WildBlue and HughesNet are two of the more common and popular Satellite ISP providers on the market and the typical subscriber is someone who is located in locations where terrestrial Internet access is not available and in locations which move frequently. However, just like dial-up, DSL and Cable, Satellite internet has its own set of pluses and minuses.
The biggest plus of using satellite internet is when you find yourself living in an area that has not been built up by the local telephone or cable company, if the infrastructure is not in place then you may find yourself unable to order even basic dial-up internet service.
But now we’ll look at the downside of using satellite internet and what some of the causes of these issues are.
Much of the slowdown associated with satellite Internet is that for each request, many roundtrips must be completed before any useful data can be received by the requester. Special IP stacks and proxies can also reduce latency through lessening the number of roundtrips, and/or simplifying and reduce the length of protocol headers. These types of technologies are generally referred to as TCP acceleration, HTTP pre-fetching and DNS caching.
Rain Fade
Satellite communications is affected by moisture and rain in the path of signal. The effects are less at lower frequencies, L and C band, but become quite severe at higher frequencies, Ku and Ka band. For satellite internet services in tropical areas with heavy rain, C band (4/6 GHz) with circular polarisation is popular. At Ka band (19/29 GHz) special techniques such as large rain margins, adaptive uplink power control and reducing bit rates during rain are applied.
The amount of time for which service is lost is reduced by increasing the size of the dish so as to gather more of the satellite signal on the downlink and producing a more intense transmission on the uplink.
Modern consumer-grade dish antennas tend to be fairly small, which reduces the rain margin or increases the required satellite downlink power and cost.
Large commercial dishes 3.7m to 13m diameter are used to achieve large rain margins and also to reduce the cost per bit by requiring far less power from the satellite.
Modern download DVB-S2 carriers, with RCS feedback, are intended to allow the modulation method to be dynamically altered, in response to rain problems at a receive site. This allows the bit rates to be increased substantially during normal clear sky conditions, thus reducing overall costs per bit.
Latency
Latency is the delay between requesting data and getting a response, or in the case of one-way communication, between the actual moment of broadcast and the time actually received at the destination. Compared to ground-based communication, all geostationary satellite communications experience high latency due to the signal having to travel 35,000 km (22,000 miles) out into space to a satellite in geostationary orbit and back to Earth again.
The signal delay can be as much as 500 milliseconds to 900 milliseconds, which makes this service unusable for applications requiring real-time user input, such as online games or remote surgery. This delay can be very unpleasant with interactive applications, such as VoIP, videoconferencing, or other person to person communication. The functionality of live interactive access to a distant computer can also be subject to the problems caused by high latency. These problems are more than tolerable for just basic email access and web browsing and in most cases are barely noticeable.
There is no simple way to get around this problem. The delay is primarily due to the speed of light being 299,792.458 km/second (186,282.397 miles per second) in a vacuum. Even if all other signalling delays could be eliminated it still takes the electromagnetic wave 233 milliseconds to travel from ground to the satellite and back to the ground, a total of 70.000 km (44,000 miles) to travel from the source to the destination. 140.000 km (88,000 miles) for a round trip (user to ISP, and then back to user). Factoring in normal delays from other network sources gives a typical connection latency of 500–700 ms. This is far worse latency than even most dial-up modem users’ experience, at typically only 150–200 ms total latency.
A proposed alternative to the geostationary relay is a special solar-powered ultralight airplane, which would fly along a circular path above a fixed ground location, operating under autonomous computer control at a height of approximately 20.000 meters. Onboard batteries would charge from solar panels covering the wings during the daylight hours, and would provide power to the plane during the night. Ground-based satellite dishes would relay signals off the plane, resulting in a greatly reduced roundtrip signal latency of only 0.12 milliseconds.
Line of Sight
Typically a completely clear line of sight between the dish and the satellite is required for the system to work. In addition to the signal being susceptible to absorption and scattering by moisture, the signal is similarly impacted by the presence of trees and other vegetation in the path of the signal. As the radio frequency decreases, to below 900 MHz, the penetration through vegetation increases, but most satellite communications operate above 2 GHz making them sensitive to even minor obstructions such as the leaves of a tree. A dish installation in the winter must factor in plant foliage growth that will appear in the spring and summer.
Fresnel Zone
The radio signal width between two satellite dishes is not perfectly straight and uniform, as if it were a beam of light. Instead as the signal propagates away from the transmitting dish, it widens towards the centerpoint between the two dishes and then narrows again as it approaches the receiving dish. This is known as the fresnel zone, and limits the usefulness of satellite dishes in locations where there is extremely limited open sky for using the dish. The signal path through space must be clear not only for direct line of sight, but must also be clear for the expanding fresnel zone, which may be several meters larger in diameter than the ground-based satellite dish.
Tags: Internet, ISP, Satellite, Technology
Posted in Technology, Writing |
