On Jun 26 05:40, 1997, Edward Rutledge of 1:218/801 wrote:
ER> host of supplimental parts.  The newest chip sets has an EEPROM 
ER> (electrically erasable programable read only memory) built into it 
ER> so that they may indeed "download" new software into it to handle 
ER> new protocols.  This is something new and you will find it on the 
ER> modems that are marked "upgradable to X2" or "upgradable to 56k". 
ER> The older ones aren't upgradable.  There are no socketed parts, like 
ER> ROMS, so there is no way to do the upgrade. 
That's not completely true.  USR "839" series Sportsters, for example, are 
upgradable to X2.  That's accomplished with a chip swap, and even after the 
swap, the modem is not upgradable through a software upload.
In other words, not all modems that are "upgradable" to 56k speeds 
necessarily can be upgraded without a chip swap.
ER> compress the data being sent.  So lets take a chunk of data - oh -
ER> lets make it 1K worth.  The modem, under ideal conditions, can and 
ER> will compress it up to four times.  
You might be surprised to learn that there is nothing magic about "4 times".  
V.42bis compression can compress a file much more than 4 times - or much 
less, depending on the file.  The "4 times" figure comes from its compression 
ratio on a standard, specified test file.  Actual results will vary, but will 
_usually_ be considerably less than 4x with most common files.
ER> So our now 1K worth is now to 
ER> 256 bytes which is sent down the "tube".  Our receiving end will 
ER> expand that back to the 1K, but if our port is set to the same speed 
ER> as the transmit/receive speed, we can only receive the 256 and we 
ER> wind up with a data overrun and lost data.  
Nope.  Flow control prevents data overruns and lost data in a properly 
configured system.  Throughput, though, would be limited by the port speed.
Rick
--- MsgedSQ 3.30
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