BANDWIDTH REQUEST MECHANISMS

During network entry and initialization processes, each SS is assigned up to three dedicated CIDs for the purpose of sending and receiving control messages. They are used for allowing a differentiated level of QoS. In WiMAX, an SS can get a bandwidth request to the BS using several methods, these include requests, grants, UGS, Unicast Polling, Multicast/Broadcast Polling, Contention-based focused bandwidth requests, Contention-based code division multiple access (CDMA) bandwidth requests, and Optional Mesh topology support. Vendors are allowed to optimize the performance of their systems by employing different combinations of these schemes. Requests refer to mechanisms used by SSs to indicate to the BS that they require Uplink allocation of bandwidth. It can be as a stand-alone bandwidth request header or as a piggyback request. The use of piggyback is optional.

Another important concept in WiMAX is bandwidth stealing. It refers to an optional strategy, adopted by an SS, in which a portion of bandwidth allocated (in response to a request by a connection) is used to send another bandwidth request rather than sending data. It has been modified in the mobile WiMAX standard, as referring to the use by a SS, of a portion of the bandwidth allocated in response to a bandwidth request for a connection to send a bandwidth request or data for any of its connections.

Polling refers to a process where the BS allocates some bandwidth to SSs specifically for making bandwidth requests. It can be for an individual (unicast) or a group (multicast/broadcast) of SSs. The contention-based bandwidth request mechanisms of WiMAX are only allowed for ertPS, nrtPS, and BE traffic classes. UGS and rtPS are not allowed to participate in the contention process. To resolve contentions, a mandatory, truncated binary exponential backoff scheme has been specified in the standard. It has an initial and a maximum backoff window controlled by the BS. Its value is specified by the uplink channel descriptor message and it follows the power-of-two rule. Readers interested in the modeling of polling and contention-based bandwidth.

To optimize the bandwidth request latency from an SS to a BS, and thus the response time, for elastic traffic generated by various sources (such as TCP or Push-to-Talk), defined the Poll-Me (PM) bit in a generic MAC header. Currently, the PM bit is part of the WiMAX standard and it may be used to request to be polled for a different non-UGS connection. The SSs with currently active UGS connections can set the PM bit in the grant management subheader to indicate to the BS that they need to be polled to request bandwidth for non-UGS connections. It is noted that except for UGS, piggyback and bandwidth stealing is allowed for all other traffic classes.

Recently, Its basic idea is to adapt polling intervals according to ON/OFF periods of traffic. During an ON period, polling intervals are short and of fixed length, whereas during an OFF period the polling intervals are lengthened exponentially, thus, reducing the signaling overhead.