The transmission protocol of LTE is divided into several layers as illustrated in Figure 3. This section describes the flow of a packet through the sub-layers of the LTE stack. The downlink direction (from network to terminal) is covered first.

a) A transport block from the physical (PHY) layer to the Medium Access Control (MAC) layer is delivered. It contains the information that was decoded off the air in the previous radio subframe. It may contain multiple or partial packets, depending on scheduling and modulation. The MAC, represented in the MAC block in Figure 3, is responsible for following functions,

• It handles the scheduling of the PDUs .
• It manages the Hybrid Automatic Repeat reQuest (HARQ) for retransmissions on the PHY.
• Hybrid ARQ function is a transport-block level automatic retry.
• When a valid transport block is available from the HARQ process, the transport channels are mapped to logical channels. A function that breaks down a transport block in to different logical channels for the higher layers. (The channels function will be described in the later part of the chapter)
• It also performs Format selection and measurements, provide information about the network that is needed for managing the entire network to the radio resource control.

b) The RLC layer is illustrated by the RLC block in Figure 3. It performs segmentation and reassembly and operates in three modes: Transparent mode (TM), Acknowledged Mode (AM) and Unacknowledged Mode (UM). These are used by different radio bearers for different purposes. The RLC provides in-sequence delivery and duplicate detection.

RLC Segmentation

The segmentation process involves unpacking an RLC PDU into RLC SDUs, or portions of SDUs. The RLC PDU size is based on transport block size. RLC PDU size is not fixed because it depends on the conditions of channels which the eNodeB assigns to every UE on the downlink. Transport block size can vary based on bandwidth requirements, distance, power levels or modulation scheme. The process also depends on the size of the packets, e.g. large packets for video or small packets for voice over IP. If an RLC SDU is large, or the available radio data rate is low (resulting in small transport blocks), the RLC SDU may be split among several RLC PDUs. If the RLC SDU is small, or the available radio data rate is high, several RLC SDUs may be packed into a single PDU. In many cases, both splitting and packing may be present. This is illustrated in Figure 4.

RLC In-order Delivery

The RLC ensures in-order delivery of SDUs. Out-of-order packets can be delivered during handover. The PDU sequence number carried by the RLC header is independent of the SDU sequence number (i.e. PDCP sequence number). An RLC SDU is built from (one or more) RLC PDUs for downlink. Packet order is corrected in the RLC using sequence numbers in the RLC header.

RLC Modes

As mentioned in previous section, the RLC operates in three modes.

Transparent mode is used only for control plane signaling for a few RLC messages during the initial connection There is effectively no header; it simply passes the message through.

Unacknowledged and acknowledged modes use the RLC header and indicate whether or not the ARQ mechanism is involved. ARQ applies to an RLC SDU, while HARQ applies to a transport block. These interactions may contain a partial SDU, one SDU, or multiple SDUs. ARQ can be used for TCP/IP or critical information; it also might use unacknowledged mode for voice over IP or when there’s no time for a retry because of latency requirements; in voice over IP, for example, a packet that does not arrive the first time is useless, and the higher layers make up for the difference. ARQ, unlike the HARQ, applies to the SDU at the top of the RLC. If the HARQ transmitter detects a failed delivery of a TB-for example, maximum retransmission limit is reached-the relevant transmitting ARQ entities are notified and potential retransmissions and re-segmentation can be initiated at the RLC layer for any number of affected PDUs.

c) At the top, the Packet Data Convergence Protocol (PDCP) layer is illustrated by the PDCP block in Figure 3. PDCP performs functions related to data integrity (like enciphering) and IP header compression. PDCP functions in the user plane include decryption, ROHC header decompression, sequence numbering and duplicate removal. PDCP functions in the control plane include decryption, integrity protection, sequence numbering and duplicate removal. There is one PDCP instance per radio bearer. The radio bearer is similar to a logical channel for user control data.

PDCP Header Compression

Header compression is important because VoIP is a critical application for LTE. Because there is no more circuit switching in LTE, all voice signals must be carried over IP and there is a need for efficiency. Various standards are being specified for use in profiles for robust header compression (ROHC), which provides a tremendous savings in the amount of header that would otherwise have to go over the air. These protocols are designed to work with the packet loss that is typical in wireless networks with higher error rates and longer round trip time. ROHC is defined in IETF RFC 3095, RFC4815, and RFC 3843. Figure 5 illustrates the Header compression, in which 40 bytes of IP header are compressed to token of 1-4 bytes.

Ciphering and Integrity Protection

Ciphering, both encryption and decryption, also occurs in the PDCP. Security has to occur below the ROHC because the ROHC can only operate on unencrypted packets. It cannot understand an encrypted header. Ciphering protects user plane data, radio resource control (RRC) data and Non Access Stratum (NAS) data. Processing order in the PDCP is as follows (as illustrated in Figure 6): For the downlink, decryption occurs first, then ROHC decompression. For the uplink, ROHC compression occurs first, then encryption. Details of LTE security architecture are still being defined. The 3GPP System Architecture Working Group 3 (SA3) is responsible for security and has decided to use either Advanced Encryption Standard (AES) or SNOW (no abbreviation, it is named like this only, the stream cipher for encryption algorithms) 3G algorithms. Specific modes for AES are still being determined; AES is a block cipher and has to use specific operational modes to operate in a streaming mode.

LTE processes on the uplink side are often similar to processes on the downlink side. Key differences are that the peak data rate is half that of downlink; access is granted by the eNodeB; there are changes in logical channels and transport channels; and random access is used for initial transmissions. The PHY uses SC-FDMA for the uplink because it has a lower peak average ratio, which allows a more power-efficient transmitter in the UE. The flow of LTE packet on uplink is illustrated in Figure 7.