1. Transmission of Digital DATA : Interfaces and Modems
Prof. Choong Seon HONG
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2. 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)

3. 6.1 Digital Data Transmission
Primary concern when considering the transmission of data from one device to another is the wiring

4. 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

5. Digital Data Transmission(cont’d)
Parallel transmission

6. Digital Data Transmission(cont’d)
Serial Transmission
one bit follows another
장점 : 단일 channel, 저가
단점 : require interface converter

7. Digital Data Transmission(cont’d)
Serial Transmission

8. 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
ex) the connection of a terminal to a computer
장점 : cheap and effective

9. Digital Data Transmission
동기전송(synchronous transmission)
Sending bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits
The receiver counts the bits as they arrive and groups them in eight-bit units
장점 : 속도(speed)

10. 6.2 DTE-DCE Interface DTE(Data Terminal equipment) : 데이터 단말장치
DCE(Data Circuit-terminating Equipment) : 데이터 회선 종단 장치

11. DTE-DCE Interface(cont’d)
DTE(Data Terminal Equipment)
~ is any device that is a source of destination for binary digital data
terminal
Personal computer
printer
fax machine and so on

12. DTE-DCE Interface(cont’d)
DCE (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
Modulator/demodulator

13. DTE-DCE Interface(cont’d)
표준안(Standards)
defining the connection between a DTE and DCE

14. DTE-DCE Interface(cont’d)
Standards
EIA (Electronic Industries Association)
EIA-232
EIA-442
EIA-449
ITU-T (International Telecommunications Union-Telecommunications Sector)
V series (전화망에 관한 데이터 통신)
X series (데이터 통신망에 대한 데이터통신)

15. DTE-DCE Interface(cont’d)
EIA-232(RS-232)
defining the mechanical, electrical and functional characteristics
기계적인 규격 (mechanical specification)
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)
전자적인 규격 (electrical specification)
define voltage levels and the type of signal(예 : NRZ-L)

16. 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

17. DTE-DCE Interface(cont’d)
Control and Timing
Only 4 wires out of 25 available in an EIA-232 interface are used for data functions
Signals must be sent using OFF less than –3 volts and ON greater than +3 volts

18. DTE-DCE Interface(cont’d)
기능 규격 (Functional Specification)

19. DTE-DCE Interface(cont’d)
Functions of Pins in EIA-232, DB-9

20. DTE-DCE Interface(cont’d)
예 : synchronous full-duplex transmission
Step1 :preparation
Step2 : Readiness
Step3 : Set up
Step1 :preparation
Step2 : Readiness
Step3 : Set up

21. DTE-DCE Interface(cont’d)
Step4:
Data Transfer
Step 5 : Clearing
Step4:Data Transfer
Step 5 : Clearing

22. DTE-DCE Interface(cont’d)
Null Modem
not needed to connect two compatible digital devices directly
Ex) connecting two DTEs in the same building
Using regular data pin connections with and without DCEs

23. DTE-DCE Interface(cont’d)
Crossing Connections
Null modem pin connection

24. 6.3 Other Interface Standards
needs more speed and/or distance
EIA-449
EIA-530
X.21

25. Other Interface Standards(cont’d)
EIA-449
DB-37 pin
DB-9 pin
DB-37 and DB-9 connections

26. 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

27. Other Interface Standards (cont’d)
DB-37 pins
Pin
Functions
Category 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
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 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
Receive Common
Terminal in service
Select ready
Signal quality
New signal
Standby indicator
Send common I II

28. 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

29. 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)

30. Other Interface Standards(cont’d)
RS-423 : Unbalanced Mode

31. Other Interface Standards(cont’d)
RS-422 : Balanced Mode

32. Other Interface Standards(cont’d)
Canceling of noise in balanced mode

33. Other Interface Standards(cont’d)
EIA-530
is a new version of EIA-449 that uses DB-25 pins

34. Other Interface Standards(cont’d)
X.21
~ is an interface designed by the ITU-T to address many of the problems existing 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 (control)
Pin 5 : equivalent of clear to send (indication)

35. 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

36. 6.4 MODEMs
The most familiar type of DCE

37. MODEMs(cont’d)
MODEM
stands for modulator/demodulator
Modulator : converts a digital signal to an analog signal
using the digital-to-analog mechanism of ASK, FSK, PSK, and QAM
Demodulator : converts an analog signal to a digital signal

38. MODEMs(cont’d) Transmission Rate 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

39. MODEMs(cont’d) Telephone line bandwidth
A telephone line has bandwidth of almost 3000 Hz

40. MODEMs(cont’d) 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.

41. MODEMs(cont’d) Baud rate in full-duplex ASK
Although ASK has a good bit rate, it is not used today because of noise

42. MODEMs(cont’d) FSK(Frequency Shift Keying)
Baud rate in half-duplex FSK
Baud rate in full-duplex FSK
Max baud rate = Max bit rate = [2400 – (fc1 – fc0)]/2

43. Baud Rate in Full-Duplex FSK
MODEMs(cont’d)
Baud Rate in Full-Duplex FSK

44. MODEMs(cont’d) Encoding Half-Duplex Full-Duplex ASK, FSK, 2-PSK
PSK and QAM(비교)
Theoretical bit rates for modems
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

45. MODEMs(cont’d) Modem Standards Bell modem ITU-T modem
The first commercial modems in the early 1970s
ITU-T modem

46. MODEMs(cont’d)
Bell Modems(cont’d)
For flow control or error control

47. MODEMs(cont’d) Bell Modems
212 --> 201 : Same technology, data rate double

48. MODEMs(cont’d)
ITU-T Modem Standards(cont’d)

49. MODEMs(cont’d)
ITU-T Modem Standards

50. MODEMs(cont’d) ITU-T Bell Baud Rate Bit Rate Modulation V.21 V.22 V.23
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
1600
2400
300
1200
2400
4800
9600
FSK
4-PSK
8-PSK
16-QAM

51. 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) ATDT

52. MODEMs(cont’d) Command Meaning Parameters A B D E H L P T
AT commands
Command
Meaning
Parameters A B D E H L P T
The number to dial
0 or 1 n
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

53. MODEMs(cont’d)
Traditional Modems

54. MODEMs(cont’d)
56K Modem

55. MODEMs(cont’d)
Cable Modem