Analog telephone lines

Leased lines are full-time connections between two specified locations, while switched lines are regular voice-telephone lines. The fastest modems today operate at an uncompressed rate of 56 kbps. Given the signal-to-noise ratio on unconditioned voice-grade telephone circuits, though, this rate is often unattainable.

Modem manufacture claims of higher bit-per-second (bps) rates are typically based on a data compression (CCITT V.42bis) algorithm that is utilized by their modems. Although V.42bis has the potential to achieve as much as four-fold reduction in data volume, compression depends on the data and rarely reaches even 50%. Data already compressed or encrypted might even increase with V.42bis applied. X2 or 56Flex extends the bps rate to 56 kbps for analog telephone lines. This is a hybrid technology that requires one end of the PPP link to be digital while the opposite end is analog. Additionally, the 56 kbps applies only when you are moving data from the digital toward the analog end of the link. This technology is well suited for connections to ISPs with the digital end of the link and hardware at their location. Typically, you can connect to a V.24 analog modem over an RS-232 serial interface with an asynchronous protocol at rates up to 115.2 kbps.

The V.90 standard put an end to the K56flex/x2 compatibility issue. The V.90 standard is the result of a compromise among the x2 and K56flex camps in the modem industry. By viewing the public switched telephone network as a digital network, V.90 technology can accelerate data from the Internet to a computer at speeds of up to 56 kbps. V.90 technology differs from other standards because it digitally encodes data instead of modulating it as analog modems do. The data transfer is a asymmetrical method, so upstream transmissions (mostly keystroke and mouse commands from a computer to the central site, which require less bandwidth) continue to flow at the conventional rates of up to 33.6 kbps. Data sent from a modem is sent as an analog transmission that mirrors the V.34 Standard. Only the downstream data transfer takes advantage of the high-speed V.90 rates.

The V.92 standard improves on V.90 by allowing upstream rates of up to 48 kbps. Additionally, connection times can be reduced because of improvements in the hand-shaking process, and modems that support a hold feature can now remain connected while the telephone line accepts in coming call or uses call-waiting.