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V.22, V.32, V.42, bis, etc. explained
Confused by all the terms? V.22, V.22bis, V.32, V.42, V.42bis and MNP?
These are all technical names given to communications standards as
recommended by the CCITT (another achronym!). V.22bis, pronounced,
"VEE-DOT TWENTY-TWO BISS" is the technical name for what we know as a
2400 baud dial-up modem. V.22 was the official designation for the 1200
baud modems and the "bis" suffix (bis meaning "second") was added when
the 2400 baud standard was settled upon.
CCITT is an achronym for a long French name which when given literal
translation means the International Telephone and Telegraph Consultative
Committee. The CCITT, through Study Groups which meet every few years,
promulgate standards and recommendations for standards to provide
compatible national as well as worldwide telecommunications circuits. If
it were not for these standards, it might be difficult or impossible to
communicate between differing brands of equipment. So we have it that
V.22 becomes a 1200 baud modem, V.22bis becomes a 2400 baud modem, V.32
becomes a 9600 baud modem. Any modem conforming to one of these
standards will be able to communicate with any other modem conforming to
the same standard.
V.42 is not another type of modem, but rather a new standard for
error-correction. V.42bis is a new standard for data compression. V.42
and V.42bis are actually improvements on the older MNP (Microcom
Networking Protocol) error correction and compression methods around for
the past 5 or 6 years or so.
From our personal experiences with data compression programs like ARC
and ZIP and the like, we have discovered certain types of files
compress better than do others. The same holds true for MNP level 5
and V.42bis compression.
Just as Phil Katz's ZIP utilities and Yoshi's LHarc generally provide
tighter compression than the older ARC standard, the new V.42bis
provides a considerable improvement over MNP-5.
MNP levels 2, 3, and 4 all refer to error-correction techniques built
into some of the more expensive modems. MNP levels 3 and 4, as part of
the error-correction process, strip the start and stop bits from your
data, thus making the data "appear" to transfer at a higher cps
throughput. The modem speed has not changed, but with all the start and
stop bits stripped out, there's nearly 20% **LESS** data to send so it
appears, on the surface at least, that the modem speed has increased.
The real reason the file travels faster is that there's less of it to
send... nearly 20% less! Your comm program will likely report that the
"cps" transfer rate has increased, when in actuality, the file shrank.
Of course, the modem on the other end reinserts the start and stop bits
on the fly, so the file arrives intact and the same size as when it was
sent. It's important that you understand that the modem speed *DID NOT*
increase. MNP level 4 is the fastest way to send already compressed
files such as .ZIP, .ARC, and .LZH files.
MNP level 5 introduces data compression, but here it's important that
you understand that MNP-5 *CANNOT* compress an already compressed file
and in fact will many times actually make the file bigger. This is
why you should never attempt to transfer already compressed files with
MNP-5 turned on. The magic of MNP-5 only works with text files and
data files. MNP-5 can, depending on how well the file compresses, nearly
double your effective throughput. Just remember, MNP-5 doesn't work
for transferring files which are already compressed.
V.42bis, the latest wrinkle in data compression, can effectively triple
or quadruple your effective transfer rate, but here again, *ONLY* with
files that are not already compressed. MNP level 4 still is the best
and fastest way to transfer compressed files.
On another subject, would you believe me if I told you your new 9600
or 14,400 bps modem actually communicates at only 2400 baud?
Voice-grade telephone circuits, the kind we have in our homes, have a
usable bandwidth of about 3100 Hz (from roughly 300Hz to 3400Hz). If you
have ever played a musical instrument, this range translates to E-flat
above middle-C on the low end to about A-sharp 3 octaves higher. In
other words, not a very broad range.
In theory, in a bandwidth of 3100 Hz we can change the state of the
signal on the line 3100 times per second, and still have it be passed
recognizably through the various analog-to-digital converters, filters,
amplifiers, and other pieces of equipment in the phone system that are
designed to carry voice signals. In reality, due to the need for
shaping of waveforms and dealing with line impairments (like delay and
attenuation distortion at the outer reaches of the band), only about
2800Hz of the band is really usable. A 9600 baud V.32 modem works
nicely in this range (it operates between 600Hz and 3000Hz).
The standards community doesn't like the term "baud" any more, since it
is so widely misused. The preferred term today is "symbols per second",
indicating the number of discrete signal elements that can be
transferred on the phone line per second. By varying the amplitude and
phase of the signal, one signal element can indicate several bits. This
is called Trellis-coded modulation, or TCM.
For example, V.22bis (2400 baud) operates at 600 symbols per second, and
uses 16 different combinations of phase and amplitude. This means
that 4 bits can be indicated per symbol, for a total of 2400 bits per
second. V.32 (9600 baud) operates at 2400 symbols per second, and
supports 32 different combinations of phase and amplitude. It actually
uses five bits per symbol, but one of the bits is a redundant encoding
of the others to provide the receiver with more information to properly
decipher the received signal. Since only four of the bits contain user
data, this means V.32 can send a maximum of 9600bps.
V.32bis also sends 2400 symbols per second, and to acheive 14,400bps
uses 128 different combinations of phase and amplitude for each symbol.
While 14,400bps is only 50% faster than 9600bps, our modulation scheme
has to be 4 times more complex. Obviously, the more complex the modem
becomes, the greater the cost.
The Courier HST uses 14,400bps modulation in one direction (2400 symbols
per second, 128 possible states per symbol, similar to V.32bis), but
only has a 450bps carrier in the reverse direction (150 symbols per
second, 8 possible states giving 3 bits per symbol).
V.32bis is in final draft stage at this time. It is backward compatible
with V.32. In addition to V.32's speeds of 4800 and 9600, V.32bis also
supports 7200, 12000, and 14400bps, *FULL-DUPLEX*, async or sync. It also
supports a "Rapid Rate Renegotiation" feature, which allows the modems
to quickly modify the data rate when necessary to respond to changing
line conditions (under 100 milliseconds, rather than the 5-10 seconds it
takes V.32 to change speeds and the 250 milliseconds it takes the USR
HST to change speeds.)
V.32bis was technically agreed to by experts in Study Group 17 at the
April, 1990, CCITT meeting in Geneva. At the next meeting, in October,
it will be submitted to the CCITT's "accelerated approval procedure".
In order to become a standard under this procedure, it must receive 100%
approval of the countries attending the meeting. It is then translated
into the official CCITT languages, and sent by mail to all of the
countries in the United Nations. They have three months to return their
final vote; of those returning a ballot, 70% must vote to approve the
standard for it to be accepted. Therefore, the earliest that V.32bis
will be an "official" CCITT Recommendation is around the end of
February, 1991.
However, we could start seeing V.32bis (14,400 baud full duplex) modems
even before the official blessing of the CCITT is received. Since the
actual technical standards have already been agreed upon, nothing
prohibits modem manufacturers from going ahead with development.
Indeed, US Robotics and Hayes have both admitted that they have
prototypes already undergoing alpha testing. Just when we'll be likely
to see the new products on the store shelves and at what price is
anyone's guess. USR has even hinted that current production models of
their expensive V.32 and HST/Dual Standard modems are already capable of
being upgraded to V.32bis, although upgrade costs are likely to be
considerable and may require that the user send their modem to USR
for the upgrade.
Do I really need the 16550 UART chip in my serial port? Maybe and maybe
not, but it can't hurt.
There are some circumstances which you should consider: if you are
multitasking, then you definitely need the buffering capabilities of the
NS16550AFN chip. For example, suppose you want to run MS-Windows with
one copy of PCBoard running in each of two windows. This means that the
CPU now has only half the time that it used to have to respond to
interrupts. The NS16550AFN chip helps alleviate this burden by
providing a larger HARDWARE serial buffer. If the serial buffer of your
8250 or 14550 is overrun, this means that blocks will have to be
retransmitted thus slowing down your effective transfer rate. However,
this does not automatically mean that file transfer rates will be
accelerated beyond their normal speed. You are simply increasing the
ability of your CPU to handle other tasks successfully while serial
activity is taking place. These activities include writing to the
screen, writing to the disk, handling timer interrupts, handling other
DOS windows, etc.
This can get to be an almost religous issue nowadays! You can't really
go wrong in putting a 16550 chip in but the question is whether you'll
really see any performance improvement. In most cases if you're working
with a dedicated machine you shouldn't have any problems with an 8250 or
a 16450. If you run under a network or a multitasker or if you have a
fetish for TSR's, the NS16550AFN UART is strongly recommended.
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