mes is that error CHAPTER 7 The IEEE 802. 16m MAC CPS (Part II) Transmitter Receiver Stop-and-Wait ARQ Protocol Transmitter Receiver 07567 Go-Back-N ARQ Protocol Transmitter Receiver 67589 Selective Repeat ARQ Protocol FIGURE 7-2 An example illustration of different ARQ schemes [4] detection requires much simpler decoding mechanisms and much less redundancy than error correction. Furthermore, ARQ is adaptive, in the sense that information is re-transmitted only when errors occur. On the other hand, FEC may be desirable instead of, or in addition to, error detection, for any of the following reasons: (1) a feedback channel is not available or ARQ delay is not tolerable; (2) the re-transmission scheme is not conveniently implemented; and (3) the expected number of errors without correction would require excessive re-transmissions. The ARQ can be used for each connection between the MS and BS. Since the use of ARQ increase the latency due to more reliability requirements, the ARQ mechanism is usually disabled for delay-sensitive applications, such as VoIP or interactive gaming. If the ARQ mechanism is enabled, the ARQ parameters are specified and negotiated during connection set-up. A connection does not contain mix of ARQ and non-ARQ traffic. The scope of a specific instance of ARQ is limited to one uni- directional flow. 7.1.2 IEEE 802. 16m ARQ Mechanism An ARQ block is generated from one or multiple MAC SDUs or MAC SDU fragments corresponding to the same flow. The ARQ blocks can be variable in size. An ARQ block is constructed by fragmenting MAC SDU or packing MAC SDUs and/or MAC SDU fragments. The fragmentation or packing information for the ARQ block is included in the extended header within the MAC PDU. When a MAC PDU is generated for transmission, the MAC PDU may contain one or more ARQ blocks. If the MAC PDU contains traffic from a single connection, the MAC PDU itself will be a single ARQ block. If 7.1 Automatic repeat request information from multiple ARQ connections is multiplexed into one MAC PDU, the MA