[Asterisk-Users] 128 kbs satelite link

[John Todd]

At 10:46 AM -0800 12/17/03, Paul Mahler wrote:
>
>While this thread is already in the archives, I'll throw my opinion on the
>table, too.
>
>The latency is about .25 seconds to and .25 seconds from the satellite.
>There is additional propagation delay in the system. Also, TCP/IP relies on
>propagation delay to optimize traffic, but doesn't handle delays this long
>well. There is special purpose hardware available the sends the traffic to
>an from the satellite with a different protocol. This helps the satellite
>calls quite a bit.
>
>http://www.mentat.com/skyx/skyx-whitepaper.html
>
>John Todd reamed me out the last time I suggested that you need this
>hardware, he says he has voice working fine over a satellite link. And he
>knows what he's talking about. I thought I would save him some time, this
>time, by putting this in. ;-)

Again, for the archives, I'll comment on why this hardware/software 
probably isn't useful for VoIP.  :-)

All of the satellite accelerators are based on TCP spoofing, which 
ACK's TCP packets before the other side actually sends the ACK.  This 
keeps things flowing faster because it "breaks" the way TCP 
functions.  Since VoIP is almost always UDP, this doesn't do any 
good.  Plus, since VoIP is real-time human-perceived communication, 
you can't make it any faster than it currently is - the speed of 
radio waves in a vacuum (and a short duration in the "slower" 
atmosphere) is fixed.  Geosync satellites are ~22,000 miles away - 
there is no speeding up that process or predicting what is being said 
before it's actually said.

I have used SIP RTP over satellite, and while the delay is 
uncomfortable it is workable if that is the only method to get to the 
endpoint at a reasonable cost.  As previously noted, discomfort is a 
function of cost; the more expensive the transport, the less 
discomfort one is willing to suffer through.

Quality of sound in UDP-based VoIP services are 100% a function of 
available bandwidth, and not a function of delay, though with IAX2 I 
can't say if that's the case or not since I am unaware of how it 
handles unreasonable duration between frames of send/receive.  The 
trunking features of IAX2 may help tremendously in saving 
transmission bandwidth, though - the removal of the IP overhead from 
subsequent sessions (after the first one) is a huge savings over SIP 
or H.323, so you should be able to get much more out of your very 
expensive bitstream than you would ordinarily be able to get with 
other VoIP protocols, so from that perspective (to the original 
poster or anyone else with IAX2 over satellite transport) please let 
us know how you make out with testing.

PS: To answer the original question, the _theoretical_ maximum for 
channels through 128kbps is 18 calls with LPC10, based on my codec 
comparisons (see the archives) with IAX2.  LPC10 sucks, and adding a 
satellite delay would make it infuriating for anyone except the most 
hardened cheapskate.  A more reasonable codec would be G.729, at a 
theoretical maximum of 11 calls, but you'd probably really only get 8 
or 9 without starting to get loss or buffer issues.

JT


>
>There is, of course, also the problem of the upload speed. The consumer
>grade satellites don't offer much upstream bandwidth. You can get better
>upload speed from some of the commercial satellite carriers.
>
>http://www.tachyon.net/
>
>Paul
>
>
>Paul Mahler
>mail:pmahler at signate.com
>phone: 650.207.9855
>fax: 877.408.0105
>
>-----Original Message-----
>From: asterisk-users-admin at lists.digium.com
>[mailto:asterisk-users-admin at lists.digium.com] On Behalf Of Bisker, Scott
>(7805)
>Sent: Wednesday, December 17, 2003 7:06 AM
>To: asterisk-users at lists.digium.com
>Subject: RE: [Asterisk-Users] 128 kbs satelite link
>
>Similar to online gaming, I would think that the propagation delay with the
>satelite connection would make calls unbearable.  Half-duplex at its worst.
>
>
>my $0.02
>
>-sb
>
>
>-----Original Message-----
>From: asterisk-users-admin at lists.digium.com
>[mailto:asterisk-users-admin at lists.digium.com]On Behalf Of Senad
>Jordanovic
>Sent: Wednesday, December 17, 2003 9:49 AM
>To: asterisk-users at lists.digium.com
>Subject: [Asterisk-Users] 128 kbs satelite link
>
>
>Hi all,
>
>Anyone has experience  using * through
>128 kbs (or bigger) satelite link?
>
>In particular I am interested to hear how many calls could be put
>through 128Kbs satelite link simultaneously?
>
>Ta
>SJ
>