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SS initiated
BS initiated
SS BS SS BS
FIGURE 4.5
Dynamic service addition (DSA)—message flow.
The DL transmission is controlled by the BS and MAC frames are broad- casted to all SSs. When an SS receives a frame, it checks for the CID in the frame. To improve the transmission performance, multiple frames are com- bined into a burst, which is then sent to the air interface for broadcasting. When an SS receives the burst, it will retrieve only the frames with its own CID and discard the other frames. Each burst includes a DL-MAP and UL- MAP, which specify the structure of the burst and how to retrieve the MAC frames from the burst. The UL data transmission is more complex as all SSs must synchronize with the BS for data transmission. The details of the UL data transmission are related to the QoS scheme discussed in Section 4.3.3.
QoS in IEEE 802.16
The IEEE 802.16 standard specifies the following four classes of services [8,9]:
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Unsolicited grant service (UGS): This service is for transmitting uncompressed voice and to emulate circuit-switched services such as DS0, n DS0, and T1. This service requires a fixed amount of data at a fixed time interval and guarantees the throughput and delay for the service.
Real-time polling service (rtPS): This service is for compressed mul- timedia (such as video streaming) and other real-time applications where the amount of bandwidth requirement may vary at each instant. This service requires the BS to implement a polling mecha- nism to the SSs at a fixed interval. Each poll asks the SS to specify the bandwidth requirement for each time interval. The polling is on the DL channel to avoid contention by the SSs.
Nonreal-time polling service (nrtPS): This service is for nonreal- time application that requires a guaranteed performance. It requires the BS to poll the SSs at a fixed time interval, but not at a rigid time interval as rtPS. If an SS does not respond to the poll after n times in a row, the BS will put the SS in a waiting group.
When the waiting group is polled, all SSs in the group will be con- tending for network access. This mechanism prevents stations with little traffic to waste valuable polls.
Best effort (BE) service: This service does not require a poll. An SS must contend with other SSs for bandwidth and network access. Requests for bandwidth are in the time slots marked in the UL- MAP as available for contention. If a request is successful, it will be indicated in the next DL-MAP and the SS can transmit the data. If it is not successful, the SS must try again later. It is possible to have collisions for the request, and the same back-off algorithm for Ethernet is applied when collision occurs.
The following is the set of performance metrics to support the above QoS classes and the application of these metrics to specify the QoS schemes is given in Table 4.1:
Maximum sustained traffic rate (MSTR): The peak data rate (in bps) of a service flow. This parameter is comparable to ATM-PCR. This service rate shall be policed for the wireless link to assure its conformance as measured in average over time.
Minimum reserved traffic rate (MRTR): The minimum reserved data rate (in bps) for a service flow. This rate is guaranteed for the ser- vice. This parameter is comparable to ATM-SCR. For example, a user subscribes to a service of MRTR of 768 Kbps with an MSTR doubling the rate to 1.544 Mbps. The user is guaranteed 768 Kbps for throughput, plus other contractual service of latency and jitter. If the service provider network has a bandwidth available beyond MRTR, the service provider would allow the user traffic to continue up to the specified MSTR. The user shall not send the data beyond the MSTR level.
Maximum traffic burst: The maximum burst size (in bytes) for a service flow. This parameter is comparable to ATM-MBS.
Maximum latency: The maximum latency (in milliseconds) between the reception of a packet by BS/SS and the transmission of the packet by SS/BS.
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