Retransmission policy (automatic repeat request – ARQ) is the set of rules or processes used by networks to define if, when, and how retransmissions of data or information will occur. Some types of services (such as real time digital audio) do not use retransmissions because the delay for retransmission would take too long to offer any benefit.
Retransmission uses error detection, feedback, retransmission processes and the retransmission of blocks of data in packets. The WiMAX system uses two forms of ARQ; hybrid automatic repeat request (HARQ) and selective repeat (SR).
Selective Repeat (SR)
Selective repeat automatic repeat request is a data transmission control process that allows the receiver to request the retransmission of selective blocks of data.
Data to be transmitted is grouped into blocks and given a block sequence number (BSN). The maximum size of a data block in the WiMAX system is 2040 bytes. When blocks are transmitted using ARQ, each block is given a sub-packet identifier (SPID). An SPID is an index value that can be used to identify specific packets that are awaiting conformation in an automatic repeat request (ARQ) process.
As blocks are transferred between the sending device and the receiving device, acknowledgement messages are sent. WiMAX acknowledgement message types include selective, cumulative and cumulative with selective. ARQ messages can be sent as separate messages or the may be combined with other messages.
Hybrid Automatic Repeat Request (HARQ)
Hybrid automatic repeat request is a data transmission flow control process that uses a combination of the physical layer (PHY) and medium access control (MAC) layer to allow the receiver to stop and restart the retransmission of data over a transmission channel.
HARQ is a variation of a stop and wait ARQ. Stop and wait ARQ is a flow control process that allows the data flow to stop when packets are not received and wait until a successful retransmission is received before the data flow is restarted.
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