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Tips and trouble-shooting hintsSpeed tipsBasic tips on how to improve your speed
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at&f OK atdt123456789 CONNECT 33600/ARQ ~ÿ}#À!}!}!} }9}"}&} }*} } }#}%Â# }%}&} }<^3}'}"}(}"L}^~~ÿ}#À!}!}"} }9}"}&} }*} } }#}%Â#€}%}&} }<^3} '}"}(}"}³~ OK ati6 U.S. Robotics 56K Message Link Diagnostics... Chars sent 3 Chars Received 219 Chars lost 0 Octets sent 3 Octets Received 146 Blocks sent 3 Blocks Received 7 Blocks resent 0 Retrains Requested 0 Retrains Granted 0 Line Reversals 0 Blers 0 Link Timeouts 0 Link Naks 0 Data Compression V42BIS 2048/32 Equalisation Long Fallback Enabled Protocol LAPM Speed 33600/33600 Current Call 00:00:10 Online OK ath OK |
The strange characters above
are associated with setting up protocols, and the
three pluses won't be echoed back to you. Instead
of hanging-up using the ath command, you
can re-enter ati6 repeatedly, although
your ISP's modem may disconnect you after a
couple of minutes.
If all goes well (and your modem supports these commands) your screen should look something like this:
CONNECT 33600 OK at%l 018 OK at%q 044 OK |
The line level reading is in -dBm,
where 018 is -18dBm. If you have a short line you
should expect to get between -13dBm and -16dBm.
Longer lines will give readings between -17dBm
and -60dBm, although most modems quote -43dBm as
the minimum required level. A level of -30dBm may
sustain a 28.8Kbps connection, but, 14.4Kbps
could be as high as possible from a -35dBm
reading.
The line quality reading represents the number of
re-transmissions the modem has performed over a
specific period of time. The lower this figure is
the better, although at speeds above 9600bps you
probably won't get zero, even on a good line,
since the number of transmissions goes up as the
speed goes up. I normally measure between 40 and
60 on a known good line at 33,600bps. If the
quality figure goes too high (99 on some modems)
the modem retrains and may re-connect at a lower
speed which should have a better quality figure.
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You can download 10 second samples of a normal connection and a connection with a retrain. Both Microsoft ADPCM WAV files are 55KBytes, and the expected download time for each file is less than 20 seconds at 28,800bps. |
CRC Errors
CRC errors, as recorded in the Point-to-Point Protocol log file PPPLOG.TXT, are errors found in the PPP packets. Put simply, as you download data, the data is broken-up into blocks of data, up to 1500 bytes in size. A mathematical calculation is applied to the data in a block and a checksum is derived. This checksum is sent along with the data in the PPP packet. At the receiving end the same calculation is applied to the block and compared to the checksum. If the checksums don't match then an error is recorded and the packet must be resent.This method of data verification is said to be 99.95% reliable.
Ideally, your PPPLOG.TXT file should not record any CRC errors. If the line is noisy, but error control is used by the modems, all packets should eventually be delivered intact. Alternatively, if error control is not being used by the modems, occurrences of line noise will cause CRC errors which will be recorded in the PPPLOG.TXT.
Overrun errors, which are also recorded in the PPPLOG.TXT file, will also be recorded as CRC errors since the CRC checksums from incomplete packets will not match.
I recently had a problem that I only
experienced with Internet Explorer while downloading JPG
graphic files. An apparent pause during downloads seemed
to coincide with CRC errors recorded in the Point-to-Point
Protocol log file. Click here
for more details.
ISP Bandwidth
The bandwidth that your Internet Service Provider
offers can greatly affect your speed. An ISP may have one
or more T1 (1.544Mbps) or E1 (2.048Mbps) to the back-bone
of the Internet, but this is of little use if they only
feed your POP (Point-of-Presence) with a 64Kbps data line
and you are not the only user on-line. When you are
connected to your POP you ultimately share bandwidth with
all other users connected to the POP a that time. So,
even if two users' modems connect to their ISP's modems
at 56Kbps, if the POP is served by a 64Kbps data line,
they both share this 64K bandwidth. If dialling directly
into the server, rather than going through a POP, or
going through the POP at quiet times of the day, is much
faster there may be not enough bandwidth to that POP.
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