CCN Lecture 4

8/3/2019 CCN Lecture 4

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Data Communication



Data Transmission

Data transmission is the transfer of data from point-to-point often represented as an

electromagnetic signal over a physical point-to-point

or point-to-multipoint communication channel A communication channel refers to the medium used

to convey information from a sender (or transmitter)

to a receiver, and it can use fully or partially the

medium.

Examples of channels: copper wires, optical fibbers

or wireless communication channels.



Characteristics of Communication

Systems

must be a Sender and Receiver A protocol is a set of rules which governs the

transfer of data between computers. Protocols allow

communication between computers and networks. Handshaking is used to establish which protocols touse. Handshaking controls the flow of data betweencomputers

protocols will determine the speed of transmission,error checking method, size of bytes, and whether synchronous or asynchronous



5 Basic Components

Every communication systemhas 5 basic requirements

Data Source (where the data

originates)

Transmitter (device used totransmit data)

Transmission Medium (cables

or non cable)

Receiver (device used toreceive data)

Destination (where the data

will be placed)



5 Basic Components



Transmission Media Speed

Bandwidth:The amount of data which can be transmittedon a medium over a fixed amount of time (second). It is

measured on Bits per Second or Baud

Bits per Second (bps): A measure of transmission speed.

The number of bits (0 0r 1) which can be transmitted in a

second .

Baud Rate: Is a measure of how fast a change of state

occurs (i.e. a change from 0 to 1)



Data Communication Channels

The following is a discussion on the THREE main types of transmission circuits (channels),simplex, half duplex and full duplex.

Simplex ± Data in a simplex channel is always one way. Simplex channels are not often used because it is not

possible to send back error or control signals to the transmit end. An example of a simplex channelin a computer system is the interface between the keyboard and the computer, in that key codes needonly be sent one way from the keyboard to the computer system.

Half Duplex ± A half duplex channel can send and receive, but not at the same time. Its like a one-lane bridge

where two way traffic must give way in order to cross. Only one end transmits at a time, the other end receives.

Full Duplex ± Data can travel in both directions simultaneously. There is no need to switch from transmit to

receive mode like in half duplex. Its like a two lane bridge on a two-lane highway.



Parallel and Serial Data

Data may be transmitted between two points in two different ways.Lets consider sending 8 bits of digital data (1 byte)

Parallel transmission ± Each bit uses a separate wire

± To transfer data on a parallel link, a separate line is used as a clock signal. Thisserves to inform the receiver when data is available. In addition, another linemay be used by the receiver to inform the sender that the data has been used,and its ready for the next data.



Parallel and Serial Data

Serial ± Each bit is sent over a single wire, one after the other

± Usually no signal lines are used to convey clock (timing information)

± There are two types of serial transmission, essentially having to do with how the clock isembedded into the serial data

Asynchronous serial transmission

Synchronous serial transmission

If no clock information was sent, the receiver would misinterpret the arriving data(due to bits being lost, going too slow).

Parallel transmission is obviously faster, in that all bits are sent at the same time,whereas serial transmission is slower, because only one bit can be sent at a time.Parallel transmission is very costly for anything except short links.



Why Not use Parallel Instead of serial?

Due to inconsistencies on channels data arrives atdifferent times

Because of the way it is transmitted packet

switching cannot be used. The above two points makes parallel slower than

serial and requires higher bandwidth.

Parallel transmissions are rarely used anymore.

CCN Lecture 4

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