The objective of the 4G architecture was to separate the various functions. First, control of the radio part between the eNodeB and the terminal, and second, management of security and mobility in the MME. How do you permit an MME to manage only the security and mobility part? How do you permit an eNodeB to concentrate on radio connection functions? These are the questions we will answer with this video. First, we have to keep in mind what the control plane is. The control plane can be defined as containing all the protocols, mechanisms, and messages that permit configuration of the switches, routers or links to enable the effective delivery of a communication service. For example, the attachment mechanism stands in the control plane. All the security exchanges, as well. They are necessary for the network to work correctly, but they do not transport any user data. In a general sense, you could say that the control plane consists of signaling exchanges. So, the dialog between a terminal, a UE, and an MME is solely in the control plane since the MME will not pass on any packets of user data; only control messages. On the control plane, one can differentiate management of mobility and of security, which is taken care of by the MME and allocation of radio resources and establishment of radio connections. These are exchanges between the UE and the eNodeB. These messages and protocols can be tightly linked with the technology used on the radio interface. We saw that, between a terminal and the eNodeB, there is a radio connection and the terminal is identified by the RNTI, the Radio Network Temporary Identifier. And that the LCID, or Logical Channel Identifier, is used to distinguish between user data and signaling. More exactly, we will use different values, one for signaling messages sent by the terminal to the MME and two others for messages exchanged with the eNodeB, 0 for urgent messages and 1 for non-urgent messages. We have seen the notion of the S1-AP connection: the messages sent by the terminal to the MME are transmitted on the radio connection and then over the S1‑AP connection. Of course, the same thing happens in the other direction from the MME to the terminal. All of these messages form the Non Access Stratum, or NAS. NAS messages, and this is very important, are forwarded by the eNodeB, but their content is never analyzed by the eNodeB. Which means that you can completely change the NAS messages, without having to change a single line of code in the eNodeB. The A. S. or Access Stratum, includes all the messages related to the radio technology, to the establishment and management of the radio connection. They are exchanged between the terminal and the eNodeB. For example, the modification of a radio bearer is an A. S. message. If we now look at the protocol stack, we’ve represented here the UE, the eNodeB and the MME. Between the eNodeB and the MME, we have an IP network, so one given layer 1, one given layer 2 and the IP protocol as the network protocol. People who specified 4G networks decided to use a transport protocol that is neither TCP nor UDP. UDP does not give reliability and TCP is a bit too complex and, importantly, it’s not message oriented. Here, what we want is to exchange signaling messages reliably. The protocol that meets our needs is SCTP, or Stream Control Transmission Protocol. This protocol is reliable and avoids unnecessary retransmissions by the upper layer. Why use this protocol? Well, sometimes the eNodeB can be connected with microwave links that are not necessarily very reliable. So, using a protocol that manages retransmission of lost messages is a good idea. Above SCTP, we have the S1-AP protocol. This protocol is connection oriented and there are as many connections as there are terminals managed by the MME. If we look at the radio interface side, we’ll see a few things we’re familiar with: the physical layer here called layer 1, the Medium Access Control, (MAC); the Radio Link Control (RLC); the Packet Data Convergence Protocol (PDCP) and, above, RRC to establish radio connections. All RRC messages are exchanged between the UE and the eNodeB. NAS messages are sent by the UE to the MME or by the MME to the UE. These NAS messages are transported in RRC messages. We have two NAS protocols: EMM, for Evolved Mobility Management and ESM for Evolved Session Management. Let’s look now on the user plane. Data are transported in tunnels, also called bearers. There should be a minimum of control to set up these tunnels. Because of that, we will also set up control tunnels. Each control tunnel is identified by a pair of TEIDs, just like the data tunnels, but the values are different. There are control tunnels between the Serving Gateway and the P-Gateway and between the MME and the Serving Gateway and the MME. These control tunnels are used for GTP-C exchanges. GTP‑C stands for GPRS Tunnel Protocol in the Control Plane. These are all the messages necessary to set up, maintain and release data tunnels. We have the protocol stack that we’ve already seen a bit for the user plane. Here, for the control plane, it is IP and above that, UDP and GTP-C, whether it’s between the Serving Gateway and the P-Gateway or between the Serving Gateway and the MME. If we try to pull back and have a larger view, still on the control plane, we have the figure here that you can take your time looking at with the slides. This figure symbolizes everything we have seen in the previous slides. There’s not just the control plane, there’s also the user plane. We can depict it in the diagram. It appears here and this gives us an overall view of the set of protocols between the UE, the eNodeB, the MME, the S-Gateway the P-Gateway and a corresponding node, an external host. We’ve used blue to depict the control plane and pink for the user plane. The NAS, or Non Access Stratum, messages are those which are exchanged between the eNodeB and the MME but which are in fact forwarded by the eNodeB. To summarize, A. S. control messages, NAS control messages and data packets are transported on different logical channels, different LCID, over the radio interface. A.S. messages are processed by the eNodeB, NAS messages are forwarded by the eNodeB to the MME on the S1-AP connections. Data messages are sent through tunnels.
