COMMUNICATION TECHNOLOGY

by

Shashi BhushanSchool of Computer and Information Sciences

Q. Discuss the relationship between data rate and bandwidth.

Answer:

Direct relationship

• Higher data rate of signal, the greater the bandwidth.

• The greater the bandwidth of a signal the higher the data rate that can be transmitted using the signal.

Q. What is the difference between bandwidth, baudrate and channel capacity?

Answer:

Bandwidth

Bandwidth is an electrical characteristic of transmission line or circuit. It indicates the range of frequencies measured in Hz. (cycles per second) which can be successfully transmitted over the line.

Baudrate

Baudrate is the number of signal elements. This defines the signalling rate on the transmission line. A signal element is a discrete voltage, phase or frequency value.

Channel Capacity

Channel Capacity is the rate at which data can be transmitted over a given Communication Path or channel under given condition.

In case of noise free channel, the limitation of data rate is simply the bandwidth of the channel.

Measuring the Channel Capacity

• Nyquist Law

• Shannon’s Law

Nyquist Law

Given a bandwidth of w, the highest signal rate that can be carried is 2w.

Nyquist Law

If the signal to be transmitted are binary (two voltage levels) then the data rate that can be supported by whz is 2 wbps

Q. Suppose a bandwidth is 3100 hz then what is the channel capacity if

(a) Signal to be transmitted are two voltage levels?

(b) If the signal to be transmitted are 8 voltage levels?

Nyquist Law

Answer:

• For two voltage levels

C =2 * 3100 = 6200 bps

(b) For 8 voltage levels with multilevel signalling the Nyquist formulation becomes

C = 2 W log2M

M = digital levels = 8

W = bandwidth

Nyquist Law

Nyquist Law

For multivoltage level

C = 2 * 3100 * log28

= 2 * 3100 * 3 = 18,600 bps

For a given bandwidth the data rate can be increased by increasing the number of different level of signals.

Shannon’s Law

• Used for noisy channel

C = W log2(1 + S/N)

S/N = Signal Power/Noise Power

Shannon’s Law

Q. Calculate a channel capacity of W = 3100 Hz, S/N =1000:1 or 30db

Answer:

C = 3100 log2 (1 +1000) = 30894 bps

Shannon’s Law

Observation:

Data rate could be increased by either increasing signal strength or bandwidth. However, as the signal strength increases, so do the nonlinearities in the system; leading to an increase intermodulation noise.

Shannon’s Law

• More amplifiers needed for increasing S/N ratio.

• Since noise is constant throughout the line the location of the amplifiers must be closely inserted.

Shannon’s Law

• Frequent spacing of amplifiers increases costs.

• Amplifiers must be carefully designed to minimise the amount of noise that is amplified along with the signal.

Transmission Impairements

• Attenuation and attenuation distortions

• Delay distortion

• Noise

Network Devices

Methods for connecting to a network or an independently connected computer to another computer are:

• Through a modem connection

• Through an ISDN connection

• Through a gateway

• Through a bridge or router

Modem

Modem converts digital data into an analog form that can be transmitted over a standard telephone line

Modem

The term modem is a composite word that refers to the two functional entities that make up the device; a modulator and a demodulator.

Modem

Normally, telephone type connections are unsuitable for digital data as they have a limited bandwidth of 300 to 400 hz. A modem must then be used to convert the digital information into an analog form that is transmittable over the telephone line.

Modem

A modulator treats a digital signal as a series of 1s and 0s and so can transform it into a completely analog signal by using the digital to analog mechanisms of ASK, FSK, PSX & QAM.

Modem

• ASK manipulates amplitude

• FSK manipulates frequency

• PSK manipulates phase

• QAM manipulates both phase and amplitude

Theoretical Bit Rates for Modems

ENCODING HALF DUPLEX

FULL DUPLEX

ASK, FSK, 2-PSK 2400 1200

4-PSK, 4-QAM 4800 2400

8-PSK, 8-QAM 7200 3600

16-QAM 9600 4800

32-QAM 12,000 6000

64-QAM 14,400 7200

128-QAM 16,800 8400

Modem StandardsToday, many of the most popular modems available are based on standards published by ITU-T.ITU-T Bell Baud-

rateBit rate

Modulation

V.21 103 300 300 PSK

V.22 212 600 1200 4-PSK

V.23 202 1200 1200 FSK

V.26 201 1200 2400 4-PSK

V.27 208 1600 4800 8-PSK

V.29 209 2400 9600 16-QAM

Routers

• Examine the network address field to determine the best route for the packets

• Supports several different types of network layer protocols

Routers

• Routers communicate with other routers to exchange routing information.

• Most network O.S. have associated routing protocols to support the transfer of routing information.

Routing Protocols

• BGP (Border Gateway Protocol)

• EGP (Exterior Gateway Protocol)

• OSPF (Open Shortest Path First)

• RIP (Routing Information Protocol)

Routing Protocols

• RIP – Most popular protocols in the past.

• Distributed with UNIX.

Routing Protocols

• RIP is based on distance vector algorithm which measures the number of hops to the destination router.

• Disadvantage – The smallest hops may not be the best route.

Routing Protocols

• With distance vector routing each router maintains a table by communicating with neighbouring routers.

• Updating the table takes time when changes occur.

Routing Protocols

Link state algorithm is based, not only on hops, but on other parameters such as delay capacity, reliability and throughput.