1. 2/26: Wide World of Modems Roll call. Video: modem Lecture: step- by-step 113 2514

2. 1. The DTR signal The PC sends a signal to the modem on line 20. This DTR (Data Terminal Ready) signal tells the modem that the PC is ready to go. 113 251420

3. 1A. The DSR signal At the same time, the PC hears a voltage, Data Set Ready (DSR), from the modem’s pin 6, saying the modem is ready. Both signals are required for anything to happen. 113 251420 6

4. 2. Transmit Data line. The software sends a command to the modem via line 2, the Transmit Data Line. It will switch to tones and pulses, dial, or disconnect. 113 251420 62

5. 3. The hailing frequency. When the modem on the other end answers, your modem lets you know by telling you in two high pitched beeps. (Much like R2D2 would.)

6. 3. Carrier detect The modem sends the PC a Carrier Detect (CD) signal on line 8. The signal is a steady tone that will later be modulated to transmit data. 113 251420 628

7. 5. The handshake. The 2 modems discuss how they will exchange data. The modems must agree on speed, number of bits, & what info to send. 113 251420 628

8. 5A. Terms. Handshake: the negotiations. Data packet: a block of data with start and stop bits. Parity bit: a doublechecking bit for error control. 113 251420 628

9. 6. The PC tries to send data. The PC sends data by first signaling on line 4 of the serial port. This Request to Send (RTS) signal asks if the modem is free to receive data from the PC. 113 251420 6284

10. 7. The modem says “okey dokey”. Unless the modem is busy, it sends a Clear To Send (CTS) signal back to the PC on line 5. The PC sends the data on line 2. 113 251420 62845

11. 7A. The modem receives data. The modem sends data it receives to the PC on line 3. The modem drops the CTS signal if it is overwhelmed. It turns it back on after it catches up. 113 251420 628453

12. 8. On the other end of the phone line. The remote modem converts the incoming analog tones into digital signals. The remote computer then processes the digital signals. 113 251420 628453

13. 9. Sign off. The PC tells the modem to break the connection. If the other computer breaks the connection, the Carrier Detect signal is dropped and the PC alerted. Goodbye. 113 251420 628453

14. In the future: Monday 3/1: web pages are due by noon. Wednesday 3/3: optional review session. Friday 3/5: midterm exam. Today’s entertainment: http://206.110.181.69/studentpages/John/Yoda.html http://206.110.181.69/studentpages/John/Yoda.html