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wimax bandwidth request procedure-BW REQ message

In wimax system, ranging procedure is initiated by SS(Subscriber Station) to establish connection with BS(Base Station). This ranging procedure are of many types mainly available for synchronization and maintenance of RF link. After Ranging is completed Base station waits for SBC REQ message transmitted by the Wimax SS. Using SBC REQ message SS informs BS of its basic capabilities. Unlike ranging request(RNG REQ) there is no dedicated slot for bandwidth request and SBC REQ. Bandwidth request can be transmitted any where in the uplink subframe except the reserved ranging slot.

The wimax BS responds to the Bandwidth request in the next broadcast message and indicates SS about when it can transmit SBC REQ message in the uplink subframe.

Figure below mentions contents of wimax bandwidth request header message.

BW REQ message format

bandwidth request header

HT (Header Type) - 1 bit (0 for Bandwdith Request MAC Header and 1 for generic mac header)

EC (Encryption Control) - 1 bit (always 0, indicating no encryption)

TYPE: Indicates the request is incremental or aggregate
000 = for incremental
001 = for aggregate

BR (Bandwidth Request) - 19 bits
The number of bytes of uplink bandwidth requested by SS.

CID (Connection Identifier) - 16 bits
Indicates the connection for which UL Bandwidth is requested

HCS (Header Check Sequence) - 8 bits

The Bandwidth Request(BW REQ) PDU shall consist of Bandwidth Request Header alone and shall not contain a payload.

BW REQ Header: Requests refer to the mechanism that SSs use to indicate to the BS that they need uplink BW allocation. Because the uplink burst profile can change dynamically, all requests for BW shall be made in terms of the number of bytes needed to carry the MAC Header and payload, but not the PHY overhead. As mentioned BW REQ message may be transmitted during any uplink allocation, except during any initial ranging interval. BW is always requested on a CID basis and BW is allocated on an SS basis.

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