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The use of wireless transmission technology will significantly extend the scope of surveillance systems towards scattered locations, such as electricity relay stations, waterworks, elements of road infrastructure and many other remote places.
Wireless transmission can be realized by:
- WLAN
- LMDS (Local Multipoint Distribution Services)
- MMDS (bi-directional version of LMDS)
- light beams
Of course, the first step is the same as in wired installations, i.e. digitalization and compression. For this purpose, the same devices are used: web cameras, web servers, DVRs, DVR cards.
Monitoring systems using WLAN networks
WLAN (Wireless Local Network) is a technology allowing to build low cost wireless data transmission systems that are characterized by satisfactory parameters and quite long ranges. Additional advantage of this technology is short time required to build a system.
In most countries there are specific regulations that should be observed when building a wireless system, or there is only a group of products that don't need any license or registration.
Frequency bands and technical parameters of data transmission devices:
- 5.6 GHz, 1W ERP (preferred),
- 2.4 GHz, 0.1W ERP.
Standards:
- 802.11a - standard for 5GHz band, bandwidth up to 54 Mbps at 5.150 - 5.350 GHz and 5.470 - 5.725 GHz
- 802.11b - standard for 2.4GHz band, bandwidth up to 11 Mbps at 2.4 - 2.4853 GHz
- 802.11g - standard for band 2.4GHz, bandwidth up to 54 Mbps at 2.4 - 2.4853 GHz
Range of radio network depends on:
- Output power of the devices
- Cable attenuation
- Gain of antennas
- Sensitivity of the devices
- Attenuation between antennas
- Interferences from other devices
Presence of barriers in the first Fresnel zone causes that radio link still doesn't work properly. More about building WLAN networks you can find in WLAN installer's self-help book
Advantages of WLAN technology:
- Flexibility of installing and work even on the move
- Possibility of communication with places where cabling is impossible or uneconomical
- Possibility of easy modification and extension of the network
Disadvantages of WLAN technology:
- Possible problems with obtaining good quality and reliability of transmission
- Fading of transmitted signals caused by multipath transmission and changeable attenuation
- At longer distances antennas must see each other (for example buildings and trees are a barrier that can make transmission impossible)
- No channel allocation makes possible mutual interferences with neighbor networks
LMDS
LMDS (Local Multipoint Distribution Services), is a wireless system, using high frequency radio transmission (3.5 - 40 GHz band), within small areas of a few kilometers diameter. The system consists of the base station and a number of small stations. The achieved transmissions rates are about 155 Mbps for ranges below 10 km.
LMDS architecture
Advantages of LMDS:
- Small sizes of transmitting and receiving devices, providing easy installation on roofs or facades
- Possibility of flexible extension of the network and increase of its range.
Disadvantages of LMDS:
- Sensitivity to electromagnetic interferences
Video surveillance system installed for Europe Economical Forum in Warsaw using Alcatel 7390 LMDS system
MMDS
MMDS (Multichannel Multipoint Distribution Service) technology was developed for distributing about a dozen television channels using radio transmission. It was meant to ensure CATV-like offer of TV programs in non-urbanized regions. But it is also used to ensure access to the Internet and for data transmission. Signals can be transmitted over distances even up to 50 km from the base station. MMDS uses 2.5-2.7 GHz band ensuring bandwidth up to 10 Mbps/channel towards the subscriber. It is planned to increase this value to 27 Mbps in the future.
Advantages of MMDS:
- Small sizes of transmitting and receiving devices, providing easy installation on roofs or buildings' elevations
- Possibility of flexible extension of the network and increase of its range
- Relatively low frequency ensuring longer range
- Relatively low frequency means cheaper devices and equipment
Disadvantages of MMDS:
- Sensitivity to electromagnetic interferences
- Low bandwidth
Wireless optical transmission
Transmitting-receiving optical link unit of GoC company
This technology employs optical radiation of wave lengths 700 - 1500 nm (infrared radiation).
Two types of light sources are used:
- Narrowband light-emitting diodes (LED). These diodes emit spectrum in infrared band with bandwidth equal around 120 nm and optical power 1 mW. Achieved transmission distances are about 1.5 km with speed up to 1.25 Gbps
- Laser diodes, giving coherent radiation that has spectral width less than 2 nm. Optical power is a few times bigger than in the case of LED diodes. Multi-beam transmitters achieve transmission speed equal 2.5 Gbps at ranges around 1 km
Different topologies of wireless optical transmissions
1. Point to point link
2.Net structure
3. Ring structure
4.Point to multipoint link
Advantages of optical transmission:
- Narrow optical beam generated by lasers makes it impossible to be intercepted without interrupting it
- Independence from radio transmission systems, resistance to electromagnetic interferences
Disadvantages of optical transmission:
- Necessity of mutual visibility of transmitter and receiver
- Possibility of interruption of transmission (caused passing an object through the beam)
- Blurred impulses in the case of lower visibility and reflections
- High attenuation of signal in the case of fog, smog, smoke, rain, snow etc
The influence of the environment on the link range depends on the following factors:
- Attenuation related to geometric diffusion of the beam
- Attenuation related to natural phenomena:
- - rain
- - snow
- - fog
- - other factors that decrease visibility (air clearance) e.g. smog
- - air turbulence
At the present level of development and technological knowledge it is possible to create highly efficient, stable and secure links. The accessibility of such links is above 98%.
