Chapter 3- Data Link Layer

• “its transform the physical layer in to reliable link”

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• Functionalities:-
• Framing- bits in to frame (units of info. Called frame)
• Physical Addressing (header added)
• Flow control
• Error control how it can be detected and corrected
• Access control
• “reliable efficient communication”

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Computer Networks

Data link layer designing issues

• Providing a well-defined service interface to the network layer.
• Dealing with transmission errors.
• Regulating the flow of data
• so that slow receivers are not swamped

Computer Networks

Computer Networks

1. Unacknowledged connectionless service.

2. Acknowledged connectionless service.

3. Acknowledged connection-oriented service.

Computer Networks

1. Unacknowledged connectionless service.
• Independent frame, no Ack, no logical conn.
• If frame lost (noise) => no attempt (detect, recover)
• service is appropriate when the error rate is very low
• e.g. real-time traffic, such as voice

2. Acknowledged connectionless service.

• reliability is acknowledged, still no logical connections
• sender knows (frame has arrived correctly or been lost)
• If not (time interval)=> it can be sent again.
• e.g. wireless systems. 802.11 (Wi-Fi)

3. Acknowledged connection-oriented service.

• establish a connection before any data are transferred.
• guarantees that each frame

(received, exactly once, in correct order)

• e.g. long-distance telephone circuit

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Ethernet Frame Structure

• Preamble:
• 7 bytes with pattern 10101010 followed by one byte with pattern 10101011
• Used to synchronize receiver, sender clock rates
• Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match
• Length: 2 bytes, length of Data field
• CRC: 4 bytes generated using CR-32, checked at receiver, if error is detected, the frame is simply dropped
• Data Payload: Maximum 1500 bytes, minimum 46 bytes
• If data is less than 46 bytes, pad with zeros to 46 bytes

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LAN Technologies

Length

Framing

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• Breaking up the bit stream into frames is more difficult
• Byte count.
• Flag bytes with byte stuffing.
• Flag bits with bit stuffing.
• Each frame begins and ends with a special bit pattern, 01111110
• Physical layer coding violations.

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Computer Networks

Computer Networks

Computer Networks

Flag bytes with byte stuffing.

Computer Networks

Flag bits with bit stuffing.

Each frame begins and ends with a special bit pattern, 0 1 1 1 1 1 0

Error control

• Reliable connection oriented service:
• Sender------receiver, back to sender (PACK, NACK)
• If ack frame lost?
• Time out mechanism, retransmission

Computer Networks

Computer Networks

�Error control� Error detection/correction ��

• Flow control:
• “Transmit frames faster than the receiver can accept them”
• feedback-based flow control
• 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.
• Rate-based flow control
• the protocol has a built-in mechanism that limits the rate at which senders may transmit data, without using feedback from the receiver.

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Computer Networks

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End of session

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Thanks.

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