Asterisk Application: IP PBX, Part - 1

sterisk was originally built as a PBX and today represents an astonishing 18% of global market for business telephone systems. The base feature set includes many of the most popular and powerful PBX functions.
Tapping the power of Asterisk requires some knowledge of Linux, telephony, basic script programming and IP networking. For those who would rather point and click than compile and script, Digium offers Switchvox, a complete IP PBX system based on Asterisk.

Step 1: Select Your Telephony Hardware

Asterisk applications that connect with legacy telephony systems (PBXs or the PSTN) require telephony interface hardware. Small system generally use analog or ISDN BRI connections. Larger systems (more than 12 lines) frequently use T1, E1 or J1 digital connections. If you're new to telephony, check out the Asterisk telephony by clicking the "More" link below.
Note that not every Asterisk implementation requires telephony hardware. Systems that are connected only by VoIP connections communicate using the host computer's Ethernet port.

Connection Types

Analog connections are commonly used in small businesses and homes. Each analog connection uses a single pair of copper wires. Asterisk connects with analog lines using an analog interface card that converts the voice and signaling information into Asterisk's native digital format.
ISDN BRI connections are digital telephone lines that have replaced analog lines in some places. BRI is very popular in Germany and is also common in businesses in the UK. BRI connections can carry up to two conversations at the same time and support some advanced features not available with analog connections. ISDN can use either a two-wire "U" interface or a four-wire "S/T" interface.
T1, E1 and J1 are standards for high capacity telephony connections. T1 "trunks" or "spans" are the standard in the United States. A T1 can carry as many as 24 simultaneous conversations. E1 is popular throughout much of the rest of the world. E1s are slightly higher bandwidth and can carry up to 30 simultaneous calls. J1 trunks are essentially the Japanese version of the US T1 standard.
There are several different services offered over T1, E1 and J1 connections. The most popular service type is ISDN PRI. PRI circuits use what is known as "out-of-band" signaling -- that is, one of the 24 channels (T1) or two of the 32 channels (E1) is reserved for sending call management messages.
T1/E1/J1 lines can also be used as data carriers for providing Internet or private data network services using the HDLC protocol.
For more information on both BRI and PRI forms of ISDN, check out the Wikipedia article.

Interface Hardware

Asterisk connects with analog and digital telephony connections through either a gateway card that is installed in the host computer (the computer running Asterisk) or through an external gateway device. Internal gateway cards generally connect through the computer's internal expansion bus. Cards are available in "PCI" and "PCI Express" (or PCIe) form factors. External gateways connect with Asterisk over the local area network (LAN) or the PC's USB bus.
Internal gateway cards are the most common means of connecting Asterisk to the PSTN or to a legacy telecom system. Cards typically fall into the same categories as telephony connections: analog, ISDN-BRI, and T1/E1/J1 devices. (There are a few hybrid devices on the market that support both analog and ISDN-BRI.)

Analog Cards

Analog cards are available in various capacities, ranging from a single port (which connects a single analog telephone line and thus a single telephone call) up to a maximum 24 ports.
Low density analog cards generally use the same kind of connector as most home and small business phone devices: the RJ-11 jack. Each jack on the back of the card takes a single RJ-11 phone cable. The other end of the cable plugs into a telco phone jack or an analog port on the legacy PBX.
Note in the image to the left that the card (an 8 port model made by Digium) the red blocks to the right-hand side of the card. These are daughter modules (small circuit cards that attach to the main card) that determine the function of each of the ports on the card. Analog ports can either connect to an analog line from the telephone company using a port connected to an "FXO" module, or can power and control an analog phone using an "FXS" module.
Most analog card manufacturers build analog cards with interchangeable modules. By including both FXO and FXS modules, a card can offer both FXO (line) and FXS (phone) capabilities. This makes it simple to build an Asterisk-based application that can both connect to the PSTN and control analog devices like fax machines, credit card terminals or TDDs.
High density analog cards often use an RJ-21 (or "amphenol") connector and require an opposite gender RJ-21 connection from the telco or PBX. Note the green modules on the card in the image to the left. This 24 port card has been configured with three four-port FXS modules (green) and three four-port FXO modules (red). This allows it to connect 12 analog devices and 12 analog phone lines.
FXS Ports In Analog Gateways
VoIP gateways built to connect legacy equipment (PBXs, key systems) with VoIP services generally use FXS ports. FXS ports provide dial tone and line voltage to a phone, exactly like the phone company's line does. This means that FXS ports can be connected to "line" or "trunk" ports on the legacy system and can emulate telco analog lines.
FXO Ports In Analog Gateways
Using FXO (line) ports to connect analog PBX ports with VoIP phones or remote VoIP servers is another common Asterisk gateway application. In this scenario, analog station ports on the PBX are connected to FXO ports on the Asterisk gateway card. When the PBX sends a call to the cross-connected analog station port, Asterisk forwards it as a VoIP call to the designated endpoint.
The limitation of this arrangement is the one-extension/one-port nature of PBX analog stations. The signaling capabilities of analog station ports are generally very limited. This means that the port can only respond to calls to the single extension number with which it is associated. It therefore cannot be used as a shared connection between the PBX and the Asterisk (and whatever connects with the Asterisk).

Digital Cards

Digital cards allow Asterisk to connect with T1, E1 and J1 digital lines (sometimes called "trunks"). Digital cards include one or more ports, each of which connects to an individual digital circuit. Trunks are often referred to as "spans", thus a single port card is a "single span" device, while a four port card is a "quad span" device. Most digital cards connect using RJ-45 jacks (the same kind of jack as is commonly used for Ethernet connections). Connections between the card and a telephone company T1 line are connected using a "straight-through" cable (exactly like the cables that connect Ethernet ports). Connections between the card and a local PBX or other "CPE" device require a "cross-over" cable or a straight-through cable with a cross-over adapter.

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