Terminology » WiMAX » WiMAX Bandwidth Request (BW REQ) Procedure

WiMAX Bandwidth Request (BW REQ) Procedure

wimax bandwidth request bw req uplink mac layer

In a WiMAX system, the Subscriber Station (SS) initiates a ranging procedure to establish a connection with the Base Station (BS). This ranging procedure encompasses various types primarily intended for synchronization and maintaining the RF link.

After the ranging process concludes, the Base Station awaits the SBC-REQ message transmitted by the WiMAX SS. The SS uses the SBC-REQ message to inform the BS of its basic capabilities.

Unlike the ranging request (RNG-REQ), there isn’t a dedicated slot for bandwidth requests and SBC-REQ.

Bandwidth requests can be transmitted anywhere within the uplink subframe, except for the reserved ranging slot. The WiMAX BS responds to the bandwidth request in the subsequent broadcast message, informing the SS when it can transmit the SBC-REQ message in the uplink subframe.

The following figure illustrates the contents of the WiMAX bandwidth request header message:

bandwidth request header Image alt: bandwidth request header

Let’s break down the fields within this header:

  • HT (Header Type): 1 bit (0 for Bandwidth Request MAC Header and 1 for generic MAC header)
  • EC (Encryption Control): 1 bit (always 0, indicating no encryption)
  • TYPE: Indicates whether 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 the SS.
  • CID (Connection Identifier): 16 bits - Indicates the connection for which uplink bandwidth is being requested.
  • HCS (Header Check Sequence): 8 bits

The Bandwidth Request (BW REQ) Protocol Data Unit (PDU) consists solely of the Bandwidth Request Header and does 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 previously, the BW REQ message can be transmitted during any uplink allocation, except during any initial ranging interval. Bandwidth is always requested on a CID basis, and bandwidth is allocated on an SS basis.