11

Transmission of Digital Transmission of Digital Data :Data :

Interface and ModemsInterface and Modems

22

Transmission of Digital Data Transmission of Digital Data ::

Interface and ModemsInterface and Modems BackgroundBackground Digital Data TransmissionDigital Data Transmission

– ParallelParallel– SerialSerial

DTE-DCE InterfaceDTE-DCE Interface ModemsModems

33

BackgroundBackground

Encoded Information

Data EncodingInformation

Transmission Process

Signal

+

Communication Link

44

Digital Data TransmissionDigital Data Transmission

Primary concern in data transmissionPrimary concern in data transmission– WiringWiring– Data streamData stream

Send one bit Send one bit Send a group of bitsSend a group of bits

Two modes of sending bit streamsTwo modes of sending bit streams– Parallel modeParallel mode– Serial modeSerial mode

55

Digital Data TransmissionDigital Data Transmission

Data Transmission

Parallel Serial

Synchronous Asynchronous

66

Parallel TransmissionParallel Transmission

Groups of Groups of n n bits bits Send one group at one timeSend one group at one time Use Use n n wires to send wires to send n n bitsbits AdvantageAdvantage

– SpeedSpeed DisadvantageDisadvantage

– Cost (Cost (nn wires to transmit wires to transmit n n bits)bits)– Limited to short distancesLimited to short distances

77

Parallel TransmissionParallel Transmission

88

Serial TransmissionSerial Transmission

One bit at one timeOne bit at one time Requires only one wireRequires only one wire Requires conversion devices at the Requires conversion devices at the

interface between interface between – The sender and the line (parallel to serial)The sender and the line (parallel to serial)– The line and the receiver (serial to The line and the receiver (serial to

parallel)parallel) AdvantageAdvantage

– Low costLow cost

99

Serial TransmissionSerial Transmission

1010

Serial Transmission Serial Transmission (Asynchronous)(Asynchronous)

AsynchronousAsynchronous– The timing of signal is unimportantThe timing of signal is unimportant– Information is retrieved and translated Information is retrieved and translated

upon patternupon pattern– Patterns : grouping bit streams into bytesPatterns : grouping bit streams into bytes

Each group (usually 8) is sent as a unitEach group (usually 8) is sent as a unit– The sendingThe sending device sends each group device sends each group

without regard to a timerwithout regard to a timer

1111

Serial Transmission Serial Transmission (Asynchronous)(Asynchronous)

1212

Serial Transmission Serial Transmission (Asynchronous) (cont.)(Asynchronous) (cont.)

MethodMethod– To alert the receiver :To alert the receiver :

An extra bit (An extra bit (start bit -- 0start bit -- 0) is added to the ) is added to the beginning of each bytebeginning of each byte

One or more extra bits are added to the end of One or more extra bits are added to the end of the bytethe byte

– Stop bitsStop bits -- 1 -- 1 A gap (idle channel/streams of stop bits) is A gap (idle channel/streams of stop bits) is

added at the end of each byteadded at the end of each byte Thus, start bit + stop bit + gap Thus, start bit + stop bit + gap alert the alert the

receiver the begin and end of each byte receiver the begin and end of each byte

1313

Serial Transmission Serial Transmission (Asynchronous) (cont.)(Asynchronous) (cont.)

AdvantageAdvantage– CheapCheap– EffectiveEffective

Example:Example:– Low speed communication Low speed communication

Terminal Terminal computer computer

1414

Serial Transmission Serial Transmission (Synchronous)(Synchronous)

1515

Serial Transmission Serial Transmission (Synchronous)(Synchronous)

Synchronous TransmissionSynchronous Transmission– Bit streams is combined into longer “frame”Bit streams is combined into longer “frame”– A frame may consist of multiple bytesA frame may consist of multiple bytes– No gap between each byte is added into a No gap between each byte is added into a

transmission link transmission link – The receiver has to separate the bit stream into The receiver has to separate the bit stream into

bytes for decoding purposebytes for decoding purpose– Timing is important in synchronous transmissionTiming is important in synchronous transmission– Byte synchronization is performed at the data link Byte synchronization is performed at the data link

layerlayer AdvantageAdvantage

– SpeedSpeed useful for high speed applicationsuseful for high speed applications

1616

DTE-DCE InterfaceDTE-DCE Interface

DTE : Data Terminal EquipmentDTE : Data Terminal Equipment DCE : Data Circuit-terminating EquipmentDCE : Data Circuit-terminating Equipment

DCEDTE DTEDCE

network

1717

DTE-DCE InterfaceDTE-DCE Interface

Sending EndSending End The DTE The DTE

– generates the data and passes them to a generates the data and passes them to a DCEDCE

The DCE The DCE – converts the signal to a format appropriate converts the signal to a format appropriate

to a transmission mediumto a transmission medium– Sends it onto the networkSends it onto the network

Receiving End Receiving End – This process is reversedThis process is reversed

1818

DTEDTE

Includes any unit that functions as a Includes any unit that functions as a source or a destination for binary datasource or a destination for binary data

At the physical layer, it can beAt the physical layer, it can be– A terminalA terminal– A computerA computer– A printerA printer– A fax machine, etc.A fax machine, etc.

DTEs do not communicate with DTEs do not communicate with other other DTE directlyDTE directly

1919

DCEDCE

IncludesIncludes any functional units that any functional units that transmits or receives analog/digital transmits or receives analog/digital signal through a networksignal through a network

At the physical layer, ex. ModemsAt the physical layer, ex. Modems Sending and receiving DCEs must use Sending and receiving DCEs must use

the modulating method (e.g. FSK) the modulating method (e.g. FSK)

2020

DTE-DCE Interface DTE-DCE Interface StandardsStandards

DCEDTE DTEDCE

network

DTE-DCE standards try to define the mechanical, electrical, and functional characteristics of the connection between the DTE and the DCE

2121

EIA-232 InterfaceEIA-232 Interface

Previously called RS-232Previously called RS-232 Defines the mechanical, electrical, and Defines the mechanical, electrical, and

functional characteristics of the functional characteristics of the interface between a DTE and a DCEinterface between a DTE and a DCE

2222

EIA-232EIA-232

Mechanical SpecificationMechanical Specification– InterfaceInterface

25-wire cable25-wire cable Male and female DB-25 pin connector Male and female DB-25 pin connector

attached to either endattached to either end The cable length not > 15 meters (50 feet)The cable length not > 15 meters (50 feet)

Electrical SpecificationElectrical Specification– Defines the voltage levels and the type of Defines the voltage levels and the type of

signal to be transmitted between DTE & DCEsignal to be transmitted between DTE & DCE

2323

EIA-232EIA-232

Sending the DataSending the Data

2424

EIA-232EIA-232

Control and TimingControl and Timing– 4 wires are used for data functions4 wires are used for data functions– 21 wires are reserved for functions like, 21 wires are reserved for functions like,

ControlControl TimingTiming GroundingGrounding TestingTesting etc.etc.

2525

EIA-232EIA-232

Control and TimingA function is considered ON if it transmits a voltage of at least +3 and OFF if it transmits a voltage with a value < -3 volts

2626

EIA-232EIA-232

Functional SpecificationFunctional Specification– DB-25DB-25– DB-9DB-9

2727

EIA-232 : DB-25 (male)EIA-232 : DB-25 (male)

2828

EIA-232 : DB-25 EIA-232 : DB-25

2929

EIA-232 : DB-25 (Control EIA-232 : DB-25 (Control Pins)Pins)

3030

EIA-232 : DB-25 (Timing EIA-232 : DB-25 (Timing Pins)Pins)

3131

EIA-232 : DB-25 (Other Pins)EIA-232 : DB-25 (Other Pins)

3232

EIA-232 : DB-9EIA-232 : DB-9

Many of the pins in DB-25 Many of the pins in DB-25 implementation are not necessaryimplementation are not necessary

A 9-pin version of EIA-232 (called DB-A 9-pin version of EIA-232 (called DB-9) was developed9) was developed

3333

EIA-232 Functioning EIA-232 Functioning ExampleExample

Synchronous

Full-Duplex

Transmission

3434

EIA-232 Functioning EIA-232 Functioning ExampleExample

3535

Null ModemNull Modem

Modems are not needed to connect two Modems are not needed to connect two computers in the same roomcomputers in the same room

Need : an interface to handle the exchangeNeed : an interface to handle the exchange– Readiness establishmentReadiness establishment– Data transferData transfer– Data receiptData receipt– etc.etc.

A standard provided by the EIA to perform A standard provided by the EIA to perform these tasks, called these tasks, called NULL MODEMNULL MODEM

3636

Null ModemNull Modem

3737

Crossing ConnectionsCrossing Connections

3838

Other Interface StandardsOther Interface Standards

IEA-449IEA-449– DB-37/DB-9DB-37/DB-9

RS-423/422RS-423/422 EIA-530EIA-530 Etc.Etc.

3939

ModemsModems

Modulator/demodulatorModulator/demodulator ModulatorModulator

– Converts a digital signal into an analog Converts a digital signal into an analog signal using ASK, FSK, PSK or QAMsignal using ASK, FSK, PSK or QAM

DemodulatorDemodulator– Converts an analog signal into a digital Converts an analog signal into a digital

signalsignal

4040

ModemsModems