1. Modem acronym for modulator-demodulator
converts a series of binary voltage pulses into an analog signal
the receiving modem translates the analog signal back into digital data

2. Date Flow: Simplex only transmit in one direction One way Monitor
Computer

3. Data Flow: Half Duplex
data may travel in both directions, but only in one direction at a time
non-simultaneous two-way communication
computers use control signals to negotiate when to send and when to receive

4. Half Duplex

5. Data Flow: Full Duplex complete two-way simultaneous transmission
faster than half-duplex communication because no turnaround time is needed

6. RS-232C
Used for connections between computer and modems, among other things
25-pin connector with a specific arrangement of leads
computer usually has male DB25 connectors while modems are female
In practice, fewer than 25 wires are generally used in applications

7. RS-232 DB-25 Connectors
DB-25 Female
DB-25 Male

8. RS-232 DB-25 Pinouts

9. RS-232 DB-9 Connectors
Limited RS-232

10. MODEM COMMANDS MEMORIZE THESE!
AT- PROCEDES ALL MODEM COMMANDS
ATDT- DIAL PHONE # USING TOUCH TONE
ATDP- DIAL PHONE # USING PULSE DIALING
ATA- ANSWER AN INCOMING CALL
ATHO- HANG UP IMMEDIATELY
AT&F- RESET MODEM TO FACTORY SETTING
ATZ- RESET TO POWER UP SETTINGS
*70- CANCEL CALL WAITING