HST: USRobotics' High Speed Technology¶

HST was USRobotics' own fast modem protocol, first sold in 1986 in the Courier HST and used into the mid 1990s. It is lopsided: one way a fast forward channel (up to 14400 bit/s, later 16800), the other way a slow back channel of 300 bit/s, and the modems turn the channels around when the heavier traffic changes direction. That suited bulletin boards and FidoNet, where most data goes one way, and HST was the favourite of BBS sysops until V.32bis and V.34 caught up. USRobotics never published it.

D-Modem's HST is built from what is known of the Courier's DSP firmware (see the technical details). Its forward and back channels are those of a real Courier, bit for bit as far as the firmware has been read. Its start-up follows a recorded call to a real Courier and the Courier's own firmware: the call connects at 2400 bit/s (V.22bis), both modems offer HST in V.42's set-up, and they then retrain in HST. Two D-Modems connect to each other this way. The HST part, which the recording does not reach, was checked against the Courier's DSP firmware itself, run in an emulator: D-Modem connects to it as caller and as answerer at 4800 to 16800 bit/s. The whole Courier, its own firmware emulated, calls D-Modem and is called by it, with HST's own link layer and the channels turning around. It has not been tried against a real Courier yet.

Using it¶

AT+MS=HST        (or AT+MS=168)

A call starts the way a Courier's does:

1. The modems connect at 2400 bit/s (V.22bis) and begin V.42.
2. In V.42's set-up (XID) each one offers HST and its speeds, in the same field a Courier uses.
3. If both offer it, they skip the rest of V.42 at 2400, drop the carrier and train in HST: 9600 bit/s if both allow it, else 7200, 4800, 12000, 14400 or 16800, whichever both have first. Over HST the modems run HST's own link layer, not V.42: error correction without compression.
4. If the other modem does not offer HST, the call stays at 2400 bit/s with V.42, as with a Courier.

The speeds offered follow +MS, for example AT+MS=HST,0,4800,7200. The answering modem sends the forward channel first; the caller sends the 300 bit/s back channel. When the side on the back channel has data to send, it asks, and the modems turn the channels around: the fast direction follows the data, every few seconds when both sides send. The rates CONNECT gives are those of the start. D-Modem does not change speed during the call (a Courier moves up from 9600 when the line allows).

With V.42 off (AT\N0), there is no set-up to offer HST in, so the modems start HST at once with D-Modem's own start-up, in which the caller measures the line and asks for the fastest speed it takes (up to 16800). A Courier does not understand that start-up. V.8 has no code for HST, so it is used only when chosen with AT+MS.

Speeds in simulation¶

Two D-Modems back to back on a simulated phone line (pump_test, with D-Modem's own start-up, which picks the speed from the line):

Every one of these calls ran without a bit error, as did calls with a 300 ppm clock difference, a 6 dB gain change, an echo, and an 8000 Hz sample rate. Below about 12 dB the start-up fails.