Up to now we have focused on what happens between an eNodeB and the UEs. In this video, we will see how this radio interface is connected to the core network and therefore with the rest of the world through the IP protocol. This is the role of the PDCP protocol which relies on the top of the radio interface protocol stack. Next, we’ll put in perspective all the principles we have seen this week to create a global view of the radio interface. PDCP stands for Packet Data Convergence Protocol. It is implemented on top of every instance of R. L. C, except those working in Transparent Mode. It provides the interface with the core of the network, in other words, with the IP protocol for users’ data and with R. R. C for signalling messages. PDCP provides three types of service. To start: header compression. As you know, applications rely on protocol stacks and each protocol adds its own header. For example, VoIP data are typically 30 bytes long. They are encapsulated in RTP which adds a 12 bytes long header, then, they are encapsulated in UDP which adds 8 bytes, and finally in IPv6 which adds 40 bytes. so, the total length of the headers is 60 bytes. So, we are transmitting twice as much protocol data as useful data. These protocol data change very little from one packet to another. Hence the possibility of compression. To do this, L. T. E uses a standard Internet protocol called ROHC or Robust Header Compression. It is no coincidence that L. T. E re-uses protocols from the Internet. Indeed, one goal of L. T. E. is to reuse when possible, existing standards, notably for the core of its network. Another role of PDCP is to avoid packet loss or the de-sequencing that can happen during handovers. We saw that R. L. C manages retransmissions and re-sequences, but only within the cell. With PDCP, when changing cell, the old eNodeB can inform the new one about the status of the current transmissions and can forward data waiting to be transmitted. The last function of PDCP is about security. Obviously, the radio link is particularly vulnerable in terms of security. Therefore, PDCP enables data cyphering to guarantee confidentiality. And it offers an integrity control system to verify that signalling data were not modified during transmission. Not all PDCP functions are used in every case. Here is a summary. Header compression, for example, will only work for data encapsulated in IP. So it can’t be implemented for R. R. C control messages. Handover-related loss prevention only works for R. L. C instances working in Acknowledged Mode. Cyphering is always possible. It is optional but strongly recommended. Finally, integrity control is only possible for control messages. But that is where it is the most important, because if the system considers a corrupted control message, it is a direct security flaw for L. T. E. And if applications need integrity control, they can implement it by themselves. This chapter about the radio interface is about to finish. Let’s sum up what we have seen this week. This diagram presents a synthesized view of the radio interface protocol stack. Going from the bottom to the top. We first see the physical layer. It takes care of error correction and modulation. Above, the MAC layer controls independently for each terminal the choice of the modulation used by physical layer. This defines the transmission capacity. On the eNodeB, the MAC layer ensures resource allocation for both the uplink and downlink. It also takes care of low-level transmission reliability with a repetition mechanism called H. A. R. Q. And builds the transport blocks by multiplexing the data from different logical channels. The R. L. C layer proposes optional re-sequencing, segmentation, and retransmission services. Several R. L. C instances run in parallel, corresponding to different applications and different levels of service. Finally, the PDCP layer implements the interface with the core network; under IP for useful data, and R. R. C for signalling messages. It proposes optional services of header compression, handover mitigation and security. This last diagram represents the path followed by data packets. User data on IP are shown in pink and an R. R. C control message is shown in blue. It’s more of a reference diagram but let’s point out the most interesting things. At the PDCP level, we see the header compression that applies to the IP messages and not to R. R. C. At the R. L. C level, the two pink messages belong to the same R. L. C instance; they have been concatenated into the same R. L. C-PDU. This is not the case for the control message. At the MAC level, we merge the MAC-SDU from the two R. L. C instances. We add headers to create a transport block that will be delivered to the physical layer. On the physical layer, the RNTI appears in a dotted box. It’s because it is not transmitted with the data, but supplied implicitly through the allocation table. Finally, this slide summaries the acronyms that have been used this week. This is the end of this chapter about the radio interface. Thank you for watching. We’ll be in touch on the forum! Bye!
