Edited Data Communication and Networking

8/10/2019 Edited Data Communication and Networking

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DATACOMMUNICATION

Prepared by Akshay

Borkar(GL)

Chetan Patil



Introduction

Terminologies of Data communication

Data- 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 linksSimple- two computers or a computer with a

printer

Complex- one or more main frame

computers with a thousand remoteterminals.



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 TransmissionCode

• 1945: Allied Governments develop the

First Large Computer

• 1950: IBM releases its first computer IBM710

• 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 Transmission

In 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 withinthe 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

arrangementsCircuit 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-

Coax

ial

cable

BNC T-Connector

Simple and low-cost

A single cable called a trunk (backbone, segment)



Star topology-

Each computer has a cable connected to asingle pointMore cabling, hence higher costAll signals transmission through the hub; ifdown, entire network down



Ring topology

Every computer serves as a repeater to boostsignals

Disadvantages

• 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 symbols

Often called as character sets, character

codes, symbol codes, character languages

Types of characters used in data

communication:

1) Data link control:

For orderly flow of data from source to destination2) 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 mustrecognize 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 CODESMorse = .-

Baudot = 5 bit (no

parity)

Int. Baudot = 6 bsit (5

data + 1 parity)

ASCII = 8 bit (7

data + 1 parity)

EBCDIC = 9 bit (8data + 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



??????



Data Communication

Hardware A multipoint data communication circuit block

diagram

Hos

t

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/transmitter Asynchronous- no 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 start/stop bits

Hardware consists of 2 sections:

Transmitterreceiver



UART TRANSMITTER



Control word

To 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 Interface

Serial 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 &

printer An interface that accepted data in same format

used internally by most computers (8 bit parallelusing TTL logic)

Comes with a 36 pin Amphenol connector (Champ

connector)Pins are classified as data, control and status

Data lines: Pins 2 to 9 are eight bit parallel databus.

Control lines: 4 control signals are used

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

AF: autofeed, active low,tells printer whether toperform line feed after it receives a carriagereturn character from computer

PRIME: also called initialize, active low, o/p fromcomputer, clears the printer’s memory, usuallyused to abort printing action suddenly

SLCTIN: select line, not mostly used, usually it isgrounded



Status line: unidirectional, and

transmission from printer to computer

ACK: 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 low

PO: paper out, active high

SLCT: select, active high, indicates whether

printer is selected or not.



Data Modems

Need for modem

To interface computers, computer networks, and

other digital terminal equipment with analog

communication lines and radio channelWhat is modem

mo dulator and dem odulator

Modem @ 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 technique

Used for mostly medium and high speed

applications(up to 57.6kbps)

Asynchronous modems:No clocking information is sent

Mostly use ASK/ FSK

Restricted to use for low speed applications(<

2.4 kbps)



Asynchronous

modems

• Example of asynchronous modems areBell 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 103modems, has full duplex over a two wire line ata 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 transmiton 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 isoch ronous t ransm iss ion



Additional requirements for

synchronous modemsSince 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 issaid 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 t it ti th l

Edited Data Communication and Networking

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