Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 60

An MS/UE communicates with the RAN of the GSM/GPRS, UMTS, LTE, and 5G systems through their respective air interface between them. The air interface is the physical interface where the actual physical link/interface/transmission media used over the air interface is the radio wave. A radio wave carries the modulated information between the UE/MS and RAN and vice versa. The air interface between the MS/UE and GSM RAN, UMTS UTRAN, LTE Evolved‐UMTS Terrestrial Radio Access Network (E‐UTRAN), and 5G RAN is illustrated, in general, in Figure 3.2 below.

Figure 3.2 Illustration: physical air interface for GSM, UMTS, LTE, and 5G NR systems.

A logical interface may consist of a protocol stack that may contain several protocol layers in it. Protocol layers are grouped into a particular logical interface which is known by a particular logical interface name for ease of its identification. Examples 3.3 and 3.4 below illustrate the typical logical interfaces found in the LTE/EPS and the 5G system.

The S1 logical interface between the LTE eNodeB and EPC contains two types of protocol stacks that are used to carry signaling and user data; see Figure 3.3a and b:

 User data transmission protocol, also called user plane, S1‐U.

 Signaling or control plane protocol, called control plane, S1‐Application Protocol (AP).

Similarly, the NG logical interface between the 5G NG‐RAN/gNB and UPF contains two types of protocol stacks that are used to carry signaling and user data; see Figure 3.3a and b:

 User data transmission protocol, also called user plane, NG‐U.

 Signaling or control plane protocol, called control plane, NG‐AP.

The S1‐U or NG‐U user plane protocol stack is used to transfer user traffic or data between the respective RAN (LTE eNodeB or 5G gNB) and its CN element (LTE/EPS MME or 5G UPF). The control plane protocol stack, S1‐AP(LTE/EPS) or NG‐AP (5G), is used to transfer signaling messages between the respective RAN (LTE eNodeB or 5G gNB) and its CN element (LTE/EPS MME or 5G AMF). Figure 3.3a shows side‐by‐side the S1‐U and NG‐U user plane protocol stacks, and Figure 3.3b shows the S1‐AP and NG‐AP signaling or control plane protocol stacks.