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Thursday, August 8, 2013

PPT On Data Link Layer


Data Link Layer Presentation Transcript:
1.Datalink Layer

2.The data link layer transforms the logical communication channel provided by the physical layer into are liable channel by splitting the data into frames which are subjected to error control and flow control procedures.
To achieve (reasonably) reliable communication between 2 adjacent computers
adjacent = physically connected by a communication channel, assumed to deliver bits in their transmitted order
errors can occur (transmit 0, receive 1)
transmission delay
propagation delay

3.Functions of the Datalink layer
Determining how bits in physical layer are framed
Acknowledgement of frames
Error checking
Flow control
 Link Protocol Types

4.Synchronous Protocol
Character-oriented (user data is a sequence of delimited characters)
Bit-oriented (user data is encapsulated by frames).
Asynchronous protocols
All  are character oriented.
Master-slave model (based on the polling technique)
Peer-to-peer model (all stations have the same status).

5.Principle of Datalink layer Communication

6. Framing can frames be defined?
time intervals between frames
these inter-frame gaps may be changed (shorter/longer) during transmission
character count–a field in the frame header specifies the number of characters in the frame
problems if the character count field is corrupted during transmission

8.starting and ending flag, with bit stuffing
flag = 0111 1110 at start and end of each frame
have to make sure flag doesn’t occur in the data part of the frame: whenever sender finds 5 consecutive 1’s in the data, it automatically inserts a 0 (bit-stuffing); when receiver sees 5 consecutive 1’s followed by a 0, it removes the 0 (de-stuffing)
the start and end flags are not bit-stuffed, so the receiver can scan the incoming bit stream for frame boundaries original

 9.Note: even if the data contains a 0 after 5 consecutive 1’s, the sender still stuffs a 0
        e.g. data = 001111101000
    If didn’t do bit stuffing, how would the receiver know which 0’s were stuff bits and which were data
Other methods also used
        e.g. special character sequences used as start and end flags, which means the data needs to be “character-stuffed” to prevent the flags occurring in the data
in practice, use a combination of these methods
    •e.g. character count + 0111 1110 flags

10.At the receiver, data contained in frame is an arbitrary bit string
this means that the receiver is not allowed to know which bit strings correspond to “legal” data and which don’t (in accordance with layered encapsulation principles)
therefore some additional bits must be added to the frame to allow errors to be detected(and possibly corrected)
Note that it is not possible to detect all errors

11.Dealing with Transmission Errors

12.Error correction
Forward Error Control(FEC)-each block transmitted contains extra information which can be used to detect the presence of errors and determine the position in the bit stream of the errors.
Backward(Feedback) Error Control(BEC)-extra information is sufficient to only detect the presence of errors. If necessary, a retransmission control scheme is used to request that another copy of the erroneous information be sent.
This is a more common method.

13.Dealing with Transmission Errors

14.Stop and Wait Flow Control

15.Sliding Window Flow Control
Sender can transmit several frames continuously before needing an ACK

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