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Wireless Backhaul

by Benchmark

Networked solutions are increasingly utilising wireless technology, which also plays an ever increasing role in a variety of industrial applications. Given the benefits for security systems, should it have a more prominent role in video surveillance?

[dropcap]T[/dropcap]he transmission of video images has always been something of a challenge. Even when 480 TV line composite video was referred to as ‘high resolution’, transmission generally delivered low quality images at reduced frame rates.

Interestingly, at the time, wireless video transmission was in use. Where infrastructure was lacking, and the required civil works to implement it were not financially viable, wireless transmission was the tool of choice. Often the transmission rates on wireless links exceeded those of hard-wired links.

Wireless transmission of video data streams is nothing new. Sites operated by Governmental departments, the police and law enforcement agencies, military bases and other critical infrastructure have all used wireless transmission where problems have been encountered running traditional hard-wired links.

The core technology that underpins high bandwidth wireless transmission is regularly employed in some of the most critical applications imaginable. Whilst it was previously a specialist tool, today the technology enjoys an increased profile.

As HD and megapixel video demands have grown, so too has the need for bandwidth. Manufacturers who offer Ethernet bridges have spotted this demand. Whilst choice is always a good thing, it is important to ensure that the companies you partner with fully comprehend security philosophies with regard to system design and continuity of service.

The power of wireless
Many video surveillance applications will use wireless technology in a ‘last mile’ capacity. The technology will either be used to bridge certain obstructions that either cannot be hard-wired or where the civil works would be too costly, or to link a number of buildings in a small geographical area.

Obviously, there are many other options, and as the level of familiarity with high bandwidth wireless bridging technologies increases, so the applications become ever more diverse. One area where the units are often employed is between satellite buildings. If a company has two or three buildings in a small geographic area, it can be cost-effective to employ wireless links rather than to attempt to install a dispersed hard-wired LAN.

Where a site is bisected by a road, railway, river or other obstacle, wireless transmission can often be the only answer.

However, when looking at ‘last mile’ applications – applications where the hard-wired infrastructure falls short of its intended recipient – wireless technology can offer a cost saving over a cabled alternative! Eliminating the need for civil works to extend traditional cabling over a short distance will make the cost of a wireless bridge seem insignificant. Not only will money be saved, but so will time on site.

Line of sight
When considering wireless transmission solutions, line of sight must be considered. This phrase can give rise to a variety of misunderstandings.

When considering line of sight, the first thing you have to do is to put aside any notion that if you can see the receiving unit, all will be well. Such a supposition relies on visuals and straight lines, but wireless signals travel in waves.

The signal between two antennae spreads as it travels, so to consider the actual transmission path, it is best to consider an elliptical shape between the two points. This shape is referred to as the Fresnel zone. The Fresnel zone is three-dimensional, so a consideration must be made for objects either in the path, above or below the path, or to either side of it.

The radius of the Fresnel zone is widest at the midpoint of the transmission. This must be accounted for when considering line of sight calculations. The actual radius of a Fresnel zone is calculated using the distance between the two antennae and the wavelength of the signal. It should be noted that the radius increases with low frequency, high wavelength signals.

As a result, it quite possible for an object that does not seem to be obstructing line of sight to impact on the reliability and quality of a wireless link.

Another consideration is that where long distances are being covered, the curvature of the earth may need to be taken into consideration. Obviously, longer transmission distances equate to a greater radius for the Fresnel zone. If antennae are not correctly mounted, it is possible for the lower side of the zone to be obstructed by the curvature.

Whilst proper calculations should always be made, it must be remembered that waves are pretty resilient, and often will find their way past obstructions. However, calculations and correct testing should be used to ensure reliability and consistency.

Other considerations
Understanding how much bandwidth is available and how much is needed is critical in planning, designing and deploying a wireless application. If this basic information is not assessed at the outset, the full potential of any surveillance system will not be reached and the risk of compromising image quality, frame rate and connectivity is higher.

The need for redundancy is another often-raised concern, and always comes with a cost. If any part of the transmission system fails, the whole system fails.

It would be unlikely there would be a requirement – or the budget available – to enable a redundancy solution for all elements in a system. Because of the nature of the signalling, it could be argued that wireless links are more robust that hard-wired options, and few have concerns with ensuring that cabled transmission is redundant! If more complex applications are considered, such as a large multi-camera wireless backhaul solutions, this may require redundancy to be in place.

In some systems, pre-configured units are held in stock for fast replacement. A higher level of protection can be obtained from placing primary and backup links side by side, but this comes at a high cost.
In general, the use of a wireless link – provided it’s a good quality robust unit – would require no more redundancy than a wired or fibre link.

In summary
Wireless technology offers networked surveillance a transmission solution where hard-wired alternatives cannot. While wireless has great flexibility, it is often used as a problem-solver rather than a first choice path. Maybe with a little more understanding, such attitudes will change.

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