Mobile Communications Systems Development. Rajib Taid

GPRS/ UMTS PS Call Establishments, Call Setup Primary and Secondary PDP Context Managements Routing Area Update, Network Attach, Detach LTE/EPS PS [Fall‐back] Call Establishments, Call Setup Default and Dedicated Bearer Managements Tracking Area Update, Network Attach, Detach

Example 3.8 LTE/EPS NAS Layer: EPS Mobility Management Layer Procedure

Let us consider the LTE UE ATTACH procedure shown in Figure 3.12 below, which is found in an LTE/EPS network. The ATTACH procedure and its messages are part of the EMM NAS protocol which is used by a UE to register with the LTE/EPS CN; see TS 24.301 [46]. This figure illustrates a successful ATTACH procedure and shows only the EMM message names without showing the contents of each message.

Figure 3.12 Illustration: LTE/EPS ATTACH procedure: NAS protocol messages.

Example 3.9 5G NAS Layer: 5G Session Management Layer Procedure

Let us consider the establishment of a PDU session, shown in Figure 3.13 below, a procedure that is initiated from a UE to the 5G core Session Management Function (SMF) network function. A PDU session is activated and is assigned to a particular network slice of a UE as part of its initial 5G UE registration procedure with a 5G CN. The establishment of a PDU session is part of the 5G Session Management (5GSM) NAS protocol. Figure 3.13 shows only the 5GSM message names without showing the contents of each message.

Figure 3.13 Illustration: NAS layer messages for a 5G PDU session establishment procedure.

In GPRS and UMTS networks also, an MS/UE registers with its CN using the ATTACH procedure for PS services. However, as far as the implementations are concerned, there are differences in the GPRS/UMTS and LTE/EPS ATTACH procedure. For example, unlike the GPRS system, an LTE/EPS ATTACH request message also contains a piggybacked session activation request to the CN.

Example 3.9 illustrates the typical messages flows associated with an SM layer (NAS) PDU session establishment procedure in the case of the 5G system.

3.4 Initialization of a Logical Interface

      In the previous sections, several logical interfaces between the concerned network elements of mobile communications networks were described and illustrated. Different functions and procedures are performed over the respective logical interfaces. Some logical interfaces do not become ready for exchanges of information between the concerned network elements. Also, following the occurrence of an erroneous event, a logical interface may become unusable. In such scenarios, a logical interface between two network elements requires to be initialized and configured or reinitialized/reconfigured, with protocol layer‐specific data, to make it ready for exchanges of information over it. The procedure for initialization and configuration with protocol layer‐specific data differs from one logical interface to another one.

Example 3.10 illustrates the typical messages flows for initialization of the S1 logical interface, which is used between the LTE/eNodeB and its MME.

      Similarly, the Gb‐interface which is used in the GPRS system is also required to be initialized. The NS protocol layer of the Gb‐interface in the BSC/PCU end initializes and sends the configuration data to the peer layer on the SGSN side of the Gb‐interface.

Example 3.10 LTE/EPS S1‐AP (eNodeB‐MME) Logical Interface Initialization

In the LTE/EPS, the S1 interface is used to exchange control or signaling‐related messages, between the eNodeB and the MME, which is also known as the S1‐AP (Application Protocol). To make the S1 interface operational for exchanges of information and other S1‐AP messages, the S1 interface is required to be setup first. For this purpose, the eNodeB sends the S1 Setup Request message, containing the eNodeB identity, Public Land Mobile Network (PLMN) identity, to the MME. The MME sends the S1 Setup Response message to the eNodeB. This is shown in Figure 3.14, which is reproduced from TS 36.413 [97]. Other examples where initialization of a logical interface is required is the 5G NG interface which is used between 5G NG‐RAN and AMF network function.

Figure 3.14 Initialization of LTE/EPS S1 interface.

      Source: © 2014. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2014, 3GPP.

3.5 Protocol Layer Termination

      The extent and working of a particular protocol layer can be further understood from the protocol layer termination point of view. The general working model of a protocol layer consists of functions and procedures that it requires to perform to facilitate the various services to be offered to the higher layer, as described later in Section 3.8.

      Protocol layer termination refers to the making available of the various services by the concerned protocol layer to its adjacent layers at the same time facilitating a peer‐to‐peer communication between two network elements over a logical interface. To understand a protocol layer termination, start from the UE/MS end and proceed toward the radio access or CN. A protocol layer terminates at the destination or peer network element or domain. Find and look at the corresponding network element containing the terminated protocol layer. Next, look at its position, e.g. Layer #2, Layer #3, and so on, within the protocol layers’ organization supported by the concerned network element. The protocol stack and its particular layer termination also identify the network elements that exchange various messages using the concerned layer protocol specification. For example, as mentioned in Section 3.3, the AS protocols terminate at the UMTS UTRAN or LTE E‐UTRAN or 5G NG‐RAN, whereas the NAS protocols terminate at the respective CN end. For more examples of protocol layer terminations, refer to TS 25.301 [49].

      

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