path: root/README

DAHDI Telephony Interface Driver
=================================
Asterisk Development Team <asteriskteam@digium.com>
$Revision$, $Date$

DAHDI stands for Digium Asterisk Hardware Device Interface.

This package contains the kernel modules for DAHDI. For the required 
userspace tools see the package dahdi-tools.

Supported Hardware
------------------
Digital Cards
~~~~~~~~~~~~~
- wct4xxp:
  * Digium TE205P/TE207P/TE210P/TE212P: PCI dual-port T1/E1/J1
  * Digium TE405P/TE407P/TE410P/TE412P: PCI quad-port T1/E1/J1
  * Digium TE220: PCI-Express dual-port T1/E1/J1
  * Digium TE420: PCI-Express quad-port T1/E1/J1
- wcte12xp:
  * Digium TE120P: PCI single-port T1/E1/J1
  * Digium TE121: PCI-Express single-port T1/E1/J1
  * Digium TE122: PCI single-port T1/E1/J1
- wcte11xp:
  * Digium TE110P: PCI single-port T1/E1/J1
- wct1xxp: 
  * Digium T100P: PCI single-port T1
  * Digium E100P: PCI single-port E1
- wcb4xxp:
  * Digium B410: PCI quad-port BRI
- tor2: Tormenta quad-span T1/E1 card from the Zapata Telephony project


Analog Cards
~~~~~~~~~~~~
- wctdm24xxp: 
  * Digium TDM2400P/AEX2400: up to 24 analog ports
  * Digium TDM800P/AEX800: up to 8 analog ports
  * Digium TDM410P/AEX410: up to 4 analog ports
- wctdm:
  * Digium TDM400P: up to 4 analog ports
- xpp: Xorcom Astribank: a USB connected unit of up to 32 ports
  (including the digital BRI and E1/T1 modules)
- wcfxo: X100P, similar and clones. A simple single-port FXO card


Other Drivers
~~~~~~~~~~~~~
- pciradio: Zapata Telephony PCI Quad Radio Interface
- wctc4xxp: Digium hardware transcoder cards (also need dahdi_transcode)
- dahdi_dynamic_eth: TDM over Ethernet (TDMoE) driver. Requires dahdi_dynamic
- dahdi_dynamic_loc: Mirror a local span. Requires dahdi_dynamic
- dahdi_dummy: A dummy driver that only provides a DAHDI timing source.


Installation
------------
If all is well, you just need to run the following:

  make
  make install

You'll need the utilities provided in the package dahdi-tools to 
configure DAHDI devices on your system.

If using `sudo` to build/install, you may need to add /sbin to your PATH.

If you still have problems, read further.


Build Requirements
~~~~~~~~~~~~~~~~~~
gcc and friends. Generally you will need to install the package gcc.
There may be cases where you will need a specific version of gcc to build
kernel modules.

TODO: copy build requirement from Zaptel README.


Installing to a Subtree
~~~~~~~~~~~~~~~~~~~~~~~
The following may be useful when testing the package or when preparing a
package for a binary distribution (such as an rpm package) installing
onto a subtree rather than on the real system. 

  make install DESTDIR=targetdir

This can be useful for any partial install target of the above (e.g:
install-modules or install-programs).

the targetdir must be an absolute path, at least if you install the
modules. To install to a relative path you can use something like:

  make install-modules DESTDIR=$PWD/target

The 'install' target might fail if run as a user to a DESTDIR when
attempting to generate device files. In that case, try:

  make install DESTDIR=$PWD/target DYNFS=


Extra Modules
~~~~~~~~~~~~~
To build extra modules / modules directory not included in the DAHDI 
distribution, use the optional variables MODULES_EXTRA and
SUBDIRS_EXTRA:

  make MODULES_EXTRA="mod1 mod2"
  make MODULES_EXTRA="mod1 mod2" SUBDIRS_EXTRA="subdir1/ subdir1/"


Installing the B410P drivers with mISDN
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DAHDI includes the wcb4xxp driver for the B410P, however, support for the
B410P was historically provided by mISDN.  If you would like to use the mISDN
driver with the B410P, please comment out the wcb4xxp line in /etc/dahdi/modules.
This will prevent DAHDI from loading wcb4xxp which will conflict with the mISDN
driver.

To install the mISDN driver for the B410P, please see http://www.misdn.org for
more information, but the following sequence of steps is roughly equivalent to
'make b410p' from previous releases.

  wget http://www.misdn.org/downloads/releases/mISDN-1_1_8.tar.gz
  wget http://www.misdn.org/downloads/releases/mISDNuser-1_1_8.tar.gz
  tar xfz mISDN-1_1_8.tar.gz
  tar xfz mISDNuser-1_1_8.tar.gz
  pushd mISDN-1_1_8
  make install
  popd
  pushd mISDNuser-1_1_8
  make install
  popd
  /usr/sbin/misdn-init config

You will then also want to make sure /etc/init.d/misdn-init is started
automatically with either 'chkconfig --add misdn-init' or 'update-rc.d
misdn-init defaults 15 30' depending on your distribution.

NOTE:  At the time this was written, misdn-1.1.8 is not compatible the
2.6.25 kernel.  Please use a kernel version 2.6.25 or earlier.


OSLEC
~~~~~
http://www.rowetel.com/ucasterisk/oslec.html[OSLEC] is an 
Open Source Line Echo Canceller. It is currently in the staging subtree
of the mainline kernel and will hopefully be fully merged at around
version 2.6.29. The echo canceller module dahdi_echocan_oslec
provides a DAHDI echo canceller module that uses the code from OSLEC. As
OSLEC has not been accepted into mainline yet, its interface is not set
in stone and thus this driver may need to change. Thus it is not
built by default.

Luckily the structure of the dahdi-linux tree matches that of the kernel
tree. Hence you can basically copy drivers/staging/echo and place it
under driver/staging/echo . In fact, dahdi_echocan_oslec assumes that
this is where the oslec code lies. If it is elsewhere you'll need to fix
the #include line.

Thus for the moment, the simplest way to build OSLEC with dahdi is:

1. Copy the directory `drivers/staging/echo` from a recent kernel tree 
   (at least 2.6.28-rc1) to the a subdirectory with the same name in the
   dahdi-linux tree.

2. Edit drivers/dahdi/Kbuild and uncomment the two lines related to OSLEC.

After doing that, you'll see the following when building (running
'make')

  ...
  CC [M] /home/tzafrir/dahdi-linux/drivers/dahdi/dahdi_echocan_oslec.o
  CC [M] /home/tzafrir/dahdi-linux/drivers/dahdi/../staging/echo/echo.o
  ...

As this is an experimental driver, problems building and using it should 
be reported on the 
https://lists.sourceforge.net/lists/listinfo/freetel-oslec[OSLEC mailing
list].

Live Install
~~~~~~~~~~~~
In many cases you already have DAHDI installed on your system but would
like to try a different version. E.g. in order to check if the latest
version fixes a bug that your current system happens to have.

DAHDI-linux includes a script to automate the task of installing DAHDI
to a subtree and using it instead of the system copy. Module loading
through modprobe cannot be used. Thus the script pre-loads the required
modules with insmod (which requires some quesswork as for which modules
to load). It also sets PATH and other environment variables to make all
the commands do the right thing.

There is an extra mode of operation to copy all the required files to a
remote host and run things there, for those who don't like to test code
on thir build system.

Live Install: The Basics
^^^^^^^^^^^^^^^^^^^^^^^^
Basic operation is through running

  ./build_tools/live_dahdi

from the root directory of the dahdi-linux tree. Using DAHDI requires
dahdi-tools as well, and the script builds and installs dahdi-tools. By
default it assumes the tree of dahdi-tools is in the directory
'dahdi-tools' alongside the dahdi-linux tree.  If you want to checkout
the trunks from SVN, use:

  svn checkout http://svn.asterisk.org/svn/dahdi/linux/trunk dahdi-linux
  svn checkout http://svn.asterisk.org/svn/dahdi/tools/trunk dahdi-tools
  cd dahdi-linux

If the tools directory resides elsewhere, you'll need to edit
live/live.conf (see later on). The usage message of live_dahdi:

 Usage:                    equivalent of:
 live_dahdi configure      ./configure
 live_dahdi install        make install
 live_dahdi config         make config
 live_dahdi unload         /etc/init.d/dahdi stop
 live_dahdi load           /etc/init.d/dahdi start
 live_dahdi reload         /etc/init.d/dahdi restart
 live_dahdi xpp-firm       (Reset and load xpp firmware)
 live_dahdi rsync TARGET   (copy filea to /tmp/live in host TARGET)
 live_dahdi exec  COMMAND  (Run COMMAND in 'live' environment)

Normally you should run:

 ./build_tools/live_dahdi configure
 ./build_tools/live_dahdi install
 ./build_tools/live_dahdi config

to build and install everything. Up until now no real change was done.
This could actually be run by a non-root user. All files are installed
under the subdirectory live/ .

Reloading the modules (and restarting Asterisk) is done by:

 ./build_tools/live_dahdi reload

Note: this stops Asterisk, unloads the DAHDI modules, loads the DAHDI
modules from the live/ subdirectory, configures the system and re-starts
Asterisk. This *can* do damage to your system. Furthermore, the DAHDI
configuration is generated by dahdi_genconf. It can be influenced by
a genconf_parameters file. But it may or may not be what you want.

If you want to run a command in the environment of the live system, use
the command 'exec':

 ./build_tools/live_dahdi lsdahdi
 ./build_tools/live_dahdi dahdi_hardware -v

Note however:

 ./build_tools/live_dahdi dahdi_cfg -c live/etc/dahdi/system.conf

Live Install Remote
^^^^^^^^^^^^^^^^^^^
As mentioned above, live_dahdi can also copy all the live system files
to a remote system and run from there. This requires rsync installed on
both system and assumes you can connect to the remove system through
ssh.

  tzafrir@hilbert $ ./build_tools/live_dahdi rsync root@david
  root@david's password:
  <f+++++++++ live_dahdi
  cd+++++++++ live/
  <f+++++++++ live/live.conf
  cd+++++++++ live/dev/
  cd+++++++++ live/dev/dahdi/
  cd+++++++++ live/etc/
  cd+++++++++ live/etc/asterisk/
  cd+++++++++ live/etc/dahdi/
  <f+++++++++ live/etc/dahdi/genconf_parameters
  <f+++++++++ live/etc/dahdi/init.conf 
  ...

As you can see, it copies the script itselfand the whole live/
subdirectory. The target directory is /tmp/live on the target directory
(changing it should probably be simple, but I never needed that).

Then, on the remove computer:

  root@david:/tmp# ./live_dahdi reload


Configuring a Live Install
^^^^^^^^^^^^^^^^^^^^^^^^^^
The live_dahdi script reads a configuration file in 'live/live.conf' if
it exists. This file has the format of a shell script snippet:

 var1=value # a '#' sign begins a comment
 var2='value'

 # comments and empty lines are ignored
 var3="value"

The variables below can also be overriden from the environment:

 var1='value' ./build_tools/live_dahdi

===== LINUX_DIR
The relative path to the dahdi-linux tree. The default is '.' and normally
there's no reason to override it.

===== TOOLS_DIR
The relative path to the dahdi-tools tree. The default is 'dahdi-tools'.

===== XPP_SYNC
Set a syncing astribank. See xpp_sync(8). Default is 'auto'.

===== AST_SCRIPT
The command for an init.d script to start/stop Asterisk. Should accept 
'start' and 'stop' commands. Use 'true' if you want to make that a
no-op. Defaults to '/etc/init.d/asterisk' .

===== MODULES_LOAD
Manual list of modules. They will be loaded by insmod. If reside in a 
subdir, add it explicitly. Modules for the drivers that are detected on
the system will be added by the script. Default: 'dahdi
dahdi_echocan_mg2'

===== REMOVE_MODULES
A list of modules to remove when unloading. Will be resolved
recursively. The default is 'dahdi'. You may also want to have 'oslec'
or 'hpec' there if you use them.


Module Parameters
-----------------
The kernel modules can be configured through module parameters. Module
parameters can optionally be set at load time. They are normally set (if
needed) by a line in a file under /etc/modprobe.d/ or in the file
/etc/modprobe.conf.

Example line:

  options dahdi debug=1

The module parameters can normally be modified at runtime through sysfs:

  pungenday:~# cat /sys/module/dahdi/parameters/debug 
  0
  pungenday:~# echo 1 >/sys/module/dahdi/parameters/debug
  pungenday:~# cat /sys/module/dahdi/parameters/debug 
  1

Viewing and setting parameters that way is possible as of kernel 2.6 .
In kernels older than 2.6.10, the sysfs "files" for the parameters
reside directly under /sys/module/'module_name' .

Useful module parameters:

debug (most modules)::
  Sets debug mode / debug level. With most modules 'debug' can be either
  disabled (0, the default value) or enabled (any other value). 
  +
  +
  wctdm and wcte1xp print several extra debugging messages if the value
  of debug is more than 1.
  +
  +
  Some modules have "debugging flags" bits - the value of debug is a
  bitmask and several messages are printed if some bits are set:
  - dahdi_dummy:
    * 1: DEBUG_GENERAL - general error messages.
    * 2: DEBUG_TICKS - Show that the module is alive :-)
  - wctdm24xxp:
    * 1: DEBUG_CARD
    * 2: DEBUG_ECHOCAN
  - wct4xxp:
    * 1: DEBUG_MAIN
    * 2: DEBUG_DTMF
    * 4: DEBUG_REGS
    * 8: DEBUG_TSI
    * 16: DEBUG_ECHOCAN
    * 32: DEBUG_RBS
    * 64: DEBUG_FRAMER
  - xpp: See also README.Astribank:
    * 1: GENERAL - General debug comments.
    * 2: PCM - PCM-related messages. Tend to flood logs.
    * 4: LEDS - Anything related to the LEDs status control. The driver
      produces a lot of messages when the option is enabled.
    * 8: SYNC - Synchronization related messages.
    * 16: SIGNAL - DAHDI signalling related messages.
    * 32: PROC - Messages related to the procfs interface.
    * 64: REGS - Reading and writing to chip registers. Tends to flood
          logs.
    * 128: DEVICES - Device instantiation, destruction and such.
    * 256 - COMMANDS - Protocol commands. Tends to flood logs.

deftaps (dahdi)::
  The default size for the echo canceller. The number is in "taps", that
  is "samples", 1/8 ms. The default is 64 - for a tail size of 8 ms.
  +
  +
  Asterisk's chan_dahdi tends to pass its own value anyway, with a
  different default size. So normally setting this doesn't change
  anything.

To get a list of parameters supported by a module, use 

  modinfo module_name

Or, for a module you have just built:

  modinfo ./module_name.ko

For the xpp modules this will also include the description and default
value of the module. You can find a list of useful xpp module parameters
in README.Astribank .


Internals
---------
DAHDI Device Files
~~~~~~~~~~~~~~~~~~~
Userspace programs will usually interact with DAHDI through device
files under the /dev/dahdi directory (pedantically: character device files 
with major number 196) . Those device files can be generated statically
or dynamically through the udev system.

* /dev/dahdi/ctl (196:0) - a general device file for various information and
  control operations on the DAHDI channels.
* /dev/dahdi/NNN (196:NNN) - for NNN in the range 1-249. A device file for
  DAHDI channel NNN. It can be used to read data from the channel
  and write data to the channel.
* /dev/dahdi/transcode (196:250) - Used to connect to a DAHDI transcoding
  device.
* /dev/dahdi/timer (196:253) - Allows setting timers. Used anywhere?
* /dev/dahdi/channel (196:254) - Can be used to open an arbitrary DAHDI
  channel. This is an alternative to /dev/dahdi/NNN that is not limited to
  249 channels.
* /dev/dahdi/pseudo (196:255) - A timing-only device. Every time you open
  it, a new DAHDI channel is created. That DAHDI channel is "pseudo" -
  DAHDI receives no data in it, and only sends garbage data with the
  same timing as the DAHDI timing master device.


DAHDI Timing
~~~~~~~~~~~~~
A PBX system should generally have a single clock. If you are connected to a
telephony provider via a digital interface (e.g: E1, T1) you should also
typically use the provider's clock (as you get through the interface). Hence
one important job of Asterisk is to provide timing to the PBX. 

DAHDI "ticks" once per millisecond (1000 times per second). On each tick every
active DAHDI channel reads and 8 bytes of data. Asterisk also uses this for
timing, through a DAHDI pseudo channel it opens.

However, not all PBX systems are connected to a telephony provider via a T1 or
similar connection. With an analog connection you are not synced to the other
party. And some systems don't have DAHDI hardware at all.  Even a digital card
may be used for other uses or is simply not connected to a provider. DAHDI
cards are also capable of providing timing from a clock on card. Cheap x100P
clone cards are sometimes used for that purpose.

If all the above fail, you can use the module dahdi_dummy to provide timing
alone without needing any DAHDI hardware. It will work with most systems and
kernels.

You can check the DAHDI timing source with dahdi_test, which is a small
utility that is included with DAHDI. It runs in cycles. In each such cycle it
tries to read 8192 bytes, and sees how long it takes. If DAHDI is not loaded
or you don't have the device files, it will fail immediately. If you lack a
timing device it will hang forever in the first cycle. Otherwise it will just
give you in each cycle the percent of how close it was. Also try running it
with the option -v for a verbose output.

To check the clock source that is built into dahdi_dummy, you can either look
at title of its span in /proc/dahdi file for a "source:" in the description.
Or even run:

  strings dahdi.ko | grep source:


Spans and Channels
~~~~~~~~~~~~~~~~~~
DAHDI provides telephony *channels* to the userspace applications. 
Those channels are channels are incorporated into logical units called
*spans*.

With digital telephony adapters (e.g: E1 or T1), a span normally 
represents a single port. With analog telephony a span typically
represents a PCI adapter or a similar logical unit.

Both channels and spans are identified by enumerating numbers (beginning
with 1). The number of the channel is the lowest unused one when it is
generated, and ditto for spans.


PROCFS Interface: /proc/dahdi
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A simple way to get the current list of spans and channels each span contains
is the files under /proc/dahdi . /proc/dahdi is generated by DAHDI as it
loads. As each span registers to DAHDI, a file under /proc/dahdi is created
for it. The name of that file is the number of that span.

Each file has a 1-line title for the span followed by an empty line and
then a line for each channel of the span. 

The title line shows the number of the span, its name and title, and 
(potentially) the alarms in which it is.

The title shows the span number and name, followed by any alarms the
span may have: For example, here is the first span in my system (with no
alarms):

  Span 1: XBUS-00/XPD-00 "Xorcom XPD #0/0: FXS"

The channel line for each channel shows its channel number, name and the
actual signalling assigned to it through dahdi_cfg. Before being configured by
dahdi_cfg: This is DAHDI channel 2, whose name is 'XPP_FXS/0/0/1'.

           2 XPP_FXS/0/0/1

After being configured by dahdi_cfg: the signalling 'FXOLS' was added. FXS
channels have FXO signalling and vice versa:

           2 XPP_FXS/0/0/1 FXOLS

If the channel is in use (typically opened by Asterisk) then you will
see an extra '(In use)':

           2 XPP_FXS/0/0/1 FXOLS (In use)

ABI Compatibility
~~~~~~~~~~~~~~~~~
Like any other kernel code, DAHDI strives to maintain a stable interface to
userspace programs. The API of DAHDI to userspace programs, dahdi/user.h, has
remained backward-compatible for a long time and is expected to remain so in
the future. With the ABI (the bits themselves) things are slightly trickier.

DAHDI's interface to userspace is mostly ioctl(3) calls. Ioctl calls
are identified by a number that stems from various things, one of which
is the size of the data structure passed between the kernel and
userspace. 

Many of the DAHDI ioctl-s use some specific structs to pass information
between kernel and userspace. In some cases the need arose to pass a few
more data members in each call. Simply adding a new member to the struct
would have meant a new number for the ioctl, as its number depends on
the size of the data passed.

Thus we would add a new ioctl with the same base number and with the
original struct.

So suppose we had the following ioctl:
----------------------------------
struct zt_example {
	int sample;
}

#define DAHDI_EXAMPLE     _IOWR (DAHDI_CODE, 62, struct zt_example)
----------------------------------

And we want to add the field 'int onemore', we won't just add it to the
struct. We will do something that is more complex:
------------------------------------
/* The original, unchanged: */
struct zt_example_v1 {
	int sample;
}

/* The new struct: */
struct zt_example {
	int sample;
	int onemore;
}

#define DAHDI_EXAMPLE_V1  _IOWR (DAHDI_CODE, 62, struct zt_example_v1)
#define DAHDI_EXAMPLE     _IOWR (DAHDI_CODE, 62, struct zt_example)
------------------------------------
We actually have here two different ioctls: the old DAHDI_EXAMPLE would be
0xC0044A3E . DAHDI_EXAMPLE_V1 would have the same value. But the new value
of DAHDI_EXAMPLE would be 0xC0084A3E .
(TODO: fix ioctl values)

Programs built with the original dahdi/user.h (before the change) use the
original ioctl, whether or not the kernel code is actually of the newer
version. Thus in most cases there are no compatibility issues.

When can we have compatibility issues? If we have code built with the new
dahdi/user.h, but the loaded kernel code (modules) are of the older version.
Thus the userspace program will try to use the newer DAHDI_EXAMPLE (0xC0084A3E).
But the kernel code has no handler for that ioctl. The result: the error 25,
ENOTTY, which means "Inappropriate ioctl for device".

As a by-product of that method, for each interface change a new #define is
added. That definition is for the old version and thus it might appear
slightly confusing in the code, but it is useful for writing code that works
with all versions of DAHDI. 


Alarm Types
~~~~~~~~~~~
An alarm indicates that a port is not available for some reason. Thus it
is probably not a good idea to try to call out through it.


Red Alarm
^^^^^^^^^
Your T1/E1 port will go into red alarm when it cannot maintain
synchronization with the remote switch.  A red alarm typically
indicates either a physical wiring problem, loss of connectivity, or a
framing and/or line-coding mismatch with the remote switch.  When your
T1/E1 port loses sync, it will transmit a yellow alarm to the remote
switch to indicate that it's having a problem receiving signal from
the remote switch.

The easy way to remember this is that the R in red stands for "right
here" and "receive"... indicating that we're having a problem right
here receiving the signal from the remote switch.


Yellow Alarm 
^^^^^^^^^^^^
(RAI -- Remote Alarm Indication)

Your T1/E1 port will go into yellow alarm when it receives a signal
from the remote switch that the port on that remote switch is in red
alarm. This essentially means that the remote switch is not able to
maintain sync with you, or is not receiving your transmission.

The easy way to remember this is that the Y in yellow stands for
"yonder"... indicating that the remote switch (over yonder) isn't able
to see what you're sending.


Blue Alarm
^^^^^^^^^^
(AIS -- Alarm Indication Signal)

Your T1/E1 port will go into blue alarm when it receives all unframed
1s on all timeslots from the remote switch.  This is a special signal
to indicate that the remote switch is having problems with its
upstream connection.  dahdi_tool and Asterisk don't correctly indicate
a blue alarm at this time.  The easy way to remember this is that
streams are blue, so a blue alarm indicates a problem upstream from
the switch you're connected to.


Recovering from Alarm
^^^^^^^^^^^^^^^^^^^^^
TODO: explain.


Loopback
^^^^^^^^
Not really an alarm. Indicates that a span is not available, as the port 
is in either a local or remote loopback mode.


Not Open
^^^^^^^^
Something is not connected. Used by e.g. the drivers of the Astribank to
indicate a span that belongs to a device that has been disconnected 
but is still being used by userspace programs and thus can't e
destroyed.


License
-------
This package is distributed under the terms of the GNU General Public License
Version 2, except for some components which are distributed under the terms of
the GNU Lesser General Public License Version 2.1. Both licenses are included
in this directory, and each file is clearly marked as to which license applies.

If you wish to use the DAHDI drivers in an application for which the license
terms are not appropriate (e.g. a proprietary embedded system), licenses under
more flexible terms can be readily obtained through Digium, Inc. at reasonable
cost.

Known Issues
------------

KB1 does not function when echocancel > 128
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
KB1 was not designed to function at greater than 128 taps, and if configured
this way, will result in the destruction of audio.  Ideally DAHDI would return
an error when a KB1 echocanceller is configured with greater than 128 taps.


Reporting Bugs
--------------
Please report bug and patches to the Asterisk bug tracker at
http://issues.asterisk.org in the "DAHDI" category.

Links
-----
- http://asterisk.org/[] - The Asterisk PBX
- http://voip-info.org/[]
- http://voip-info.org/wiki/view/DAHDI[]
- http://docs.tzafrir.org.il/dahdi-linux/README.html[Up-to-date HTML version
  of this file]