Traffic Scheduling

Our proposed service policy is achieved by uplink/downlink traffic schedulers located at a base station which schedule uplink/downlink traffic in order to fulfil the services committed to flows. Even though uplink and downlink support the same service classes, the ways that both the traffic schedulers work should be different because the downlink is a broadcast channel while the uplink is a shared channel. We explain below how both the traffic schedulers operate to achieve the committed service. The detail queuing disciplines and multiple access protocol will be covered in the following chapters.

T o W ired N etw ork

From W ired N etw ork

Proxy P acket Generator

A C ,W C , BC packet AC, W C, BC packet PC packet PC packet PC A W F Q PC A W FQ U plink Scheduler D ow n lin k Scheduler Slot T ype Selector Priority Q ueuing Data Packet Re-queuing Data Packet Proxy packet or Signal Re-queuing

Slot A llocation C ontroller Reservation Request

A

U plink S lot B roadcast D o w n lin k Slot

Figure 4.4 The proposed traffic scheduling system for a base station

4 .3 .1 .1 D ow nlink traffic scheduling

To achieve the committed class services in a downlink, we adopt a downlink scheduler which multiplexes downlink traffic coming from fixed networks. The downlink scheduler consists o f PCA W F Q and FCFS (First Come First Served) scheduling disciplines. The PCA W FQ, which we develop and explain details in Chapter 5, serves the three classes o f traffic - absolute, weighted and best-effort - as we defined in the above section, and the FCFS serves periodic traffic.

The data packets may be fragmented or assembled to the slot size before being queued in the downlink scheduler, and the downlink scheduler places the incoming packets in either FCFS or PCAWFQ queue according to the packet class type. FCFS always has higher priority than PCAWFQ in using downlink slots. Thus, PCAWFQ uses the downlink slots only when FCFS queue is empty: i.e. the PCAWFQ uses the residual slots after serving the periodic traffic packets. Whenever SAC assigns a slot to the downlink, the downlink scheduler chooses one queue among both queues according to the explained rule and transfers the packet at the head of the chosen queue to SAC. Then SAC broadcasts the downlink slot data to the mobile host. Figure 4.4 illustrates how the downlink scheduler works.

4.3.1.2 Uplink traffic scheduling

A multiple access protocol enables mobile hosts to share a common channel. To provide uplink flows with QoS, a multiple access protocol should associate with a scheduling discipline which can schedule uplink channel slots to convey packets scattered over the mobile hosts according to the committed QoS. Even though many multiple access protocols [42], [69], [64], [97], [13], [4], [83] have been proposed, those cannot be used in our proposed service architecture since they support only simple QoS. Therefore, we propose a slot based FQMA (Fair Queuing Multiple Access) protocol, which can serve uplink traffic according to our proposed class service policy.

Although the FQMA performs uplink traffic scheduling to achieve the same class based services as a downlink does, the used methods are slightly different from those of the downlink since the uplink is a shared medium. Unlike the downlink traffic scheduling, we do not know whether or not the queues of uplink sources in the mobile hosts are empty or how long their packets have waited for service. This information is essential to serve the periodic traffic with higher priority than random traffic, maximizing channel utilization. For instance, an uplink slot may be wasted if it is assigned to a periodic source which queue is empty, or it is difficult to decide when is the time to give an uplink slot to random traffic. Thus without those information we cannot serve random traffic without affecting the periodic traffic service, maximizing the channel utilization. FQMA can estimate the periodic traffic queues status of mobile hosts using parameters passed from mobile hosts. Based on this parameters, Slot Type Selector (STS) shown in Figure 4.4 allocates uplink slots to both traffic achieving those goals. The details of FQMA and STS will be presented in Chapter 6.

We propose two scheduling disciplines to allocate uplink slots in FQMA: PEDQ (Packet Endurable Delay Queuing) and PCAWFQ'. The PEDQ schedules the uplink periodic traffic in

ascending order of the remaining lifetimes of packets to maximise channel usage - we will explain PEDQ in detail in Chapter 6. It is more efficient than FCFS, which is adopted for the downlink periodic traffic scheduling. However, we cannot apply PEDQ to the downlink scheduling because we cannot estimate the remaining lifetime of a downlink packet. The packet header of a usual transport protocol does not embody any clue to calculate it. Although uplink periodic traffic scheduling performs better than the FCFS since it can use more information from the mobile hosts through MAC level packet header, the uplink periodic traffic scheduling should spend additional channel costs for contention among mobile hosts and communicating with mobile hosts. After all, the uplink and downlink scheduler for periodic traffic have performance reduction factor respectively: i.e. the uplink scheduler wastes channels due to channel sharing, while the downlink scheduler does since it cannot access the remaining lifetime of packets. We will show how the negative factors affect both scheduler performances in Chapter 6.

PCAWFQ' is a modified PCAWFQ and is designed for scheduling uplink random traffic. Since PCAWFQ is designed for single queue, it cannot be applied to the uplink scheduling directly. The PCAWFQ' schedules uplink random packets as if real packets are queued in a single queue using proxy packets which are representing the real packets in the mobile hosts. After all, PCAWFQ' is the same discipline as PCAWFQ except that it uses proxy packets. We will explain the PCAWFQ' in detail in Chapter 6. The uplink schedulers in Figure 4.4 illustrate what we have presented in this section.

Meanwhile, when a mobile host receives its uplink slot, it transmits a slot of data. The data arrived at the base station is queued in the FCFS queue as can be seen in Figure 4.4 and transmitted to the wired network.