mobile communications system and network. There are also protocol and signaling procedures that are similar in nature, but they are implementations dependent on a particular communications system.
3.12.1 Similar Functions and Procedures over the CN Interfaces
Several similar protocol functions and procedures are performed over the LTE/EPS S1‐interface, UMTS Iu‐interface, GSM A‐interface, GPRS Gb‐interfaces, and 5G NG interface. Some of them are mentioned below. However, these high‐level functions and procedures are implementation‐dependent in terms of different signaling messages names and their contents but are similar in nature with respect to their applicability from GSM to the 5G system.
Between the MS/UE and the CNThe CS or PS domain Session Management, Call Management, and MM layer signaling messages that are exchanged between an MS/UE and the CN are not processed by the GSM or UMTS or LTE or 5G RAN, but they are forwarded to the CN. Forwarding of such signaling messages by the RAN to the CN is done through the direct transfer of messages. In the GSM system, it is known as the DTAP as shown earlier in Figure 3.21; in the UMTS system, it is known as the DIRECT TRANSFER; in the LTE and 5G systems, it is known as the NAS transport.
A Session Management, Call Management, and MM‐related message that is exchanged between an MS/UE and the CN is embedded in a DTAP (GSM) or DIRECT TRANSFER (UMTS) or NAS TRANSPORT (LTE/EPS) message. A GSM DTAP or UMTS DIRECT TRANSFER or an LTE/EPS or 5G NAS TRANSPORT message is exchanged between the radio access and its CN only. However, the initial Layer 3, i.e. CC, MM related, message from a ME/UE toward the CN is exchanged through the GSM INITIAL MS message in UMTS and the INITIAL UE message in the LTE/EPS and 5G systems.
Example 3.15 below illustrates the typical messages flow between an LTE/eNodeB and its MME through NAS transport messages in uplink and downlink direction. Such NAS transport messages between an LTE/eNodeB and its MME are used to forward signaling messages exchanged between a UE and LTE/MME and vice‐versa.
Example 3.15 LTE/EPS: PS Domain NAS Transport Messages
Figure 3.12 shown earlier illustrates the LTE/EPS MM procedures and their signaling messages. This figure shows snapshots of a few end‐to‐end signaling messages for a PS domain LTE/EPS ATTACH procedure. In fact, between the eNodeB and the MME, the individual NAS signaling messages of a particular LTE/EPS NAS procedure will be exchanged using the uplink or downlink NAS TRANSPORT message as illustrated in Figure 3.22 below.
Figure 3.22 Illustration: LTE/EPS NAS transport between eNodeB and MME over S1 interface.
As
