 where bandwidth is scarce. The IEEE 802.16 standard defines a native header compression algorithm that is part of the convergence sub- layer. The Payload Header Suppression (PHS) defined in the IEEE 802.16 standard compresses the repetitive or redundant parts of the payload header received from network layer. The PHS operation is based on the PHS rules, which provide all the parameters corresponding to header suppression of the SDU. Other standard header compression algorithms, such as Robust Header Compression (RoHC) defined by IETF [21], are also supported. 3.1 The IEEE 802.1 16m reference model The RoHC scheme may be used as an alternative to PHS to compress the RTP/UDP/IP header of an IP packet. When RoHC is enabled for a service flow, the service flow constitutes what in IETF RFC 3095 is referred to as a RoHC channel [21,3]. Two service flows cannot share an RoHC channel, and two RoHC channels cannot share the same service flow. On a service flow for which RoHC has been enabled, all of the IP packet passes through the RoHC compressor on the transmitter side and the decompressor on the receiver side. The support of RoHC is negotiated between the BS and MS during capability negotiation [3]. The data-plane part of MAC CPS includes functions such as Automatic Repeat reQuest (ARQ), Packet Fragmentation/Packing, MAC PDU formation and encryption. The scheduler on the BS side allocates radio resources and multiplexes the users, and selects the appropriate MIMO mode, modulation and coding scheme based on the measurement reports that are received from the mobile stations. The ARQ is an error control mechanism at data link layer where the receiver may request the transmitter to resend a block of data that was erroneously detected or not received. An ARQ block is a distinct unit of data that is carried on an ARQ-enabled connection. Such a data unit is assigned a sequence number and is managed as a distinct entity by the ARQ state machines. The ARQ block size is a parameter that is negotiated during connection establish