5.5 Ensuring Quality of Service
While the PHY supplies the means of transferring data over the air interface, the management and control functionality of the network is part of the MAC layer. One of the primary goals of the 802.16 standard is to ensure a certain quality of service for a data stream. While some applications require constant bandwidth, low delay and no jitter (variations in the delay over time), other applications are delay tolerant but have rapidly changing bandwidth requirements. To accommodate these different requirements, a connection-oriented model is used that transfers data over unidirectional connections. A connection is identified via its CID, which is part of the MAC header of each packet as mentioned in Section 5.4. For an IP session between a user and the network, one CID is used in the downlink direction and another CID is used in the uplink direction. This allows the network to control the properties of the downlink and uplink independently. The maximum guaranteed bandwidth is one of the properties that might be different between the downlink and uplink directions. The 802.16 standard defines the following QoS classes to accommodate the requirements of different applications.
The Unsolicited Grant Service (UGS)
This service guarantees a fixed bandwidth and constant delay for a connection. The base station assigns sufficient uplink opportunities to a subscriber station for the allocated bandwidth and ensures that the SS can use the uplink at the correct ...