HOME WORK TITLE NO. : 2COURSE CODE: CAP 306COURSE INSTRUCTOR: LECT. JASLEENDOA:DOS: OCT 10, 2009ROLLNO: D3803A03SECTION NO: D3803

DECLARATION:

I declare that this assignment is my individual work. I have not copied from any other students’ work or from any other source, except where due acknowledgement is made explicitly in the text, not has any part being written for me by other person.

KIRTI SINGH

Evaluator comments…………………………………………………………………………..

Marks obtained ………. out of …………….

PART A

Ques1. The Nyquist theorem is one of the deciding factors in data communication. The fiber 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?

Ans: - Nyquist theorem 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. The concept behind digitizing sound. Working at Bell Labs, Harry Nyquist discovered that it was not necessary to capture the entire analog waveform; rather samples of the wave could be taken at various points. He also found that in order to have enough information in the sample pool to reconstruct the original waveform, the sampling rate must be at least twice the signal bandwidth.Nyquist theorem is a sampling theory, doesn't matter what the media is, as long as the transmission is on a TDM system.

Ques2. 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.

Ans: - In order to study the effects of noise upon radio communication, an amplitude-modulation radio system was set up in the laboratory and provision was made for generating electrical interference and introducing it, together with the speech-modulated carrier from the transmitter, into the receiver. In addition, arrangements were made for producing ambient noise at the talkers' and listeners' positions, so that the separate effects of acoustic noise and radio noise could be compared and their combined effects could be studied. By means of word articulation tests, the intelligibility of speech heard over the radio system was determined as a function of signal-to-noise ratio for each of a number of types of noise. Several principles of noise reduction were studied, and basic parameters of the radio link were varied systematically so that their influence upon the effects of noise could be determined. With regard to ambient noise, it was found that exclusion of noise at the microphone is even more important as a prerequisite for effective radio communication than it is for effective interphone communication, especially if either compression or premodulation clipping is employed in the radio transmitter. Noise exclusion at the listeners' end of the line is also important because, for optimal intelligibility under difficult conditions, it is necessary for the noise reaching the ear through or under the earphone cushions to be at least 10 decibels less intense than the noise coming through the earphones from the receiver. The deleterious effect of electrical interference was found to depend greatly upon the relation between certain characteristics of the noise and corresponding characteristics of the receiver circuits. In general, interferences with continuous spectra are more detrimental than those with line spectra, and non-impulsive types more detrimental than impulsive types. Noise-reducing circuits were ineffective against random fluctuation noise, but in the presence of certain types of impulse interference, limiters and canceling circuits provided such great improvement in performance that it was possible to maintain satisfactory communication despite a 35-decibel reduction in carrier intensity. As a general principle, it appears that, whenever there is a characteristic difference between the wave forms or the spectra of the signal and the interference, the impairment of intelligibility by electrical interference may be reduced by employing amplitude-selective or frequency-selective circuits in the radio receiver.

Ques3. 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?

Ans: - Given:Number of bits in each signal element= 4Number of signal elements sent each second= 10000

Now, baud rate= number of signal elements, andBit rate= baud rate × number of bits per signal element.

So baud rate= 10000 bauds per secondBit rate= 10000 × 4= 40000 bps

Ques4. What are the reasons for the imperfection caused in transmission media? How the perfection can be measured?

Ans: - A device including an element for receiving an electrical signal comprising a high-frequency data signal component and a low-frequency power supply component. The electrical signal is conveyed in an electrical cable of an electrical installation. The device further includes impedance matching operative in a band of frequencies of the high-frequency signal component, the impedance matching being determined as a function of one or more characteristics of the electrical cable. Such a device can be integrated into a socket outlet or an electrical device or take the form of a socket adaptor. Signals travel through the transmission media, which are not perfect. The imperfections cause impairment in signal. This means that the signal at the beginning and end of the medium are not same. What is sent in and what is received. There are three types of impairment usually occur. These are attenuation, distortion and noise. Attenuation means the loss of energy. When a signal, simple or composite, travels through a medium, it loses some of its energy so that it can overcome the resistance of the medium. That is why a wire carrying electrical signals gets worm, if not hot, after a while. Some of the electrical energy in signal is converted to heat. To compensate for this loss, the amplifiers are used to amplify the signal.

Distortion means that signal changes its form or shape. Distortion occurs in a composite signal that is made of different frequencies. Each signal component has its own propagation speed through a medium and, therefore its own delay in arriving at the final destination. Noise is another problem occurred during the transmission of data.  Several types of noise like thermal noise, induced noise, crosstalk, and impulse noise may corrupt the signal. Thermal noise is the random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter.  Induced noise comes from the sources such as motors and appliances. These devices act as a sending antenna and transmission medium acts as the receiving antenna. Crosstalk is the effect of one wire on other.

Ques5. 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?

Ans: - Multiplexing Techniques

Multiplexing is the process where multiple channels are combined for transmission over a common transmission path.

There are two predominant ways to multiplex:

Frequency Division Multiplexing Time Division Multiplexing

Frequency Division Multiplexing (FDM)

In FDM, multiple channels are combined onto a single aggregate signal for transmission. The channels are separated in the aggregate by their FREQUENCY.

There are always some unused frequency spaces between channels, known as "guard bands". These guard bands reduce the effects of "bleedover" between adjacent channels, a condition more commonly referred to as "crosstalk".

FDM was the first multiplexing scheme to enjoy wide scale network deployment, and such systems are still in use today. However, Time Division Multiplexing is the preferred approach today, due to its ability to support native data I/O (Input/Output) channels.

Time Division Multiplexing

Timeplex is probably the best in the business (IMHO) at Time Division Multiplexing, as it has 25+ years or experience. When Timeplex was started by a couple of ex-Western Union guys in 1969 it was among the first commercial TDM companies in the United States. In fact, "Timeplex" was derived from TIME division multiPLEXing!

In Time Division Multiplexing, channels "share" the common aggregate based upon time

Ques6. While transferring the data from the transmission medium there are various aspects of your data getting tempered by other users? What in your opinion is the most secure and insecure transmission medium? Justify your answer with an example.

Ans: - There are three broad categories of media: Wire, fiber and wireless. On a very high level, it could be said that fiber is the most secure as it is the hardest to tap. Cable is a little more secure, as physical access is not hard to tap and sniff the passing traffic. Then there is wireless, it broadcasts point-to-point or well beyond the facility in all possible directions. Anyone that can pick up the signal may be able to sniff sensitive information. Although there is a second item we must consider, and that is the physical and technical controls that have been implemented. Wireless can be made more secure by using WPA, encryption or 802.1x. A cabled network can be fully switched, use encryption and have implemented VLANS. So the point would be that even seemingly weaker systems can have controls implemented to make their security more robust.

PART B

Ques1. Assume a stream is made of ten 0s .Encode this stream, using following encoding schemes .How many can you find for each scheme?

Unipolar NRZ-L NRZ-I RZ Manchester Differential Manchester

Ans: -

Unipolar: -

0 0 0 0 0 00 0 0 0

NRZ-L: -

0 0 0 0 0 00 0 0 0

NRZ-I: -

0 0 0 0 0 00 0 0 0

Ques2. 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. Answer the following:

What is the size of frame in bits What is the frame rate? What is the duration of a frame? What is the data rate?

Ans: -

Ques3. Contrast & compare sampling rate & received signal?

Ans: - sampling rate: 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), or hertz (Hz). The inverse of the sampling frequency is the sampling period or sampling interval, which is the time between samples.

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.

Sampling theorem

The Nyquist–Shannon sampling theorem states that perfect reconstruction of a signal is possible when the sampling frequency is greater than twice the maximum frequency of the signal being sampled, or equivalently, when the Nyquist frequency (half the sample rate) exceeds the highest frequency of the signal being sampled. If lower sampling rates are used, the original signal's information may not be completely recoverable from the sampled signal.

For example, if a signal has an upper band limit of 100 Hz, a sampling frequency greater than 200 Hz will avoid aliasing and allow theoretically perfect reconstruction.Also called a sample rate. Typically expressed in samples per second, or hertz (Hz), the rate at which samples of an analog signal are taken in order to be converted into digital form. A PC’s sound card typically will sample a received analog signal, such as through a microphone, and digitize it for use by the computer. A higher sampling rate provides a better quality reproduction than a lower sampling rate.

In telecommunications, received signal strength indication (RSSI) is a measurement of the power present in a received radio signal.

RSSI is generic radio receiver technology metric, which is usually invisible to the user of device containing the receiver, but is directly known to users of wireless networking of IEEE 802.11 protocol family.

RSSI is often done in the intermediate frequency (IF) stage before the IF amplifier. In zero-IF systems, it is done in the baseband signal chain, before the baseband amplifier. RSSI output is often a DC analog level. It can also be sampled by an internal ADC and the resulting codes available directly or via peripheral or internal processor bus.

Ques4. Synchronization is the problem in data communication. Explain?

Ans: - 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 received by the wrong channel. For this 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 synchronize with the incoming steam so that it can separate the time slots accurately. in most cases, this synchronization information consists of one bit

Ques5. Can bit rate be less than the pulse rate? Why or why not?

Ans: - In telecommunications and computing, bitrate (sometimes written bit rate, data rate or as a variable R or fb) is the number of bits that are conveyed or processed per unit of time.

The bit rate is quantified using the bits per second (bit/s or bps) unit, often in conjunction with an SI prefix such as kilo- (kbit/s or kbps), mega- (Mbit/s or Mbps), giga- (Gbit/s or Gbps) or tera- (Tbit/s or Tbps). 1 kbit/s has almost always meant 1,000 bit/s, not 1,024 bit/s, also before 1999 when SI prefixes were defined for units of information in an IEC standard.

The formal abbreviation for "bits per second" is "bit/s" (not "bits/s"). In less formal contexts the abbreviations "b/s" or "bps" are often used, though this risks confusion with "bytes per second" ("B/s", "Bps").A pulse generating system generating a series of relatively low-speed or low-frequency pulses from a high-speed stable crystal oscillator, utilizes a 66-stage MOS shift register as a divisor. A second source of very high-frequency clock pulses provides a control of the pulse width of the several pulses within the pulse generating system. Disclosed herein are two systems which function in a similar manner and each are provided with suitable control members to control the generation of the lower frequency output pulses from the pulse generating system.

Ques6. A signal is sampled. Each sample represents one of four levels. How many bits are needed to represent each sample? If sampling rate is 8000 samples per second, what is the bit rate?

Ans: - first of all, we need to calculate number of bits per sample. Actually, we need 2 bits. One bit for the sign and 1 bit for the value. A 3-bit value can represent 23=8 levels, which is more than what we need.

Therefore, number of bits per sample= 2Sampling rate= 8000 samples per second [given]

Now, bit rate= sampling rate × number of bits per second

Therefore, bit rate= 80000 × 2= 160000 kbps