Wimax standards and Security The Wimax

rtPS Scheduling

The rtPS is designed to support real-time service with variable-size data pack- ets on a fixed interval, such as streaming audio and video. The service allows an SS to specify the size of the desired grant and it has more request overhead than UGS. The BS issues request opportunities for the SSs to obtain UL trans- mission opportunity. Multiple connections of rtPS share the same bandwidth for the UL data transmission and a connection can send the data on the UL channel only when it is polled. The implementation of the polling service is not specified in the standard and each vendor may design its own polling mechanism. The following is an example of the polling service:

1. 
   An SS requests and is granted a connection of the rtPS service with a guaranteed bandwidth of 378 Kbps and a delay of 50 ms.
   

2. 
   The BS shall poll the SS at a fixed interval shorter than 50 ms.
   
3. 
   Whenever the SS has data to send it first waits for its polling, which is sent from the BS to the SS on the broadcast DL channel. The SS checks for its own polling as indicated in the UL-MAP with its own CID.
   
4. 
   When the SS gets its polling period, the SS retrieves the bandwidth allocation information from UL-MAP and uses the bandwidth to send the data on the assigned UL channel.
   
5. 
   If there is more data to send than the allocation, the SS shall build the frame according to its guaranteed bandwidth of 378 Kbps and maximum traffic burst.
   
6. 
   After sending the data burst to the air interface, the SS waits for its next poll.
   

3. 
   nrtPS Scheduling
   

The nrtPS provides polls on a regular basis and assures that the service flow receives request opportunity even under network congestion. In general, the BS polls nrtPS connections at an interval of 1s or less. The BS shall provide the request opportunities to SSs as specified by the QoS parameters. In addition, the SSs are allowed to use contention request opportunities to obtain grants.

4. 
   BE Scheduling
   

The BE service is to provide an efficient operation for best effort traffic. The SSs are allowed to use contention request opportunities to obtain grants. Collision could happen when multiple stations are transmitting requests at the same time. When collision happens, each SS uses a back-off algorithm similar to 802.11, except that the contention window is controlled by the BS, which uses the DL channel to specify the contention window size for individual SSs. The grants to SSs are sent via the DL channel, which uses the UL-MAP to specify the channel for UL data transmission. Note that the BS does not have a scheduler for the BE requests as they are operating in the contention mode. The BS scheduler, however, handles BE requests and provides grants (which are UL channels) for UL data transmission. There is no contention for UL data transmission.

4.