The need to
transmit information in large volumes and in more compact forms
is felt these days more than ever before. To provide the bandwidth
necessary to fulfill the ever-increasing demand, the copper networks
have been upgraded and nowadays to a great extend replaced with
optical fiber networks. Though initially these were deployed as
point-to-point interconnections, real optical networking using
optical switches is possible today. Since the advent of optical
amplifiers allowed the deployment of dense wavelength division
multiplexing (DWDM), the bandwidth available on a single fiber
has grown significantly.
Optical communication
can take place in one of the two ways - either circuit switching
or else packet switching. In circuit switching, the route and
bandwidth allocated to the stream remain constant over the lifetime
of the stream. The capacity of each channel is divided into a
number of fixed-rate logical channels, called circuits. Optical
cross connects (OXCs) switch wavelengths from their input ports
to their output ports. To the client layer of the optical network,
the connections realized by the network of OXCs are seen as a
virtual topology, possibly different from physical topology (containing
WDM links). To set up the connections, as in the old telephony
world, a so called control plane is necessary to allow for signaling.
Enabling automatic setup of connections through such a control
plane is the focus of the work in the automatically switched optical
network (ASON) framework. Since the light paths that have to be
set up in such an ASON will have a relatively long lifetime (typically
in the range of hours to days), the switching time requirements
on OXCs are not very demanding.
It is clear
that the main disadvantage of such circuit switched networks is
that they are not able to adequately cope with highly variable
traffic. Since the capacity offered by a single wavelength ranges
up to a few tens of gigabits per second, poor utilization of the
available bandwidth is likely. A packet switched concept, where
bandwidth is effectively consumed when data is being sent, clearly
allows more efficient handling of traffic that greatly varies
in both volume and communication endpoints, such as in currently
dominant internet traffic.
In packet
switching, the data stream originating at the source is divided
into packets of fixed or variable size. In this method the bandwidth
is effectively consumed when data is being sent and so allows
a more efficient handling of traffic that greatly varies in both
volume and communication endpoints.
In the last
decade, various research groups have focused on optical packet
switching (OPS), aimed at more efficiently using the huge bandwidths
offered by such networks. The idea is to use optical fiber to
transport optical packets, rather than continuous streams of light.
Optical packets consist of a header and a payload. In an OPS node,
the transported data is kept in the optical domain, but the header
information is extracted and processed using mature control electronics,
as optical processing is still in its infancy. To limit the amount
of header processing, client layer traffic (e.g., IP traffic)
will be aggregated into fairly large packets.
To unlock
the possibilities of OPS, several issues arise and are being solved
today. To be competitive with the other solutions, the OPS cost
node needs to be limited, and the architectures should be future
proof (i.e., scalable). In this context, the work of Clos on multistage
architectures has been inspiring.
