# Asterisk Project : Channel Variables

Added by mdavenport , edited by russell on Aug 31, 2010

What's a channel variable? Read on to find out why they're important and how they'll improve your quality of life.

There are two levels of parameter evaluation done in the Asterisk dial plan in extensions.conf.

1. The first, and most frequently used, is the substitution of variable references with their values.
2. Then there are the evaluations of expressions done in $[ .. ]. This will be discussed below.

Asterisk has user-defined variables and standard variables set by various modules in Asterisk. These standard variables are listed at the end of this document.