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COMP1321 Digital Infrastructure Week 13

COMP1321 Digital Infrastructure Week 13. Richard Henson January 2019. Week 13: WANs, LANs, Open Systems Interconnect. Objectives: Naming systems for LANs and WANs Need for a standard OSI model and why seven layers Roles of individual layers Simplifying the OSI model.

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COMP1321 Digital Infrastructure Week 13

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  1. COMP1321Digital InfrastructureWeek 13 Richard Henson January 2019

  2. Week 13: WANs, LANs, Open Systems Interconnect • Objectives: • Naming systems for LANs and WANs • Need for a standard • OSI model and why seven layers • Roles of individual layers • Simplifying the OSI model

  3. Intro (& some revision?) • Remember the legendary Carrie-Ann Philbin? • https://www.youtube.com/watch?v=AEaKrq3SpW8

  4. Network Topologies • Modern LANs • usually some kind of star arrangement • at one time, a central spine was popular

  5. Network Topologies • WANs • usually some kind of mesh arrangement • Need to ensure that an alternative route is available in case the preferred route cannot carry a signal

  6. Device Naming • No duplicate names allowed • essential to be logical • enforced by “not null” in database field • Early efforts • based on telephone networks • Town… then number within that town • e.g. Whitehall 1212 (Scotland Yard) • development of an all number system • Known as STD code (e.g. 01905 for Worcester) • no two telephone numbers alike…

  7. Naming and Computer Networks • Within a network… • same principle… no duplicates • Names used to navigate data round the network • naming needs to be designed into any network communications protocol • e.g. MAC address, IP address

  8. WANs, LANs and getting the message through (Routing) • In principle, routing is the same whether LAN or WAN • In practice… • LANs infrastructure owned and managed by organisation themselves (or a third party on their behalf) • WAN infrastructure managed by a specialist organisation and offered as a service

  9. Naming and Addressing • Words used almost interchangeably… • “address” tends to be numerical • “name” tends to be based on words • Fulfill essentially the same purpose: • unique network identity for each device • provides location for data to be sent to • provides source location for data sent

  10. LAN naming systems • IEEE responcible for naming • 802 spec based on Feb’ 1980 start • defined and used “hardware” address of any networkable device • MAC address • Data sent as “frames” (not packets!) • Typical MAC address: • xx.xx.xx.xx.xx.xx (where x= a hex number)

  11. Packet-based LAN naming • Associated with Internet (TCP/IP) • NetBIOS names • WINS names • Active Directory naming… • IP addresses

  12. IP address systems • Originates from TCP/IP naming system • typical IPv4 name: • x.x.x.x • where x = a number, 0 to 255 decimal • now IPv6 • IPv4 running out of unique numbers!

  13. WAN and LAN combination • Organisational data often travels through LANs and WANs… • Navigation may need a combination of naming systems… • dealt with through software that will deliver via a unique name

  14. Communications Protocols and Software Layers • Rapidly got very complicated • Different computer manufacturers through their protocol was best • Prevented communications development via computer for many years

  15. The OSI Seven Layer Protocol • Software for a WAN communication protocol needs to cover SEVEN (!) layers • software has to manage a diverse range of newtork issues and concepts • compromise between IBM and Internet community, but other players involved

  16. The OSI Model (a 1978 compromise…) • Benefits of OSI compatible software: • 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

  17. 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…

  18. 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

  19. 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…

  20. 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 model Receive Station Transmit Station Link

  21. 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!

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

  23. 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

  24. 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

  25. 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

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

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

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

  29. 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

  30. 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)

  31. 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

  32. 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

  33. Network Layer • Responsibilities: • packet (IP) addressing and sequencing • determining to route from source to destination computer • Routers operate up to this layer • use IP addresses to navigate packets

  34. 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

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

  36. Data Link Layer • Responsible for error free physical transmission of data, using frames • may include an error recovery mechanism and also a flow control mechanism, although this may be done at the transport layer • Mechanism (down): • data from the upper layers (i.e. the network layer) is converted by the data link layer into frames • Mechanism (up) • arranges raw data bits received via the physical layer into frames, for passing on to the upper layers • Bridges/Switches operate up to this level • Virtual LANs use frames • https://www.pluralsight.com/blog/it-ops/virtual-lan-vlan-basics • frames navigated through MAC addresses

  37. Physical Layer • Responsible for communicating with the network media • bits (0,1) 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 no specified • Allows improved technology to be easily included • Repeaters work only at this level

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