Using VOIP over a satellite link

Thanks Gary. Just for comparison what are typical delays when using ADSL?

The quality of VoIP calls using an ATA-186 over a satellite link will depend entirely on the technology used by the satellite network operator. Most broadband satellite systems are not designed specifically to support VoIP with business-class quality. Some may not work at all, some may have poor call quality and very long delays as the voice sits in jitter buffers waiting for voice data to catch up.

To obtain consistent, clear VoIP quality over satellite requires either:

a) SCPC service – this is 100% dedicated bandwidth and it is expensive. This is what Gary refers to above.

b) a broadband satellite service that can allocate CIR (committed information rate) quickly and consistently. Shared broadband satellite services based on iDirect-enabled technology are optimized for VoIP and have the most advanced technology today to deliver business class VoIP quality at reasonable prices.

VoIP requires dedicated bandwidth. Unlike browsing that is intermittent and bursty with lots of idle time, a voice call is a continuous stream of bandwidth. To provide good voice quality, the system must be capable of QoS so that it can recognize and prioritize voice, and CIR that can be allocated in sub-second time so that the call can be protected with dedicated bandwidth.

One caveat with regard to the ATA-186. It supports standard codecs, however it can only support one G.723 call at a time. If G.723 is used for compression on one line, the second line will use G.711 (i.e. it will not be compressed). On VSAT systems, G.723 is most popular because it uses the least amount of bandwidth – about 16 Kbps per call. The iDirect technology also includes cRTP header compression, which further reduces the bandwidth requirements to about 11 Kbps for G.723 and 14 Kbps for G.729. Since CIR has a cost associated with it, this is a useful feature to keep the bandwidth costs minimized. For example, two G.729 calls will require 28 Kbps of CIR. One G.723 and one G.711 call will require about 86 Kbps of CIR and that will be expensive.

Voice also requires consistent latency. As Gary indicated above, latency over consumer class satellite services such as DirecWay and Starband can range from 600 to 2400 ms. It’s not the length of the latency, but rather the consistency that creates problems – i.e. jitter.

SCPC and the iDirect-enabled services mentioned above can deliver consistent latency in the 550-650ms range depending on where the sites are located. Latency is a function of total distance to and from the satellite (22,300 miles above equator), and back again, divided by the speed of light (186,000 miles per sec.) Most teleports are not located under the satellite on the equator as this limits the number of satellites they can see, and most remote sites are not located on the equator directly under the satellite, so a good average round trip figure is 92,000 miles, which divided by the speed of light yields .495 seconds. Add to this hardware delays for modems, routers, gateways, proxy servers, Internet link to site pinged, etc. and you will come up with about 550 – 650 ms on average. This really is not much more than two people talking with each other on cell phones. Cell phones have almost 1/4 second latency, so when two of them talk to each other, you end up with just under 1/2 second latency which is just a bit less than what’s experienced over a good satellite link. People get used to this very quickly (particularly when you consider the cost savings for wireline long distance calls!).

Satellite services can be an excellent way to transport VoIP from remote locations where other options don’t exist – however there it requires a satellite service that has the right technology if you want a consistent, reliable service. Using DirecWay to call Aunt Rose is fine late at night when the nework isn’t loaded, but you need the right technology if you want to support business class VoIP service over satellite.

Patrick Gannon
Business Satellite Solutions, LLC
http://www.bsatellite.com
pgannon@bsatellite.com