Wireless and Satellite Technology

Spread spectrum transmission was developed during World War II to provide a wireless, secure means of communication for the military. The technique used was called ‘frequency hopping’; a transmission would move from frequency to frequency based on a specific algorithm. The length of time spent on any one frequency was a fraction of a second, making it difficult for an enemy to jam a transmission, if they were able to find it in the first place.

A second type of spread spectrum transmission is called Direct Sequence Modulation. The transmission is spread over a wider frequency, and orthogonal digital spreading codes are used to enable multiple users to share the same band. Instead of avoiding interference, the effect of narrow band interference is reduced by spreading it over the band in the receiver.

Wireless LANs have been evolving over the last decade as an alternative to wired LAN structures. Wireless LANs primarily operate within the 900 MHz, 2.4 GHz and 5.8 GHz frequency bands. The devices used are generally designed to contain their signalling within a 100 to 250 metre range.

The earliest wireless LANs operated in the 900 MHz band. The devices had a nominal data rate of 500 Kbps.

The next generation of wireless devices evolved in the 2.40 – 2.483 GHz band. Microwave ovens are a major user of this frequency band, making it necessary to use more complex modulation schemes. Data rates of up to 2 Mbps were achieved.

A third generation of devices, still operating in the 2.4 GHz band, using still more complex modulation formats is allowing a data rate of about 7 Mbps. However, cordless telephones are being developed operating in this frequency band. There is a danger of the band becoming too crowded.

Developments are also being made in the 5GHz band. This frequency band has fewer potential interferors. Data transfer rates of 10 Mbps are being achieved.

Wireless networks are implemented with two basic types of components, a Network Adapter which is the electronic interface between the client computer and the network, and an Access Point which provides a bridge between the wireless network and a wired network. A wireless network can consist entirely of Network Adapters or a combined network in which wired and wireless connectivity is employed.

Wireless implementations utilise protocols compatible with TCP/IP. Rather than using CSMA/CD which is not practical for wireless applications, a Collision Avoidance protocol is employed. A ‘listen before send’ procedure is established; if the channel is in use, the user attempting to send the data stops, and, as in Ethernet, waits a randomly chosen time and then attempts the transmission again.

The IEEE standard 802.11 covers wireless LAN in the 2.4 GHz frequency band. As yet there is no standard for 5 GHz. This is not surprising given the time it takes to approve a new standard.

There are three types of satellite network:

Geostationary satellites orbit about 36,000 km above the Earth. This orbit is the region of space where the Earth’s gravity is counter-balanced by the centrifugal force of the satellite, enabling it to remain ‘fixed’ above a particular point on the Earth’s surface.
Low Earth Orbit satellites orbit between 450 and 1000km above the Earth’s surface. As they are relatively close to the Earth’s surface, a large number of satellites are necessary to provide global coverage.
Medium Earth Orbit satellites orbit about 15,000km above the Earth. The greater distance means fewer satellite are required to provide the same coverage.

Information is transmitted across the satellite networks using microwaves. Ground-based amplifiers transmit the signals to the receiving satellite. The satellite’s transponder receives the signal and then downlinks it to another base station or to another satellite. The re-transmission is normally on another frequency to ensure there is no interference between the received and the re-transmitted signals.

There is a growing interest in the use of satellites to provide routes for data traffic. It offers a solution to the difficulty of sites which cannot be connected by cabling, and potentially offers additional bandwidth for existing overloaded cable networks. A number of companies are developing an interest in the use of satellites for Internet access.

Satellite networking currently accounts for about five per cent of the telecomms market and is expected to grow between 10 and 15 per cent over the next ten years.

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