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Competing Optical Transmission Technologies


So far we have discussed the technical properties of different optical network architectures and focused the discussion on WDM links and wavelength routing networks. In this section we compare these networks to other high-speed networking solutions which are based on electrical switching. The electrical networking example considered for the purposes of this comparison is the current SDH-TDM networking. The amount of processing required for ATM cell switching does not currently enable it to compete as a very-high-speed alternative (2.5 Gb/s and above).

The following solutions are the main alternatives to very high speed networking (see Fig 9 for a graphical demonstration).



Very-High-Speed SDH TDM Links and Switches


One obvious solution is to push current high-speed TDM networks to even higher speeds. This solution works for fairly high speeds: current SDH links operate at 10 Gb/s (OC-192), and could be pushed up to 40 Gb/s (OC-768). However, the technology seems to already be approaching its limits, as dictated by the maximum speed of current electronics. Optical transmitters and receivers are currently limited to 10 Gb/s speeds. Furthermore, in some cases where old fiber is installed in the ground, polarization mode dispersion limits the bit rate to less than 10 Gb/s, 40 Gb/s for long distances. i


Parallel Fibers, Parallel Electrical Switches


Another straightforward solution is to use parallel fibers between sites connected by lower-speed electrical switches. This is indeed a good idea in places with a rich optical infrastructure. However, for long distances this is still a costly solution, since each of these fibers requires its own set of optical amplifiers every 80120 km. Such equipment constitutes a large portion of the fiber cost, needs to be managed, and considerably complicates the system. This solution is not very scalable, as additional parallel fibers will result in linear increases in the cost. In places where there is not enough fiber, the high costs of laying more fiber in the ground and legal complexities involved in getting the "right-of-way" permission from land owners to install it are the major disadvantages of this solution.



Figure 9 - Alternatives for very high speed links  i


WDM Links, Parallel Electrical Switches


Here, the parallel fibers of the previous solution are replaced by separate channels of a single WDM link. These channels are interconnected by lower-speed existing TDM equipment. An important cost advantage of this solution over the previous one is that there is no need for an amplifier per channel, and all the wavelengths are amplified together by a single optical amplifier. A central advantage of combining optical transmission with electrical switching over all-optical networks (OTNs) is that it is based on today's technology, and is thus cheaper, more reliable, and more flexible.

However, the current deployment of point-to-point WDM technology supports little in terms of networking functionality, and does not yet perform traffic protection or restoration. In cases of fiber cuts or network failures, synchronous digital hierarchy (SDH) equipment would usually provide these functions, with WDM used strictly for fiber capacity expansion.  i


All-Optical Networks-OTN


Recent advances in WDM technology -- WDM add/drop multiplexers (ADMs), optical cross-connects (OXCs) -- with the ability to add, drop, and in effect construct wavelength-switched and wavelength-routed networks, are now beginning to shift the focus more toward optical networking and network-level issues. As such, it presents an attractive opportunity to evolve WDM technology toward an optical networking infrastructure with transport, multiplexing, routing, supervision, and survivability supported at the optical layer.

Wavelength routing networks allow the setup of lightpaths which remain in the optical domain across the network. Thus, they enable the creation of configurable higher-level (logical) topologies based on traffic analysis, and easy reconfiguration as traffic demands change. In any case, it is clear that they offer an almost unlimited upgrade path for the future, which is not the case with the other solutions.  i




The all-optical solution based on OTN has the following advantages over all the other solutions:

  • Transparency

  • Future-Proofness

  • Reduced Processing

  • Reduced Management


Since no electrical processing is involved, wavelength routing networks are not aware of the structure of the data, and can carry diverse protocols and bit coding structures. Electrical solutions carry a single form of traffic and require costly conversion devices from other protocols to the supported standard, which also complicate the management of the network. Another type of transparency supported by wavelength routing networks (although to a lesser extent) is bit rate transparency. Such networks will carry quite a large spectrum of bit rates, up to a maximum rate determined by the design point of the system.


A corollary of the above is that all-optical networks will carry most future protocols at many different bit rates without having to replace components of the network. Thus, the investment in this technology is protected against future developments.

Reduced Processing

Electrical solutions involve considerably more processing than their all-optical counterparts, a fact which implies more hardware or more expensive hardware. Consider a bit in a connection between nodes A and D in Fig 9-a-c. This bit is converted to the electrical domain, re-clocked and processed by each and every node on the path (nodes B and C in this example). In Fig 9d, however, there is a lightpath from node A to node D, and our bit remains in the optical domain at nodes B and C. Thus, the electrical switches of nodes B and C are not bothered by it and can be made smaller, and less costly.

Reduced Management

Whenever a bit is interpreted, an error may occur. In turn, this event must be detected and reported (especially in the telco world, where network management is much less oblivious to such events). Thus, if bits are interpreted only at the border of a network, much less quality-of-service-related management is necessary (fault management is, however, still necessary).




Disadvantages of wavelength routing networks are the following.

  • Immaturity

  • System Design Problems

  • Standards are not ready yet (Fig. 10)


At this stage, optical components are not yet mature. Some of them are technically mature -- for example, distributed feedback (DFB) lasers. Other components are technically immature (e.g., optical switches), as indicated by their large physical dimensions and their less reliable nature. However, it seems that this is not an inherent problem and will be resolved in the near future. i

System Design Problems

Many design issues for wavelength routing network systems are not yet fully understood and solved. Examples of such problems are the wavelength allocation problem and the dynamic gain equalization problem. There has been quite extensive research on wavelength allocation, but the problem is far from being resolved efficiently, even for simple network topologies. This fact limits the scalability of the network, especially if the number of wavelengths per fiber is low.

A much more severe obstacle for having scalable wavelength routing networks is the physical layer design, particularly variation in the signal quality of individual lightpaths, which is very hard to control. Since optical amplifiers do not amplify all wavelengths by the same amount, and some lightpaths travel many hops while others travel a single hop, the energy of some lightpaths may be very low at their destination, while others have high energy. Thus, it is necessary to equalize the gain, for example, by having adaptive filters or transmitting different energy levels depending on the route of lightpaths. To further complicate the picture, the network has to react to sudden changes in the configuration of lightpaths (e.g., due to link failures). This dynamic gain equalization problem is very complex and far from being well understood or solved.i 


State of ITU standards


As obvious from Figure 10, the standards as regards to OTN are not ready yet, and most current solutions are vendor dependent.


Figure 10 - Expected time frame of ITU optical networking standards.  i


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