Introduction to Basic Asterisk Components

Asterisk is a full-featured IP PBX in software. It was primarily developed on the GNU/ Linux for x86, but it also runs on other operating systems, including BSD and Mac. With Asterisk, you have access to the source code, allowing you to port it to other operating systems or even make modifications if you have the requisite skill. Asterisk provides a rich set of features and supports multiple VoIP protocols. Asterisk provides voicemail, directory services, conferencing, Interactive Voice Response (IVR), and other features. For a full list, check out http://www.asterisk.org/features.

Asterisk is designed to be flexible and extensible, and provides a central switching core , with APIs for the addition of telephony applications, hardware interfaces, file-format handling, and codecs.

Asterisk (in other words, Digium) invented the open -source Inter-Asterisk eXchange (IAX) channel protocol (used to tunnel signaling and media between Asterisk-compliant IP PBXs). Asterisk also supports the proprietary Cisco SCCP protocol. For this book, we used SIP for the IP phones and IAX between the phones and the servers running Asterisk.

Trixbox (www.trixbox.com) provides a single CD containing Linux, Asterisk software, and everything you need to create a working Asterisk system.

IP PBX Hardware

The Asterisk software provides a function similar to the Cisco Unified CallManager or Avaya Communication Manager running on a media server. Asterisk does not require specialized hardware for Voice over IP. It only requires a server/PC running one of its supporting operating systems. In contrast to other telephony equipment providers such as Cisco and Avaya, you aren't required to use a specific vendor-provided platform to host Asterisk. Of course, you will have to select a platform that is suitable for your needs and ensure that you take the necessary steps to secure it.

Media Gateways

To provide media gateway functions and interact with legacy telephony devices, Asterisk is able to interface with a variety of equipment, including hardware distributed by Digium. Digium provides hardware such as T1/E1 interface cards, channel banks, and analog cards, allowing you to connect Asterisk to traditional analog and telephony using cards you can plug into your PC. This interface hardware is similar in function to the media gateways provided by Cisco and Avaya. Asterisk also supports the use of a variety of IP phones, which can be purchased from various vendors , as well as an increasing number of other SIP gateways and devices.

Asterisk offers the advantage that you can buy your media gateways and IP phones from different vendors. This is in contrast to Cisco, Avaya, and most of the larger telephony equipment providers, where at least for now, you must buy the majority of equipment from them, including server platforms and media gateways. Of course with Asterisk, you still have to get all the equipment to work together.

A large number of manufacturers produce hardware that is known to work with Asterisk. See http://voip- info .org/wiki/view/Asterisk+hardware.

Hard Phones/Communication Devices

Asterisk supports legacy analog phones by using interface cards with their appropriate drivers. Of course, Asterisk can also support analog devices through use of an Analog Telephone Adaptor (ATA), which converts the analog signaling/media to VoIP. Most legacy digital phones use proprietary protocols and are not supported. Standards-based IP phones and softphones, using SIP and H.323, are supported. Asterisk also supports the proprietary Cisco SCCP protocol. We used a variety of SIP phones and all worked well with Asterisk.

Communication Between IP Phones and Asterisk

Asterisk supports the most popular standards-based telephony signaling protocols, including

• H.323

• SIP (Session Initiation Protocol)

• MGCP (Media Gateway Control Protocol)

Asterisk also supports the IAX protocol, which is used to interconnect multiple instances of the Asterisk IP PBX. Asterisk also supports Cisco's proprietary SCCP.

Management Systems

Asterisk includes an API that can be used to provide management interfaces, including what Asterisk calls the Management Interface. The Trixbox version of Asterisk provides a web-based GUI that simplifies administration. With the growing popularity of Asterisk, it is likely that improved management systems will be provided by Digium and/or a third party. The following site lists several management GUIs for Asterisk: http://www.voip-info.org/wiki/view/Asterisk+GUI.

Switches and Routing

Asterisk/Digium does not manufacture their own networking infrastructure. Asterisk is compatible with popular switching infrastructure from vendors such as Cisco, Extreme, 3COM, Nortel, HP, and so on. For information on network-based attacks, including those unique to Cisco equipment, see Chapters 4, 5, 6, and 7.