INTRODUCTION TO QoS SCHEDULING IN WiMAX

One of the main objectives of WiMAX is to manage bandwidth resources at the radio interface in an efficient manner, while ensuring that QoS levels, negotiated at the time of connection setup, are met in an appropriate way. In the sequel, the provision of guaranteed levels of QoS in WiMAX is fundamentally dependent upon traffic policing, traffic shaping, connection admission control, and packet scheduling. To utilize the bandwidth most efficiently, the IEEE 802.16 standards employ operations of concatenation, fragmentation, and packing of MAC protocol data units (PDUs) and MAC SDUs.

Due to the highly variable nature of multimedia traffic, subscriber stations (SSs) of WiMAX can perform traffic shaping and policing for efficient utilization of resources and conformance to service level agreements. The nonconforming traffic can be penalized or rejected by an SS. A centralized connection admission control guarantees that newly admitted traffic will not cause congestion or degradation of services in the existing traffic. In WiMAX, admission control is implemented at the base station (BS). Despite the importance of the aforementioned mechanisms, the most important component for QoS provisioning is the Packet Scheduler. Thus, providing efficient scheduling mechanisms in WiMAX is the main focus of this chapter. However, several other related concepts are also described briefly.

In its broadest sense, scheduling refers to the mechanism for serving the enqueued resource requests of various users. A scheduling discipline has two orthogonal components: deciding the order of servicing the users’ requests and management of the service queues. A sketch of basic operations in a typical wireless scheduler is presented in Figure 1. Scheduling is important in both best effort and QoS networks. In the former case, the fair allocation of network bandwidth among a wide variety of network users is a prime objective. Whereas, in networks providing QoS guarantees (such as WiMAX), scheduling disciplines play a key role in ensuring that negotiated service level agreements are fully complied with. It should be noted that the requirements for scheduling algorithms in wired and wireless networks, such as WiMAX, are different from each other, and this will be explained in more detail in forthcoming sections.

Figure 1: Basic operation of scheduler in a wireless network. (Adapted from Bhagwat, P., Bhattacharya, P., Krishna, A., and Tripathi, S.K., IEEE INFOCOM, 3, 1133, March 1996.)

To provide QoS provisioning, the following three methods have been devised for WiMAX: Service flow QoS scheduling, dynamic service establishment, and two-phase activation model. A service flow in WiMAX has been defined as a MAC transport service that provides unidirectional transport of data packets either to uplink packets transmitted by the SS or to downlink packets transmitted by the BS. It is characterized by latency, jitter, and throughput assurances. It has the following major attributes:

§  Service flow ID (SFID): It is assigned to each existing service flow and serves as its principal identifier in the network

§  Connection ID (CID): It is a mapping to the SFID that exists only when a connection has been admitted or it is an active service flow

§  ProvisionedQoSParameterSet: It is a set of QoS parameters that is provisioned from outside the standard, such as a network management system belonging to the provider

§  AdmittedQoSParameterSet: It defines a set of QoS parameters for which both the BS and the SS reserve resources (bandwidth, memory, and other time-based resources)

§  ActiveQoSParameterSet: It is a set that defines the service actually being provided to active service flows

§  Authorization module: It is a logical module within the BS that approves or denies every change to the QoS parameters and classifiers associated with a service flow

The relationship between the various sets of QoS parameters has been depicted in Figure 2. It can be noticed that the ActiveQoSParameterSet is always a subset of the AdmittedQoSParameterSet, which in turn is always a subset of the authorized envelope. The scheduling algorithms to be used at the SSs of WiMAX will need to comply with values of the QoS parameters as indicated by the envelope.

Figure 2: Envelopes for provisioned and dynamic authorization models. (From IEEE 802.16e-2004, IEEE standard for local and metropolitan area networks—Part 16: An interface for fixed and mobile broadband wireless access systems, October 2004. With permission.)

Note that the automatic repeat request (ARQ) mechanism is optional in WiMAX. If implemented, it is done on a per connection basis and is specified and negotiated at the time of creation of the connection. Also, a connection cannot have a mixture of ARQ and non-ARQ traffic.