Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 83

In Section 3.2, we have discussed the separation of protocol layers of various logical interfaces into a control plane and user plane categories which are applicable, in general, in all the mobile communications systems, i.e. GSM, GPRS, UMTS, LTE and 5G. In Section 3.3, we have also discussed the grouping of UMTS, LTE, and 5G systems only protocol layers into AS and NAS categories from the protocol layer termination at UMTS UTRAN, LTE E‐UTRAN, 5G NG‐RAN, and CN point of view.

The grouping of protocol layers into the AS and NAS layers in the UMTS, LTE, and 5G systems is done from their respective air interface point of view, where the air interface is the physical interface. On the other hand, the UMTS UTRAN, LTE E‐UTRAN, and 5G NG‐RAN communicate with their CN elements and another network element of a UTRAN, E‐UTRAN, and 5G NG‐RAN using the standard data transport network, e.g. ATM, IP, which is the standard protocol. It may be noted that the protocol stack and its logical interface between UMTS UTRAN and its CN; LTE E‐UTRAN and its CN; and 5G NG‐RAN and its CN are logically independent of the underlying data transport network used by them. Based on this, the protocol stack of a logical interface, i.e. Iu interface between UMTS UTRAN – CN; S1, X2 interface between E‐UTRAN and MME or E‐UTRAN; NG interface between 5G NG‐RAN and 5G core, is further modeled with the following horizontal‐layered structures:

 Radio Network Layer (RNL)

 Transport Network Layer (TNL)

The above protocol model of the UMTS UTRAN, LTE E‐UTRAN, and 5G NG‐RAN is illustrated in Figure 3.18. For more information on these layered structures, refer to UMTS TS 25.401 [51], LTE TS 36.401 [95], 5G TS 38.401 [117], and TS 38.410 [118].

Figure 3.18 Illustration: general protocol layer model of UTRAN, E‐UTRAN, and 5G NG‐RAN.

Figure 3.19 Illustration: general protocol layer model of UTRAN and E‐UTRAN.

The RNL deals with the UTRAN or E‐UTRAN or 5G NG‐RAN‐specific related various functions and procedures, for example, radio bearer management, paging, and so on, in terms of an AP. The data TNL deals with the particular transport method, e.g. ATM, IP, and so on, and possibly through the intermediate network also, to be used to transport RNL procedures messages.

RNL and TNL layers are logically independent of each other, which makes it possible to make UMTS UTRAN or LTE E‐UTRAN or 5G NG‐RAN protocol‐related changes in the RNL without affecting the TNL. As shown in Figure 3.18, the protocol layers of the logical interfaces between the UMTS UTRAN or LTE E‐UTRAN or 5G NG‐RAN and their respective CN or other network element are still separated as the control plane and user plane, but they share the same physical layer. Figure 3.19 shows a side‐by‐side RNL and TNL of the LTE/EPS S1‐AP control plane between LTE eNodeB and MME, and UMTS Iu‐PS user plane protocols between RNC and SGSN.