[3816] in linux-net channel archive
CSU/DSU's, async RS-232, and PPP. How?
daemon@ATHENA.MIT.EDU (Kevin D. McCormick)
Tue Jul 23 02:31:55 1996
Date: Mon, 22 Jul 1996 02:56:33 -0400 (EDT)
From: "Kevin D. McCormick" <fbyte@sub-zero.mit.edu>
To: linux-net@vger.rutgers.edu
I've looked through the HOWTOs and FAQs, yet I can't find a confirmation
that what I want to do here actually works. Can someone let me know if
I'm on the right track?
I am setting up a number of dedicated 56k lines. One of the major
constraints on this project is per-line cost, so we have chosen to use
CSU/DSU's with an asynchronous interface that will hang off a standard
(well, 16550A) serial port on a 486DX/4-120 running Linux. Synchronous
interfaces (especially v.35 interfaces) cost more than we want to spend.
Maybe.
Here's the part where I'm confused: Is it possible to just shove bytes
down the RS-232 line to the CSU/DSU and have them properly reassembled at
the other end? Or do I have to apply asynchronous HDLC framing to the
bytestrean before giving it to the CSU/DSU? If no framing is necessary,
then it seems that all I have to do is set up the Linux PPP driver on the
serial port and I'm home free.
However, if framing is needed, it looks like I'd have to hack up
something in the PPP driver to do HDLC encapsulation right before data is
sent out (or received from) the serial port. I think the ISDN driver has
HDLC encapsulation, but that's for ISDN cards, not a standard serial
port. Also, this project may eventually move to Frame Relay (to save
money in the CSU/DSU department on the hub end of the 56k lines), so is
there a Frame Relay driver for Linux that can be applied to async RS-232
lines?
BUT, if there exsits somewhere a cheap, low-speed v.35 interface we could
buy, then we could use a $199 CSU/DSU that has a v.35 interface only,
saving us about $400 in the CSU/DSU department, plus we would have a real
synchronous interface. But the cheapest v.35 cards I've seen are $500
each, and handle around 10 Mbps, which is insanely excessive for 56k lines.
By the way, the CSU/DSU's we picked are made by Bat Electronics, model
5696, if I remember, for something like $398. Does anyone have any
good/bad comments about these boxes (or maybe suggestions for something
cheaper...) ?
-------------------------------------------------------------------------------
Kevin "Frostbyte" McCormick kmccorm@mit.edu - root@sub-zero.mit.edu
Massachusetts Institute of Technology You should be running Linux!
Q: How many Microsoft engineers does it take to screw in a light bulb?
A: None. They just define darkness as an industry standard.
