RAS and US Robotics HST (97879)

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HST is a proprietary high speed modulation scheme owned by US Robotics. With HST, data flow is asymmetrical - 14,400 bps high band width in one direction and 450 bps low band width in the return direction. This seems like a reasonable thing to do for most typical terminal type applications where most work is done in one direction only. HST modems will automatically switch the high\low speed lines based on which direction has the heaviest traffic.

However, RAS extends a fully bi-directional network across the asynchronous connection. For a 14.4 kbps connection, AsyBeui [or NBF] adjusts its timing parameters [T1, T2, TI, and so on] based on the assumption that data frame acknowledgments [ACKs] will be returning faster on a high speed connection. LAN Manager may pause until it receives this ACK. If the returning line is very slow compared to the send line, this will cause unnecessary delays. If LAN Manager does not get an ACK with a given period of time it will automatically timeout and possibly retransmit. These return delays and retransmissions on a 14.4 kbps RAS "connection" with the HST modem will actually get closer to 2400 bps throughput. In fact, a standard v.22bis 2400 bps connection is faster than HST with RAS.

USR even recommends in their manuals that you use v.32bis for interactive applications, which is essentially what RAS is.

To explicitly specify V.32bis mode [14.4 kbps in both directions] on USR HST Dual Standard modems, add "B0" to COMMAND= the in the Init string in the MODEMS.INF file. See the "RAS Administrator's Guide" for more information on editing the MODEMS.INF file.