The service-specific Convergence Sublayer (CS), often simply known as CS, is the top sublayer of the MAC Layer in WiMAX/802.16 (Figure 7.1). The CS accepts higher-layer PDUs from the higher layers and transmits them to the MAC CPS where classical type MAC procedures are applied (see Chapter 8). Classifying and mapping the MSDUs into appropriate CIDs (Connection IDentifier) made by the CS are basic functions of the QoS mechanisms of WiMAX/802.16. Among other functions of the CS is the optional Payload Header Suppression (PHS), the process of suppressing repetitive parts of payload headers at the sender and restoring the headers at the receiver. The classification and mapping made by a QoS management module allow full advantage to be taken of the different PHYsical layer features presented in the two previous chapters of this book.
In the present version of the 802.16-2004 standard, two CS specifications are provided and described in Section 5 of standards  and . The first CS specification is the ATM CS. The Asynchronous Transfer Mode (ATM) CS is a logical interface that associates different ATM services with the MAC CPS SAP. The ATM CS accepts ATM cells from the ATM layer, performs classification and, if provisioned, PHS. Then the ATM CS delivers CS PDUs to the appropriate MAC SAP.
The other available CS specification is the packet CS. The packet CS is used for the transport of all packet-based protocols such as the Internet Protocol (IP), IPv4, IPv6. Point-to-Point Protocol (PPP) and the IEEE standard 802.3 (Ethernet). Classification and, if provisioned, PHS are also defined for the packet CS.
The standard states that other CSs may be specified in the future. For the moment, no implementation of the ATM CS is planned, although it is detailed in the standard. In the rest of this chapter only the packet CS will be considered.
IEEE 802.16-2004, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems, October 2004.
IEEE 802.16e, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems, Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1, February 2006 (Approved: 7 December 2005).