# The fibre optics

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### PART A

1. The Nyquist theorem is one of the deciding factor in data communication. The fibre optics as well as the copper wires are communication mediums. Do you think the theorem is valid for the fiber optics or for the copper wires.

The Nyquist theorem is a property of mathematics and has nothing to do with technology. It says that if you have a function whose Fourier spectrum does not contain any sines or cosines above f, then by sampling the function at a frequency of 2f you capture all the information there is. Thus, the Nyquist theorem is true for all media.

2. Noise affects all the signals which are there in air. There are some communicating modulation techniques. Noise affects which of the modulation technique the most.

AM is more noise susceptible than FM because you hear noise from lightning on AM radio but not on FM radio. In reality it is the high bandwidth of FM that allows the effects of noise to be minimized.

3. An analog signal carries 4 bits in each signal element. If 10,000 signal elements are sent per sec, find the Baud Rate and Bit Rate?

Baud rate = number of sgnal elements=1000 bauds per sec.

Bit rate=baud rate*number of bits per signal elements=1000*4=4000bps.

4. What are the reasons for the imperfection caused in tramsmission medias ? how the perfection can be measured?

Communication is one of the most important functions performed by computers. It is easy to understand that a computer must calculate, compare, and store data. It is also easy to see that the computer must input and output data used to communicate with the input and output devices. Also, within the computer, data must be transferred from one location-for example, from read only memory to random access memory (RAM), or from permanent storage such as a compact disc to temporary memory, etc. Transferring data, therefore, is just another way of referring to communication.

Communicating from one device to another within the computer is usually done through a bus, which is essentially a set of printed circuit boardtraces or wires within the computer. Buses are usually local to the device. Very special wire arrangements, called transmission lines, must be used when the required communications travel a significant distance or are executed at a very high speed. For high-speed computers, these transmission line techniques are even used for the internal computer buses.

A very simple transmission line consists of a pair of wires that are kept precisely side by side. Since the wires are often twisted to keep them together, they have earned the title “twisted pair.” The twisted pair was one of the first transmission lines originally used, and remains in use for telephone systems. The twisted pair is inexpensive to manufacture and is also used extensively for computer communications.

Twisted pairs are used in many local area networks (LANs). There is a large selection of standard cable for use in LANs. Some twisted pairs include a shield, which reduces the amount of egress and ingress. The two basic types of twisted pairs are unshielded twisted pair (or UTP) and shielded twisted pair (or STP).

Although twisted pairs can have very little ingress and egress, they are not perfect, particularly at higher frequencies and data rates. The imperfections in transmission lines become more pronounced when the data rates are very high. One transmission line topology that reduces the amount of egress and ingress for very high data rates, extending to the gigabit per second range, is the coaxial transmission line, commonly called coaxial cable or co-ax. In this transmission line design, one conductor is actually a hollow cylinder and the other conductor is placed in the center of the cylinder. Egress and ingress can be reduced to very low levels but coaxial cable is much more expensive than unshielded twisted pairs. Coaxial cable is used for television distribution despite the increased cost because of the very broad frequency range of television signals.

The loss of energy due to radiation and from other losses within the transmission line subtracts from the signal energy in the line. This means that the signal has to be amplified or restored if the transmission line is long. Since there are more losses at higher frequencies, more amplification is required for the higher frequencies. Very high-speed data communications systems will require more amplifiers or repeaters for the same length of cable.

The best transmission line for very high-speed data or wide bandwidth is a glass fiber. It is often difficult to view the glass fiber as a transmission line because the signals within the fiber are not electrical but light waves.But, light waves are electromagnetic waves just like radio. The glass fiber has characteristics exactly like a wire transmission line. However, the transmission rate is generally higher.

Many computer communications applications require a “wireless” communications medium. Of course, in modern terminology it must be understood that wireless also implies “fiber optic-less.” Clearly this is the only communications solution for portable and vehicle-mounted devices. Wireless transmission is accomplished through electromagnetic waves, radio, and light. These electromagnetic waves require no physical medium because they are able to flourish through a vacuum better than through any substance. In fact, wireless signals can be partially blocked by common building materials causing difficulties with wireless systems used indoors.

For transmission through short distances, “wireless modems” are used. These modems are low powered radio transmitters and receivers that require no government license. Long distance data communications using radio waves include terrestrial microwave links and satellite data links. These applications involve much higher power transmitters and require government licensing to insure that users do not interfere with other users. Microwave links propagate in straight lines. Depending on the height of the transmitting and receiving antennas, the microwave links are seldom more than 100 kilometers (or approximately 62 miles) apart because of the curvature of the Earth. As with wired communications, repeaters are required to extend microwave communications.

There are two basic types of satellite links, LEO for low earth orbiting and GEO for geostationary orbit. LEO satellites orbit the Earth in less than two hours and are visible to the user for only 20 minutes or so. To provide continuous communications, a number of satellites called a “constellation” is required. Thus when one satellite “sets” or is no longer in view, another satellite can be used for communications. Because the satellites are close to the Earth, typically only 800 kilometers (497 miles) or so, a modest antenna and transmitter power will provide reliable communications. On the other hand, because the user must switch from one satellite to another, a complex system must be employed to switch the communications channel between satellites much like a cellular telephone system in space.

The GEO satellite is always in view and the antenna is pointed at the satellite. Since the satellite never sets, only one satellite is used. Most GEO satellite systems are used by large organizations. This is because the uplink transmitter must use a rather large antenna and needs to be licensed by the government. For small users and individuals, satellite systems are available where the uplink is provided via a conventional telephone line and the downlink is via the satellite. Generally, the uplink data rate required by the individual user is much less than the downlink and this arrangement is acceptable.

For short distance communications, such as from a large room of computers to a LAN, infrared radiation may be used. Low-powered infrared radiation from a light emitting diode (LED) provides the transmitter while the receiver is a phototransistor or diode. This type of infrared technology has been used for many years for remote control devices for consumer entertainmentequipment such as television. The range of these systems can be as much as 30 meters (98.5 feet), but the light energy can be blocked easily.

5. There are numerous multiplexing techniques available. What in your opinion is the most appropriate multiplexing technique for the fibre optics as well as copper wires?

Multiplexing is defined as the process of combing multiple signals together in order to share a transportation medium. Two popular multiplexing techniques for fiber optic communication systems are Time Domain Multiplexing (TDM) and Wavelength Division Multiplexing (WDM). Multiplexing saves the number of fibers needed to transmit signals.

### TDM

Time domain multiplexing; is accomplished in the electrical domain. Multiple parallel signals are simultaneously applied to a multiplexor that will only allow each input signal to transmit through the communication link at certain times. Since multiplexing is done in the time domain, TDM can be used with multimode fiber. Since the output bit rate is an addition of all the input signals plus some overhead, the output data rate quickly becomes large. Timing and latency issues are to be considered when using a TDM network.

### WDM

Wavelength Division Multiplexing is done entirely in the optical domain. Input electrical signals are each assigned a wavelength, which are combined on one fiber for transmission and separated before being received. Each electrical input signal can operate at an independent bit rate and will not interfere with any of the other input signals. Wavelength division multiplexing is available in multiple channels. Evertz offers an economical 1310nm/1550nm WDM solution consisting of two wavelengths only. Also available are 4, 8, 12 and 16 channel CWDM systems that have a 20nm channel spacing. 8, 16, 24, 32, and 40 channel dense wavelength division multiplexing systems are also now available for high signal capacity networks.

6. While transferring the data from the transmission medium there are various aspects of your data getting tempered by other users. What in your opinin is the most secure and insecure transmission medium. Justify your answer with an example.

Trasmission through ethernet cable would bemore secure as it would be difficult for intruders to access the user resources. Wirelessmedium is the unsafe of all as it could be easily hacked without a good encryption like WPA personel or WPA2..

### PART B

1. Assume a stream is made of ten 0s. Encode this stream, using following encoding schemes. How many can you find for each scheme ?

Â· Unipolar

Unipolar encoding is a line code. A positive voltage represents a binary 1, and zero volts indicates a binary 0. It is the simplest line code, directly encoding the bitstream, and is analogous to on-off keying in modulation.

Its drawbacks are that it is not self-clocking and it has a significant DC component, which can be halved by using return-to-zero, where the signal returns to zero in the middle of the bit period. With a 50% duty cycle each rectangular pulse is only at a positive voltage for half of the bit period. This is ideal if one symbol is sent much more often than the other and power considerations are necessary, and also makes the signal self-clocking.

Traditionally, a unipolar scheme was designed as a non-return-to-zero scheme, in which the positive voltage defines bit 1 and the zero voltage defines bit 0. It is called NRZ because the signal does not return to zero at the middle of the bit.

Compared with its polar counterpart, Polar NRZ, this scheme is very expensive. The normalized power (power required to send 1 bit per unit lne resistance) is double that for polar NRZ. For this reason, this scheme is not normally used in data communications today.

Â· NRZ-L

NRZ-L: [Non-Return-to-Zero-Level]: In NRZ-L encoding, the polarity of the signal changes only when the incoming signal changes from a one to a zero or from a zero to a one. NRZ-L method looks just like the NRZ method, except for the first input one data bit. This is because NRZ does not consider the first data bit to be a polarity change, where NRZ-L does

Â· NRZ-I

NRZI [Non-Return-to-Zero-Inverted Encoding]: A ‘0' is encoded as no change in the level. However a ‘1' is encoded depending on the current state of the line. If the current state is ‘0' [low] the ‘1' will be encoded as a high, if the current state is ‘1' [high] the ‘1' will be encoded as a low. Used with FDDI and USB for example.

Â· RZ

Return-to-zero (RZ) describes a line code used in telecommunications signals in which the signal drops (returns) to zero between each pulse. This takes place even if a number of consecutive 0's or 1's occur in the signal. The signal is self-clocking. This means that a separate clock does not need to be sent alongside the signal, but suffers from using twice the bandwidth to achieve the same data-rate as compared to non-return-to-zero format.

The “zero” between each bit is a neutral or rest condition, such as a zero amplitude in pulse amplitude modulation (PAM), zero phase shift in phase-shift keying (PSK), or mid-frequency in frequency-shift keying (FSK). That “zero” condition is typically halfway between the significant condition representing a 1 bit and the other significant condition representing a 0 bit.

Although return-to-zero (RZ) contains a provision for synchronization, it still has a DC component resulting in "baseline wander" during long strings of 0 or 1 bits, just like the line code non-return-to-zero.

Â· Manchester

In telecommunication, Manchester code (also known as Phase Encoding, or PE) is a line code in which the encoding of each data bit has at least one transition and occupies the same time. It is, therefore, self-clocking, which means that a clock signal can be recovered from the encoded data.

Manchester code is widely-used (e.g. in Ethernet. See also RFID). There are more complex codes e.g. 8B/10B encoding which use less bandwidth to achieve the same data rate (but which may be less tolerant of frequency errors and jitter in the transmitter and receiver reference clocks

An example of Manchester encoding showing both conventions

Â· Differential Manchester

Differential Manchester encoding (also known as CDP; Conditioned Diphase encoding) is a method of encoding data in which data and clock signals are combined to form a single self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value. This gives it several advantages over standard Manchester encoding:

• Detecting transitions is often less error-prone than comparing against a threshold in a noisy environment.
• Because only the presence of a transition is important, polarity is not. Differential coding schemes will work exactly the same if the signal is inverted (wires swapped). (Other line codes with this property include NRZI, bipolar encoding, biphase mark code, coded mark inversion, and MLT-3 encoding).

A ‘1' bit is indicated by making the first half of the signal equal to the last half of the previous bit's signal i.e. no transition at the start of the bit-time. A ‘0' bit is indicated by making the first half of the signal opposite to the last half of the previous bit's signal i.e. a zero bit is indicated by a transition at the beginning of the bit-time. In the middle of the bit-time there is always a transition, whether from high to low, or low to high. A reversed scheme is possible, and no advantage is given by using either scheme.

2. Two channels, one with bit rate of 150kbps and another with a bit rate of 140kbps,are to be multiplexed using pulse stuffing TDM with no synchronization bits.Answere the following:

Â· What is the size of frame in bits

The sizes are categorized into three categories:- Small frame, medium frame and large frame. The reason for this is that obviously bone structures vary in size and density from person to person. Equally obviously men and women have different structures. Bone mass and muscle mass all play a part in determining your optimal weight. Large boned people. There are two simple methods of determining frame size:-

• Measuring the circumference of your wrist. This is by far the most straight forward.

Â· What is the frame rate ?

Frame rate, or frame frequency, is the measurement of the frequency (rate) at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems. Frame rate is most often expressed in frames per second (FPS) and in progressive-scan monitors as hertz (Hz).

Â· What is the duration of a frame?

The time between the beginning of a frame and the end of that frame. Note: For fixed-length frames, at a fixed data rate, frame duration is constant.

Â· What is the data rate?

Data rate can refer to:

• Bit rate
• Data signaling rate
• Data transfer rate
• Data rate units

3. Contrast & compare sampling rate & received signal?

The sampling rate, sample rate, or sampling frequency defines the number of samples per second (or per other unit) taken from a continuous signal to make a discrete signal. For time-domain signals, it can be measured in samples per second (S/s),[1] or hertz (Hz).[2] The inverse of the sampling frequency is the sampling period or sampling interval, which is the time between samples.[3]

The concept of sampling frequency can only be applied to samplers in which samples are taken periodically. Some samplers may sample at a non-periodic rate.

The common notation for sampling frequency is fs which stands for frequency (subscript) resulting sampled signal particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in voltage per length or signal power received by a reference antenna. High-powered transmissions, such as those used in broadcasting, are expressed in dB-millivolts per metre (dBmV/m). For very low-power systems, such as mobile phones, signal strength is usually expressed in dB-microvolts per metre (dBÂµV/m) or in decibels above a reference level of one milliwatt (dBm). In broadcasting terminology, 1 mV/m is 1000 ÂµV/m or 60 dBÂµ (often written dBu).

4. Synchronization is the problem in data communication. Explain?

synchronization between the multiplexer and demultiplexer is a major issue in data transmission. if the multiplexer and demultiplexer are out of synchronization a bit belonging to one channel may be recieved by the wrong channel. for dis reason, one or more synchronization bits are usually added to the beginning of each frame. these bits, called framing bit, follow a pattern, frame to frame, that allow the demultiplexer to syncronize with the incoming steam so that it can separate the time slots accurately. in most cases, this syncronization information consists of one bit.

5. Can bit rate be less than the pulse rate? Why or why not?