egrity protected in the PDCP layer. This chapter provides a top-down systematic description of convergence sub-layers supported in IEEE 802.16m and their constituent protocols. An overview of 3GPP LTE PDCP layer is further provided to enable the readers to contrast the corresponding functionalities. 5.1 HEADER COMPRESSION In some services and applications, such as voice-over-IP, interactive gaming, multimedia messaging, etc., the data payload of the IP packet is almost the same size or even smaller than the header. Over the end-to-end connection comprising multiple hops, these protocol headers are extremely important, but over a single link these headers serve no useful purpose. It is possible to compress these headers, and thus save the bandwidth and use the expensive radio resources efficiently. The header compression also provides other important benefits, such as reduction in packet loss and improved interactive response time [2]. Payload header compression is the process of suppressing the repetitive portion of payload headers at the sender and restoring them at the receiver side of a low-bandwidth/capacity-limited link. The use of header compression has a well-established history in transport of IP-based payloads over capacity-limited wireless links where more bandwidth efficient transport methods are required. The Internet Engineering Task Force (IETF) has developed several header compression protocols that are widely used in telecommunication systems. Let's consider some examples of the compression ratios that can be achieved using header compression schemes. The IP version 4 protocol header consists of 20 bytes [3], when combined with 5.1 Header compression User Datagram Protocol (UDP)¹ header of 8 bytes and the Real-time Transport Protocol (RTP) header of 12 bytes this results in an IPv4/UDP/RTP header size of 40 bytes. A header compression scheme typically compresses such headers to 2-4 bytes in the steady-state. Note that the RTP payload size of some commonly used voice codecs is approximately 20-40 b