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Q97879: RAS and US Robotics HST

Article: Q97879
Product(s): Microsoft LAN Manager
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 30-JUL-2001

SUMMARY
=======

Do not use US Robotics HST (High Speed Technology) mode on USR Dual Standard or
USR HST modems with Microsoft Remote Access Service (RAS). It will actually slow
down data communication rates.

MORE INFORMATION
================

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.

Additional query words: 2.20 compression correction tuning baud

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