1

6 장 Transmission of Digital Data:Interfaces and MODEMs

6.1 Digital Data Transmission

6.2 DTE-DCE Interface

6.3 Other Interface Standards

6.4 모뎀 (MODEM)

6.5 요약

2

6.1 Digital Data Transmission

~ of primary concern when considering the transmission of data from one device to another is the wiring

3

Digital Data Transmission(cont’d)

parallel Transmission~ By grouping, we can send data n bits at a time inst

ead of one

장점 : 속도 (speed) 단점 : 고가 (expansive)

전송거리 (short distance): 최대 25 feets

4

Digital Data Transmission(cont’d)

Parallel transmission

5

Digital Data Transmission(cont’d)

Serial Transmission~ one bit follows another

장점 : 단일 channel, 저가 단점 : require interface converter

6

Digital Data Transmission(cont’d)

Serial Transmission

7

Digital Data Transmission(cont’d)

비동기 전송 (Asynchronous Transmission)~ send one start bit(0) at the beginning and one or

more stop bits(1) at the end of each byte

8

Digital Data Transmission

동기전송 (synchronous transmission)~ send bits one after another without start/stop bits

or gaps. It is the responsibility of the receiver to group the bits

장점 : 속도 (speed)

9

6.2 DTE-DCE Interface

DTE(Data Terminal equipment) : 데 이 터 단말장치

DCE(Data Circuit-terminating Equipment) : 데이터 처리 종단 장치

10

DTE-DCE Interface(cont’d)

DTE(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

11

DTE-DCE Interface(cont’d)

DCE(Data Circuit-Terminating Equipment) ~ is any device that transmits or receives data in the

form of an analog or digital signal through a network

Modulator/demodulator

12

DTE-DCE Interface(cont’d)

표준안 (Standards)~ to define the connection between a DTE and DCE

13

DTE-DCE Interface(cont’d)

Standards EIA

EIA-232 EIA-442 EIA-449

ITU-T V series ( 전화망에 관한 데이터 통신 ) X series ( 데이터 통신망에 대한 데이터통신 )

14

DTE-DCE Interface(cont’d)

EIA-232(RS-232)~ defines the mechanical, electrical and functional

characteristics

기계적인 면 (mechanical)~ define the interface as a 25-wire cable with a male and a

female DB-25 pin connector

~ length may not exceed 15m(50 feet)

전자적인 면 (electrical specification)~ define voltage levels and the type of signal( 예 : NRZ-L)

15

DTE-DCE Interface(cont’d)

sending the data~ To be recognized as data, the amplitude of a signal

must fall between 3 and 15 volts or between –3 and –15 volts

16

DTE-DCE Interface(cont’d)

control and timing~ Signals must be sent using OFF less than –3 volts

and ON greater than +3 volts

17

DTE-DCE Interface(cont’d)

기능 적인 면 (Functional Specification)

18

DTE-DCE Interface(cont’d)

예 : synchronous full-duplex transmission

19

DTE-DCE Interface(cont’d)

20

DTE-DCE Interface(cont’d)

Null Modem ~ not needed to connect two compatible digital devic

es directlyUsing regular data pin connections with and without

DCEs

21

DTE-DCE Interface(cont’d)

Crossing ConnectionsNull modem pin connection

22

6.3 Other Interface Standards

~ needs more speed and/or distance

EIA-449

EIA-530

X.21

23

Other Interface Standards(cont’d)EIA-449

DB-37 pin DB-9 pin

DB-37 and DB-9 connections

24

Other Interface Standards(cont’d)

Pin Functions

category I pins~ are compatible with those of EIA-232

category II pins~ have no equivalent in EIA-232

25

Other Interface Standards(cont’d)

DB-37 pinsPin Functions Category Pin Functions Category

123456789

10111213141516171819

ShieldSignal rate indicatorUnassignedSend dataSend timingReceive dataRequest to sendReceive timingClear to sendLocal loopbackData modeTerminal readyReceive readyRemote loopbackIncoming callSelect frequencyTerminal timingTest modeSignal ground

202122232425262728293031323334353637

Receive CommonUnassignedSend dataSend timingReceive dataRequest to sendReceive timingClear to sendTerminal in serviceData modeTerminal readyReceive readySelect readySignal qualityNew signalTerminal timingStandby indicatorSend common

IIIIIIIIIIIII

IIIII

IIIIIIIIIIIIIIII

IIIIIII

26

Other Interface Standards(cont’d)

DB-9 pins

Pin Function EIA-232 Equivalent

123456789

ShieldSecondary receive readySecondary send dataSecondary receive dataSignal groundReceive commonSecondary request to sendSecondary clear to sendSend common

1

14167

121913

27

Other Interface Standards(cont’d)

Electrical specification : RS-423 and RS-422~ EIA-449 uses two other standards to define its

electrical specification

RS-423 (unbalanced circuits)

RS-422 (balanced circuits)

28

Other Interface Standards(cont’d)

RS-423 : Unbalanced Mode

29

Other Interface Standards(cont’d)

RS-422 : Balanced Mode

30

Other Interface Standards(cont’d)

Canceling of noise using balanced mode

31

Other Interface Standards(cont’d)

EIA-530~ is a new version of EIA-449 that uses DB-25 pins

32

Other Interface Standards(cont’d)

X.21~ is an interface designed by the ITU-T

DB-15 connector

33

Other Interface Standards

DB-15 pins

Pin Function

12345678

Pin

9101112131415

Function

ShieldTransmit data or controlControlReceive data or controlIndicationSignal element timingBye timingSignal ground

Transmit data or controlControlReceive data or controlIndicationSignal element timingByte timingReserved

34

6.4 MODEMs

~ the most familiar type of DCE

35

MODEMs(cont’d)

MODEM~ stands for modulator/demodulator

Modulator : converts a digital signal to an analog signal

Demodulator : converts an analog signal to a digital signal

36

MODEMs(cont’d)

Transmission Bandwidth

~ 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

37

MODEMs(cont’d)Telephone line bandwidth

~ A telephone line has bandwidth of almost 3000 Hz

38

MODEMs(cont’d)

Modem speed ASK(Amplitude Shift Keying)

Baud rate in half-duplex ASK

39

MODEMs(cont’d)Baud rate in full-duplex ASK

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

of noise

40

MODEMs(cont’d)

FSK(Frequency Shift Keying)

Baud rate in half-duplex FSK

41

MODEMs(cont’d)PSK and QAM( 비교 )

Encoding Half-Duplex Full-Duplex

ASK, FSK, 2-PSK4-PSK, 4-QAM8-PSK, 8-QAM16-QAM32-QAM64-QAM128-QAM

256-QAM

2400480072009600

12,00014,40016,800

1200240036004800600072008400

19,200 9600

42

MODEMs(cont’d)

Modem Standards Bell modem ITU-T modem

43

MODEMs(cont’d)

Bell Modems(cont’d)

44

MODEMs(cont’d)

Bell Modems

45

MODEMs(cont’d)

ITU-T Modem Standards(cont’d)

46

MODEMs(cont’d)

ITU-T Modem Standards

47

MODEMs(cont’d)

ITU-T/Bell Compatibility

ITU-T Bell Baud Rate Bit Rate Modulation

V.21

V.22

V.23

V.26

V.27

V.29

103

212

202

201

208

209

300

600

1200

1200

1600

2400

300

1200

1200

2400

4800

9600

FSK

4-PSK

FSK

4-PSK

8-PSK

16-QAM

48

MODEMs(cont’d)

Intelligent Modems~ contain software to support a number of additional

function, such as automatic answering and dialing

AT Command[parameter] command[parameter]… Ex) ATDT4088648902

49

MODEMs(cont’d)AT commands

Command Meaning Parameters

ABDEHLPT

Put modem in answer modeUse V.22bis at 1200 bpsDial the numberEnable/disable echo printingPut modem on/off hookAdjust speaker volumeUse pulse dialingUse tone dialing

The number to dial0 or 10 or 1n