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Figure 4.8 shows the definition of the downlink NAS transport message from the LTE/EPC MME to the eNodeB.

Figure 4.8 LTE/EPS MME‐ENodeB: S1‐AP: downlink NAS transport.

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.

 Between GSM BSC and SGSN: Gb‐Interface

The Gb‐interface is used to exchange signaling messages between the GSM BSC and the SGSN. Over the Gb‐interface, the following types of messages are exchanged between the BSC and SGSN of a GPRS network.

 Network Service protocol layer; refer to 3GPP TS 48.016 [135].

 BSS GPRS Protocol (BSSGP) protocol layer; refer to 3GPP TS 48.018 [136].

NS and BSSGP layer messages are also encoded/decoded and are described in a tabular format similar to the air interface Layer 3 messages. However, unlike the Layer 3 messages, NS and BSSGP layer messages do not contain a message header. Each NS and BSSGP layer message starts with their message type, followed by the associated IEs.

 Between UMTS RNC – CN; LTE E‐UTRAN and CN; 5G NG‐RAN and CN

The UMTS RANAP, over the Iu interface between RNC and CN, uses the ASN.1 notation for describing the signaling message between RNC and the CN. The LTE/EPS S1‐AP, between eNodeB and MME, and X2‐AP, between two eNodeBs, also uses the ASN.1 notation for encoding and decoding of signaling message over the S1 and X2 interface. Similarly, the 5G system NG‐AP, between NG‐RAN/gNB and AMF, and XN‐AP, between two gNB, also use the ASN.1 notation for encoding and decoding of signaling message over the NG and Xn interfaces. Protocol messages specifications using ASN.1 notation was described earlier in Section 4.1.5. Note that the messages exchanged between UMTS RNC – CN; LTE E‐UTRAN and CN; 5G NG‐RAN and 5G core are also described using tabular notation. Example 4.8 illustrates the tabular definitions of a message described in a tabular format.

The NAS‐PDU IE, which is a mandatory one, in the downlink NAS transport message contains the message to be communicated from the MME to the UE.