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A DWDM in the metro

Metro Optical Networking is gaining significance due to the ever-increasing demand for bandwidth and diminishing fiber availability. With an exponential rise in data traffic, especially for innovative and sophisticated communication services, service providers need to deliver services in the right way at the right price. John C. Collins, Director Asia-Pacific, Optical Internet Engineering, Nortel Networks, asserts Metro Optical Networking is the answer to the burgeoning demand for bandwidth, and says DWDM is the ideal enabling technology. by Minu Sirsalewala

What is a metro network and how is it different from a long haul network?

It is a network running across the city, or it may span a metropolitan area wherein several cities are connected on close proximity. In a typical scenario this might be in a range of 200-400 kilometers.

We are focussed on developing optical networks for high-density metropolitan areas, which are centers of business and commerce. The business encompasses a variety of optical networking technologies that help service providers relieve network congestion, that is common in metropolitan areas.

A metro network uses sophisticated switching and routing mechanisms for efficient data transfer. It has the ability to transmit data in its native protocol. Long haul networks do not use sophisticated switching and routing technologies and do not transmit signals in their native protocols.

Where does a metro network fit in better, enterprises or carriers?

A metro network is better suited for enterprises. It functions optimally for industries like banking, finance, airlines, and medical centers. It is highly recommended in an industry where operations are spread over geographically displaced campuses. In a metro, a carrier can provide its service to another carrier in order to get a better market reach.

Which technology is more suitable for a metro network: SDH or DWDM?

In the past, SDH (Synchronous Digital Hierarchy) which has a North American equivalent named SONET (Synchronous Optical Network) has been the core component in a metro network. The problem with this technology is that it is highly structured, has very specific interfaces, offers limited bandwidth, and does not offer the versatility that's demanded by enterprises. It is also very difficult and complex to deploy Ethernet over SDH. It requires high capital expenses and also higher operational costs.

DWDM (Dense Wavelength Division Multiplexing) technology has emerged as the alternative to SDH in metro networks. A DWDM environment offers versatility and flexibility and requires enterprises and carriers to deploy more sophisticated technology. A DWDM metro network is typically bit-rate protocol independent and allows you to carry native databases. This lowers capital expenses and operational expenses, and offers greater sophistication over SDH-enabled metro networks.

Another important development, which is visible, is the commonality of Ethernet. We have Ethernet in VoIP (Voice over IP), Ethernet for videos etc. Instead of relying on a SONET infrastructure, which is optimized for voice and expensive to deploy, metropolitan Ethernet providers use a combination of fiber, DWDM and Ethernet boxes. The combination of DWDM equipment and simple Ethernet gear is much less expensive than SONET equipment, enabling metropolitan Ethernet providers to offer cut-rate pricing.

What are the emerging trends in optical networking?

There is a visible and definite move to the DWDM environment—worldwide, nations are shifting to DWDM. Europe and North America are already using it. In the Asia-Pacific region we are seeing penetration in Shanghai, Beijing, and several cities in China. Companies like SingTel in Singapore, companies in Australia are all deploying metro networks because it is a multi-services platform, bit-rate protocol independent, less expensive, and offers the capability to support new services.

It is difficult to offer new services in an SDH environment. SDH is a finite building block and does not allow the flexibility to just plug services as and when required. For instance, in a DWDM environment, Ethernet can be plugged in and related services like video conferencing, VoIP etc. are made available. But to plug services in an SDH environment requires various manipulations, like lowering the bandwidth for services.

DWDM is going to be the trend and Ethernet is going to play an important role because of its ubiquity, commonality, and cost. However, to move over long distances, say over a few kilometers, it requires that Ethernet is on some kind of signal, and that signal is the wavelength within the DWDM. Ethernet is going to be present in 98 percent of interfaces around the world. Just as an example we can put 160, 10 GB channels on the same piece of fiber i.e. 1.6 terabytes and still expand further in the future when need arises.

DWDM is flexible, agile, presents low capital cost, low operational cost and offers high revenue generation.

What kind of services can hop onto a metro network?

As mentioned earlier it is difficult and expensive to put Ethernet on SDH, but DWDM opens doors for Ethernet. Systems like SAN (Storage Area Network) or NAS (Network Attached Storage) are good examples of the kind of services that can be offered.

To quote statistics from a consulting house, they say "mankind has created 12 exabytes of data till date, humans will create another 12 exabytes of data in the next two years." You need to have the capacity to store such huge data and also move this data around to put it to use. It is not fruitful to have the data stored and not put it to use. It is important to have the capability and the network to move this data around.

There are services like HDTV (high definition television), and exotic services like holographic imagery. For instance you could be sitting 1,000 miles away and have a conversation with me as if we were in the same room. You'll see my 3-D image and you could be shaking hands with me even though we are present at locations several miles apart.

Data, voice and video are already on the network, but in the future we are going to have touch, 3-D images et al. These services are some years away but are certainly coming in, being thought about and scientifically pursued. DWDM can enable all of this and more.

How will metro networks shape up in India?

In India, within the metro areas the population is so great it is inefficient to use an SDH platform. In SDH today, typically an STM 16 system gives you 2.5 gigabits and at the same time its interfaces are very structured. Whereas with DWDM you can get 32 triple eight G5 interfaces, and can further take it up to 10 GB. With this capacity you can carry SDH on one wavelength, ESCON on the other wavelength, Ethernet on another and also HDTV on another wavelength.

DWDM can be deployed easily, with low cost investment and in very specific environments too. It can expand as far as possible and does not have any scalability issues.

From a vendor standpoint, in the next 6 months we expect to see more implementations of DWDM in a metro network. A metro network can be deployed very inexpensively, easily and in a very specific environment. It is possible to expand it as far as possible. It is scalable, flexible and agile.

What is DWDM?

DWDM (Dense Wavelength Division Multiplexing) enables data from different sources to be transmitted on a solitary optical fiber. Each signal is carried at the same time on its own separate wavelength. Up to 80 separate wavelengths or channels of data can be multiplexed into a light stream. Every channel carries a time division multiplexed (TDM) signal and is also known as wave division multiplexing (WDM). In a system where each channel carries 2.5 Gbps, up to 200 billion bits a second can be delivered by optical fiber.

As each channel gets demultiplexed at the end of the transmission into the original source, different data formats at different data rates can be transmitted together.

DWDM technology uses current electronics and fibers, and simply share fibers by transmitting different channels at different wavelengths (colors) of light. Systems that already use fiber optic amplifiers as repeaters do not require upgrading for most WDM systems. The key benefit of DWDM is that its protocol and bit-rate independent.

Minu Sirsalewala can be reached at minus@networkmagazineindia.com

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