UNIT IVDATA COMMUNICATION

Prepared by

P SAJID AKRAM

Lecturer/ ECE

CAHCET

Introduction

Terminologies of Data communicationData- Information that has been processed, organized

and stored.Data communication-transmission, reception &

processing of digital information.Network/ nodes/ stations- set of devices interconnected

by media links

Simple- two computers or a computer with a printer

Complex- one or more main frame computers with a thousand remote terminals.

What is Data Communications?

Exchange of digital information between two digital devices is data communication

History of Data Communication

• 1838: Samuel Morse & Alfred Veil Invent Morse Code

Telegraph System

• 1876: Alexander Graham Bell invented Telephone

• 1910:Howard Krum developed Start/Stop Synchronisation

• 1930: Development of ASCII Transmission Code

• 1945: Allied Governments develop the First Large Computer

• 1950: IBM releases its first computer IBM 710

• 1960: IBM releases the First Commercial Computer IBM 360

Standard Organization for Data CommunicationInternational Standards Organization(ISO)

International Telecommunications Union-Telecommunication Sector(ITU-T)

American National Standard Institute(ANSI)

Institute of Electrical and Electronics Engineers(IEEE)

Electronics Industry Association(EIA)

Data Communication Circuit• Simplified block diagram of data communication network

Data Transmission

• Data Transmission means movement of the bits over a

transmission medium connecting two devices

• Two types of Data Transmission are:

Parallel Transmission

Serial Transmission

Parallel TransmissionIn this all the bits of a byte are transmitted simultaneously on separate wires.

Practically, if two devices are close to each other e.g. Computer to Printer, Communication within the Computer

High speed but complex circuit

Serial Transmission• Bits are transmitted one after the other

• Usually the Least Significant Bit (LSB) is transmitted first

• Suitable for Transmission over Long distance• Less speed but simple circuit

Data Communication circuit arrangements

Circuit Configuration:Two point configuration

Multipoint configuration

Transmission Modes• Simplex

• signals transmitted in one direction• eg. Television

• Half duplex• both stations transmit, but only one at a time

• eg. police radio

• Full duplex• simultaneous transmissions

• eg. telephone

Network topologies• It describes the layout or appearance of a network• A multi point topology connects 3 or more stations through

a single transmission medium

Eg:

star, bus, ring, mesh & hybrid

Bus topology-

Coaxial cable

BNC T-Connector

Simple and low-costA single cable called a trunk (backbone, segment)

Star topology- Each computer has a cable connected to a single pointMore cabling, hence higher costAll signals transmission through the hub; if down, entire network down

Ring topologyEvery computer serves as a repeater to boost signalsDisadvantages

• If one computer fails, whole network fails

Mesh topology• Each and every node of the network is interconnected

Hybrid topology• Combination of two or more topologies

Data Communication CodesThese are prescribed bit sequence used for encoding

characters and symbolsOften called as character sets, character codes, symbol

codes, character languagesTypes of characters used in data communication:

1) Data link control:For orderly flow of data from source to destination

2) Graphic control:Presentation of data at the receivers

3) Alpha/numeric characters:Various alphabets, numbers, etc.

CHARACTER CODES• Various character codes have been used in data

communication including:• Morse, Baudot• EBCDIC, ASCII• Unicode• Bar code

• Regardless of the character code, both the terminal/ host or sender/receiver must recognize the same coding scheme

MORSE CODE

BAUDOT CODE• One of first codes developed for machine to machine

communication• Uses 1’s and 0’s instead of dots and dashes• For transmitting telex messages (punch tape)• Fixed character length (5-bits)

• 32 different codes• increased capacity by using two codes for shifting

• 11111 (32) Shift to Lower (letters)• 11011 (27) Shift to Upper (digits, punctuation)

• 4 special codes for SP, CR, LF & blank• Total = 26 + 26 + 4 = 56 different characters

BAUDOT CODE (cont.)• Problems:

• required shift code to switch between character sets• no lower case, few special characters• no error detection mechanism• characters not ordered by binary value• designed for transmitting data, not for data processing

• International Baudot• Added a 6th bit for parity• Used to detect errors within a single character

BAUDOT CODE

EBCDIC• Extended Binary Coded Decimal Interchange Code• 8-bit character code developed by IBM

• used for data communication, processing and storage• extended earlier proprietary 6-bit BCD code• designed for backward compatibility or marketing?• still in use today on some mainframes and legacy systems.

• Allows for 256 different character representations (28)• includes upper and lower case• lots of special characters (non-printable)• lots of blank (non-used codes)

• assigned to international characters in various versions• used with/without parity (block transmissions)

EBCDIC CODE

ASCII CODE• American Standard Code for Information Interchange• 7-bit code developed by the American National Standards Institute

(ANSI)• most popular data communication character code today

• Allows for 128 different character representations (27)• includes upper and lower case• lots of special characters (non-printable)• generally used with an added parity bit• better binary ordering of characters than EBCDIC

• Extended ASCII uses 8 data bits and no parity• Used for processing and storage of data• Allows for international characters• 8th bit stripped of for transmission of standard character set

7-BIT ASCII CODE

SUMMARY OF CHARACTER CODES

Morse = .-

Baudot = 5 bit (no parity)

Int. Baudot = 6 bsit (5 data + 1 parity)

ASCII = 8 bit (7 data + 1 parity)

EBCDIC = 9 bit (8 data + 1 parity)

UNICODE = 16 bits (no parity)

• Normally terminals and hosts must use the same code• However, code conversion hardware/software can be used to allow

different machines to communicate

Error control • What is error?• Types of error:

• Single bit error• Burst error

Methods : 1) error detection

2) error correction

Error detection:i. redundancy

ii. parity

iii. checksum

iv. longitudinal and horizontal redundancy check

v. Cyclic redundancy check(CRC)

Error Correction

1. Retransmission resending of message when it is received

incorrectly often called as ARQ- Automatic Repeat reQuest for

retransmission positive and negative acknowledgment

2. Forward error correction(FEC)

only technique which detects and corrects errors at the receiver without the need for retransmission

Eg : Hamming code

Hamming code

Number of redundancy bits needed• Let data bits = m• Redundancy bits = n

Total message sent = m+r

The value of n must satisfy the following relation:

2n ≥ m+n+1

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Data Communication Hardware• A multipoint data communication circuit block diagram

Host

Secondary or remote

Line control unit(LCU)@ primary:

Directs traffic to and from many circuits(which have different characteristics)

@ secondary:

Directs traffic between one data link and other few devices(all of similar char)

LCU with a software is called as front end processor(FEP)

Usually LCU of primary is an FEP

• LCU operates only on digital data

• Most of the functions of LCU are performed by single IC called as UART/USRT

• UART- Universal asynchronous receiver/transmitter

• USRT- Universal synchronous receiver/transmitter

• INTEL’s USART 8251• Motorola’s UART – asynchronous communication interface

adapter(ACIA)

UART- Universal asynchronous receiver/transmitterAsynchronous- no clocking information is transferred

between DTE and DCEPrimary functions of UART:

To perform serial to parallel and vice-versaTo perform error detection(parity bits)To insert and detect start/stop bits

Hardware consists of 2 sections:Transmitterreceiver

UART TRANSMITTER

Control wordTo indicate no of bits, nature of parity, and the no of stop

bits

Timing diagram of UART• TBMT-Transmit Buffer Empty • TD-Transmit Data• TDS-Transmit Data Strobe• TEOC-Transmit End of Char• TSO-Transmit serial out

Receiver of UART

Timing diagram of UART Receiver

• RSI-receive serial input• RDA-receive data available• RPE- receive parity error• SWE- status word enable• RDE-receive data enable• RDAR-receive data available

reset

USRT- Universal synchronous receiver/transmitterSynchronous- clocking information is transferred between

DTE and DCE

Primary functions of UART:To perform serial to parallel and vice-versa

To perform error detection(parity bits)

To insert and detect SYN characters(difference between USRT& UART)

Transceiver of USRT

Interfaces

Serial InterfaceSerial communication is the most

simplistic form of communication between two devices.RS-232 is a standard by which two serial devices

communicate• The connection must be no longer than 50 feet.• Transmission voltages are –15V and +15V.• It is designed around transmission of characters (of 7 bits of

length).• Defines a 25 wire cable with a DB 25S/9S connector.• Data rate of up-to 20kbps for a distance of 50ft • driver, terminator, noise margin

RS 232 Pin functions- data pins

Control pins

Timing pins

Miscellaneous

Parallel interface• Transfers data between two devices eight or more bits at

a time.• Also referred to as serial by word transmission• advantage: faster transmission• Disadvantage: higher cost for transmission• Eg: Centronics parallel interface, IEEE 488 bus

Interface btw computer & printerAn interface that accepted data in same format used internally by

most computers (8 bit parallel using TTL logic)Comes with a 36 pin Amphenol connector (Champ connector)Pins are classified as data, control and statusData lines: Pins 2 to 9 are eight bit parallel data bus.Control lines: 4 control signals are used

STB: active low, edge triggered, o/p from computer & tells printer to accept data from data lines

AF: autofeed, active low,tells printer whether to perform line feed after it receives a carriage return character from computer

PRIME: also called initialize, active low, o/p from computer, clears the printer’s memory, usually used to abort printing action suddenly

SLCTIN: select line, not mostly used, usually it is grounded

Status line: unidirectional, and transmission from printer to computerACK: acknowledge, active low, response to STB lineBusy: active high, and goes high anytime printer is busy

When printer is busy?

1. when printer is accepting data from computer

2. when printer is printing

3. when printer is switched off or offline

4. when printer’s ERROR line is lowPO: paper out, active highSLCT: select, active high, indicates whether printer is selected or

not.

Data Modems

Need for modemTo interface computers, computer networks, and other digital terminal equipment with analog communication lines and radio channel

What is modem

modulator and demodulatorModem @ transmitter:Digital signals modulate an analog carrier

Modem @ receiver:Analog signals are demodulated and converted to digital signals

Also called as DCE, data set, dataphone.

Types of modems

Broad classification can be made as:Synchronous modems:

Clocking information is recovered at the receiver Use PSK or QAM modulation techniqueUsed for mostly medium and high speed applications(up to

57.6kbps)

Asynchronous modems:No clocking information is sentMostly use ASK/ FSK Restricted to use for low speed applications(< 2.4 kbps)

Asynchronous modems • Example of asynchronous modems are

Bell Systems 202 T/S modems, uses FSK • 202 T- full duplex, four wire operation• 202 S- half duplex, two wire operation

• 202T modems use 1700Hz carrier• Another modem standard is Bell Systems 103 modems, has

full duplex over a two wire line at a rate of 300bps• Has 2 data channels,(low band/ high band) with each mark &

space frequency• Low band channel- originate channel• High band channel-answer channel

• Circuit which originates the call has to transmit on low band and the receiver has to respond on the high band channel

Synchronous modems• Used for medium and high speed modems• For medium speed:

• QPSK for 2.4Kbps(eg: Bell Systems 201C)• 8-PSK for 4.8 Kbps(eg: Bell Systems208A)• Both are full duplex, 4 wire systems

• For high speed:• 16-QAM for a 9.6Kbps(eg: Bell Systems 209A)• Full duplex, four wire transmission

• Sometimes asynchronous data format is used in a synchronous modem, this is referred to as isochronous transmission

Additional requirements for synchronous modems

Since the medium and high speed modems are synchronous, these modems contain the following additional hardware: clock recovery

scramblers and descrambler circuit

equalizers

Modem control• Initial modems- dumb modems

• Basic function include only modulation and demodulation

• Intelligent modems- smart modems• Consists of mostly microprocessor which performs routine function

• Smart modems are controlled by a set of system commands.

• Most widely used are AT command set or Hayes command set

• Has two modes of operation:• AT command mode• AT on-line mode

AT command mode• When a modem is not communicating with any other

modem, then it is command mode• All commands begin with ASCII character AT(attention)

• AT online mode:• Once communication begins, modem is said to be in

online mode• In this, modem accepts information and allows them to

modulate its carrier before transmission• To switch from online to command mode, DTE transmits

consecutive three plus sign(+++). This sequence is called as escape code.

Any doubts????????????