Another protocol layer of the same network element may send a signaling message as a series of octets with octet alignments. To sum up, as far as the development of a protocol layer is concerned, two aspects of signaling messages defined in its technical specifications are required to be considered:
The method used to describe a signaling message
The method used for encoding and decoding of a signaling message to transfer/receive among network elements.
It may be noted that the method of description, i.e. tabular format, and encoding/decoding of signaling messages differs from layer to layer. In the subsequent sections that follow, the following methods of descriptions, encoding, and decoding of mobile communications networks signaling messages over their respective air interfaces are discussed.
1 Encoding and Decoding of Air Interface Layer 3 Messages
This method is used by the GSM air interface Radio Resource (RR) sublayer of Layer 3 protocol between an MS and the base station controller (BSC). This method is also used by the Call Control (CC) and Mobility Management (MM) sublayers of GSM; GPRS Mobility Management (GMM), Session Management (SM) layers of Universal Mobile Telecommunication System (UMTS); Evolved Packet System Session Management (ESM) and Evolved Packet System Mobility Management (EMM) layers of Long‐Term Evolution (LTE)/Evolved Packet System (EPS) system; and 5GMM and 5GSM layers of 5G system. These protocol layers work between an MS/UE and the CN.
1 Concrete Syntax Notation.1 (CSN.1) Encoding/Decoding
This method is used by the General Packet Radio Service (GPRS) air interface Layer 2 radio link control (RLC)/Medium Access Control (MAC) protocol between the MS and BSC.
1 Abstract Syntax Notation.1 (ASN.1) Encoding/Decoding Using Packed Encoding Rule (PER)
This method is used by the following protocol layers over their respective logical interfaces:
UMTS Radio Resource Control (RRC) air interface Layer 3 between the UE and UMTS Terrestrial Radio Access Network (UTRAN)/Radio Network Controller (RNC),
LTE RRC air interface Layer 3 between the UE and Evolved‐UMTS Terrestrial Radio Access Network (E‐UTRAN)/eNodeB, and
5G New Radio (NR) RRC air interface Layer 3 between the UE and Next Generation Radio Access Network (NG‐RAN)/5G Base Station (gNB).
4.1.1 Encoding/Decoding: Air Interface Layer 3 Messages
To describe and encode/decode the air interface Layer 3 and its sublayers signaling messages, the basic tabular form of definition is used. Except for the UMTS, LTE, and 5G RRC layers, the following layers share the same tabular format to describe their air interface Layer 3 and Non‐access Stratum (NAS) layers messages.
GSM RR, MM, and Connection Management (CM) layers,
GPRS/UMTS GMM and SM layers,
LTE/EPS NAS layers – EMM and ESM NAS, and
5G System NAS layers – 5GMM and 5GSM.
An air interface Layer 3/NAS layers signaling message consists of an ordered series of octets (1 octet: 8 bits) and each message being with a protocol header. The protocol header is followed by protocol information fields. Each field is known as the information element (IE). An IE has certain attributes such as its unique identifier and presence requirements in the message, length, and value as described below. IEs and their attributes are described in a tabular format.
IE and its Identifier
A signaling message carries various information from a sender to a receiver through a collection of IEs. Each IE indicates particular information of a protocol layer to a receiver and is uniquely identified by a so‐called Information Element Identifier (IEI). An IE has a name and is represented by assigning a hexadecimal value through the IEI. IEs of a signaling message may have different lengths such as 1 octet, 2 octets, and so on. An IE of a message has the following components, also shown graphically in Figure 4.1.
Figure 4.1 Components of an IE of a protocol message.
Type (T), represented by the IEI,
Length (L), in octets, and
Value (V), i.e. an actual value of an IE.
Presence Requirements of IE
The presence of an IE in a signaling message may not be required always. Based on this, the presence of an IE is classified as shown in Figure 4.2:
Mandatory (M) – An IE must be present always; if it is not, the receiver will consider the message as an erroneous one and reports a protocol error.
Conditional (C) – Presence depends on the value of another IE. If a condition is met and the IE is not present, the receiver will consider the message as an erroneous one; else, it will accept the message.
Optional (O) – The receiver will accept the received message irrespective of the presence of the IE.
IE Formats
As shown in Figure 4.1, each IE of a signaling message has the type (T), represented by the IEI, along with a defined range of values (V), including reserved value, and its length (L). The type (T), length (L), and allowed value (V) of IEs of signaling messages of a particular protocol layer are defined by its corresponding 3rd Generation Partnership Project (3GPP) technical specification. Using the Type, Length and Value (TLV), several formats of an IE can be defined as shown in Figure 4.3; refer to TS 24.007 [44]. The formats “LV‐E” and “TLV‐E” indicate that these IE formats are used in the case of the LTE/EPS system only for its air interface Layer 3 MM and SM messages. GSM, GPRS, UMTS, LTE, and 5G NR air interface messages defined and encoded in TLV formats are called Standard Messages.
IE Types
Figure 4.2 Presence requirements of an IE of a protocol message.
Figure 4.3 Standard formats of air interface layer
