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Engineering LibreTexts

3.9: Fixed Wireless

  • Page ID
    11109
  • This category includes all wireless-service-provider systems where the subscriber’s location does not change. Often, but not always, the subscriber will have an outdoor antenna for improved reception and range. Fixed-wireless systems can involve relay through satellites, or can be terrestrial.

    3.9.1   Terrestrial Wireless

    Terrestrial wireless – also called terrestrial broadband or fixed-wireless broadband – involves direct (non-satellite) radio communication between subscribers and a central access point. Access points are usually tower-mounted and serve multiple subscribers, though single-subscriber point-to-point “microwave links” also exist. A multi-subscriber access point may serve an area with radius up to several tens of miles, depending on the technology, though more common ranges are under ten miles. WiMAX 802.16d is one form of terrestrial wireless, but there are several others. Frequencies may be either licensed or unlicensed. Unlicensed frequency bands are available at around 900 MHz, 2.4 GHz, and 5 GHz. Nominally all three bands require that line-of-sight transmission be used, though that requirement becomes stricter as the frequency increases. Lower frequencies tend to be better at “seeing” through trees and other obstructions.

    Trees vs Signal

    me_in_tree_cropped_small.png

    Photo of the author attempting to improve his 2.4 GHz terrestrial-wireless signal via tree trimming.

    Terrestrial fixed wireless was originally popularized for rural areas, where residential density is too low for economical cable connections. However, some fixed-wireless ISPs now operate in urban areas, often using WiMAX. One advantage of terrestrial fixed-wireless in remote areas is that the antennas covers a much smaller geographical area than a satellite, generally meaning that there is more data bandwidth available per user and the cost per megabyte is much lower.

    Outdoor subscriber antennas often use a parabolic dish to improve reception; sizes range from 10 to 50 cm in diameter. The size of the dish may depend on the distance to the central tower.

    While there are standardized fixed-wireless systems, such as WiMAX, there are also a number of proprietary alternatives, including systems from Trango and Canopy. Fixed-wireless systems might, in fact, be considered one of the last bastions of proprietary LAN protocols. This lack of standardization is due to a variety of factors; two primary ones are the relatively modest overall demand for this service and the the fact that most antennas need to be professionally installed by the ISP to ensure that they are “properly mounted, aligned, grounded and protected from lightning”.

    3.9.2   Satellite Internet

    An extreme case of fixed wireless is satellite Internet, in which signals pass through a satellite in geosynchronous orbit (35,786 km above the earth’s surface). Residential customers have parabolic antennas typically from 70 to 100 cm in diameter, larger than those used for terrestrial wireless but smaller than the dish antennas used at access points. The geosynchronous satellite orbit means that the antennas need to be pointed only once, at installation. Transmitter power is typically 1-2 watts, remarkably low for a signal that travels 35,786 km.

    The primary problem associated with satellite Internet is very long RTTs. The the speed-of-light round-trip propagation delay is about 500 ms to which must be added queuing delays for the often-backlogged access point (my own personal experience suggested that RTTs of close to 1,000 ms were the norm). These long delays affect real-time traffic such as VoIP and gaming, but as we shall see in 14.11   The Satellite-Link TCP Problem bulk TCP transfers also perform poorly with very long RTTs. To provide partial compensation for the TCP issue, many satellite ISPs provide some sort of “acceleration” for bulk downloads: a web page, for example, would be downloaded rapidly by the access point and streamed to the satellite and back down to the user via a proprietary mechanism. Acceleration, however, cannot help interactive connections such as VPNs.

    Another common feature of satellite Internet is a low daily utilization cap, typically in the hundreds of megabytes. Utilization caps are directly tied to the cost of maintaining satellites, but also to the fact that one satellite covers a great deal of ground, and so its available capacity is shared by a large number of users.

    The delay issues associated with satellite Internet would go away if satellites were in so-called low-earth orbits, a few hundred km above the earth. RTTs would then be comparable with terrestrial Internet. Fixed-direction antennas could no longer be used. A large number of satellites would need to be launched to provide 24-hour coverage even at one location. To data (2016), such a network of low-earth satellites has been proposed, but not yet launched.