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Data Communications

Chapter 1: An Introduction

For thousands of years communication was based on auditory or visual senses (sounds, words, letters) or smoke signals-remote

Some History

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BreakthroughBreakthrough: invention of : invention of telegraph (1837) by Samuel Morse telegraph (1837) by Samuel Morse --signals send via electrical impulses signals send via electrical impulses (dots and dashes)(dots and dashes)

In 1876 telephone is invented by In 1876 telephone is invented by Alexander Graham Bell Alexander Graham Bell -- direct direct conversion of voice into electrical conversion of voice into electrical impulsesimpulses

Telephone communications progressed steadily, from point-to-point link, through a switchboard, to becoming a popular utility

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Phone-Call RoutingManual (“voice activated”)ElectromechanicalComputer-controlled (after 1960’s)

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1945 - invention of first electronic computer – ENIAC

1980’s - introduction of massive amounts of PC’s, electronic storing of information, computer networking(WAN’s, LAN’s)

1990’s – widespread use of Internet, WWW, e-mailetc.

Broadcasting

Applications

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E-mail

Major Other ApplicationsFaxVoice and videoTeleconferencingCell phonesInformation servicesE-commerce

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Computer Communication IssuesHow to connect computers (by wires, fiber optics, etc.?)Connection routes form a network of “roads” with limited capacities and different “maximum speeds”Rules of communication (“traffic rules”)-protocols

Computer Communication IssuesEase of useSecurity (encryption, etc.)Incompatibility of different systems (e.g. UNIX vs. PC) – use of Open Systems

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Computer Network“a system connecting computers, printers, drives etc. is a computer network”Allows easy information and resource sharing

Computer Networks

Computer System as a network

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Network Topologies

Network Topology: a connection strategy (structure)

Common Bus Topology

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Common Bus TopologyEach device has an addressed listener interface (NIC) sensing the bus activity and reading the data destined for each stationIt can only accept the data designated for itInterface is also sensing when the bus is not busy to transfer the dataCollisions of signals can happen

Common Bus TopologyThe collision noise is being detected by the interface and results in another attempt to send (CSMA/CD technique)Typical implementation: EthernetEasy to add new devices, no device has to be aware of othersFailure of one device does not affect the network operations

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Star Topology

Star TopologyCentral computer controls the network traffic (any communication is possible only via central machine)Focal point of responsibilityDifficult to add new devices (they have to register with the central machine)Failure of the focal machine leads to the network failure

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Ring Topology

Ring TopologyIndirect communications onlyCan be be unidirectional or bi-directionalTypical implementation: “token ring network” of IBM (token is used to avoid collisions, to synchronize transmissions)Too much information relying resulting in delaysFailure of one node affects the whole network

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Fully Connected Topology

Fully Connected TopologyVery expensiveVery fast and reliableMay have underused connections

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Combined Topology

Open System

Open System is a set of protocols to allow incompatible systems to communicate

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Open Systems Interconnect Model

Developed by International Standards Organization (ISO)Seven layersEach layer performs a specific function and communicates with its neighboursImplementation of each layer is transparent to the “parent”Each child provides service to the parent

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Functions of the Layers7. Application: Provides electronic mail, file transfers and other user services6. Presentation: Translates data formats, encrypts, decrypts data5. Session: Synchronizes users, recovers from errors, and brackets operations4. Transport: Determines network, may de-assemble and re-assemble packets3. Network: Determines routes, manages billing information2. Data Link: Detects or corrects errors, defines frames1. Physical: Transmits physical data

OSI-Main IdeasLogically, each layer communicates directly with its counterpart on the other sidePhysically, each layer communicates with the layers immediately above and below it

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A B

A B

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Physical LayerConcerned with:- transmission media- connection strategies

Physical Layer-Transmission Media

Twisted pairCoaxial cableOptical fibreSateliteMicrowave towersRadio waves

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Connection Strategies

Sample Computer Network

How to connect A to F?

Connection Strategies-Circuit Switching

Dedicated lineRecipient must answer before connection possibleHigh speedHigh cost

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Connection Strategies-Message Switching

No dedicated lineRecipient does not have to answer Low speed (buffering)Route selection based on cost and availability of connectionLong message may cause disruption (exceeding buffer size)

Connection Strategies-Packet Switching

Good network utilizationDelays due to buffering and packet re-assembly

Packet: fixed-size unit of data with destination address

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Packet Routing ProtocolsDatagram: packets routed independently, at each node a routing decision is made- high cost- packets may arrive out of orderVirtual Circuits: route to the end is established before sending the packet- route is not dedicated

Note: For larger groups of packets, virtual circuit is betterdue to lower cost (the overhead of routing a packet is high)

Data Link LayerControls operation of the physical layerIssues: - Contention- Collision detection- Error detection, error correction

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Dealing with Contention

CSMA/CD: Carrier Sense, Multiple Access with Collision DetectionToken passing technique

Token Ring Network

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Error DetectionUsing extra parity bit and Parity Checking

Network Layer

Top layer of the group of layers dealing with net communicationsThe layers above net layer deal with user servicesConcerned with:Routing- finding the best route between two pointsBilling

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Route Costs

Routing NotesGood, cheap route can attract too much traffic and overload the systemSome of the messages may be lostDetouring can information to bounce from node to node

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Transport LayerGoal: provide a reliable and efficient network connection

allow the 3 layers above to perform their tasks independent of a specific network architectureuse the features of the 3 layers below tocontrol the actual network operations

Typical functions:multiplexing, buffering, and connection management

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Transport LayerMultiplexing

it is common for multiple applications to make use of the network simultaneouslythe transport layer allows multiple applications to use the same networkensures that incoming data go to the correct application

Multiplexing

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Transport LayerBuffering (at the source and destination)

data to be sent is divided into transport protocol data units (TPDUs)the transport layer typically requires acknowledgement that each TPDU is receivedthis requires that outgoing TPDUs be buffered at source until acknowledged (and resent if no ack comes)on the receiving end TPDUs are buffered until thesession layer needs it

Transport LayerConnection Management

the protocol by which connections between nodes are established and releasedtwo-way handshake protocol is common (ask, accept, connect)delays in responses must be addressed in the protocol

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Two-Way handshake

Session LayerContains the protocols necessary for establishing and maintaining a connection or session between two end user applications.The difference between this layer and the transport layer is that the transport layer is making connections between nodes, but the session layer is making connections between applications.The session layer can also support things like full-duplex connections even if the underlying network is half duplex.

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Multiple Sessions Using One Transport Connection

Note: One session is serviced at a time

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Session Layer IssuesDialog ManagementFull duplexHalf duplexSynchronization pointsMeans of ensuring recovery from transmission errorsActivitiesSeries of requests (dialog units) fulfilling a specific function (eg. Data base query)

Presentation LayerDeals with the difficulties of different computer systems representing data in different formats

text formats (ASCII vs EBCDIC)numerical formats (one’s compliment vs. two’s compliment, various floating point formats)

Compression and data encryption can occur at this layer

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Application LayerCommunication with the application programs.Provided protocols for:

file transfers over the networkvirtual terminalsaccess to distributed systems

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Internet Layers

1. physical layer

2. data link layer

3. network layer

4. transport layer

5. application layer

transmission medium

five layersfirst four roughly correspond to the firstfour OSI layersfifth layer is for high-level Internet protocols (HTTP, FTP, SMTP, etc.)

Internet LayersLayers 1 and 2 (physical and data link) are actually not specified: depends on the type of network (Ethernet over twisted pair, fibre optics, etc.)Layer 3 (network) defines packet formats, routing procedures, types of services, etc. (IP exists here)Layer 4 (transport) provides end-to-end protocols (TCP and UDP exist here)