ntrol (RLC) protocol; - medium access control (MAC) protocol. Three types of channels are defined (Figure 4.1): - the logical channel defines the data structure at the interface between the RLC and MAC sub-layers; - the transport channel defines the data structure at the interface between the MAC sub-layer and the physical layer; - the physical channel defines the data structure between the two parts making up the physical layer, firstly, the coding and, secondly, the modulation and the multiplexing. The RRC messages can carry non-access stratum (NAS) messages exchanged between the mobile and the mobility management entity (MME). VoLTE and ViLTE: Voice and Conversational Video Services over the 4G Mobile Network, First Edition. André Perez. © ISTE Ltd 2016. Published by ISTE Ltd and John Wiley & Sons, Inc. VoLTE and ViLTE message Traffic flow Control flows message Packet sub-layer sub-layer Logical channels sub-layer Transport (UL-SCH) RACH DL-SCH channels Physical layer Physical PUCCH PUSCH (PRACH (PDCCH)PDSCH Channels Uplink (PCFICH) PHICH Downlink Figure 4.1. Radio interface structure. For a color version of the figure, see www.iste.co.uk/perez/volte.zip 4.1.1. Data link sub-layer 4.1.1.1. PDCP protocol The PDCP protocol is used for RRC signaling messages, relating to dedicated control data and for the traffic IP packets. The PDCP protocol performs the following functions: - compression of the data traffic headers using the robust header compression (ROHC) mechanism; - security of data traffic (confidentiality) and RRC signaling (integrity and confidentiality); - the delivery in sequence of the RRC messages and the IP packets; - the recovery of PDCP frames lost during the handover. Several PDCP instances can be activated simultaneously: - two instances for signaling radio bearers (SRB1 and SRB2) relating to RRC signaling data: Radio Interface Procedures - the SRB1 bearer is used for the transmission of an RRC message that can carry a NAS message, - the SRB2 bearer is used for the transmission of a NAS message 