Chapter 1 SCHEDULING ALGORITHMS FOR UNICAST, MULTICAST, AND BROADCAST George N. Rouskas Department of Computer Science North Carolina State University Raleigh, NC 27695-7534
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Abstract
In this chapter we present a survey of algorithms for scheduling packet traffic in broadcast optical WDM networks. We first describe the context and motivations of the scheduling problem. We then review the current literature in the field with an emphasis on scheduling techniques for providing best-effort service as well as guaranteed service for both unicast and multi-destination traffic. We provide alternative formulations of the problem, and we compare the formulations and theoretical results, as well as algorithms and heuristics.
Keywords: Broadcast optical networks, Wavelength division multiplexing (WDM), Scheduling algorithms, Open shop scheduling, Quality of service (QoS), Multicast
1.
INTRODUCTION
The broadcast WDM network architecture has been widely studied as an approach to building optically interconnected networks. Under one widely adopted scenario for the evolution of the optical network infrastructure (Kam et al., 1998), broadcast WDM subnetworks will be used to provide local access, and the subnetworks will be interconnected through wavelength routed MANs and WANs. One of the potentially difficult issues that arise in a broadcast WDM environment, is that of coordinating the various transmitters/receivers. Some form of coordination is necessary because (a) a transmitter and a receiver must both be tuned to the same channel for the duration of a packet’s transmission, and (b) a
1
2 Transmitting side
Receiving side
Node 1
Node 1 tunable laser
tunable filter
λ ... λ 1
λ ... λ 1
C
C
. . .
. . . Node N
Node N passive star
λ ... λ 1
Figure 1.1
C
λ ... λ 1
C
A broadcast WDM network with N nodes and C channels
simultaneous transmission by one or more nodes on the same channel will result in a collision. The issue of coordination is further complicated by the fact that tunable transceivers may need a non-negligible amount of time to switch between wavelengths. Thus, at the heart of media access control (MAC) protocols for broadcast WDM subnetworks is a scheduling algorithm responsible for coordinating access to the available channels (wavelengths). In this chapter we survey a number of algorithms for traffic scheduling in a packet-switched broadcast WDM with N nodes and C channels, N ≥ C, as shown in Fig. 1.1. Unless otherwise specified, it is assumed that each node has exactly one tunable transmitter and one tunable receiver. We let ∆ denote the transceiver tuning latency, i.e., the time it takes a transmitter or receiver to tune from one wavelength to another. Packets in the network are of fixed size. Time is slotted, with a slot time equal to the packet time plus some guard band, which depends on the MAC protocol and the corresponding scheduling algorithm. The chapter is organized as follows. In Section 2. we discuss the role of scheduling within the context of broadcast WDM networks, and we examine its relationship to reservation protocols and load balancing. In Section 3. we review scheduling algorithms for both best-effort and guaranteed-service unicast traffic. In Section 4. we discuss approaches to scheduling multi-destination traffic. We conclude the chapter in Section 5..
Scheduling Algorithms for Unicast, Multicast, and Broadcast
2.
3
LOAD BALANCING, RESERVATIONS, AND SCHEDULING
While in this chapter we are mainly concerned with scheduling algorithms, one should keep in mind that, in ensuring an acceptable level of network performance, scheduling is only one piece of the puzzle. In this section we briefly review two other components critical to the operation of broadcast WDM networks, namely, load balancing and reservation protocols, and we discuss their relationship to scheduling. We distinguish two levels of network operation, differing mainly in the time scales at which they take place. At the media access control level, connectivity among the network nodes is provided by a reservation protocol, whose main function is to collect information regarding traffic demands, and a scheduling algorithm whose objective is to provide collision-free communication among the nodes while optimizing some performance measure of interest (e.g., schedule length). At the network dimensioning level, which takes place at significantly longer time scales, the objective is to allocate resources in a way that optimizes network performance. In this context, the shared resource of interest is bandwidth, and load balancing algorithms are needed to ensure good performance and fairness at this level of network operation.
2.1
LOAD BALANCING AND RECONFIGURATION
In optical WDM networks, each channel will have to be shared by multiple receivers, and the problem of assigning receive wavelengths arises. A wavelength assignment (hereafter referred to as WLA) implies an allocation of the bandwidth to the various network nodes. Intuition suggests that if the traffic load is not well balanced across the available channels, the result will be poor network performance. A recent study on the performance of the HiPeR-` reservation protocol (Sivaraman and Rouskas, 1997) has confirmed this intuition. Let us define parameter b such that b) times the total traffic offered to the no channel carries more than (1+ C network. In other words, b is a measure of the degree of load balancing of the network; under perfect load balancing, b = 0. It was shown in (Sivaraman and Rouskas, 1997) that the maximum sustained throughput γ (i.e., the number of packets successfully transmitted per packet time) is directly affected by b through the following stability condition:
γ
