9.1 Overview

24.3013GPPNon-Access-Stratum (NAS) protocol for Evolved Packet System (EPS)Release 18Stage 3TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

Within the protocols defined in the present document, every message, except the SERVICE REQUEST message, is a standard L3 message as defined in 3GPP TS 24.007 [12]. This means that the message consists of the following parts:

1) if the message is a plain NAS message:

a) protocol discriminator;

b) EPS bearer identity or security header type;

c) procedure transaction identity;

d) message type;

e) other information elements, as required.

2) if the message is a security protected NAS message:

a) protocol discriminator;

b) security header type;

c) message authentication code;

d) sequence number;

e) plain NAS message, as defined in item 1.

The organization of a plain NAS message is illustrated in the example shown in figure 9.1.1.

8	7	6	5	4	3	2	1
EPS bearer identity or Security header type				Protocol discriminator				octet 1
Procedure transaction identity								octet 1a*
Message type								octet 2
								octet 3
Other information elements as required
								octet n

Figure 9.1.1: General message organization example for a plain NAS message

The organization of a security protected NAS message is illustrated in the example shown in figure 9.1.2.

8	7	6	5	4	3	2	1
Security header type				Protocol discriminator				octet 1
								octet 2
Message authentication code

								octet 5
Sequence number								octet 6
								octet 7
NAS message
								octet n

Figure 9.1.2: General message organization example for a security protected NAS message

The EPS bearer identity and the procedure transaction identity are only used in messages with protocol discriminator EPS session management. Octet 1a with the procedure transaction identity shall only be included in these messages.

Unless specified otherwise in the message descriptions of clause 8, a particular information element shall not be present more than once in a given message.

When a field extends over more than one octet, the order of bit values progressively decreases as the octet number increases. The most significant bit of the field is represented by the highest numbered bit of the lowest numbered octet of the field. The least significant bit of the field is represented by the lowest numbered bit of the highest numbered octet of the field.