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

[Steve Kann]

On Feb 6, 2005, at 1:11 AM, Steve Underwood wrote:
[EMAIL PROTECTED] wrote:
On Sun, 6 Feb 2005, Steve Underwood wrote:

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.


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
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.
I have no doubt that you could use somewhat fewer bits if you made the 
code more complicated, but I don't think you'd probably find you need a 
variable number of bits to do it effectively.

There's a number of properties of (present) IAX2 and VoIP streams and 
IAX2 trunks that are different than TDM telephony streams:

1) The loss rate is (potentially) much higher;
2) packets may arrive out-of-order.
3) There's jitter.
4) There's discontinuous transmission of frames.
Now, you can potentially de-jitter each of the sub-streams before 
putting them together on an iax2 trunk, but then you would need to also 
de-jitter them _again_ after they've been been sent across the trunk.  
If you have multiple hops, this all adds to latency in an expensive 
way.

IAX2 streams presently have a number of "issues" that make this more 
difficult, especially problems where timestamps are sent pretty screwed 
up (i.e. the case where you have a call that gets bridged, either 
natively or not, and you switch you timing source from the local box, 
to the bridged channel, often there's a 2-4 second jump in timestamps). 
 I guess this is really a separate issue to solve, though..

-SteveK
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