AON test still needed

The struggle for bandwidth is as old as the telecoms industry and remains a hot topic. The capability of optical fibre and the development of DWDM allowed extremely fast deployment of extra bandwidth which, combined with a very heated investment climate, led to over-capacity in the backbone network (particularly in the US). However, the demand for more bandwidth, driven by continued Internet use, will continue. In developed countries, the next few years will be characterised by solving the current bottleneck in the metro and access networks, while in many developing countries basic backbone infrastructure will continue to be deployed. But the dilemma of service providers - falling profit margins along with rising revenues but faster rising costs and traffic - needs to be solved.

One way of reducing operational expenditure is the all-optical network (AON), which requires an optically transparent cross-connect (OXCs) - currently under development by network equipment manufacturers. These networks will lower cost by reducing the amount of electronics, optimising bandwidth to where it is needed rather than deploying large amounts of redundant bandwidth, and by allowing easy network upgrades and optimisations due to the network being bit-rate and protocol independent. In this sense, the OXC will herald a revolution in the telecom industry, similar to the optical amplifier in the early 1990s, when point-to-point links suddenly became optically transparent.

The AON creates a need for several new test applications:

• The intensive use of wavelengths, including for routing purposes through GMPLS schemes, needs more thorough analysis of the wavelength domain, also in conjunction with functional analysis
• The parallel trend to use the network for higher speeds at 40Gbit/s and beyond opens up the need for new tools for dispersion analysis
• The ability for re-configuration on-the-fly mandates more built-in test as the physical behaviour of individual links changes substantially during operation. Parameters like loss, used wavelengths and dispersion are very hard to compensate up-front in all possible configuration states
• The variable number of optical amplifications of a configurable link leads to new requirements for testing on a per-wavelength basis, e.g. optical signal-to-noise-ratio (OSNR) or dispersion.

Even more important is network management. To manage the network, you need the SONET network element and an OTDR.

This applied where optical links were basically point-to-point links and the SONET/SDH network element would alert users of any power, signal or synchronisation loss, and the OTDR was needed to determine potential problems on the fibre inbetween.

But for AONs the capabilities of the SONET network elements are no longer available at branching points like OXCs. To make these networks manageable, a replacement for the capabilities of the SONET layer is needed, moreover one cost-effective enough to be deployed in an optical network. Some solutions have been proposed, but all have deficiencies that reduce the value of the AON:

• Some use a stripped-down functional test capability, which requires an O/E conversion and access to the data-stream. This causes the network to lose the bit-rate and protocol independence
• Others solutions monitor just the OSNR, but this is insufficient because any signal distortion due to jitter, chirp or dispersion cannot be captured.

In that respect, it is advantageous that the deployment of optical networking has been delayed. This will allow companies to develop appropriate solutions for a monitor that provides quasi-BER (bit error rate) information - or equivalent - to manage the quality of transmission in the optical domain.

So, the all-optical network will be deployed, but at a later date than originally anticipated.

It will require test tools for installation which are of the more traditional instrument type, and tools for maintenance and network management which are embedded into the network or even integrated into the network equipment itself. Without a compelling solution for the network management challenge in the coming years, the emergence of optical networking could be delayed even further.