(ASK). The light pulses travel down the
fiber and eventually encounter a photo
diode at the receiver that converts the
light back into a binary signal.
Data in the long distance and
Internet back bones are usually sent
with a protocol standard known as the
Synchronous Optical Network or Sonet.
What it does is package data into blocks
or frames of 810 bytes of data and send
it at one of several standard data rates;
the slowest is 51. 84 Mb/s and the fastest
is 39.812 Gb/s. Most data exchange is
done at the 2.488 and 9.953 Gb/s rates.
See Table 1 for the Sonet standard rates.
Sonet networks are either point-to-point or in rings. The long haul
networks are known as wide area
networks (WANs). When Sonet is used
in smaller city or town rings, it is called
a metropolitan area network (MAN).
Ethernet — the local area networking (LAN) standard — now sends data
over fiber (and copper) LANs at 1 Gb/s
and 10 Gb/s. Data is by packets rather
than synchronous frames so the format
is less complex. High speed Ethernet is
also being more widely used in MANs
and in some cases, WANs like Sonet.
The IEEE (Institute of Electrical and
Electronics Engineers), who standardizes Ethernet, is working on a 100 Gb/s
standard for long haul fiber networks.
FASTER AND FARTHER
In the computer, electronics, and
communications fields, the ultimate and
on-going goal is to make everything go
faster and at a longer distance. The
same is true in fiber optics. For years,
the average long distance data rate over
the backbone was 2. 5 Gb/s (actually
2.488 Gb/s Sonet rate). Today, that has
been increased to an average of 10
Gb/s. Most long distance calls and
virtually all Internet messages go at that
10 Gb/s rate through the main routers
in the system. The distance is roughly a
maximum of several hundred kilometers
before the light signals get too weak.
They then need to be converted back
to electrical signals where they are
amplified, reshaped, and otherwise
rejuvenated before being converted
back to light and sent on their way.
Now the push is on to boost data
rates and extend the range of the light
signals. Before the technology crash in
2001, there was an effort to boost the
speed to 40 Gb/s ( 39.812 Gb/s Sonet
rate). That work has now been
renewed and there are lots of new
products to support that data rate.
Network providers are beginning to
offer services at that speed. Verizon
recently announced that some of their
network segments in the northeast US
have already been bumped up to the
40 Gb/s rate. And work is on the way
to move that up to 100 Gb/s.
Besides the IEEE effort to build a
100 Gb/s Ethernet, other efforts are
under way to boost the data rate to 100
Gb/s and beyond. Some of these are
Sonet type systems while others are
proprietary, but progress is being made.
One of the easiest ways to get to 100
Gb/s over a single fiber is to use what is
called dense wavelength division multiplexing (DWDM). This is a technique
where a very high speed data stream is
divided up into many slower streams.
These streams are then transmitted
simultaneously down the same fiber, but
with each stream modulating a different
light wavelength. The different “colors”
of light travel down the fiber together
and are separated out by filters at the
receiver where they are then recombined back into the fast single stream.
Most of the first 100 Gb/s systems
are using DWDM. But single wavelength 100 Gb/s is tough. One of the
main reasons is that glass fiber greatly
attenuates and distorts the light. These
problems are called polarization mode
distortion (PMD) and chromatic dispersion. New types of fiber help to mitigate
the problems but mostly the problems
are dealt with by an equalization technique implemented at the receiver. The
signals are predistorted in a special way
and the receiver corrects for them using
fast digital signal processing techniques.
Another solution lies in using
different modulation methods. The
standard OOK or ASK is being replaced
by more sophisticated modulation
techniques like differential phase shift
keying (DPSK) and differential quadrature phase shift keying (DQPSK). An
old telephone system modulation
method called duobinary has also been
applied. All of these help overcome the
attenuation and distortion allowing
TABLE 1. Sonet Data
Standards and Rates
STS means synchronous transport
signal and OC means optical carrier.
Sonet Level Data Rate
STS-1/OC-1 51. 84 Mb/s
STS-3/OC- 3 155.52 Mb/s
STS-12/OC- 12 622.08 Mb/s
STS-48/OC- 48 2.488 Gb/s
STS-192/OC-192 9.953 Gb/s
STS-768/OC-768 39.812 Gb/s
faster data rates over longer distances.
There are lots of demo 100+ Gb/s
systems. It will be a few more years
before we see standards and equipment
for those rates, but they are on the way.
Why? Because of the greater load the
Internet carries with the ever-increasing
music downloads and the forthcoming
Internet video. Companies will begin
offering movies-on-demand or video-on-demand, as well as Internet protocol
television (IPTV) that is designed to
compete with the cable TV companies.
The current Internet structure and
bandwidth can handle some video now,
but it will eventually be overwhelmed if
video becomes popular on the Internet.
The new 100+ Gb/s technology is on
the way to solve this problem.
Another hot optical topic today is
the PON. Passive Optical Networks
(PONs) are fiber optical networks
designed so they do not use an active
repeater or other equipment between
the source central office and a home or
business. Right now, most individuals
■ Splitter component.
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