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comp.windows.x Frequently Asked Questions (FAQ) 1/7
Section - 13)! What is "low-bandwidth X" (X.fast/LBX)? XRemote? PPP? SLIP? CSLIP?

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X.fast is the new name of LBX.

The one-line summary of LBX is:
	LBX  =  "XRemote"  +  reply/event/error compaction  +  caching

There are several options for using X over serial lines:

SLIP - Serial Line IP; this is both a mechanism and a protocol for sending IP
packets over point-to-point serial links. It has been around for several
years, and implementations are available for many of the major TCP/IP
implementations.  Most X Terminal vendors supply this as a checkoff item,
although nobody really ever uses it since it is horribly slow.  The TCP/IP
headers add 40 bytes per packet and the TCP/IP encoding of the X protocol is
rather verbose (rightfully so; it is optimized for packing and unpacking over
high-speed links).

CSLIP - Compressed header SLIP; this is a variant of SLIP that compresses the
40 bytes of TCP/IP headers down to about 5 or 6 bytes.  It still doesn't do
anything about reencoding the X protocol.  Modems that do compression can
help, but they increase packet latency (it takes time to dribble the
uncompressed data through typical serial interfaces, plus the compression
assembly time).

PPP - Point-to-Point Protocol; this is an emerging standard for point-to-point
links over serial lines that has a more complete set of option negotiation
than SLIP.  A growing number of people see the combination of PPP for the
serial line management and CSLIP for the header compression as becoming common
for running normal TCP/IP protocols over serial lines.  Running raw X over the
wire still needs compression somewhere to make it usable.

XRemote - this is the name of both a protocol and set of products originally
developed by NCD for squeezing the X protocol over serial lines.  In addition
to using a low level transport mechanism similar to PPP/CSLIP, XRemote removes
redundancies in the X protocol by sending deltas against previous packets and
using LZW to compress the entire data stream.  This work is done by either a
pseudo-X server or "proxy" running on the host or in a terminal server.  There
are several advantages to doing compression outside the modem:
   (1)  You don't *have* to have compressing modems in there if you wouldn't
	otherwise be using them (e.g. if you were going to be directly
	connected), and 
   (2)  It reduces the I/O overhead by cutting down on the number of bytes 
	that have to cross the serial interface, and 
   (3)  In addition to the effects of #2, it reduces the latency in delivering
	packets by not requiring the modem to buffer up the data waiting for
	blocks to compress.

LBX - Low Bandwidth X; this is an X Consortium project that is working on a
standard for this area.  It is being chaired by NCD and Xerox and is using
NCD's XRemote protocol as a stepping stone in developing the new protocol.
LBX will go beyond XRemote by adding proxy caching of commonly-used
information (e.g. connection setup data, large window properties, font
metrics, keymaps, etc.) and a more efficient encoding of the X protocol. The
hope is to have a Standard ready for public review in the first half of next
year and a sample implementation available in R6.

Additional technical information about how XRemote works and a few notes on
how LBX might be different are available via anonymous ftp from ftp.x.org in
R5contrib/ in the following files:
	XRemote-slides.ps               slides describing XRemote
	XRemote-LBX-diffs.ps            more slides describing some of LBX

[information provided by Jim Fulton, jim@ncd.com; 7/92]

There is also a set of slides on ftp.x.org from Jim Fulton's talk at the 7th
X Technical Conference.

LBX is designated as a work in progress in R6.  See workInProgress/README and
workInProgress/lbx/README in the R6 distribution for more information.

LBX was withdrawn from the X11R6.1 release until it can be finished. It is
being worked on within the X Consortium and is intended to be released as a
Consortium standard, with a Consortium implementation, as part of Broadway.
[3/96]

Note: dxpc is an X11 compressor that improves the performance of X
applications run over low-bandwidth network connections (e.g. 28.8Kb/s).  It
runs on most UNIXes.  dxpc achieves compression performance between
3:1 and 6:1 compression for a wide variety of applications.  Sources are on
ftp://sunsite.unc.edu/pub/Linux/X11/xutils/dxpc-3.3.0.tar.gz and
ftp://ftp.x.org/contrib/utilities/dxpc-3.3.0.tar.gz .  Information: Brian Pane
(bpane@bnc.bellcore.com) [9/96]

Note: X/lbX/server is the implementation of the server side of the X11R6 LBX
"protocol", totally independent from the X Window server. It is an LBX proxy
for the client side containing some performance enhancements and bug fixes.
You may use any X Consortium LBX proxy, too. The software is in beta state.
See http://www.x-software.com/Software/Xlbx/ for more information. [8/96]

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Top Document: comp.windows.x Frequently Asked Questions (FAQ) 1/7
Previous Document: 12) Just what are OPEN LOOK and Motif?
Next Document: 14)+ TOPIC: X CONSORTIUM FUTURES

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