Video Conference on  SCPC /DVB

Audio and video are demanding services, requiring a transmission path able to support a particular bit rate, per second and for each and every consecutive second.

For example, a single audio and video may need a steady average of 500 kbit/s.

This could be successfully sent over a 550 kbit/s SCPC link.

A 10 Mbit/s DVB-S link would have many other users sharing, and if some users were downloading very large files this might easily cause unpleasant interruptions of the video conference. However, if the router at the sending end were programmed to allocate sufficient regularly spaced frame time slots for packets of data from the Video conference IP address then a dedicated link of say 550 kbit/s might be effectively be created within the DVB-S data stream.  This will work fine.

It is important to minimise the required bit rate below what you have available over the satellite. I note that the Polycom 1500 device has settings of 5 to 30 for the video frame rate and settings of 1 to 6 for image quality. These selections are critical to the resulting data rate. Remarkably, I could find nothing in the Polycom document that said what bit rate was actually produced by video and audio quality options.

Most VSAT systems have a DVB-S or similar high speed shared outlink carrier and a TDMA (time shared burst mode) carrier for the return link from the VSAT to the hub.

The only way to get a 500 kbit/s TDMA return link to carry video/audio is to fill it up with contiguous regular bursts and this blocks out its use by all other VSAT sites from sharing that uplink carrier slot.  You would have to have spare VSAT return links to hub for the other sites. You might as well have a temporary SCPC carrier put up for the duration of the video conference. You could have Comtech Vipersat SCPC/mesh DAMA which does precisely what you want. Video and audio are demanding and expensive applications as they need high average bit rates.

Normal PC internet web browsing and emails needs an intermittent peak bit rate of say 1 Mbits/ but an average of only about 16 kbit/s.  Speech comes in at an average of 50 kbit/s and small screen monochrome video at an average of 450 kbit/s. Cost difference ratio 450/16 = 28 times more than a PC used for web browsing/emails!

Best regards, Eric.

Hello africa-vsats,

Some satellite network operators oversubscribe their DVB(S2) downlinks in order to decrease prices, appear more attractive to customers and increase their revenues. If the price for the DVB(S2) downlink you have been offered was significantly off the SCPC uplink then this is a good indicator that your downlink may be oversubscribed, i.e. it is not true CIR bandwidth. This stems from the fact that both DVB-S2 and SCPC are usually very similar in terms of spectral efficiency, i.e. you need the same X MHz to provide Y Mbps of IP bandwidth using both DVB-S2 and SCPC. There are some benefits offered by DVB-S2 compared to SCPC, such as absence of guard bands at each side of the carrier, availability of ACM and others, however the price for true, dedicated DVB-S2 downlink can not be way off the SCPC.

If your downlink is oversubscribed then this means you may get jitter and unavailability of minimum guaranteed bandwidth level required for the videoconferencing. So you really have two options here. A) Switch to SCPC on the downlink and B) Migrate to a different satellite network operator who is not messing with their bandwidth pools. If you are paying for dedicated CIR bandwidth then it can not be oversubscribed.  

Another thing you may want to check is the way the routing is set up. Some DVB downlink scenarios may involve NAT on the teleport side. Sometimes VTC appliances have troubles traversing the NAT and this may result in, say, total loss of sound or picture, one-way like you describe. You said the remote party can not hear you, so I wonder if that's all the time or you actually experience choppy sound or sporadic loss of sound during the session?