ADMISSION CONTROL AND BANDWIDTH ALLOCATION

In general, admission control is a network’s QoS mechanism that determines whether a new session (or connection), with given bandwidth and delay requirements, can be established or not. For providing QoS, this procedure has been applied to both wireline and wireless networks. In the case of WiMAX, whenever a new session wants to make use of the wireless network, an admission control request is sent by the SS to the BS. This admission control request will be accepted by the BS if there is enough available bandwidth, QoS guarantees for bandwidth and delay can be met and the QoS of existing connections is not disturbed. An admission control scheme for WiMAX has been proposed together with the derivation of rules for each of the four classes of WiMAX. In addition, a token bucket based admission control for rtPS flows has been proposed. Omitting any further discussion involving admission control, we now present a brief overview of bandwidth allocation mechanisms in WiMAX.

The BS allocates bandwidth on a per SS basis, known as the grant per subscriber station (GPSS); further, each SS distributes this bandwidth among all of its active connections. The SS can efficiently distribute the allocated bandwidth as it has up-to-date information about the queue status of each connection. Thus, GPSS requires a packet scheduler at each SS, which may increase the complexity and the cost of an SS. However, GPSS is scalable to a large number of SSs and is, therefore, the only bandwidth allocation mechanism being employed in the current WiMAX.^[*]

With GPSS, each SS treats various connections separately at its own level and these are then pooled together as one entity for bandwidth allocation at the BS. Thus, the scheduler at the BS will only need a small amount of information about the overall bandwidth required by a particular SS. This approach has the additional advantage of avoiding the time lag in receiving updated information about individual connections at the SS. Once a lump of bandwidth has been granted to a particular SS, then it is responsible for the appropriate scheduling, according to priorities and the QoS for each active connection. This process greatly reduces the workload on the BS. For instance, suppose that an urgent packet arrives at the SS, then the BS does not need to have information about it and it is the duty of the scheduler at the SS to provide the required throughput and delay.

BS and SS communicate with each other by using a bidirectional path, viz., Uplink (UL: SS to BS) and Downlink (DL: BS to SS); whereas the bandwidth requirements are made by the UL and grants are made by the DL. WiMAX supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes as shown in Figures 1 and 2, respectively.

Figure 1: An example of Burst FDD bandwidth allocation. (From IEEE-802.16-2004, IEEE standard for local and metropolitan area networks—Part 16: An interface for fixed and mobile broadband wireless access systems, October 2004. With permission.)

Figure 2: Frame structure of TDD. (From IEEE-802.16-2004, IEEE standard for local and metropolitan area networks—Part 16: An interface for fixed and mobile broadband wireless access systems, October 2004. With permission.)

In FDD mode, both the UL and DL are operating at separate frequencies and DL data can be sent in bursts. To facilitate various types of modulation, a fixed duration frame is used for both the DL and UL transmissions. Also, it allows the simultaneous use of both full and half duplex SSs; a full duplex SS can transmit and receive data at the same time whereas a half duplex SS can either transmit or receive data at any given time. If half duplex SSs are used, then bandwidth controller will not allocate UL bandwidth at the same time that it is expecting to receive data on DL channel. It should also take into account the allowance for propagation delay, SS transmit/receive transition gap, and SS receive/transmit transition gap. In FDD mode, the use of a fixed duration frame, for both the DL and UL channels, also helps in simplifying the design of algorithms for bandwidth allocation. It can be noted that a full duplex SS can listen to a DL channel continuously, whereas the half duplex SS can only listen to a DL when it is not transmitting on the UL channel.

In the case of TDD, the UL and DL transmissions occur at different time intervals, while usually employing the same frequency. A TDD frame is also of fixed duration and is composed of one DL and one UL subframe. For easy partitioning of bandwidth, a TDD frame is divided into an integer number of physical slots. Also, TDD framing is adaptive and the bandwidth allocated to the UL and DL parts can vary and is controlled by the higher layers. The DL-MAP and UL-MAP messages define the usage of the corresponding transmission intervals. The BS also regularly transmits DL and UL channel descriptors, DCD and UCD, for the physical description of the corresponding channel. The complete list of MAC management messages. It should be noted that a WiMAX network can be planned with either FDD or TDD, but the former mode has been discussed more frequently in the literature.

BANDWIDTH REQUEST MECHANISMS

During network entry and initialization processes, each SS is assigned up to three dedicated CIDs for the purpose of sending and receiving control messages. They are used for allowing a differentiated level of QoS. In WiMAX, an SS can get a bandwidth request to the BS using several methods, these include requests, grants, UGS, Unicast Polling, Multicast/Broadcast Polling, Contention-based focused bandwidth requests, Contention-based code division multiple access (CDMA) bandwidth requests, and Optional Mesh topology support. Vendors are allowed to optimize the performance of their systems by employing different combinations of these schemes. Requests refer to mechanisms used by SSs to indicate to the BS that they require Uplink allocation of bandwidth. It can be as a stand-alone bandwidth request header or as a piggyback request. The use of piggyback is optional.

Another important concept in WiMAX is bandwidth stealing. It refers to an optional strategy, adopted by an SS, in which a portion of bandwidth allocated (in response to a request by a connection) is used to send another bandwidth request rather than sending data. It has been modified in the mobile WiMAX standard, as referring to the use by a SS, of a portion of the bandwidth allocated in response to a bandwidth request for a connection to send a bandwidth request or data for any of its connections.

Polling refers to a process where the BS allocates some bandwidth to SSs specifically for making bandwidth requests. It can be for an individual (unicast) or a group (multicast/broadcast) of SSs. The contention-based bandwidth request mechanisms of WiMAX are only allowed for ertPS, nrtPS, and BE traffic classes. UGS and rtPS are not allowed to participate in the contention process. To resolve contentions, a mandatory, truncated binary exponential backoff scheme has been specified in the standard. It has an initial and a maximum backoff window controlled by the BS. Its value is specified by the uplink channel descriptor message and it follows the power-of-two rule. Readers interested in the modeling of polling and contention-based bandwidth.

To optimize the bandwidth request latency from an SS to a BS, and thus the response time, for elastic traffic generated by various sources (such as TCP or Push-to-Talk), defined the Poll-Me (PM) bit in a generic MAC header. Currently, the PM bit is part of the WiMAX standard and it may be used to request to be polled for a different non-UGS connection. The SSs with currently active UGS connections can set the PM bit in the grant management subheader to indicate to the BS that they need to be polled to request bandwidth for non-UGS connections. It is noted that except for UGS, piggyback and bandwidth stealing is allowed for all other traffic classes.

Recently, Its basic idea is to adapt polling intervals according to ON/OFF periods of traffic. During an ON period, polling intervals are short and of fixed length, whereas during an OFF period the polling intervals are lengthened exponentially, thus, reducing the signaling overhead.

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.