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19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 L1/1 COM342 Networks and Data Communications Ian McCrumRoom 5B18 Tel: 90 366364 voice mail on 6 th.

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Presentation on theme: "19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 L1/1 COM342 Networks and Data Communications Ian McCrumRoom 5B18 Tel: 90 366364 voice mail on 6 th."— Presentation transcript:

1 19/09/04 L1/1 COM342 Networks and Data Communications Ian McCrumRoom 5B18 Tel: voice mail on 6 th ring Web site:

2 19/09/04 L1/2 Networks and Data Communications Lectures Practicals Tutorials Assessment (on-line) Examples - Software Reading List

3 19/09/04 L1/3 Lectures 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

4 19/09/04 L1/4 Practicals 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.

5 19/09/04 L1/5 Tutorials on a regular basis work through mathematical problems and discussion illuminate lectures and practicals help with assessment and examination

6 19/09/04 L1/6 Assessment 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.

7 19/09/04 L1/7 Examples WWW generally, I will provide links Will be on WebCT I will leave lectures + other materials on WebCT demonstrations where possible

8 19/09/04 L1/8 Reading List Essential: Computer Networks Andrew S. Tanenbaum Prentice Hall (2002) Recommended: Data Communications, Networks and Open Systems. Fred Halsall Addison Wesley

9 19/09/04 L1/9 Professionalism Arrive on time Apply oneself diligently Acquire the text Submit coursework on time Good attendance

10 19/09/04 L1/10 Networks overview Some definitions networks and interconnections broadcast and point-to point LANs and WANs Topology Software

11 19/09/04 L1/11 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)

12 19/09/04 L1/12 Interconnection enables the exchange of data. and information, using various media. examples of media: –Copper telephone, LANs –Microwaves telephone, satellite –fibre optics light, telephone, data

13 19/09/04 L1/13 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.

14 19/09/04 L1/14 Informatics network

15 19/09/04 L1/15 Wider Network

16 19/09/04 L1/16 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.

17 19/09/04 L1/17 Fig 1.1 Tanenbaum

18 19/09/04 L1/18 Individuals uses of Computer Networks Information Internet, home-banking, stock trading. Communication , video-conferencing, IRC, news groups. Entertainment video on demand, multi-user doom, swapping of games.

19 19/09/04 L1/19 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.

20 19/09/04 L1/20 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” “everyone who 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.

21 19/09/04 L1/21 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.

22 19/09/04 L1/22 Which uses which network? small, compact (geog.) tend to use broadcast networks. larger, more distributed will be usually point-to-point.

23 19/09/04 L1/23 LAN (Bus) Conductor Computer

24 19/09/04 L1/24 LANs (Ring) Computer Conductor

25 19/09/04 L1/25 LANs size limited by transmission time (nanosec per foot) Ethernet IEEE CSMA/CD Token Ring IEEE static allocation of resource round robin - wasteful dynamic –de-centralised –centralised

26 19/09/04 L1/26 Metropolitan Area Networks larger version of LAN supports voice and data typical Mbps Distributed Queue Dual Bus IEEE see Fig.1-4 A.T.

27 19/09/04 L1/27 Fig 1.4 Tanenbaum

28 19/09/04 L1/28 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.

29 19/09/04 L1/29 Fig 1.5 Tanenbaum

30 19/09/04 L1/30 WANs 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

31 19/09/04 L1/31 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)

32 19/09/04 L1/32 Fig 1.6 Tanenbaum

33 19/09/04 L1/33 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

34 19/09/04 L1/34 Fig 1.7 Tanenbaum

35 19/09/04 L1/35 Terms Subnet + hosts = WAN distinct WAN + distinct WAN = internet(work)

36 19/09/04 L1/36 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

37 19/09/04 L1/37 Fig 1.9 Tanenbaum

38 19/09/04 L1/38 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.

39 19/09/04 L1/39 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.

40 19/09/04 L1/40

41 19/09/04 L1/41 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.

42 19/09/04 L1/42 Services 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)

43 19/09/04 L1/43 Interface 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.

44 19/09/04 L1/44 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.

45 19/09/04 L1/45 Fig 1.13 Tanenbaum


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