Shaul Karl wrote:
>>
>>Have you tried the Modems section (21) of the LDP's Hardware-HOWTO?
>>
>>And a word (question?) about 56k connectivity:
>>
>> * You might be unrealistic about how much available bandwidth is on
>>your
>> modem line. Lets do the math for a typical 56k modem connection:
>>
>> 1. 56k modems = 56,000 bits per second.
>>
>> 2. You really DON'T have a 56k modem but a 52k modem per US FCC
>> limitations.
>>
>> 3. You'll almost NEVER get 52k, the best connection I used to get
>>was
>> 48k
>>
>> 4. 48,000 bits per second is 4,800 BYTES per second (8 bits to a
>>byte +
>> 2 bits for the START and STOP RS-232 serial bits)
>>
>> 5. With an MTU of 1500, you will get (3.2) packets in one second.
>>Since
>> this will involve fragmentation, you need to round DOWN to (3)
>> packets per second.
>>
>> 6. Again with MTU of 1500, thats 3.2 x 40 bytes of TCP/IP overhead
>>(8%)
>>
>> 7. So the BEST throughput you could hope for is 4.68KB/s w/o
>> compression. Compression, be it v.42bis hardware compression,
>>MNP5,
>> or MS/Stac compression can yeild impressive numbers on highly
>> compressable stuff like TEXT files, but acutally slow things
>>down
>> when transfering pre-compressed files like ZIPs, MP3s, etc.
>>
That last sentance is utter bullshit. Both modems keep their compression
tables up to date, but stop compressing as soon as the compression ratio
drops below 1:1. This means that you will only get slowdowns due to
compression if you have input that is alternating highly compressible
and lowly compressible, in a calculated way. Almost any actual transfer
will yield compression ratio of no less than 1:1.
>>
>>Source: The LDP's IP-Masquerade-HOWTO.gz.
>>
Is it just me, or did you count the MTU overhead twice? Also, are you
sure the stop and start bits are inside the counting?
In any case, what is the speed people are actually GETTING when
transferring via 56K modems? I think that is the more interesting question.
Shachar
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