Wireless video links part 4

Wireless transfer of video signals, for example from surveillance cameras, has until now mostly been used in special applications. It has been a matter of surveillance of sites which have been spread over a large area or surveillance of industrial processes, vehicles, ships or military installations. In such cases microwave links have often been used to transfer the image information.
Now there are some signs of a change. In the new network based camera surveillance systems, cameras with built in wireless net links (WLAN) have recently been introduced. The operational range of these links is relatively short, in the order of 100 meters, but on the other hand it is possible to take advantage of the low cost of this technology. It rides on the back of the price development in the computer industry.

The classic microwave link
A classic video link works in the microwave band and is established as a powerful directional pointto- point connection between two carefully aimed antennae or dishes mounted on rooftops, in masts or attached to house walls. Such a link is characterised by a high transmission ability, long range and a relatively high price. A similar technology and frequency band is, among other things, used for distribution of broadband in areas with low population density or for handling programme collection within the radio and TV industry, which gives a hint of what kind of signal bandwidths are available.
A powerful aimed link of this kind offers the potential to transfer one or more picture channels, and works over relatively long distances – up to several tens of kilometres. But for the link to work, a free line of sight between the aerials is necessary, and over such long distances the transmission can be affected by heavy rain. The general use is limited by the fact that the equipment hardly can be regarded as cheap. Therefore microwave links are used primarily where the installation environment makes it necessary, for example to bridge long distances where alternative cable routes are not available.
The link communication takes place over licensed frequency bands, which normally require a licence from the relevant PTT organisations. Frequency planning is necessary, which means that it is mandatory to specify where the link passage will run.

The first link jump
Because the traditional analogue video signal is relatively demanding in terms of bandwidth (in the order of 4 - 8 MHz) the basic principle has always been to get it on to a cable or a fibre link for onwards transmission to a surveillance centre as early as possible. In practice we now talk about the first link jump, from the camera to a mux, video switch or corresponding equipment.
This air link is sensitive from a security point of view. Analogue signals transmitted via a radio link are difficult to protect, a fact that has been known for a long time, particularly in military contexts. In fact it is a well known strategy to force the communication of the enemy up in the air, as it becomes easier to intercept.
In this context it is however worth mentioning that the very accurate and directional aim of a microwave link makes it difficult for an assailant to listen in to the signal. The strength of the signal diminishes very quickly as soon as one departs from the link line, which means that in practice it is necessary to be precisely in-between the aerials in order to be able to intercept anything at all.

Encryption offers protection
In order to protect radio traffic it is common to digitize and encrypt it. Earlier methods to distort an analogue signal through so called scrambling is no longer in general use today. These methods are based on security-by-obscurity and there are not very many different ways to distort the signal. This is why only digital image transfer should be used in new installations, or digitised signals from analogue cameras, which can be sent directly over a digital link.
In these cases the signal is normally protected by encryption of the whole link. This is a convenient way to do it. All traffic is encapsulated – and the encryption process can be carried out by the link equipment, which through this process forms a kind of secure encapsulation of the image traffic. The video signal is transferred in plain language inside this encapsulation, and it is also possible alarm communication inside the protected ”encryption shell”.

Network cameras
We have already mentioned network surveillance cameras. These are a relatively new phenomenon in the surveillance market. Here one can take advantage of the technical development in recent years in the computer industry and simply connect the surveillance camera to a wireless network board.
The technology in such a network camera is normally based on WLAN (Wireless Local Area Network), a standardised link technology which in practice is nothing but a radio version of Ethernet, the dominating link technology in normal company networks (LAN, Local Area Network). Both these technologies have been standardised by the US engineering organisation IEEE. Ethernet has the designation 802.3 and WLAN 802.11. The difference is in the method of ”listening to the line” and the timing of transmission of signals over the net link in order to establish contact (establish a link). With WLAN it is relatively easy to connect the video image directly to for example a company network. There may however be some justification for building a separate network dedicated to surveillance and alarm communication. It is however not necessary to build a physically separated network. The surveillance net can easily be realised by means of WLAN-technology (virtual LAN) which connects to selected ports in the same network switch as in the normal enterprise network.

A relatively short range
WLAN uses a licence free frequency band around 2,45 GHz. The slot is a so called ISM-band (Industrial, Scientific, Medical), which can be used for other equipment as well, like for example microwave ovens. This is normally not a problem but one needs to be aware of noise pollution which occasionally may interfere with the equipment. The range is normally limited to approximately 100 metres, but by adding directional aerials it is often possible to achieve satisfactory transmission over distances up to a kilometre, sometimes a bit further.
The great advantages with WLAN-links are the price and the simplicity. Several camera manufacturers have today built network connections for Ethernet or WLAN into the cameras. If there is a WLAN connection in the camera, all that is needed is to install it and link it to an access point (AP) for WLAN nearby which in turn is connected to the traditional company network. Should WLAN be missing, but the camera be equipped with an Ethernet network connection, then it can be connected to a low cost external WLAN unit - which in turn can be connected to an access point in the enterprise network. Installing cameras of this kind is usually swift and easy. The part of the radio spectrum which WLAN uses doesn’t require any frequency planning, which makes it possible to rearrange the position of the unit freely.

The durability varies
The drawback with the quickto- install WLAN technology is on the one hand the reliability and secondly the security of the transfer of data. Let’s start with the reliability. Network cameras are to a certain extent the result of the development within the computer industry over the last ten years, and the product generations supersede one another in a never ending stream. The protocol in use for WLAN-communication has by now stabilised, and often there are programme updates available. It may be worth keeping an eye on the additions to IEEE 802.11 which have been published (a/b/g), where higher transmission speed has been made available. In order to benefit from the higher transmission speed without any problems, all units should be of the same brand and type.
Components in the physical construction are not always of the highest quality. WLAN equipment for consumer use is not always designed for 24 h. use over several years. A common example of a weaknesses of this kind is the net adaptors in low cost WLAN links. They usually break after around one year of use. It is therefore important – particularly in security applications – to purchase equipment which has been designed for continuous long term operation. The WLAN manufacturers have today broadly three different product lines on offer: consumer links, units for enterprise use and pure operator equipment.

Link security
The security is a second dilemma with WLAN cameras. Basically WLAN is an open link technology, which needs to be complemented with link encryption in order to protect the traffic channel. This can be achieved in several different ways: by means of a fixed encryption key, rotating encryption keys or keys controlled against a database (a radius-server or similar solutions).
Unfortunately the methods of attacks have also been developed, which means that the available methods of protecting the link itself today no longer offer full protection. To break into the radio link itself is no major task. In order to secure the confidentiality - the protection of the image information in the video link - one should encrypt the traffic also at a higher level. The level above the link level is usually called the net level and at this level one usually protects the traffic by means of tunnelling according to a standard family called IPsec (IP security). When using IPsec all IP traffic sent via the radio link is protected in an efficient way.