Satellite Internet Access

Satellite Internet access

If high speed is needed in cost effective data communication and where DSL or Cable Line is not available, The satellite internet access (Satellite Broadband) could be an answer. satellite internet access provides a very good download speeds and upload speeds.

Satellite

Satellite transmission uses radio waves to transmit data. A satellite dish or antenna, known as the uplink station, transmits data to the satellite, which orbits the earth. Transponders on the satellite then repeat the signal, which is received by another satellite dish, known as the downlink station. Conventional satellites typically orbit approximately 22230miles above the earths surface; any satellite dish pointed at the satellite can receive the signal, provided that it is tuned to the proper frequency, much the way we tune in to a radio station.

To help understand satellite internet access, picture a strange mirror orbiting above the earth. Standing in London , we shine a very powerful flashlight pointed right at this mirror. This special mirror then reflects the light over the entire country. Anyone in Leeds can then read outside in the light provided by this mirror. We could not light up the whole country with the powerful flashlight without the help of the mirror to reflect the light everywhere. Similarly, the satellite takes our data and repeats it so that everyone within reach of the satellite can receive the signal. We simply point our dish at the satellite and begin transmitting. Since the satellite is so high, the same satellite can transmit data to almost a third of the world; going further simply requires relaying the signal to another satellite. A major advantage of satellites is their ability to communicate from almost anywhere to almost anywhere else.

The bandwidth of satellite internet access depends on the equipment used in the uplink and downlink stations and the number of satellite channels used; each satellite is capable of handling many channels at one time, each of different transmission frequencies. Satellite bandwidth can extend from 64 kbps to many Mbps and is almost unlimited if enough channels are used. Satellite transmission is usually cheaper over long distances than fibre optics or twisted pair, because there is no cabling cost between two locations. The satellite dishes are a one-time purchase and do not have to be leased monthly from telephone carriers, though the satellite channels must be leased.

However, satellite internet access are subject to noise and long delays. Since every transmission must go from the ground to the satellite and back to the ground, each transmission takes approximately a 45,000 mile trip. This transmission occurs at roughly the speed of light, so the trip takes a quarter of a second. Sending a message and receiving a response means at least a half-second delay fro the round-trip. A half-second wait may seem trivial to us, but a computer might be able to transmit 4800 bits during the time.

The long delays of satellite transmission can produce the so-called ?tunnel effect?, where annoying echoes are heard during long-distance calls. Though these echoes can be prevented by using echo cancellers, nothing can be done about the delay between transmission and reception. A transcontinental call over twisted pair wire will probably go no farther than 5000 miles, no matter how convoluted the call routing; the same call using a satellite travels nine times farther and therefore can markedly slow down host computers waiting to transmit data.

New technologies in Satellite

While these high-altitude satellites have carried voice and data communications for decades, a new breed of satellite, known as a low-earth orbit satellite or LEO satellite, has been launched in recent years. These satellites orbit a few hundred miles above the earth. Because they are closer to the earth, more satellites are required to provide global coverage. For example, one such satellite network operates 66 satellites orbiting 485 miles above the earth. Another LEO satellite network operates 48 satellites at an altitude of 736miles. In between these and conventional satellites falls another type as a medium-earth orbit satellite ore MEO satellite. One of these networks operates 12 satellites at latitude of 6434 miles. In contrast, a conventional satellites system, based on a geosynchronous-earth orbit satellite or GEO satellite, require only 3 satellites at an altitude of approximately 22,230 miles to cover the globe. This particular altitude is convenient because the satellite orbit at the same speed as the earth rotates, so dish antennas can be aimed once and the satellite remains in the same relative position. The face at all times, so these antenna systems are designed to allow for transmission and reception without directly pointing at one particular satellite. The orbiting satellites in these systems hand off calls to one another as they pass near the user. These handoffs are similar in nature to those conducted by a ground-based cellular telephone system.

Radio/Microwave

Terrestrial microwave transmission uses radio frequencies similar to those found in satellite transmission. Instead of bouncing radio waves off a satellite, however, the users build tall towers and point the dishes at each other, as shown in the figure. Alternatively, the dishes can be put on top of buildings, as long as there is a clear line of sight between the two dishes.

If there is an obstruction, like a mountain or building, between the receiver and the destination, another tower can be placed on top of the obstruction and used as a repeater. This additional tower simply receives a signal on one side and transmits to the other side.

The signal used for microwave transmission can be sent only 20 to 30 miles. Longer distances require using multiple hops, with additional towers in between. These intermediary towers act as repeaters, receiving the signal and passing it on. Microwave transmission bandwidth can exceed 25Mbps.

The difference between Microwave and satellite

The key difference between microwave transmission and satellite transmission is that microwave transmission requires a line of sight between the two dish antennas, there can be no obstructions between the two antennas; if we climbed one of the tower on a clear night, we should be able to shine the powerful flashlight at the second tower and anyone at the top of the second tower would see the light. If the other tower was not the final destination but only a repeater station, anyone would see the flashlight and then point his own flashlight at the next tower.

The cost advantages of microwave transmission lie in relatively short-haul, high-bandwidth applications. The towers and dishes are a one-time purchases cost, there are no satellite channels to lease, and there is currently no shortage of microwave frequencies. Like conventional satellite transmission, there is no cabling cost between the locations, because the signal travels through the air. However, each added hop significantly increases the cost, since every repeater requires a separate tower and two dish antennas, one pointed in each direction. Microwave transmission is subjected to the same noise and interference as satellite transmission, though it is less likely that we will be encountering interference on a 30-mile route than on a 45,000 mile transmission.

Wireless (Optical Broadband)

One of the biggest problems in getting broadband can be the last distance to the location. For example, it wouldn't be very cost effective if you're in a remote location if you had to have land dug up to lay cables for the computer network. An alternative might be to use optical wireless for the short distance from a relay station to the remote location.

An optical wireless system (OW) is similar to a fixed wireless (Microwave or RF) system except that information is carried by optical or infrared beam rather than by a microwave or RF carrier. OW systems generally have smaller transceiver units and much smaller beam widths. These are also referred to as free space optical or infrared communications systems. Most OW transmitters use light-emitting diodes (LEDs) instead of lasers to generate the infrared beam. Like fixed wireless, OW requires a direct line of sight between locations.

The systems are designed to work in the infrared region of the electromagnetic spectrum, a region invisible to the eye. The operating wavelength is typically in the region of 850Nm corresponding to a frequency of 340,000GHz, which is 4 to 5 orders of magnitude greater than that used for microwave links.

Each end of the connection (or node) has some equipment installed, usually up high, e.g. on a roof. One side of the network at the relay station is connected tot the internet by a normal high speed link. The other side is connected to the optical device that relays data to/from remote location up to about 1000m away.

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