and the core network element transport network may provide provisions for configuring multiple physical layer interfaces. This ensures a backup, in case one physical interface fails the other physical interface can be brought into service to continue communications services. The upper layers of a protocol stack are independent of the type of underlying data link layer and its physical interface being used to transport higher‐layer data.
Example 3.13 GPRS Gb‐Interface Multiple Physical Layer Interfaces
Consider Figure 3.5 illustrated earlier. As shown in that figure, the GSM BSC and the SGSN can exchange the Gb‐interface protocol stack messages using either the Frame Relay network protocol or IP network. The Sub‐Network control protocol takes care of the data link layer protocol (Frame Relay or IP) to be used by the Gb‐interface. The higher‐layer applications are independent of the sub‐Network control protocol. In case the Frame Relay protocol and its physical E1 interface go down, the IP Transport/Ethernet interface can be brought into services immediately so that the Gb‐interface is not affected and GPRS services are not down permanently.
Similarly, between the UMTS UTRAN and its CN, the Iu‐interface can be configured to use either the ATM over STM or IP transport network physical layer; refer to TS 25.412 [53].
Reason for Multiple Data Link Layer Provisions
Typical reasons to support multiple data links by a network element are as follows:
Make the higher layer(s), e.g. RNL, independent of the TNL being used for the introduction of a new transport network without affecting the application or RNL;
Create a backup transport network; and
System and application requirements, e.g. use ATM to transport multimedia contents, whereas frame relay can transport data only.
3.11 How to Identify and Understand Protocol Architectures
3GPP defines a larger number of protocol logical interfaces, starting with the alphabet “A”, stacks, and layers which are central to the interworking of a network element with another network element of mobile communications systems and networks. In this chapter, we have covered only the following logical interfaces of mobile communications networks:
Air interfaces, i.e. Uu, Um, between UE/MS and RAN of GSM, UMTS, LTE, and 5G systems.
Network interfaces (A, Gb, S1, Iu, X2, Gn, NG, and so on) between the GSM, UMTS, LTE, and 5G RANs and their respective CNs.
In fact, the majority of the logical interfaces are found only in the CNs domain along with the other CN elements such as the Home Location Register/Home Subscriber Server (HLR/HSS), Visitor Location Register (VLR), and Policy Charging and Restriction Function (PCRF). Other logical interfaces are also available that can be configured to support interworking and interoperations, e.g. CS fallback, Single Radio Voice Call Continuity, and so on, between two mobile communications networks. Some of these interoperation facilities may be configured as an optional and separately licensed feature.
A developer must put the focus on a particular network element and its logical interfaces at a time. The air interface and its protocol stack is the most interesting one that consists of advanced wireless communications theories. The air interface differentiates one system from its predecessor. As a starting point, the reader is advised to go through and familiarize themselves with the list of 3GPP TSs mentioned in the Reference section of this book. There are 3GPP TSs describing the protocol layers of the respective air interface of the GSM, GPRS, UMTS, LTE, and 5G systems. The reader may, then, proceed gradually toward the other logical interfaces and their protocol stack.
3.11.1 Identifying a Logical Interface, Protocol Stack, and Its Layers
To identify and understand a particular protocol stack architecture, its layers, and their corresponding 3GPP technical specifications, the following steps may be taken:
Choose a particular mobile communications system such as the GSM, GPRS, and UMTS, LTE, or 5G as your area of interest. Use the information available in the 3GPP site [2] (second, third, fourth column) as mentioned in Section 2.5.6.
Next, decide the particular network element of interest such as GSM MS, BTS, BSC, and MSC; UMTS NodeB, and RNC; or LTE eNodeB, MME, and S‐GW or 5G gNB and 5G core.
Now, look at the logical interfaces supported by the chosen network element. Pick a particular logical interface and look at its protocol stack and layers. A logical interface and its protocol stack cover different subjects and specifications area. Look at the subject and specifications areas mentioned in the 3GPP site [2], first column, and pick a particular subject area. Against this chosen subject area, e.g. signaling, requirements, and so on, or a particular protocol layer, attempt to identify the corresponding technical specifications series and its specifications from the column 2, 3, or 4, 3GPP site [2].
Study the protocol layer architecture, its functions and procedures, and other details from the identified technical specification.
Example 3.14 describes the typical steps to be used to derive the 3GPP technical specification of a protocol layer and its sub‐layer.
Figure 3.21 shows the protocol stack of the A‐interface on the CN side.
Example 3.14 GSM Circuit‐Switched BSS and Air Interface Layer 3 Technical Specifications
Suppose a developer is interested to study the GSM BSS that consists of BTSs and BSC network elements. Further, suppose that developer is interested in the air interface Layer 3 protocol, between an MS and the BSC, of a GSM system. Figure 3.21 below shows the GSM air interface Layer 3 protocols along with its sublayer protocol, as follows:
Call Control Management (CM),
MM, and
Radio Resource Management (RR).
Figure 3.21 Illustration: GSM air interface Layer 3 protocol stack.
1 As shown in Figure 3.21, the GSM air interface Layer 3 consists of the CM, MM, and RR layers. The developer may be further interested in the Radio Resource Management, (RR) sublayer of the GSM Layer 3 protocol stack. The RR layer of a BSC deals with the signaling functions/messages of the GSM Layer 3 protocol stack. Now refer to the 3GPP site [2].
2 For
