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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.

Digital spans can be configured to carry telephone calls in several formats. (Note that the RJ-45 format is the current standard in North America but that physical form factors vary depending on region and telephone service provider.)

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Telephony systems are generally mission-critical components and therefore need to be as reliable as possible. So while you can build an Asterisk server using a Start with a reasonably powerful and reliable server-class system as your platform. While not required, it's generally a good idea to use a system with redundancy features including mirrored hard disk drives (RAID 1, 5 or 10) and dual power supplies.

Cooling is also an important issue. PSTN interface cards can add to the overall heat load of the server. Failure to provide proper ventillation can cause stability issues and can lead to premature failure of critical components.

CPU and memory requirements vary depending on the application. Small systems can be built on embedded processors using only a few megabytes of memory. Large scale system that process thousands of simultaneous calls require significantly more horsepower and memory. The safest bet is to go with the most powerful CPU and the most memory that fits within your budget.

If your application calls for TDM hardware, be sure that your server includes the correct type of card slot. Cards are available in PCI and PCI Express form factors and at a variety of voltages. See the "Select Your Telephony Hardware" section for more information on the various interconnect formats.

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Download CentOS Linux 5.3

The first step is to download a copy of the CentOS 5.3 installation image. There are two supported system architectures available, a 32-bit version of Linux and can run on either 32-bit or 64-bit systems, and a 64-bit version that runs only on x86_64 hardware. If in doubt, download the 32-bit version. Most CentOS mirrors offer the distribution as either 5 CD images or a single DVD image. Either method works for the purposes of this tutorial. Use the CentOS 5 mirrors list to select a mirror. Most mirrors host the CD images and a BitTorrent seed file for downloading the DVD image. To download the DVD image use a BitTorrent client.

Download or copy the .iso image to a computer with a CD or DVD burner (writer). Keep in mind that the images are roughly 700 MB each and the DVD image is over 3 GB.

Burn The CentOS .iso To CDs or DVD

Use your CD or DVD burner software to burn the ISO image to an actual CD. Note that if you are installing on virtual machine you can generally use the ISO image without burning it to physical media.

Install CentOS

To install CentOS Linux, insert the newly burned CD in the CD or DVD drive of the target computer and boot. Be sure that your system is set to boot from the CD or DVD drive. (You can either adjust the boot order in the system BIOS or use a one-time boot menu if your system supports it.)

Full details of the CentOS installation are beyond the scope of this tutorial. Several excellent quick-start tutorials can be found at Howto Forge, including this one. For more detailed installation instructions you can also refer to the CentOS Installation Guide . In general it is safe to select the default options throughout the installation process. Be sure to select a secure root password when prompted. IP Telephony systems are a frequent target for hackers and maintaining system security is extremely important.

Note that the installation of graphical environments (Gnome, KDE, etc.) is perfectly acceptable on systems used for development or unit testing. When building production servers or systems designed for load testing it is recommended that the graphical interfaces and subsystems (frame buffers) be omitted.

Add Asterisk Yum Repositories

After the install is complete and your new CentOS Linux system is up and running, log in as the root user with the password you set during the installation. (Note that you generally want to avoid logging in as root. Create a non-privileged user account for day to day operations.)

Now, use the text editor of your choice to create a new file named "centos-asterisk.repo" in the "/etc/yum.repos.d" folder. Add the following text to the file:

[asterisk-tested]
name=CentOS-$releasever - Asterisk - Tested
baseurl=http://packages.asterisk.org/centos/$releasever/tested/$basearch/
enabled=0
gpgcheck=0
#gpgkey=http://packages.asterisk.org/RPM-GPG-KEY-Digium

[asterisk-current]
name=CentOS-$releasever - Asterisk - Current
baseurl=http://packages.asterisk.org/centos/$releasever/current/$basearch/
enabled=1
gpgcheck=0
#gpgkey=http://packages.asterisk.org/RPM-GPG-KEY-Digium

Save the new file and create another named "centos-digium.repo" and insert the following text:

[digium-tested]
name=CentOS-$releasever - Digium - Tested
baseurl=http://packages.digium.com/centos/$releasever/tested/$basearch/
enabled=0
gpgcheck=0
#gpgkey=http://packages.digium.com/RPM-GPG-KEY-Digium

[digium-current]
name=CentOS-$releasever - Digium - Current
baseurl=http://packages.digium.com/centos/$releasever/current/$basearch/
enabled=1
gpgcheck=0
#gpgkey=http://packages.digium.com/RPM-GPG-KEY-Digium

Now you should be ready to install Asterisk. To start the installation, open a terminal window and type the following:

[root@localhost~]# yum install asterisk16 asterisk16-configs asterisk16-voicemail dahdi-linux dahdi-tools libpri

The system will connect with the Asterisk and Digium yum servers, download the necessary packages for Asterisk 1.6 and install them. For a detailed view of sample output from the install see the Yum installation page.

When the installation is complete, reboot your system to activate Asterisk and DAHDi. By default DAHDi will start automatically. To enable auto-start of Asterisk, run the following command:

[root@localhost~]# chkconfig asterisk on

To manually start Asterisk you can use the following:

[root@localhost~]# service asterisk start

To stop Asterisk, execute:

[root@localhost~]# service asterisk stop

Test Your Asterisk Installation

Once the Asterisk service is started you should be able to access the Asterisk command line interface from the Linux command line as follows:

[root@localhost~]# asterisk -r

The system should respond with something similar to:

Asterisk 1.6.0.15, Copyright (C) 1999 - 2009 Digium, Inc. and others.
Created by Mark Spencer 
Asterisk comes with ABSOLUTELY NO WARRANTY; type 'core show warranty' for details.
This is free software, with components licensed under the GNU General Public
License version 2 and other licenses; you are welcome to redistribute it under
certain conditions. Type 'core show license' for details.
=========================================================================
Connected to Asterisk 1.6.0.15 currently running on localhost (pid = 3052)
Verbosity is at least 3
localhost*CLI> 

Congratulations, you now have Asterisk installed and running. To exit from the Asterisk CLI, simply type 'exit'.