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 What is a Signaling Message?

One important aspect of a mobile communications network is the “signal” that is exchanged between the network elements. Mobile communications signaling is nothing but an exchange of information between the concerned network elements prior to providing communications services to a subscriber or another network element. Signaling messages among the network elements are exchanged to reserve resources as part of the call setup and release the resources as part of the call release procedure. In addition to this, signaling messages are exchanged to carry out various functions and procedures among the network elements of GSM/GPRS, UMTS, LTE/EPS, and 5G networks. By exchange of signaling information among the network elements, necessary logical objects, or contexts, a database of various information is created at the UE/MS, RAN, and CN end. Information such as the capabilities of a particular MS/UE, radio access, and CN capabilities and their features, information about a subscriber, and so on are also exchanged only through signaling, which is updated at the UE/MS, RAN, and CN end.

In a traditional Public Switched Telephone Network (PSTN) fixed network also, signaling functions are performed for call establishing, maintaining, and supervision and call release. Note that control/signaling plane messages last for a fraction of a second only to setup and release a call or to perform other procedures among the network elements. Before a user can start communications services, a signaling procedure or phase must be completed successfully between a UE/MS and the RAN, between the RAN and CN, and between the UE and CN. For example, an MS/UE must be registered with the CN through a successful GSM location area or GPRS routing area or LTE/EPS tracking area update or 5G Registration Request procedure. Prior to performing such signaling procedures, a UE/MS must request and be allocated the necessary radio resources by the GSM BSC, UMTS RNC, or LTE eNodeB or 5G NG‐RAN through signaling only. All such signaling procedures in a particular mobile communications network are performed by its control plane protocol stack and its layers only.

If there is a signaling congestion situation in a network, an MS/UE would not get the requested resources from the network. Thus, a mobile user would not be able to make a circuit‐switched (CS) or a packet‐switched (PS) call. This would further require the troubleshooting of the issue or a proper radio network planning by the operator.

 LTE/EPS Network Control Plane Protocols: UE – eNodeB – MME

Figure 3.7 shows the end‐to‐end control plane protocol layers of an LTE network, which is reproduced from TS 23.401 [39].

The LTE air interface control plane protocol layers between the UE and E‐UTRAN are Radio Resource Control (RRC), Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), and Media Access Control (MAC) layer. Between the UE and the MME, the control plane protocol layer is known as the Non‐Access Stratum (NAS) layer, consisting of the EPS Mobility Management (EMM) and ESM protocols. On the CN side, the control plane protocol is S1‐AP/MME AP between eNodeB and MME. The S1‐AP control messages are transported over the SCTP [17]. The EPS mobility and SM layer performs similar functions and procedures as described above for the GPRS control plane protocols. The RRC protocol layer is responsible for providing a reliable link between the UE and the MME in the case of the LTE/EPS network (Figure 3.7). Beyond the LTE/EPC MME, it uses the GTP control plane [11] protocol to perform tunnel management procedures with the S‐GW.

The Relay as shown in Figure 3.7 is not a protocol layer but an application module that is responsible for the reconstruction or reassembly of information transmitted by the lower layer, i.e. RLC or RRC. A common module also available in GPRS, UMTS, LTE RAN, and 5G NG‐RAN is the Relay module. In the GPRS/UMTS system, the Relay module forwards the reformatted data to the GPRS BSSGP or UMTS Radio Access Network Application Part (RANAP) layer in terms of PDUs for onward transmission to the SGSN over the Gb or Iu‐PS interface. Similarly, in LTE/EPS, the Relay module forwards data to the MME over the S1 interface. In 5GS, the Relay module forwards data to the AMF and UPF. The relay module is required to follow the underlying protocol layer details to reassemble their data for onward transmission to the CN.

 Functions of Control PlaneProtocol: Air interface

The control plane protocol stack performs different functions according to its logical interface. From the air interface point of view, some of the common functions and procedures, in general, performed by the control plane protocol stack and layers in the GSM, GPRS, UMTS, LTE/EPS, and 5G system are as follows. These functions are similar though their protocol layer specification and implementation aspects are different:

 Broadcasting of network system information to mobile devices,

 Radio resource allocation for CS (GSM) and PS services.

 CS (GSM) and PS call setup, supervise, and its release,Figure 3.7 LTE network end‐to‐end control plane protocol layers.Source: © 2015. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2015, 3GPP.

 MM functions and procedures such as the GSM location update, handover, GPRS routing area, LTE/EPS tracking area update procedure, and 5G registration area update procedure, and

 GPRS, LTE/EPS SM functions and procedures such as Packet Data Protocol Context Creation and Bearer Allocation.

However, the air interface control plane protocol stack and layers also perform different functions and procedures that are specific to a particular communications system, i.e. GSM, GPRS, UMTS, LTE, and 5G systems. For example, the following functions are performed by the UMTS, LTE, and 5G air interface control plane protocol stacks only:

 Header compression,

 Establishments of radio bearers,

 Configurations of lower layers, e.g. PDCP and RLC, by another higher‐layer RRC,

 Ensuring the ciphering, integrity, and security protection of the information exchanged between UE and RAN and CN, and

 Provision for a transparent mode (TM) of user data transfer.

Using the control plane protocol layers functions and procedures, the RAN or CN controls and commands the behavior of an MS/UE. For more information on the control plane protocol functions and procedures by the individual protocol layer of a logical interface, refer to the 3GPP TSs mentioned in the References section of this book.