Asterisk: The Definitive Guide, 4th Edition

Chapter 14. Device States

Out of clutter, find simplicity.

—Albert Einstein

It is often useful to be able to determine the state of the devices that are attached to a telephone system. For example, a receptionist might require the ability to see the statuses of everyone in the office in order to determine whether somebody can take a phone call. Asterisk itself needs this same information. As another example, if you were building a call queue, as discussed in Chapter 13, Asterisk needs to know when an agent is available so that another call can be delivered. This chapter discusses device state concepts in Asterisk, as well as how devices and applications use and access this information.

Device States

There are two types of devices that device states refer to: real devices and virtual devices. Real devices are telephony endpoints that can make or receive calls, such as SIP phones. Virtual devices include things that are inside Asterisk but provide useful state information. Table 14-1 lists the available virtual devices in Asterisk.

Table 14-1. Virtual devices in Asterisk

Virtual device	Description
MeetMe:<conference bridge>	The state of a MeetMe conference bridge. The state will reflect whether or not the conference bridge currently has participants called in. More information on using MeetMe() for call conferencing can be found in Conferencing with MeetMe().
SLA:<shared line>	Shared Line Appearance state information. This state is manipulated by the SLATrunk() and SLAStation() applications. ...