PPP server doesnt't support 56K speed (by RH6.0)

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>>>>> "kang" == kang  <kang@kinyo.com.tw> writes:

    >> No, that won't help.  It is a hardware problem.  A 56K modem
    >> sends at 56K, but only receives at 28.8K.  :-(
    >> 
    >> This is because of analog bandwidth limitations on the phone
    >> line.  If you want to be able to act as a server and receive
    >> calls and send data to the caller at 56K, then you must be on a
    >> digital line, such as ISDN.  The USR Courrier I-modem from 3com
    >> will do the trick, but you must also have an ISDN line at the
    >> server end.  The caller can be on an analog POTS line, but the
    >> server must be on ISDN with appropriate equipment, such as the
    >> I-modemt
    kang> thanks for your reply, i doesn't really know what you means?

    kang> (caller PC) --------------------------------( Exchange Site
    kang> ) --------------------------(PPP server) PSDN ISDN You means
    kang> that? So if i want to active as a small ISP, i need to rent
    kang> ISDN for my ppp server site?

If you want to provide 56K service to your customers, yes, you need
ISDN for the lines with the modems they call in to.

A regular analog 56K modem sends data at 28.8 KBPS and receives data
at 56 KBPS.  As a server, you need to work it the other way around.
You need to send data at 56 KBPS and receive data at 28.8 KBPS.

In order for the 56 KBPS speed to be achieved, one end of the
connection can be analog, but the other end *MUST* be digital.  It has 
to do with the errors introduced into the signal by the analog to
digital conversion.  

All modern telephone exchanges operate digitally internally.  So the
analog signal from a customer's line is converted to a digital signal
at the telephone company's end of the line.  This digital signal stays 
digital as long as it is passed around thru central office switches,
over long distance lines, etc.  Only when it is at the last stage of
its journey, just before it leaves the access equipment and heads down 
the last wire to the other customer's phone line, is it converted back 
to an analog signal.

So we see that in the normal case, there are 2 conversions, an analog
to digital at one end, and a digital to analog at the other end.  Each 
of these conversions introduces distortion, frequency dependent phase
shift, and subsequent bandwidth limitations into the overall circuit.
By eliminating one of these conversions, we can up the bandwidth.  How 
do we eliminate one conversion?  We use a digital line, such as ISDN,
at one end of the circuit.

So, with analog at both ends, we can run at 28.8 KBPS in each
direction.  With digital at one end, we can run at 56 KBPS in one
direction and 28.8 KBPS in the other direction.  If we use digital at
both ends, we can run 56 KBPS in both directions.  With ISDN, we can
put the signalling channel on a seperate C-channel, and get a full 64
KBPS in each direction.  Since ISDN allows 2 64 KBPS B-channels on one 
copper twisted pair, we can run MPP on both B-channels at once and get 
128 KBPS in both directions if we are connected to another ISDN line
at the other end.

To do better than that, we need to use DSL, but that's another story. 

-- 
--------  "And there came a writing to him from Elijah"  [2Ch 21:12]  --------
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Elijah Laboratories Inc. 457 Signal Lane, Grayslake IL 60030  fax 847 543-4061
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