Data Link Layer.ppt 1s373z

Data-Link Layer

COMPUTER NETWORKS (DATA-LINK LAYER)

BITS edu campus

Data-Link Layer

Question to be answered ? Computer Networks

• Why it is important to have Data–link Layer?

What are Functions performed on Data-link layer? •

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Data-Link Layer

the need for Data-link layer Computer Networks

1) Communication circuits make errors occasionally. 2) Communication circuits have a limited date rate. 3) There is a nonzero propagation delay between the time a bit is sent and the time it is received. 3

Data-Link Layer

Functions performed on Data-link layer are: Computer Networks

1- Framing 2- Flow Control 3- Error Control 4- Error Detection & Correction 4

Data-Link Layer

Framing Computer Networks

• The transmitted bit stream by the physical layer is not guaranteed to be error free. • It is up to the data-link layer to detect, and if necessary, to correct errors. • The data-link layer breaks the bit stream up into discrete frames and compute the checksum for each frame. • When a frame arrives at the destination, the checksum is recomputed.

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Data-Link Layer

Sending machine

Receiving machine Computer Networks

Packet

Packet Frame

Header

Payload field

Trailer

Header

Payload field

Trailer

Relationship between packets and frames 6

Data-Link Layer

SERVICES PROVIDED TO NETWORK LAYER(1)

(a) Virtual communication. (b) Actual communication.

Data-Link Layer

SERVICES PROVIDED TO NETWORK LAYER (2)

Placement of the data link protocol.

Data-Link Layer

Framing

• Networks rarely make any guarantees about timing. Other Framing Methods are: 1) Character count. 2) Flag bytes with byte stuffing 3)Starting and encoding flags, with bit stuffing 4) Physical layer coding violations 9

Computer Networks

• The easiest way to achieve framing is to insert time gaps between frames.

Data-Link Layer

Framing

Character count

4

1

2

3

6

1

2

3

4

5

2

Computer Networks

1) Character count framing method uses a field in the header to specify the number of characters in the frame.

1

Frame 1

Frame 2

Frame 3

4 Characters

6 Characters

2Characters 10

Data-Link Layer

Framing

Error

4

1

2

3

8

1

2

3

Frame 1

Frame 2 (wrong)

4 Characters

6 Characters

4

5

Computer Networks

The problem with Character count algorithm is that the count can be distorted by a transmission error.

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1

Frame 3 11

Data-Link Layer

Framing

FLAG

Header

Payload field

Trailer

A Frame delimited by flag bytes

Computer Networks

2) The second method uses in each frame start and end special bytes (Flag byte) to get around the problem of resynchronisation after an error occurred. - Flag bytes used as a delimiters.

FLAG

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FRAMING (2)

(a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing.

FRAMING (3)

Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver’s memory after destuffing.

Data-Link Layer

Framing Computer Networks

- A problem occurs with this method when binary data are being transmitted. It may easily happen that the flag byte’s bit pattern occurs in the data. - one way to solve this problem is to have the sender’s data-link layer insert a special escape byte (ESC) just before each accidental flag byte in the data. - this technique is called byte stuffing or character stuffing. 15

Data-Link Layer

Framing

- A major disadvantage of this framing method is that it is closely tied to the use of 8bit characters.

Computer Networks

- The byte-stuffing scheme with a slight simplification is used in the PPP protocol.

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Data-Link Layer

Error Control

- The technique of acknowledgment and retransmission is used to ensure reliable delivery. - This can be achieved by using timer and asg sequence number for each frame.

Computer Networks

- Error Control is needed to make sure all frames are eventually delivered to the network layer at the destination and in the proper order.

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Data-Link Layer

Flow Control Computer Networks

- Flow Control is needed to deal with the situation when the sender is running on a fast computer and the receiver is running on a slow machine. - the sender keeps transmitting the frames out at a higher rate until the receiver is completely swamped (flooded). - At a certain point the receiver will simply be unable to cope with the frames as they 18 arrive, and will start to lose them.

Data-Link Layer

Flow Control -

There are two approaches for flow control: Computer Networks

1) -based flow control, where the receiver sends back information to the sender giving it permission to send more data.

2) Rate-based flow control, where there is a rate limit at which senders may transmit data, without using from the 19 receiver.

Data-Link Layer

Error Detection & Correction Two basic strategies to deal with errors: Computer Networks

1) Error-correcting codes, includes enough redundant information with each sent data, to enable receiver to deduce what the transmitted data must have been.

2) error-detecting codes, includes only enough information to allow the receiver to deduce that an error occurred, but not which error, and have it request a 20 retransmission.

Data-Link Layer

Error Detection & Correction

- In this scheme a single parity bit is appended to the data.

Computer Networks

- The simplest error-detection scheme is a Parity Check.

- The parity bit is chosen so that the number of 1 bits in the code is even (or odd). 21

Data-Link Layer Transmitter E=f(DATA)

DATA

Computer Networks

DATA

E

Receiver

DATA

E

E, E’ = Error-detecting codes f

= Error-detecting code function

E’=f(DATA)

22 Compare

Data-Link Layer

Error Detection & Correction

- With wireless links, where unacceptable delay caused by repetition Error-correction is more suitable.

Computer Networks

- Error-detection and retransmission is efficient where error rate is low (over copper wire or optical fiber).

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