chapter6(interfaces and modem)

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Kyung Hee

University

Transmission of Digital DATA :Transmission of Digital DATA :Interfaces and ModemsInterfaces and Modems



Kyung Hee

University

Transmission of Digital Data:Transmission of Digital Data:Interfaces and MODEMsInterfaces and MODEMs

6.1 Digital Data Transmission

6.2 DTE-DCE Interface

6.3 Other Interface Standards

6.4 MODEM



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6.1 Digital Data Transmission6.1 Digital Data Transmission

t Primary concern when considering the transmission of data

from one device to another is the wiring



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Digital Data Transmission(cont’d)Digital Data Transmission(cont’d)

t Parallel Transmission

By grouping, we can send data n bits at a time instead of one

speed

expansive



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tParallel transmission



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tSerial Transmission

one bit follows another

channel,

require interface converter



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t Serial Transmission



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tAsynchronous Transmission

send one start bit(0) at the beginning and one or more stop

bits(1) at the end of each byteeg) the connection of a terminal to a computer

cheap and effective



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Asynchronous here means “asynchronous at the byte level,”

but the bits are still synchronized;

their durations are the same.



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Digital Data TransmissionDigital Data Transmission

t synchronous transmission

Sending bits one after another without start/stop bits or gaps. Itis the responsibility of the receiver to group the bits

The receiver counts the bits as they arrive and groups them in

eight-bit units

t 장점 : 속도 (speed)



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6.2 DTE-DCE Interface6.2 DTE-DCE Interface

tDTE(Data Terminal equipment)

tDCE(Data Circuit-terminating Equipment)



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DTE-DCE Interface(cont’d)DTE-DCE Interface(cont’d)

tDTE(Data Terminal Equipment)

~ is any device that is a source of destination for binary digital

data

terminal

microcomputer

computer

printer

fax machine and so on



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tDCE (Data Circuit-Terminating Equipment)

~ is any device (functional unit) that transmits or

receives data in the form of an analog or digital

signal through a network

q Modulator/demodulator



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t Standards

defining the connection between a DTE and DCE



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tStandards

EIA (Electronic Industries Association)q EIA-232

q EIA-442

q EIA-449

ITU-T (International Telecommunication Union-Telecommunication Sector)

q V series

q X series



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t EIA-232(RS-232)

defining the mechanical, electrical and functionalcharacteristics

tmechanical

defining the interface as a 25-wire cable with a male (plug) and

a female (receptacle) DB-25 pin connector length may not exceed 15m(50 feet)

t electrical specification

define voltage levels and the type of signal(NRZ-L)



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t sending the data

To be recognized as data, the amplitude of a signal must fallbetween 3 and 15 volts or between –3 and –15 volts



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t control and timing

Only 4 wires out of 25 available in an EIA-232 interface are usedfor data functions

Signals must be sent using OFF less than –3 volts and ON

greater than +3 volts



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tFunctional Specification



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t synchronous full-duplex transmission



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t Null Modem

not needed to connect two compatible digital devices directly

t Using regular data pin connections with and without DCEs



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t Crossing Connections

tNull modem pin connection

6 3 O h I f S d d



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6.3 Other Interface Standards6.3 Other Interface Standards

tneeds more speed and/or distance

EIA-449

EIA-530

X.21

O h I f S d d ( ’d)Oth I t f St d d ( t’d)



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Other Interface Standards(cont’d)Other Interface Standards(cont’d)

t EIA-449

DB-37 pin

DB-9 pin

t DB-37 and DB-9 connections

Oth I t f St d d ( t’d)Oth I t f St d d ( t’d)



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tPin Functions

category I pins

~ are compatible with those of EIA-232

category II pins

~ have no equivalent in EIA-232

Oth I t f St d d ( t’d)Oth I t f St d d ( t’d)



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t DB-37 pins

Pin Functions Category Pin Functions Category1234567

8910111213141516171819

ShieldSignal rate indicator UnassignedSend dataSend timingReceive dataRequest to send

Receive timingClear to sendLocal loopback Data modeTerminal readyReceive readyRemote loopback Incoming callSelect frequency

Terminal timingTest modeSignal ground

20212223242526

2728293031323334353637

Receive CommonUnassignedSend dataSend timingReceive dataRequest to sendReceive timing

Clear to sendTerminal in serviceData modeTerminal readyReceive readySelect readySignal quality New signalTerminal timing

Standby indicator Send common

I

I

I

I

I

I

II

I

I

I

II

II

I

II

II

I

I

I

I

I

I

I

II

I

I

I

II

II

I

II

II

Oth I t f St d d ( t’d)Other Interface Standards(cont’d)



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t DB-9 pins

Pin Function EIA-232 Equivalent

1

2

3

4

5

6

7

89

Shield

Secondary receive ready

Secondary send data

Secondary receive data

Signal ground

Receive common

Secondary request to send

Secondary clear to sendSend common

1

14

16

7

12

19

13




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tElectrical specification : RS-423 and RS-422

EIA-449 uses two other standards to define its electricalspecification

RS-423 (unbalanced circuits)

RS-422 (balanced circuits)




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Other Interface Standards(cont’d)Other Interface Standards(cont d)

t RS-423 : Unbalanced Mode




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Other Interface Standards(cont d)Other Interface Standards(cont d)

t RS-422 : Balanced Mode




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tCanceling of noise using balanced mode




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tEIA-530

is a new version of EIA-449 that uses DB-25 pins




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tX.21

~ is an interface designed by the ITU-T to address many of the problemsexisting in the EIA

eliminating most of the control circuits of the EIA standards

DB-15 connector

Byte timing (pins 7 and 14) : byte synchronization

Control and indication

– Pin 3 : equivalent of request to send

– Pin 5 : equivalent of clear to send

Other Interface StandardsOther Interface Standards



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Other Interface StandardsOther Interface Standards

t DB-15 pins

Pin Function

1

2

3

4

5

6

7

8

Pin

9

10

11

12

13

14

15

Function

Shield

Transmit data or control

Control

Receive data or control

Indication

Signal element timing

Bye timing

Signal ground

Transmit data or control

Control

Receive data or control

Indication

Signal element timing

Byte timing

Reserved

6 4 MODEMs6.4 MODEMs



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6.4 MODEMs6.4 MODEMs

t the most familiar type of DCE

MODEMs(cont’d)MODEMs(cont’d)



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MODEMs(cont d)MODEMs(cont d)

tMODEM

stands for modulator/demodulator

Modulator : converts a digital signal to an analog signal

q using the digital-to-analog mechanism of ASK, FSK,

PSK, and QAM

Demodulator : converts an analog signal to a digital

signal




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MODEMs(cont d)MODEMs(cont d)

tTransmission Rate

Bandwidthq Each line has an upper limit and a lower limit on the

frequencies of the signals it can carry. This limited range is

called the bandwidth




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MODEMs(cont d)( )

t Telephone line bandwidth

A telephone line has bandwidth of almost 3000 Hz

Used for dataUsed for voice




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( )( )

t Modem speed

The bandwidth required for ASK transmission is equal to baud rate of

signal

in half-duplex ASK, because the effective bandwidth of a telephone

line is 2400Hz, the maximum bit rate is also 2400.




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( )( )

t Baud rate in full-duplex ASK

Although ASK has a good bit rate, it is not used today because of noise




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( )( )

t FSK(Frequency Shift Keying)

Baud rate in half-duplex FSK

Baud rate in full-duplex FSK




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( )

t PSK and QAM

Encoding Half-Duplex Full-Duplex

ASK, FSK, 2-PSK

4-PSK, 4-QAM

8-PSK, 8-QAM16-QAM

32-QAM

64-QAM

128-QAM

256-QAM

2400

4800

72009600

12,000

14,400

16,800

1200

2400

36004800

6000

7200

8400

19,200 9600

Theoretical bit rates for modemsTheoretical bit rates for modems




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tModem Standards

Bell modem

q The first commercial modems in the early 1970s

ITU-T modem




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t Bell Modems(cont’d)

For flow control or error control




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t Bell Modems

212 --> 201 : Same technology, data rate double




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t ITU-T Modem Standards(cont’d)




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t ITU-T Modem Standards




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t ITU-T/Bell Compatibility

ITU-T Bell Baud Rate Bit Rate Modulation

V.21

V.22

V.23V.26

V.27

V.29

103

212

202201

208

209

300

600

12001200

1600

2400

300

1200

12002400

4800

9600

FSK

4-PSK

FSK 4-PSK

8-PSK

16-QAM




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t Intelligent Modems

~ contain software to support a number of additional function,such as automatic answering and dialing

AT Command[parameter] command[parameter]…

Ex) ATDT4088648902




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t AT commands

Command Meaning Parameters

A

B

D

E

H

L

P

T

Put modem in answer mode

Use V.22bis at 1200 bps

Dial the number

Enable/disable echo printing

Put modem on/off hook

Adjust speaker volume

Use pulse dialing

Use tone dialing

The number to dial

0 or 10 or 1

n

chapter6(interfaces and modem)

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