of the NAS message is used only for transferring the initial NAS message during connection setup. For example, the LTE/EPS ATTACH Request message is send piggybacked with the RRCConnectionSetupComplete message over the LTE air interface between UE and E‐UTRAN.
Similarly, in the 5G NR system, the NAS layer RegistrationRequest (toward the 5G Core Access and Mobility Management Function (AMF)) message is piggybacked to the RRCSetupComplete message from a UE to the NG‐RAN.
CN Signaling Messages
Example 4.7 Piggybacking GTPv2 Control Plane Messages
In the LTE/EPS and 5G systems, the GTPv2 control plane signaling messages are used in a couple of logical interfaces, for example, between the LTE/EPS MME and the S‐GW and S‐GW and P‐GW. The fifth bit of a GTPv2 control plane header, 3GPP TS 29.274 [70], contains a field called “P” which indicates the presence of a further piggybacked GTPv2C message along with the current GTP GTPv2C messages. For example, a Create Session Response message from the S‐GW to the MME may contain the Create Bearer Request message as well as the piggybacked message for the MME. In this case, the “P” flag in the header of the Create Session Response message shall be set to 1.
4.2 Encoding/Decoding of Signaling Messages: RAN and CN
In the previous sections, we have discussed the encoding and decoding of signaling messages between a UE/MS and their respective RAN of the GSM, UMTS, LTE, and 5G NR systems over their respective air interface. To perform certain functions and procedures between the RAN and the CN, they also exchange signaling or control plane messages over the respective logical interface. The IEs of a signaling or control plane message exchanged over the concerned logical interface between the RAN and CN are packed and unpacked using a particular encoding and decoding method, which are described below:
Between GSM BSC and MSC: A‐Interface
The logical A‐interface is used to exchange signaling messages between the GSM BSC and the MSC. Over the A‐interface, the following types of messages are exchanged between the BSC and MSC.
Base Station Management Application Part (BSSMAP)
Signaling messages for various functions and procedures performed between the BSC and MSC are classified into BSSMAP types. BSSMAP messages, between BSC and MSC, 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, BSSMAP messages do not contain a message header. Each BSSAMP message starts with its message type, followed by the associated IEs.
Direct Transfer Application Part (DTAP)
DTAP messages are exchanged between the UE and MSC only. All the air interface Layer 3 CC and MM signaling messages that are transparently forwarded by the BSC, received from the MS to the MSC without processing by BSC, are classified into DTAP types. DTAP messages are the air interface Layer 3 messages with a header containing protocol discriminator information in the header of every message that is exchanged between the MS and MSC. Because of this, the DTAP message is encoded and decoded as described in Section 4.1.1.
Example 4.8 LTE NAS Layer: Downlink NAS Transport: MME to eNodeB
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.
Chapter Summary
This chapter presented the CSN.1, ASN.1, direct, indirect, tabular format, and so on and methods of describing and encoding/decoding protocol layer messages across the different mobile communications systems from the GSM to the 5G system. The peer protocol layers of network elements of a mobile communications network exchange signaling messages using a particular encoding/decoding method as described in this chapter. These encoding and decoding methods are used over their respective logical interfaces, e.g. air interface Layer 2, Layer 3, NAS layer, between RAN and CN, and so on, to exchange control plane information. In comparison to the other encoding methods described in this chapter, the CSN.1 encoding method produces more compact protocol layer messages that are to be transmitted over the GPRS air interface.
An encoding and decoding method represents the data structures of IEs of messages that are used during the software design and development of protocol layers supported by the network elements of a mobile communications network. An IE of a message may contain and may be encoded with several information. Unlike the traditional IP layer header and packet, information may be encoded/decoded even at a single bit level in a mobile communications message. Correctly encoding and decoding of an IE, its components,
