[Freeswitch-users] recommendations for Wifi SIP phones?

[Karl Schmidt]

On 09/19/2013 06:15 PM, Nathan Neulinger wrote:
> Anyone got any recommendations for decent quality wifi sip phones? Obviously the 'android+soft client' model will work,
> but what about actual handsets built for the task.
>

I dug into this issue a bit - sounds like wifi and sip don't exactly like each other.  The problem 
people get is stuttering. It is the old bit where it is easy to confuse the difference between speed 
and latency. If you look at the through-put - wifi appears like it should work, but there are cases 
where packets can be delayed - and if part of an audio stream, either you get excessive latency from 
buffering or it misses a time slot and stutters.

Now if you were watching a video - no problem ( it is if you need to mix video inputs) - just delay 
the playback via a big buffer - but phone conversations are sort-of realtime.

So this is why there is a market for a substitute and what I've heard is that Siemens- Gigaset ( 
DECT )  phones are a good substitute for wifi.

This is looking like it would block using a softphone via wifi.  I've wondered if some of the low 
data rate codex would make it sort of work?  Once I get the rest of freeswitch working I will be 
building a updated linphone ( the stable version in Debian has problems) to test with.  My hunch is 
that the low data rate won't fix the problem with maximum delay of packets. (Could be a function of 
the wifi access-point - and how many are sharing it).

See myth 16 here
http://www.cisco.com/en/US/prod/collateral/wireless/ps9391/ps9393/ps9394/prod_white_paper0900aecd807395a9.html

<warning boring EE digression follows>

A bit more about latency. Landlines use data paths like in T1 bits where every call channel has a 
place waiting for the next bit of data - ( 8 bit words where they steal a bit for signaling - thus 
56kbits instead of 64kbit.) Every frame of the T1 data has a place reserved for your phone 
conversation - no sharing (at least at this level).

Land-line latency was so short no one noticed or even talked about it.  Enter cell phones. Bad nasty 
latency - I measured up to 500ms (anything over 50ms bugs me)( plug a mike into a storage 
oscilloscope and have the cell phone call your land-line on speaker phone - clap - measure the time 
between the clap and the clap coming out of the speakerphone.).

This is bad. In WWII the Nazi's used wire recorders to insert latency and found that if the latency 
was bad enough - people would get mad at each other for interrupting. ( I suppose the latency 
introduced with cell phones is why everyone started hating each other.)..

Now introduce wifi.  The lovely folks in government did not want it to work. It potentially competed 
with services that people paid for and those providers provided bribes^h^h^h^h^h^h investment 
opportunities (The type the little guy never sees) to the ruling elite to be sure it would die. So 
after stalling they came up with a plan - put wifi on top of the frequency u-wave ovens use - lots 
of interference and they are tuned for water absorption so the range should be limited - and specify 
a very low amount of power - limit the spectrum.

BUT! - EEs are a stubborn bunch - they took what the FCC gave them and made it work anyway.

But, it sort of sucks. The kluges that resend missed packets and the like induce latency all over 
the place.

Part of the problem is the SIP protocol overhead - could be IAX would make it work a bit better?, 
but right now we live in a SIP world. I've not been able to find any numbers on IAX vs SIP over wifi 
- I bet it still has problems.

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Karl Schmidt                                  EMail Karl at xtronics.com
Transtronics, Inc.                              WEB http://secure.transtronics.com
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Lawrence, KS 66049                              FAX (785) 841-0434

Never put off till to-morrow what you
can do the day after to-morrow just as well. -- Mark Twain
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