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The Data Link Layer Chapter 3. Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control.

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Presentation on theme: "The Data Link Layer Chapter 3. Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control."— Presentation transcript:

1 The Data Link Layer Chapter 3

2 Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control

3 Functions of the Data Link Layer Provide service interface to the network layer Dealing with transmission errors Regulating data flow

4 Services Provided to Network Layer (a) Virtual communication. (b) Actual communication. Three Types of Communication 1.Unacknowledged connection less service 2.acknowledged connection less service 3.acknowledged connection Oriented service

5 Design Issues / Functions of DL Layer  Service Provide To Network layer :  Transfer data from one network layer to another network layer.  Frame Synchronization :  the beginning and End of the frame should be recognized.  Flow Control :  The sender should not send frames faster then the speed of receiver.  Error Control :  Error during transmission in bits must be corrected.  Addressing :  On multipoint link the identity of sender and receiver must be specified.

6 Design Issues / Functions of DL Layer  Control and data on same link :  The receiver must be able to distinguish control information from the data being transmitted.  Link management :  Procedures for the management of initiation, maintenance and termination of the link between sender and receiver.

7 Framing  Large block of data may be broken up into small frames at the source because:  limited buffer size at the receiver  A larger block of data has higher probability of error  With smaller frames, errors are detected sooner, and only a smaller amount of data needs to be retransmitted  On a shared medium, such as Ethernet and Wireless LAN, small frame size can prevent one station from occupying medium for long periods

8 Questions ????  Design Issues for DLL ?  What is flow control ?  What is addressing ?  What is link management ?  What is control and data on same link ?  Three types of services provided by DLL  What is framing ?  What is time based framing ?  Why time based framing is not successful ?  What is char count method for framing ?

9 Character Count Method A character stream. (a) Without errors. (b) With one error.

10 Byte Stuffing  Also referred to as character stuffing.  ASCII characters are used as framing delimiters (e.g. DLE STX and DLE ETX) DLESTXDLEETXTransparent Data

11 Byte Stuffing  The problem occurs when these character patterns occur within the “transparent” data.  Solution: sender stuffs an extra DLE into the data stream just before each occurrence of an “accidental” DLE in the data stream.  The data link layer on the receiving end unstuffs the DLE before giving the data to the network layer. DLESTXDLEETXABDLEHW STXDLEETXABDLEHW

12 Bit Stuffing  Each frame begins and ends with a special bit pattern called a flag byte [01111110].  Whenever sender data link layer encounters five consecutive ones in the data stream, it automatically stuffs a 0 bit into the outgoing stream.  When the receiver sees five consecutive incoming ones followed by a 0 bit, it automatically destuffs the 0 bit before sending the data to the network layer.

13 Input Stream Stuffed Stream Unstuffed Stream 0110111111100111110111111111100000 01101111101100111110011111011111000000 0110111111100111110111111111100000 Stuffed bits Bit Stuffing

14 Error Control  When Sender Sends frames to receiver he will reply either positive or negative.  But the receiver will give reply only if It gets the frames  If because of noise frame lost then ???  So the concept of time was introduce it will manage  Time required to reach to destination  Time required to process it  Time required to reply  If acknowledgement is not received in the time the sender will re transmit frame.

15 Error Detection: Types of Error  An error occurs when a bit is altered between transmission and reception  Single bit errors  One bit is altered  Adjacent bits are not affected  Burst errors  A cluster of bits with Length B  the first and the last and a number of intermediate bits in error (not necessarily all the bits in the cluster suffer an error)  More common and more difficult to deal with

16 Flow Control  Handles mismatch between sender’s and receiver’s speed.  Receiver’s buffer limitation.  Feedback-based flow control.  Explicit permission from receiver.  Rate-based flow control.  Implicit mechanism for limiting sending rate.  DLL typically uses feedback-based flow control.

17 Important Definitions  Code Words :  It is the n bits encoded block that contains message bits and also parity bits  Code Rate :  It is the ratio of the no of Message bits to total no of bits.  Code rate R = k/n  Hamming weight [W(x)] :  X is defined as the no of non zero elements in codeword.  Hamming Distance :  The no of location in which their respective element differs  1 1 0 1 0 1 0 0 Codeword 1  0 1 0 1 1 1 1 0 Codeword 2

18 Example  Hamming weight for X = 1 1 1 0 0 0 1 1  Ans is = 5  Hamming Distance For  1 1 1 1 0 0 1 0  1 1 0 0 1 0 1 1  Ans is : 4

19 Error Detection  When data is transmitted there is a chance of error in bits.  We can get error bits by comparing the codeword  There are several methods for error detection  Parity check  Checksum Error Detection  Cyclic Redundancy Check

20 Parity Check  Append a parity bit to the end of a block of data  Value of parity bit is such that the new data has even (even parity) or odd (odd parity) number of ones  E.g., original data 1110001 -> 11100011 (odd parity)  Even number of bit errors goes undetected  E.g., 11100011 -> 11010011 (undetected!)

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