in Figure 4.4. It may be noted that though the message description in tabular format is the same, the protocol headers used across the different air interfaces are different.
Example 4.1 Illustration of LTE/EPS NAS MM Layer Message
Consider the LTE/EPS Attach Complete NAS layer EMM message (see Figure 4.4) reproduced from TS 24.301 [46], which is sent from the UE to the Mobility Management Entity (MME) through the eNodeB. They are described in the tabular format and also their encoding method, i.e. IE format‐TLV, is the same. But there is a difference between the GSM, UMTS, GPRS and LTE/EPS, and 5G NAS layer messages. The GSM, GPRS, and UMTS air interface Mobility Management (GMM) messages are not security protected, but every LTE/EPS, 5GS NAS, MM layer message is security protected. For this purpose, the header of every LTE/EPS EMM or 5G MM message contains the Security Header Type IE.
Figure 4.4 LTE/EPS NAS layer 3 attach complete message.
Source: © 2014. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2014, 3GPP.
Example 4.2 Illustration of LTE/EPS NAS SM Layer Message
Consider the LTE/EPS ESM Information Request NAS message (see Figure 4.5) which is sent from the MME to the UE through the eNodeB. It is described in a tabular format and their encoding method, i.e. IE format, is the same. Unlike the GSM/GPRS Layer 3 SM messages, the protocol header of every LTE/EPS NAS layer message contains an EPS Bearer Identity and a Procedure Transaction Identity; refer to TS 24.301 [46]. In the case of the 5G system NAS layer, a 5GSM message contains a PDU Session ID and a Procedure Transaction Identity.
Figure 4.5 LTE/EPS NAS layer 3 ESM information request message.
Source: © 2014. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2014, 3GPP.
Example 4.3 Illustration of Encoding and Transmissions of Layer 3/NAS Layer Message
LTE/EPS air interface NAS layer messages and their tabular definitions/descriptions have been presented in the previous examples. It has been observed that the types, in TLV format, of the different IEs of a particular message are not the same, but they are mixed in nature, i.e. Type 1, Type 2, and so on. Figure 4.6 illustrates one such typical encoding of IEs of a Layer 3 messages header (1 octet, value only); message type (1 octet, value only); and followed by a typical IE in TLV format for transferring between an MS and the RAN over the respective air interface. The IEs of a message are encoded at the sender and decoded at the receiver in the same order as it is defined in its concerned technical specification.
Figure 4.6 Illustration: encoding and transmission of air interface message.
4.1.2 Encoding/Decoding: LTE and 5G NR Layer 2: RLC Protocol
The LTE and 5G NR air interface RLC layers provide the capability to exchange information between a UE and the LTE E‐UTRAN or between a UE and the 5G NG‐RAN in terms of the PDU. An RLC layer PDU facilitates transfer of higher‐layer data in Transparent (TM), Unacknowledged (UNACK) or Acknowledged (ACK) mode. A PDU consists of a header part that is further followed by the data part of the PDU.
PDU Description
An RLC PDU, ACK, and UNACK mode header consist of several fields with different lengths in bits. Thus, the encoding and decoding of each field are different. Nevertheless, the protocol header and the data part of RLC PDU are octet aligned and is described in a tabular format. The TM PDU of the RLC layer does not contain the header part and is used to transfer messages such as paging and system information messages. Neither the sending RLC nor the receiving RLC layer performs any operations on a TM PDU. There is another PDU called Control PDU, which is used by the receiving RLC layer to inform the sending RLC layer on the status, i.e. lost or successfully decoded, of a PDU being received.
Encoding of RLC PDU
Though the RLC PDU is described in a tabular format, the header and data part is encoded as bit strings where the leftmost bit of the first line of the table is considered as the most significant bit and the rightmost bit of the last line of the table is considered as the least significant bit. Depending on the length of Sequence Number (SN) used in an RLC header, the length of the RLC header may take 1 or 2 octets at the beginning of the table and is different for the ACK mode and UNACK mode of data transfers. The 5G NR air interface RLC layer and its PDU formats are described later in Chapter 19. For more information on the RLC layer protocol header, its different parameters, and their encoding requirements, refer to TS 38.322 [114] for 5G NR.
4.1.3 Encoding/Decoding: LTE and 5G NR Layer 2: MAC Protocol
The LTE and 5G NR MAC layer facilitates the exchange of air interface Layer 2 information in terms of a PDU between a UE and LTE E‐UTRAN or between UE and 5G NG‐RAN. A MAC PDU consists of a MAC header and MAC SDU, which is received from the RLC layer. The method of description and encoding of the MAC layer PDU is similar to the method used by the LTE and NR RLC layers as described above. Similar to the RLC layer, the encoded bit strings of LTE or 5G NR MAC layer is an octet (8 bits) aligned.
Also, note that unlike the air interface Layer 3 message header, the MAC header and the MAC SDU are variable in size. For more information on the MAC layer protocol header, its different parameters, and their encoding requirements, refer to TS 36.321 [93] for LTE and TS 38.321 [113] for 5G NR. NR air interface MAC layer and its PDU formats are described later in Chapter 19.
4.1.4 CSN.1 Encoding/Decoding: GPRS Layer 2 Protocol (RLC/MAC)
The CSN.1 [10] notation method for describing, encoding, and decoding of air interface Layer 2 signaling messages are used by the GPRS RLC/MAC layer. It was described in Section 4.1.1 that the air interface Layer 3 messages are encoded and decoded on the octet (8 bits) level in TLV format. However, using the CSN.1 notation, only the value, and not type or length, of the IEs of a signaling message are encoded and decoded on a bit level. This results in a compact bits stream for transmission over the GPRS air interface. A CSN.1 encoded signaling message contains a bits string having an ordered sequence of symbols, i.e. 0 and 1. The resulting bits string may not be multiple of 8, i.e. an octet. An example CSN.1