Date: Tue, 19 Mar 96 13:52:53 MET From: Greg Lehey <lehey.pad@sni.de> To: hackers@freebsd.org (FreeBSD hackers), isdn@muc.ditec.de (Distribution List; FreeBSD ISDN) Subject: ISDN: "modem" or board? (Was: Microsoft "Get ISDN"?) Message-ID: <199603191255.NAA08040@nixpbe.pdb.sni.de>
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Well, I've been away from the list for a few days, and so much has
come in that I don't think it would make sense to respond to each
message individually, so here's a summary:
1. Speed of a connection. Some people say "the bottleneck is the B
channel, so you can use async instead". Well, yes, assuming your
machine isn't doing anything else. To run 2 B channels flat out,
you need a 230 kb/s line, which with standard el cheapo 16550As
will give you 23000 interrupts per second. This is enough to
max out a slow 386. In my experience, it's also enough to cause
overruns even on fast machines. By contrast, I run my current
ISDN card on an 8 MHz 286, and if it drops anything, it doesn't
notice :-) In any case, I get full B channel throughput.
2. Setup time. Could be that I've bitten off more than I can chew
here. Of course, the setup time is dependent on the switch you
connect to, but it also depends on the way you talk to it. On an
ISDN board, the software talks directly to the D channel. On an
ISDN "modem", you need first to establish (serial) connection with
the "modem", it then needs to interpret your commands and talk to
the D channel. This is bound to take longer, but the question
remains whether the difference is noticable. If it is, then I
consider it also objectionable: I think the 2 second setup time I
have to be too long.
3. Somebody said that it's nice to be able to leverage off existing
technology. That's a nice way of saying nothing: after all, the
ISDN boards are existing technology too (ISA bus interface--not
fast, but a whole lot faster than a serial interface).
Conceptually, think of them as slow Ethernet boards.
4. Price. I think some people have not read the messages carefully.
I just ordered another batch of boards for DM 133 each, about $90.
What combination of ISDN "modem" and serial board can you get for
that?
Other points:
Darryl says that an Ascend P50 can peak 42 kB/s. That seems
reasonable with builtin compression. How many B channels? Is this
rate sustainable?
Hellmuth mentions a number of points:
- you can do other things than IP over a board.
- 99% of ISDN IP traffic over here is raw IP over HDLC. I disagree.
I know a number of ISPs who use PPP, probably because they don't
know any better. The difference is important, because currently the
FreeBSD implementation can't handle PPP (probably a bug somewhere).
- He mentions CAPI. For those of you who didn't want to ask, CAPI is
a German API for ISDN boards. To the best of my knowledge, the only
implementations run on DOS. Since the CAPI is supplied with the
board, this is a serious limitation.
dennis says "the \"future\" is in high-speed async". It's difficult
to form an opinion about that from my perspective here. Certainly
there's not much activity in high-speed async over here. But I
suppose it's a case of absolute crud driving out the crud.
I think that's covered most of the topics.
Greg
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