2.3. Telephony Hardware

If you are going to connect Asterisk to any legacy telecommunications equipment, you will need the correct hardware . The hardware you require will be determined by what it is you want to achieve.

2.3.1. Connecting to the PSTN

Asterisk allows you to seamlessly bridge circuit-switched telecommunications networks^[*] with packet-switched data networks.^[] Because of Asterisk's open architecture (and open source code), it is ultimately possible to connect any standards-compliant interface hardware. The selection of open source telephony interface boards is currently limited, but as interest in Asterisk grows, that will rapidly change.^[] At the moment, one of the most popular and cost-effective ways to connect to the PSTN is to use the interface cards that evolved from the work of the Zapata Telephony Project (http://www.zapatatelephony.org).

^[*] Often referred to as TDM networks, due to the Time Division Multiplexing used to carry traffic through the PSTN.

^[] Popularly called VoIP networks, although Voice over IP is not the only method of transmitting voice over packet networks (Voice over Frame Relay was very popular in the late 1990s).

^[] The evolution of inexpensive, commodity-based telephony hardware is only slightly behind the telephony software revolution. New companies spring up on a weekly basis, each one bringing new and inexpensive standards-based devices into the market.

2.3.1.1. Analog interface cards

Unless you need a lot of channels (or a have lot of money to spend each month on telecommunications facilities), chances are that your PSTN interface will consist of one or more analog circuits, each of which will require a Foreign eXchange Office (FXO) port.

Digium, the company that sponsors Asterisk development, produces the most popular analog interface card for Asterisk , known as the TDM400P.^[*] The TDM400P is a four-port base card that allows for the insertion of up to four daughter cards, which deliver either FXO or Foreign eXchange Station (FXS) ports. The TDM400P can be purchased with these cards preinstalled, and Digium has designated part numbers to describe these configurations. The naming convention is TDM x y B, where x and y are numbers representing the quantity of FXS and FXO^[*] cards on the board, respectively. Check out Digium's web site (http://www.digium.com) for more information about this card.

^[*] The TDM400P is not, in fact, a TDM card at all. It is analog.

^[*] FXS and FXO refer to the opposing ends of an analog circuit. Which one you need will be determined by what you want to connect to. Chapter 7 discusses these in more detail.

An older card produced by Digium was known as the X100P. It is no longer available from Digium, but you may be able to find a clone of this card.

Another company that produces Asterisk-compatible analog cards is Voicetronix. They have three Asterisk cards in their analog lineup: OpenLine4, OpenSwitch6, and OpenSwitch12.

2.3.1.2. Digital interface cards

If you require more than 10 circuits, or require digital connectivity, chances are you're going to be in the market for a T1 or E1 card.^[] Bear in mind, though, that the monthly charges for a digital PSTN circuit vary widely. In some places, as few as five circuits can justify a digital circuit; in others, the technology may never be cost-justifiable. The more competition there is in your area, the better chance you have of finding a good deal. Be sure to shop around.

^[] T1 and E1 are digital telephony circuits. We'll discuss them further in Chapter 7.

The Zapata Telephony Project originally produced a T1 card, the Tormenta, that is the ancestor of most Asterisk-compatible T1 cards. The original Tormenta cards are now considered obsolete, but they do still work with Asterisk. Currently, the only company known to be producing these cards is Varion.

Digium makes several different digital circuit interface cards. The features on the cards are the same; the primary differences are whether they provide T1 or E1 interfaces, and how many interfaces each card provides. Although it's technically possible, the general consensus in the Asterisk community is that no more than one of these cards should be deployed in a single system.

Sangoma, who have been producing open source WAN cards for many years, have recently added Asterisk support for their T1/E1 cards.^[*] Sangoma's cards contain powerful field-programmable gate arrays (FPGAs), which make them extremely flexible. In an Asterisk environment, for example, they have been programmed to interface with the Zapata channel driver.

^[*] It should be noted that a Sangoma Frame Relay card figured prominently in the original development of Asterisk (see http://linuxdevices.com/articles/AT8678310302.html)Sangoma has a long history of supporting open source WAN interfaces with Linux.

2.3.1.3. Channel banks

A channel bank is loosely defined as a device that allows a digital circuit to be de-multiplexed into several analog circuits (and vice versa). More specifically, a channel bank lets you connect analog telephones and lines into a system across a T1 line. Figure 2-2 shows how a channel bank fits into a typical office phone system.

Figure 2-2. Channel bank

Although they can be expensive to purchase, many people feel very strongly that the only proper way to integrate analog circuits and devices into Asterisk is through a channel bank.

2.3.1.4. Other types of PSTN interfaces

Many VoIP gateways exist that can be configured to provide access to PSTN circuits. Generally speaking, these will be of most use in a smaller system (one or two lines). They can also be very complicated to configure, as the interaction between the various networks and devices requires a solid understanding of both telephony and VoIP fundamentals. For that reason, we will not discuss these devices in detail in this book. They are worth looking into, howeverpopular units are made by Sipura, Grandstream, Digium, and many other companies.

Another way to connect to the PSTN is through the use of Basic Rate Interface (BRI ) ISDN circuits . BRI is a digital telecom standard that specifies a two-channel circuit that can carry up to 144 kbps of traffic. It is very rarely used in North America and most of the rest of the world, but it's quite popular in Europe. Due to the variety of different ways this technology has been implemented, we will not be discussing BRI in very much detail in this book.

2.3.2. Connecting Exclusively to a Packet-Based Telephone Network

If you do not need to connect to the PSTN, Asterisk requires no hardware other than a server with a Network Interface Card.

However, if you are going to be providing music on hold or conferencing and you have no physical timing source , you will need the ztdummy Linux kernel module. ztdummy is a clocking mechanism designed to provide a timing source to a system where no hardware timing source exists. In Version 2.4 of the Linux kernel, to use ztdummy you must have a UHCI-type USB controller on your motherboard. In Linux 2.6, that requirement is no more.