1. Link Layer Protocols Martin Weiss

2. Slide 2 Objectives of this Meeting u Explain what a protocol is u Compare connection-oriented with connectionless protocol types u Describe some protocol mechanisms u Describe different link layer protocols

3. Slide 3 Review of the Last Meeting u Error detection u Flow control u Performance of flow control techniques

4. Slide 4 Data Link Layer Issues u Type u Error detection and control u Flow control

5. Slide 5 OSI Reference Model End User Application Process Application Services: FTAM, MHS, EDI Data Representation, Transfer Syntax, Data Transformation Dialog Synchronization and Control End-to-End Message Transfer Network Routing and Addressing Data Link Control, Error Checking, Framing Mechanical and Electrical Interface Data Communications Network Physical Connection Network Independent Message Interchange Syntax-Independent Message Interchange Distributed Information Services Application Presentation Session Transport Network Link Physical OSI Layer

6. Slide 6 Structure of Protocols Source APDesintation AP Application Process (AP) to AP Messages Source Link Layer Desintation Link Layer L_DATA.request L_DATA.indication Link Layer PDU’s Physical Transmission Medium

7. Slide 7 Link Management u Connection-Oriented u Connectionless

8. Slide 8 Review of Protocol Types u Protocol Environments u Byte oriented protocol u Bit oriented protocol

9. Slide 9 Kermit Protocol Structure SOHLENSEQTYPEdataBCCCR

10. Slide 10 Kermit Details u SOH = Start of Header u LEN = Number of Bytes in the Frame u SEQ = Sequence Number of the Current Frame

11. Slide 11 Kermit Details u TYPE = Type of Frame –S: Send Initiation Parameters –F: Filename –D: File Data –Z: End of File –B: End of Transaction –Y: ACK –N: NAK –E: Error (Fatal)

12. Slide 12 Kermit Details u Data = Contents of Frame u BCC = Block Check Character u CR = End of Block Marker (ASCII CR Character)

13. Slide 13 Operation of Kermit Kermit Time Kermit User Kermit Connect Receive Send (File) Data Block [1] Sent Call Setup V(S) = 0 V(R) = 0 V(S) = 1 I[0,S,Init] ACK[0,Y,Init] I[1,F,Filename] ACK[1,Y] V(R) = 1 V(R) = 2 V(S) = 2 I[2,D,data] ACK[2,Y] V(R) = 3 V(S) = 3 Data Block [1] Received

14. Slide 14 Operation of Kermit (Error) Kermit Time Kermit User Data Block [1] Sent V(R) = 2 V(S) = 2 I[2,D,data] ACK[2,Y] V(R) = 3 V(S) = 3 Data Block [2] Sent I[3,D,data] NACK[3,N] V(R) = 3 V(S) = 4 I[3,D,data] ACK[3,Y] V(R) = 4 Data Block [1] Received Data Block [2] Received

15. Slide 15 Operation of Kermit (Lost Frame) Kermit Time Kermit User Data Block [1] Sent V(R) = 2 V(S) = 2 I[2,D,data] ACK[2,Y] V(R) = 3 V(S) = 3 Data Block [2] Sent I[3,D,data] V(S) = 4 I[3,D,data] ACK[3,Y] V(R) = 4 Timeout Data Block [1] Received Data Block [2] Received

16. Slide 16 Operation of Kermit (Lost ACK) Kermit Time Kermit User Data Block [1] Sent V(R) = 2 V(S) = 2 I[2,D,data] ACK[2,Y] V(R) = 3 V(S) = 3 Data Block [2] Sent I[3,D,data] V(S) = 4 I[3,D,data] ACK[3,Y] V(R) = 4 Timeout ACK[3,Y] V(R) = 4 Data Block [1] Received Data Block [2] Received

17. Slide 17 Operation of Kermit Destination Computer Kermit Time Kermit User Data Block [n*] Sent EOF Exit Data Block [n*] Received EOF EOT Exit V(S) = n* I[n*,D,data] ACK[n*,Y] V(R) = n* I[(n+1)*,Z] ACK[(n+1)*,Y] I[(n+2)*,B] ACK[(n+2)*,Y]

18. Slide 18 Sliding Window Protocols u Frequently assumes a peer to peer protocol u Acknowledgements can be “piggybacked”

19. Slide 19 Bit-Oriented Protocols u IBM’s Synchronous Data Link Control (SDLC) u ISO’s High-Level Data Link Control (HDLC) –ITU’s Link Access Protocol-Balanced (LAP-B) –ITU’s Link Access Protocol for the ISDN D channel (LAP-D) –ITU’s Link Access Protocol for Modems (LAP-M)

20. Slide 20 HDLC u Bit oriented protocol u Can operate in several modes –Normal Response Mode (NRM) –Asynchronous Response Mode (ARM) –Asynchronous Balanced Mode (ABM)

21. Slide 21 HDLC Frame Types u Unnumbered frames –Used for connection establishment and disconnection –Do not contain acknowledgements u Information frames –Carry information between nodes –Can use piggyback acknowledgements in ARM and ABM

22. Slide 22 HDLC Frame Types u Supervisory frames –Used for error and flow control –Contain sequence numbers

23. Slide 23 HDLC Frame Format FlagAddressControlInformationFCSFlag 88/16 0 to N16/328 Frame Format Control Field for Normal Information Frame 0N(S)N(R)P/F Control Field for Normal Supervisory Frame 0N(R)P/F1S Control Field for Normal Information Frame MP/FM10 P/F = Poll/Final Bit N(S) = Send Sequence Number N(R) = Receive Sequence Number S: RR RNR REJ SREJ M: Defines 9 Commands and 4 Unnumbered Responses

24. Slide 24 Operation of HDLC u Normal mode u Components –Received frame counter (V(R)) –Sent frame counter (V(S)) –Contents of frame »V(R), S(R) »Frame type »Data –Unacknowledged list

25. Slide 25 Unidirectional Data (ACK and NAK) SenderReceiver UnACK’ed List V(S)V(R)UnACK’ed List V(S) I(0,0) 010 000 N(S) = V(R) => Frame OK RR(1) - I(1,0) I(2,0) 20 30 N(S) = V(R) => Frame OK 1 1,2 RR(2)RR(3) 2 - V(R) 0 01 02 03 (N(S), N(R))

26. Slide 26 Unidirectional Data (With Error) SenderReceiver UnACK’edV(S)V(R)UnACK’ed List V(S) I(3,0) 340 300 I(4,0) 50 N(S) = V(R) => Frame OK 3,4 REJ(3)RR(5) - V(R) 3 04 (N(S), N(R)) N(S) = 4 V(R) = 3 ERROR! I(3,0) I(4,0) N(S) = V(R) => Frame OK 05 -

27. Slide 27 Example 2: Bidirectional Data SenderReceiver UnACK’ed List V(S)V(R)UnACK’ed List V(S) I(0,0) 010 000 I(0,1) - I(1,1) I(2,1) 21 31 1 1,2 I(1,3) - V(R) 0 01 12 23 (N(S), N(R)) 11011 - 13- 1 32 RR(2) 23-

28. Slide 28 Modems Error Correction (LAPM) DTE Modem (DCE) User Interface Part (UIP) Error Corr- ecting Part (ECP) Public Telephone Network Modem (DCE) DTE User Interface Part (UIP) Error Corr- ecting Part (ECP)

29. Slide 29 Point to Point Protocol (PPP) u Used as the link layer in dialup Internet access

30. Slide 30 Summary u Description of a protocol u Types of protocols u Protocol primitives and mechanisms u Examples