How Does Satellite Internet Work?

Satellite Internet isn't the best way to the internet, since it's not as fast or reliable as the DSL available in the city, but its major advantage is that it works pretty much anywhere. So even if you live on a farm surrounded by thousands of acres of corn, or in my case, on a mountain ridge in the middle of a national forest it's one of the best things ever, since it's still a huge upgrade from dial-up. First generation Wild Blue and third generation Hughes Net were capable of delivering up to 3 Mbps or so, while the newer exede and Hughes Net Gen 4 systems can reach up to 10-15 Mbps.

So, we'll start at the beginning. There are currently two major providers, ViaSat, which owns the Wild Blue and exede trademarks, and Hughes Net, which is now a part of Echostar.

Overview

Each provider has a Network Operations Center (NOC), which is connected to the internet via a high speed optical carrier such as SONET. The traffic from everyone that connects through that provider is routed through the NOC. It's why when you check with an IP locator it tells you you're in Cheyenne, Wyoming.

Satellite Internet is what's referred to as a "bent pipe" network, which is a stupid name but pretty accurate when you think about it. Each end user has their own Earth station which is capable of bi-directional data transfer, sending a request to a remote server transmits the data through the user's antenna, to an orbiting satellite, which filters, analyzes, decodes, and re-modulates the signal to be sent back to Earth, to the provider's NOC. Once at the NOC, the request is sent out over the public Internet infrastructure, through various routers to the destination IP. When the server responds, the process is reversed; the response is sent over the public Internet to the NOC, to the satellite, and back down to the user's Earth station.

This accounts for the lag satellite users experience when using real time network applications like gaming and IP telephony. The satellite is orbiting 36,000 km (22,400 mi) above the equator. From my Earth station in the southeastern US, Echostar XVII at 107° which is the satellite that carries most of the HughesNet traffic is 38,000 km (23,600 mi) away. The signal travels at the speed of light, 300,000 km/s, so each trip to the satellite and back takes approximately 400 ms, almost half a second more than a land based internet connection.

The User's Earth Station

This consists of a modem and a satellite antenna, which in the industry is referred to as an outdoor unit (ODU).

Modem

The modem takes care of translating IP packets into a format compatible with transmission over a satellite link. Its ouput is a modulated carrier wave in the L band (950-1450 MHz). Earlier Hughes Net and the original Wild Blue used DVB (Digital Video Broadcast) modulation, the same type of coding used for satellite TV. It had the advantage of already being widely deployed, so the hardware was already relatively cheap. New advances in technology made it economical to use WIMAX (Worldwide Interpretable Microwave Access) for exede, while Hughes Net Gen4 uses IPoS (Internet Protocol over Satellite).

Both of these schemes support adaptive coding and modulation (ACM). What this means is that the modem can change the modulation and level of error correction based on the current conditions, while older systems could only change the transmit power. By default the system will use a high order modulation such as 16-PSK, which can carry 4 bits per symbol, and the lowest level of forward error correction to give the highest possible data throughput. Rain, atmospheric noise, and solar activity can adversely affect satellite communications by increasing the level of noise in the system. High order modulation schemes can carry more data, but are more affected by noise. The signal level is referred to as the energy per bit over noise (Eb/No), which is the ratio of total received power to bitrate. When the Eb/No drops below a certain threshold, as would happen in a rain storm, the modem switches to a lower order modulation, such as 8PSK (3 bits per symbol) or QPSK (2 bits per symbol) , which lowers the throughput, increasing the Eb/No.

ODU

This type of system falls into a category of antennas called very small aperture transmission (VSAT), meaning that the dish a lot smaller than the 4 meter and up antennas normally used for satellite uplinks. The uplink and downlink are both Ka band circular polarized signals. The Ka band (20 GHz) is the next band up from the Ku band used for TV broadcasts (12 GHz). There are two reasons for using the Ka band, one is that most satellites already carry some Ku band payload which is doing something else, and two, you can use a smaller antenna.

Higher frequency means shorter wavelengths. A Ku band carrier at 12 GHz has a wavelength (λ) of 25 mm, while a Ka band carrier at 20 GHz has a wavelength of 15 mm. With dish type antennas, gain increases as wavelength decreases. With higher gain comes narrower beamwidths, which means less interference with adjacent satellites. The Skyware Global 75 cm VSAT antenna used with Hughes Net has a gain of dB and a beamwidth of ° at 20 GHz compared to dB and ° at 12 GHz.

Instead of an LNB, like you would find on a receive only satellite antenna, an Internet ODU will have what is called a transmit-receive integrated assembly (TRIA). This single device combines to take the place of several, a block-upconverter (BUC), a solid state power amplifier (SSPA), a block-downconverter (LNB), a multiplexer, and a feedhorn. The block upconverter takes the L band signal from the modem and converts it to the carrier frequency, the SSPA amplifies the signal up to the required transmit power, which can be as high as 1 watt in some of the newer models. The block downconverter does the same thing as a regular LNB, converts the downlink signal to L band and sends it to the modem. The multiplexer allows the BUC and LNB to be connected to the same feedhorn, by ensuring that only one is operating at a time. The feedhorn collects signals from the dish, separates the uplink and downlink frequencies, and selects the correct polarity. Unlike most TV dishes, the standard Internet antenna has a fixed polarity, to receive RHCP signals you need an RHCP TRIA, likewise with LHCP. The Hughes Net Gen 4 uses the same TRIA for both polarities, the feedhorn is removeable and can be flipped to receive the opposite polarity.

Older models had separate lines for transmit and receive, which were connected directly to the TRIA's internal BUC and LNB respectively. The new models use a single line, which is connected to a multiplexer to reverse the direction when needed. All of this equipment is powered by the modem over the coaxial cable. The receive components and BUC don't use very much electricity, generally under 3W, but the SSPA can use over 50W. For this reason the modem supplies 48 VDC instead of the standard 18 VDC to keep the current under 1A. Most coaxial cable has a copper plated steel core and can't carry enough current without overheating, so only cable with a solid copper core is used between the modem and TRIA.

Satellite

The satellites used for internet are purpose built for the job and substantially more complicated than a standard communications satellite. If you read my article on calculating the bandwidth of a satellite, you already know that the absolute maximum that a normal satellite using a normal array of transponders can carry is 2 Gb/s, while Echostar XVII is said to be able to carry 50 times more, a total of 100 Gb/s.

The reason that this works comes down to one term: frequency reuse. Normal satellites cover a wide area, such as the continental US or even an entire hemisphere, which limits them to a total of 24 (or in the case of DBS, 36) transponders. Satellites such as Echostar XVII have dozens of transponders, which are configured as spot beams, meaning that they only illuminate a small region of a few hundred to a few thousand square miles. This is the same technology that makes it possible for satellite TV providers to offer local network channels. This allows them to use the same frequencies for areas where the spot beams don't overlap. For example, they could use 19580 MHz for Maine, Lousiana, and Oregon. This type of system is so flexible that the newer satellites even have programmable spot beams. The beam can be widened or narrowed, or moved to another region to keep up with demand.

Another feature unique to internet satellites is the addition of onboard signal processing and routing. A television satellite has one function: take the signal it's sent, convert it to a lower frequency, and send it back to Earth. Echostar XVII has built in routing and switching capability that can intelligently decide when traffic needs to be directed to another customer served by the same satellite, so that for direct peer to peer applications like video conferencing and file sharing the data is sent to the satellite and back down immediately. This lightens the load of the NOC, and speeds things up since there are fewer hops from Earth to space.

NOC

This is the final part of the link, the provider's Earth station. Although I've not had the opportunity to work at a NOC (yet), so I don't know all of the exact details, but I can provide you with a description of what kind of equipment would be common to any such Earth station.

Each Earth station has an array of really big antennas, one for each satellite. By really big I mean really big, such as the one pictured at right which is 18.3 meters. Large antennas are necessary for two resons, one: the transmissions can be extremely powerful, up to 700 watts, so it's important to have a narrow beamwidth so as to not interfere with adjacent satellites, and two: the downlink has a lot of data crammed into it.

This next paragraph is essentially speculation. You'll recall that I mentioned earlier that Echostar XVII is said to have up to 100 Gb/s of bandwidth. A standard Ka band downlink would be 1 GHz wide, with a combined symbol rate of around 720 MS/s, which we'll round up to 1 GS/s, 8PSK would only give us 3 Gb/s, so it's pretty obvious that they're using a higher order modulation for their downlink. Even 256-QAM, which is unheard of in satellite downlinks, would only give us a data rate of 8 Gb/s, so it's safe to assume that they're using a wider downlink as well.

As for the rest of the station, the RF parts would be essentially the same as any other satellite Earth station, racks of equipment containing modems to convert the incoming signal to data and vice versa, and high power amplifiers to provide the uplink frequency and power required to drive the satellite's tranponders to saturation. On the network side, the equipment would be similar to what is found in any other ISP, racks of routers and switches, which are interfaced together via 10 gigabit ethernet, and which communicate with the wider internet over a SONET link, the fiber optic "backbone" of the public Internet and telephone system.