href="#ulink_b13dd309-38ff-53b6-ad30-20fcdb987729">Figure 3.22, the LTE air interface initial NAS layer messages, e.g. ATTACH REQUEST, TAU, received from a UE is sent through the InitiaUEMessage from the eNodeB to the MME. The subsequent NAS messages between eNodeB and the MME are exchanged using the DownlinkNASTransport and UplinkNASTransport messages. The MME sends the ATTACH ACCEPT message to the eNodeB through the InitialContextSetup message. For more information on the NAS TRANSPORT messages and their contents, refer to TS 36.413 [97].
Between the RAN and the CN
Several similar procedures are performed between the RAN and its CN only over their respective logical interfaces, e.g. LTE/EPS S1 and X2; 5G NG; UMTS‐IuPS; and IuCS. Typical similar procedures are mentioned below:
Handover procedure performed in case of the GSM and LTE system; relocation procedure performed in case of UMTS system. A handover or relocation procedure is executed to transfer an ongoing call from one cell to another suitable cell.
Interface management – to setup, initialize, and release the respective interfaces, i.e. A, Gb, Iu, S1, and 5G system NG interface.
Security and encryption.
Paging – to notify an incoming call for an MS/UE.
UE tracking – to track a particular UE/MS.
Overload control from CN – which indicates the RAN to reduce the signaling load toward the CN.
Several functions are performed by a network element to complete a particular protocol layer procedure as highlighted above, which are described later in different chapters. For more information on the above functions and procedures, refer to TS 25.413 [54], TS 36.413 [97], TS 38.413 [119], and TS 48.008 [134].
3.12.2 Specific Functions and Procedures over the CN Interfaces
Some functions and procedures performed over a CN logical interface are specific to a GSM, UMTS, LTE, or 5G system. Typical examples of such GSM, UMTS, and LTE system‐specific functions over their specific logical interface are mentioned below:
BSSMAP [between GSM BSC – MSC]Speech transcoding from 64 kbps, at MSC end, to 16 kbps, at BSC end
RANAP [between UMTS/UTRAN RNC – MSC]UMTS Radio Access Bearer management, i.e. establish, modify, release, create, and allocate radio access bearer to the UETransport and RNL link management functions
S1‐AP [between LTE/E‐UTRAN eNodeB – MME]LTE/EPS Radio Access Bearer management, i.e. setup, modify, release, to create and allocate radio access bearer to the UETransport and RNL link management functions
NG‐AP [between 5G NG‐RAN – AMF]5G PDU Session Management, i.e. setup, modify, and release, a PDU session to a UE, MM, UE context management, and so onTransport and RNL link management functions.
For more information on the specific functions and procedures, refer to TS 25.413 [54], TS 36.413 [97], TS 48.008 [134], and TS 38.413 [119].
Chapter Summary
This chapter has introduced the core aspects of the understanding, design, and development of logical interfaces, their protocol stack, and its layers of mobile communications systems and networks. Logical interfaces are used to communicate among network elements of a mobile communications network. Logical interfaces are also used for the interworking and interoperations of mobile communications systems and networks, which shall be described later in Chapter 6.
Different terminologies are used to describe a logical interface, its protocol stack, and layers. Protocol layers are classified into user plane and control or signaling plane distinctly based on the nature of the information that is transmitted over a particular logical interface. Within the control plane only, protocol layers are also grouped into the AS and NAS categories, especially in the UMTS, LTE, and 5G networks and systems.
We presented the protocol layer termination and sublayering of a particular protocol layer found in mobile communications systems and networks. We also presented the general working model of a protocol layer that is used to communicate with its peer layer and provide services to an upper layer. Apart from this, we presented the general protocol model and layers of the UMTS UTRAN, LTE E‐UTRAN, and 5G NG‐RAN and their respective CNs.
A 3GPP technical specification may cover the description of GSM, GPRS, UMTS, LTE, and 5G system protocol functions and procedures. A method to identify the functions and procedures description that applies to a particular communications system was presented. Finally, the reader is recommended to focus on a particular logical interface, protocol stack at a time, and its specifications as mentioned in the references section and then, proceed gradually toward other logical interfaces and their protocol stack.
4 Encoding and Decoding of Messages
Introduction
This chapter covers the methods for encoding and decoding of control plane or signaling messages and protocol data units (PDUs) that are exchanged between the peer protocols layers of network elements of mobile communications networks, from the Global System for Mobile Communication (GSM) to the 5G system. The method of a description of signaling messages and their encoding and decoding is part of a protocol layer specification, which differs from one logical interface to another one. Messages are exchanged between the network elements of a mobile communications network to facilitate various communication services to users. A source network element creates a protocol layer message in a predefined format using a particular encoding/packing method and sends it to the peer network element over the concerned logical interface. The peer network element decodes or unpacks a message using the same method that was used to encode it.
We begin with the description of encoding and decoding methods of air interface Layer 3 signaling messages, followed by the Layer 2 signaling messages exchanged between a Mobile station (MS)/User Equipment (UE) and the network. We also cover the encoding method used by the Radio Access Network (RAN) and core network (CN) elements. We close this chapter with the method of embedding a control plane message within another control plane message to reduce signaling overhead between two network elements, especially over the air interface.
4.1 Description and Encoding/Decoding of Air Interface Messages
One important aspect of a mobile communications network is the “signaling message” that is exchanged between its network elements. A signaling message is nothing but an exchange of a series of information between the network elements to establish, maintain, and release of resources allocated for communication services being provided to the service users. Information in a particular signaling message being transmitted is encoded (packed) and decoded (unpacked) differently across the mobile communications systems, i.e. from the GSM to 5G. Depending on the protocol stack and its layers supported by a network element, it may use different methods of encoding and decoding of signaling messages at each protocol layer. Also, a network element may decode the contents of a message that it receives or may not decode but transparently forward to the destination network element using another encoding method.
A protocol layer of a network element may send a signaling message to its peer layer like a long series of ordered
