TDMoE Timing Requires Zaptel -Newsgroup article reference-

Comment on Asterisk TDMoE
bluecrow76
Joined: Tue 18 of Oct, 2005

TDMoE Timing Requires Zaptel -Newsgroup article reference-

Posted:Fri 09 of Jun, 2006 (05:35 UTC)
Here's a link to a news posting regarding the use of ztdummy as a timing source for TDMoE:

http://lists.digium.com/pipermail/asterisk-dev/2006-February/018951.html

Relevant excerpt:

As the one who added 2.6 RTC support to ztdummy, I'm qualified to comment!
The primary reason for ztdummy is to support SIP, IAX and MeetMe in a
system with no Zaptel cards. It is quite adequate for that, although
there is a patch on the bugtracker that aims to address a perceived
weakness in it.

However, I don't think ztdummy as it stands is quite accurate enough
for governing TDMoE, because it doesn't maintain a 1ms spacing. Since
the RTC can only do power-of-2 subdivisions of a second, it is set to
1/1024 sec (0.9765625ms), but then 3 interrupts out of every 128 are
skipped (evenly spaced) to give an average of 1000 per second. This
is ok for VoIP with a 20ms or 30ms frame size, but not for TDMoE with
a packet every 1ms. I would probably try making ztdummy use 4096 RTC
interrupts per second and skip 3096 of them. This would make the
interval between two consecutive packets either 0.9765625ms or
1.220703125ms.