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COM342 Networks and Data Communications. Ian McCrum Room 5B18 Tel: 90 366364 voice mail on 6 th ring Email: 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


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
  • 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