Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 93

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 illustrated in Figure 3.22, the LTE air interface initial NAS layer messages, e.g. ATTACH REQUEST, TAU, received from a UE is sent through the InitiaUEMessage from the eNodeB to the MME. The subsequent NAS messages between eNodeB and the MME are exchanged using the DownlinkNASTransport and UplinkNASTransport messages. The MME sends the ATTACH ACCEPT message to the eNodeB through the InitialContextSetup message. For more information on the NAS TRANSPORT messages and their contents, refer to TS 36.413 [97].

 Between the RAN and the CN

Several similar procedures are performed between the RAN and its CN only over their respective logical interfaces, e.g. LTE/EPS S1 and X2; 5G NG; UMTS‐IuPS; and IuCS. Typical similar procedures are mentioned below:

 Handover procedure performed in case of the GSM and LTE system; relocation procedure performed in case of UMTS system. A handover or relocation procedure is executed to transfer an ongoing call from one cell to another suitable cell.

 Interface management – to setup, initialize, and release the respective interfaces, i.e. A, Gb, Iu, S1, and 5G system NG interface.

 Security and encryption.

 Paging – to notify an incoming call for an MS/UE.

 UE tracking – to track a particular UE/MS.

 Overload control from CN – which indicates the RAN to reduce the signaling load toward the CN.

Several functions are performed by a network element to complete a particular protocol layer procedure as highlighted above, which are described later in different chapters. For more information on the above functions and procedures, refer to TS 25.413 [54], TS 36.413 [97], TS 38.413 [119], and TS 48.008 [134].