Dial-up connection steps
- Calling modem goes off hook
- Telephone exchange sends a dial tone
- Calling modem dials 1-570-234-0003 via DTMF, a number in Pennsylvania
Phase 1: Network interaction between calling modem and answering modem
- Answering modem INIT: initiates a V.8 _bis _transaction
- Answering modem CR: asks calling modem to list its capabilities
- Calling modem responds to V.8 bis initiation, agrees to list its capabilities
- Calling modem CRd and ESr: requests to escape from “telephony” into “information transfer” mode
- Calling modem CL: sends FSK data @ 300 bps: (01111110 01111110…) “I’m capable of full V.8. I can transmit ACK. By the way, my country is the U.S. I was manufactured by Net2phone Inc.”
- Answering modem MS: replies with FSK data @ 300 bps: “Why don’t we use V.8. then.”
- Calling modem ACK: replies: “Okay, mode acknowledged. Terminating V.8 bis transaction.”
- Answering modem disables echo suppressors and cancellers in PSTN
- Calling modem sends FSK data @ 300 bps, repeated 6 times:
“Modules modes available:
- PCM V.90/V.92 analogue
- V.34 duplex
- V.32/V.32 bis
- V.23 duplex
- V.22/V.22 bis
V.42 LAPM is available.
PSTN connection is over a regular landline (not cellular). My network is analogue.”
- Answering modem sends FSK data @ 300 bps, repeated 3 times: “Oh, I can do those as well. Except my V.90/V.92 is digital. PSTN connection is over a regular landline (not cellular). My network is digital.”
Phase 2: Probing/ranging
- Answering modem sends DPSK data @ 600 bps: “I can transmit at either carrier frequency at whichever standard symbol rate except 3429. Tx and Rx symbol rates must be equal. I can reduce power if needed. I can support up to 1664-point signal constellations. My clock source is internal.
- Calling modem sends DPSK data @ 600 bps: “I can transit at either carrier frequency at whichever standard rate except 3429. Tx and Rx symbol rates must be equal. I can reduce power if needed. I can support up to 1664-point signal constellations. My clock source is external.
- Both modems send a wide-spectrum probling signal in both directions to do measurements of the telephone line
- Calling modem sends DPSK data @ 600 bps: “Please don’t reduce your power by more than 6dB. I’m not equipped to accurately measure the frequency offset of the tones you sent. Using different symbol rates, we could achieve the following:
|symrate||ur pre-emph α||bps|
- Answering modem sends DPSK data @ 600 bps: “Please don’t reduce power any further. Your tones were offset by 0 Hz. Let’s use a symbol rate of 3200. We can achieve a maximum of 4800 bps. For you pre-emphasis filter, select the parameters β = 1.0 dB and γ = 2.0 dB. Please use carrier at 1920 Hz for transmission, I’ll use 1829.
Phase 3: Equalizer and echo canceller training
- Both modems go to scrambled data and learn how the other modem sounds over the channel. The final bitrate and signal constellation are also decided on.
“Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers.”
“known under the trademark Touch-Tone […], starting in 1963.”
“Multi-frequency signaling (MF) is a group of signaling methods that use a mixture of two pure tone (pure sine wave) sounds”
“The DTMF telephone keypad is laid out as a matrix of push buttons in which each row represents the low frequency component and each column represents the high frequency component of the DTMF signal. The commonly used keypad has four rows and three columns, but a fourth column is present for some applications. Pressing a key sends a combination of the row and column frequencies. For example, the 1 key produces a superimposition of a 697 Hz low tone and a 1209 Hz high tone.”