dahdi.conf.sample: Add description for "buffers" setting.

This contains an edited version of the patch originally created by John
Bigelow.

(closes issue ASTERISK-14435)
Reported by: John Bigelow
Patches:
      buffers.patch (license #5091) patch uploaded by John Bigelow
      0001-dahdi.conf.sample-Add-description-for-buffers-settin.patch (license #5417) patch uploaded by Shaun Ruffell
      Modified
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Merged revisions 374727 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 374728 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 374729 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374730 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/chan_dahdi.conf.sample

@@ -1051,6 +1051,33 @@ pickupgroup=1
 ;
 ;faxbuffers=>6,full
 ;
+; Configure the default number of DAHDI buffers and the transmit policy to use.
+; This can be used to eliminate data drops when scheduling jitter prevents
+; Asterisk from writing to a DAHDI channel regularly. Most users will probably
+; want "faxbuffers" instead of "buffers".
+;
+; The policies are:
+; immediate - DAHDI will immediately start sending the data to the hardware after
+;             Asterisk writes to the channel. This is the default mode. It
+;             introduces the least amount of latency but has an increased chance for
+;             hardware under runs if Asterisk is not able to keep the DAHDI write
+;             queue from going empty.
+; half      - DAHDI will wait until half of the configured buffers are full before
+;             starting to transmit. This adds latency to the audio but reduces
+;             the chance of under runs. Essentially, this is like an in-kernel jitter
+;             buffer.
+; full      - DAHDI will not start transmitting until all buffers are full.
+;             Introduces the most amount of latency and is susceptible to over
+;             runs from the Asterisk process.
+;
+; The receive policy is never changed. DAHDI will always pass up audio as soon
+; as possible.
+;
+; The default number of buffers is 4 (from jitterbuffers) and the default policy
+; is immediate.
+;
+;buffers=4,immediate
+;
 ; This option specifies what to do when the channel's bridged peer puts the
 ; ISDN channel on hold.  Settable per logical ISDN span.
 ; moh:          Generate music-on-hold to the remote party.