K u v e m p u U n i v e r s i t y

Assignments for B.Sc.(IT)

Subject: Data Communication

Subject Code: BSIT – 23

Assignment TA (Compulsory)

1. Discuss the different entities involved in a simple communication network model.Ans: The different entities are:

i> Source: Device that generates the data to be passed on to the Destination device. It could be user computer trying to make a query to a server computer.

ii> Transmitter: If the data generated by the source device has to be transmitted through transmission channel or transmission system then it has to be presented in a form that is acceptable to the transmission system. This job is done by the transmitter.

iii> Transmission system: This can be a single transmission line connecting the two systems communicating or a complex network to which numerous communicating systems are connected.

iv> Receivers: This receives the signal from the transmission system and converts it into a form that is suitable to the destination device.

v> Destination: Device to which the source device sends data.

2. Discuss the TCP/IP protocol architecture.

Ans: TCP/IP protocols map to a four-layer conceptual model known as the DARPA model , named after the U.S. government agency that initially developed TCP/IP. The four layers of the DARPA model are: Application, Transport, Internet, and Network Interface. Each layer in the DARPA model corresponds to one or more layers of the seven-layer Open Systems Interconnection (OSI) model.

3. Discuss why digital transmission is preferred over analog transmission.

Ans: A transmission system makes use of a physical transmissionmedia or channel that allows the propagation of electromagnetic energy in the form of pulses or variation in voltage, current, or light intensity. In analog communication the objective is to transmit a signal waveform, which is a function that varies continuously with time. In digital transmission the objective is to transmit a given symbol that is selected from some finite set of possibilities. The task of the receiver is to receive the signal and determine the input symbol with high probability. The positive or negative pulses that were transmitted for the given symbols can undergo a great degree of distortion. Where signaling uses positive or negative voltages, the system will operate correctly as long as the receiver can determine whether the original voltage was positive or negative the cost advantages of digital over analog transmission become apparent when transmitting over a long distance.

4. Explain different modulation techniques.

Ans: In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal (high frequency signal), with a modulating signal that typically contains information to be transmitted.

In telecommunications, modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform transforms a baseband message signal into a pass band signal.

A modulator is a device that performs modulation. A demodulator (sometimes detector or demod) is a device that performs demodulation, the inverse of modulation. A modem (from modulator–demodulator) can perform both operations.

The different modulation techniques are:-

Quadrature PSK (QPSK), using M=4 symbols

8PSK, using M=8 symbols

16PSK, using M=16 symbols

Differential PSK (DPSK)

Differential QPSK (DQPSK)

Offset QPSK

π/4–QPSK5. List different transmission media. Discuss properties of optical cable.

Ans: The transmission media are:-Copper twisted pairCopper coaxial cable, andOptical fiber.

An optical fiber (or optical fiber) is a flexible, transparent fiber made of high quality extruded glass (silica) or plastic, slightly thicker than a human hair. It can function as a waveguide, or “light pipe”, to transmit light between the two ends of the fiber. Power over Fiber (PoF) optic cables can also work to deliver an electric current for low power electric devices. The field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics.

6. Explain circuit switching in detail

Ans: Circuit switching is a methodology of implementing a telecommunications network in which two networ nodes establish a dedicated communications channel (circuit) through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the duration of the communication session. The circuit functions as if the nodes were physically connected as with an electrical circuit.

The defining example of a circuit-switched network is the early analog telephone network. When a call is made from one telephone to another, switches within the telephone exchanges create a continuous wire circuit between the two telephones, for as long as the call lasts.

Circuit switching contrasts with packet switching which divides the data to be transmitted into packets transmitted through the network independently. In packet switching, instead of being dedicated to one communication session at a time, network links are shared by packets from multiple competing communication sessions, resulting in the loss of the quality of service guarantees that are provided by circuit switching.

7. Write a note on modems and digital modulation.

Ans: A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information and demodulates the signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used with any means of transmitting analog signals, from light emitting diodes to radio. The most familiar type is a voice band modem that turns the digital data of a computer into modulated electrical signals in the voice frequency range of a telephone channel. These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data.

In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods

can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as

analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M

alternative symbols (the modulation alphabet).

A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not

digital bits (zeros and ones). Computers may however communicate over a telephone line by means of

modems, which are representing the digital bits by tones, called symbols. If there are four alternative

symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the

first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem

plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or baud.

Since each tone (i.e., symbol) represents a message consisting of two digital bits in this example, the bit

rate is twice the symbol rate, i.e. 2000 bits per second. This is similar to the technique used by dialup

modems as opposed to DSL modems.

8. Discuss crossbar switch. How the cross-points required are reduced in the multistage switches?

Ans: In electronics, a crossbar switch (also known as cross-point switch, cross-point switch, or matrix switch)

is a switch connecting multiple inputs to multiple outputs in a matrix manner. Originally the term was used

literally, for a matrix switch controlled by a grid of crossing metal bars, and later was broadened to matrix

switches in general. It is one of the principal switch architectures, together with a rotary switch, memory

switch and a crossover switch.

The number of cross points required in a three-stage switch is the sum of the following components:-

N/n input switches x nk cross points/input switch.

K intermediate switches c (N/n)2 cross points/intermediate switch.

N/n output switches x nk cross points/output switch.

In this case the total number of cross point is: 2Nk + k(N/n)2

The number of cross points required to make the switch non blocking is: 2N(2n----- 1) + (2n-1)(N/n)2

The number of cross points can be minimized through the choice of group size n. By differentiating the

above expression with respect to n, we find that the number of cross points is then 4N((2N)1/2 -1). We then

see that the minimum number of cross points grows at a rate proportional to N1.5 which is less than the N2

growth rate of a crossbar switch.