Not long ago, there seemed to be a bandwidth glut in optical transport networks. The questions surrounding broadband services, including slow copper access lines, competitive regulations and developing standards, were distractions from the evolution taking place. Network interfaces of 40 gigabit (Gb) speeds were hardly even being considered for deployment. Not until the realization of services such as IPTV (Internet protocol television), VoD (video on demand), and nPVR (network personal video recorder) was the importance of high-capacity, reconfigurable metro networks truly appreciated.

Dense wave division multiplexing (DWDM) combines different wavelengths in a single fiber and then separates them again. Most metro optical networks employing DWDM are now engineered to handle 10 Gb wavelength channels. In addition to the general need of more capacity for growth, 10 Gb channels are important for carrying Ethernet traffic. Ethernet-based protocols have become the de facto standard for multi-service transport in telecom networks. Until recently, most metro Ethernet traffic was transported in Gigabit Ethernet (GbE) streams at a rate of about one gigabit per second (1 Gb/s). The next step up in speed for Ethernet is to 10 GbE, which requires a DWDM channel with at least a 10 Gb/s rate.

10 Gb DWDM with sub-wavelength services

IPTV providers and wholesale carriers especially must be able to transport these 10 GbE services effectively. They need a combination of flexible service delivery and high capacity at the optical layer. Moreover, there is a trend of smaller independent operating companies (IOC) to pool resources in a shared backbone network for the distribution of video and other services. This consortium approach gives ownership and low-cost access to statewide and interstate facilities previously not viable for IOCs on their own. A metro DWDM platform that can reach across regional networks is ideal for this application.

As more and more bandwidth traverses larger serving areas network owners are looking for DWDM systems that can reach even further without the high cost of traditional long haul equipment. Metro DWDM has evolved from the simple and relatively inexpensive to intelligent and agile. The requirement driving this evolution is the shift in function from fiber exhaust relief to packet-based service delivery. Where feasible, carriers want a single, more scalable DWDM system rather than a series of different platforms. This not only makes economic sense, it also guarantees network simplicity and ease of operation, a fact not lost on both large and small companies. Smaller independent operating companies are particularly appreciative of such systems.

Single system for metro and inter-metro connections

The latest metro/regional DWDM systems can go at least 600 km and some well over 1,000 km without using costly regeneration equipment or exotic components. This all-range concept combines nimble, service-aware optical transport with integrated long reach technology. The result is a reconfigurable metro network well-suited for service delivery that can also span distances to connect city networks to each other.

Taking it to a whole new level

Although the desire is for a unified, scalable network, economics dictates that such a network be engineered for the speed and distances in use. With today’s reconfigurable networks it is rather easy to scale up by increasing the number of channels as needed. However, as demand continues to grow the transmission capacity of each channel must increase as well. The next step in the optical transport hierarchy is to 40 Gb/s, as even 10Gbps is rapidly becoming saturated.
Core IP (Internet protocol) routers—the main traffic controllers—already support connections of 40 Gb/s between nodes. To transport this traffic in the metro, DWDM systems must also be upgraded to handle 40 Gb channels. Similar to the move from 2.5 Gb to 10 Gb, basic physics says that more power and shorter node spans are needed to support transmission of 40 Gb signals.

Growth of service bandwidth over DWDM

ECI Telecom, a leading supplier of best-in-class networking infrastructure equipment for carrier and service provider networks worldwide, understands that deployment of 40 Gb systems is restricted by multiple physical effects and design considerations. Such factors are generally not accounted for in current networks designed for 10 Gb.

How to circumvent these challenges and provide higher capacity on the current infrastructure is the fundamental question that needs answering. ECI has shown that 40 Gb is feasible over dynamic and versatile metro DWDM in the industry’s first successful demonstration of 43 Gbps transmission over a 1,000 km, 80-channel reconfigurable network. This demonstration was accepted for presentation at this year's OFC/NFOEC, which took place in March 2007.

In this demo, ECI showed that its XDM® All Range™ multi-service ROADM (Reconfigurable Add-Drop Multiplexer) enables carriers to deploy a cost-effective solution for high capacity reconfigurable networks. The demonstration exemplified new modulation techniques that enable inter-metro transmission of 43 Gb signals, while maintaining high tolerance to noise and non-linear effects—similar to that of 10 Gb signals.

The demonstrated solution is unique in that it allows carriers to build reconfigurable ROADM-based networks with a total capacity of 80 channels x 40 Gb/s (or 3.2 Terabits per fiber) and distances suitable for metro, regional and long haul applications. This solution provides carriers with the flexibility necessary given today's networks' hard-to-predict growth patterns. The ROADM facilitates remote wavelength provisioning without re-engineering and costly tuning adjustments in the field. And there is no need to pre-plan the channel layout and wavelength connections.

The multi-degree ROADM enables meshed optical networks and subtended rings to be turned up rapidly without using back-to-back systems. This greatly reduces equipment cost and stranded bandwidth. Multi-degree allows the wavelength layer to be dynamic and used as a provisionable and managed transport entity, similar to SONET/SDH.

Conclusion

Industry experts and analysts agree that carriers worldwide are currently deploying ROADMs in their metro and core networks and they are looking to gradually introduce 40 Gb services. The demonstration by ECI shows that there is a cost-effective and forecast-tolerant answer to this demand.

A key benefit of this technology is that now 40 Gb/s can effectively be used to upgrade capacity-constrained 10 Gb/s systems, without a forklift upgrade or wholesale replacement of equipment. Since 40 Gb/s channels can be overlaid on existing 10 Gb/s systems, the transition to 40 Gb/s data rates will be a seamless and natural evolution from today’s 10 Gb/s systems.