WiMAX QoS

Support for QoS is a fundamental part of the WiMAX MAC-layer design. WiMAX borrows some of the basic ideas behind its QoS design from the DOCSIS cable modem standard. Strong QoS control is achieved by using a connection-oriented MAC architecture, where all downlink and uplink connections are controlled by the serving BS. 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. In addition to connections for transferring user data, the WiMAX MAC defines three management connectionsthe basic, primary, and secondary connectionsthat are used for such functions as ranging.

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. Service flows may be provisioned through a network management system or created dynamically through defined signaling mechanisms in the standard. The base station is responsible for issuing the SFID and mapping it to unique CIDs. Service flows can also be mapped to DiffServ code points or MPLS flow labels to enable end-to-end IP-based QoS. To support a wide variety of applications, WiMAX defines five scheduling services that should be supported by the base station MAC scheduler for data transport over a connection:

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 emulation and VoIP without silence suppression. The mandatory service flow parameters that define this service are maximum sustained traffic rate, maximum latency, tolerated jitter, and request/transmission policy.
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. The mandatory service flow parameters that define this service are minimum reserved traffic rate, maximum sustained traffic rate, maximum latency, and request/transmission policy.
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. The mandatory service flow parameters to define this service are minimum reserved traffic rate, maximum sustained traffic rate, traffic priority, and request/transmission policy.
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. The mandatory service flow parameters to define this service are maximum sustained traffic rate, traffic priority, and request/ transmission policy.
5. Extended real-time variable rate (ERT-VR) service: 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. This is also referred to as extended real-time polling service (ErtPS).

Although it does not define the scheduler per se, WiMAX does define several parametersand features that facilitate the implementation of an effective scheduler:

• Support for a detailed parametric definition of QoS requirements and a variety of mechanisms to effectively signal traffic conditions and detailed QoS requirements in the uplink.
• Support for three-dimensional dynamic resource allocation in the MAC layer. Resources can be allocated in time (time slots), frequency (subcarriers), and space (multiple antennas) on a frame-by-frame basis.
• Support for fast channel-quality information feedback to enable the scheduler to select the appropriate coding and modulation (burst profile) for each allocation.
• Support for contiguous subcarrier permutations, such as AMC, that allow the scheduler to exploit multiuser diversity by allocating each subscriber to its corresponding strongest sub-channel.

It should be noted that the implementation of an effective scheduler is critical to the overall capacity and performance of a WiMAX system.

 

Source: Fundamentals of WiMAX : understanding broadband wireless networking / Jeffrey G. Andrews, Arunabha Ghosh, Rias Muhamed. PrenticeHall. 2007.