Wireless Bridging and Internetworking is generally used for connecting buildings and facilities on campuses, within metropolitan areas or between offices in different planetary locations by satellite.
For wireless bridging that does not involve satellites, Frequency Hopping Spread Spectrum (FHSS) radio is generally used. Most land-based wireless bridging and products have a range of 3 miles to 6 miles but some have a 25 mile range. While typically in the 2Mbps range, more recent wireless products can operate in the 10Mbps range.
Microwave Transmission
A common form of wireless media used for bridging and internetworking is the microwave, which was first used for communications after World War II. Essentially, microwaves are a radio frequency technology that use a high frequency electromagnetic wave that vibrates at one GigaHertz and above—at approximately one billion cycles per second. Microwaves use higher frequencies than radio waves and are better able to deliver throughput and communications performance.
There are two basic types of microwave transmission systems.
- Terrestrial
- Satellite
- Terrestrial Microwave Transmission Systems
Terrestrial microwave transmissions systems are used when a local cable network would be too extensive, expensive or impractical. Natural or manmade barriers often present nearly insurmountable obstacles, and prohibit the use of cables for transmission without a great deal of expense. Microwave systems can be a practical solution.
Terrestrial microwave transmission systems use directional parabolic antennas (or satellite dishes) that send and receive data as waves of low-range GigaHertz (GHz) signals. Both Time Division Multiplexors (TDM) and Frequency Division Multiplexors are used in microwave systems with TDM being more prominent in digital systems. Radio transmission is usually frequency or amplitude modulated.
Microwave transmissions are made up of highly focused signals that require a physical line-of-sight path. In other words, microwave transmissions are concentrated energy waves requiring a clear point-to-point path between the sending and receiving antennas. Often, intermittent antennas are used to relay the signals over long distances.
In many cases, terrestrial microwave transmission systems must operate over a licensed frequency: A frequency owned by an organization—such as a government agency—that monitors, leases and regulates a particular frequency. Use of a licensed frequency can incur added costs and time limits due to licensing commissions and restraints.
Terrestrial microwave systems are commonly used to transmit signals from building to building. They can also be used to transmit data within a building, such as in an office setting. Omni-directional hubs using microwave technology are easy to set up and allow communication between workstations via tiny microwave transmitters.
Terrestrial microwave transmission systems operate at frequencies between 4 to 6 GHz, and 21 through 23 GHz. Terrestrial transmission system costs vary because there are a number of options. Terrestrial microwave systems used within a single building can be relatively inexpensive and highly effective up to hundreds of meters.
However, larger terrestrial microwave systems designed to cover longer distances can dramatically increase a system’s price. Consequently, some companies lease terrestrial microwave systems. Leasing eliminates the cost of building a system. Over time, however, a terrestrial system's accrued leasing cost can outstrip a new system's purchase price.
There are a number of advantages for implementing a terrestrial microwave system, as it is comparatively simple to implement; however, heavy rain and other factors can interfere with the signal.
| Terrestrial Microwave Systems | |
|---|---|
| Advantages | Disadvantages |
| Long distance transmission | FCC licensing required |
| Speedy MHz and GHz transmission | Performance degraded by weather |
| Flexible analog or digital transmission | Frequency congestion |
| Transmits voice, video, and data | Power lines can interfere |
| Avoid most right of way problems | Flash light pattern transmission |
- Satellite Microwave Transmission Systems
Satellite transmission systems are similar to terrestrial systems. Satellite systems use parabolic antennas, operate on low GHz frequency and rely on line-of-sight (point-to-point) antennas.
Most satellite transmission systems, however, use orbiting antennas that are located 22,500 miles above the earth's surface at the equator. These floating antennas are orbital satellites and rotate in accord with the earth's rotation in geo-synchronous orbits. This way the satellite rotates at the same rate as the Earth and turns so that the satellite is always in the same position over the Earth. They are also separated by 1.5 to 3 degrees of arc so that signals do not interfere with each other.
The satellite is made up of transponders and antennas. The transponders amplify signals from the Earth and send them on to other Earth receiving stations.
Orbital satellites make it possible for a satellite transmission system to send a signal halfway around the earth. In addition, satellite systems can reach extremely remote receivers and mobile computers.
How Satellite Systems Work
Satellite systems consist of a space component, a signal component, and a ground component. Of course the space segment consists of the leased satellite transponder. The signal portions include the frequencies and protocols used for communicating with the satellite, distance factors, and any sources of signal interference. The ground portion includes the Earth stations, the antennas, and the multiplexors that access the satellite systems.
Antennas, or satellite dishes, receive a signal from a LAN through various media. The received data is transformed into binary-coded microwaves and transmitted to an orbital satellite.
The signal is then redirected off the satellite toward a location on earth or toward another orbital satellite. If the location of the receiving antenna is on the other side of the earth, the signal may visit several orbital satellites before reaching its intended destination.
Because microwave satellite systems use orbital satellites to complete a transmission, sending a message down the street or across the involves the same process as sending the message to another continent. Due to the vast distances a signal must travel, there can often be fluctuations, delays, or changes in the signal. These variations are propagation delays, and can last anywhere between .5 and 5 seconds.
Satellite microwave transmission systems operate between 4 and 14 GHz, and depend on highly expensive orbital satellites. Many satellites use C-band transmissions that operate in the 6/4 GHz range (6 uplink/ 4 downlink). Although these signals are relatively weak and require large earth station antennas, they are relatively resistant to atmospheric interference and can operate in fog and rain. C-band signals can also be used in terrestrial point-to-point microwave transmissions.
Complementing the C-band signals are Ku-band transmissions, which are often located near the centers of cities. These operate at 14/12 GHz (14 uplink/ 12 downlink) and are reasonably immune to any terrestrial microwave network interference. Although Ku-band signals are much stronger than C-band signals, they are subject to weather interference.
Several large companies actually own and control individual orbital satellites, leasing their satellites to smaller companies that cannot afford to launch their own. Sometimes, having determined that building and launching a satellite costs less than building a cable network, a company will launch a satellite system of its own.
Installing a satellite microwave transmission system is an extremely technical and exacting process. Only well-versed technical professionals can perform the difficult installation that requires precise measurements and adjustments.
Satellite data rates are generally between 1Mbps to 10Mbps. Interference, can be attributed to a number of factors including the size and limitations of the antennas, exact frequency, power source, weather and atmosphere. In addition, microwave systems can be intercepted, though most signals are encrypted as a matter of security.
Satellite Broadcasting
In the past it was extremely difficult to communicate from a number of remote places across the globe. You would have been required to carry huge satellite phones that would have to align perfectly and would have to remain in one spot to stay connected. Communicating around the world was often a hassle with multiple telephone contracts, multiple phone numbers and multiple bills to pay.
Enter Iridium services. First developed by Motorola engineers in 1988 and commercially available for a brief time in 1999, Iridium service allowed the user to connect from practically anywhere on the globe with much smaller satellite phones. Multiple accounts were eliminated and users could receive just a single telephone bill.
Unfortunately, Iridium has gone out of business, but others are picking up with the same concept: Teledesic and Odyssey. Their goal is to provide affordable telecommunications services worldwide for businesses, educational institutions and individuals world wide. These services include teleconferencing, videoconferencing, voice and data communications and broadband Internet access.
