VSAT inroute and outroute capacity calculations

There is little published clear information about this problem.

The general advice seems to be "What do your customers want?" then calculate the answer.

If they have some clear purpose, e.g. a dedicated 1000 kbit/s per site, all of the time, then the calculation is easy.

It is not so easy if the customer wants "Internet access".  Internet users vary from those doing a few emails per day to people watching streamed video, doing skype voice calls, or uploading video. Even claimed 'low users' may inadvertently download massive software updates or 'sync' many image files with the cloud.

Overall my impression, for large networks, is that you need to allow about 30kbit/s outlink and 6 kbit/s return link per PC in the network. A big network example would be an outlink carrier of 67 Mbit/s shared with 2200 customers, each rate limited to 20 Mbit/s and sold as "Up to 20 Mbit/s service".

These 30 and 6 kbit/s figures are about double that of 5 years ago, reflecting the recent increased use of video and images in social media. It will no doubt get worse with new operating systems that secretly 'phone home' for app updates all the time and report on your browsing habits.

Obviously figures like 30 and 6 kbit/s don't go down well with the marketing/sales folk but the reality is that satellite internet costs a lot and only by sharing does the cost per PC come down.

If you have 100 sites, each with 1 PC then the outlink needs to be 100 x 30k = 3 Mbit/s. You can then sell this as "Up to 3 Mbit/s download" which sounds good. The problem is that congestion will slow things down a lot at times. The return link might be 100 x 6 kbit/s = 600 kbit/s, TDMA. This could be implemented as 2 x 300k carriers, with 2 hub receivers, if your remote BUC power is too low or remote dish size too small.

With fewer sites the congestion problem gets worse, due to lack of averaging, and more than 30kbit/s + 6 kbit/s per site will be needed to keep users happy. It helps to have a clearly understandable fair access policy (FAP) during the day and a free-for-all at night, plus rules that discourage or prohibit file sharing, video, viruses, mass emailing etc. A firewall at the hub will help.

Once you have determined your wanted information bit rate convert to symbol rate and bandwidth using the FEC ratio and modulation method. e.g. 3 Mbit/s information rate. 0.75 FEC ratio. QPSK=2 bits/symbol. Symbol rate = 3 x 1/0.75 x 1/2 = 2 Msps. This need approx 2.6 MHz of transponder bandwidth. 8PSK (3 bit per symbol) would reduce this by 2/3, but check your remote dish size is big enough.

If you are reading this and agree or disagree with what I have written above, please add your ideas below.

Best regards, Eric.