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.

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

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

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.

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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