Which protocols are used to exchange data packets between nodes of a 4G network? We know that IP is used but is it the only protocol? Just like Robinson Crusoe alone on his island? That is the question we are going to answer in this video. Remember encapsulation : when a packet that arrives from an external network, for example, the Internet it is taken up by the P Gateway, then placed in another IP packet that is sent to the S Gateway and once there, the S Gateway takes it inside and places it in another packet sent to the eNode B. Between the P Gateway and the S Gateway, we have an IP network. If we take the entire stack of IP protocols, we get the idea, over IP, of a transport protocol between the endpoints. At this level, the endpoints are the P Gateway and S Gateway. There are two well-known transport protocols: TCP and UDP. TCP is used a lot, but is relatively complex due to reliability mechanisms such as the retransmission of lost packets and also a complex congestion control. UDP has the advantage of being much simpler. So, it’s UDP that was chosen as the transport protocol. If we need reliability and retransmission mechanisms, these will be placed over the user IP protocol, in other words, between the server and the mobile terminal: at that point, using UDP is a very good choice. So, UDP is used between the Serving Gateway and the Packet Gateway and between the Serving Gateway and the eNode B. The moment you speak of a protocol, you’re speaking of headers that correspond to this protocol. So, IP packets include a source address (here the address of the P Gateway), a destination address (here the address of the Serving Gateway) and finally the UDP header with the port numbers. We could easily say, “Let’s put our user IP packet directly inside the IP datagram.” But we know that there are several variations of the IP protocol: IPv 4, IPv 6, and so on; we want to keep the possibility of evolution and permit the transport of other protocols. To do this, we will develop an additional protocol that’s called GTP: GPRS Tunneling Protocol. The idea is to add a format that allows consistent processing for all user protocols, whether they be IPv4 IPv6 or any other. Therefore, we have different levels of headers, and on this diagram we see the level of the additional header at the GTP layer. We’ll explain in the next video what is in this GTP header. Now, if we look at the whole protocol stack, we have, between the Serving Gateway and the P Gateway, an IP network. In accordance with the principles of IP, the physical layer (1) and the data link layer (2) can be anything. The only thing that counts is having IP on the Serving Gateway and the Packet Gateway. Above the IP layer we have UDP, above UDP, GTP. To make it clear that we consider user packets, for example the packets that contain the video stream if I’m watching a video on my smart phone, we talk of GTP-U, U stands for “user plane”. Above GTP-U we find the user IP. If we consider everything, we have the protocol stack that we saw earlier between the Serving Gateway and the Packet Gateway, and we have the same protocol stack between the eNode B and the Serving Gateway. GTP-U above UDP, above IP and above the physical and data link layers. We thus find GTP-U on the S1-U interface as well as on the S5/S8 interface. To summarize, user IP packets are put in GTP-U packets, which are themselves put in UDP packets transported in IP packets! The protocol stack between the S-Gateway and the P-Gateway and between the S- Gateway and the eNodeB includes IP/UDP/GTP-U/IP. In the end, IP is not at all alone – unlike Robinson Crusoe.
