The protocol stack used on radio interface Uu is shown in Figure 1.71. The physical layer in this stack is represented by OFDM in the DL and SC-FDMA in the UL. Then we see the MAC protocol that is
Figure 1.71 Protocol stack LTE Uu interface.
responsible for mapping the transport channels onto the physical channels, but also for such important tasks as packet scheduling and timing advance control. RLC provides reliable transport services and can be used to segment/reassemble large frames. The main purpose of PDCP is the compression of larger IP headers as well as ciphering of user plane data and integrity protection of both user plane and control plane data.
On top of PDCP the stack is split into the user plane and control plane parts. On the control plane side we see RRC protocol, that is, the expression for the communication between the UE and eNB.
RRC provides all the necessary functions to set up, maintain, and release a radio connection for a particular subscriber. Details of these functions are described in Section 1.10.7.
RRC also serves as a transport protocol for NAS signaling messages. NAS is the expression for the communication between the UE and MME in which MME represents the core network.
On the user plane side we see IP as the transport layer for end-to-end applications. On the Uu stack the IP is always end-to-end IP, which means that all these IP packets are transparently routed, often tunneled through the mobile network. The user plane IP frames we see on Uu are the same IP frames that can be monitored at SGi reference points before or behind the PDN-GW.
The IP version can be Internet Protocol version 4 (IPv4) or Internet Protocol version 6 (IPv6). In the case of VPN (Virtual Private Network) traffic, IPsec will be used.
The applications on top of IP in the user plane stack are all protocols of the TCP/IP suite, such as the File Transfer Protocol (FTP), HTTP (web-browsing), and POP3/SMTP (for e-mail), but also Real-Time Transport Protocol (RTP) and SIP for real-time services like VoIP.
1.9.2 S1 – Control/User Plane
On the S1 reference point the physical layer L1 will in most cases be realized by Gigabit Ethernet cables. L2 in this case will be Ethernet. On top of Ethernet we find IP, but used as a transport protocol between two network nodes: eNB and MME. This lower layer IP does not represent the user plane frames.
Instead, the user plane IP frames (higher layer IP) are carried by the GTP Tunneling Packet Data Unit (T-PDU). The GTP is responsible for the transport of payload frames through the IP tunnels on S1-U. The transport layer for GTP-U is the User Datagram Protocol (UDP). As IP this protocol may be found twice in the user plane stack: lower UDP for transport between the eNB and MME and higher UDP (not shown in Figure 1.72) that is transparently routed through the mobile network as the
Figure 1.72 Protocol stack S1 control/user plane.
transport protocol for real-time application data. The higher layer IP on top of GTP-U as well as all application data on top of this higher layer IP are identical with the user plane information described in the previous section.
On the control plane side, the Streaming Control Transport Protocol (SCTP) provides reliable trans- port functionality for the very important signaling messages. S1AP is the communication expression between MME and S-GW while NAS – as already explained in the previous section – is used for communication between the UE and MME.
1.9.3 X2 – User/Control Plane
On the X2 interface the user plane protocol stack is identical to that of the S1 reference point.
However, as shown in Figure 1.73, on the control plane there is a different protocol: X2 Application Part (X2-AP).
The main purpose of X2-AP is to provide inter-eNB handover functionality. It also is used to exchange traffic-related and radio quality measurement reports between different eNBs.
1.9.4 S6a – Control Plane
There is no user plane at the S6a reference point due to the fact that we find here the connection between the MME and HSS, which is a plain signaling communication.
L1, L2, IP, and SCTP shown in Figure 1.74 provide the same functionality as explained in the section on the S1 protocol stack.
The new player in the S6a stack is the DIAMETER protocol. In the EPC network DIAMETER has taken over the role of the Mobile Application Part (MAP). It is used to update the HSS about the current location of the subscriber and to provide crucial subscriber attributes stored in HSS databases to the MME so that network access can be granted and the subscriber’s traffic can be routed according to these parameters. DIAMETER is also involved in the security functions of the network: subscriber authentication, integrity protection, and ciphering.
The protocol stack on S6a is identical to the protocol stack of the S13 reference point between the MME and EIR – in case an EIR exists in the network.
Figure 1.73 Protocol stack X2 control/user plane.
Figure 1.74 Protocol stack S6a.
Figure 1.75 Protocol stack S3S4/S5/S8/S10/S11.
1.9.5 S3/S4/S5/S8/S10/S11 – Control Plane/User Plane
At the S3, S4, S5, S8, S10, and S11 reference points we find the same protocol stack as shown in Figure 1.75. The reason is that all these interfaces are used to tunnel IP payload transparently from one network node to the other. The tunnel management is provided by GTP-C while the IP payload is transported using GTP-U T-PDUs. Indeed, the user plane protocol stack is the same as on the S1 reference point.
Simplifying the functionality of GTP-C, it can be said that this protocol is used to create, modify, and delete user plane tunnels. It also supports mobility of subscribers between core network nodes.