Comparison of ISO-OSI and TCP/IP Suit. Functions of Data Link Layer:

Transcription

1

2 Comparison of ISO-OSI and TCP/IP Suit Functions of Data Link Layer: 1. Frame generation ( Character Count, Character Stuffing, Bit Stuffing) 2. Error Processing (Parity(EVEN or ODD), Block Parity, Hamming Codes, CRC) 3. Flow Control ( STOP & WAIT, SLIDING WINDOW and all related versions)

3 Comparison between TCP/IP and ISO-OSI Model ISO_OSI Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer TCP/IP Application Transport Layer Internet Layer Network Layer Physical Layer HTTP, SMTP, POP3, FTP TCP UDP IP ICMP ARP RARP ETHERNET

4 Functions of Data Link Layer: 1. Frame generation ( Character Count, Character Stuffing, Bit Stuffing) 2. Error Processing (Parity(EVEN or ODD), Block Parity, Hamming Codes, CRC) 3. Flow Control ( STOP & WAIT, SLIDING WINDOW and all related versions)

5 Character Count: 1. Uses a field in the header to specify the # of characters in the frame. 2. When destination sees the character count, it knows how many characters follow, and hence where the end of the frame is. 3. If the count is garbled by a transmission error, the destination will lose sync and will be unable to locate the start of the next frame. So, this method is rarely used.

6 Character Count:

7 Character stuffing: In the second method, each frame starts with the ASCII character sequence. DLE: Data Link Escape STX: Start of TeXt ETX: End of TeXt Every data begins with DLE STX and ends with DLE ETX. Example 1: Message is Come to PVR this Evening DLE STX Come to PVR this Evening ETX DLE

8 Character stuffing: Example 2

9 Bit stuffing: An arbitrary number of bits are used, at the start and end of each frame is a flag byte. The specified pattern is IMPORTANT: Whenever the sender's data link layer encounters five consecutive 1s in the data, it automatically stuffs a zero bit into the outgoing bit stream. This technique is called bit stuffing. When the receiver sees five consecutive 1s in the incoming data stream, followed by a zero bit, it automatically de-stuffs the 0 bit. The boundary between two frames can be determined by locating the flag pattern.

10 Example of Bit Stuffing: 1: Data Bits as Input: Frame Definition: : Data Bits as input: Frame Definition: At the Receiver end: Remove a zero after 5 consecutive 1. 3: Data Bits as Input : Frame Definition: At the Receiver end: Remove zero after 5 consecutive 1 and retain next 0 as a part of data.

11 Flow Control Basic Assumptions: 1. It is a flow control between neighbor machine. 2. FRAMES are transmitted. 3. ACK field is transmitted upon reception of FRAME. +ACK for successful transmission -ACK In case of failure transmission Various flow control Protocols are 1. STOP & WAIT 2. SLIDING WINDOW 3. GO BACK n 4. SELECTIVE REPEAT

12 STOP & WAIT Flow Control Protocol 1. Error Free Transmission 2. DROP of FRAME 3. DROP of ACK

13 Error Free Transmission with ACK Process of STOP & WAIT t i m e Sender Sender is transmitting a frame. f0 t i m e Receiver Receiver receives a f 0 frame. +ACK f1 Invoke ERROR CONTROLL SOFTWARE NO ERROR REPORTED Receiver receives a f1 frame. -ACK +ACK f1 Invoke ERROR CONTROLL SOFTWARE ERROR NOT RECOVERED Invoke ERROR CONTROLL NO ERROR REPORTED.

14 DROP of FRAME in STOP & WAIT t i m e Sender Sender is transmitting a frame. f0 +ACK f1 drops t i m e Receiver Receiver receives a f 0 frame. Invoke ERROR CONTROLL SOFTWARE NO ERROR REPORTED Sender Waiting for ACK Receiver waiting for f1 frame. f1 +ACK NO ERROR REPORTED

15 DROP of ACK in STOP & WAIT t i m e Sender Sender is transmitting a frame. f0 +ACK +ACK drops t i m e Receiver Receiver receives a f 0 frame. Invoke ERROR CONTROLL SOFTWARE NO ERROR REPORTED Receiver Waiting for f1, sender timer expire before receiver, so sender has to take action.. +ACK f0 Duplicate Arrival of f0,means it was a case of DROP of +ACK Discard f0 and resend +ACK

16 Various Parameters used in STOP & WAIT Flow Control Protocol

17 Timing Considerations in STOP & WAIT t i m e 1010 Sender Sender is transmitting a frame. f0 t i m e t prop Receiver t frame +ACK t proc ERROR CONTROLL SOFTWARE, Addressing, Frame ID etc t prop 1010 t ack t proc ERROR CONTROLL SOFTWARE, Addressing,

18 Concept of Sliding Window Flow Control Protocol Main drawback of STOP & WAIT is It takes too much time in communication and it posses control over every frame Practically it is difficult to find errors in every frame.. Hence such a fine control in every frame is waste of energy. Main points of Sliding Window Protocol Multiple frames are send without waiting for ACK. Number of frames send without ACK is called window size. No Negative ACK, but ACK n means all (n-1) frames received successfully and receiver is waiting for n th frame.

19

20 Systems Details (Sliding Window Flow Controll): Three bits are used to number frames Maximum 8 frames will be addressed (0,1,2,3,4,5,6,7) Now we will define appropriate size of Window Ws.. Let us evaluate system having Ws=8

21 Sender Finding Proper Window Size (8 for 3 bits or not.?) Receiver Sender ready to transfer all 8 frames, without receiving ACK. 0 Receiver ready to receive all 8 frames, without sending ACK. 7 ACK 0 All 8 frames (f0,f1,f2,f3,f4,f5,f6,f7) received Successfully. Sender ready to transfer all 8 frames, without receiving ACK. 0 Receiver ready to receive all 8 frames, without sending ACK. ACK 0 7 Receiver still waiting (f0,f1,f2,f3,f4,f5,f6,f7) Frames. TIME OUT

22 Solution is to define Window Size 7 but not 8. In other words, for 3 bit address size we can address 8 frames ( f0, f7) and define window size as 7. In general, for n bit address we can address 2 n frames, and window size would be (2 n 1). Hence for 2 bits we can address 4 frames and window size would be = 3. for 3 bits we can address 8 frames and window size would be = 7. for 4 bits we ca address 16 frames and window size would be = 15.

23 Sliding Window Example WINDOW Size = 8 Maximum transmission without ACK = 8-1 =7 Frame numbers are etc

24 Example of complete Window Exaust t i m e Sender t i m e Receiver Sender ready to transfer all 7 frames, without receiving ACK. 0 Receiver ready to receive all 7 frames, without sending ACK. Buffer not updated because no +ACK received, Window shirked because one frame transmitted.. Buffer not updated because no +ACK transmitted, Window shirked because one frame received.. 6 WINDOW EXAUSTED WINDOW EXAUSTED

25 Main Operations on SENDER Machine Window Shrink Window Expansion Buffer Updation

26 Windows operations on SENDER machine t i m e Sender t i m e Receiver 0 Sender ready to transfer all 7 frames, without receiving ACK. Receiver ready to receive all 7 frames, without sending ACK. 1 2 ACK 1 ACK 2

27 Windows operations on Receiver machine t i m e Sender t i m e Receiver Sender ready to transfer all 7 frames, without receiving ACK. 0 1 Receiver ready to receive all 7 frames, without sending ACK. 2 3 ACK ,0 ACK 1

28 Switching Technique Circuit Switching

29 Switching Technique Message Switching

30 Switching Technique Packet Switching

31 Trace Finite State Machine for STOP and WAIT Questions on Flow Controll Go Back-n Flow control ARQ Selective Repeat ARQ

32 Protocol Verification STOP & WAIT <S, R, C> S=0 means Sender sent f0 1 means sender sent f1 R=0 means receiver expecting f0 =1 means receiver expecting f1 C= 0 channel carrying f0 =1 channel carrying f1 =A channel carrying ACK = - means channel carrying nothing

33 Protocol Verification STOP & WAIT <S, R, C> Loss of Frame/ACK Transition 0 Channel delivered f0 1 ACK of f0 accepted by sender 2 Channel delivered f1 3 ACK of f1 accepted by sender 4 Duplicated arrival of f0, receiver resend ACK 5 Duplicated arrival of f1, receiver resend ACK 6 Sender time out for f0 7 Sender time out for f1 8 Sequence 1: f0, A, f0, f0, f0 Sequence 2: f0, A, f0, f0, A Sequence 3: f0, f0, f0, f0, f0 Sequence 1: f0, A, f1, A, f1, A, f1 Transitions: Transitions: Transitions: Transitions:

34 What are the key functions of error control techniques..?

35

36 Mention Key Advantages and Disadvantages of STOP & WAIT..?

37

38 Question: Consider the use of 10kbits of frame size and 10Mbps satellite channels with 270 micro second delay. What is the link utilization in STOP and WAIT ARQ, if the probability of drop of frame in 10 Pow -3

39

40 Under What condition STOP and WAIT functions efficiently..?

41

42 Question :What do you mean by Piggy Backing..

43

44 Go-Back-n Flow Controll ARQ Selective Repeat Flow Control ARQ

45 Go back-n Protocol t i m e Sender t i m e Receiver Out of sequence reception, discard f2 GO Back 1 3 Out of sequence reception, discard f

46 Selective Repeat Protocol t i m e Sender t i m e Receiver Out of sequence reception. Selective repeat 1 Out of sequence reception

47 Question: Proof that RTT (Round Trip Time) is independent to the value of a. Where a = T prop/t frame.