Data Communication and Networking332 Hardware Components of Data Communications

Lecture Overview

• Simplex, Half Duplex & Duplex• Serial & Parallel transmission• Synchronous & Asynchronous• Bit & Baud Rate

Communication ModesVarious modes of

communication/transmission:Simplex, Half Duplex & Full DuplexSerial & Parallel transmissionSynchronous & Asynchronous

Simplex, Half-Duplex & DuplexSimplex: Transmit in one direction. • Example: a radio station broadcasting

and data logging system Half-duplex: Two interconnected

devices exchanges information (data) alternately • Example: a “walkie-talkie”

Duplex: To interconnected devices transmit and receive simultaneously. • Example: A telephony system

Serial & Parallel

Serial – the process of sending data one bit at a time, sequentially, over a communication channel• Example: Modem

Parallel – the process of sending data multi bits at a time.• Example: Printer

Synchronous TransmissionSynchronous data transmission is a data

transfer method in which a continuous stream of data signals is accompanied by timing signals (generated by an electronic clock) to ensure that the transmitter and the receiver are in step (synchronized) with one another.

The data is sent in blocks (called frames or packets) spaced by fixed time intervals.

In brief, synchronous communication is direct communication where the communicators are time synchronized

E.g. a chat room event and instant messaging

Synchronous Transmission The transmitter sends out one or more unique

characters called SYNC characters are inserted at the beginning of the bit stream by the sender to mark the start of transmission

These characters are used to synchronize a block of information

When data is not being sent through a synchronous data link the line is held in a marking condition.

Two ways of organizing a synchronous data link : Character (or byte) oriented and bit oriented.

Byte Oriented & Bit Oriented

character-oriented: Also known as byte-oriented protocols Interpret a transmission frame as a

succession of characters

bit-oriented: Interpret a transmission frame as a

succession of individual bits Control information in a bit-oriented protocol

can be one or multiple bits depending on the information embodied in the pattern

Asynchronous Transmission• The information is received and translated by

agreed upon patterns• Data are generated at random intervals• It is necessary for the receiver to resynchronize

at the beginning of each character. • This is done by adding additional bits to the

beginning and end ( start and stop bits respectively) to encapsulate the original data bits• Examples are e-mail messages, discussion boards, blogging, and text messaging over cell phones

BIT & BAUD RATE

Data Element & Signal Element• Data is what we need to send.

Signal is what we can send. So, signal is the carrier which carriers data.• Smallest entity of the data that

can represent a piece of information is called data element.• Shortest meaningful unit of a

signal is called signal element

Data Element & Signal Element• Consider a train.• Each carriage is a signal

element, each passenger inside the train is a data element.• The train as whole is a signal

and all passengers together represent a data.

Data Rate & Signal Rate

• Data rate – number of data elements transmitted per second• Signal rate – number of signal

elements transmitted per second• Unit of data rate is bit rate.• Unit of signal rate is baud rate.

Bit Rate

The bit rate is the number of bits sent in 1s, expressed in bits per second (bps)

Bit Rate (Example)

• Assume we need to download text documents at the rate of 100 pages per minute. What is the required bit rate of the channel?– Note: A page is an average of 24

lines with 80 character in each line. One character requires average 8 bits.

Bit rate = 100 x 24 x 80 x 8 = 1,636,00 bps = 1.636

Mbps

Bit Rate (Example)• HDTV uses digital signal to broadcast

high quality video signals. The HDTV Screen is normally a ratio of 16:9 (in contrast to 4:3 for regular TV), which means the screen is wider. There are 1920 by 1080 pixels per screen, and the screen is renewed 30 times per second. Twenty four bits represent one color pixel.Bit rate = 1920 x 1080 x 30 x 24 = 1,492,992,000 bps = 1.5 Gbps

Baud Rate

• The baud rate refers to the number of signal elements or symbol per second.• A symbol may have 2 or more

bits represented by it.• In the analog transmission of

digital data, the baud rate is less than or equal to the bit rate.

Baud Rate (cont’d)

• Baud Rate is calculated using the below formula:

S = baud rateN = data rate (bps)r (rand)= the number of data elements carried in

one signal element

The value of r in analog transmission is r = log2 L, where L is the type of signal element, not the level.

Baud Rate (Example)

An analog signal carries 4 bits per signal element. If 1000 signal elements are sent per second, find the bit rate.

R = 4, S = 1000N = S x r = 1000 x 4 = 4000 bps

Baud Rate (Example)An analog signal has a bit rate of 8000 bps

and a baud rate of 1000 baud. How many data elements are carried by each signal element? How many signal elements do we need?In this example, S = 1000, N =8000, and rand L

are unknown. We find first the value of randthen the value of L.

r = N / S = 8000 / 1000 = 8r= log2 LL = 2r = 28 = 256

QUESTIONS?