3.2. Installing Legacy Interface Cards
Using PCI interface cards and USB devices from Digium, VoiceTronix, Quicknet, and others, Asterisk can communicate with POTS, FXO/FXS, and T1/E1 phone lines:
Many of the examples in this book use IP telephones, which communicate with the Asterisk server using Ethernet and therefore don't need specialized interface hardware to access the PBX. A legacy interface card is not required in order to use Asterisk for VoIPin fact, VoIP can supplant all traditional telephony technologies. But since the PSTN will be here for years to come, analog trunks and phones are still important complements to VoIP. Since most phone companies don't yet offer dialtone trunks over IP, you'll need a legacy interface to connect your Asterisk server to the phone company.
3.2.1. The X100P Card
Most of the examples in this book use a single analog telephone line (POTS line) to access the telephone company. In order to use this line, your Asterisk server will need to be equipped with a Digium X100P analog trunk interface card, which provides a connector to plug in a single telephone company line.
22.214.171.124 The POTS pass-through connector
The X100P card has a second connector, a pass-through that you can connect an analog telephone to. You can't use this telephone with the Asterisk system, but when it's connected to the second interface, you can use it to tell whether the phone line connected to the Asterisk system is active with a phone company dial-tone. The setup of a basic VoIP-enabled telephony network is shown in Figure 3-2.
Figure 3-2. The basic Asterisk setup used throughout this book
126.96.36.199 Installing an X100P
To install the X100P, unplug the power from your PC, open the case, and snap the card into an empty PCI slot. Once it is installed, connect your telephone company wall jack to the primary jack of the X100Pthe one marked with an etching of a telephone company plug. Optionally, connect a standard residential-style analog phone into the secondary jack, which is marked with an etched image of a tiny telephone. (If you don't have a spare analog phone, you can skip it.) Now hook your Linux system to your Ethernet network (wired or wireless), configure its IP settings (avoid DHCP), and you're ready to install the Asterisk software.
3.2.2. The TDM400P Card
The Digium TDM400P card (Figure 3-3) is a more powerful interface card, and it is not required for any of the projects in this book.
Like the X100P, the TDM400P is a PCI card that allows connections to the PSTN through a POTS line, as well as connections to an analog telephone using a regular two-wire phone cord. Unlike its simple cousin, the TDM400P has four modular ports, so you can use up to four POTS lines and/or analog phones.
These ports accept one of the two Digium proprietary hybrid interface modules, FXS signaling for analog trunks (POTS lines) and FXO signaling for analog phones. The card can host them in any combination. This card also has a Molex power connector, like a hard drive, for supplying analog phones with the line power they're accustomed to. Don't forget to connect it.
Figure 3-3. The TDM400P interface cardTDM400P interface card
Multiple TDM400P cards can be used in the same computer simultaneously . Since they have four ports per card, they are better than the X100P for high-density applications. Due to capacity issues on the PCI bus, however, you shouldn't use more than two TDM400P cards. A T1 interface is a great alternative, though: it allows you to connect two dozen phones at a time using a device called a channel bank. More on this later.
We've chosen the X100P for connecting our VoIP server to the PSTN because it's less costly than the TDM400P. Of course, you could substitute a TDM400P card with an FXO module in place of the X100P. Asterisk distinguishes little between a TDM400P connected to a POTS line and an X100P connected to a POTS line, so either card should work fine for most of our projects.
If you decide to use the TDM400P instead of the X100P for your VoIP test server, it needs a minimum of one FXO interface module, connected to a POTS line as if it were a single-line X100P card.
3.2.3. Other Interfaces
Besides the X100P and the TDM400P, Digium offers some other specialized interface cards. Other vendors make interface devices that can be used with Asterisk, too. Table 3-1 describes some of the devices that are available, including the X100P and TDM400P.
Table 3-1. Some Asterisk-compatible VoIP interface devices
In addition to these, there are USB-based telephony devices. They can turn a USB-attached phone handset into a VoIP endpoint. These can be useful with softphones, though most serious enterprise implementers will avoid them because, like softphones, they are reliant upon the PC. For roughly the same money, you can purchase a low-end IP phone like the Grandstream Budgetone series.