computer networks unit-6

8/7/2019 Computer Networks UNIT-6

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C o n f i d e n t i a l

Course : BCA

Semester : IV

Subject Code : BC 0048

Subject Name : Computer Networks

Unit number : 6

Unit Title : Data Link Layer

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Unit-6 Data link Layer

Data Link Layer

Objectives

After going through the presentation, you should be able to:

Describe Design issues of data link layer.

Discuss Error Detection and correction

Describe data link layer protocols.

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Lecture outline

Introduction

Design issues of DLL

Error Detection and Correction

Data Link Layer Protocols

Summary

Data Link Layer

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Introduction

The data link layer is the second layer from bottom of the seven-layer

OSI model

It responds to service requests from the network layer. It issues

service requests to the physical layer. This is the layer, which transfersdata between adjacent network nodes in a wide area network or

between nodes on the same local area network segment.

The data link layer provides the functional and procedural means to

transfer data between network entities and might provide the means to

detect and possibly correct errors that may occur in the Physical layer.

The data link layer is split into MAC and LLC sub layers.

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The data link layer takes the packets from the network layer and

encapsulates bit stream into units called frames for transmission.

Each frame contains a frame header, a payload field for holding the

packet, and a frame trailer.

The DLL design issues are listed below:

In general, provide service to the network layer. The network layer

wants to be able to send packets to its neighbors without worrying

about the details of getting it there in one piece.

Framing: Encapsulating the packets

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Sender checksums the frame and sends checksum together with

data. The checksum allows the receiver to determine when a frame

has been damaged in transit.

Receiver recomputes the checksum and compares it with thereceived value. If they differ, an error has occurred and the frame is

discarded.

Perhaps return a positive or negative acknowledgment to the sender.

A positive acknowledgment indicates the frame was received without

errors, while a negative acknowledgment indicates the opposite.

Flow control

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Services provided to the network layer :

The function of the data link layer is to provide services to the network

layer.

The principal service is transferring data from the network layer on the

source machine to the network layer on the destination machine.

Other services provided to the network layer are

The network layer is interested in getting messages to the

corresponding network layer module on an adjacent machine.

The remote Network Layer peer should receive the identical message

generated by the sender .

The Network Layer wants to be sure that all messages it sends, will

be delivered correctly (e.g., none lost, no corruption).

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The DLL provides three types of services

1. Unacknowledged connectionless service

2. Acknowledged connectionless service

3. Acknowledged connection-oriented service

4. Unacknowledged connectionless service :

This service consists of having the source machine send independent

frames to the destination machine without having the destination

machine acknowledge them. No logical connection is established

beforehand or released afterward. If a frame is lost due to noise on the

line, no attempt is made to detect the loss or recover from it in the DLL.

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2. Acknowledged connectionless service :

Here no logical connections are used, but each frame is individually

acknowledged.

Thus the sender knows whether a frame is been delivered to the

destination or not. If the acknowledgement has not arrived within a

specified period then the frame is retransmitted.

This service is useful over unreliable channels, such as wireless system.

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3. Acknowledged connection-oriented service :

This service is the most sophisticated service of DLL.

With this service the source and destination machines establish a

connection before any data are transferred.

Each frame sent is indeed received. Each frame sent over the connection

is numbered. It guarantees that each frame sent is received.

Again this services calls for the three phases:

Connection establishment:

where source and destination stations initializes variables and counters or

buffers required for the data transferor to keep trackof received frames etc.

Data transfer: frames are actually transmitted

Connection termination: where a connection is released.

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Framing :

The DLL translates the physical layer's raw bit stream into discrete units

(messages) called frames.

That is because the physical layer just accepts a raw bit stream and to

deliver it to the destination. This bit stream is not guaranteed to be error

free.

The number of bits received may be less than, equal to, or more than the

number of bits transmitted. They may also have different values. It is up

to DLL to detect and if necessary correct errors. If not correct at least

detect errors and take proper action like ask for retransmission etc.

The usual approach of DLL is to break the bit stream up into discrete

frames and then for the purpose of error detection or correction, it uses

some coding technique

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Error control :

Error control is concerned with insuring that all frames are eventually

delivered and possibly in order to a destination as they are sent from the

source.Reliable delivery is achieved using the ``acknowledgments with

retransmission. Here the receiver returns a special acknowledgment

(ACK) frame to the sender indicating the correct receipt of a frame.

In some systems, the receiver also returns a negative acknowledgment

(NACK) for incorrectly-received frames. This is nothing more than a hint

to the sender so that it can retransmit a frame right away without waiting

for a timer to expire.

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Flow control :

Flow control deals with throttling the speed of the sender to match that

of the receiver.

Usually, this is a dynamic process, as the receiving speed depends onsuch changing factors as the load, and availability of buffer space.

There are two approaches

1. feedback-based flow control

2. rate based flow control

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Flow control : ( continued)

1. Feedback-based flow control : in this the receiver sends back

information to the sender giving it permission to send more data or

at least telling the sender how the receiver is doing.

2. Rate based flow control: In this the protocol has a built in

mechanism that limits the rate at which senders may transmit the

data without using feedback from the receiver.

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There are two types of attacks against errors:

Error Detecting Codes:

Include enough redundancy bits to detect errors and use ACKs andretransmissions to recover from the errors.

Error Correcting Codes:

Include enough redundancy to detect and correct errors.

To understand and deal with errors, we consider the following some ofthe terms and conventions used:

1. Messages (frames) consist of m data (message) bits and rredundancy bits, yielding an n = (m+r)-bit codeword.

2. Hamming Distance: Given any two codewords, we can determinehow many of the bits differ. Simply exclusive or (XOR) the twowords, and count the number of 1 bits in the result.

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3. Significance? If two codewords are d bits apart, d errors are required

to convert one to the other.

4. A code's Hamming Distance is defined as the minimum Hamming

Distance between any two of its legal codewords (from all possiblecodewords).

5. n general, all possible data words are legal. However, by choosing

check bits carefully, the resulting codewords will have a large

Hamming Distance. The larger the Hamming distance, the better able

the code can detect errors.

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Parity Bits :

parity: A single parity bit is appended to each data block (e.g. each

character in ASCII systems) so that the number of 1 bits always adds up

to an even (odd) number.

If the total number of bits including the appended bit is even, the paritybit appended is called even parity. Similarly, If the total number of bits

including the appended bit is odd, the parity bit appended is called odd

parity

of even parity:

Data is 1000000

After using even parity the code becomes 10000001

Data is 1111101

After using even parity the code becomes 11111010

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Various transmission control protocol are

Asynchronous DLC protocols (ADLC)

Binary Synchronous protocol (Bisync or BSC)

Synchronous DLC (SDLC) High Level DLC (HDLC)

Asynchronous DLC protocols :

An asynchronous DLC protocol is primarily used for low speed data

communication. Framing is done at byte level.

Framing start bit a 0 bit & Stop 1 bit. Often parity is used for each

character.

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Binary Synchronous protocol (Bisync or BSC) :

Binary Synchronous protocol (Bisync or BSC) is developed by IBM in

1966, and is also a byte oriented protocol.

It organizes data into block of 512 characters. Receiver transmits positive

acknowledgement (ACK) or Negative acknowledgement (NAK).

It supports error checking on block checking character (BCC). Receiver

calculates BCC independently and compares the block for error.

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Synchronous Data Link control (SDLC) Protocol :

SDLC developed again by IBM in mid of 1970s to supports high speed

transmission.

It is a Bit oriented protocol & can support either half duplex or Full duplex

modes of transmission.]It uses Cyclic Redundant Code, known as FCS for error detection.

High-level Data Link Control(HDLC) Protocol

HDLC is adopted as part of X.25. It is Bit oriented that is it uses bit

stuffing and bit delimiters.

It uses 3-bit sequence numbers and up to 7 unacknowledged frames can

be outstanding at any time

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Summary

The data link layer is the second layer from bottom of the seven-

layer OSI model.

The data link layer is split into MAC and LLC sub layers.

The data link layer takes the packets from the network layer and

encapsulates bit stream into units called frames for transmission. If the total number of bits including the appended bit is even, the

parity bit appended is called even parity.

The most popular error detection code is polynomial codes or cyclic

redundancycodes (CRCs).

Most common Data Link Layer Protocols are

Asynchronous DLC protocols(ADLC), Binary Synchronous protocol (Bisync

or BSC), Synchronous DLC (SDLC) and High Level DLC (HDLC)

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