Photonic Network Communications, 1:2, 161±178 (1999)
# 1999 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands.
Ef®cient Scheduling Algorithms for Real-Time Service on WDM
M. Ma, B. Hamidzadeh, M. Hamdi*
Department of Computer Science, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
Received July 17, 1998; Revised December 21, 1998
Abstract. One of the important issues in the design of future generation high-speed networks is the provision of real-time services to different
types of traf®c with various time constraints. In this paper we study the problem of providing real-time service to hard and soft real-time
messages in Wavelength-Division-Multiplexing (WDM) optical networks. We propose a set of scheduling algorithms which prioritize and
manage message transmissions in single-hop WDM passive star networks based on speci®c message time constraints. In particular, we develop
time-based priority schemes for scheduling message transmissions in order to increase the real-time performance of a WDM network topology.
We formulated an analytical model and conducted extensive discrete-event simulations to evaluate the performance of the proposed algorithms.
We compared their performances with that of the state-of-the-art WDM scheduling algorithms which typically do not consider the time
constraint of the transmitted messages. This study suggests that when scheduling real-time messages in WDM networks, one has to consider not
only the problem of resources allocation in the network but also the problem of sequencing messages based on their time constraints.
Keywords: optical networks, Wavelength-Division-Multiplexing (WDM), real-time scheduling, multiple-access protocols
With the proliferation of the world-wide-web
(WWW) in all aspects of networking, current local
and wide area networks can barely cope with the huge
demand for network bandwidth. As a result, there is
a world-wide effort in upgrading current networks
with high-bandwidth ®ber-optic links that can
potentially deliver Tera-bits/sec. Wavelength-
Division-Multiplexing (WDM) is an effective tech-
nique for utilizing the large bandwidth of an optical
®ber. This technique, by allowing multiple messages
to be transmitted in parallel on a number of channels,
has the potential to signi®cantly improve the
performance of optical networks. The nodes in such
a network can transmit and receive messages on any
of the available channels by using and tuning one or
more tunable transmitter(s) and/or tunable receiver(s).
The system can be con®gured as a broadcast-and-
select network in which all of the inputs from various
nodes are combined in a WDM passive star coupler,
and the mixed optical information is broadcast to all
destinations. Several topologies have been proposed
for WDM networks [1,2]. The most popular one is the
single-hop passive star-coupled topology .
To unleash the potential of single-hop WDM
passive star networks, ef®cient multiple-access pro-
tocols and scheduling algorithms are needed to
allocate and coordinate the system resources opti-
mally, while satisfying the message and the system
constraints . The multiple-access protocols in the
single-hop WDM passive star network environment
can be divided into two main classes, namely pre-
allocation-based and on-demand adaptive protocols.
Pre-allocation-based techniques use all channels of a
®ber to transmit messages. These techniques assign
transmission rights to different nodes in a static and
pre-determined manner. The examples of prealloca-
tion-based protocols can be found in [3±6]. On the
other hand, there are two types of common on-
demand adaptive protocols. One is the token-passing
protocol. The other is the reservation-based protocol,
examples of which can be found in [7±11].
Reservation-based techniques allocate a channel as
the control channel to transmit global information
regarding messages to all nodes in the system. Once
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*Author for Correspondence: M. Hamdi, E-mail: email@example.com