2/26: Wide World of Modems Roll call. Video: modem Lecture: step- by-step 113 2514.
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Transcript of 2/26: Wide World of Modems Roll call. Video: modem Lecture: step- by-step 113 2514.
2/26: Wide World of Modems
• Roll call.
• Video: modem
• Lecture: step-by-step
113
25 14
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.
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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.
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6
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.
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6 2
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.)
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.
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6 28
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.
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5A. Terms.
• Handshake: the negotiations.
• Data packet: a block of data with start and stop bits.
• Parity bit: a doublechecking bit for error control.
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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.
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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.
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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.
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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.
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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.
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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