Quality of Service (QOS)

QOS refers to meeting certain requirement - e.g. throughput, packet error rate, delay, and jitter - associated with given application. Broadband wireless networks must support a variety of applications, such as voice, data, video, and multimedia, and each of these has different traffic patterns and QoS requirements. The variability in the QoS requirements across applications, services, makes it a challenge to accommodate all these on a single-access network, particularly wireless networks, where bandwidth is at a premium.

The problem of providing QoS in broadband wireless systems is one of managing radio resources effectively. Effective scheduling algorithms that balance the QoS requirements of each application and user with the available radio resources need to be developed. In other words, capacity needs to be allocated in the right proportions among users and applications at the right time. This is the challenge that the MAC-layer protocol must meet: simultaneously handling multiple types of traffic flows - bursty and continuous, varying throughputs and latency requirements. Also needed are an effective signaling mechanism for users and applications to indicate their QoS requirements and for the network to differentiate among various flows.

Before any data transmission happens, the BS and the MS establish a unidirectional logical link, called a connection, between the two MAC-layer peers. Each connection is identified by a connection identifier (CID), which serves as a temporary address for data transmissions over the particular link.

WiMAX also defines a concept of a service flow. A service flow is a unidirectional flow of packets with a particular set of QoS parameters and is identified by a service flow identifier (SFID). The QoS parameters could include traffic priority, maximum sustained traffic rate, maximum burst rate, minimum tolerable rate, scheduling type, ARQ type, maximum delay, tolerated jitter, service data unit type and size, bandwidth request mechanism to be used, transmission PDU formation rules, and so on. These parameters are managed using the DSA and DSC messages. The base station is responsible for issuing the SFID and mapping it to unique CIDs.

To support a wide variety of applications, WiMAX defines five scheduling services:

1. Unsolicited grant services (UGS): This is designed to support fixed-size data packets at a constant bit rate (CBR). Examples of applications that may use this service are T1/E1 and VoIP.

2. Real-time polling services (rtPS): This service is designed to support real-time service flows, such as MPEG video, that generate variable-size data packets on a periodic basis.

3. Non-real-time polling service (nrtPS): This service is designed to support delay-tolerant data streams, such as an FTP, that require variable-size data grants at a minimum guaranteed rate.

4. Best-effort (BE) service: This service is designed to support data streams, such as Web browsing, that do not require a minimum service-level guarantee.

5. Extended real-time polling service (ErtPS): This service is designed to support real-time applications, such as VoIP with silence suppression, that have variable data rates, but require guaranteed data rate and delay. This service is defined only in IEEE 802.16e-2005, not in IEEE 802.16-2004.