Photonic Network Communications, 1, 49±64 (1999)
# 1999 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands.
Recon®guration and Dynamic Load Balancing
in Broadcast WDM Networks*
MCNC, RTP, NC 27709, USA
George N. Rouskas
Department of Computer Science, North Carolina State University, Raleigh, NC 27695-7534, USA
Received September 10, 1998; Revised November 20, 1998
Abstract. In optical WDM networks, an assignment of transceivers to channels implies an allocation of the bandwidth to the various network
nodes. Intuition suggests, and our recent study has con®rmed, that if the traf®c load is not well balanced across the available channels, the result
is poor network performance. Hence, the time-varying conditions expected in this type of environment call for mechanisms that periodically
adjust the bandwidth allocation to ensure that each channel carries an almost equal share of the corresponding offered load. In this paper we
study the problem of dynamic load balancing in broadcast WDM networks by retuning a subset of transceivers in response to changes in the
overall traf®c pattern. Assuming an existing wavelength assignment and some information regarding the new traf®c demands, we present two
approaches to obtaining a new wavelength assignment such that (a) the new traf®c load is balanced across the channels, and (b) the number of
transceivers that need to be retuned is minimized. The latter objective is motivated by the fact that tunable transceivers take a non-negligible
amount of time to switch between wavelengths during which parts of the network are unavailable for normal operation. Furthermore, this
variation in traf®c is expected to take place over larger time scales (i.e., retuning will be a relatively infrequent event), making slowly tunable
devices a cost effective solution. Our main contribution is a new approximation algorithm for the load balancing problem that provides for
tradeoff selection, using a single parameter, between two con¯icting goals, namely, the degree of load balancing and the number of transceivers
that need to be retuned. This algorithm leads to a scalable approach to recon®guring the network since, in addition to providing guarantees in
terms of load balancing, the expected number of retunings scales with the number of channels, not the number of nodes in the network.
Keywords: broadcast optical networks, wavelength division multiplexing (WDM), recon®guration, dynamic load balancing
Single-hop lightwave networks have been proposed
for Local and Metropolitan Area Networks (LANs
and MANs) [1,2]. The single-hop architecture
employs Wavelength Division Multiplexing (WDM)
to provide connectivity among the network nodes.
The WDM channels are dynamically shared by the
attached nodes, and the logical connections change on
a packet-by-packet basis creating all-optical paths
between sources and destinations. Single-hop net-
works require the use of rapidly tunable optical lasers
and/or ®lters that can switch between channels at high
speeds. Such devices do exist today ; however, they
have to be custom-built and they tend to be extremely
expensive, accounting for a signi®cant fraction of the
overall cost of building a lightwave network.
Consequently, media access protocols such as
HiPeR- , FatMAC , DT-WDMA , and
Rainbow  that require tunability only at one end
have the potential of keeping the overall cost at
reasonable levels, leading to network architectures
that can be realized cost effectively.
When tunability only at one end, say, at the
transmitters, is employed, each ®xed receiver is
permanently assigned to one of the wavelengths
used for packet transmissions. In a typical near-term
WDM environment, the number of channels that will
be supported within the optical medium is expected to
be smaller than the number of attached nodes. As a
result, each channel will have to be shared by multiple
receivers, and the problem of assigning receive
*This work was supported by the NSF under grant NCR-9701113.
This work was performed while the author was with the Department of Computer Science, North Carolina State University.