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COMP2221 Networks in Organisations

This article discusses the OSI model and its benefits in enabling communication between different systems. It explores the seven software layers of the OSI model, the communication process in each layer, and the simplified categorization of layers. The interworking layers, including the application, presentation, and session layers, are also explained.

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COMP2221 Networks in Organisations

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  1. COMP2221Networks in Organisations Richard Henson February 2012

  2. Open Systems Interconnect (OSI) • Bold aim: • produce a set of communication protocols that would allow any system to exchange information with any other system • Many proprietary models • IBM biggest and most influential, major say • Open systems? • Internet & TCP/IP

  3. Open Systems Interconnect (OSI) • All manufacturers wished to have their own communication models represented • Principle: • “A new layer must be created for each new level of abstraction” • Result: • 7software layers!!!

  4. Open Systems Interconnect (OSI) • OSI layers universal - cover all types of networks • client/server • peer-peer • Define standards for: • interconnecting (linking data) • physical -> network • interworking (presenting data) • transport -> application

  5. OSI Model – Benefits • All manufacturers… • target to aspire towards • Benefits of OSI compatible products: • other manufacturers products would be able to communicate with their own • consumer would no longer be “locked in” to specific vendor products • vendors would be able to produce products that work at specific layers only • specialise and hence produce better products

  6. Layer Communication (Sending) • Each layer in the OSI model considers itself to be talking to a peer layer in another computer • adds/removes its own “header” (formatting info) • e.g. application layer • adds a header to the user data on screen • passed to the presentation layer as a single block e.g. presentation layer • adds its header to the block of data • passed on to session layer as a single block… • and so on…

  7. AH AH AH AH AH AH DATA DATA DATA DATA DATA DATA Application Layer Application Layer DATA AH Presentation Layer Presentation Layer PH DATA AH PH Session Layer Session Layer SH SH SH SH PH PH PH PH DATA AH PH SH Transport Layer Transport Layer TH DATA AH PH SH TH Network Layer Network Layer NH TH Data link Layer Data link Layer LH NH TH LT Physical Layer Physical Layer LT DATA AH PH SH TH NH LH DATA AH PH SH TH NH The OSI reference model Receive Station Transmit Station Link

  8. Layer Communication (Receiving) • Each layer in the OSI model strips away its own header • e.g. physical layer • removes header from data block • passed to the data link layer • e.g. data link layer • removes header to the block of data • passed on to network layer • and so on…

  9. AH AH AH AH AH AH DATA DATA DATA DATA DATA DATA Application Layer Application Layer DATA AH Presentation Layer Presentation Layer PH DATA AH PH Session Layer Session Layer SH SH SH SH PH PH PH PH DATA AH PH SH Transport Layer Transport Layer TH DATA AH PH SH TH Network Layer Network Layer NH TH Data link Layer Data link Layer LH NH TH LT Physical Layer Physical Layer LT DATA AH PH SH TH NH LH DATA AH PH SH TH NH The OSI reference model Receive Station Transmit Station Link

  10. Simplifying The OSI model • Layers can be sub-divided into two groups • The top 3 layers (interworking layers) • user applications and support services • The lower 4 layers (interconnection layers) • the network (and navigation of packets) • Memory aids: • PDNTSPA • Please Do Not Throw Sausage Pizza Away!

  11. Interworking Layers • All about servicing needs of users • support for the application layer… • includes presentation layer • and session layer

  12. Application Layer • Interface for applications to use to gain access to network services: • Networked file transfer • Message handling • Database query processing • Controls generalised network access: • supports applications which exchange data • provides error & status information for applications • If network is peer-peer… • authenticates peer partners • determines if peers are ready to communicate

  13. Presentation Layer • Responsible (sending) for converting data from • application-specific format • to a generic (machine-independent) format that can be passed across a network • Receiving… • for converting incoming data from a generic format to one that makes sense to the receiving application • Also responsible for protocol conversion, encryption & decryption, and graphics commands • The redirector (software for handling service requests) also operates at this layer: • If a service cannot be resolved locally, it sends the request out to the network resource that can offer the required service

  14. Session Layer • Sets up a logical connection between machines called a “session”, which allows networked resources to communicate • Manages the setting up of a user “session”, exchange of information, and “tear down”as the session ends • Manages issues such as who may transmit data at a certain time, and for how long, also ensuring that the system doesn’t “time out” after inactivity • Ensures data is routed to the correct application on the local machine • Synchronises services between tasks at each end of the communications channel in half duplex communications

  15. Interconnection Layers • Concerned with packets of data • and navigating them through the network • Transport • Network • Data Link • Physical

  16. The Four Layers Model • Introduced with Unix (mid-1970s, pre-OSI) • based on Internet protocols… “application” “transport” “network” “physical”

  17. TCP/IP • Evolved with the Unix four layers… Application, presentation, session TCP IP Connecting with physical medium

  18. Transport Layer (from Unix) • Manages the transmission of level 4 data from sender to corresponding layer in receiver • segments data streams into chunks of a given packet size for the medium being used • checks for errors due to corruption, requests retransmission etc. • Gateways can operate at this layer

  19. Transport Layer (from Unix) • Other roles: • managing flow control • providing acknowledgement of successful transmission of chunks of data • software multiplexing • routing in an Internetwork • Manages OSI levels 1-4 so messages travel between network nodes via pairs of “sockets” socket A (sender) socket B (receiver)

  20. End User End User Upper OSI Layers Upper OSI Layers Transport Layer Transport Layer Network Layer Network Layer Network Layer Network Layer Data link Layer Data link Layer Data link Layer Data link Layer Physical Layer Physical Layer Physical Layer Physical Layer Transport layer Peer-to-Peer communications Socket A Socket B Network B Network A

  21. End-end v logical neighbour communications • Top four OSI layers communicate logically with remote peer… • regardless of topology or distance • The lower layers all communicate physically with their nearest neighbour in a network • dependent on topology and routing to get the packets through

  22. Network Layer • Responsibilities: • packet (IP) addressing and sequencing • determining to route from source to destination computer • Routers operate up to this level

  23. Network Layer Functions • Provides messages with an address for delivery (e.g. IP address) • Translates logical network addresses/names into physical equivalents • Handles packet switching and routes packets to their destination on the local network • Controls network packet congestion • Ensures packets conform to the network's format

  24. Network Layer User Specifies Service Transport Layer Network Service Network Layer Network provides Service Network layer service definitions

  25. Data Link Layer • Responsible for error free transmission, using data frames • A frame is a basic unit for network traffic, and has a highly structured format • Mechanism: • data from the upper layers (ie the network layer) is converted by the data link layer into frames • groups raw data bits received via the physical layer into frames, for passing on to the upper layers • may include an error recovery mechanism and also a flow control mechanism, although this may be done at the transport layer • Bridges operate up to this level

  26. Physical Layer • Responsible for communicating with the network media • Bits are converted into electrical signals and vice versa • Issues include modulation of signals and timing • Manages the interface between a computer and the network medium, but cable type and speeds of transmission are deliberately omitted to allow future technology to be easily included • Repeaters work only at this level

  27. Standards and the OSI reference model • OSI designed to promote the development of protocols… • that support open systems interconnection • Become an agreed standard in 1984 • ISO 7498 (the International Standard) • BSI 6568 (the identical British Standard) • CCITT recommendation X.200

  28. The IEEE 802 Specifications and the OSI model • Emerged from IEEE-OSI meeting: February ‘80 • applied mainly to lower level OSI layers (1/2) • found it necessary to extend the data link layer into two parts • Essential for development of LANs • IEEE layer 1/2 definitions used by manufacturers when defining specifications for hardware such as network interface cards

  29. Effect of IEEE 802 on the OSI model • To cover all different types of interfaces, IEEE divided the Data Link Layer into two sub-layers: • Layer 2 (upper) Logical Link Control - used by 802.1 & 802.2 • Layer 2 (lower) Media Access Control - 802.3, 4, 5, 11, 12, etc.

  30. Layer 2 (upper) Logical Link Control • Focuses on IEEE 802.1 & 802.2 • Controls transfer of data to the network layer • Uses logical interface points called SAPs (service access points)

  31. Layer 2 (lower) Media Access Control • Direct communication with the network card • provides packets with MAC address • Focuses on IEEE 802.3, 4, 5, 11, 12… • Provides shared access for multiple network interface cards to the physical layer • Responsible for ensuring error-free communication across the network

  32. OSI layer software and Network cards • Layer 1 and 2 software supplied with the network card • card itself should contain software (on ROM) that conforms to one of the sixteen IEEE 802 specifications • Cards for wired connections have connectors for cables: • usually IEEE 802.3 • more rarely… IEEE 802.5 • Wireless Cards • usually based on IEEE802.11

  33. “Binding” Network Card Software • Data received by the network card needs to be passed on to level 3 software • Normally held on the computer hard disk • Configuration: • level 2 software needs to combine with level 3 • achieved through “binding” OSI Level 3 software binding OSI Level 1/2 software

  34. Now for the practical…P.S. Homework: check out the IEEE 802 standards and decide which are most important in 2012

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