Asterisk: The Future of Telephony, 2nd Edition

Music on Hold

Any popular PBX system offers the ability to supply a source of music to be played for callers while on hold. Asterisk allows for a lot of creativity in this regard.

Nowadays, everyone is familiar with the MP3 music format, and there is a lot of interest in using MP3s as a music-on-hold source. The concept sure seems like a good idea, but there are a few things that we think should be given some consideration:

• MP3 files are extremely complex, and require a substantial amount of CPU to decode. If you have a lot of channels pulling music from the system (for example, people sometimes like to listen to music through their phone, or a call center may have several callers on hold), the load on the CPU caused by all of the transcoding of the stored MP3 files could place too much demand on a machine that is otherwise suitable to the performance needs of the system.

• Current-generation hard drives hold a lot of data, so there may not be any reason to worry about cutting down hard drive use. Compressed audio makes sense from a distribution standpoint (an MP3 is a much smaller download than the equivalent in .wav format), but once on your system, do we really care how much space they take up?

• MP3 files don’t usually come with the right sort of licensing. ;-)

Taking all of this into consideration, we recommend that you convert your music sources into the native format of the various codecs you may be supporting. For example, if you support μlaw for your internal phones, and G.729 on ...