Wavelength Routing Networks

A scalable optical network can be constructed by taking several WDM links and connecting them at a node by a switching subsystem. Using such nodes (also called wavelength routers) interconnected by fibers, diverse networks with complex and large topologies can be devised (Fig. 3).

Each wavelength router makes its routing decision based on the input port and wavelength of a connection going through it. Thus, if a light signal enters a router at a port x it is switched to some output port y. At the other end of the fiber, attached to y, the signal enters another router in which a similar routing decision is made. This process continues until the signal is switched to an output port of the system (Fig.1). Another optical signal coming into the same router on a different wavelength will be routed differently. Such an end-to-end connection is called a lightpath, and it provides a high-speed transparent pipe to its end users. At the same time, another lightpath can reuse the same wavelength in some other part of the network, as long as both lightpaths do not use it on the same fiber. Since such "spatial reuse" of wavelengths is supported by wavelength routing networks, they are much more scalable than broadcast-and-select networks.

Another important characteristic which enables these networks to span long distances is that the energy invested in a lightpath is not split to irrelevant destinations. There is a large diversity of capabilities that a wavelength router can provide, depending on the components in use and design of the node. Most notably, nodes may provide

· configurable lightpaths versus
· fixed routing,
 
· full wavelength conversion versus
· limited conversion versus
· no conversion at all,
 
· fault tolerance in the optical layer versus
· reliance on higher layers.
 

Nodes also vary in their

· scalability to increasing numbers of local or network ports.
 

Node design options

As for the design of the node itself, current commercial technology enables either of the first two of the following designs (Fig.6). The third design relies on large optical switches and wavelength converters, a technology far from commercially available and therefore a longer-term option:

· Electro-Optical Node
· Simple All-Optical Node
· Full-Conversion All-Optical Node

Figure 6 - Three designs for a wavelength routing node

Electro-Optical Node

Converts the optical signal into the electrical domain, performs the switching in this domain, and regenerates the optical signal at the outputs (Fig 6a). This design easily enables wavelength conversion and maintains a high-quality signal for multiple hops. On the other hand, it does not support transparency. This design represents an evolutionary phase toward all-optical networks.

Simple All-Optical Node

Separates the different wavelengths from each input and sends all channels of _i to the same switch, which optically switches them to the output ports (Fig. 6b). This design does not allow wavelength conversion, thereby restricting the reuse of wavelengths in the system. This may prove to be a cost-effective solution because it does not require a (costly) transceiver per channel per node.

Full-Conversion All-Optical Node

Enables each wavelength to be converted to any other wavelength. It is based on a large optical switch which takes a channel and switches it to any other channel (on any fiber). Before being multiplexed into the fiber, each channel is converted to the appropriate wavelength by fixed wavelength converters (Fig.6c).

Figure 7 - The architecture of a hybrid fiber-wavelength-packet (FWP) switching node of the OTN.

Enabling Wavelength Routing Technologies

To enable wavelength routing networks, the maturity of the device technology needed to manufacture tunable and switched sources, tunable filters, wavelength converters, wavelength routers, and switching elements is a key issue. Most recent reported analyses indicate that, for instance, some of these have a high maturity and can easily be inserted in real systems (e.g., tunable filters); some others are still not mature enough for employment in practical systems (e.g., wavelength converters). However, we can expect that in the following two or three years most of them will gain a technological maturity which will allow the implementation of more complex WDM systems.

Most WDM network architectures presented so far are based on static or semi-permanent wavelength routing. This means that the status of the network and its devices changes very slowly with time, on the order of hours or days.

Nowadays, advanced network architectures (intelligent) are hypothesized, which will allow more flexible and dynamic use of wavelength resources, depending on the variation of traffic dynamics (Fig.7). This is particularly true in the case of optical networks for data traffic (e.g. optical Internet, or optical networks which interconnect several IP routers).

A relevant example is represented by multi-protocol lambda-switched optical networks, which are optical networks compatible with the multi-protocol label switching scheme proposed for Internet routing. In this case, dynamic routing in a timeframe of seconds or even less is required. WDM networks which employ such dynamic and flexible routing schemes need wavelength agility, that is, the property of optical devices to rapidly change their working conditions.

Wavelength agile devices have already been demonstrated. In particular, burst mode operating receivers and agile wavelength converters have been realized.

The wavelength agility characteristic requires technological efforts to render such devices reliable and well performing. This means that much effort should be made to push the maturity of the technology at a reasonable level. Wavelength agility is the key function for realizing wavelength/time-division multiple access (WDMA/TDMA) access systems, which require fast tunable transmitters and receivers to set up individual customer connections through a single wavelength router (suitably replicated for resilience). In such networks connection among users is realized by a double dimension resource: wavelength and time slots.

A further step is represented by the realization of devices suitable for optical packet switching (OPS). In this case it is much more difficult to foresee when and if the maturity of these devices will be such that OPS networks can practically be realized.

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