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

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

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

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

Device Naming No duplicate names allowed Early efforts 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…

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

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

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

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)

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

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!

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

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

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

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

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…

The OSI reference model Transmit Station Receive Station AH DATA PH SH TH NH LH LT Physical Layer Data link Network Transport Session Presentation Application Physical Layer Data link Network Transport Session Presentation Application DATA AH PH SH TH NH LT LH Initially, the diagram shows the seven network software layers in the sending computer. It then shows how extra header data is added to screen data by each of the network layers until it is send out onto the physical network via the physical layer. It then shows all this data arriving at the receiving computer and the header data being stripped away layer by layer until what appears on the screen is just the data that was originally on the sending computer’s screen. Link

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…

The OSI model Link Transmit Receive Station Station Physical Layer AH DATA PH SH TH NH LH LT Physical Layer Data link Network Transport Session Presentation Application Physical Layer Data link Network Transport Session Presentation Application DATA AH PH SH TH NH LT LH Repeat of earlier slide Link

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!

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

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

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

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

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

The Four Layers Model Introduced with Unix (mid-1970s, pre-OSI) based on Internet protocols… “application” “transport” Shows four names in boxes: application, transport, network, physical “network” “physical”

TCP/IP Evolved with the Unix four layers… Application, presentation, session TCP IP As previous slide but application, presentation, session all together in top layer Next layer: TCP Third layer: IP Fourth layer: connecting with physical medium Connecting with physical medium

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

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” After slide text: socket A (sender) in a box, arrow from box to socket B (receiver) in another box socket A (sender) socket B (receiver)

Transport layer Socket A Socket B End User Peer-to-Peer communications Physical Layer Data link Network Transport Upper OSI Layers End User Peer-to-Peer communications Socket A Socket B Similar to previous diagrams showing all seven network layers. This time, transport layer on sender identified at socket A, and transport layer on receiver is socket B. Arrow linking to two together as “peer-peer” communication, Diagram also shows two intermediate routers, where data is not converted up the stack beyond the network layer, before forwarding. Network A Network B

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

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

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

Network Layer Transport Layer Network Layer User Specifies Service Transport Layer Network Service Network Layer Diagram shows the all important communication between transport layer (user oriented service) and network layer (network oriented service) Network provides Service Network layer service definitions

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

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