Communications technology has seen many advances. Telephony is still here (albeit now mostly digital), but it is apparent that with the advent of the Internet, a large portion of traffic now consists of data rather than voice. Still, the concepts of the “old” telephony world are still in use. In essence, classical telephony is a circuit-switched concept: communication between two parties is realized by establishing a connection, which is reserved for only their use throughout the duration of their conversation. Prior to communication, signaling takes place through the exchange of messages to set up the connection through the various switches on the path between the two parties. This same idea of connection-oriented communications prevails today, and a circuit-switched approach is also taken in so-called backbone networks to provide high-bandwidth interconnections between, for example, telephone private branch exchanges (PBXs). However, in the Internet world, a packet-switched concept dominates. Instead of reserving a certain amount of bandwidth (a circuit) for a certain period of time, data are sent in packets. These packets have a header containing the information necessary for the switching nodes to be able to route them correctly, quite similar to postal services [1].

To provide the bandwidth necessary to fulfill the ever-increasing demand (Internet growth), the copper networks have been upgraded and nowadays to a great extent replaced with optical fiber networks. Since the advent of optical amplifiers (erbiumdoped fiber amplifiers, EDFAs) allowed the deployment of dense wavelength division multiplexing (DWDM), the bandwidth available on a single fiber has grown significantly. Whereas at first these high-capacity links were mainly deployed as point-to-point interconnections, real optical networking using optical switches is possible today. The resulting optical communication network is still exploited in a circuit-switched manner: so-called lightpaths (making up an entire wavelength) are provisioned [1]. Optical cross-connects (OXCs) switch wavelengths from their input to 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 the physical topology (containing WDM link,), as indicated in Figure 11.1 [1]. These links in the logical plane thus have wavelength capacity. 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 lightpaths 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.

Figure 11.1 WDM, logical links consist of wavelength(s) on these fibers interconnected via OXCs, such as logical circuit switching with OXCs. Physical links (black lines) carry multiple wavelengths in (D)link IP2–IP3 using OXC1 (dotted).

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 only effectively consumed when data are 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 [1].

Figure 11.2 Optical packet switching: a network with packets rather than the circuits shown in Figure 11.1.

Therefore, during the past decade, various research groups have focused on optical packet switching (OPS), aimed at more efficiently using the huge bandwidths offered by WDM networks. The idea is to use optical fiber to transport optical packets rather than continuous streams of light as sketched in Figure 11.2 [1]. Optical packets consist of a header and a payload. In an OPS node, the transported data (payload) are 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 (IP traffic) will be aggregated into fairly large packets. To unlock the possibilities of OPS, several issues arise and are being solved today. A major issue is the lack of optical random access memory (RAM), which would be very welcome to assist in a contention resolution that arises when two or more packets simultaneously want to use the same outgoing switch port. Still, workarounds for the contention resolution problems have been found in optics [1]. Since the timescales at which a switch fabric needs to be reconfigured in OPS are much smaller than in, say, the ASON case, other switching technologies have been devised to unlock the possibilities of OPS. These packetswitched networks can be operated in two different modes: synchronous, in which packets can start at only certain discrete moments in time, and in each timeslot packets on different channels are aligned; and asynchronous, in which packets can arrive at any moment in time, without any alignment.

The major architectures for OPS switches will be discussed shortly. To be competitive with other solutions (electronic or ASON-like), the OPS node cost needs to be limited, and the architectures should be future-proof (scalable). In this context, the driving factors that lead to multistage architectures were reducing switch complexity (thus cost) and circumventing technological constraints [1].