rel="nofollow" href="#ulink_9a9404ce-0d51-5c85-9e8f-cb233c1e8776">Section 4.1.4 that the GPRS RLC/MAC layer messages are encoded/decoded at bit level using CSN.1 [10] method. Even the presence or absence of a single bit of information in a particular communications signaling message matters a lot for the receiver of the message because it may not be able to decode the rest of the received message successfully. Sometimes, if the complete information to be sent does not fit in a single signaling message, the same may be transferred in a segmented way, and this is indicated by a presence of a bit in the first message segment already sent to the receiver.
Similarly, the segmentation of the New Radio (NR) RRC layer messages i.e. RRCReconfiguration or RRCResume, in DL direction, and UECapabilityInformation, in UL direction, has been introduced as part of the 5G system Release 16. This is due to the limitation of the maximum PDU size of 1900 bytes as supported by the NR PDCP layer. The 5G NR air interface control plane protocol layer shall be described later in Chapter 18
4.1.8 Piggybacking a Signaling Message
To reduce the exchange of signaling messages overhead between mobile devices and the network over the air interface or other interface, sometimes a signaling message is attached to another signaling message that is being scheduled and already transmitted. Such a mechanism of attaching a signaling message as part of another signaling message is known as the piggybacking of a message. The piggybacked message may be used to request additional RRs from the network to start data transfer in the opposite direction or to trigger the start of a new signaling procedure between the mobile device and the network. The following Examples 4.5 to 4.7 shall clarify the concept and purpose of piggybacking a signaling message into another message.
Air Interface Signaling Messages
Example 4.5 Piggybacking of GSM Air Interface Complete Layer 3 Information Using SCCP
In the case of the GSM system, the SS7 protocol stack is used to exchange messages between the GSM and the MSC over the A‐interface. The Signaling Connection Control Part (SCCP) layer is a part of the SS7 protocol stack. GSM Complete Layer 3 Information is used to transport all the initial signaling connection messages, between the MS and MSC, piggyback with the SCCP protocol message. Examples of GSM system initial messages are – CM SERVICE REQUEST, which is used for a mobile originated call, LOCATION UPDATE REQUEST, which is used for location area update request from MS to the CN, etc.
Example 4.6 Piggybacking Using LTE or 5G NAS Layer Signaling Message
In the LTE or 5G system also, piggybacking of a signaling message, e.g. piggybacking an SM layer message to an MM layer message, is used. In the LTE/EPS, piggybacking of NAS messages is used for the bearer management procedure in the downlink direction. In the uplink direction, piggybacking 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