Mobile Communications Systems Development. Rajib Taid

control plane.

Figure 3.3 shows that the S1‐U and NG‐U or the S1‐AP and the NG‐AP have the same protocol stack over the IP transport network. Protocol layer classifications under user plane and control plane protocols are described later in Section 3.2.

Example 3.4 LTE Logical Interfaces with the Same Protocol Stack

In the LTE/EPS mobile communications network, several logical interfaces may have the same protocol stack and its transport network. For example, consider the IP transport‐based logical interfaces: S3 (MME‐SGSN), S4 (S‐GW‐SGSN), S5 and S8 (S‐GW – P‐GW), S10 (MME‐MME), and S11 (MME‐S‐GW) that are used among the several network elements of an LTE/EPS network. These logical interfaces have the same protocol stack on top of the IP transport network, i.e. UDP/IP, which is used to tunnel IP payload from one network element to its peer. Refer to the illustration shown in Figure 3.4. IP payload is tunneled through a protocol called GPRS Tunneling Protocol (GTP) in an encapsulated manner. GTP PDUs between two network elements are transported on top of the underlying IP transport network.

Figure 3.4 LTE/EPS logical interfaces: S3, S4, S5, S8, S10, and S11 protocol stack.

      Note that in the case of the LTE/EPS system, the GTP control plane Version 2 (GTP‐C v2) [70] is used in all the logical interfaces mentioned above to carry signaling information between their respective network elements. But in the case of the GPRS/UMTS system, GTP control plane Version 1 (GTP‐C v1) TS 29.060 [67] is used. Similarly, the GTP‐user plane, TS 29.281 [72], protocol is used to carry user traffic/data between their respective network elements.

      To achieve successful interoperability between two network elements from two different vendors, it is important to understand the concerned protocol specification, encode/decode, and implement correctly each message/PDUs defined in a particular logical interface and its protocol specification. Different logical interfaces may use different physical transmission media to transport the messages of a logical interface. All the IP transport‐based mobile communications protocols will use theStream Control Transmission Protocol (SCTP) RFC 4960 [17] or UDP protocol suite and UTP CAT cable as physical media.

      

      3.1.3 Logical Interfaces’ Names and Their Protocol Stack

      In a computer network, the TCP/IP protocol suite has a collection of protocols that are classified and grouped into four layers, namely the application layer, transport layer, network layer, and the physical layer. By referring to the TCP/IP protocol suite, one can easily recall the various layers and protocols available under it.

Similarly, a logical interface in mobile communications networks also consists of a protocol stack and its layers. To identify and refer such protocol stack and its layers, each logical interface is given an identification name that begins with a capitalized English alphabet letter. For example, in the previous section, the LTE/EPS logical interface S1 or 5G NG logical interface was discussed. Similar examples of other logical interfaces are A‐interface between the BSC and MSC, Gs interface between the MSC and SGSN, A‐bis interface between the BTS and BSC, Iu interface between RNC and MSC, and so on.

There are large numbers of logical interfaces connecting different network elements of GSM, GPRS, UMTS, LTE/EPS, and 5G networks. To provide a brief overview to the reader, the following Tables 3.1 to 3.4 lists the names of some of the logical interfaces, second row, along with their network elements, first row, used in the GSM, GPRS, UMTS, and LTE/EPS networks.

      The reader may refer to the corresponding technical specification(s) which are available on the 3GPP site [1] for further information on the logical interfaces mentioned in Tables 3.1 to 3.4.

Table 3.5 summarizes the various logical as well as the physical interfaces being used between the respective RAN and the CN in the GSM, GPRS, UMTS, LTE, and 5G systems.

Table 3.1 GSM and GPRS system network elements and logical interfaces.

Air Interface (MS‐BSC)	BTS‐BSC	BSC‐MSC	BSC‐SGSN	SGSN‐GGSN
Um	A‐bis	A	Gb	Gn

Table 3.2 UMTS system network elements and logical interfaces.

Air Interface (UE‐NodeB)	Node‐RNC	RNC‐MSC	RNC‐SGSN	SGSN‐GGSN	GGSN‐WWW
Uu	Iub	Iu‐CS	Iu‐PS	Gn	Gi

Table 3.3 LTE/EPS network elements and logical interfaces.

Air Interface (UE‐eNodeB)	eNodeB‐MME	eNodeB‐S‐GW	S‐GW ‐ P‐GW	P‐GW ‐ WWW
Uu	S1‐AP	S1‐U	S5	SGi

Table 3.4 Interworking features and their logical interfaces.

3GPP Feature: CSFB (MME‐MSC)	Feature: SRVCC (MME‐MSC, MSC‐SGSN)
SGs	Sv

Table 3.5 Logical and physical interface between RAN and CN elements.

System	Interface Between Network Element	Logical Interface Name	Physical Interface Name
GSM	BSC	MSC	A‐interface	E1