8/8/2019 UNIT I-CN

1/43

UNIT I

INTRODUCTION:

yA computer network is a group of interconnectedcomputers

y A collection of computers and devices connected

to each other.yAllows computers to communicate with each other

and share resources and information.

8/8/2019 UNIT I-CN

2/43

Building a Network

y To build a network

Identify the set of constraints and

requirements based onApplication programmer

Network designer

Network provider

8/8/2019 UNIT I-CN

3/43

yRequirements:y Connectivity

y point to point or multiple access

y

Links - physical mediumy Nodes,clouds - computer

y Switched Network

y Circuit Switched

y Packet Switched

y Uses store and forward

y Establishes dedicated circuit

y More efficient in working

8/8/2019 UNIT I-CN

4/43

y Routingy Provides Systematic procedure for forwarding messages

y Unicasting

y Multicasting

y Cost effective Resources sharing

How system resource is shared effectively by multiple users

multiplexing

8/8/2019 UNIT I-CN

5/43

y Multiplexing methods

y STDM - Synchronous time division multiplexing

y FDM - Frequency division multiplexing

8/8/2019 UNIT I-CN

6/43

Network Architecture

y Provides a general, effective, fair, and robustconnectivity of computers

y Provides a blueprint

y Types

y OSI Architecturey Internet Architecture

8/8/2019 UNIT I-CN

7/43

OSI ARCHITECTURE

y Open Systems Interconnection (OSI) model is areference model developed by ISO (InternationalOrganization for Standardization) in 1984

OSI model defines the communications process into Layers

Provides a standards for communication in thenetwork

Primary architectural model for inter-computing and Internetworking communications.

network communication protocols have a structure basedon OSI Model

8/8/2019 UNIT I-CN

8/43

OSI Architecture

8/8/2019 UNIT I-CN

9/43

Internet Architecture

y TCP/IP Architecture

y Four Layer model

y

TCP,UDP,FTP,HTTP,SMTP Protocolsused

y Internet Protocol Graph

8/8/2019 UNIT I-CN

10/43

Direct Links: Outliney Physical Layer

y Link technologiesy Encoding

y Link Layery Framingy ErrorDetectiony ReliableTransmission (ARQ protocols)

y Medium Access Control:

y Existing protocols: Ethernet, Token Rings,Wireless

8/8/2019 UNIT I-CN

11/43

Link Technologies

yCables:y Cat 5 twisted pair, 10-100Mbps, 100m

y

Thin-net coax, 10-100Mbps, 200my Thick-net coax, 10-100Mbps, 500m

y Fiber, 100Mbps-2.4Gbps, 2-40km

y Leased Lines:y

Copper based:T1 (1.544Mbps), T3 (44.736Mbps)y Optical fiber: STS-1 (51.84Mbps), STS-N (N*51.84Mbps)

8/8/2019 UNIT I-CN

12/43

Link Technologies

y Last-Mile Links:

y POTS (56Kbps), ISDN (2*64Kbps)

y xDSL: ADSL (16-640Kbps, 1.554-8.448Mbps), VDSL(12.96Mbps-55.2Mbps)

y CATV: 40Mbps downstream, 20Mbps upstream

y

Wireless Links:Cellular, Satellite, Wireless LocalLoop

8/8/2019 UNIT I-CN

13/43

FRAMING

yAn efficient data transmission technique

y node A wishes to transmit a frame to node B

yInstructs adapter to transmit a frame from thenodes memory

y Sequence of bits are transmitted

yAdapter on B collects the bits and deposit in Bs

memory

y Start and end point-problem faced by adapter

8/8/2019 UNIT I-CN

14/43

Approachesy Byte oriented Protocol(PPP)

BISYNC

Binary SynchronousCommunication

DDCMPDigital Data Communication Message

Protocol

y Bit oriented Protocol(HDLC)

y Clock based Framing(SONET)

8/8/2019 UNIT I-CN

15/43

8/8/2019 UNIT I-CN

16/43

DDCMP Frame Format

SYN SYN Cla Cou t Heade

r

Body CRC

8/8/2019 UNIT I-CN

17/43

Bit Oriented Protocol(HDLC)

y Collection of Bits

1.HDLC

High-Level Data LinkControl

2.Closed Based Framing(SONET)

Synchronous Optical Network

8/8/2019 UNIT I-CN

18/43

HDLC Frame Format

Be i ieque e

Header Body CRC E dieque e

Bit Stufffing

After 5 consecutive 1s insert 0

Next bit is 0 end of frame, stuffed removedNext bit is 1 erorr has occurred

8/8/2019 UNIT I-CN

19/43

Clock Based Framing(SONET)

y STS-1 Frame9 rows of 90 byte each

First 3 byte for overhead rest contains data

Payload bytes scrambled- exclusive OR

Supports Multiplexing

90 columuns

Payloads

9 rows

8/8/2019 UNIT I-CN

20/43

E O DETE TIO

Types of Redundancy Checks

y Parity Check

y Simple Parity Check

y Two Dimensional Parity Check / Longitudinal

Redundancy Check (LRC)

y Cyclic Redundancy Check (CRC)

y

Check Sum

8/8/2019 UNIT I-CN

21/43

EVE D ODD ITY

yA redundant bit called Parity Bit is addedto every data unit

y

Even Parity: total number of 1s in the dataunit becomes even

yOdd Parity: total number of 1s in the data

unit becomes odd

8/8/2019 UNIT I-CN

22/43

E EN PARITY

8/8/2019 UNIT I-CN

23/43

sender receiver

01101011

ODD parity

paritynumber of alltransmitted 1sremains ODD

8/8/2019 UNIT I-CN

24/43

TwoDimensional Parity

7 bit of data 8 bit i ludiparity

Number of 1 eve odd

0000000 (0) 00000000 100000000

1010001 (3) 11010001 01010001

1101001 (4) 01101001 111010011111111 (7) 11111111 01111111

8/8/2019 UNIT I-CN

25/43

Transmission sent using even parity:

yA wants to transmit: 1001

yA computes parity bit value: 1^0^0^1 = 0

y A adds parity bit and sends: 10010

yB receives: 10010 B computes parity: 1^0^0^1^0= 0

yB reports correct transmission after observingexpected even result.

8/8/2019 UNIT I-CN

26/43

Transmission sent using odd parity:

yA wants to transmit: 1001

yA computes parity bit value: ~(1^0^0^1) = 1

yA adds parity bit and sends: 10011

y B receives: 10011

yB computes overall parity: 1^0^0^1^1 = 1

yB reports correct transmission after observingexpected odd result.

8/8/2019 UNIT I-CN

27/43

PARITY CHECK

y Can detect all single-bit errors

y Can also detect burst errors if the total number of bitschanged is odd (1,3,5,..)

y Cannot detect errors where the total number of bitschanged is even

y Detects about 50% of errors

8/8/2019 UNIT I-CN

28/43

Longitudinal Redundancy Check (LRC)

y Adds an additional character (instead of a bit)

y A block of bits is organized in a table

y he Parity Bit for each data unit is calculated

y hen Parity Bit for each column is calculated

y Parity Bits are attached to the data unit

8/8/2019 UNIT I-CN

29/43

L -EX LE

8/8/2019 UNIT I-CN

30/43

LRC

yDetects all burst errors up to length n

(number of columns)

y If two bits in one data unit are damagedand two bits in exactly same positions in

another data unit are also damaged, the

checker will not detect an error

8/8/2019 UNIT I-CN

31/43

CYCLIC REDU DANCY CHECK

Powerful error detection scheme

Rather than addition, binary division is used

A sequence of redundant bits, called CRC or CRC

remainder is appended to the data unit, so that theresulting data unit becomes divisible by apredetermined binary number

At the receiver side, the incoming data unit is dividedby the same predetermined number.

If there is no remainder, the data unit is accepted

If there is a remainder, the receiver indicates that thedata unit has been damaged during transmission

8/8/2019 UNIT I-CN

32/43

CRC

8/8/2019 UNIT I-CN

33/43

CRC GENERATOR

8/8/2019 UNIT I-CN

34/43

CRC CHECKER

8/8/2019 UNIT I-CN

35/43

CRC PO NOMI A S

y he divisor in the CRC generator is most

often represented as an algebraic

Polynomial.yReasons:

yIt is short

yIt can be used to prove the concept

mathematically

8/8/2019 UNIT I-CN

36/43

CRC-POLYNOMIAL

8/8/2019 UNIT I-CN

37/43

PERFORMANCE

y CRC can detect all burst errors that affect an odd number

of bits

y CRC can detect all burst errors of length less than or equal

to the degree of the polynomial

8/8/2019 UNIT I-CN

38/43

CHECKSUM

y The Check Sum generator subdivides the data unitinto equal segments of n bits (usually 16)

y These segments are added using ones complement

arithmetic in such a way that the total is also n bitslong

y Total is complemented and appended to the end of the

original data unit as redundancy bits, called the checksum field

8/8/2019 UNIT I-CN

39/43

The sender follows these steps:The sender follows these steps:

yyThe data unit is divided into k sections,The data unit is divided into k sections,

each of n bitseach of n bits

yyAll sections are added using onesAll sections are added using ones

complement to get the sumcomplement to get the sum

yyThe sum is complemented and becomes theThe sum is complemented and becomes the

checksum.checksum.

yyThe checksum is appended and sent with theThe checksum is appended and sent with the

data.data.

8/8/2019 UNIT I-CN

40/43

yy The receiver follows these steps:The receiver follows these steps:

yyThe unit is divided into k sections, each ofThe unit is divided into k sections, each of

n bitsn bits

yyAll sections are added using onesAll sections are added using ones

complement to get the sum.complement to get the sum.

yyThe sum is complemented.The sum is complemented.

yy If the result is zero, the data are accepted;If the result is zero, the data are accepted;otherwise, rejectedotherwise, rejected

8/8/2019 UNIT I-CN

41/43

CHECK UM

8/8/2019 UNIT I-CN

42/43

CHECKS M-EXAMP E

Data:

10101001 00111001

Computing Checksum:

1010100100111001

---------------

Sum 11100010

CheckSum 00011101

Data Sent :

10101001 00111001 00011101

8/8/2019 UNIT I-CN

43/43

CHECKS M-EXAMP E

Receiver Side:

10101001

0011100100011101

---------------

Sum 11111111

Complement 00000000