Mel Rees writes:
>I just bought abother load of USR Sportster V.34 modems and discovered
>that they can now run at 33.6K baud. An upgrade is available for
>older V.34 models.
Before anyone goes rushing off to buy 33.6K baud modems, they should know
that this change extends only the top speed of the modems and does not
change the speed vs. line characteristics curve. Motorola has been using
the 33.6Kb technology in their high-end modems for about six months, and
tests of those modems bear this out.
What this means is that if your 28.8Kb modems normally connect at the high
end of the range -- 26-28.8Kb -- you may see some performance improvement
from the 33.6KB technology. However, if they normally connect at 21-24Kb,
buying new modems won't help; you need better phone lines first.
By default, most modems report the serial port speed on a connection, so
they'll report "CONNECT 57600" if your AT command went out at 57600 baud,
regardless of the connection speed. Try ATW2 before dialing in order to get
the modem to report the actual communication speed. This will vary from
line to line, and usually from call to call. If you don't often see 26400
or 28800 as the connection speed, don't expect much gain from a
33.6Kb-capable modem.
If you really need the extra speed and ISDN is available in your area,
basic-rate ISDN is a better bet. ISDN provides 56Kb or 64Kb synchronous,
depending on the telco switching equipment serving your area. This
translates into 70Kb or 80Kb asynchronous; I routinely see throughput of
6.5K bytes/sec doing FTP through our local 56KB ISDN service.
Consumer-grade terminal adaptors (TA's -- the ISDN equivalent of modems)
can be had for under $300, making them competitive with high-end
consumer-grade analog modems. If you need still more speed, many TAs --
including the low-end ones mentioned above -- can use both high-speed data
channels (B channels) on one call, for a total line speed of 14 or 16
kbytes/sec depending on your ISDN rate. The call costs twice as much, so
you don't save on phone costs by doing this. However, if your ISDN tariff
works like then one around here -- $75/month fixed for up to 200 hours of
use -- the high speed is nice to have.
The devices are not trivial to set up; you can't just take one out of the
box, plug it in and expect it to work. There are a bunch of parameters that
must be set before the TA will talk to the phone line, and several more to
work out to insure that you can talk to the device at the far end. In many
areas, though, you can get away with a cookbook procedure for the setup
once you know the necessary information. I also note that there's a book
called "ISDN for Dummies," a sure sign that the technology will soon be
ready for prime time.
One advantage to ISDN for home use is that a basic-rate ISDN line is like
getting two lines for the price of two, but on one twisted pair. As I
mentioned earlier, the ISDN line has two high-speed data channels. Many TAs
allow you to use the second data channel for voice calls when it's not
being used for data. The audio quality on consumer-grade TAs won't win any
hi-fi magazine awards, but it's perfectly serviceable once you get used to
it.
Another advantage to ISDN relative to analog modems is that call setup time
is very short. I usually see two or three seconds on local calls and four
or five seconds on long-distance calls. This makes quite a contrast with
the 20-30 second call setup times for analog modems and makes possible some
tricks to cut phone costs. We use ISDN to connect to one client site; we
have a dial-up router-to-router connection and use NS to get between the
systems on either side. Each router is set to dial automatically when
there's traffic for the other side. Because the call setup time is so
short, I have the inactivity timers on the routers set for one minute. If
the timer drops the line, it takes about five seconds to get a response
from the remote computer, certainly an acceptable response level. The trick
only works if _both_ ends can autodial, though, and if the call setup
charge isn't very high (about $0.05 in this area).
Several companies, including USR and Motorola, make dual ISDN/28.8Kb analog
terminal adaptors. They're expensive, but they allow your central site to
answer either kind of call on the same phone line and serial port. I have
no experience with these dual-protocol devices.
-- Bruce
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Bruce Toback Tel: (602) 996-8601| My candle burns at both ends;
OPT, Inc. (800) 858-4507| It will not last the night;
11801 N. Tatum Blvd. Ste. 142 | But ah, my foes, and oh, my friends -
Phoenix AZ 85028 | It gives a lovely light.
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