1. การส่งข้อมูลผ่าน PSTN
DTE : Data Terminal Equipment is any device that is a source of or destination for binary digital data (PC)
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 (Modem)
PSTN
DCE
DTE
DTE
DCE

2. Data communication interfacing
DTE -DCE Interfacing
Requirement : Use the same encoding scheme and have common standards
Mechanical characteristics : plugs, circuits
Electrical characteristics : voltage-level, timing
Functional characteristics : meaning of each interchange circuits เช่นขา 7 คือ Ground

3. V.24/EIA-232 (RS 232)
Electronic Industries Association (EIA)

4. Mechanical characteristics
(ISO 2110) 25-pin connector (but can be substituted by 9-pin (DB-9) : 8, 3, 2, 20, 7, 6, 4, 5, 22)

5. Electrical characteristics
-3 to -15 V=binary 1 or OFF condition
+3 to 15 V =binary 0 or ON condition
Maximum bit rate 20 kbps

6. Functional specification

7. Control Signal Name To Function Request to send DCE
DTE wishes to transmit
Clear to send
DTE
DCE is ready to receive
DCE ready
DCE is ready to operate
DTE ready
DTE is ready to operate
Ring indicator
DCE is receiving a ringing signal
Received line signal detector
DCE is receiving a signal
(carrier)

9. Null modem

10. MODEMS MOdulator/DEModulator
Bandwidth 600 Hz to 3000 Hz

11. PSTN

12. Dial-up operation
DTE 20