Modems perform modulation of the analog signal they put out, and demodulation of the signal they receive. They do this by mixing two signals in a digital signal processor and transmitting the analog result to the phone system as the electrical equivalent of sound. Here is a list of the protocols. The Bell protocols were developed by Bell Telephone prior to the government's breakup of that monopoly. The CCITT is an international standards body, which changed it's name later to the ITU (International Telecommunications Union).
| MODULATION | SPEED | IN USE |
|
Bell 103 |
300 bps | Dead |
|
Bell 212A |
600 bps | Dead |
|
CCITT V.21 |
900 bps | Rare |
|
ITU V.22 |
1,200 bps | Rare |
|
ITU V.29 |
9,600 | Uncommon |
|
ITU V.32 |
14,400 | Uncommon |
|
ITU V.32bis |
28,800 downlink/uplink | Yes |
|
ITU V.34 |
33,600 downlink 28,800 uplink |
Yes |
| 56k downlink* ~28,800 Uplink |
Yes | |
| 56k downlink* 33k Uplink |
Yes |
All these protocols use one of four types of modulation:
- Frequency Shift Keying (FM)
- Phase Shift Keying (PSK)
- Quadrature Amplitude Modulation (QAM)
- Pulse Code Modulation (PCM)