Deck 25: Optical Communication Systems

Question 1

What types of emitters, detectors, and fiber are most common for short-range fiber systems

Answer 1

The optical fibers are used to transmit the signals from transmitter to receiver in optical communication systems and it can be either single mode fibers or multimode fibers and can be used depending up on the distance over to travel the signals.
Commonly for short range distances such as within the factories and offices, multimode optical fibers are used, which can allow electronic devices operated at lower wavelengths such as light-emitting diode (LED) used as a transmitter and PIN diode used as a receiver, because of which they operate at lower wavelengths of 850 nm and also which have low cost. But in case of single mode fibers, which operate at higher wavelengths as compared to multimode fibers for transmission.

Question 2

How is a SONET STS-3 signal constructed from STS-1 signals

Answer 2

SONET stands for synchronous optical network, which is a North American standard but it is design such that it is an international standard. It is used for synchronously transferring multiple bit streams over a fiber optical network. The basic data rate for SONET is 51.840 Mb/s and it is used in multiples of this data rate. A signal, which uses basic SONET data rate of 51.84, Mb/s, is termed as STS-1 (that is, synchronous transport signal level 1).
SONET STS-3 (that is, synchronous transport signal level 3) signal is constructed by multiplexing three STS-1 signals such that the line rate of STS-3 signal (that is, 155.52 Mb/s) is three times the line rate of STS-1 signal. SONET STS-3 signal frame structure has 2430 bytes (9
270 bytes) in all, that is, it has nine rows with 270 bytes in each row. SONET STS-3 signal has two-dimensional structure. SONET STS-3 frame are also transmitted in 125 microseconds, just as SONET STS-1 signal frames. Therefore, when 1 byte is transmitted from each of the STS-1 signals in turn, it helps to form STS-3 signal.

Question 3

A fiber has a loss of 0.5 dB/km and a bandwidth-distance product of 1 GHz-km. Which of these specifications limits the distance over which a signal with a bandwidth of 50 MHz can be transmitted, with a loss of 20 dB or less

Answer 3

NO ANSWER

Question 4

What types of emitters, detectors, and fibers are most common in long-distance, high-data-rate systems

Question 5

Why are fiber-optic cables more popular in telephone trunk applications than for local loops

Question 6

An optical communications link is to be built using fiber rated for 5000 MHz-km. The light source is a laser diode producing 0.25 mW. The fiber has losses of 0.4 dB/km and is available in 2 km lengths. It can be spliced with a loss of 0.2 dB per splice, and there will be a 5 dB loss in the connectors throughout the system. The receiver used has a sensitivity of -30 dBm. The source rise time is 2 ns, and the receiver rise time is 4 ns.
(a) Prepare a loss budget for a Unk 20 krn long, and calculate the system margin in dB.
(b) What would be the maximum bandwidth that could be used with the system in (a)

Question 7

What are the most commonly used wavelengths for short-and long-range fiber-optic links

Question 8

How can the equivalent of a superheterodyne receiver be implemented in an optical communication system

Question 9

A fiber-optic cable has a rise time of 2 ns/km and a length of 40 km. It is used with a receiver with a rise time of 20 ns and a transmitter with a rise time of 50 ns. Calculate the total rise time and bandwidth of the system.

Question 10

What is meant by a loss budget for a fiber-optic system

Question 11

Discuss the advantages and disadvantages of the use of fiber optics in local-area networks.

Question 12

Over what distance could a signal be transmitted using the cable in Problem 35 and a data rate of 100 Mb/s with RZ code Neglect the transmitter and receiver rise times, and consider only the cable.

Question 13

What can be done to improve a system that does not have sufficient power at the receiver

Question 14

Which network topology is used by the FDDI standard Why

Question 15

What is the maximum total system rise time that would be acceptable for a 10 Mb/s signal using NRZ code

Question 16

What is system margin Why is it necessary

Question 17

What is the standard data rate for FDDI

Question 18

Calculate the bandwidth-distance product for a cable with a rise time of 0.3 ns/km.

Question 19

Why do even single-mode fibers have some dispersion

Question 20

What other medium, in addition to optical fiber, can be used with FDDI

Question 21

What is the total possible data rate for a fiber on which 30 wavelengths with data rates of 10 Mb/s each are multiplexed

Question 22

Explain the relationship between bandwidth-distance product and rise time.

Question 23

What physical and logical topologies are used with Ethernet on fiber

Question 24

A fiber system uses DWDM to achieve a data rate of 200 Mb/s. How many wavelengths are needed if the technology used allows 5 Mb/s on each

Question 25

Explain why the permissible amount of pulse stretching is different for RZ and NRZ codes.

Question 26

What is the maximum distance limitation for gigabet Ethernet on fiber with single-mode cable

Question 27

Sketch one frame of a SONET STS-3 signal.

Question 28

What advantage does a regenerative repeater have over an amplifier as a means of extending the length of a fiber-optics system

Question 29

Distinguish between FITL and FTTC, and comment on the relative practicality of each.

Question 30

Approximately how many telephone calls could be carried by an STM-16 signal

Question 31

Describe the construction and operation of an erbium-doped fiber amplifier.

Question 32

Why do cable television systems still use analog signals with their fiber-optic trunk lines

Question 33

Draw a block diagram of a cable television system, indicating areas that are likely candidates for conversion to fiber optics.

Question 34

Does a fiber amplifier have any advantages over a regenerative repeater If so, state them.

Question 35

What are solitons How does their use improve the performance of a fiber-optic system

Question 36

Draw a block diagram showing how an optical heterodyne system can be implemented.

Question 37

How can full-duplex communication be implemented on a single optical fiber

Question 38

Describe the fiber-optic equivalent of a superheterodyne receiver.

Question 39

A fiber system uses single mode fiber with a loss of 0.15 dB/km and dispersion of 0.7 ps/km. The laser diode sources used with this system have a power output of 4 mW and a rise and fall time of 50 ps. The receivers have a rise and fall time of 30 ps and require a power level of -30 dBm to operate. The system operates with a data rate of 2.5 Gb/s using an NRZ code. Allow 10 dB for losses other than those of the cable.
(a) What is the maximum distance over which this system can operate without either a repeator an amplifier
(b) Determine whether the maximum distance for a single link in this system is determined by dispersion or by loss.
(c) Suppose that you needed to communicate over a distance of 300 km with this system. Would you suggest using an optical amplifier or a repeater Why

Question 40

Compare copper and fiber optics as media for submarine communications cables.

Question 41

A communications link has a length of 50 km. The transmitter power output is 3 mW, and the losses are as follows:
Connector loss (total): 5 dB
Splice loss: 0.3 dB per splice (splices are 2 km apart)
Fiber loss: 1.5 dB/km
Calculate the power level at the receiver, in dBm.

Question 42

Suppose the system in Problem 45 were converted to use DWDM with 30 wavelengths. What data rate would be possible with the setup described in Problem 45, over the same distance

Question 43

Why is the SONET system better adapted to optical fiber than the T-carrier system

Question 44

A laser diode emits a power of 1 mW. It is to be used in a fiber-optic system with a receiver that requires a power of at least 1 W for the required bit error rate. Determine whether the system will work over a 10 km distance. Assume that it will be necessary to have a splice every 2 km of cable. The losses in the system are as follows:
Coupling and connector losses, transmitter to cable: 10 dB
Cable loss: 0.5 dB per km
Splice loss: 0.2 dB per splice
Connector loss between cable and receiver: 2 dB

Question 45

How does SONET deal with problems in synchronization among the systems with which it must interface

Question 46

A fiber-optic cable has a bandwidth-distance product of 500 MHz-km. What bandwidth can be used with a cable thal runs 50 km between repeaters