Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 92

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.