2 days ago
- Cellular modems historically acted as independent devices with direct analog audio paths, later transitioning to digital but remaining separate from the main processor.
- The Hayes Smartmodem (1981) introduced the AT command set and standalone modem control over serial, a legacy still present in modern 5G modems via USB-CDC ACM interfaces.
- Voice support emerged in 1992 with Rockwell chipsets, using a proprietary AT# command set and Fax Class 8 mode, standardized later as V.253 in 1998.
- Audio data was transmitted over the same serial channel as commands, using PCM or ADPCM codecs and escape codes (e.g., DLE) for control.
- Voice modems enabled PC-based IVR systems and voicemail, marketed heavily to small businesses seeking a 'big business' phone presence.
- Consumer adoption stalled due to PCs not running 24/7, compatibility issues across chipsets, and competition from DSL and VoIP.
- Even modern cellular modems persist with serial-based PCM audio transport, described as an 'ugly hack' by developers, echoing the Smartmodem's architecture.
- The technology saw significant investment (Microsoft TAPI extensions) but ultimately faded as SIP and VoIP became simpler and more standardized.