Praise for Fundamentals of WiMAX

9.1 Convergence Sublayer 3099.1 Convergence SublayerTable 9.1 shows the various higher-layer protocol convergence sublayers—or combinations—that are supported in WiMAX. Apart from header compression, the CS is also responsible formapping higher-layer addresses, such as IP addresses, of the SDUs onto the identity of the PHYand MAC connections to be used for its transmission. This functionality is required becausethere is no visibility of higher-layer addresses at the MAC and PHY layers.The WiMAX MAC layer is connection oriented and identifies a logical connection betweenthe BS and the MS by a unidirectional connection indentifier (CID). The CIDs for UL and DLconnections are different. The CID can be viewed as a temporary and dynamic layer 2 addressassigned by the BS to identify a unidirectional connection between the peer MAC/PHY entitiesand is used for carrying data and control plane traffic. In order to map the higher-layer address tothe CID, the CS needs to keep track of the mapping between the destination address and therespective CID. It is quite likely that SDUs belonging to a specific destination address might becarried over different connections, depending on their QoS requirements, in which case the CSdetermines the appropriate CID, based on not only the destination address but also various otherfactors, such as service flow 1 ID (SFID) and source address. As shown in Table 9.1 the IEEE802.16 suite of standards defines a CS for ATM (asynchronous transfer mode) services andpacket service. However, the WiMAX Forum has decided to implement only IP and Ethernet(802.3) CS.9.1.1 Packet Header SuppressionOne of the key tasks of the CS is to perform packet header suppression (PHS). At the transmitter,this involves removing the repetitive part of the header of each SDU. For example, if the SDUsdelivered to the CS are IP packets, the source and destination IP addresses contained in theheader of each IP packet do not change from one packet to the next and thus can be removedbefore being transmitted over the air. Similarly at the receiver: The repetitive part of the headercan be reinserted into the SDU before being delivered to the higher layers. The PHS protocolestablishes and maintains the required degree of synchronization between the CSs at the transmitterand the receiver.In WiMAX, PHS implementation is optional; however, most systems are likely to implementthis feature, since it improves the efficiency of the network to deliver such services as VoIP.The PHS operation is based on the PHS rule, which provides all the parameters related to headersuppression of the SDU. When a SDU arrives, the CS determines the PHS rule to be used, basedon such parameters as destination and source addresses. Once a matching rule is found, it providesa SFID, a CID and PHS-related parameters to be used for the SDU. The PHS rule can bedependent on the type of service, such as VoIP, HTTP, or FTP, since the number of bytes that canbe suppressed in the header is dependent on the nature of the service. In of VoIP, for example,the repetitive part of the header includes not only the source and destination IP addresses but1. The concept of service flow is discussed in Section 9.2.