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19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 L1/1
COM342Networks and Data Communications
Ian McCrum Room 5B18
Tel: 90 366364 voice mail on 6th ring
Email: [email protected]
Web site: http://www.eej.ulst.ac.uk
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 L1/2
Networks and Data Communications
• Lectures
• Practicals
• Tutorials
• Assessment (on-line)
• Examples - Software
• Reading List
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 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
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 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.
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 L1/5
Tutorials
• on a regular basis
• work through mathematical problems and discussion
• illuminate lectures and practicals
• help with assessment and examination
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 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.
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 L1/7
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
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 L1/8
Reading List
• Essential:Computer Networks Andrew S. TanenbaumPrentice Hall (2002)
• Recommended:Data Communications, Networks and Open Systems.Fred HalsallAddison Wesley
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 L1/9
Professionalism
• Arrive on time
• Apply oneself diligently
• Acquire the text
• Submit coursework on time
• Good attendance
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 L1/10
Networks overview
• Some definitions
• networks and interconnections
• broadcast and point-to point
• LANs and WANs
• Topology
• Software
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 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)
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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
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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.
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 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.
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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.
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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.
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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.
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 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.
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Which uses which network?
• small, compact (geog.) tend to use broadcast networks.
• larger, more distributed will be usually point-to-point.
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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
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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.
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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.
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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
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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)
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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
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Terms
• Subnet + hosts = WAN
• distinct WAN + distinct WAN = internet(work)
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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
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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.
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 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.
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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.
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 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)
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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.
19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM342/L1 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.