Digium TDM400P and TDM2400P Bundles Explained



Digium is a manufacturer of analog and digital interface PCI cards based out of Huntsville, Alabama. Digium also knows a thing or two about open source telephony because it is the creator and primary developer of Asterisk – the premier open source IP PBX.

This article will go over the Digium TDM400P, the Digium TDM2400P, and the modules for these cards, what bundles you can build using these components and what functionality these bundles will provide.

The Digium TDM400P is a half length PCI card that fits into any modern computer motherboard and includes four RJ11 ports on the external face of the card.  The RJ11 ports are numbered 1 through 4 and each have a corresponding module slot on the circuit board. Each of these module slots can accept an FXS (S100M) or FXO (X100M) module for up to fourteen different combinations of FXS / FXO port configurations. Adding an FXS or FXO port to the TDM400P activates the corresponding RJ11 port and creates a 'bundle' which have model numbers of their own such as TDM11B.

From a bundle model number you can tell how many FXS and FXO ports the bundle has. The first number denotes how many FXS ports and the second number shows how many FXO ports the bundle has. For instance the TDM31B has three FXS port and one FXO ports, a TDM04B has no FXS ports and four FXO ports.
 

 
What is an FXS and FXO port?

This is probably one of the most asked questions on our support line. FXS and FXO ports are what connect a voice over IP phone system to analog telephony equipment. An FXS port connects an analog phone to the PBX and an FXO port connects the PBX to a phone company like AT&T. So if you want to use analog phones with Asterisk you would need an FXS port for each analog phone you want to use. If you want to connect Asterisk to a regular analog phone line (provided by a phone company such as AT&T) you would need an FXO port for each analog line.

Connecting to the analog world has several advantages, such as not having to replace your existing analog phones with VoIP phones and the ability to connect Asterisk to analog telephone service for back up lines or to use in conjunction with VoIP lines.

Going back to the TDM400P, since it has four module slots for FXS or FXO modules you can connect up four analog phones, four analog phone lines or any combination in between. But what if you buy a TDM11B bundle (one FXS port and one FXO port) and you decide you want to add another analog phone to your system later on? That is not a problem because you can have a total of four modules and can add up to this many at any time. All you need to do is buy the additional modules necessary, shut down the Asterisk server and remove the card. Once the card is removed you can add the additional modules in the provided module slots.

* Someone familiar with computer hardware, ESD procedures and Asterisk should perform this upgrade.

Because of its relative low, cost a TDM400P bundle is perfect for home users and small to medium sized businesses. Although only four ports are available for each TDM400P card, more than one card can be added to an Asterisk server depending on how many PCI slots the motherboard has available. So an Asterisk server using TDM400P's could have one to sixteen or more FXS or FXO ports which offers quite a bit of flexibility and scalability to the server. The one downside to TDM400P series bundles is the lack of a hardware echo cancellation option. If you are experiencing echo problems and cannot solve with the standard software techniques, the TDM400P does not have a hardware option but the next card that I will discuss does.


Digium TDM2400P

The Digium TDM2400P is a full length PCI card that also fits into any modern computer motherboard and provides up to twenty four FXS / FXO ports to an Asterisk server. It is similar to the TDM400P but has a few key differences. First, it does not have RJ11 jacks on the external face of the PCI card like the TDM400P, you must attach a cable and some form of breakout. The breakout attaches to the opposite end of the cable and is what houses the RJ11 jacks. You generally have two different options for a breakout, a surface mount or a rack mount. The surface mount option allows you to attach the breakout to a wall, desk or any available surface close to the server. The rack mount option fits into a standard 19 inch rack and is a good option if you have a rack.

The second difference is the TDM2400P accepts up to six modules and it uses different modules than the TDM400P. The TDM2400P modules are physically larger and they will activate four ports each on the breakout. This means that a TDM2400P bundle can have either four, six, eight, twelve, sixteen, twenty or twenty four ports. The model numbers for the TDM2400P modules are S400M which activates four FXS ports and the X400M activates four FXO ports.

The third major difference is the TDM2400P has an optional hardware echo cancellation module. The causes of echo problems are beyond the scope of this article but if you are experiencing echo and cannot get rid of it via standard software techniques - the echo cancellation module is probably what you need. The echo cancellation module is a black rectangular printed circuit board and can be added later if needed much like adding a new FXS or FXO module. 

TDM2400P bundle model numbers are similar to the TDM400P bundle model numbers but with one major difference. The first digit after the '24' tells you how many FXS ports are in the card and the second digit after the '24' tells you how many FXO ports are in the card. The difference is the FXS and FXO modules light up four ports per module. So to calculate how many FXS ports a bundle has multiply the FXS number by four and to calculate the number of FXO ports a bundle has, multiply the number in the FXO number by four.

The final letter in the bundle model number describes whether the bundle includes hardware echo cancellation. If the last letter is a 'B' the bundle does not include a hardware echo cancellation module, if the last letter is an 'E' the bundle includes hardware echo cancellation.

For instance the bundle module number TDM2411B, includes four FXS and four FXO ports activated and does not include hardware echo cancellation. The bundle model number TDM2422E has eight FXS ports activated and eight FXO ports activated as well as a hardware echo cancellation module.
 

Conclusion

Digium TDM cards provide a very useful service - they bridge Asterisk with the analog. And while they now have several competitors in the market place (Sangoma, Rhino and OpenVox) they are easy to get up and running with Asterisk and in the unlikely event you do have a problem there is plenty of documentation on the web for troubleshooting.

The TDM400P series of bundles is a good choice for a small office / home office set up that requires relatively few ports. They are small cards and more than one can be added to an Asterisk server given it has more available PCI slots. But if you require a higher port density, hardware echo cancellation or might need these options in the future a TDM2400P series bundle is the way to go.