NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/comp/sys/sun/hardware/3890

Text File | 1992-08-15 | 1.7 KB | 38 lines

Newsgroups: comp.sys.sun.hardware
Path: sparky!uunet!zaphod.mps.ohio-state.edu!magnus.acs.ohio-state.edu!usenet.ins.cwru.edu!neoucom.edu!wtm
From: [e-mail] (Bill Mayhew)
Subject: Re: Does anybody REALLY need 115.2K baud for a modem?
Message-ID: <[e-mail]>
Keywords: modem serial RS-232
Organization: Northeastern Ohio Universities College of Medicine
References: <[e-mail]> <[e-mail]>
Date: Sun, 16 Aug 1992 03:44:30 GMT
Lines: 26

Without going into a mathematical discussion, Shannon's theorem
implies that the maximum information rate that one should expect
from an average voice grade telephone circuit is around 24,000 bits
per second.  The Telebit Worldblazer modem promises around 23,000
bits per second, half duplex.  These are uncompressed base rate
figures.  With factal algebra or something new, who knows what
we'll be able to achieve.  Even LZW compression can obtain up to
3:1 ratios on plain text, so you need more than 57.6Kbuad (a
standard interface rate) to get full use from the modem.  115Kbuad
is the next standard rate that is commonly used.

Driving the modem async is not terribly efficient at such high data
rates.  Of course, the Worldblazer also supports SDLC sync
connections.  Of course, also, sync connections are pretty alien to
the Unix world.

If one does the compression in the host, an interface good enough
to support the base rate will suffice.  Many modern modems,
however, offload the compression to a procesor in the modem
itself.


--
Bill Mayhew      NEOUCOM Computer Services Department
Rootstown, OH  44272-9995  USA    phone: [phone]
[e-mail]   (140.220.1.1)