A $10 Linux Answering Machine

A Linux telephone answering machine

Architecture of a Linux Telephone Interface
Install Drivers and Libraries
Select and Install the Softmodem
A Simple Answering Machine
Next Steps ...




This article describes how to build a Linux based telephone answering machine using a low cost winmodem (softmodem) for the telephone interface. We describe how to install the drivers and libraries, and how to select and install the proper modem card. Our answering machine program is a couple of hundred lines of C code in a single file. If you've ever used a telephone you should have no trouble understanding the code.

The requirements for the project are:
    - An Intel 537 based modem (softmodem)
    - A PCI slot that does not share interrupts

Intel 537 based modems are readily available for less than ten dollars. You can probably find one on the web by searching on the terms "price" and "intel 537". The driver we are going to use requires the Intel chip, so be sure you have the right modem before starting.



Architecture of a Linux Telephone Interface

Architecture of a Linux Telephone Application Our answering machine uses drivers and libraries which are part of the Asterisk open-source PBX. Asterisk can work as an answering machine but Asterisk is huge and has a steep learning curve for installation and configuration. The uncompressed Asterisk package is over 16 megabytes, well in excess of our 5 kilobyte source file. The overall architecture of our answering machine is shown in the attached diagram.

The driver for the modem card is the wcfxo module. This interfaces to a higher level driver called "zaptel". The division of the driver into two pieces helps the Asterisk developers minimize how much code they need to write for new types of PC telephony interface cards. The zaptel driver delivers a 64 Kbps stream of mu-law samples to the user space application. Zaptel also has IO control routines (ioctl's) to go on-hook, off-hook, and to indicate that the phone line is ringing.

The Zapata library (libzap) processes the 64 Kbps stream of data to extract caller ID and DTMF signals. The competence of libzap is why our answering machine application is so simple.

Our answering machine is pretty minimal as telephony applications go. It extracts and displays to standard out the caller ID information for incoming calls. If the phone rings more than four times, it answers the phone and plays a greeting. The greeting message asks the caller to press one to leave a message. Having the caller press a button eliminates messages from so called "bulk dialers". Voice mail is left as mu-law encoded data in a file with the date and time encoded into the file name.



Install Drivers and Libraries

Our answering machine requires the zaptel drivers and the Zapata library, both of which are available from the Asterisk web site.


Installing the Drivers

Two drivers, wcfxo and zaptel, are required. Both are in the zaptel package. These drivers are in the older 1.0.9 release which is archieved here, but you should probably get the latest package from the Downloads section on the Asterisk home page or from Digium which hosts the download site.


Untar the file, do a 'make linux26' and a 'make install' (as root). If you are using udev, be sure to follow the directions in README.udev. Specifically, you need to add the following lines to your /etc/udev/rules.d/50-udev.rules file:

   KERNEL="zapctl",     NAME="zap/ctl"
   KERNEL="zaptimer",   NAME="zap/timer"
   KERNEL="zapchannel", NAME="zap/channel"
   KERNEL="zappseudo",  NAME="zap/pseudo"
   KERNEL="zap[0-9]*",  NAME="zap/%n"

I run the answering machine as myself so I've added the following line to the udev permissions file (/etc/udev/permissions.d/00-udev.permissions).

The final piece of zaptel configuration is to tell it that we have one interface to the telephone "central office". Edit /etc/zaptel.conf and add the following line to the end.

If you don't live in the US, you should should make one other change to zaptel.conf. You need to uncomment the line with your country code and make that country code the defaultzone.


Installing the Zapata Library

The Zapata library is no longer used by Asterisk, so it has been relegated to the 'old' directory on the download site. An archieved version of the 1.0.1 release is available here but other, later versions may be on the Digium FTP site.


Untar the file, do a 'make' and a 'make install' (as root). The Zapata library does not need any configuration.



Select and Install the Intel Modem

As mentioned above, you need to be sure to get an Intel 537 compatible modem. These modems should be easy to find and should be relatively inexpensive.

To call it a "modem" is not quite correct. It is really more of a "phone line interface". Its 64 Kbps stream is 8000 bytes per second, which the wcfxo/zaptel driver delivers in 8 byte chunks. This means a thousand interrupts per second! It is the interrupt load that makes installing the modem card a little tricky. You need to install it in a PCI slot with an interrupt that is not shared by any other device. Check the documentation for your motherboard to find which interrupts are used for which slots and how those interrupts are used by the internal peripherals. If you are very unlucky, you may find that the interrupt for every slot on your motherboard is shared and used. You can try disabling internal peripherals but wcfxo just won't work on some motherboards.

The other, less common problem is that the wcfxo driver does not recognize your modem. To fix this you need to edit the wcfxo_pci_tbl table near the bottom of wcfxo.c in the zaptel build directory. Use 'lspci -nv' to get the needed vendor ID, product ID, and subsystem ID. The following vendor, product, and subsystem IDs are already recognized by wcfxo:
    - e159:0001 8085
    - e159:0001 8086
    - e159:0001 8087


System Check-out

If you've completed the hardware and software installation, you can verify your system with a few commands.

Install the modules and examine the resulting syslog output.

   #modprobe zaptel
   #modprobe wcfxo
   #tail /var/log/messages
   Sep 18 23:30:41 kernel: Zapata Telephony Interface Registered on major 196
   Sep 18 23:30:51 kernel: PCI: Found IRQ 5 for device 0000:00:0b.0
   Sep 18 23:30:51 kernel: PCI: Sharing IRQ 5 with 0000:00:07.5
   Sep 18 23:30:51 kernel: wcfxo: DAA mode is 'FCC'
   Sep 18 23:30:51 kernel: Found a Wildcard FXO: Generic Clone
   Sep 18 23:30:51 kernel: Registered tone zone 0 (United States / North America)
Don't worry about the "Sharing IRQ 5" log message. This reports that the IRQ can be shared but does not imply that another Linux device is using it.

An lsmod should show the following three modules.

   # lsmod
   Module                  Size  Used by
   wcfxo                  10848  0 
   zaptel                185572  3 wcfxo
   crc-ccitt               1664  1 zaptel

Verify that the wcfxo driver is generating a thousand interrupts per second and that it is not shared with any other devices.

cat /proc/interrupts; sleep 10; cat /proc/interrupts

The wcfxo entry in /proc/interrupts should be on a line by itself, and the interrupt count should have gone up by ten thousand during the above 10 second sleep. If wcfxo is not on a line by itself, you need to move the modem to another PCI slot. Specifically, you want something like this:

     5:   3003856        XT-PIC  wcfxo
not something like this:
     9:         0        XT-PIC  uhci_hcd, uhci_hcd, wcfxo

The zaptel driver includes a utility to view the driver configuration. Use it to verify that it has one interface. If this fails, check the fxsks=1 line in the configuration file.

   # ztcfg -v
   Zaptel Configuration
   1 channels configured.



A Simple Answering Machine

Our answering machine displays caller ID information to standard out and saves voice mail in files with the date and time encoded into the file name.


Installation and Use of the Answering Machine

The code for the answering machine is in a single C file which is available here. Compile the program with the command:
   gcc -lzap -o answering_machine answering_machine.c
Run the program with the command:

The program expects to find an 8 KHz mu-law encoded greeting message called "leave_a_msg.ul" in the working directory. You can record the greeting using any tool you like and then convert the WAV file to mu-law using sox. The command is:

   sox leave_a_msg.wav leave_a_msg.ul

Sox interprets files with the .ul extension as mu-law encoded audio at 8 KHz. You can hear your voice mail using the play command. For example:

   play 2005_09_22_13_30_22.ul


Code Review of the Answering Machine

The hard work in the answering machine is done by the Zapata library. It does the caller ID, DTMF tone detection, and reads and writes mu-law audio files. Here is a skeleton of the code in the answering machine.

  zp = zap_open("/dev/zap/1", 0);

  while (1) {
    /* Wait for a ring and get caller ID info */
    zap_clid(zp, cidnumber, cidname);
    { Display caller ID to user.  Do logs or black list checking here. }

    /* Wait for an additional RINGS-1 rings before answering */
    zap_waitcall(zp, (RINGS - 1), ZAP_OFFHOOK, TM_OUT/1000);

    /* Tell Zapata that we want DTMF digits */
    zap_digitmode(zp, ZAP_DTMF) && zap_clrdtmf(zp);

    /* Play our outgoing message.  Abort on DTMF or hang-up. */
    zap_playf(zp, "leave_a_msg.ul", ZAP_DTMFINT | ZAP_HOOKEXIT);
    { Continue if caller hung up }

    /* Wait up to TM_OUT ms for the caller to enter a DTMF digit */
    zap_getdtmf(zp, 1, (char *)0, 0, 0, TM_OUT, ZAP_HOOKEXIT);
    { Hang-up/continue if time-out and no DTMF digit. }

    /* Get the DTMF digit the caller entered.  Hang-up if wrong digit */
    pDigits = zap_dtmfbuf(zp);
    { Hang-up/continue if caller entered wrong digit }

    /* Save voice mail to 'date_time' file. */
    zap_recf(zp, date_time, 0, ZAP_BEEPTONE | ZAP_SILENCEINT);

    /* Done with message.  Hang-up and wait for next call. */
    zap_sethook(zp, ZAP_ONHOOK);

The above code should give you a feel for how to write telephony applications using the Zapata library. The library also has routines that you can use to make outgoing calls and to help make conference calls.



Next Steps ...

For a simple answering machine, this program works surprisingly well. Its simplicity is its best feature.

If we were to add features to the answering machine, we would probably add:

The Zapata library uses blocking IO, making it a little difficult to use a select() loop to manage a user interface while waiting for incoming calls. Perhaps a threaded application could step around this problem.