1st lec- introduction to optical networks

Berzingi

Introduction to Optical Networks

1 March 2014 1 Dr. Shavan Askar

The tremendous growth of the Internet and the World Wide Web, both

in terms of number of users and the amount of time, and thus

bandwidth taken by each user, is a major factor for the dramatic

changes in the telecommunications industry.

Internet traffic has been growing rapidly for many years. Estimates of

growth have varied considerably over the years, with some early

growth estimates showing a doubling every four to six months. Recent

estimates of about 50% annual increase of traffic.

However, broadband access technologies have been deployed widely: in

2008, 55% of the adults in the United States had broadband access at

home while only 10% had access through dialup lines of 28-56 Kb/s.

Fiber to the home (FTTH) has shown steady growth with Asian

markets showing the highest market penetration.

1 March 2014 Dr. Shavan Askar 2

Businesses today rely on high-speed networks to conduct their

businesses. These networks are used to interconnect multiple

locations within a company as well as between companies for

business-to-business transactions.

Large corporations that used to lease 155 Mb/s lines to

interconnect their internal sites are commonly leasing 1 Gb/s

connections today.

• There is a strong correlation between the increase in demand and

the cost of bandwidth. Technological advances have succeeded in

continuously reducing the cost of bandwidth. This reduced cost of

bandwidth in turn spurs the development of a new set of

applications that make use of more BW.


Optical fiber offers much higher bandwidth than copper cables and is less

susceptible to various kinds of electromagnetic interferences and other

undesirable effects.

As a result, it is the preferred medium for transmission of data at

anything more than few tens of megabits per second over any distance

more than a kilometer.

It is also used with short distances (a few meters to hundreds of meters),

high-speed (gigabits per second and above) interconnections inside large

systems.

The amount of deployment of fiber is often measured in sheath miles.

Sheath miles is the total length of fiber cables, where each route in a

network comprises many fiber cables.


Optical Networks

A 10-mile-long route using three fiber cables is said to have 10 route

miles and 30 sheath (cable) miles. Each cable contains many fibers.

If each cable has 20 fibers, the same route is said to have 600 fiber

miles. A city or telecommunications company may present its fiber

deployment in sheath miles.


First Generation In the first generation, optics was essentially used for transmission and

simply to provide capacity.

Optical fiber provided lower bit error rates and higher capacities than

copper cables.

All the switching and other intelligent network functions were handled

by electronics.

Examples of first-generation optical networks are SONET

(synchronous optical network) and the essentially similar SDH

(synchronous digital hierarchy) networks, which form the core of the

telecommunications infrastructure in North America and in Europe

and Asia.


Point-to-Point Optical Networks In 1980, optical fiber was primarily used to build and study point-to-

point transmission systems.

An optical point-to-point link provides an optical single-hop connection

between two nodes without any (electrical) intermediate node in

between.

Optical point-to-point links may be used to interconnect two different

sites for data transmission and reception.


Point-to-Point Optical Networks At the transmitting side, the electrical data is converted into an optical

signal (EO conversion) and substantially sent on the optical fiber.

At the receiving side, the arriving optical signal is converted back into

the electrical domain (OE conversion) for electronic processing and

storage.

To interconnect more than two network nodes, multiple optical single-

hop point-to-point links may be used to form various network toplogies.


Point-to-Point Optical Networks The Figure shown below depicts how point-to-point links can be

combined by means of a star coupler to build optical single-hop star

network.

The star coupler is basically an optical device that combines all incoming

optical signals and equally distributes them among all its output ports.

In other words, the star coupler is an optical broadcast device where an

optical signal arriving at any input port is forwarded to all output ports

without undergoing any EO or OE conversion at the star coupler..


Point-to-Point Optical Networks Optical ring networks can be realized by interconnecting each pair of

adjacent ring nodes with a separate optical single-hop point-to-point

fiber link.

The combined OE and EO conversion is usually referred to as OEO

conversion.

A good example of an optical ring network with OEO conversion at

each node is the fiber distribution data interface (FDDI) standard.


Multiplexing Techniques


Multiplexing Techniques

The need for multiplexing is driven by the fact that in most

applications it is much more economical to transmit data at higher

rates over a single fiber than it is to transmit at lower rates over

multiple fibers.

There are fundamentally two ways of increasing the transmission

capacity on a fiber. The first is to increase the bit rate.

This requires higher-speed stream at the transmission bit rate by

means of electronic time division multiplexing (TDM).

The multiplexer typically interleaves the lower-speed streams to

obtain the higher-speed stream.


TDM continued…. For example, it could pick 1 byte of data from the first stream, the next

byte from the second stream, and so on.

As an example, sixty four 155 Mb/s streams may be multiplexed into a

single 10 Gb/s stream.

Today, the highest transmission rate in commercially available systems is

40 Gb/s TDM technology. To push TDM technology beyond these rates,

researchers are working on methods to perform the multiplexing and

demultiplexing functions optically. This approach is called optical time

division multiplexing (OTDM).


TDM and WDM Laboratory experiments have demonstrated the

multiplexing/demultiplexing of several 10 Gb/s streams into/from a

250 Gb/s stream, although commercial implementation of OTDM is

not yet viable.

Another way to increase the capacity is by a technique called

wavelength division multiplexing (WDM).

The idea of WDM is to transmit data simultaneously at multiple carrier

wavelengths over a fiber.

These wavelengths do not interfere with each other provided they are

kept sufficiently far apart.

Therefore, WDM provides virtual fibers, in that it makes a single fiber

look like multiple fibers, with each virtual fiber carrying a single data

stream.


WDM WDM systems are widely deployed today in long-haul and undersea

networks and are being deployed in metro networks as well.

WDM and TDM both provide ways to increase the transmission

capacity and are complementary to each other. Therefore, networks

today use a combination of TDM and WDM.

The questions is what combination of TDM and WDM to use? For

example, suppose a carrier wants to install an 160 Gb/s link. Should

we deploy 64 WDM channels at 2.5 Gb/s each, or should we deploy 16

WDM channels at 10 Gb/s each?

The answer depends on a number of factors, including the type and

parameters of the fiber used in the link and the services that the carrier

wishes to provide using that link.


SDM (Space Divion Multiplexing) SDM is well suited for short-distance transmission but becomes less

practical and more costly for increasing distances due to the fact that

multiple fibers need to be installed and operated.

It is an approach used to avoid the electro-optical bottleneck, where

multiple fibers are used in parallel instead of a single fiber.

Each of these parallel fibers may operate at any arbitrary line rate.


1st lec- introduction to optical networks

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