OSI MODEL AND DATA LINK LAYER

2ND UNITCOMPUTER NETWORK AND

SECURITY

Chapter…….

• Network Models• Error Detection and Correction• Flow and Error Control

Network Models

• Protocol Set of rules that enable two device to

connect and transmit data to one another.

• Service Set of opration that layer provides to

the layer above or below it.

Key Elements of Protocol

• Syntax Structure or format of the data.• Semantics meaning of each Section of bit.• Timing When data should be sent and how

fast they can be sent.

Standards

• De facto De facto standards are often

established originally by manufacturers who request to define the functionality of a new product or technology.

• De jure Thos standards that have legislated by

an officially recognized body are de jure standards.

OSI Model

• Developed by the International standard Organization(ISO).

• The model is called ISO OSI(Open System Interconnection)

• The OSI model has Seven layers.• H- Header Information(Source & Destination

address)• T- Trailer Information(Error control)

OSI Layers

1. Application layer2. Presentation Layer3. Session Layer4. Transport layer5. Network Layer6. Data link layer7. Physical layer

OSI Reference model

Session

Transport

Network

Physical

Presentation

Application Application

Presentation

Session

Transport

Network

PhysicalData Unit(Bits)

DH Data Unit DT

TH Data Unit

SH Data Unit

PH Data Unit

AH Application data

ApplicationData Receiving DeviceSending Device

Physical Layer

• The physical layer is responsible for transmitting individual bits from one node to the next.

Data Link Layer

• The data link layer is responsible for transmitting frames from one node to the next.

Functions of Data link layer

• Framing Divides the data in to smaller unit.• Physical addressing Hardware address is added in Header.• Flow control Control the flow of data between sender and

receiver• Error control generally error control mechanisms are added

in Data link Trailer(DT)

Network Layer

The network layer is responsible for the delivery of packets from the original source to the final destination.

Functions of Network Layer

• Logical addressing The physical addressing

implemented by the data link layer handles the addressing problem locally.

• Routing Finding the shortest path

between source and destination.

Transport Layer

• The transport layer is responsible for delivery of a message from one process to another.

Functions of Transport Layer

• Service point addressing Used to identify the process• Segmentation and reassembly A message is divided into transmittable

segments, each having a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arrival at the destination.

• Flow control.• Error control.

Session Layer

• Allow a user to log into a remote system or to transfer a file between two machines.

Functions of Session Layer:• Dialogue control Control the conversation between two

system • Synchronization Coortinate the interaction among

communication system.

Presentation Layer

• It concerned with the syntax and semantics of the information transmitted.

Functions of Session Layer:• Translation Own format information is exchange between two

different system• Encryption Original message into secret form• Compression Reduce the number of bit s contained in the

information

Application layer

• The application layer is responsible for providing services to the user.

Functions of Application Layer

• Network Virtual Terminal It allow a user to log on to the remote

computer.• File Transfer, Access and Management Allow user to access file from remote

computer.• Mail Service It provide the basic for E-Mail forwarding

and Storage.

Error detection and Correction

• Error: unpredictable changes of bits from 1->0 or 0->1

• Types:– Single bit error– Burst error(Multiple)

Single bit error• A 0 is changed to 1 or a 1 is changed to 0

1 0 0 1 1 1 0 0 1 0 1 1 0 0 1 0 1 0 0 1 0 1

Error

Received Sent

Burst Error(Multiple)• 2 or more bits in the data unit have changed from 1

to 0 or from 0 to 1

0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1

0 1 0 1 1 1 0 0 1 1 1 1 0 0 1 1

Corrupted Bits

Length of burst error (8 bits)

CRC• Cyclic codes are special block codes with one extra

property.• A codeword is cyclically shifted (rotated) the result is

another codeword.

MSB LSB MSB LSB

1 0 1 1 0 0 0 0 1 1 0 0 0 1

CRC Encoder & Decoder

Data word - 1001(data to be sent)Divisor - 1011(predetermined constant)Code word- Data word + Remainder

Code word-100110

1 0 0 1 0 0 0

1 0 1 0

1 0 1 1

0 1 0 00 0 0 0

1 0 0 0

1 0 1 1

0 1 1 00 0 0 0

1 1 0

Quotient

1 0 1 1

Remainder

CRC Encoder & Decoder

Data word - 1001(data to be sent)Divisor - 1011(predetermined constant)Code word- Data word + Remainder-1001110

Code word-100110

1 0 0 1 1 1 0

1 0 1 0

1 0 1 1

0 1 0 00 0 0 0

1 0 0 0

1 0 1 1

0 0 0 00 0 0 0

0 0 0

Quotient

1 0 1 1

Remainder

Parity Check

• Parity is a system in which each transmitted character contain one additional bit.

• Two system of parity are normally used: odd parity even parity• Odd parity means the total number of binary 1’s is in

the character,including the parity bit is odd.• Even parity means that the number of binary 1’s bit in

the character,including the parity bit is even.• This technique only detects the single bit error.

Checksum

• Several protocols still use the checksum for error detection.

• For example, if the set of number is(7,10,3,11,5),we send(7,10,3,11,5,36),where 36 is the sum of (7+10+3+11+5) the original number.

• The receiver adds the five numbers and compares the result with the sum.

• If the two are the same,the receiver assumes no error,accepts the five numbers and omits the sum.

Check sum

• There is an error somewhere and the data are not accepted

• The traditional checksum uses a small number of bits(16) to detect error in a massage of any size.

• It is not strong as CRC in error checking capability.• For example,If the value of one word is increamented

and the value of another word is decreamented by the same amount,the two error cannot be detected because the sum remains the same.

Flow control

• Flow control The management of data flow

between computers or device or between nodes in a network so that the data can be handled at an efficient pace

• Types of flow control Stop – and – wait flow control Sliding window flow control

Stop and Wait Flow control

• Sender sends a frame and wait for acknowledgement from the receiver.

• After receiving the acknowledgement from the receiver,sender sends next frame.

Stop and wait flow control

• Advantage Gurantee for the delivery of

every frame.Because sender sends the next frame only after receiving the acknowledgement of previous frame.

• Disadvantage It is time consuming.

Sliding window flow control

• The flow control technique that allows multiple frames to be in transit on the line at one time is called sliding window .

Error control

• Error control is a method that can be used to recover the corrupted data whenever possible.

• Types of error control backward error control forward error control

Damaged frame

• A recognizable frame does arrive,but some of the bit are in error.

Lost frame

• A frame fail to arrive at the other side.

Lost acknowledgement

• An acknowledgement fails at the source.the sender is not aware that acknowledgement has been transmitted from the receiver.

ARQ

• Full form --Automatic Repeat Request• The purpose of ARQ is to change an

unreliable data link into a reliable one.• Version of ARQ Stop-and –wait ARQ Go-Back-N ARQ Selective-repeat ARQ

Stop-and-wait ARQ

• Sender transmits a single frame and then twaits for acknowledgement(ACK).

Go back N ARQ

• A station may send a series of frames sequentially numbered with some maximum value.

• It give a NACK from the receiver.

Selective Repeat ARQ