Wimax standards and Security The Wimax

Bandwidth Allocation Scheme

During network entry and initialization, every SS is assigned up to three dedicated CIDs for the purpose of sending and receiving control messages. These connection pairs are used to allow differentiated levels of QoS service to be applied to the different connections carrying MAC management traffic. Changing bandwidth requirements is necessary for all services except con- stant bit rate UGS connections. Demand assigned multiple access (DAMA) services will provide resources on a demand assignment basis, as the need arises. When an SS needs to ask for bandwidth on a connection with BE scheduling service, it sends a message to the BS containing the immediate requirements of the DAMA connection. QoS for the connection was estab- lished at connection establishment and is looked up by the BS. There are numerous methods by which the SS can get the bandwidth request message to the BS.

1. 
   Requests
   

Requests are for SSs to indicate to the BS that they need uplink bandwidth allocation. A request may come as a stand-alone bandwidth request header or it may come as a piggyback request. As the uplink burst profile can change dynamically, all requests for bandwidth shall be made in terms of the number of bytes needed to carry the MAC header and payload, but not the PHY over- head. The bandwidth request message may be transmitted during an uplink allocation except during an initial ranging interval. Bandwidth requests may be incremental or aggregate. When the BS receives an incremental bandwidth request, it shall add the quantity of bandwidth requested to its current per- ception of the bandwidth needs of the connection. When the BS receives an aggregate bandwidth request, it shall replace its perception of the bandwidth needs of the connection with the quantity of bandwidth requested. The piggy- backed bandwidth requests shall always be incremental. The self-correcting

nature of the request/grant protocol requires that the SSs shall periodically use aggregate bandwidth requests. The period may be a function of the QoS of a service and of the link quality. Owing to the possibility of collisions, bandwidth requests transmitted in broadcast or multicast request IEs should be aggregate requests.

2. 
   Grants
   

For an SS, bandwidth requests are not to individual connections while each bandwidth grant is addressed to the SS’s basic CID. In all cases, based on the latest information received from the BS and the status of the request, the SS may decide to perform backoff and request again or to discard the MAC SDU. An SS may use request IEs that are broadcast, directed at a multicast polling group it is a member of, or directed at its basic CID. In all cases, the request IE burst profile is used, even if the BS is capable of receiving the SS with a more efficient burst profile. To take advantage of a more efficient burst profile, the SS should transmit in an interval defined by a data grant IE directed at its basic CID. Owing to this, unicast polling of an SS would normally be done by allocating a data grant IE directed at its basic CID. Also note that in a data grant IE directed at its basic CID, the SS may make bandwidth requests for any of its connections.

3. 
   Request Transmission Schemes
   

There are two ways to issue the bandwidth requests. In the rtPS and nrtPS services, the requests will be issued by the control of polling scheme or contention. In the BE service, the requests will be issued mainly by contention.

1. 
   Polling
   

Polling is the process by which the BS allocates to the SSs bandwidth specifi- cally for the purpose of making bandwidth requests. These allocations may be to individual SSs or to groups of SSs. Allocations to groups of connections or SSs actually define bandwidth request contention IEs. The allocations are not in the form of an explicit message but are contained as a series of IEs within the UL-MAP. Polling is done on SS basis. Bandwidth is always requested on a connection basis and bandwidth is allocated on an SS basis.
When an SS is polled individually, it is the unicast polling scheme with- out an explicit message that is transmitted to poll the SS. Rather, the SS is allocated, in the UL-MAP, bandwidth sufficient to respond with a bandwidth request. If the SS does not need bandwidth, the allocation is padded. SSs that have an active UGS connection of sufficient bandwidth shall not be polled individually unless they set the PM bit in the header of a packet on the UGS connection. This saves bandwidth over polling all SSs individually. Note that unicast polling would normally be done on a per-SS basis by allocating a data grant IE directed at its basic CID.
If insufficient bandwidth is available to individually poll many inactive SSs, some SSs may be polled in multicast groups or a broadcast poll may be issued. As with individual polling, the poll is not an explicit message,

but bandwidth allocated in the UL-MAP. The difference is that, rather than associating allocated bandwidth with an SS’s basic CID, the allocation is to a multicast or broadcast CID.

When the poll is directed at a multicast or broadcast CID, an SS belonging to the polled group may request bandwidth during any request interval allo- cated to that CID in the UL-MAP by a request IE. To reduce the likelihood of collision with multicast and broadcast polling, only SS’s needing bandwidth reply. They shall take the contention resolution algorithm to select the time slot in which to transmit the initial bandwidth request. The SS shall assume that the transmission has been unsuccessful if no grant has been received in the number of subsequent UL-MAP messages specified by the parameter contention-based reservation timeout. Note that, with a frame-based PHY with UL-MAPs occurring at predetermined instants, erroneous UL-MAPs may be counted towards this number. If the request is made in a multicast or broadcast opportunity, the SS continues to run the contention resolution algorithm.

2. 
   Contention Resolution
   

The mandatory contention resolution method is the truncated binary expo- nential backoff with the initial backoff window and the maximum backoff window controlled by the BS. When an SS has information to send and wants to enter the contention resolution process, it sets its internal backoff window equal to the request backoff start defined in the UCD message referenced by the UCD count in the UL-MAP message currently in effect. The SS shall randomly select a number within its backoff window. This random value indi- cates the number of contention transmission opportunities that the SS shall defer before transmitting. An SS shall consider only contention transmission opportunities for which this transmission would have been eligible. These are defined by request IEs in the UL-MAP messages.
The SS shall now increase its backoff window by a factor of two, as long as it is less than the maximum backoff window. The SS shall randomly select a number within its new backoff window and repeat the deferring process described above. This retry process continues until the maximum number (i.e., request retries for bandwidth requests and contention ranging retries for initial ranging) of retries has been reached. At this time, for bandwidth requests, the PDU shall be discarded.
For bandwidth requests, if the SS receives a unicast request IE or data grant burst type IE at any time while deferring for this CID, it shall stop the contention resolution process and use the explicit transmission opportunity. The BS has much flexibility in controlling the contention resolution. At one extreme, the BS may choose to set up the request (or ranging) backoff start and request (or ranging) backoff end to emulate an Ethernet-style backoff with its associated simplicity and distributed nature as well as its fairness and efficiency issues.
A transmission opportunity is defined as an allocation provided in a UL-MAP or part thereof intended for a group of SSs authorized to transmit

bandwidth requests or initial ranging requests. This group may include either all SSs having an intention to join the cell or all registered SSs or a multicast polling group. The number of transmission opportunities associated with a particular IE in a map is dependent on the total size of the allocation as well as the size of an individual transmission. The size of an individual transmission opportunity for each type of contention IE shall be published in each trans- mitted UCD message. The BS shall always allocate bandwidth for contention IEs in integer multiples of these published values (Figure 3.2).

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