COM342 Networks and Data Communications

COM342Networks and Data Communications Ian McCrum Room 5B18 Tel: 90 366364 voice mail on 6th ring Email: IJ.McCrum@Ulster.ac.uk Web site: http://www.eej.ulst.ac.uk www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Networks and Data Communications • Lectures • Practicals • Tutorials • Assessment (on-line) • Examples - Software • Reading List www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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 www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Tutorials • on a regular basis • work through mathematical problems and discussion • illuminate lectures and practicals • help with assessment and examination www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Examples • WWW generally, I will provide links • http://www.eej.ulst.ac.uk/~ian/modules/COM347.html • Will be on WebCT • I will leave lectures + other materials on WebCT • demonstrations where possible www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Reading List • Essential: Computer Networks Andrew S. Tanenbaum Prentice Hall (2002) • Recommended: Data Communications, Networks and Open Systems. Fred Halsall Addison Wesley www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Professionalism • Arrive on time • Apply oneself diligently • Acquire the text • Submit coursework on time • Good attendance www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Networks overview • Some definitions • networks and interconnections • broadcast and point-to point • LANs and WANs • Topology • Software www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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) www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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 www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Informatics network www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Wider Network www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Fig 1.1 Tanenbaum www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Which uses which network? • small, compact (geog.) tend to use broadcast networks. • larger, more distributed will be usually point-to-point. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Computer Conductor LAN (Bus) www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Computer Conductor LANs (Ring) www.eej.ulster.ac.uk/~ian/modules/COM342/L1

LANs • 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 www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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 www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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) www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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 www.eej.ulster.ac.uk/~ian/modules/COM342/L1

Terms • Subnet + hosts = WAN • distinct WAN + distinct WAN = internet(work) www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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 www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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) www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

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. www.eej.ulster.ac.uk/~ian/modules/COM342/L1

COM342 Networks and Data Communications