COM347J1 Networks and Data Communications

19/09/04 www.eej.ulster.ac.uk/~ian/modules/COM347J1/L1 L1/1

COM347J1Networks and Data Communications

Ian McCrum Room 5D03BTel: 90 366364 voice mail on 6th ringEmail: [email protected] site: http://www.eej.ulst.ac.uk


Networks and Data Communications

• Lectures• Practicals• Tutorials• Assessment (on-line)• Examples - Software• Reading List


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


Practicals

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

• will take place in 16G26/27• safety rules apply• If you don’t understand , ask!• under development with potential

flexibility, so make suggestions.


Tutorials

• on a regular basis• work through mathematical problems and

discussion• illuminate lectures and practicals• help with assessment and examination


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.


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


Reading List

• Essential:Computer Networks Andrew S. TanenbaumPrentice Hall (2002)

• Recommended:Data Communications, Networks and Open Systems.Fred HalsallAddison Wesley


Professionalism

• Arrive on time• Apply oneself diligently• Acquire the text• Submit coursework on time• Good attendance


Networks overview

• Some definitions• networks and interconnections• broadcast and point-to point• LANs and WANs • Topology• Software


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)


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


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


Wider Network


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


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• 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• 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.


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?

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

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


LAN (Bus)

Conductor

Computer


LANs (Ring)Computer

Conductor


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


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


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


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


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


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


Terms

• Subnet + hosts = WAN

• distinct WAN + distinct WAN = internet(work)


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


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

• 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• 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.


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)


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.


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

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