RO>Dan can you give me a bit of help on selecting a modem.  I see that
RO>modems have become very specialized and maybe to difficult to totally
RO>understand by the average user.  Judging from the amount of traffic
RO>going back and forth over the conference a lot of people are pretty
RO>confused about all the various types of modems and how to get them to
RO>work as they think or thought they were going to work!  I still use a
RO>2400 baud internal for working the BBS and have tried it once or twice
RO>on the internet.  It is obvious that I will need to upgrade soon to
RO>something faster, but being of the old school I'm slow to change to
RO>something that is not straight forward to set up and that the average
RO>person with some technical experience can't figure out.  I don't want
RO>something that is only software configureable or that runs only with DOS
RO>Windows etc.
First off, I started out with a 2,400bps no-name modem without
compression and error correction.  A modem with a DTE rate of 2,400bps
is commonly referred to as a "2400 baud" modem but it is actually
running at 600 baud since, with modern modems, each symbol encodes
multiple bits - in this case 4 bits/baud). The best transfer speed I
could get out of this was about 232cps under Zmodem. This certainly
whetted my appetite but by today's standards it's a piece of junk.
Next I bought a Maestro v32bis modem. This is an Australian-made modem,
but since the Australian market is relatively small, it's a
straightforward Rockwell v32bis chipset clone. Most of these clones have
the same basic command set, sometimes with a few of the manufacturer's
command extensions. The important thing here is that high-speed modems
have v42bis data-compression and v42 error-correction. (Successful
transmission of modem-compressed (on-the-fly) data requires that EC be
operational.) The max transfer rate of an already compressed (e.g. zip)
file is about 1640-1660cps using Zmodem.  BBS listing and menu screens
will compress on-the-fly somewhat so the screen display will be more
snappy with compression turned on than with it turned off i.e. the
effective cps rate for this type of data will be higher than
1640-1660cps. To achieve this good performance you need to:
1) Lock the comms program's com port access speed (DTE) at 2-4 times
higher than the DCE speed (v32 DCE is 14,400bps). "Locking" means that
the DTE speed is fixed i.e. not "Autobauding" (free to match the DCE
rate).
2) Tell the comms program to use hardware flow control. Most highspeed
modems default to using hardware flow control (aka RTS/CTS flow
control). This does not put extra control characters in the DTE link
(comms program-modem) to stop modem datastream overflows. Instead it
makes use of the extra wires in the modem's connecting cable (hence
the term "hardware"). This is available in the majority of situations.
Using hardware FC means that no space in the DTE datastream is
wasted on flow control so potentially you get a better throughput.
3) If you are running straight DOS then you can probably use up to a
57,600bps or higher DTE rate. If you are multitasking (e.g. OS/2 or
Windows) then the OS overhead means that the comms prog may not be able
to get each byte from the UART (the chip in the computer that converts
between the serial data stream use by the modem and the parallel data
stream used by the computer's data bus) in time before the next byte arrives
from the remote modem. This will cause a data overrun as well. To
prevent this, the comms port your modem is attached to needs to be
equipped with a 16550A (or better) UART. Modern Pentium-class machines
come with this as standard; on my 486DX-50 clone I had to buy this chip.
Again this may not be necessary if you run straight DOS (i.e. not using
DESQview).
An internal high-speed modem will probably either have a 16550A UART
built-in or emulate the behaviour of it. However I much prefer external
modems. With an external modem if you get a bad connect you can reset
a stuck modem by turning the modem off and then on again. With a stuck
internal modem you usually have to restart the computer, and if you
have lots of programs open at once, this is a major hassle.
The presence of v.42 error-correction means that the transfer of
screen data as well as file data will be error-corrected (through the
retransmission of faulty data). With an non-error-correcting modem
link you can still transfer files but you have to rely on the file
transfer protocol (e.g. Zmodem) to detect and resend affected data
packets. There are two operational differences between the old non-EC
modems and modern-EC modems: The DCE link (modem-modem) operates
synchronously (no precious bandwidth is wasted on the transmission of
start and stop bits around every character - this means a 14,400bps DCE
rate can transfer date at a rate of 1,600cps+ rather than at a little
less than 1,440cps with an async DCE link; While the modems are still
operating in synchronous EC mode you should never see any phone-line
noise junk on the screen.
Now I have a USR Courier V.Everything (33,600bps) external modem and for
the last 5 years I've worked for an ISP/dial-in data service where we
have hundreds of Maestro modems at our end but have to set up our
clients to connect to us. In most cases, modem strings are simple. In
our case with the Rockwell clones (pretty common) it's:  AT&F (reset to
factory defaults) &K3 (hardware flow control - may already be a factory
default) &D2 (hang up if the DTR light turns off - may already be
factory default). With the v34+ modem on a good line (low error rate)
you can expect 3,200-3400cps depending on whether your DCE rate is
28,000bps, 31,2000bps or 33,600bps. With these modems you should operate
at a DTE of 57,600bps, preferably 115,200bps to give enough headroom for
efficient v42bis compression.
I'm not familar with all the modems on the US market but most should
be close to the mark with factory defaults. You may have to check up
on whether compression is enabled or not. With most Rockwells the
command to check settings is AT&V. With the Courier it's ATI6.
Many ISP software installation packages come with the Shiva Dialer. The
installation package may offer to match you modem. Our Netscape
installation package doesn't know about Maestro's (we use yank software)
so we pick Hayes-compatable v34. It works fine. Our netscape package
seems to use strings that disable compression. I've found that it it
worthwhile (with us at least) to enable compression for Internet access
so I go in and modify the installation package a little, but it's not a
serious problem if you don't. (As a techo type it makes me feel better.)
In Australia you can now get a v34+ modem for $170 AUS. I believe in
the US this is around the $100 USD or lower mark. This is not a lot of
money by general PC hardware standards when you consider how much a
fast modem can extend the scope of your computing. In my opinion, any
money I've every spent on modems over the years ($220, $200
(replacement internal), $500 and $450) have provided the best return
of any hardware purchase I've made.
As regards OS interchangibility, do not get a "Winmodem" or "RPI"
(gutless wonder) modem. Just a bog-standard v34 or v34+ should be
fine. I use Maestro and USR at home and at work under DOS, OS/2,
Windows 3.11 and Win 95.
Since there is no 56K ITU standard yet I'd keep away from these too.
Cheers, Dan Bridges, Brisbug PCUG.
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