Looking ahead to the increasing availability of fiber paths and the customers who need them to serve their high-bandwidth needs, the all-optical networking community is hard at work trying to open up more and more of the 25,000 GHz of fiber bandwidth to convenient and economical access to end users. Already, Pirelli has shipped a four-wavelength system and ATandT has announced an 8-wavelength system. We at IBM recently announced the 9729 20-wavelength system, and have already installed many units worlwide. Our customers find these systems very useful for achieving fiber rental cost savings. The rationale behind both all these commercialization efforts involves not only getting more bandwidth out of existing fiber, but making the installation "multi-protocol" or "future-proof" by taking advantage of the fact that each wavelength can carry an arbitrary bit rate and framing convention, up to some maximum speed set by the losses on the link.
These successful realizations of simple multi-wavelength links represent the simplest case of the three different kinds of all-optical systems, as shown above. In addition to the two-station WDM link (with multiple ports per station), the figure shows the two forms taken by full networks, structures in which there are many stations (nodes), with perhaps only one or a few ports per node.
The second type, the broadcast and select network usually works by assigning to the transmit side of each node in the network a fixed optical frequency, merging all the transmitted signals at the center of the network in an optical star coupler and then broadcasing the merge to the receive sides of all nodes. The entire inner structure consisting of fiber strands and the star coupler is completely passive and unpowered. By means of a suitable protocol, when a node wants to talk to another (either by setting up a fixed lightpath "circuit" or by exchanging packets, the latter's receiver tunes to the former's transmit wavelength and vice versa. Broadcast and select networks have been prototyped and, while still considered not quite in the commercialization cost range, have been used in live application situations, for digital video distribution (Rainbow-1, 1991) and for gigabit supercomputer interconnection at 1 gigabit per second rates (Rainbow-2, 1995).
Aside from high cost, which is currently a problem with all WDM systems, there are two other things wrong with broadcast and select networks. The power from each transmitter, being broadcast to all receivers, is mostly wasted on receivers that do not use it. Secondly, the number of nodes the network can have can be no larger than size of the wavelength pool, the number of resolvable wavelengths. Today, even though there are 25,000 gigahertz of fiber capacity waiting to be tapped, the wavelength resolving technology is rather crude, allowing only up to about 100 wavelengths to be built, so far. Both the cost and number of wavelengths problems are gradually being solved, often by the imaginative use of the same tool that brought cost reductions to electronics two decades ago: lithography.
Clearly, a network architecture that only allows 100 nodes does not consitute a networking revolution; some means must be provided for achieving scalability by using each wavelength many places in the network at the same time. Wavelength routing accomplishes this, and also avoids wastage of transmitted power, by channeling the energy transmitted by each node along a restricted path the the receiver instead of letting it spread over the entire network, as with the broadcast and select architecture. As the name implies, at each intermediate node between the end nodes, light coming in on one port at a given wavelength gets routed out of one and only one port. The quantitative tradeoffs and needed distributed control algorithms for wavelength routing networks have now proceeded to the point that a well-defined Optical Layer can be defined and standardized to be substituted for the present Physical Layer of protocol stacks like TCP/IP or ATM.
The components to build broadcast and select networks have been available on the street for four years, but optical wavelength routers are still a reality only in the laboratory. A prototype wavelength routing network was recently demonstrated by Bellcore.
Capacity of broadcast-and-select networks
The ultimate capacity of optical networking is enormous, as shown by the above figure, and especially great with wavelength routing. (next figure)
Capacity of wavelength-routed networks
The first of these two figures shows how one might divide the 25,000 GHz into many low-bitrate connections or a smaller number of higher bitrate connections. For example, in principle one could carry 10,000 uncompressed 1 Gb/s HDTV channels on each fiber. The figure also shows that erbium amplifiers, needed for long distances, narrow down the 25,000 GHz figure to about 5,000 GHz, and also that the commercially available tunable optical receiver technology is capable of resolving no more than about 100 channels.
With broadcast and select networks, the number of supportable connections is limited to the number of available wavelengths in the pool of wavelengths. However, with wavelength routing, the number of supportable connections is the available number of wavelengths multiplied by a {\em wavelength reuse factor} that grows with the topological connectedness of the network. This reuse factor is shown in the last figure. For example, for a 1000-node network of nodes with a number of ports (the degree) equal to four, the reuse factor is around fifty, meaning that with 100 wavelengths, there could, in principle, be fifty connections supportable for each of the 1000 nodes.
As far as the end user is concerned, there is sometimes a preference for circuit switching and sometimes for packet switching. The former provides protocol transparency during the data transfer interval, and the latter provides concurrency (many apparently simultaneous data flows over the same physical port, by the use of time-slicing). In both cases, very large bit rates are possible without the electronics needing to handle traffic bits from extraneous nodes other than the communicating partners. Wavelength routing networks are usually considered to be intrinsically wide area networks, and therefore circuit switched, simply because of the significant propagation time between the controllers of the various wavelength routers.
It seems pretty widely agreed now that while many of us have all been talking about the U.S. National Information Infrastructure or its global counterpart, it has actually been happening under our noses in the form of the Web. As we have seen, the date when all-optical networking could become a commercially practical part of everyday networking depends on three factors, (1) whether the investment will be made to continue or accelerate the installation of fiber to the premises and desktop, (2) whether it proves feasible to reduce component costs by two to three orders of magnitude below today's values, and (3) the extent to which providers offer the fiber paths in the form of "dark fiber", that is, without any electronic conversions between path ends.
This last problem seems to be solving itself in metropolitan and suburban areas, simply by the intense competition between providers, but the problems of long dark fiber paths that cross jurisdictions and require amplification have yet to be faced. The FCC has viewed dark fiber as being equivalent to copper, within the meaning of the Communication Act of 1934; that is, the public interest requires making dark fiber ends available, since one of the monopoly obligations implied by monopoly privileges is that the public should be offered it at a fair price.
The optoelectronic component cost issue is also under active attack. Considering that there are significant efforts under way to use lithography for cost reduction of tunable and multichannel WDM transmitters and receivers, it seems possible to predict a one order of magnitude decrease in price by 2000 and two orders of magnitude by 2005. This implies that, in 2000 and 2005, all the terminal equipment, especially including the optoelectronics for each end of WDM links of 32 wavelengths should cost $15K and 1.5K, respectively, and that the optoelectronics in each node of a broadcast and select network of 32 to 128 nodes should cost $1000 and $100, respectively. If these last numbers are correct, this means that broadcast and select MANs and LANs should be usable by desktop machines some time between 2000 and 2005, since the costs would be competitive with the several hundred dollars people typically spend year after year on each modem or LAN card for PCs. These things are bought by the consumer like TV sets; people have been spending $400-500 for TV sets for years while they evolve into better and better TV sets.
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