Optical Communication and Networks Two Marks Questions With Answers 2014

Anna University, Chennai




1. List out the services provided by optical networks

Many types of services are offered by carrriers to their custom,ers are

Connection oriented services

Connectionless services

2. What are the advantage of optival networks

1. Flexible bandwidth

2. High capacity and immune to EMI

3. Low power requirement

4. Low crosstalk

3. What are the examples of 1st genertation optical networks


4. Define coupling ratio

It takes a fraction α of the power ffrom input 1 and places it on output 1 and the remaining fraction 1- α on output 2. Similarly a fraction 1- α of the power from input 2 is

distributed to output 1 and the ramining power to output 2. α is a coupling ratio.

5. Define insertion loss

Insertion loss occurs in forward direction and which should be as small as possible. It is around 1db

6. Define couplers

To combine and split the optical signals in an optical network

7. Define isolation loss

It is occur in reverse direction and which should be as large as possible. It is around 40-


8. Write the application s of circulator Duplex transmitter/receiver system WDM

Chromic dispersion compensator

Double pass erbium doped amplifier

9. Define blazing

The light energy in this zeroth order interference maximum is wasted since the wavelengths are not separated. Thus gratings must be designed so that the light energy is

maximum at one of the other maxima. This is done using a technique called balzing

10. Define optical filter

Filter is a two port device that selects one wavelength and rejects all others. It may have an additional third port on which the rejected wavelengths can be obtained.

11. Write the functions of optical multiplexers

A multiplexer combines signals at different wavelengths on its inputs ports onto a common output port and a demultiplexer performs the opposite function.

12. What are the uses of fiber gratings

Fiber gratings are attractive devices that can be used for a variety of applications, including filtering, add/drop functions,and compensating for accumulated dispersion in the


13. Define stark splitting

Due to the presence of erbium ions in silica, each energy level is split into multiple energy levels. This effect is known as stark splitting

14. Define raman amplifier

Using pump laer at wavelength around 1480nm, the power transfer from low wavelength photons can provide amplification to signals around wavelength 1550nm. Such an optical amplifier is called raman amplifier

15. Write the applications of optical switches

Provisioning of lightpaths

Protection switching

16. Define population inversion

A lightwave with frequency f such that hf=E2-E1 can bring atoms to energy level

E2,causing a population inversion where the number of atoms at E2 is greater than that E1. Number of atoms in energy level 2 is greater than number of atoms in energy level1.

1. Define SONET

Unit 2- SONET and SDH networks

Synchronous optical network is the transport technology commonly used in backbone and customer access networks. SONET is a set of standard defining the rates and formats of optical networks.

2. Write the function of path layer

Its main function is to map the signals into a format required by a line layer

Its functions includes reading modifying the POH and interpreting

3. Define TOH

Transport overhead includes section overhead, Line overhead

Section overhead is used for communications between adjacent network elements such as regenerators

4. List out the SONET layer

Path layer

Line layer Section layer Physical layer

5. What are the advantages of SONET Point to point monitoring

Easy error contingency and isolation

Support for new high speed services

Reduction in equipment requirements

6. Write the line rate for SONET STS-1 frame

Actual line rate -51.85 MBps

7. What are the uses of SONET multiplexing?

This process is used when multiple lower order path layer signals are adapted into a higher order path signal, or when the higher order path signals are adapted into the line


8. What are the SONET topologies?

Point to point configuration

Hubbed configuration

Linear add/ drop configuration

Ring configuration

9. What are the fundamental types of protection mechanisms PTP link?

1+1 protection

1:1 protection

1:N protection

10. Define protection paths

Protect paths provide an alternate path to carry the traffic in case of failures

11. Define accounting management

It is responsible for billing and for developing lifetime histories of the network components. This function is the same for optical networks compared to other networks

12. What are the advantages of SDH?

High transmission rates


High capacity


13. List out the SONET network management functions

Performance management

Fault management

Configuration management Security management Accounting management

14. Why we use SDH?

In response to the demand for increased bandwidth, reliability, and high quality service, SDH developed steadily during the 1980s eliminating many of the disadvantages inherent in PDH

15. What are the types of protection switching

Unidirectional protection switching

Bidirectional protection switching


1.Define broadcast and select networks.

A broadcast and select network is a second generation network in which each transmitter sends messages at a fixed frequency to the central star coupler through fiber path.

2.Write the disadvantages of broadcast and select networks.

Power from each transmitter is collected and broadcast to all stations, even to those who do not use it. This leads to a lot of wasted power. Since simple splitters and combiners are

used to split and combine signals, there is a loss of 1/N for each such device. The overall

loss is 1/N2.

3.What are the most popular topologies for broadcast and select networks.

The two most popular topologies for broadcast and select networks

1.Bus topology

2.Star topology

4.What are the major components used in star topology.

The major components used in star topology are

1.Power splitter

2.Power combiner

5.Define Single-hop networks.

In single-hop networks, information in the form of light is transmitted in to destination node without converting in to electrical form at each intermediate node.

These networks are all optical by the nature as signals continue to remain in the optical domain throughtout their transmission. This results in very fast communication.

6.Write the characteristics of single hop networks.

1. All nodes are connected to central hub node.

2.Single hop WDM networks are based on central passive star coupler or array waveguide grating.

7.What are the Available single hop networks?




8.Define Multi-Hop networks

Multi hop networks are used in suitations where a direct link between a source and a destination node is, in general, not available. Signals that require a multi-hop path are converted to electrical domain at each intermediate node and then converted back to the optical domain for further propogation.

9. What are the characteristics of multihop network.

1.The average no. of hops that apacket should pass to reach a destination from any source is low.

2.There is no more than one path between any pair of nodes.

10.Write the classification of the MAC protocols.

1.ALOHA- Based protocols

2.TDMA - Based protocols

3.WDMA- Based protocols

4.CDMA- Based protocols

5.Token - Based protocols

11.Write the assumptions of slotted Aloha.

1.Time is divided into slots of equal length.

2.Each packet length is equal to one time unit or one time slot.

12.Write the concepts of slotted Aloha.

1.When at least one data packet is available , a user transmits a packet in the next time slot.

2.Upon finding that a collision has occurred , a user retransmits the packet after waiting a random number of time slots.

13.Define Tell and GO property.

At a node, once a packet is available for transmission, that node simply tells the other node about this packet in a control slot and goes ahead and transmits the packet in the next data slot. This feature of the protol is called as tell and go.

14. What are the design criteria of logical topology.

To ensure a high throughput and low cost, a logical topology should be such that

1.G has a small diameter

2.G has a low average hop distance.

3.The in-degree and out-degree of each node should be low.

15.Define Access delay.

The access delay is defined as the delay between the time at which a packet is available for transmission at a node and the time at which it is transmitted.


1. Define wavelength routed network

A wavelength division multiplexing network in which a transmission wavelength can be reused in different parts of the network is referred to as wavelength routed network

2. What are the types of wavelength conversion?

Fixed wavelength conversion

Limited wavelength conversion

Full wavelength conversion

No wavelength conversion

3. Define limited wavelength conversion

In limited wavelength conversion ,a signal is allowed to be converted from one

wavelength to a limited subset of other wavelength ie., λ€ s(λj).

4. Classify the light path requests

Offline light paths

Online light paths

5. Define static networks

It does not use switches inside the wavelength crossconnect nodes. The set of light paths that can be established between users is fixed for static network. The static

network is fixed.

6. Define RWA

Given a set of connections, the problem of setting up lightpaths by routing and assigning a wavelength to each connection is called the routing and wavelength assingnment problem.

7. What are the basic approaches are used for the routing sub-problem?

Fixed routing

Fixed alternate routing

Adaptive routing

8. What are the advantages of fixed alternate routing?

It can significantly reduce the connction blocking probability compared to fixed routing. It has also been shown that, for certain networks, having as few as two alternate routes provides significantly lower blocking probabilities than having full wavelength conversion at each node with fixed routing

9. What are the conditions required for wavelength assignment?

Two light paths must not be assigned the same wavelength on a given link

If no wavelength conversion is available, then a light path must be assigned the same wavelength on all the links in its route.

10. What are the different combinations of types of light paths and network edges

Unidirectional light path , undirected edge

Directed light path, undirected edge UnDirected light path, directed edge Directed light path, directed edge

11. What are the design parameters that need to be considered in determining the cost of the network?

Router ports



12. List out different light path topologies

A point to point WDM ring

Hub topology

Full mesh topology

13. What are the various wavelength testbeds?

AON NTT ring



Alcatel WDM ring

Africa ONE/SEA Me We-3

14. Write short notes on adaptive routing

In adaptive routing, the route form a source node to a destination node is chosen dynamically, depending on the network state. This approach has lower connection blocking than fixed and fixed alternative routing, but it is more computationally intensive. In adaptive routing, the route from a source node to a destination node is chosen dynamically, depending on the network state.

15. What are the wavelength assignment problems in terms of two abstracted models?

Min fiber

Min conversion


1.What are the ways to increase transmission capacity.


Space division multiplexing (SDM) approach: it keep the bit rate same but use more

Time division multiplexing (TDM) approach: This approach is used to increase the transmission bit rate on the fiber.

Wavelength division multiplexing (WDM) approach: This approach is used to add additional wavelengths to carry data at different bit rates and protocols formats.

2.Write the advantages of WDM approach?

WDM systems can be desinged to be transparent systems. This allows different wavelength to carry data at different bit rates and protocol formats.

3.What are the types of TDM?

1.Electronic time domain multiplexing(ETDM)

2. Optical time domain multiplexing(OTDM)

4.What is the data rates for OTDM?

OTDM can take tens of gigabit streams to several (100 Gb/s) hundreds of gigabits streams.

5.What are the advantages of OTDM.

1.Simpler management and control

2.The provision of truly flexible bandwidth on demand(BOD) service.

3.Packet switching or self-routing of packets.

6.What are the methods used for OTDM.

1.Bit interleaving

2.Packet interleaving

7.Define Bit interleaving.

Optical signals representing data streams from multiple sources are interleaved in time to produce a single data stream. The interleaving can be done on a packet by packet basis. Framing pulses can be used.

8. Define Packet interleaving.

Optical signals representing data streams from multiple sources are interleaved in time to produce a single data stream. The interleaving can be done on a bit-by-bit basis. Framing pulses can be produced.

9.What is the purpose of using guard time in OTDM.

1. It is to provide for some tolerance in the multiplexing and demultiplexing operations.

2. It is used to prevent the undesirable interaction between adjacent pulses.

10.Write about NOLM.

The nonlinear optical loop mirror(NOLM) consists of a 3db directional coupler, a fiber loop connecting both outputs of the coupler, and a nonlinear element (NLE) located asymmetrically in the fiber.

11.Define TOAD.

To realize an AND gate, it require an NLE whose nonlinear properties can be conveniently controlled by the use of control pulses. This configuration has both of these properties and is called the terahertz optical asymmetric demultiplexer(TOAD).

12.Write the advantages of TOAD.

1.Because of the short length of the fiber loop the polarization state of the pulses is maintained even if standard single-mode fiber is used.

2.If the fiber loop is long, it must be constructed using polarization-maintaining fiber.

13.Write the functions of routers in photonic packet switching.




4.Header processing




14.Define Broadcast OTDM networks.

Broadcast OTDM networks are based on the optical delay alone. In fact, broadcast OTDM can produce broadcast storms quickly and render a large network useless. OTDM is based on the topology

1.star topology

2.bus topology

15.How synchronization is achieved in optical phase loc loop?

To achieve synchronization between stream pulses 1 and 2, a tunable delay element can be used to adjust their relative delays.