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OPTICAL NETWORKING: Cost Arbitrage

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VoicenData Bureau
New Update

Optical networks have been making ‘waves’ recently, following the opening

up of the national long-distance market. A few Indian operators have started

installing national optical networking backbones. Some of these backbones will

be based on the newer dense wavelength division multiplexing (DWDM) technology,

as DWDM provides enormous bandwidth at much lower unit costs.

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Optical

Communication



Optical communication is the transfer of signals from one point to another

using optical fiber as the communication medium. Simply put, optical fiber is an

extremely thin glass pipe that has enormous communication capacity. The

communication is effected by turning a light source, usually a laser, ‘on’

to indicate a ‘1’ bit and ‘off’ to indicate a ‘0’ bit. The rate at

which the laser is turned on and off indicates the rate of communication. It is

called the ‘bit rate’ and is measured in bits per second. Optical fiber has

a virtual monopoly as the communication medium whenever the distances exceed a

few hundred meters, or the bit rate exceeds a few Mbps. Today, optical fiber has

been widely deployed in the telecommunications backbone of many countries, and

optical communication systems have been in use for over a decade.

Multiplexing Techniques



The capacity of the fiber communication systems had to keep pace with the

increasing communication needs (traffic). Initially, this was accomplished by

increasing the bit rate from 155 Mbps to 622 Mbps, then to 2,488 Mbps, and

finally to 10 Gbps. These are the standard rates of the synchronous digital

hierarchy (SDH) networks, which are widely deployed globally, including India. A

higher rate SDH stream carries a number of lower rate SDH streams by

interleaving them in time. This technique is called time-division multiplexing (TDM).

However, the communication capacity of many backbone networks needed to be at

rates higher than 10 Gbps and lasers simply could not be turned on and off much

faster. This led to the use of several signals at different frequencies or

equivalently different wavelengths, akin to the transmission of different radio

or television signals at different frequencies. For example, we can transmit

many different 10 Gbps signals, each using a laser transmitting at a different

wavelength, simultaneously over the same optical fiber. This technique is called

DWDM.

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The first DWDM systems operated with 4—16 wavelengths. Today, DWDM systems

supporting 160 wavelengths transmitting at a speed of 10 Gbps are commercially

available. Such a system has a capacity of 1.6 Tbps. This is the equivalent of

25-million telephone calls, which means that the traffic resulting from all the

phones in India can be carried on a single strand of fiber even if all these

phones are active simultaneously.

Optical Amplification



It is possible to send each signal over a different fiber rather than at a

different wavelength over the same fiber, thus avoiding the use of DWDM.

However, such a multi-fiber solution, which amounts to the use of multiple SDH

networks in parallel, is inefficient for the following reason. After traversing

some distance, typically 60—80 km, the light signals become feeble. At this

point, we can detect the feeble signals and use another set of lasers to

retransmit the signals. This process is called regeneration. Multiple SDH

networks require multiple regenerators, one for each fiber, thus increasing the

cost. DWDM proves to be a cheaper alternative because all the signals are

transmitted on a single fiber. Thus, we can optically amplify all the light

signals when they become feeble with a single optical amplifier. The use of

optical amplification every 60—80 km made it possible to transmit DWDM signals

without regeneration over distances of several hundreds of kilometers. The

availability of cost-effective optical amplification technology is the primary

reason for the widespread use of DWDM technology today, and the consequent

dramatic reduction in the costs of long-distance communication.

Bridging the gap
The cost of the US domestic long distance is about 5 cents per minute, or about Rs 2.50 coast-to-coast
In comparison, the Bangalore—Delhi STD rate, for instance, is Rs 33.60 per minute
Optical networking backbones can pull down bandwidth costs
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New optical amplifier technologies, such as Raman amplifiers, have made

possible the realization of even longer optical DWDM links. Today, DWDM signals

may be transmitted without being regenerated (detected and retransmitted) for

thousands of kilometers–across the length and breadth of India, the US, and

most oceans of the world.

Intelligent Optical Networking



Optical communication uses optics merely for transmission and is only the

first step in optical networking. A significant amount of research and

development effort has been spent over the last decade to add more functionality

at the optical level, which leads us to the next generation of optical networks

that are being put in place today. These networks make use of optical routing of

signals, in addition to optical transmission, to realize even more significant

cost savings. Today, these networks are commonly referred to as intelligent

optical networks.

Consider an intelligent optical network consisting of Internet routers being

linked to New Delhi-Mumbai, and Mumbai-Bangalore fiber links. If we can arrange

for the data being sent between Bangalore and New Delhi to remain on wavelengths

separate from those carrying data to/from Mumbai, the former wavelengths can be

optically switched at Mumbai. This can obviate the need for the data carried in

them to be processed by the routers at Mumbai. This leads to significant savings

in network equipment costs, since it is much cheaper to switch the wavelengths

optically than to process all the data being carried on them, resulting in

further decreases in the bandwidth costs.

Conclusion



The high cost of long-distance communication has stifled the widespread use

of Internet and Web technologies in the country, and has even curtailed the

usage of telephone. Compare the cost of the US domestic long distance, which is

about 5 cents per minute, or about Rs 2.50 coast-to-coast, with the STD rates in

India, which for Bangalore-Delhi is Rs 33.60 per minute. The deployment of

state-of-the-art optical networking backbones in the country, by competing with

the long distance carriers, will see a marked decrease in bandwidth costs,

enabling India to take active part in the global communications revolution.

Kumar N Sivarajan chief

technology officer Tejas Networks

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