Very Computer

     I was wondering what electronically happens when I tell setserial to
set my 28.8 modem to spd_vhi.
     I know I can get 115k off of my 28.8 modem but I'd like to know how
that works.  My ISP tech warned me not to do this as it has "a possiblity
in his experience to*up the hardware in my modem".  It is to his
advantage to give me the fastest speed I can get off of my hardware to his
system as well as correct advice.  So now I'm confused.
     Comments?  Al Longyear?

Thanks,
Michael

It does nothing to your modem per se. It sets up the UART (typically a
16550) to 115.2k BPS. If you have an external modem this means that
your serial card and your modem talk to each other at 115.2k. If it's
an internal modem then essentially the cable between the UART part
and the modem part disappears...

Quote:>     I know I can get 115k off of my 28.8 modem but I'd like to know how
>that works.

One word: compression. The modem/modem link can compress the data over
the 28.8 link giving a faster effective speed.

Quote:> My ISP tech warned me not to do this as it has "a possiblity
>in his experience to*up the hardware in my modem".  It is to his
>advantage to give me the fastest speed I can get off of my hardware to his
>system as well as correct advice.  So now I'm confused.

I'd sure like to hear about his experiences. I just can't see how changing
the bit rate can affect the hardware. Now of course if the hardware or
the OS can't handle the extra speed you may have some overruns but there's
no way you can get physical damage from changing the speed.

BAJ
--
Another random extraction from the mental bit stream of...
Byron A. Jeff - PhD student operating in parallel - And Using Linux!

Quote:>     I was wondering what electronically happens when I tell setserial to
>set my 28.8 modem to spd_vhi.

It changes the divisor for the bit clock in the UART.

Quote:>     I know I can get 115k off of my 28.8 modem but I'd like to know how
>that works.  My ISP tech warned me not to do this as it has "a possiblity
>in his experience to*up the hardware in my modem".  It is to his

He's full of it.  What might happen is you lose data because your
computer can't keep up with the interrupts at 115K.  But it won't
hurt your modem and you can fix lost data by lowering the speed to
something the computer can handle.

If you're not losing data or getting errors, then don't worry,
be happy.

---
 Bob Hauck                              Wasatch Communcations Group

This sounds like garbage to me.

What setserial does is tell your serial port to communicate at the
higher speed with your modem - which should be able to talk at 115200
baud anyway. All you are doing is allowing Linux to use the serail port
at the higher rate.

There should be no possibility of screwing up your modem.  28.8 modems
HAVE to be able to talk to a serial port at 115200 baud, otherwise, what
is the point?

Std. Disclaimer: I accept NO responsibility if your or anyone else's
hardware IS fried after taking my advice.  Think about it yourself.

Rogan.
--

490-7327
"All power corrupts, but we need electricity." - fortune

Hi,

:      I was wondering what electronically happens when I tell setserial to
: set my 28.8 modem to spd_vhi.

Nothing out of the ordinary. The only reason that this is required is that
originally Unix did not support speeds above 38400.  Linux does now support
at least 57600 and 115200, but the a.out C library does not.  In order to
get round this limitation, the spd_hi and spd_vhi flags were introduced.

The ELF C library does know about the higher speeds and does not require
the flags to be set separately.  If you compile an appliction (e.g. ppp)
with the ELF library and that application itself supports 115200, you
can forget about the setserial flags and just use the value 115200 on
the command line.

:      I know I can get 115k off of my 28.8 modem but I'd like to know how
: that works.  My ISP tech warned me not to do this as it has "a possiblity
: in his experience to*up the hardware in my modem".  It is to his
: advantage to give me the fastest speed I can get off of my hardware to his
: system as well as correct advice.  So now I'm confused.
:      Comments?  Al Longyear?

I would say that the tech has had some very strange experiences.  I would
be amazed if anybody would design a modem which can be damaged by talking
to it at the wrong speed.

A higher speeds, there is a greater risk of overruns (basically a character
arrives at the PC before its got round to dealing with the previous one),
but whether you are affected depends on the type of UART and the device
drivers you are using (e.g. using IDE drives will commonly cause problems
with 16450s at speeds above 38400 - there are ways round this - read the
manual page for hdparm).  Generally, I have no problems with overruns
running at 115200 with 16550 UARTs.  Note that most of this stuff about
UARTs only applies to external modems, internal modems should have less
problems in this respect.

Remember that there is only a tiny amount of benefit to be gained by
connecting your modem to your PC at 115200 if your ISP connects to his
modems at 38400.

                Dale.

--
******************************************************************************
*  Dale Shuttleworth                                                         *

******************************************************************************

: :      I was wondering what electronically happens when I tell setserial to
: : set my 28.8 modem to spd_vhi.

: Nothing out of the ordinary. The only reason that this is required is that
: originally Unix did not support speeds above 38400.  Linux does now support
: at least 57600 and 115200, but the a.out C library does not.  In order to
: get round this limitation, the spd_hi and spd_vhi flags were introduced.

: The ELF C library does know about the higher speeds and does not require
: the flags to be set separately.  If you compile an appliction (e.g. ppp)
: with the ELF library and that application itself supports 115200, you
: can forget about the setserial flags and just use the value 115200 on
: the command line.

: :      I know I can get 115k off of my 28.8 modem but I'd like to know how
: : that works.  My ISP tech warned me not to do this as it has "a possiblity
: : in his experience to*up the hardware in my modem".  It is to his
: : advantage to give me the fastest speed I can get off of my hardware to his
: : system as well as correct advice.  So now I'm confused.

You cannot use 115K or 57K even with a library that supports it since core
tools do "segfault" on these baudrates. They do some arithmetic with the baudrate
and that causes weird things. Try a "rsh" or "rlogin" when connected at 115K.

The best is to use 38K and then use setserial to make it really 115K. All tools
will work since they only see the conventional 38K.

: Remember that there is only a tiny amount of benefit to be gained by
: connecting your modem to your PC at 115200 if your ISP connects to his
: modems at 38400.
But you might gain 2-3 times the throughput if you convince your ISP to set
it up correctly.
--
-----------------------------------------------------------------------------
 Christoph Lameter            FTS Box 466, Pasadena, CA 91182
 Internet Administrator       Who is like Jesus... who is like God...?

   I'm afraid you're wrong here; I'm running a strictly a.out
system, and I can set comm speed to 115200 without having to
diddle with the spd_xx flags.  From looking at termios.h, it's
pretty obvious that it's setting a spd_xx flag, but as far as
the applications go, all they know is that B57600, B115200(*), and
B230400 are legitimate speeds.

                 ____
   david parsons \bi/ Or *zoooom* on the rare days that the net isn't
                  \/                             completely saturated.

+-----



| >:      I was wondering what electronically happens when I tell setserial to
| >: set my 28.8 modem to spd_vhi.
| >
| >Nothing out of the ordinary. The only reason that this is required is that
| >originally Unix did not support speeds above 38400.  Linux does now support
|
|    I'm afraid you're wrong here; I'm running a strictly a.out
| system, and I can set comm speed to 115200 without having to
| diddle with the spd_xx flags.  From looking at termios.h, it's
+--->8

POSIX and earlier Unix (pseudo-)standards did not specify beyond B38400 (go
back far enough and they didn't specify anything over B9600... but EXTA=19200
and EXTB=38400 were so common that they were simply renamed in POSIX).  As a
result, portable *ix programs *do not know* about speeds beyond 38400.  The
spd_hi and spd_vhi flags allow you to work around this by redefining B38400
as one of the higher speeds and then telling the program to use 38400.
--