Physical Layer OSI

8/13/2019 Physical Layer OSI

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Chapter 6Chapter 6 -- Physical LayerPhysical Layer

It is about the characteristics of Physical Layer, theIt is about the characteristics of Physical Layer, thelowest layer in the ISO/OSI reference model.lowest layer in the ISO/OSI reference model.

Most of the network problems such as noiseMost of the network problems such as noiseinterference, incompatible cable connection and lineinterference, incompatible cable connection and line

disconnection come from this layer.disconnection come from this layer.

It is about the characteristics of Physical Layer, theIt is about the characteristics of Physical Layer, thelowest layer in the ISO/OSI reference model.lowest layer in the ISO/OSI reference model.

Most of the network problems such as noiseMost of the network problems such as noiseinterference, incompatible cable connection and lineinterference, incompatible cable connection and line

disconnection come from this layer.disconnection come from this layer.


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ObjectiveObjective

Understand various definition ofUnderstand various definition of

standards related to this layerstandards related to this layer

Identify the characteristics and role ofIdentify the characteristics and role ofphysical layerphysical layer

Understand the PC communications usingUnderstand the PC communications using8250 UART communication chip8250 UART communication chip


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Physical LayerPhysical Layer The lowest layer in ISO/OSI reference model and isThe lowest layer in ISO/OSI reference model and is

about the transmission of data from one point toabout the transmission of data from one point toanother over the communications mediaanother over the communications media

This layer is dealt with the physical dataThis layer is dealt with the physical datatransmission and is related to:transmission and is related to:

Type of transmission medium (Copper wire, air, opticalType of transmission medium (Copper wire, air, opticalfiber etc.)fiber etc.)

The transmission media such as telephone cable areThe transmission media such as telephone cable aresometimes regarded as Layer 0.sometimes regarded as Layer 0.

Modulation and demodulation scheme (FSK, PSK, digitalModulation and demodulation scheme (FSK, PSK, digitaldata).data).

Message switching techniquesMessage switching techniques


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Four SpecificationFour Specification

MechanicalMechanical specification such as socket layout,specification such as socket layout,pin size etc.pin size etc.

ElectricalElectrical specification such as the electronicspecification such as the electronicsignals for certain pins.signals for certain pins.

FunctionalFunctional specification such as timing or clockingspecification such as timing or clocking

signalssignals

ProceduralProcedural specification such as the sequence ofspecification such as the sequence ofevents required to effect the data transfer across aevents required to effect the data transfer across apair of modems.pair of modems.


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EIAEIA--232232--DD

The EIA standard RSThe EIA standard RS--232C (RS refers to232C (RS refers to

Recommended Standard) was developed inRecommended Standard) was developed in1969 is still widely used.1969 is still widely used.

The latest version is called RSThe latest version is called RS--232D.232D.

Electrical, ITUElectrical, ITU--T V.28T V.28 Functional, ITUFunctional, ITU--T V.28T V.28

Mechanical, ISO 2110Mechanical, ISO 2110

Procedural, CCITT V.24Procedural, CCITT V.24


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CharacteristicsCharacteristics

This is a 25This is a 25--pin connector with data, control, timing andpin connector with data, control, timing andground signal pins.ground signal pins.

Not all of them are used for data transmissionNot all of them are used for data transmission


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DTEDTE--DTE (Computer to Computer port)DTE (Computer to Computer port)

When terminals are connected directly to a computer port, itWhen terminals are connected directly to a computer port, itis necessary to cross over some of the wires in order tois necessary to cross over some of the wires in order to

communicate properly.communicate properly.

When two computer ports are connected running atWhen two computer ports are connected running atsynchronous protocol, either one should provide the correctsynchronous protocol, either one should provide the correcttiming or a null modem with proper timing signals is required.timing or a null modem with proper timing signals is required.


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PC to PCPC to PC

Three Pins, Transmit, Receive and SignalThree Pins, Transmit, Receive and Signal

ReturnReturn

Transmit

Receive

Signal Ground

NullModemTransmit


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DTEDTE--DCE (Synchronous)DCE (Synchronous) This is a normal configuration of the V24This is a normal configuration of the V24interface between the computer andinterface between the computer andmodem.modem.

It therefore uses the straight throughIt therefore uses the straight throughcable. That is to say there is no cablecable. That is to say there is no cablecross.cross.


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Serial communicationsSerial communications

It takes place by transmitting and receivingIt takes place by transmitting and receivingdata in a stream of consecutive electricaldata in a stream of consecutive electricalpulses that represents bits.pulses that represents bits.

The EIA (Electronic Industries Association)The EIA (Electronic Industries Association)has recommended several standards forhas recommended several standards forserial communication such as:serial communication such as:

RSRS--232232--D (Similar to ITUD (Similar to ITU--T V.24)T V.24)

RSRS--422422

RSRS--423423

RSRS--449449


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Wiring DiagramWiring Diagram Apart from the conventional wiringApart from the conventional wiring

diagrams between computer & modemdiagrams between computer & modemand computer & computer.and computer & computer.


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Serial communication controllerSerial communication controller The fundamental element of the serial port is an ICThe fundamental element of the serial port is an IC

called 8250, universal asynchronous receiver andcalled 8250, universal asynchronous receiver andtransmitter (UART).transmitter (UART).


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Modem CommandsModem Commands The modem supports the standard and extended Hayes* ATThe modem supports the standard and extended Hayes* AT

command set.command set.

The AT prefix (also known as the Attention Code), signals theThe AT prefix (also known as the Attention Code), signals themodem that one or more commands are to follow.modem that one or more commands are to follow.

These commands are industry standard language used toThese commands are industry standard language used tocommunicate with the modem.communicate with the modem.

The modem is always either in the command mode, or theThe modem is always either in the command mode, or theonon--line mode.line mode.

Commands are only accepted by the modem when it is inCommands are only accepted by the modem when it is incommand mode.command mode.

Commands may be entered from the terminal mode of mostCommands may be entered from the terminal mode of mostcommunications software packages.communications software packages.


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Basic CommandsBasic Commands

ATAT, This prefix begins all but two commands you issue to the, This prefix begins all but two commands you issue to themodem locally,, and tells the modem "ATtention! commandsmodem locally,, and tells the modem "ATtention! commandsto follow".to follow".

DD, Dial. Use the D command to dial a telephone number from, Dial. Use the D command to dial a telephone number fromthe command line. The format of the command is ATD andthe command line. The format of the command is ATD and

the string parameter can contain up to 45 charactersthe string parameter can contain up to 45 characters ++++++, Escape Character Sequence. After the users have, Escape Character Sequence. After the users have

connected to another modemconnected to another modem

A/,A/, Repeat. This command does not use the AT prefix norRepeat. This command does not use the AT prefix nordoes it require a carriage return to enter. Typing thisdoes it require a carriage return to enter. Typing this

command causes the modem to repeat the last command linecommand causes the modem to repeat the last command lineenteredentered

ZnZn, Reset Modem. This command (in which n=0 or 1) resets, Reset Modem. This command (in which n=0 or 1) resetsthe modem to the configuration profile stored in nonthe modem to the configuration profile stored in non--volatilevolatilememory location 0 or 1.memory location 0 or 1.


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An example: ATD 27888639An example: ATD 27888639

Below shows the connection to CityU's Link Plus to access theBelow shows the connection to CityU's Link Plus to access theInternet service.Internet service.

The AT commands include ATZ, AT10, ATTD2xxxxxx etc.The AT commands include ATZ, AT10, ATTD2xxxxxx etc.


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X.20 and X.21 RecommendationX.20 and X.21 Recommendation

For digital networks such as X.25 packet switching network,For digital networks such as X.25 packet switching network,

X.20 and X.21 are another interface standards.X.20 and X.21 are another interface standards.

X.21 is a 15X.21 is a 15--pin connector. However, only eight lines aspin connector. However, only eight lines asshown below are required.shown below are required.


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EIA RSEIA RS--449/RS449/RS--422/RS422/RS--423423

RS449 corresponding to CCITT (ITURS449 corresponding to CCITT (ITU--T)T)V.35 is a 37V.35 is a 37--pin connector and supportspin connector and supportstransmission rates up to 2Mbps withtransmission rates up to 2Mbps withcable length to 60 meters.cable length to 60 meters.

It defines both mechanical (pin and plugIt defines both mechanical (pin and plugconfiguration) and procedural (signalconfiguration) and procedural (signaldescriptions) aspects similar to RS232.descriptions) aspects similar to RS232.

The electrical specifications (balanced orThe electrical specifications (balanced orunbalanced circuit) are defined by RS422unbalanced circuit) are defined by RS422and RS423 respectively.and RS423 respectively.


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Current LoopCurrent Loop

AnAn obsoleteobsolete technology to extend thetechnology to extend thephysical distance up to 300 meters.physical distance up to 300 meters.

It uses the current signal, presence ofIt uses the current signal, presence of

current signal or absence (no current), tocurrent signal or absence (no current), torepresent binary 0 or 1 (mark or space)represent binary 0 or 1 (mark or space)instead of voltage level in RS232D.instead of voltage level in RS232D.

The most common current values are 20 mAThe most common current values are 20 mA

and 60 mA.and 60 mA.

This transmission method could support halfThis transmission method could support halfor full duplex, which requires 2 or 4 wireor full duplex, which requires 2 or 4 wire

connectionconnection

Physical Layer OSI

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