6장 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
1
6.1 Digital Data Transmission
~ of primary concern when considering the
transmission of data from one device to
another is the wiring
2
Digital Data Transmission(cont’d)
parallel Transmission
~ By grouping, we can send data n bits at a time
instead of one
장점 : 속도(speed)
단점 : 고가(expansive)
전송거리(short distance): 최대 25 feets
3
Digital Data Transmission(cont’d)
Parallel transmission
4
Digital Data Transmission(cont’d)
Serial Transmission
~ one bit follows another
장점 : 단일 channel, 저가
단점 : require interface converter
5
Digital Data Transmission(cont’d)
Serial Transmission
6
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
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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)
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6.2 DTE-DCE Interface
DTE(Data Terminal equipment) : 데이터 단
말장치
DCE(Data Circuit-terminating Equipment) :
데이터 처리 종단 장치
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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
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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
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DTE-DCE Interface(cont’d)
표준안(Standards)
~ to define the connection between a DTE and DCE
12
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)
13
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)
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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
15
DTE-DCE Interface(cont’d)
control and timing
~ Signals must be sent using OFF less than –3 volts
and ON greater than +3 volts
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DTE-DCE Interface(cont’d)
기능 적인 면 (Functional Specification)
17
DTE-DCE Interface(cont’d)
예 : synchronous full-duplex transmission
18
DTE-DCE Interface(cont’d)
19
DTE-DCE Interface(cont’d)
Null Modem
~ not needed to connect two compatible digital
devices directly
Using regular data pin connections with and without
DCEs
20
DTE-DCE Interface(cont’d)
Crossing Connections
Null modem pin connection
21
6.3 Other Interface Standards
~ needs more speed and/or distance
EIA-449
EIA-530
X.21
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Other Interface Standards(cont’d)
EIA-449
DB-37 pin
DB-9 pin
DB-37 and DB-9 connections
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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
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Other Interface Standards(cont’d)
DB-37 pins
Pin
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Functions
Shield
Signal rate indicator
Unassigned
Send data
Send timing
Receive data
Request to send
Receive timing
Clear to send
Local loopback
Data mode
Terminal ready
Receive ready
Remote loopback
Incoming call
Select frequency
Terminal timing
Test mode
Signal ground
Category
Pin
I
I
I
I
I
I
II
I
I
I
II
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
II
I
II
Functions
Receive Common
Unassigned
Send data
Send timing
Receive data
Request to send
Receive timing
Clear to send
Terminal in service
Data mode
Terminal ready
Receive ready
Select ready
Signal quality
New signal
Terminal timing
Standby indicator
Send common
Category
II
I
I
I
I
I
I
I
II
I
I
I
II
II
I
II
II
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Other Interface Standards(cont’d)
DB-9 pins
Pin
Function
EIA-232 Equivalent
1
2
3
4
5
6
7
8
9
Shield
Secondary receive ready
Secondary send data
Secondary receive data
Signal ground
Receive common
Secondary request to send
Secondary clear to send
Send common
1
14
16
7
12
19
13
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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)
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Other Interface Standards(cont’d)
RS-423 : Unbalanced Mode
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Other Interface Standards(cont’d)
RS-422 : Balanced Mode
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Other Interface Standards(cont’d)
Canceling of noise using balanced mode
30
Other Interface Standards(cont’d)
EIA-530
~ is a new version of EIA-449 that uses DB-25 pins
31
Other Interface Standards(cont’d)
X.21
~ is an interface designed by the ITU-T
DB-15 connector
32
Other Interface Standards
DB-15 pins
Pin
Function
Pin
Function
1
2
3
4
5
6
7
8
Shield
Transmit data or control
Control
Receive data or control
Indication
Signal element timing
Bye timing
Signal ground
9
10
11
12
13
14
15
Transmit data or control
Control
Receive data or control
Indication
Signal element timing
Byte timing
Reserved
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6.4 MODEMs
~ the most familiar type of DCE
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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
35
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
36
MODEMs(cont’d)
Telephone line bandwidth
~ A telephone line has bandwidth of almost 3000 Hz37
MODEMs(cont’d)
Modem speed
ASK(Amplitude Shift Keying)
Baud rate in half-duplex ASK
38
MODEMs(cont’d)
Baud rate in full-duplex ASK
~ Although ASK has a good bit rate, it is not used
today because of noise
39
MODEMs(cont’d)
FSK(Frequency Shift Keying)
Baud rate in half-duplex FSK
40
MODEMs(cont’d)
PSK and QAM(비교)
Encoding
Half-Duplex
Full-Duplex
ASK, FSK, 2-PSK
4-PSK, 4-QAM
8-PSK, 8-QAM
16-QAM
32-QAM
64-QAM
128-QAM
2400
4800
7200
9600
12,000
14,400
16,800
1200
2400
3600
4800
6000
7200
8400
256-QAM
19,200
9600
41
MODEMs(cont’d)
Modem Standards
Bell modem
ITU-T modem
42
MODEMs(cont’d)
Bell Modems(cont’d)
43
MODEMs(cont’d)
Bell Modems
44
MODEMs(cont’d)
ITU-T Modem Standards(cont’d)
45
MODEMs(cont’d)
ITU-T Modem Standards
46
MODEMs(cont’d)
ITU-T/Bell Compatibility
ITU-T
Bell
Baud Rate
Bit Rate
Modulation
V.21
103
300
300
FSK
V.22
212
600
1200
4-PSK
V.23
202
1200
1200
FSK
V.26
201
1200
2400
4-PSK
V.27
208
1600
4800
8-PSK
V.29
209
2400
9600
16-QAM
47
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
48
MODEMs(cont’d)
AT commands
Command
A
B
D
E
H
L
P
T
Meaning
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
Parameters
The number to dial
0 or 1
0 or 1
n
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