3.1.5: Data Transmission

3.1.5: Data Transmission

Specification:

QuestionsJune 14 - 1 LAN and 7 Packet switching, errorsJune 13 3 errors and 6 transmissionJan 13 6 circuit and packet switching, 7 protocolJune 12 6 WAN, hardware, transmission, rules, bit ratesJan 12 4 LAN, handshake, protocol, 7c errorsJune 11 2 transmission, 8 rules/protocols, 9 packet switchingJan 11 6 LAN, echoes, packet and circuit switching and 9 protocols, bit ratesJune 10 1c LAN, hardware, errors, 6 protocols, linksJan 10 2 packet and circuit switching, 7 bit rate, data transmission, handshakeJune 09 2b transmission, packet switching, 8 LAN WAN, errorsJan 09 6 protocols, 7 hardware and software, bit rates,

LANs and WANs

What is a network?Collection of two or more computers connected together to share resources

Network Types

LANWANLocal Area NetworkWide Area NetworkOver a small geographical areaComputers are remoteWithin a building (25 km)e.g. a schoolEach computer has its own peripherals typicallyCommunication via third party telephone lines or satellites

Characteristics of LANs and WANsLANsWANsDistanceOver short distancesOver geographically remote distancesCommunicationHardwiredShort wireless rangeThird part communication linksSecurityMore secureSubject to attackHardwareNo extra communication device neededDevices sharedRequires a modemEach PC has own device

Software & Hardware for a LANWhat are they and how are they used?

Network Operating SystemDrivers for HardwareNetwork Interface CardsCabling or Wireless Access PointsServer, Hub or Switch

Hardware for a WANRouterActs as gateway to WANCommunication link (fibre optic, broadband connection, telephone line etc.)Provided by 3rd party, rented from providerFirewallStops hacking, communications restricted to authorised usersProxy serverAllows all computers to access WAN as though it were one computerWhy not Anti-virus software?

RoutersAllows links between networks gateway to WANsRouters Explained

Firewalls

Monitors where data has come from and is going to determines if its allowedChecks a list of pre-defined rulesMore advanced firewalls analyses each packet of data

Proxies Knows your external IP (e.g. your location)Knows cookies you have on PCKnows browser and plugins you useYour PCServerServerYour PCProxy serverCan block cookiesIP isnt disclosed (proxy connects to server)Can block malicious websitesCan also save bandwidth

Data TransmissionSerial, parallelSimplex, half-duplex and duplex

Specification:

Serial and Parallel Transmission

How the data is sent (transmitted)

Serial: Individual data bits sent One bit at a timeOften along a single wireDown a single wireAs a single streamParallel: multiple/all bits at oncesimultaneouslyDown more than one wire Usually through 8 wires bitsAs many streamsTransmission

Applications of Parallel Transmission Sending data from hard drive/games console to processorVideo streamingUse of graphics card

Where speed is criticalData volume is greatData is time sensitive

Simplex CommunicationData can only be transferred in one directionDoes not suffer collisionsPoor for mutual communicationsE.g. Cable television Pager

Half-Duplex CommunicationData can be transferred in both directionsBut not at the same timeI.e. Data can only be sent from one device at a timeMost likely to cause collisionsData collides and is corruptedSimilar to phones being busyE.g. walkie talkie

Full-Duplex Communication Data can be transferred in both directions simultaneouslyVirtually no collisions take placeE.g. smart phones, video calling etc.

Data Transmission: Bit/Baud rateMore wires connecting two devices more data that can be transferred in a period of time

Rate at which data is transferred (not speed)

How many bits are transferred within a certain time e.g. Per second

Measured in baud

Bit rate and Time SensitivityDifferent bit rates are suitable for different types of applicationsInfluenced by time sensitivity of informationFile use not type

Compare the following...

Streaming video

Saving a word processed document

Bit Rate and Time SensitivityWord processed documents tend to have less data to transmitSmaller than video filesWord processed documents not needed immediatelyTherefore bit rate unimportant time for data to download even at a slow rateVideo file larger used immediatelyTherefore data transmitted is time sensitiveRequires a high bit rate otherwise data would be useless

Data TransmissionTransmission errorsParity, checksum and echo checks

Specification:

Purpose of ChecksEnsures data retains its integrityChecks data transmitted is same as when it is receivedNo corruptionCompleted through various ways:

ParityCheck sumEchoing

Parity ChecksData is sent in fixed byte sizes (normally 8 bits)

Data contained in first 7 bits

8th bit is parity bit 8th bit used to make number of 1s sent either odd or evenData is either odd or even parity (determined by number of 1s present including 8th bit)

Even number of 1s Parity bit: 0Odd number of 1s Parity bit: 1System set to expect either all bytes to have odd or even number of 1sIf an error request sent for re-transmission

Parity Check ExampleWhich byte is corrupted?Why?

Parity Check Examplea) 01101101 = 5 (odd) bitsb) 10111010 = 5 (odd) bitsc) 10101010 = 4 (even) bitsd) 10100010 = 3 (odd) bits

c) must be corrupted only byte with even parity

Parity Check QuestionOne error checking method is to use parity. One of the following bytes has been corrupted in transmission. 00010010010111011010100111000011State which the corrupt byte is and explain why it is corrupt (3)

Parity Check QuestionOne error checking method is to use parity. One of the following bytes has been corrupted in transmission. 00010010010111011010100111000011State which the corrupt byte is and explain why it is corrupt (3)01011101Has an odd number of onesAll the other bytes have an even number an even parity

Check Sum ChecksData consists of binary numbersThese numbers are added together to create a value checksumIf there is any carryover (overflow) this is ignoredChecksum value sent with dataIf sent value matches destination value no error

Check Sum Check Example

Check Sum Check Example 0110110110000001+100010000111011 0(1)1

Bytes are added togetherand carry over (overflow) ignored

Sent with dataSame sum done at receivers end results compared

Echo ChecksBlock of data sent from device A to device BData at device B is sent back to device AThis data is then comparedIf data matches no errorIf different original block of data sent again

QuestionsJune 14 - 1 LAN and 7 Packet switching, errorsJune 13 3 errors and 6 transmissionJan 13 6 circuit and packet switching, 7 protocolJune 12 6 WAN, hardware, transmission, rules, bit ratesJan 12 4 LAN, handshake, protocol, 7c errorsJune 11 2 transmission, 8 rules/protocols, 9 packet switchingJan 11 6 LAN, echoes, packet and circuit switching and 9 protocols, bit ratesJune 10 1c LAN, hardware, errors, 6 protocols, linksJan 10 2 packet and circuit switching, 7 bit rate, data transmission, handshakeJune 09 2b transmission, packet switching, 8 LAN WAN, errorsJan 09 6 protocols, 7 hardware and software, bit rates,