-=> Quoting Robert Osborne to Rick Collins/All <=-
 RO> Are all UART chips clearly marked with the 16550 or 8250 designation?
Discrete UART chips marked 8250 or 16450 need uprating if using a modem
faster than V32bis.  A 16450 here carries the marks:
S+B8648
NS16450N
and measures some 52mm long by 13mm wide.  They are interchangable with
16550AFN and 16650 series.  If soldered to motherboard take care with
change since some motherboards are multi-layered - the sandwiched layer
may have track you can't see connected to any of the legs.  Best way is
to carefully snip all the old chip's existing legs leaving them proud of
the m/board then solder appropriate holder to them.  If you've no
experience of this then take your PC to someone who has
 RO> chip?  If it is an external modem, do you have to buy a special serial
 RO> board to handle the extra speed of the 16550?
Yes if you only have an 8250 or 16450 to handle comms above 14k4 line
speeds - especially in Windows.  Internals will have a "16550 compatable"
UART on them although there's no parallel to series data stream conversion
occuring because the computer's data bus in use is already paralell
 RO> that speed.  Other wise they shifted to a common lower speed?  How does
 RO> this work?  Do you have to software set your speed to some upper limit?
Your maximum port speed should be fixed to higher than your modem's
speed.  The port speed is the data rate that your modem speaks with
your computer - min 19k2 for a V32bis, min 38k4 for a V.FC or V34
Your maximum line speed is automatically negotiated with the remote
modem according to line conditions, and can be renegotiated during
the connect if line conditions change
 RO> I also thought that there is some limiting speed over the
 RO> communication media being used.  Ie 28.8 was the upper limit for
 RO> telephone lines?
Techniques using higher symbol rates allow these and 31k2 and
33k6 speeds.  Some don't ever get over 26k4 tho.  This is usually due
to a bandwidth limitation somewhere along the link between your
modem's phone jack all the way to the remote modem's phone jack.
Currently, speeds greater than 33k6 can be achieved if there's no
more than one digital to analog conversion in the link - the remote
will only be able to download to you if it's connected to a digital
link, like some ISPs have, at these extra speeds typically 43k3 and up.
You'll need a modem that's also either X2 (USR series) or K56flex or
Flex2 capable (Supra, Zoom, Hayes, Lucent, etc) if your ISP offers
either protocol.  Get one with FlashROM for when the 56k standard's
agreed and you can upload to your modem the new code when available.
 RO> rates, but instead compress and decompress the data on the fly to
 RO> obtain faster rates of data thru put?
The ITU recognised test file 1X30.TST (a repeating pattern of same
characters) flies thru links at speeds approaching your port speed:

----------  Mon 17 Mar 97, FD 2.12.SW
+ 12:19:49  Calling Explosion BBS!, 2:440/222, 01732-773369@
= 12:20:12  CONNECT 28800/ARQ/LAPM/V42BIS

* 12:20:15  Sent C:\BBS\FD\OUTBOUND\000C4E2D.PKT; 3371b, 0 CPS
* 12:21:41  Sent C:\BBS\FD\FILES\1X30.TST; 983040b, 11430 CPS
+ 12:21:46  Mail transfer completed
$ 12:21:46  To 2:440/222,  1:34, 24.
(between two Zoom model 275, not local loop)
This 11,430cps is achieved by stripping start and stop
bits off of each character (reducing the 10 bits to eight) but mostly
by the V42bis compression which recognises a repeating pattern when
it sees it.  Similar to the way that PkZip would compress the same
file.  That's "on the fly" compression which doesn't happen if the
file is fully pre-compressed before transmission - V42bis recognises that
and won't try to compress further (in PkZip terms compressing with
the -ex switch)
 RO> When did they start all this and
 RO> at what modem speed did compression take place?
V42bis was in use in the mid/late 80s and became the preferred method
of compression by ITU(t) in (thinx) '90(?)  It's generally held
that it compresses by up to four times (if there's enough
compressability in the file to be transmitted for this)  Which is
why it's ususal for your port to be locked at 4x maximum line speed:
38k4 for a V32bis, 115k2 for a 28k8 although some versions of Windows
will only do a max 57k6 port speed (Win'95 and NT/4 are alright at
115k2)  Prior to V42bis there was MNP5 (MNP4 error correction with
max 2:1 compression)  This compression is dumb in that it tries to
compress even the uncompressable
 RO> There is also some specifications that deal with error correction and
 RO> being able to recognize compressed data?
There's LAP-M, MNP4, and MNP10 in common use.  LAP-M is the
ITU-T preferred form of error correction.  There's also an enhanced
varient called LAP-M Selective REJect which will only retransmit
that portion of a block of data destroyed by say line noise, not the
whole block.  MNP10 is good for very bad links, and when combined with
some other wizardry (MNP10ECC or MNP10 with ACE) is good for cellular
analog links
 RO> I would also like to know any thing you can provide me with on the
 RO> subject of faxing via computer.  Does the modem have anything to do
 RO> with the faxing capability or is this all just software that you buy
 RO> and install on the computer?
There are software modems that will only do fax in Class1 mode (software
handshaking and the fax program does all the work)  Some of these
"modems" (really just a line interface on a card) won't adaptively
answer so are no good for BBSs where you want the modem to go into
fax mode for and incoming fax, but data mode for data calls.
Unlike Class2 or Class2.0 (hardware handshaking the faxmodem does all
the work)  Modems have become very price competitive nowadays so there's
also models that only have Class1 but offer other features which take up
the ROM space formerly used by Class2 (RPI+ and Videophone mode)
 RO> I see modems listed as 2400 baud data and
 RO> 9600 baud fax or something like that?
Those are very old now but good if you just want a cheap fax option
with occasional data.  Check your modem's "Group III" compatible.
Max fax speed is now 14k4
 RO> are out there and what is coming in the future before i make a decision
 RO> on purchasing one.  Do you work for one of the modem companies?
You decision will be based on what you want to use the modem for.
Have no need for VoiceMail here so havn't got Zoom's latest n'greatest
ComStar DSVD K56flex (Digital Switched Voice over Data, you can talk to
the remote whilst transferring a file, capable of upto 53k3 downloads in
US from a digital source)
rgdZ
Richard
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