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COM342 Networks and Data Communications. Ian McCrum Room 5B18 Tel: 90 366364 voice mail on 6 th ring Email: [email protected] Web site: . Networks and Data Communications. Lectures Practicals Tutorials Assessment (on-line) Examples - Software

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Com342 networks and data communications

COM342Networks and Data Communications

Ian McCrum Room 5B18

Tel: 90 366364 voice mail on 6th ring

Email: [email protected]

Web site:

Networks and data communications
Networks and Data Communications

  • Lectures

  • Practicals

  • Tutorials

  • Assessment (on-line)

  • Examples - Software

  • Reading List


  • will start promptly at time specified

  • will be 50 mins in length

  • illuminate the text chosen and help achieve the learning outcomes of the module

  • are simplex with opportunities to reverse the line

  • help pass the examination


  • give you the opportunity to look at computers and communications in a new way

  • will take place in 6c49

  • safety rules apply

  • If you don’t understand , ask!

  • under development with potential flexibility, so make suggestions.


  • on a regular basis

  • work through mathematical problems and discussion

  • illuminate lectures and practicals

  • help with assessment and examination


  • Mainly via WebCT or other computer based testing

  • assessments using various weightings shown on WebCT page for module

  • Examination weighting of 75%

  • some past papers available on intranet and WebCT. More later.


  • WWW generally, I will provide links


  • Will be on WebCT

  • I will leave lectures + other materials on WebCT

  • demonstrations where possible

Reading list
Reading List

  • Essential:

    Computer Networks

    Andrew S. Tanenbaum

    Prentice Hall (2002)

  • Recommended:

    Data Communications, Networks and Open Systems.

    Fred Halsall

    Addison Wesley


  • Arrive on time

  • Apply oneself diligently

  • Acquire the text

  • Submit coursework on time

  • Good attendance

Networks overview
Networks overview

  • Some definitions

  • networks and interconnections

  • broadcast and point-to point

  • LANs and WANs

  • Topology

  • Software

Computer networks
Computer Networks

  • Definition: “A computer network is an interconnected collection of autonomous computers”

  • autonomous(a). possessed of autonomy

  • autonomy(n). right of self government; personal freedom; freedom of will

    (concise oxford dictionary)


  • enables the exchange of data. and information, using various media.

  • examples of media:

    • Copper

      • telephone, LANs

    • Microwaves

      • telephone, satellite

    • fibre optics

      • light, telephone, data

Distributed system
Distributed system

  • A user has the perception of using a system, not a single or greater number of computers.

  • The distributed system determines where execution and storage of results should take place.

  • uses a network to carry out its tasks transparently.

Informatics network
Informatics network

Wider network
Wider Network

Companies uses of computer networks
Companies uses of Computer Networks

  • Resource sharing

    • physical, software, data

    • elimination of geographic constraints

  • High reliability

    • multiple copies of information

    • multiple computers

  • Finance

    • more bang/buck for small computers give rise to file servers and clients. see Fig 1.1

  • Co-operation

    • among separated colleagues.


    Fig 1 1 tanenbaum
    Fig 1.1 Tanenbaum

    Individuals uses of computer networks
    Individuals uses of Computer Networks

    • Information

      • Internet, home-banking, stock trading.

  • Communication

    • email, video-conferencing, IRC, news groups.

  • Entertainment

    • video on demand, multi-user doom, swapping of games.


    Social implications
    Social Implications

    • Working from home.

      • flexible working patterns, parents of young children.

  • Cheaper Offices

    • hot desking, less direct contact with co-workers. +/-

  • Freedom

    • with increasing laxity in regulation due to growth rate.

    • exploited by fascists, pornographers, freedom fighters

  • Whistle blowers

    • politics, child abuse, Deep Throat, etc.


    Broadcast networks
    Broadcast networks

    • a single comms channel shared by all communicators.

    • messages comprised of packets sent by one machine can be received by all others

    • each packet has a destination address which is scrutinised by all receivers and only acted upon by the machine which is intended to be the recipient.

      • “Hi Jamie, the bookshop says that the text you ordered has arrived”

      • “everyonewho attends the student union bar at six o’clock will get pints at £1.00”

    • This latter is broadcasting, subsets multi-casting

  • Geographically localised networks are usually of this type.


    Point to point networks
    Point-to-Point Networks

    • a communications channel is shared by only two machines.

    • to travel from a source to destination a packet may pass through intermediate machines.

      • “Hi Jamie, the bookshop says that the text you ordered has arrived”

    • intermediate machines must know how to forward that message to Jamie with accuracy and not like Chinese whispers.

    • multiple routes are possible, routing algorithms are employed.

    Which uses which network
    Which uses which network?

    • small, compact (geog.) tend to use broadcast networks.

    • larger, more distributed will be usually point-to-point.

    Lan bus



    LAN (Bus)

    Lans ring



    LANs (Ring)


    • size limited by transmission time (nanosec per foot)

    • Ethernet IEEE 802.3

    • CSMA/CD

    • Token Ring IEEE 802.5

    • static allocation of resource round robin - wasteful

    • dynamic

      • de-centralised

      • centralised

    Metropolitan area networks
    Metropolitan Area Networks

    • larger version of LAN

    • supports voice and data

    • typical 160km @ 44.736Mbps

    • Distributed Queue Dual Bus

    • IEEE 802.6

      see Fig.1-4 A.T.

    Fig 1 4 tanenbaum
    Fig 1.4 Tanenbaum

    Wide area networks
    Wide Area Networks

    • Large geographical separation

    • hosts

      • machine which run applications

    • subnet

      • carries messages from host to host

      • transmission lines

        • circuits or channels or trunks

      • switching elements

        • computers which chose an onward path for incoming data.

          see Fig. 1-5 A.T.

    Fig 1 5 tanenbaum
    Fig 1.5 Tanenbaum


    • network consists of cables or telephone lines connecting a pair of routers.

    • should non-interconnected routers wish to communicate they must use intermediate routers

    • store-and-forward or packet-switched subnet

    Topology fig 1 6 a t
    Topology (Fig. 1-6) A.T.

    • LANs usually have a symmetrical topology

    • WANs are typically irregular topologies.

    • Satellite can be used but usually in broadcast mode ( in contrast with the point-to-point usual in WANs)

    Fig 1 6 tanenbaum
    Fig 1.6 Tanenbaum

    Wireless networks
    Wireless Networks

    • Notebooks and PDAs need to talk to office machines whilst on the move.

    • wireless means that the machine has no physical connector onto a network.

    • mobile means that the machine can be easily moved from one place to the next

      See Fig. 1-7

    Fig 1 7 tanenbaum
    Fig 1.7 Tanenbaum


    • Subnet + hosts = WAN

    • distinct WAN + distinct WAN = internet(work)

    Network software
    Network ‘Software’

    • must be structured

    • consists of layers

      • a layer offers a pre-determined service to a higher layer, without divulging how its implemented.

    • A layer(n) on one machine can communicate with the layer(n) on another machine using rules and conventions known as the layer n protocol.

      see Fig. 1-9

    Fig 1 9 tanenbaum
    Fig 1.9 Tanenbaum

    Protocol hierarchies
    Protocol Hierarchies

    • peers are the entities which comprise the corresponding layers on different machines.

    • The physical medium is the only communications path.

    • an interface exists between adjacent pair of layers

      • objective is have simple, clean-cut interfaces with complexity within the layer, enables improvement etc..

  • A set of layers and protocols is an network architecture

  • a list of protocols, one per layer, is a protocol stack.


    Multi layer example
    multi-layer example

    • one philosopher speaks Urdu and English, other speaks Chinese and French see Fig 1-10

    • each protocol is independent of the others so long as the interfaces are unchanged. Thus translators could agree upon another intermediate language while not changing each interface with layer 1 and 3.

    Five layer network example
    Five-layer network example

    • application in layer 5 produces M tx

    • Layer 4 supplies header id (no limit on M size)

    • Layer 3 max packet size therefore Layer 4 message is sub-divided with header added to each packet.

    • Layer 2 adds both header and trailer.

    • Layer 1 does physical transfer.

    • at rx end messages moves up from layer to layer with headers and trailers being stripped.

    • n.b. think that comms are horizontal.


    • an active element in a layer is called an entity.

      • entity can be hardware or software

  • entities in layer n implement a service used by layer n+1, layer n is the service provider and layer n+1 the service user.

    • the service provider many offer different classes of service, speed cost or quality

  • services are available at Service Access Points (SAPs)



    • layer n SAPs are where layer n+1 can access the services

    • SAP have unique addresses.

    • Layer n+1 passes Interface Data Unit though the SAP, this consists of Interface Control Information and Service Data Unit.

    • The SDU is passed to the peer entity on the destination.

    Services see fig 1 13
    Services see Fig 1-13

    • Connection-oriented service, like telephone, establish a fixed route through the network.

    • Connectionless service, like postal service, independent routing for each component.

    • quality of service

      • reliable, unreliable

  • Datagrams

    • with acknowledgement and without

  • Request-reply.


    Fig 1 13 tanenbaum
    Fig 1.13 Tanenbaum