Wireless communication? Wireless communication uses a different communication medium than cables. In reality, wireless networks connect most of the time to wired networks and thus constitute mixed or hybrid networks.
Wireless communication is mainly suitable in two situations:
- Networks or mobile stations
- Temporary connections
Wireless communication requires transmitters and receivers. When these transmission devices belong to a company, we speak of “local networks” or “wide area networks”, when they belong to a telecommunications company or a public service (AT&T, MCI or SPRINT in the United States), we speak of “mobile computing”.
To establish a wireless connection to a wired network, one of the computers in the wired network must be used as an access point for the wireless stations. The computer used as an access point and mobile computers must be equipped with a wireless network card and a transceiver.
The advantages of a wireless network
The benefits of a wireless network are many, but opportunities to take advantage of them are rare:
Temporary connections
Emergency equipment
Mobile equipment
Increase the extent of a network and the distance between substations
Enjoy the benefits of a network in places where there are no cables
Transmission techniques
There are four transmission techniques:
The infrared
The laser
Narrowband radio (single frequency)
Spread spectrum radio
The infrared
Infrared is a beam of light.
Infrared transmissions must be very intense so that there is no confusion with the many light sources that exist in a room (windows, neon lights, television, bulbs, etc.)
Infrared light has a wide bandwidth, the flow rates are relatively high, but the range is low:
10 Mb/s.
30 meters
An infrared network is convenient, fast, but sensitive to light interference. The beam must never be cut or the transmission is interrupted.
Types of infrared networks
There are four types of infrared networks:
- Line of sight networks (transmitters and receivers must be close to each other)
- Diffusion infrared networks (infrared signals are reflected on walls and ceilings over a distance of 30 metres, but the flow rate is slow due to signal rebounds)
- Reflective networks (computer transceivers transmit signals to the same point that acts as a router and redirects them to the destination computer)
- Broadband optical link networks (the performance is comparable to a cable network and allows multimedia files to be transmitted)
The laser
The laser is a technology similar to infrared in that it requires direct visibility. The laser is also called “coherent light”.
Narrowband radio (single frequency)
The narrowband radio (single frequency) works like a radio, the transmitter and receiver must be set to the same frequency.
Narrowband radio (single frequency) does not require line-of-sight, the broadcast range is wide, but the transmission speed is low:
1650 meters
4.8 Mb/s
Transmission by radio waves requires a license or authorization from local authorities:
The Federal Communications Commission in the United States
The Ministry of the Interior in France
Spread spectrum radio
Spread spectrum radio transmission technology broadcasts signals over a certain frequency range.
The bandwidth is divided into several communication channels. Network cards for “spread spectrum” are set for a predetermined time on one of the channels, then switch to another channel, this is called frequency hopping. All computers are synchronized to “jump” at the same time. To intercept such signals, it is necessary to know the channel change algorithm….
The baud rate is low but the range is high:
25 Kb/s to 4 Mb/s
250 meters inside and 3000 meters outside
The spread spectrum radio transmission technique makes it possible to create a true wireless network.
Point-to-point transmission technology
The point-to-point transmission technique is not a true network transmission technique. This technique allows signals to be transmitted in series only between two computers.
The point-to-point transmission technique uses a point-to-point radio link:
Point-to-point radio waves pass through walls, floors and ceilings
The signal transmission is carried out in series
1.2 Kb/s to 38.4 Kb/s
70 metres inside and 500 metres in direct visibility
Point-to-point transmission technology requires specific components:
An isolated transceiver
A host transceiver
Transmission over wide area networks
Transmission over wide area networks consists of transmitting signals over long distances, but requires special equipment.
Wireless bridges (WIRELESS BRIDGE) for wireless local area networks allow two networks to be connected 5000 meters apart, so two buildings can be connected without the need for cables. The AIRLAN/BRIDGE PLUS bridge, which uses spread spectrum radio transmission technology, creates a wireless main link (a backbone or BACKBONE) and thus links several remote sites. This type of equipment is relatively expensive but saves, among other things, earthmoving costs or line rental….
Long-range wireless bridges can connect sites 40 kilometres apart without using American T1 digital lines (1,544 Mb/s) or microwave connections.
Mobile computing
Mobile computing is carried out through telecommunications companies or public services. Mobile computing is very useful for people who are often on the move and who need to exchange e-mail or transmit files.
Mobile computing works with different types of hardware:
Laptops and laptops
PDAs (Personal Digital Assistant)
Baud rates are slow, and can be slowed down if they include error correction:
8 Kb/s to 19 Kb/s
Mobile transmission techniques
Mobile computing signals are transmitted by different means:
Packet radiocommunication
Cellular networks
Satellite stations
Packet radiocommunication
Packet radiocommunication is a transmission technique that divides a message into packets. A package is always composed of the same elements:
The address of the source
The address of the destination
The information necessary for error correction
Part of the message
The packets are sent to a satellite that forwards them to the recipient computer.
Cellular networks
The cellular network must connect to a cable network. The Canadian company NORTEL manufactures an equipment, the Ethernet Interface Unit (UIE), which connects a cellular network to a cable network.
The Cellular Digital Packet Data (CDPD) system of cellular phones allows computer data to be transmitted over analogue voice networks.
Satellite stations
The transmissions pass through a satellite that rotates around the planet “earth”.
Internet access (download) can be carried by satellite for isolated people who do not have close telephone relays.
Microwave transmission technology
Microwave transmission technology allows buildings spread over relatively small areas to be interconnected, but requires direct visibility.
Microwaves are widely used in the United States:
In a university campus
In an industrial area
Between a satellite and a terrestrial facility
Between two buildings
On the water or in a desert
A microwave installation is composed of several elements:
Two radio transceivers, one to receive, the other to transmit
Two directional antennas oriented towards the transceivers and installed as high as possible
