The OSI Reference Model Data Communications and Networks Mr. Greg Vogl Uganda Martyrs University Lecture 3, 21 March 2003

19 March 2003 Data Communications and Networking: OSI Model 2 Protocols and Standards n Ensure data is recognised by receiver n Hide low level details from end user n Communicate despite differences in: –hardware, software, companies, countries n Formalise existing informal conventions –Often companies set de facto standards n Tradeoff: innovation vs. standardisation

19 March 2003 Data Communications and Networking: OSI Model 3 Organisations and Standards n International Standards Organisation - OSI n American National Standards Institute - FDDI n Electronics Industry Association – RS232C n Inst. Electrical & Electronics Engineers -LANs n CCITT & Int’l Telecomm. Union – V.24 n Internet Activities Board – TCP/IP n Proprietary (IBM, MS, Novell, DEC etc.)

19 March 2003 Data Communications and Networking: OSI Model 4 OSI Model n Open architecture/systems –available to anyone, not proprietary n Open Systems Interconnect (OSI) –ISO-defined open network architecture –Reference Model for designing networks –flow of information appears peer to peer at each level –actual info. transfer occurs at physical level

19 March 2003 Data Communications and Networking: OSI Model 5 IEEE 802.x LAN standards n Used for media access n CSMA/CD (Ethernet) n Token bus n Token ring n Metropolitan Area Networks n cableless or wireless n Mbit/s

19 March 2003 Data Communications and Networking: OSI Model 6 Functions of Network Protocols n Segmentation and Reassembly n Encapsulation and Delineation of data n Connection Control n Ordered Delivery n Flow Control n Error Control: Detection and Correction n Application Synchronisation n Addressing n Data Transparency (Bit/Byte oriented) n Data Compression n Multiplexing n Transmission services

19 March 2003 Data Communications and Networking: OSI Model 7 The 7-Layer OSI Model

19 March 2003 Data Communications and Networking: OSI Model 8 Why layers? 1. Like engineering and software design 2. Breaks large problem into small ones 3. Each layer can be developed separately 4. Each layer can be tested separately 5. Layer details hidden from other layers 6. Layers replaced with improved versions

19 March 2003 Data Communications and Networking: OSI Model 9 Physical Layer n How bits are represented n How data is physically signalled n Spec’s of plugs, sockets, other equipment n Actual transfer of information

19 March 2003 Data Communications and Networking: OSI Model 10 Data Link Layer n Structure of bits n Point-to-point links n Framing n Transferring blocks/packets of data n Error detection/correction/resending n Flow control n Logical link, medium access sublayers

19 March 2003 Data Communications and Networking: OSI Model 11 Network Layer n Network routing and addressing n E.g. Internet Protocol, IP addresses

19 March 2003 Data Communications and Networking: OSI Model 12 Transport Layer n End-to-end transfer –two way, reliable, low cost n secure connections n error or flow control n segmentation n multiplexing and resizing blocks n sequencing

19 March 2003 Data Communications and Networking: OSI Model 13 Session Layer n Dialogue and synchronisation control n establishes calls

19 March 2003 Data Communications and Networking: OSI Model 14 Presentation Layer n Data representation and transformation n Conversion between transmission codes –e.g. ASCII to EBCDIC n Formatting of messages n Security, encryption, and compression of data

19 March 2003 Data Communications and Networking: OSI Model 15 Application Layer n File transfer, access and management n Document and message interchange n n Directory services n Used by application programs that are communicating with each other

19 March 2003 Data Communications and Networking: OSI Model 16 See Also n BITDCO lecture 1 n Hodson Ch. 7 n Stamper pp