On Nov 14, 2007, at 7:16 AM, Peter Todd wrote:

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> On Wed, Nov 14, 2007 at 08:46:01AM +0100, wouter van ooijen wrote:
>>> http://arastra.com/recruit/quiz.py
>>
>> 1. UDP is preferred over TCP for VoIP mainly because:
>>
>> IMHO the correct answer is: Compared to UDP, TCP provides additional
>> services, like reliability using an acknowledge/timeout/resend
>> mechanism. Such additional services (like delayed reliability) are  
>> of no
>> use in a voice context.
>
> Yeah, gotta agree with you on that one. Simply put, a late TCP  
> packet is
> still usefull, a late VOIP packet isn't.


Depends on how big your buffers are, and if you have enough CPU and/or  
DSP horsepower to reassemble the correct order fast enough to recover  
-- before the end user hears it.  ;-)

Problem is -- in most VoIP applications, any appreciable delay in  
audio coming out the handset into a typical "not perfect" hybrid on  
any standard 2-wire phone, creates echo back to the calling party.

This happens because you can't get too far behind or you fall out of  
the window of the PSTN carrier's DSP-based echo-cancellation buffers  
-- especially on International circuits/calls which are heavily DSP  
processed and echo-cancelled because of the natural latency of the  
distances involved.

  :-) :-) :-)

Ah, the joys of the modern PSTN.

--
Nate Duehr
nate@natetech.com
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