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 3 messages IEs.
Source: © 2011. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2011, 3GPP.
The IEs of the GSM, GPRS Layer 3 and UMTS, LTE, and 5G NR NAS layer signaling messages are octet aligned. Depending on the usage of the IEI, length (L) of an IEI, and its value (V), the IE can be represented in one of the formats as shown in Figure 4.3. The particular format used to encode an IE represents its corresponding type. The different types of IE are mentioned below:
Type 1: IE format – V (with a half octet in length) and TV (each is half octet in length),
Type 2: IE format – T, i.e. IEI only (1 octet in length),
Type 3: IE format – TV (length of IEI is 1 octet; length of value: 1..n octet),
Type 4: IE format – LV, TLV (length of IEI is 1 octet, length of value: 1..n octet, and length of length indicator: 1 octet), and
Type 6: IE format – LV‐E, TLV‐E (length of IEI is one octet, length of value: 1..n octet, and length of length indicator: 2 octets).
For more information on the above IE, its format, and types, refer to the 3GPP TS 24.007 [44].
Encoding/Decoding of IEsThe IEs of a signaling message is encoded by the sender and decoded by the receiver in order of their appearances in the tabular description. IEs are encoded as octet aligned.Only the IEI/Type (T), Length (L), and Value (V) of an IE are encoded/decoded as per their tabular descriptions of the concerned message. Because of the TLV encoding, the overall message size becomes larger for transmission over the air interface.
In case an IE in a message is encoded incorrectly, a protocol error is detected and flagged by the receiver of the message which is notified to the sending network element about the detected error so that the predefined actions may be taken. For example, if an MS/UE sent an IE to the CN that is not encoded correctly, the CN element will report an error to the RAN. Further, the RAN may report about the error to the operation and maintenance personal through a predefined alarm.
Usages of IEs, their formats/lengths, i.e. TLV, and presence requirements in a NAS signaling message are illustrated through Example s 4.1 to 4.3.
From the LTE/EPS Attach Complete message definition which is shown in Figure 4.4, the following observations may be made:
Tabular Description of Air Interface NAS Layer Signaling Message
All the air interface Layer 3 and NAS layer messages and their IEs are described in a tabular format with six columns. IEIs are arranged in the order they are transmitted.
For an IE having its format type (T), the corresponding IEI is mentioned in the first column, IEI, of the table.
Figure 4.4 contains IEs with format types: V and LV.
Note that the GSM, GPRS, UMTS, air interface Layer 3, and LTE and 5G NAS layer signaling messages have protocol header part followed by IEs for user information/data part that is defined in