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The ISO-OSI Model Tanenbaum 3 edition: 28-42 OSI-ISO model rd 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 Computer Systems (159.253) ~ 122 ~ Data Communications: © P.Lyons 2004 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 Computer Systems (159.253) ~ 123 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model Standardisation 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" Elephant standardisation activity research investment just right time Computer Systems (159.253) ~ 124 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model Layering LAYERING A divide and conquer approach layer n+1 layer n+1 Layer n service requests Layer n service requests SAP - (Service Access point) Layer n Protocol entity PDUs (Protocol data units) Layer n-1 service requests Layer n-1 service requests layer n-1 Computer Systems (159.253) Layer n Protocol entity layer n-1 ~ 125 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model Seven layer model THE SEVEN-LAYER MODEL sender data APDU receiver Application Application data Presentation PPDU Presentation SPDU Session Session TPDU Transport Transport Packet Network Frame Data Link router1 router1 Network Network Network Data Link Data Link Data Link Physical bitstream Computer Systems (159.253) ~ 126 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model Seven layer model THE SEVEN-LAYER MODEL sender data receiver Application Application Presentation Presentation Session Session Transport Transport Network Data Link router1 router1 Network Network Network Data Link Data Link data Data Link Physical bitstream bitstream Computer Systems (159.253) ~ 127 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model Seven layer model THE SEVEN-LAYER MODEL sender data receiver Application Application data Presentation Presentation data Session Session data Transport Transport Network Data Link data router1 router1 Network Network data Data Link Data Link data Network Data Link Physical bitstream Computer Systems (159.253) bitstream ~ 128 ~ data Data Communications: © P.Lyons 2004 The ISO-OSI Model Seven layer model THE SEVEN-LAYER MODEL sender data receiver Application Application Presentation Presentation Session Session Transport Transport Network Data Link router1 router1 Network Network Network Data Link Data Link Data Link Physical Computer Systems (159.253) ~ 129 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model Seven layer model THE SEVEN-LAYER MODEL Upper (user) layers End-to-end Application Application Presentation Presentation Session Session Transport Transport Responsibility Standard data comms apps Email, WWW, file transfer Data representation, encryption, compression Sets up and administers sessions, synchronises after upper-layer errors Administers connections, QOS, transfers error-free data (end-to-end ) ASCII↔EBCDIC, ASN.1, PGP, Lempel- Ziv compression Map between user sessions and transport connections 5 transport protocol classes allow for range of network service standards Routing (tables-based, flooding), address translation Error correction, data frames, ack frames Network Delivers data Network Data Link Data Link Physical Physical Protocol Transfers error-free data (point to point ) Maps bitstream onto medium Volts, timing, mechanical specs Node-To-Node Network Computer Systems (159.253) ~ 130 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model Seven layer model THE SEVEN-LAYER MODEL Upper (user) layers End-to-end Application Application Presentation Presentation Session Session Transport Transport Responsibility Standard data comms apps Email, WWW, file transfer Data representation, encryption, compression Sets up and administers sessions, synchronises after upper-layer errors Administers connections, QOS, transfers error-free data (end-to-end ) ASCII↔EBCDIC, ASN.1, PGP, Lempel- Ziv compression Map between user sessions and transport connections 5 transport protocol classes allow for range of network service standards Routing (tables-based, flooding), address translation Error correction, data frames, ack frames Network Network Delivers data Data Link Transfers error-free data (point to point ) Physical Physical Protocol Maps bitstream onto medium Volts, timing, mechanical specs Node-To-Node Network Computer Systems (159.253) ~ 131 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model Layer 2 (Data Link Layer) LAYER 2 (THE DATA LINK LAYER) Network Link establishment and termination; messages received and for transmission Data Link Data Link Bit sequence only (virtual) communication peer-to-peer Physical real communication link management; transfer of error-free messages Computer Systems (159.253) ~ 132 ~ Data Communications: © P.Lyons 2004 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 Computer Systems (159.253) ~ 133 ~ Data Communications: © P.Lyons 2004 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 Computer Systems (159.253) ~ 134 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model HDLC HDLC Asynchronous techniques use frequent synchronisation events to stay in synch HDLC is synchronous Sender inserts SYN character occurs at start and end of large block of data operates between adjacent nodes in a network Receiver recognises bit pattern of SYN & setsmay its clock to sample be full duplexsignal in the middle of each bit need not operate over a reliable medium is responsible for error-free data transfer uses sliding window acknowledgement Computer Systems (159.253) ~ 135 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model The HDLC Frame HDLC FRAME FLAG 01111110 FRAME CHECK 10-bit CRC check on everything between flags INFORMATION FIELD 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 Computer Systems (159.253) ~ 136 ~ Data Communications: © P.Lyons 2004 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 Computer Systems (159.253) ~ 137 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model HDLC Information Frame HDLC CONTROL FIELD : INFORMATION FRAME Distinguishes between 0 Poll/final Information frames Send count bit Numbered supervisory frames Unnumbered supervisory frames Receive count 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 Computer Systems (159.253) ~ 138 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model Numbered Supervisory ONTROL FIELD : N HDLC CHDLC INFORMATION UMBERED SUPERVISORY FRAME FRAME Frame 10 0 Send Function count Poll/final bit Receive count 10 denotes numbered supervisory frame Frame carries information payload (hence number), and specifies a supervisory function Function field 00 Receive Ready 01 Reject notification of sequence error Must retransmit all frames from Receive Count onwards 10 Receive Not Ready 11 Selective Reject need only retransmit specified frame Computer Systems (159.253) ~ 139 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model HDLC Unnumbered Supervisory Frame HDLC CONTROL FIELD : N UUMBERED NNUMBERED SUPERVISORY SUPERVISORY FRAME FRAME 1 01 function Function part A Poll/final bit Receive count function Part B 11 denotes unnumbered supervisory frame Function field 5-bit code for network housekeeping Reset counters Disconnect Query identity Test etc Computer Systems (159.253) ~ 140 ~ Data Communications: © P.Lyons 2004 HDLC : BIT STUFFING The ISO-OSI Model 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 Computer Systems (159.253) ~ 141 ~ Data Communications: © P.Lyons 2004 0111111x HDLC: BIT STUFFING The ISO-OSI Model HDLC Bit Stuffing Computer Systems (159.253) HDLC 0111111x x10111110 HDLC ~ 142 ~ Data Communications: © P.Lyons 2004 HDLC: MODES NRM The ISO-OSI Model HDLC Modes 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 Computer Systems (159.253) ~ 143 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model HDLC Sliding Window Acknowledgement HDLC: SLIDING WINDOW ACKNOWLEDGEMENT Send count 6 10 5 4 Receive count 0 7 1 6 2 5 3 0 Send count 0 4 Poll/final bit Receive count 7 0 7 Send count 0 7 0 1 6 1 6 1 2 5 2 5 2 4 3 3 4 3 Receive count frame.sendCount := sendCount frame.receiveCount := receiveCount frame.Data := data … frame.send sendCount++ Computer Systems (159.253) If checksOutOK(frame.CRC) then If frame.sendCount = receiveCount then begin Frame.accept sendCount := frame.receiveCount receiveCount++ end ~ 144 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model HDLC Frame Transfer Diagrams HDLC: FRAME TRANSFER DIAGRAMS abbreviated representation of admin. information transfer and updating Symbol Meaning 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 Computer Systems (159.253) ~ 145 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model HDLC Frame Transfer Diagrams HDLC: FRAME TRANSFER DIAGRAMS NRM (error-free operation) Primary Secondary Next frame to send N(S) Next frame to receive N(R) 0 0 1 0 I(0,0) Next frame to send N(S) Next frame to receive N(R) 0 0 0 1 0 2 0 3 1 3 .. .. Time I(1,0) 2 0 3 0 3 1 .. 3 .. I(2,0),P I(0,3) I(1,3)F 2 Computer Systems (159.253) 2 3 ~ 146 ~ Data Communications: © P.Lyons 2004 The ISO-OSI Model HDLC Frame Transfer Diagrams HDLC: FRAME TRANSFER DIAGRAMS NRM (error-free operation) (transmission error): Primary Secondary Next frame to send N(S) Next frame to receive N(R) 5 3 6 3 7 3 0 3 1 3 6 4 .. 7 .. I(5,3) I(6,3) I(7,3) I(0,3),P I(3,6),F I(6,4)F 4 Computer Systems (159.253) Next frame to send N(S) Next frame to receive N(R) 3 5 3 6 Error: ignore 3 6 Incorrect N(S): ignore 3 6 Incorrect N(S): ignore data, accept P bit 3 6 4 6 .. .. 4 Time 7 ~ 147 ~ Data Communications: © P.Lyons 2004