91 lines
5.1 KiB
Plaintext
91 lines
5.1 KiB
Plaintext
Asterisk Timing Interfaces
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-------------------------------------
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In the past, if internal timing were desired for an Asterisk system, then the
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only source acceptable was from DAHDI. Beginning with Asterisk 1.6.1, a new
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timing API was introduced which allows for various timing modules to be used.
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Included with Asterisk are the following timing modules:
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res_timing_pthread.so
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res_timing_dahdi.so
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res_timing_timerfd.so (Beginning with Asterisk 1.6.2)
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res_timing_pthread uses the POSIX pthreads library in order to provide timing.
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Since the code uses a commonly-implemented set of functions, res_timing_pthread
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is portable to many types of systems. In fact, this is the only timing source
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currently usable on a non-Linux system. Due to the fact that a single userspace
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thread is used to provide timing for all users of the timer, res_timing_pthread
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is also the least efficient of the timing sources and has been known to lose
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its effectiveness in a heavily-loaded environment.
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res_timing_dahdi uses timing mechanisms provided by DAHDI. This method of
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timing was previously the only means by which Asterisk could receive timing. It
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has the benefit of being efficient, and if a system is already going to use
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DAHDI hardware, then it makes good sense to use this timing source. If,
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however, there is no need for DAHDI other than as a timing source, this timing
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source may seem unattractive. For people who are upgrading from 1.4 and are used
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to the old ztdummy timing interface, res_timing_dahdi provides the interface
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to dahdi_dummy, which is ztdummy's replacement.
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res_timing_timerfd uses a timing mechanism provided directly by the Linux
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kernel. This timing interface is only available on Linux systems using a kernel
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version at least 2.6.25 and a glibc version at least 2.8. This interface has
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the benefit of being very efficient, but at the time this is being written,
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it is a relatively new feature on Linux, meaning that its availability is not
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widespread.
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What Asterisk does with the Timing Interfaces
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---------------------------------------------
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By default, Asterisk will build and load all of the timing interfaces. These
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timing interfaces are "ordered" based on a hard-coded priority number defined
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in each of the modules. As of the time of this writing, the preferences for the
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modules is the following: res_timing_timerfd.so, res_timing_dahdi.so,
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res_timing_pthread.so.
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The only functionality that requires internal timing is IAX2 trunking. It may
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also be used when generating audio for playback, such as from a file. Even
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though internal timing is not a requirement for most Asterisk functionality,
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it may be advantageous to use it since the alternative is to use timing based
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on incoming frames of audio. If there are no incoming frames or if the incoming
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frames of audio are from an unreliable or jittery source, then the
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corresponding outgoing audio will also be unreliable, or even worse,
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nonexistent. Using internal timing prevents such unreliability.
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Customizations/Troubleshooting
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---------------------------------------------
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Now that you know Asterisk's default preferences for timing modules, you may
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decide that you have a different preference. Maybe you're on a timerfd-capable
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system but you would prefer to get your timing from DAHDI since you already are
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using DAHDI to drive your hardware.
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Alternatively, you may have been directed to this document due to an error you
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are currently experiencing with Asterisk. If you receive an error message
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regarding timing not working correctly, then you can use one of the following
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suggestions to disable a faulty timing module.
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1. Don't build the timing modules you know you will not use. You can disable
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the compilation of any of the timing modules using menuselect. The modules are
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listed in the "Resource Modules" section. Note that if you have already built
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Asterisk and have received an error about a timing module not working properly,
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it is not sufficient to disable it from being built. You will need to remove
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the module from your modules directory (by default, /usr/lib/asterisk/modules)
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to make sure that it does not get loaded again.
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2. Build, but do not load the timing modules you know you will not use. You can
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edit modules.conf using "noload" lines to disable the loading of specific
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timing modules by default. Based on the note in the section above, you may
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realize that your Asterisk setup does not require internal timing at all. If
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this is the case, you can safely noload all timing modules.
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Important Notes
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----------------------------------------------
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Some confusion has arisen regarding the fact that non-DAHDI timing interfaces
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are available now. One common misconception which has arisen is that since
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timing can be provided elsewhere, DAHDI is no longer required for using the
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MeetMe application. Unfortunately, this is not the case. In addition to
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providing timing, DAHDI also provides a conferencing engine which the MeetMe
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application requires.
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Starting with Asterisk 1.6.2, however, there will be a new application,
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ConfBridge, which will be capable of conference bridging without the use
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of DAHDI's built-in mixing engine.
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