Latency time of a satellite VSAT link is a crucial factor when choosing to use a satellite system, it can affect the performance of your network and some applications experience issues when the latency is too high, not to mention a poor user experience.
Radio Frequency (RF) waves travel at the speed of light which is a little less than 300,000 km per second (299,792.458 km/s). So, we can calculate how long a signal will take to travel from the antenna to the satellite and back down to the distant end of the link. The latency is unaffected by the frequency band of the link.
But not all satellites were created equal. Depending on the type of orbit (i.e distance) of the satellite and therefore the time taken to travel will vary.
Low Earth Orbit (LEO)
Low Earth Orbit satellites orbit from 160 to 2000km above the earth and take approximately 90 minutes for a full orbit. These only cover a portion of the earth’s surface.
Medium Earth Orbit (MEO)
Medium Earth Orbit satellites are located in between LEO and GEO satellites and typically travel in an elliptical orbit over the North and South Pole or in an equatorial orbit. The most common orbital distance is 20,200km.
Geostationary Earth Orbit (GEO)
Geostationary Earth Orbit satellites orbit at 35,786km or 22,282 miles above the equator in the same direction and speed as the earth rotates on its axis, making them appear as fixed in the sky.
VSAT Latency Calculation
We can just use the simple speed, distance and time equation to calculate the time:
speed = distance / time
time = distance / speed
A Geo VSAT Satellite Example
time = 35,786 km / 299,792 km/s
time to satellite = 0.11937 seconds
So therefore, the time for the RF signal to reach the satellite is 0.11937 seconds but we must come back down again so it’s really 0.23874 sec or approx. 0.25 sec for a signal (or information, data, packet) to travel across a satellite link. This is called one ‘hop’.
TCP applications (like Pings, internet browsing) require an ‘ack’ and the Return Trip Time (RTT) of a VSAT link is just under 0.5 seconds or 500 msec.
VSAT Latency Times (PING)
Acceptable latency times over a VSAT link are between 550 – 650 msec. This is more than the 500 msec return trip time due to processing of the data packet by the VSAT router, Teleport equipment, etc.
To get a true reading of the satellite latency a ping to a device at the other end of the link should be used i.e the distant VSAT modem or hub, Teleport router or switch. To find a suitable device you can perform a trace route to google and this will show the connected equipment in the route. Pick a device just after the satellite link, check the latency times!
If the ping time of the link is greater than 900 msec then we need to consider what other factors might be causing the additional time.
These might include some of the following:
- Traffic congestion – The local traffic is demand is much greater than your VSAT bandwidth
- Terrestrial links – What route is your traffic taking to reach the destination
- Server location and load – The ping server might be under load and running at max CPU usage
- Distant end equipment
- Oversubscribed VSAT network – the most common cause of VSAT satellite latency and you need to speak with your satellite provider!
Reducing VSAT Latency
The latency time can be reduced by:
- Using a MEO satellite service like O3B
- Reduce local traffic like software updates – schedule when office is unattended
- Ensure there is adequate network bandwidth available and the service is not over subscribed
- Trace route to google to check where the issue is
- Ping a device at the distant end (Teleport)
VSAT satellite latency should be between 550 and 650msec on a good link.