HETEROGENOUS FRAMEWORKS

These frameworks employ more than one type of scheduler for the various traffic classes of WiMAX.

WRR has been adopted for rtPS and nrtPS traffic classes, whereas simple RR has been employed for BE. It does not consider the ertPS traffic class and nor does it specify clearly whether or not GPSS is used.

Based on priority scheduling and dynamic bandwidth allocation, a QoS architecture for WiMAX. The contention slot allocator has been proposed to be used by the BS to dynamically adjust the ratio of the bandwidth allocated to the contention and reservation slots. WRR has been used as an upstream scheduler at the BS. At the SS level, there are two levels of scheduler, viz., (1) strict priority scheduling for distribution of allocated bandwidth among various classes and (2) schedulers for each of four classes. At the first level of scheduling at SS, the Multiclass Priority Scheduler has been suggested; whereas, at the second level of scheduling at the SS IWFQ has been proposed for higher priority, WRR for middle priority, and first-in-first-out (FIFO) for lower priority traffic classes of WiMAX. No experimental or simulation results have been presented.

A scheduling framework has been proposed, which is based on a centralized mechanism similar to the GPC mode of operation. The proposed UL packet scheduler requires three modules at the BS: an information module, a scheduling database module, and a service assignment module. The information module performs the following functions: retrieving the queue size information for each connection from bandwidth request messages, determining the number of packets that have arrived from an rtPS connection in the previous time frame by using arrival service concept, determining rtPS packets’ arrival time/deadlines and updating the scheduling database, queueing information from nrtPS, and BE request messages are directly passed to the scheduling database. The proposed scheduling database module will provide scheduling information to all of the connections; whereas the service assignment module determines UL-MAP by using information in the scheduling database. A strict priority scheduler is used for allocation of bandwidth from UGS, rtPS, nrtPS, and BE. Further, it has been proposed that there is no scheduler for UGS, EDF for rtPS, WFQ for nrtPS, and FIFO for BE. 

Deficit fair priority queueing (DFPQ) has been proposed for the first layer of scheduling at SSs. It has been derived from the DRR discipline. At the second layer of scheduling, EDF has been proposed for rtPS, WFQ for nrtPS, and WRR for BE. Some simulation results have also been presented. It is not clear whether GPSS or GPC has been employed for this work. However, the idea of DFPQ can be easily adopted for other advanced scheduling frameworks involving WiMAX.

A multiclass uplink fair scheduling structure (MUFSS), to support delay and bandwidth requirements of IEEE 802.16 QoS. The BS adopts modified WRR, but these details are not provided. At the SS, a modified WFQ has been suggested for UGS and rtPS classes; whereas, modified WRR and FIFO have been suggested for nrtPS and BE classes. Although MUFSS can provide guarantees for delay sensitive real-time traffic, it has less throughput overall and higher processing time.

HOMOGENOUS FRAMEWORKS

These frameworks employ a single kind of scheduler for all types of WiMAX traffic.

Simulations have been used to evaluate the QoS performance of WiMAX. DRR has been selected as a DL scheduler and WRR has been chosen as an UL scheduler. Similarly, both WRR and DRR have been selected, independently, to be employed at SSs. This chapter does not consider the application of a wide variety of other schedulers for the various traffic classes.

Two sets of simulation experiments are performed: first EDF for all traffic classes and second WFQ for all traffic classes. It has been reported in these studies that, if the total traffic load is under 100%, both schedulers can satisfy traffic class performance requirements. However, when one traffic class starts consuming more than its fair share of bandwidth, EDF favors streams with more crucial deadlines, whereas WFQ strives for fairness and punishes the flow that is unfair. It has been concluded that using either EDF or WFQ for all traffic classes is not optimal.

Based on the fair queueing model proposes a hierarchical scheduling model for WiMAX traffic classes. It specifies three types of scheduling servers, viz., hard-QoS, soft-QoS, and best effort servers. UGS traffic is mapped into the hard-QoS server and rtPS traffic into the soft-QoS server; whereas nrtPS can be mapped into either. All servers implement the WF²Q algorithm.

A token bucket based call admission control and an UL packet scheduler employing EDF. The focus of their algorithm is to meet the bandwidth and delay requirements of rtPS flows. It employs the arrival service curve and a two-dimensional rtPS database. However, this algorithm does not consider the ertPS traffic class. A token rate estimation model, for Poisson traffic fed to both an infinite and a finite queue, has been developed by employing well-known Markov chain models. The integration of admission control and scheduling has been left as future work. Thus, this approach needs to be investigated further before it can be adopted for use in WiMAX. It is also known that a potential problem with token bucket-based schemes involves allowing large bursts of traffic, for which a leaky bucket can be arranged after the token bucket 

SCHEDULING SETUP IN WiMAX

IEEE 802.16e has been developed to serve mobile SSs through a centralized BS by employing point-to-multi point (PMP) as well as through the optional mesh mode architecture of a wireless network topology. In the former operating mode, the downlink from a BS to an SS operates on the basis of PMP. However, in the mesh mode, there are no separate DL and UL subframes and there can be SSs that are not directly connected to the BS but only through intermediary SSs, which is in contrast to the PMP mode. Hence, a larger number of SS can be supported in the mesh mode than in the PMP mode or equivalently, mesh mode can offer the least number of BSs for economy. Furthermore, in the mesh mode of WiMAX the SSs can consume less power, thereby efficiently using battery life, as it is not mandatory to be always connected to the BS. The intermediate SSs will greatly reduce the power consumption of far off users.

Three types of scheduling are supported in the mesh mode of WiMAX; they are coordinated distributed, uncoordinated distributed, and centralized scheduling. In coordinated scheduling, all nodes coordinate in their two-hop neighborhood and broadcast their schedules (available resources, requests and grants) to all of their neighbors; whereas, in uncoordinated scheduling there are direct uncoordinated requests and grants between two nodes. The main difference between the two types of distributed scheduling methods is in the use of the control subframe: transmitting collision-free scheduling messages in the coordinated type and with possible collision in the uncoordinated type. In the centralized method, the resources are distributed centrally and this is similar to the PMP case.

The performance of coordinated distributed scheduling has been investigated. It has been reported that this mechanism has a scalability problem that leads to poor performance in dense networks and aggravates QoS provisioning. To overcome these problems, the XmtHoldoffTime has been made adaptive at every node, which has been shown to improve contention, and thus enhance the throughput in dense meshes. A combined distributed and centralized scheduling scheme has been proposed for mesh networks in WiMAX; wherein, through simulation studies, it has been shown that the minislot^[*] utilization can be significantly improved with the proposed scheme.

For synchronization of distributed and centralized control mesh networks, the WiMAX standard provides network configuration (MSH-NCFG) and network entry (MSH-NENT) packets as a basic level of communication between various nodes. The scheduling of transmission for the next MSH-NCFG is done by a mesh-election procedure. It is carried out among all eligible competing and local nodes. The NetEntry scheduling protocol provides slots for transmission of MSH-NENT packets by new nodes that are not yet fully functional members of the mesh .

In contrast to mesh mode, a detailed QoS architecture has been defined for the PMP mode. Scheduling services refer to data-handling mechanisms supported by the MAC scheduler for data transport on each connection. A single scheduling service will be associated with each data connection. Each of the data services will be characterized by a set of parameters that will quantify the QoS aspects of its behavior. These QoS parameters are managed by dynamic service addition (DSA), dynamic service change (DSC), and dynamic service deletion (DSD) message dialogues, where each of these signaling schemes can be initiated by either a BS or an SS.

It can be seen that scheduling mechanisms for a PMP mode are also applicable to a mesh mode; however, since all transmission between two nodes is managed by a link, PMP scheduling is not directly applicable to the mesh mode. By default, at the time of connection establishment, each mesh SS is assigned a unique node identifier; a Service Adaptive QoS has been proposed for mesh mode, which assigns five node IDs to each SS instead of a single ID. These five virtual nodes correspond to five traffic classes, and each node requests bandwidth individually and the mesh mode BS handles these requests on the basis of their scheduling services. Hence, mesh mode WiMAX can be treated by scheduling the services of the PMP mode. Therefore, subsequently in this chapter, we shall only consider scheduling in the PMP mode for WiMAX networks.