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=========[ PC-OHIO Main Bulletin 14 - 05-19-91 ]===========================
WHAT IS THE BENEFIT OF MNP?
By David Terry, Salt Air BBS
There are three benefits and I'll discuss both MNP -and- v.42 since they
have the same benefits and the same reasons for being:
1) MNP (or v.42) provides you with an ERROR CORRECTED session between
your modem and the modem at the other end of the phone line.
If you have ever logged onto a system and found that you could barely
read or write messages due to all of the line noise .. then you can
appreciate the difference between a "clean line" and a "noisy line".
When both modems have MNP (or v.42) then they are capable of
filtering out the line noise. BUT, make no mistake about it - the
line noise may STILL be there .. it just does not get printed on
your screen nor the host screen because the modems have filtered it
out.
This "filtering process" is similar to the error correction protocols
such as Xmodem or Ymodem. They send a block of data and a CRC
together and if the receiving modem finds a different CRC value then
the two modems resend the data until it is corrected. So, in the
same manner that a file transfered with Ymodem is pretty much
guaranteed to be "correct" after it arrives (even though line noise
may have caused several re-sends of the data) the same is true of
data that you see on your screen when using error correcting modems.
2) The second benefit of MNP (or v.42) is that while it is creating
data packets for the "error correction protocol" it is able to
reduce the size of the data by stripping out start and stop bits.
For instance, a normal character takes up 8 bits plus 1 start bit and
1 stop bit for a total of 10 bits. On that basis you can figure that
a 2400 bits per second modem will give you a maximum throughput of
240 characters per second (because each character is 10 bits long).
The MNP (or v.42) protocol can strip the start and stop bits which
subtracts 20% of the data and gives you a 20% increase in speed
(minus a few percentage points for the protocol overhead).
Therefore, without even compressing the data you can expect to see
as much as 270 characters per second on a 2400 BPS line (versus the
"norm" of about 235 cps on the same line).
3) The third benefit of MNP (or v.42) is DATA COMPRESSION.
In the BBS world you are probably aware of files that are ARC'ed or
ZIP'ed. The reason for using ARC or ZIP is to decrease the size of
the file before storing it on disk - and then uncompress the file
when you want to use it. This saves disk storage. When performing
file transfers it also saves time!
The data compression capabilities of MNP and v.42 are not nearly as
good as either ARC or ZIP. But on straight ASCII text they are still
capable of decreasing the data to about 50% of its size. Decreasing
by 50% means that you can DOUBLE the throughput on the line so that a
2400 bps modem can effectively transmit 480 cps (the speed of a 4800
bps modem!).
Now the drawbacks...
1) You only get the benefits of MNP (or v.42) if the modem at the OTHER
END also has MNP (or v.42) built into it.
2) Data Compression between modems is only effective if the data being
transferred is NOT ALREADY COMPRESSED. This means that you can
expect to see fast transfers on ascii text files - but transferring
a file that is already compressed (such as an ARC or ZIP file) will
actually be SLOWER than if the modems did not perform any data
compression.
Unfortunately, in the BBS world compressed data is more common than
non-compressed data. Sure, you'll be able to read messages faster
(if you can move your eyes that fast!) and you can download bulletins
and other non-compressed data faster. But downloads of most files on
BBS's will actually be slower.
Fortunately, you can usually tell your modem to turn data compression
off (prior to making the phone call) so as not to slow down your
file transfers.
HIGH SPEED MODEMS
By David Terry, Salt Air BBS
The following text basically discusses the US Robotics HST 9600 bps modems
and the Hayes V-Series 9600 bps modems. It also covers the subject of V.32
modems.
The old USR HST had a top transmission speed of 9600 bps. This is before
taking into account any kind of MNP compression. Typical throughputs with
the old HST ranged from 1150 cps on a compressed file with the
modem-compression-DISABLED to 1900 cps on a regular text file with
modem-compression-ENABLED.
The HST will only transmit at 9600 bps when connected to another HST but
will connect at 300/1200/2400 baud to other standard modems.
The new USR HST (termed the 1440) is able to transmit data at 14400 bps
(again, this is before taking into account MNP compression, etc). Typical
throughputs with the new HST will range from about 1500-1700 cps on a
compressed file with modem-compression-DISABLED to about 2300-2400 cps on a
text file with modem-compression-ENABLED -- this is assuming that you've
opened your comm port at 38400 bps.
The HST will only transmit at 9600 bps when connected to another HST but
will connect at 300/1200/2400 baud to other standard modems.
The Hayes V-Series 9600 modems are similar to the old USR HST described in
#1 above. You will typically see throughputs as high as 1900 cps on text
files but only about 960 cps on compressed files.
The Hayes V-Series 9600 will only transmit at 9600 bps when connected to
another V-Series 9600 modem but will connect at 300/1200/2400 baud to other
standard modems.
Hayes has recently begun shipping its V-Series modems with new ROM chips in
them giving them v.42 compatibility. This means that the V-Series 9600
modems can now provide an error-corrected session when connected to any
regular MNP modems at 2400 bps. This is because v.42 implements MNP levels
1 through 4 (which excludes MNP compression). You will typically see
throughputs of about 260-280 cps on a 2400 bps line due to MNP's stripping
of the start and stop bits.
The v.32 modems (such as those made by US Robotics and MultiTech) run at
9600 bps and will give you similar throughputs to those described in #1
above (ie, v.32 will give you slower transmission speeds than will the new
HST's running at 14400 described in #2). However, the advantages of v.32
are that it provides you with better "interactive response times" (such as
when typing) and that because v.32 is a CCITT "standard" they will connect
at 9600 bps to modems made by OTHER manufacturers. By "other" I mean that
you can connect US Robotics v.32's to MultiTech v.32's to any other v.32's.
The v.32 standard appears to be one that remain for some time to come so
purchasing a v.32 modem may be a better investment if you are concerned
about future compatibility. However, v.32 still costs more than the
proprietary standards such as the HST 9600 or the V-Series 9600.
The USR Dual Standard is BOTH a v.32 and an HST modem. When it is in the
"HST mode" everything said in #2 above (about the new 1440 HST's) is true.
When it is in "v.32 mode" then everything said in #5 (about v.32 modems) is
true. In other words in v.32 mode you will not get the full speed
advantage of the Dual Standard for file transfers. However, one BIG
advantage to the Dual Standard is that it is compatible not only with the
v.32 standard but with all of the existing HST modems as well.
V.32, V.32BIS, V.42, V.42BIS .. IT'S ENOUGH TO DRIVE YOU CRAZY!
By David Terry, Salt Air BBS
With so many v. (pronounced VEE DOT) standards you can get confused just
thinking about them and even more confused if you try to pronounced them
all in the same sentence. I'm going to attempt to explain the standards
below in hopes of clearing some of the air surrounding these topics.
WHAT IS THE V.32 STANDARD?
The v.32 standard is a "modulation" standard. I like to compare it to the
AM and FM standards used in radio broadcasting. Not only are they at
different frequencies but they use different modulation techniques. There
are different modulation standards for 300, 1200 and 2400 baud. The v.32
standard is a full duplex (data going both ways simultaneously at the rated
speed) standard for 4800 and 9600 bps connections.
WHAT ABOUT V.32BIS?
The "bis" means simply that it is an enhanced v.32 standard. Modems
incorporating v.32bis are capable of transmitting data not only at the 4800
and 9600 bps standards but also at the higher 12000 and 14400 bps
standards. Again, like the slower v.32 speeds the data can move in both
directions simultaneously at the rated speed.
SO WHAT IS V.42 ALL ABOUT?
The v.42 standard is an error correction standard. It is a method by which
data is packetized and sent between modems to ensure that the data that
arrives at the receiving end is the same as what was transmitted.
MNP is another error correction standard. In fact, the v.42 standard
includes MNP as an "alternate" method in case a modem is not v.42 compliant
.. in other words v.42 modems can connect with MNP modems and achieve a
"reliable" connection.
THEN V.42BIS IS AN ENHANCED V.42?
Yes. The v.42bis standard adds a high level of data compression to the
error correcting facilities built into the v.42 standard. For example, an
ascii text file that is 100K in length could, while being transmitted, be
compressed down to just 25K making a 4:1 reduction in size. On the
receiving side the transmitted data is expanded out to the original 100K
size. In effect, the transfer rate of the modem can be double, triple or
even quadruple the rated speed of the connection by compressing the data
before transmitting it and uncompressing it on the other end.
It should be noted that this type of data compression, while very useful
for downloading ascii files, bulletins, messages, etc, is useless when
downloading ZIP or ARC files which are already compressed.
WHAT ABOUT V.32 AND V.42 TOGETHER?
A commonly asked question is if v.32 modems will work with v.42 and the
answer is yes. If you asked the question "can I transmit ENGLISH over an
FM RADIO FREQUENCY and have the listeners understand" the answer would be
the same and for virtually the same reasons (here we're likening the v.42
method of packetizing data to ENGLISH and the v.32 method of modulation to
FM RADIO TRANSMISSIONS).
The v.42 and v.32 standards are for two completely different (but
complimentary) areas of communication. In fact, you'll most likely
discover that every v.32 modem you find has v.42, MNP or some other kind of
error correction control built into it.
So, combined together, a modem with v.32bis, with its higher transmission
speeds, and v.42bis, with its greater compression ability, can be very fast
when transmitting and receiving data.
For example, a v.32 modem's raw throughput at 9600 bps would deliver 960
cps. Adding v.42 brings you up to 1150 cps and if the data is compressible
then with the compression capability of v.42bis you could see even greater
speeds (2 to 4 times greater depending on the data being compressed).
A v.32bis modem's raw throughput of 14400 bps delivers 1440 cps. Adding
v.42 brings you to 1700 cps and with data compression, again, it is
possible to achieve 2 to 4 times that speed depending on the
compressibility of the data being transferred.