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2/26: Wide World of Modems

2/26: Wide World of Modems. 13. 1. Roll call. Video: modem Lecture: step-by-step. 25. 14. 1. The DTR signal. 13. 1. 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. 25. 20. 14. 1A. The DSR signal.

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2/26: Wide World of Modems

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  1. 2/26: Wide World of Modems 13 1 • Roll call. • Video: modem • Lecture: step-by-step 25 14

  2. 1. The DTR signal 13 1 • 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. 25 20 14

  3. 1A. The DSR signal 13 6 1 • 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. 25 20 14

  4. 2. Transmit Data line. 13 6 2 1 • 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. 25 20 14

  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 13 8 6 2 1 • 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. 25 20 14

  7. 5. The handshake. 13 8 6 2 1 • The 2 modems discuss how they will exchange data. • The modems must agree on speed, number of bits, & what info to send. 25 20 14

  8. 5A. Terms. 13 8 6 2 1 • Handshake: the negotiations. • Data packet: a block of data with start and stop bits. • Parity bit: a doublechecking bit for error control. 25 20 14

  9. 6. The PC tries to send data. 13 8 6 4 2 1 • 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. 25 20 14

  10. 7. The modem says “okey dokey”. 13 8 6 5 4 2 1 • 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. 25 20 14

  11. 7A. The modem receives data. 13 8 6 5 4 3 2 1 • 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. 25 20 14

  12. 8. On the other end of the phone line. 13 8 6 5 4 3 2 1 • The remote modem converts the incoming analog tones into digital signals. • The remote computer then processes the digital signals. 25 20 14

  13. 9. Sign off. 13 8 6 5 4 3 2 1 • 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. 25 20 14 Goodbye.

  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

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