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OSI-ISO model

T he ISO-OSI M odel. OSI-ISO model. Tanenbaum 3 rd edition: 28-42. Data communications Immensely complex Many manufacturers Many types of data. We need tools for Facilitating interconnection of heterogeneous systems Standards Reducing complexity Layering.

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OSI-ISO model

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  1. The ISO-OSI Model OSI-ISO model Tanenbaum 3rd edition: 28-42 • Data communications • Immensely complex • Many manufacturers • Many types of data • We need tools for • Facilitating interconnection of heterogeneous systems • Standards • Reducing complexity • Layering • Need a model to bring this all together

  2. The ISO-OSI Model OSI-ISO model • The model of data communications • Facilitates communication about data communication • discussion of functionality in commonly understood terms • Not a widely implemented set of protocols • overtaken by success of the Internet (TCP/IP) • TCP/IP not completely consistent with ISO model

  3. The ISO-OSI Model Standardisation Elephant Elephant Elephant activity just right time STANDARDISATION • Essential ingredient in a multi-manufacturer industry • Timing of standardisation critical • Too soon - research continues after standardisation • result - non-compliant systems • Too late - multimillion dollar investments in nonstandard technology • result - non-compliant system "Apocalypse of the two elephants" standardisation too late too soon research investment

  4. The ISO-OSI Model Layering layer n+1 layer n+1 PDUs Layer n Layer n (Protocol data units) Protocol entity Protocol entity layer n-1 layer n-1 LAYERING A divide and conquer approach Layer n service requests Layer n service requests SAP - (Service Access point) Layer n-1 service requests Layer n-1 service requests

  5. The ISO-OSI Model Seven layer model sender receiver Application data data Presentation Session Transport router1 router1 Network Network Network Data Link Data Link Data Link bitstream THE SEVEN-LAYER MODEL APDU Application PPDU Presentation SPDU Session TPDU Transport Packet Network Frame Data Link Physical

  6. The ISO-OSI Model Seven layer model sender receiver Application data data Presentation Session Transport router1 router1 Network Network Network Data Link Data Link bitstream bitstream THE SEVEN-LAYER MODEL Application Presentation Session Transport Network Data Link Data Link Physical

  7. The ISO-OSI Model Seven layer model sender receiver Application data data data data data data data data router1 router1 bitstream bitstream THE SEVEN-LAYER MODEL Application Presentation Presentation Session Session Transport Transport Network Network Network Network Data Link Data Link Data Link Data Link Physical

  8. The ISO-OSI Model Seven layer model sender receiver Application data Presentation Session Transport router1 router1 Network Network Network Data Link Data Link Data Link THE SEVEN-LAYER MODEL Application Presentation Session Transport Network Data Link Physical

  9. The ISO-OSI Model Seven layer model Upper (user) layers Standard data comms apps Email, WWW, file transfer Data representation, encryption, compression ASCII↔EBCDIC, ASN.1, PGP, Lempel- Ziv compression Sets up and administers sessions, synchronises after upper-layer errors Map between user sessions and transport connections Administers connections, QOS, transfers error-free data (end-to-end ) 5 transport protocol classes allow for range of network service standards Routing (tables-based, flooding), address translation Delivers data Error correction, data frames, ack frames Transfers error-free data (point to point ) Maps bitstream onto medium Volts, timing, mechanical specs Node-To-Node End-to-end Network THE SEVEN-LAYER MODEL Responsibility Protocol Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical

  10. The ISO-OSI Model Seven layer model Upper (user) layers Responsibility Protocol Application Standard data comms apps Email, WWW, file transfer Application Data representation, encryption, compression ASCII↔EBCDIC, ASN.1, PGP, Lempel- Ziv compression Presentation Presentation Sets up and administers sessions, synchronises after upper-layer errors Map between user sessions and transport connections Session Session Administers connections, QOS, transfers error-free data (end-to-end ) 5 transport protocol classes allow for range of network service standards Transport Transport Routing (tables-based, flooding), address translation Network Delivers data Network Error correction, data frames, ack frames Transfers error-free data (point to point ) Physical Maps bitstream onto medium Volts, timing, mechanical specs Physical Node-To-Node End-to-end Network THE SEVEN-LAYER MODEL Data Link

  11. The ISO-OSI Model Layer 2 (Data Link Layer) Data Link LAYER 2 (THE DATA LINK LAYER) Network Link establishment and termination; messages received and for transmission Data Link Bit sequence only peer-to-peer (virtual) communication Physical link management; transfer of error-free messages real communication

  12. The ISO-OSI Model HDLC HDLC - AN IMPLEMENTATION OF LAYER 2 A synchronous communications technique Asynchronous techniques allow for clock drift between sender and receiver Raw data Sampling times • Manchester encoding is used for asynchronous communications • Every bit involves a transition • Data acts as its own synch pulse Drawback; “housekeeping” transitions increase the bandwidth requirement

  13. The ISO-OSI Model HDLC HDLC - AN IMPLEMENTATION OF LAYER 2 • HDLC is synchronous • Sender inserts SYN character occurs at start and end of large block of data • Receiver recognises bit pattern of SYN & • sets its clock to sample signal in the middle of each bit Synchronisation must last for the whole of the current block

  14. The ISO-OSI Model HDLC HDLC Asynchronous techniques use frequent synchronisation events tostay in synch • HDLC is synchronous • Sender inserts SYN character occurs at start and end of large block of data • Receiver recognises bit pattern of SYN & • sets its clock to sample signal in the middle of each bit operates between adjacent nodes in a network may be full duplex need not operate over a reliable medium is responsible for error-free data transfer uses sliding window acknowledgement

  15. The ISO-OSI Model The HDLC Frame HDLC FRAME FLAG 01111110 FRAME CHECK 10-bit CRC check on everything between flags INFORMATIONFIELD ANY combination of 0 or more bits CONTROL FIELD Octet containing sequencing and protocol information • ADDRESS FIELD • Octet (8-bit sequence) specifying destination terminal for frame when using multidrop line

  16. The ISO-OSI Model HDLC Control Field HDLC CONTROL FIELD • Distinguishes between • Information frames • Numbered supervisory frames • Unnumbered supervisory frames CONTROL FIELD Octet containing sequencing and protocol information

  17. The ISO-OSI Model HDLC Information Frame Poll/final Receive count 0 Send count bit CONTROL FIELD HDLC : INFORMATION FRAME • Distinguishes between • Information frames • Numbered supervisory frames • Unnumbered supervisory frames 0 denotes information frame • Send count is sequence number of current frame • When frames arrive correctly, receiving station stores arriving send count + 1 as receive count • Arriving frame’s send count should always equal receiving station’s receive count • Receiving station does not increment its receive count till arriving frame checks out • Receive count in the HDLC frame is seq no. of next frame expected by the sender • Receiver compares incoming receive count with its send count • Sends frames starting with incoming receive count value P/F bit is set by primary station when polling, by secondary when finished

  18. The ISO-OSI Model HDLC Numbered Supervisory Frame 1 0 Poll/final Receive count 0 Send count bit CONTROL FIELD HDLC : NUMBERED SUPERVISORY FRAME : INFORMATION FRAME Function • 10 denotes numbered supervisory frame • Frame carries information payload (hence number), and specifies a supervisory function Function field • 00Receive Ready • 01Reject • notification of sequence error • Must retransmit all frames from Receive Count onwards • 10Receive Not Ready • 11 Selective Reject • need only retransmit specified frame

  19. The ISO-OSI Model HDLC Unnumbered Supervisory Frame 1 1 0 1 Poll/final bit CONTROL FIELD HDLC : UNNUMBERED SUPERVISORY FRAME : NUMBERED SUPERVISORY FRAME Function function Part B function part A Receive count 11 denotes unnumbered supervisory frame Function field • 5-bit code for network housekeeping • Reset counters • Disconnect • Query identity • Test • etc

  20. The ISO-OSI Model HDLC Bit Stuffing HDLC : BIT STUFFING • Problem • HDLC’s flag sequence is 01111110 • Data may include 01111110 • arbitrary length, so flag’s position can’t be predicted • conflict if data gets mistaken for flag • Solution • Accept data containing flag sequence from level 3 • Deliver data containing flag sequence to level 3 • BUT at level 2 transmitter, add an extra bit to the data • prevents flag sequence from occurring in data part of transmitted bit stream

  21. The ISO-OSI Model HDLC Bit Stuffing HDLC : BIT STUFFING 0111111x x10111110 HDLC HDLC 0111111x

  22. The ISO-OSI Model HDLC Modes HDLC : MODES • NRM Normal Response Mode • Secondary is polled, starts transmitting frame sequence • Secondary sets F bit in final frame • May not transmit again till polled again • ARM Asynchronous Response Mode • Secondary may be polled but • may initiate transmission without being polled • may result in contention (cf. CSMA) • ABRM Asynchronous Balanced Response Mode • Two stations:Each sends as primary, receives as secondary

  23. The ISO-OSI Model HDLC Sliding Window Acknowledgement Send count Receive count Receive count Send count 0 0 7 7 0 0 7 7 1 1 1 1 6 6 6 6 5 5 2 2 5 5 2 2 3 3 3 4 3 4 4 4 0 Poll/final Send count bit Receive count HDLC : SLIDING WINDOW ACKNOWLEDGEMENT 0 0 0 0 1 frame.sendCount := sendCount frame.sendCount := sendCount frame.receiveCount := receiveCount frame.sendCount := sendCount frame.receiveCount := receiveCount frame.Data := data frame.sendCount := sendCount frame.receiveCount := receiveCount frame.Data := data … frame.send frame.sendCount := sendCount frame.receiveCount := receiveCount frame.Data := data … frame.send sendCount++ If checksOutOK(frame.CRC) then If frame.sendCount = receiveCount then begin Frame.accept sendCount := frame.receiveCount receiveCount++ end

  24. The ISO-OSI Model HDLC Frame Transfer Diagrams Meaning Symbol I(0,0) Information Frame, SN = 0, RN = 0, P/F bit = FALSE I(1,0)P Information Frame, SN = 1, RN = 0, Poll RR(4)F Receive Ready Frame, no SN, RN = 4, Final HDLC : FRAME TRANSFER DIAGRAMS abbreviated representation of admin. information transfer and updating

  25. The ISO-OSI Model HDLC Frame Transfer Diagrams NRM (error-free operation) Primary Secondary Next frame Next frame Next frame Next frame to receive to send to receive to send N(R) N(S) N(R) N(S) 0 0 0 0 I(0,0) 0 0 1 1 0 0 2 2 0 0 3 3 3 1 1 3 . . . . . . . . 3 2 2 3 HDLC : FRAME TRANSFER DIAGRAMS Time I(1,0) I(2,0),P I(0,3) I(1,3)F

  26. The ISO-OSI Model HDLC Frame Transfer Diagrams NRM NRM (error-free operation) (transmission error): Primary Secondary Next frame Next frame Next frame Next frame to receive to send to receive to send N(R) N(S) N(R) N(S) 5 5 3 3 I(5,3) 3 3 3 3 6 6 7 6 3 3 6 0 3 3 6 1 6 4 4 6 . . . . . . . . 7 4 4 7 HDLC : FRAME TRANSFER DIAGRAMS I(6,3) Error: ignore Time I(7,3) Incorrect N(S): ignore I(0,3),P Incorrect N(S): ignore data, accept P bit I(3,6),F I(6,4)F

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