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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
GPRS	BSC	SGSN	Gb‐interface	E1, IP Layer 1
UMTS	RNC	MSC	Iu‐CS	PDH, SDH, IP Layer 1
SGSN	Iu‐PS	PDH, SDH, IP Layer 1
LTE	E‐UTRAN	MME	S1(S1‐AP)	IP Layer 1
S‐GW	S1(S1‐U)	IP Layer 1
5G	NG‐RAN	AMF	NG (NG‐AP)	IP Layer 1

For more information on all the 3GPP defined logical interfaces, their protocol stack/layers, and network elements of mobile communications networks, refer to the 3GPP TS 23.003 [30]. The next section illustrates the logical interfaces and their physical transmission network used in a typical GSM and GPRS network.