2/26: Wide World of Modems

• Roll call.

• Video: modem

• Lecture: step-by-step

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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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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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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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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