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A case for using hybrid optical switching in long-haul networks

Posted: 16 Jul 2003 ?? ?Print Version ?Bookmark and Share

Keywords:hybrid optical switching? long-haul network? fibre connection? ulr? edfa?

There is a fundamental difference between using transparent elements at line sites--degree two junctions with only two fibre directions--and using them at switch sites that have a degree greater than two with three or more fibre directions. If we are optimizing total network costs, we need to consider the impact that the switches will have on the line system costs.

The first such impact is on the reach requirement of the line systems. We might imagine that this is a simple matter of understanding the distribution of demand distances, and then using the appropriate ultra-long reach (ULR) transmission technology so that most of those demands are met without using regenerators.

This is slightly disingenuous. The extra reach does not come free. ULR can be achieved by using better transponders than in LR systems, or by using distributed Raman amplifiers in conjunction with the normal EDFA, or by using both techniques together. The point is that both techniques increase costs.

The bottom line is: savings that might be expected by reducing the number of transponders in a transparent network are offset by the higher cost of the ULR transponders and line systems required. We save less than might be expected just by counting transponders.

Switching is used to make more efficient use of the transmission systems in three ways: sharing working bandwidth when the demands change, sharing protection bandwidth and grooming demands to pack wavelengths more efficiently.

Generally, protection path lengths are longer than the shortest path from source to destination because the shortest path is used for the working bandwidth. To be effective against backhoe fade, the protection path must be different from the working path. In some cases, the protection path may be longer than the working path, depending on the fibre topology of the network in question. And, if we are doing the protection switching transparently, then the transmission path from laser to detector will change on the protection switching event. If we are using some of the more efficient protection schemes, then the protected path length may not be easily predictable.

Regenerating the signal

Since we cannot achieve infinite reach, even for infinite money, there will be cases where the required reach exceeds the available reach, following a protection switching event or even a normal demand routing event. In this case, we must regenerate the signal, and the only cost-effective way to do that is with a transponder.

Transparently switched networks cannot groom. This does not matter if the services provided by the network are simple bit-streams at the modulation rate of the wavelengths (10G or 40G), and if all the services and transponders are the same across the network.

Is there any cost saving to be made by using transparent elements at the degree two nodes? It is a good question, and the answer is that considerable cost can be removed by using optical add/drop multiplexer (OADM) network elements at these locations, if the percentage of add/drop traffic is less than maybe 70 percent of the total traffic through that node. There are still challenges, but less severe ones.

To be efficient in a dynamic demand environment, the OADMs need to be reconfigurable.

Many believe that the high costs of transparent networks are simply a function of the maturity of the optical components used, and that over time, transparent networks will become cheaper than hybrid networks. This turns out not to be the case. It is not the costs of the switches that matter; almost all of the network costs are in the line systems, and most of the line system costs are in the transponders.

Transponder for life

The first future cost problem that transparent networks face concerns the rapid evolution of transponder technology. A non-blocking transparent network can only use one transponder type. This means that once chosen, the same transponders must be used for the life of the network.

In contrast, the opaque or hybrid networks can use new technology on a link by link basis, and with some constraints, even on an existing link. We are always free to optimize the transmission technology for lowest cost for each link, at any time.

Therefore, over time, the transparent network built today will be at an increasing cost disadvantage to the hybrid or opaque networks. But transponder technology is evolving very fast, and the cost of building a 10G LR transponder has dropped by a factor of ten over the last few years. There are developments in process which will give us another factor of ten over the next few years, with another tenfold decrease conceivable after that. So, the "$100 transponder" is at least conceivable.

However, the technologies that allow this kind of radical cost reduction do not allow ULR performance. And the economics of ULR are balanced on a knife edge. It would always be cheaper to regenerate. And since transparent networks depend on ULR transmission for their economics, they will be at an increasing disadvantage.

This has nothing to do with the cost or scalability or reliability of MEMS switch fabrics. Even if transparent switches were free, infinitely scalable and infinitely reliable; it would still be cheaper not to use them.

- Ian Wright

Chief Technology Officer

Altamar Network

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