UNIT-V

OPTICAL NETWORKS

Point to point lines:

Point to point fiber optic lines are the simplest transmission line. This type of link

places the least demand on optical fiber technology and thus sets the basis for

examine more complex system architecture.

Fig: Simple point to point link

The repeaters may be opto electronic (or) optical repeaters. In this system, the repeater

spacing is a major design factor spacing between repeater increases, it reduces the

system cost spacing between transmitters receiver increases, it will also increases

system cost.(ie transmission distance and increases)

If L increases then bit rate reduces because of dispersion thus, product of B(bit

rate) and transmission distance(L) is a measure of system performance and its

depends on operating wavelength

Operating wavelength BL product

0.854m 1Gb/s-Kn

1.34µm 1 Tb/s-Km

1.55µm 100Tb/s-km

To analyze the point to point link. One should know the system requirements such

as the maximum transmission distance, required data rate and allowed bit error rate

(BER).

To satisfy these requirements the system should be designed based on the

components available and their characteristics.

1. Multimode (or) single mode fiber (transmission media)

(a)Core radius (b) fiber repactive index profile (c) bandwidth (or) dispession

(d) fiber attenuation (e) numerical aperture

2. Optical sources (LED or laser diode)

(a) Emission wavelength (b) output power (c) spectral line width (d)

radiation pattern (e) radiating area (f) no. of emitted modes (g)

stability and life time.

3. Light detectors (PIN (or) APD)

(a) Responsivity (b) efficiency (c) operating wavelength (d) speed (e)

sensitivity (f)noise figure

System considerations:

a. Operating wavelength selection:

First generation optical wavelength are in the range of 0.8µm to 0.9µm. Here the

transmission less is maximum and dispersion is also maximum.

Today we choose the wavelength around 1.3µm to 1.55µm. Here the attenuation and

dispersion are very small in silica fibers are used for long distance transmission.

b. System performance:

System performance is decided by three major blocks (or) the optical fiber

transmission. They are transmitter, optical fiber links and receiver. The designer

should choose proper light source, proper optical fiber and proper photo detector to get

high bit rate and high S/N ratio.

Regarding optical fiber, the single mode step index fiber is the proper choice. Even in

that to reduce dispersion proper choice of the refractive index profile is necessary. These

single mode step index fibers are preferable.

Regarding optical sources, single mode laser diodes are suitable for single mode stop

index fibers. For multi mode fibers, heterojunction LEDs chosen based on economy.

Regarding optical receivers, the P-i-n photodiodes and avalanche photodiodes are

preperable. Here also they should be quantum noise limited.

The maximum transmission distance is limited by the net less of fiber cable such that

L=10/α log10(Pt/pr)

α=not loss (in dB/1cm)

Pt=average power from transmitter

Pr=average power detected at receiver =NphvB

Np=minimum no. of photons/bit required

Hv=energy of photon

B=bit rate Link Power Budget:

The main aim of power budget is to have enough power at the receiver so as to maintain

reliable performance during the life time of the entire fiber optic system. The minimum

power required at the receiver is the receiver sensitivity Pmin. The average power

launched at the transmitter is Ptr.

Generally less can be calculated by

Loss(dB)=10log (Pout/pin)

Where Pin and pout =optical power emerging in and out of the element

Let Ptr=transmit power

Pmin=minimum power required at the receiver(decides receiver sensitivity)

Ploss=Total power loss produced by the fiber optic channel

Psm=system margin

Fig: optical power loss model for point to point links

System margin is the link power margin which is normally added with the total

power less in the analysis of power budget

where αfL=fiber attenuation (dB/Km)

Lc =connector loss.

lsp=splicing loss

Total power loss

PT=Ptr-Pmin

Subs (2) in (3)

Therefore, PT= αfL+ lc+lsp +system margin (Psm ) for simplicity add connector loss and

splicing loss

ie lc=lsp

Usually system margin is 6 to 8 dB

Rise budget:

In linear system, the rise time is defined as the time during which the response increases

From 10% to 90% of the final output value when the input is changed abruptly in a stop

wise manner.

τr = rise time

τf=fall time

Rise time budget analysis is a method for determining the dispersion limitation of an

optical fiber link. The rise time of the system is given by

∆ti= Rise time of each component in the system

There are four basis element that limit the system spped of optical communication

system.

1. Transmitter rise time

2. Receiver rise time

3. Material dispession rise time

4. Modal dispession rise time

1. Transmission rise time (ttx):

It depends on light source, drive circuit of light source. It is the calculated value and it

is specified optical fiber data sheet.

2. Receiver rise time (trx):

It depends on photodetector response and receiver front and response . Receiver front

and response depends on first order low pass filter in the receiver having step response and it

is given by

u(t)=step frequency

Brx=3dB Bandwidth of receiver

The rise time of the receiver is defined as the time interval between g(t)=0.1 to

g(t)=0.9. This is known as 10 to 90% rise time. It is generally given by

Trx=350/ Brx (or) 350/B B= Brx

3. Material dispession rise time:

σλ=half of spectral width of the source

D=dispession parameter (Its value change from fiber

to fiber) L=Length of the optical fiber

4. Modal dispession rise time:

The relationship between B.W and rise time of the fiber is given by

∆t modal =0.44/BM(L)

BM(L)=Bandwidth BM in a link length L can be expressed by approximate emphrical relation as,

BM(L)=Bo/Lq

Where Bo=B.W of 1Km length of fiber cable

Q=empirically fit parameter

Q=1 indicates steady state modal equilibrium is not reached

Q=0.5 indicates steady state modal equilibrium is reached

∆t modal=0.44/ Bo/Lq = 0.44Lq/Bo

Total rise time of optical system is given by

Where all time ∆t sys is in nano seconds

SONET: (Synchronous Optical Network)

SONET is the set of standards developed by Bell core and then adopted by 170-

7(cc 177) as an international standard designated Synchronous Digital Hierarchy

(SDH).

SONET specifies the rates, formats and parameters of all physical transmission media.

Even though SONET is called an optical network, it can support traditional electrical

transmission. The electrical signal in SONET are called Synchronous Transport

Signal(STS).STS level 1,designated as STS-1 transmit at 51.84 Mbits/sec. All other

levels are multiples of STS-1 as shown in the following table.

Electrical signal Optical signal Bit rate STS-1 OC-1 51.84

STS-3 OC-3 155.52

STS-9 OC-9 466.56 STS-12 OC-12 633.08

STS-18 OC-18 933.12 STS-24 OC-24 1244.16

STS-48 OC-48 2488.32

Higher bit rate signals, starting with STS-9 are not defined as electrical standards

and they never actually transmitted in electrical form

Structure of STS-1 format:

An STS frame consists of 9 rows and 90 columns of 8-bit bytes for a total of 810

bytes (6480 bits).The duration of each frame is 125µs.

Path overhead:

It provides end to end communication99 bytes length column wise only).It is inserted to

user data or payload by the end transmitter. It remains attached until the data reaches the end

receiver. It monitors end equipment, status, signal labeling and a user channel.

Transport overhead:

It provides communication between STS-N multiplexer and adjacent nodes such as

regenerators (ie,) it carries network management function.

CONVERSION OF DIGITAL SIGNAL TO OPTICAL SIGNAL (OC):

Elements of SONET network:

SONET uses terminal multiplexers(TM),add-drop multiplexer

(ADM),digital cross connects(DCS) and generators.

Terminal multiplexer:

Low speed links connected to SONET through terminal multiplexers. It is also called

line terminating equipment(LTE).If a TM is connected to digital cross connect, we have a

point to point link.

Add drop multiplexer:

It is used to pick out one or more low speed streams from a high speed streams and

add one or more low speed streams to high speed streams.

Digital cross connect:

It is used to manage all the transmission facilities in the central office. It is actually a

digital switch. It also monitors network performance and do multiplexing.

Regenerators:

It is used to boost a signal for long distance transmission

Figure : Elements of a SONET infrastructure

Elements of SONET infra structure:

SONET layers:

1.Physical layer:

It is responsible for the transmission of bits over physical media.

The SONET standard specifies such physical media as optical fibre and light sources

with operating wavelengths that depends on the bit rates and distance for which the

SONET network id designed.

SONET systems are sub-divided into three networks:

a. Short reach networks (SR)

b. Intermediate reach networks (IR)

c. Long reach networks (LR)

For short reach network, SONET recommended 1310nm operating wavelength

LED and multimode fiber.

For long reach network, SONET recommended 1550nm operating wavelength DFB

laser diode and single mode fiber. Physical layer is needed as each node.

2. Section layer:

It is responsible for sending data to the physical layer.

It interface with the physical layer by adding appropriate bytes to the frame overhead.

It also provides error monitoring and control.

It regenerates the signal and protect the multiplexing and switching operations.

Section layer is needed at each regenerator.

3. Line layer:

It is responsible for multiplexing many path layer connections onto a single link

between adjacent nodes.

This layer also provides most of the operating, administrative management and

provisioning (OAM& P) functions, including network protections.

A line layer needed at ADMS and TMS.

4. Path layer:

It is an end to end layer.

It is responsible for connections between a source and destination.

It accepts data from line layer, adds routing and error control information.

It also monitors and tracks the status of connection.

A path layer is needed at each SONET terminal multiplexer.

Passive Optical Network (PON):

PON use some form of passive components such as optical star coupler or static

wavelength router as the remote node.

Simple PON architecture uses a separate fiber pair from the CO to each ONU. The main

Problem with this approach is that cost of CO equipment scales with the number of ONU’s.

Moreover, the operator needs to install and maintain all these fiber pairs. This type of

architecture used to provide high speed service.

Instead of providing a fiber pair to each ONU, a single fiber can be used with Bidirectional

transmission. However the same wavelength cannot be used to transmit data simultaneously

in both the directions because of the uncontrolled reflection in the fiber.

One way is to use time division multiplexing so that both the ends does not

transmit simultaneously. Another is to use different wavelength (1.3 and

1.55µm,for example)for the different directions.

In PON architecture, fiber pair can also shared by many users.

Common example for such network is SONET/SDH rings. This type of network

provides high speed services to large business customers. An ONU is a SONET

add drop multiplexer(ADM),which can drop its information at particular

wavelength.

PON architecture types:

T PON-Passive Optical Network for Telephony.

W PON-Wavelength Division Multiplexing(WDM) Passive Optical Network.

WR PON-Wavelength Routing Passive Optical Network.

Disadvantages of PON:

The cost of CO equipment scales with number of ONU’s.

Operator needs to install and maintain all the pair of fibers coming from each ONU’s to

CO.

Advantages of PON:

Since this architecture is made from passive components, its reliability is very high.

Ease of maintenance.

Fiber infra structure itself is transparent to bit rate modulation formats and the overall

network can be upgraded in the future without changing the infra structure itself.

a. Passive Optical Network for Telephony(TPON):

The CO broadcasts its signal downstream to all the ONU’s using a passive star coupler.

The ONU shares an upstream channel in a time multiplexed fashion. In this case,

upstream and downstream signals are carried using different wavelength over a single

fiber.

In TDM approach, the ONU’s need to be synchronized to a common clock. This is

done by a process called ‘RANGING’, where each ONU measures its delay from CO

and adjusts its clock such that all the ONU’s are synchronized relative to the CO.

The CO transmitted can be LED or fabry penot laser and receiver is PIN FET receiver.

ONU’s transmitter and receiver can also be LED or laser and PIN FET receiver.

Number of ONU’s is limited by splitting loss in the star coupler.

There is a tradeoff between transmitted power, receiver sensitivity, bit rate and the

number of ONU’s and total distance covered.

TPON’s may be cost effective at offering low speed services compared to SONET/SDH

rings or Ethernet based offerings.

b. WDM PON:

It is an upgraded version of the basic PON architecture. In this case, the CO

broadcasts multiple wavelengths to all the ONU’s and each ONU select a

particular wavelength.

In this case, a single transceiver at the CO with WDM array of transmitters or

single tunable transmitter to yield (WDM PON).

This approach allows each ONU’s to have electronics running only at the rate it

receives data, and not at the aggregate bit rate.

However it is still limited by the power splitting at the star coupler.

c. WR PON- Wavelength routing PON:

In this case, a passive arrayed waveguide grating (AWG) is used to route different

wavelength to different ONUS in the down stream directions, without is curring a

splitting loss.

AS in the TPON and WPON architectures, the ONUS time shared wavelength for

Upstream transmission

It allows point to point dedicated services to be provided to ONUS.

FTTH: Fiber to the home

IN FTTC ie Fiber to the curb (or) Fiber to the building, data is transmitted digitally

over optical fiber from the hub, or central office, to fiber terminating nodes called

optical network units(ONU). The expectation is that fiber would get much closer to

the subscriber with this architecture.

IN FTTH (fiber to the home) architecture, the ONUS would perform the function of

NIU. Here the optical fiber is used to transmit data from central office to remote node

(RN) and RN to home.

In network from the co to ONU is typically a passive optical network(PON). The

remote node is a simple passive device such as an optical star coupler and it may some

be collocated in the central office itself rather than in the field.

Although many different architectural alternatives can be used for FTTC, the term

FTTC is usually used to describe a version where the signals are broadcast from the

central office to the ONUS, and the ONUS share a common total bandwidth in time

division multiplexed fashion.

In FTTC, the fiber is within about 100m of the end user. In this case, there is an

additional distribution network from the ONUS to the NIUS with the fiber to the

cabinet (FTTcab) approach, the fiber is terminated in a cabinet in the neighbourhood

and is within about 11cm of the end user.

AON: All optical network

Figure: A Helical LAN topology proposed to be used in the AON TDM

All optical network (AON) consortium consisting of AT&T Bell laboratories, digital

equipment co- operation and Massachusetts institute of technology developed a test bed

for light wave communication.

The aim of the test bed was to demonstrate a single routing mode in a network

operating at a transmission rate of 100 bits/s.

Packet interleaving was used and packets from electronic sources at 100 Mb/s were

optically compressed to the 100 lib/s rate using optical time division multiplexing.

AON supported different classes of service, specifically guaranteed bandwidth

service and bandwidth-on-demand service.

The topology used in the above diagram is bus topology where users transmit in the top

half of the bus and receive from the bottom half. However, each user is attached for

transmission to two points on the bus such that the guranteeded bandwidth transmission

are always upstream from the bandwidth-on-demand transmission since it having helical

shape, the name helical LAN(HLAN) for this network.

Wavelength division Multiplexing (WDM):

A powerful aspect of an optical communication link is that many different wavelengths

can be sent along a single fiber simultaneously in the 1300 to 1600nm spectral band. The

technology of combining a number of wavelengths on to the same fiber is known as

wavelength division multiplexing or WDN.

Features of WDN:

Capacity upgrade:

If each wavelength support an independent network signal of perhaps a few giga bits

per second, then WDN can increase the capacity of fiber optic network dramatically.

Transparency:

Using different wavelengths, fast (Or) slow asynchronous and synchronous digital

data and analog information can be sent simultaneously and independently, over the same

fiber, without the need for a common signal structure.

Wavelength routing:

The use of wavelength sensitive optical routing devices makes it possible to use

wavelength as another dimension, in addition to the time and space in designing

communication networks and switches. In wavelength routed networks, use the actual wavelength as

intermediate (or) final address.

Wavelength switching:

Wavelength routed network-rigid configuration (can not be altered)

Wavelength switched network (WSN):

Allow the reconfiguration of optical network. Key components needed for WSN add

drop multiplexed .Optical cross connects and wavelength converters.

Operation principle of WDM.

Figure: A Unidirectional WDM system that combines N independent input signal for

transmission over a single fiber

Figure : Schematic representation of a bidirectional WDM system

Here N fibers come together at an optical combiner (or)wavelength multiplexer,

each with its energy present at different wavelength.

The N light beams are combined (or) multiplexed on to a single shared fiber for

transmission to a distance destination.

At far end, the beam is split up over many fibers as there were on the input side.

Each output fiber contains a short, specially constructed core that filters act all but

one wavelength.

The resulting signals can be rated to their destination (or) recombined in different

ways for additional multiplexed transport.

The only difference with electrical FDM is that on optical system using a diffraction

grating is completely passive and thus highly reliable.

The first commercial system had eight channels of 2.5 Gpbs per channel . By 2001,

there were products with 96 channels of 10 Gpbs , for a total of 960 Gbps.

When the number of channels is very large and wavelength are spaced close

together, for example 0.1nm, the system often referred to as DWDM (Dense

WDM).

By running many channels in parallel on different wavelength, the aggregate bandwidth is

increased linearly with the no. of channels. Since the bandwidth of single fiber band is

about 25,000 GHZ, there is theoretically room for 2500 10 GPPS channels even at 1

bit/HZ. (for DWDM, write same explanation with the diagram)