[Asterisk-Dev] IAX2 trunk really should send timestamps as part of iax2_meta_trunk_entry..

[Steve Underwood]

steve at daviesfam.org wrote:

>On Sun, 6 Feb 2005, Steve Underwood wrote:
>
>  
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>>Think plesiochronous :-) [if you don't know what that means, look it 
>>up]. T1s and E1s come from the days of plesiochronous telecoms (PDH - 
>>the plesiochronous digital heirarchy). SDH - the synchronous digital 
>>heirarchy - replaced that, and the complexity of multiplexing and 
>>demultiplexing dropped dramatically.
>>
>>What goes into an IAX trunk packet is a bunch of plesiochronous streams. 
>>They each have their own timing. A common timestamp for the whole group 
>>doesn't cut it.
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>>
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>Just before Astrican I tried to convince Mark that the timestamps had to 
>be there, but couldn't.  Perhaps someone else will be more successful.
>
>Steve
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>
OK. Think more than plesiochronous. Think plesiochronous digital 
heirarchy :-) Because the streams are plesiochronous (unless one of the 
sources has a really screwy clock), you can just encode how much ahead 
or behind the group an individual stream is, which is a tiny value. That 
is basically how PDH does it.

In PDH, the higher order muxing (everything above T1/E1) forces the 
lower streams into a single synchronous stream, which is a little faster 
than the sum of the streams it carries. This allows some slots for 
carrying extra bits, when an incoming stream is a little fast, and 
tagging that bits are missing when an incoming stream is a little slow. 
The amount of fudging data you need to handle is very small, because the 
worst case mismatch in speeds in small (100ppm in the case of PDH). For 
IAX2 trunking, just one or two bits per channel per packet could do the 
job. Without a bit of work, I'm not sure how messy the software might 
look, but in terms of dense coding on the wire its makes sense.

Regards,
Steve