From: goldstein@carafe.enet.dec.com (Fred R. Goldstein)
Subject: Re: Asynchronous TAs
Organization: Digital Equipment Corp., Littleton MA USA
Date: Thu, 24 Dec 1992 05:30:15 GMT


In article <1992Dec23.192710.3006@xenitec.on.ca>, vances@xenitec.on.ca (Vance Shipley) writes...
>Of all the ISDN terminal adaptors I have seen very few provide asynchronous
>speeds above 19.2K.  None of the TAs I have seen provide faster than 38.4K.
>Of course they all provide 56 or 64K synchronous.  I'm sure this makes
>sense when viewed from the position of evolving the current telecommunications
>solutions to ISDN.  Currently modems provide asynchronous interfaces at
>speeds up to 19.2K and sometimes 38.4K while DDS provides leased 56K
>synchronous connections.  The problem here is that by maintaining this
>limited view the TA manufacturers have ensured that they will cater solely
>to the high end users.
> 
>For the billion PC users out there without synchronous cards and software
>there is no benefit in getting ISDN.  These people have, or could get, V.42bis
>modems which they believe will get them 19.2K throughput.  If they get an ISDN
>terminal adaptor they could achieve a real 19.2K conection.  There would be
>a gain here if you could explain the issue of compression.  If they were to
>get a TA which gave them 38.4K they would have a better than two fold gain.
>This is just enough to make it worth the trouble, maybe.

Indeed, compression IS the trick, and what makes the comparison of
oranges to juice a bit unfair.  Most high-end modems nowadays perform
compression, and claim 2:1 to 4:1, and use flow-controlled DTE
interfaces at maybe 57.6 kbps or even higher.  But the modem itself
is usually 14.4 kbps or slower (V.fast going up to 24k and maybe
28k on a really, really swell analog line).  A TA with a "raw"
async speed of 38.4k is better.  Sadly, there are many current
TAs (like the Fujitsu for which a flyer just came in the mail)
that only go to 19.2!  What a waste.

>I read an article recently about a group of astronomers who were working at
>home with X terminals over ISDN.  Because the X terminals only provided an
>asynchronous serial interface (in addition to the ethernet) they were only
>using 38.4K.  The article was giving this as an example of how ISDN allowed
>users to do things never before practical.  While this was essentially true
>to me it was a great example of what is wrong with the cuurent state of
>affairs in the ISDN world.

Yep.  What's wrong is that customers don't always get the right
equipment.  I'm typing this over an ISDN circuit, momentarily at
64 kbps, though I sometimes use 56kbps "data over speech" just to
exercise that option.  (We don't have interexchange data ISDN yet
here, and some of our ISDN users are interexchange.)  My clone PC
has a bog-standard DEC Ethernet card, tied via 10baseT to a 
Gandalf Premier 5510 bridge.  It makes a nice X setup!  I can also
transfer files at 7000 Bytes/second, using DECnet NFT or FTP.

ISDN PC cards that go 64 or 56 kbps sync do exist, but there are
many issues of compatibility (protocol stacks).  BTW (in response
to KPH's comment), T-Link is a Northern Telecom rate adaptation
protocol.  Introduced with the SL-1 Add-on Data Module in 1979 or
so, it was used on the DEC CPI-32 in 1982.  It's smarter than
V.110, though also bit-oriented (not HDLC like V.120).  A notable
feature: At 9600 and down, all info is sent with triple-mode
redundancy, so a one-bit error can be outvoted by a smart receiver.
---
Fred R. Goldstein   goldstein@carafe.tay2.dec.com 
k1io             or goldstein@delni.enet.dec.com   voice:+1 508 952 3274
Standard Disclaimer:  Opinions are mine alone; sharing requires permission.
Newsgroups: comp.dcom.isdn