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.
3.2.2 User Plane Protocols
User plane protocol layers perform the functions required to transmit and receive the user/application traffic between the source and destination network element and vice versa. As an example, Figure 3.8 shows the end‐to‐end LTE system, TS 23.401 [39], user plane protocol layers structure starting from the UE, eNodeB, and S‐GW to P‐GW.
At the UE end, the user plane protocols consist of the application layer, IP layer, PDCP, RLC, MAC, and physical layer. Note that the application and IP layers terminate at the P‐GW. The IP layer contains the source and destination IP addresses using which the P‐GW receives/route the user application data packets toward the external packet data network. The air interface layers (RLC, MAC, and PDCP) terminate at the eNodeB end.
Figure 3.8 LTE UE – P‐GW user 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.
Example 3.5 File Transfer Protocol (FTP) Protocol
Consider the File Transfer Protocol (FTP) that is used to upload and download files between two different hosts. FTP uses port number 20 to transfer user data, whereas it uses port number 21 to exchange control commands between the two communicating hosts. Here, one will find that the FTP also uses the so‐called user and control plane concept to facilitate the exchange of control and data transfer between two hosts.
Example 3.6 GPRS Tunneling Protocol
The GTP has the control as well as the user plane protocols. The tunnel management is taken care of by the so‐called GPRS tunneling Control Protocol, whereas the user IP data packets/payload is transported using the GTP‐User Plane T‐PDU (tunneled PDU) as shown later in Figure 8.1. GTP is used in a couple of interfaces in GPRS, UMTS, LTE/EPS, and 5G CN. For more information on the GTP control and user plane protocol, refer to TS 29.060 [67], 29.274 [70], and TS 29.281 [72].
At the LTE/EPC end, the user plane of the eNodeB, S‐GW, and P‐GW consists of the GTP‐user plane [12] to tunnel user data on top of the UDP/IP transport network. Figure 3.8 also shows the respective user plane logical interfaces, i.e. S1‐U and S5/S8, for carrying user data between two network elements of an LTE/EPS network.
From Figures 3.7 and 3.8, it is observed that in the LTE system, the radio interface protocol layers – PDCP, RLC, and MAC, are available in both the control plane and the user plane protocol stack. In the user plane protocol stack,