The world is increasingly moving towards a wireless future, and the benefits are obvious. Wireless technology also plays an ever increasing role in a wider variety of industrial applications. Given the obvious benefits which the technology delivers to security systems, should it have a prominent role in data transmissions, or do questions over its reliability makes it something that those involved in video surveillance should avoid?
The transmission of video images has always been something of a challenge. Back when 480 TV line composite video was referred to as ‘high resolution’, and mainstream video was in the region of 330 TV lines, systems such as slow scan transmission were implemented. These delivered low quality images, at very low frame rates, across PSTN lines. Even then, long before anything even approximating today’s data delivery speeds, people found the process infuriating. Slow scan was indeed slow, but it was also necessary! Because of this, the frailties of video transmission were accepted.
Interestingly, at the time, wireless video transmission was used for ‘problematic’ sites. 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. However, inevitably when the video data returned to a cabled link, the inevitable bottleneck developed. For those who simply could not justify the expense of an optical fibre network, video transmission was painful!
The point of this reminiscence is simple; wireless transmission of multiple video data streams is nothing new. Within the security industry, it has been used for a number of vital systems protecting high risk applications. 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.
Indeed, the core technology that underpins high bandwidth wireless transmission is regularly employed in some of the most critical applications imaginable. Whilst the profile of such solutions have risen in recent years, it is important to understand that there have been a core of manufacturers concentrating on the needs of the security sector for decades, with regard to wireless connectivity for video transmission.
The reasons for the increased profile are simple. As HD and megapixel video demands have grown, so too has the need for bandwidth. Manufacturers who offer Ethernet bridges to the IT sector have spotted this demand, and see video transmission as something of a target area. Whilst choice is always a good thing, it is important to ensure that the companies you partner with have an understanding of the market, but more importantly, fully comprehend security philosophies with regard to system design and continuity of service. If you get the feeling that they are paying lip service to video surveillance, viewing it as just another potential application, then you might be better off walking away! There are plenty of options out there, many of whom are well versed in the intricacies of security solutions.
Typically, video surveillance applications will use wireless technology in a ‘last mile’ capacity. It is still uncommon to find applications where the entirety – or even the majority – of the transmission path is wireless. 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 are certain to become ever more diverse. One area where the units are often employed is between satellite buildings, which are located in different parts of a locality. Where a single site exists, there will typically be a LAN infrastructure that covers the various buildings. However, if a company has two or three buildings in a small geographic area, it can be more cost-effective to employ wireless links than to attempt to install dedicated infrastructure. Such situations not only carry the cost of civil works, but often having to gain necessary permissions just makes the entire process something of a non-starter.
Even councils often take this approach as the preferred option if linking residential tower blocks. The lack of problems with line of sight make this a simple task, and the solution allows one building on an estate to receive signals from all others, and this central point can then transmit the video over traditional links to the local police, council control rooms, etc..
Where a site is bisected by something that makes hard-wiring problematic, wireless transmission can often be the only answer. The problem could be a road, a river or a railway line which needs to be crossed, and even if bridges exist, it is not best practice to have cables for security solutions where they can be accessed.
However, when looking at ‘last mile’ applications – applications where the hard-wired infrastructure falls short of its intended recipient – the most common reason to select wireless technology over a cabled option is cost! The cost of the required civil works to extend traditional cabling over a short distance such as one kilometre will make the expense of a wireless bridge seem insignificant. Not only will money be saved, but time too. It is, quite simply, a ‘no-brainer’.
Understanding how much bandwidth is available and how much is needed is critical in planning, designing and deploying a wireless video surveillance 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 connection to a network is higher.
The need for redundancy is another often-raised concern, and always comes with a cost. That much is true of bother wireless and hard-wired links. If any part of the tranmission system fails, the whole system fails.
The solution to the issue is the same for both technologies. It would be very unlikely that there would be a requirement or the cost 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 fully redundant!
If more complex higher risk applications are considered, such as a large multi-camera wireless backhaul solutions, this would probably require a redundancy system to be in place.
In some systems, spare previously configured units are held in stock for fast replacement should the link fail; a cost effective option but not instant. A higher level of protection can be obtained from placing primary and backup links side by side and run onto the network so that if one fails the other automatically kicks in, 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 backup than a wired, fibre or broadband link.
Wireless technology offers solutions where hard-wired alternatives cannot. While the technology has great flexibility, it is used as a problem-solver rather than a first choice in security solutions. Maybe with a little more understanding, such attitudes will change.
|Line of SightOften, when people discuss wireless transmission solutions, they refer to line of sight. This phrase can give rise to a variety of misunderstandings. Indeed, one prominent vendor of point-to-point wireless bridges describes line of sight as such: ‘Imagine your head is in the middle of the antenna, and if you can see the corresponding second antenna, you have line of sight’. Such descriptions are unhelpful, as they don’t really tell the whole story.|
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. Wireless signals don’t rely on visuals or straight lines, so that’s the first problem. 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, named after Augustin Fresnel, who is also the creator of the lens design used in many modern day detectors (his original lens design was used in lighthouses). 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 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 should also 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.