Double Knock Detection

Double knock is a concept that has long been at the heart of security system design. Whilst often viewed as an intruder detection technique, the core principle can be used across a wide range of security applications, regardless of the technologies being implemented. The deployment of verified alarm activations during the early days of the ACPO Security Systems Policy use a form of double knock alarms, and with the growth of smarts systems the approach not only enhances security but can also add business benefits for a wide variety of users. The Benchmark Smart Solutions project considers how this simple technique can add value for many mainstream applications.

They say that the best ideas are often the simplest ones, and this is true when creating advanced security systems. Solutions are often designed to deliver layers of protection, an approach which allows appropriate actions to be taken as the potential risk of an event or incident increases.

For example, someone entering a protected area might be an intruder or criminal, and may well represent a threat against the site. However, if there is a chance that they might also be someone who is acting innocuously, has entered a protected area in error or who has genuine cause to be where they are, then generating an alarm is not always the best outcome.

However, the first detection – whether generated by a detection device, video analytics or any other relevant trigger – does give the site users a chance to establish some type of parameter. It could, for example, trigger an announcement during working hours, directing the person to an office or gatehouse. It might switch signage to indicate that the area is private. It might even send a notification to on-site personnel to inform that activity has been detected.

In some cases, it might actually do nothing at all except to create a pre-alarm state in the system itself.
If the detected person fails to heed announcements or signage, or deviates from expected behaviour such as going directly to an office or reception area, then a secondary alarm of event confirms that the event is probably not innocuous and that action should be taken.

This approach to layering security goes by many names: confirmed activation, sequential alarm, staged alarm, double knock, etc.. In truth, the term ‘double knock’ has been around for many years, but has recently been predominantly used by manufacturers of intruder alarm control panels. The reason for this is that in many intruder alarm systems, the implementation of a double knock or sequential alarm is necessary to meet standards for first police response. As a result the intruder alarm definition is slightly different to systems using other technologies.

Any technology

In theory, the use of double knock techniques can involve a wide range of technologies. For example, two detection devices triggering in a single zone in a specified time window is a typical use of the approach, as is two adjacent zones triggering in a specific order within a defined time frame. Some would even argue that a single device triggering twice could be viewed as a double knock application.

The important issue when designing a smart solution is to not become bogged down with alarm standards-based definitions, but to consider first the application of the double knock approach.

What needs to be achieved, and what are the best and most reliable triggers that can be implemented? Is the technique being deployed to ensure that activations are not caused by environmental issues, or will a threat be indicated by a sequence of events?

For example, an external passive infrared detector might generate an nuisance alarm due to animals or wind-borne debris. In order to ensure that this is not the case, video analytics covering the same area might need to detect a person or vehicle in order for an alarm to be passed to an operator. This could see a variety of technologies – PIR detection, photo-electric beams, video analytics, etc. – deployed so that each technology effectively verifies the others.

An increasingly common approach, and one which lends itself well to business intelligence, is for a sequence of events to be analysed to identify unusual behaviours. For example, if someone enters a site they could be there for good reason, or they might not present a threat, even if they should not be there.

However, if they then progress further into the site and enter specific areas, the risk becomes real and the generation of an alarm is required. This effectively allows the creation of a pre-alarm state.

When it comes to business intelligence uses, defining a sequence of events can be helpful with a number of site management tasks. For example, detecting the arrival of a delivery vehicle rather than a customer’s car can allow a set of actions to be implemented, such a directing a driver to an empty loading bay or opening a gate to depot area. If the expected actions are not followed, the vehicle can be stopped and a notification sent to a relevant member of staff.

Even in domestic applications, a double knock approach can add value by increasing security for homeowners. For example, a detection device might sense somebody approaching the premises on a path or driveway. This might be a milkman, a post man or a neighbour, and will not require notification to the user. However, if they then avoid the front door, or carry on beyond it to access the rear of the property or an outbuilding, a notification or alarm may be required.

The inclusion of a double knock scenario should always be driven by the site requirements, but with today’s range of smart solutions, the implementation is often simpler than many think!

Implementing the technique

The important elements of any double knock detection implementation are initial and secondary trigger events (and maybe even a tertiary event), the creation of a time window (or a ‘stop’ event to end the pre-alarm stage) and the interaction between the various technologies.

As already mentioned, it is not necessary for double knock events to use different technologies. While verified alarms in intruder alarm systems do, and so will implementations designed to filter out nuisance alarms, the implementation of double knock systems for sequential actions can use the same technology.

Double knock scenario 1

If we take the scenario discussed earlier for a domestic premises with sequential alarms creating a notification, this is easily achieved.

The Benchmark Smart Solutions project uses the Texecom Connect system for its intruder detection, and establishing such a scenario is simple using Recipes from the platform’s Smart App.
For this example let’s assume a property is accessed via a gate and pathway leading to the front door.

The gate area is protected by a Premier External TD-W wireless detector (named External 1 for this example). Additional external detectors are located at the back door (External 2) and on a side path leading to a garage (External 3).

The householder doesn’t want a notification or alarm is someone approaches the house; the likelihood is that it will be an everyday event. However, if the caller deviates from going to the front door, either going around to the back of the house or approaching the garage, they will want to receive a notification.

To achieve this, the user creates a Recipe whereby the detector on the pathway (External 1) initiates a ‘cause’ alarm when triggered, and this remains active for a prescribed period. This period can be set by the installer or integrator via the Premier Elite control panel’s pulse timer function. For example, if a pulse timer is set to 5 minutes and this is the selected period, then the detector (External 1) will remain active for that length of time.

Both External 2 and External 3 can be set to be active on ‘cause’, and linked to External 1 using an AND condition.

If External 1 is triggered but is not followed by an activation from the other detectors, no action is taken.
However, the Recipe will trigger an action when External 1 and External 2, or External 1 and External 3 are activated, with either External 2 or External 3 triggering within the prescribed period following an activation from External 1.

The ‘effect’ of the cause can be to send a push notification to the user, switch on a video surveillance camera, trigger another device, generate an alarm condition, etc..

Texecom offers the Premier External TD-W which is a wireless Ricochet-enabled passive infrared detector. Optex, the Benchmark Smart Solutions project’s external and perimeter detection partner, also offers a wide range of external detection devices which are compatible with the Ricochet wireless mesh platform, allowing flexibility with regard to detection options.

Double knock scenario 2

In the past, video surveillance was predominantly used as a verification tool when linked with alarms. When an alarm condition was generated, video would be referred to as an aide to ascertaining the cause. Of course, in systems that are not monitored, this is a far from ideal situation.

Thankfully today’s smart solutions can make use of a wide variety of video analytics tools which can automate advanced detection. As a result, video can move from being a verification tool to becoming an essential part of a double knock scenario.

Axis Communications, the video capture partner of the Benchmark Smart Solutions project, supports third party video analytics applications which are simply added to a camera much as one would download an app for a smart device. This allows the use of highly specialised tools, such as line crossing, loitering, object left or removed, etc..

Where this can add to the potential of a double knock scenario is that a trigger need not be limited to simple detection. The issue with detection is that whilst it offers a powerful tool when detecting intrusion, it does not discriminate between different types of activity. However, video analytics can add behavioural filtering.

By way of an example, external detectors could be used to protect a depot. Such sites will typically have established perimeters which an intruder would have to breach in order to enter the site. However, this would not the case with animals, birds or wind-borne debris.

Using perimeter protection-based video analytics such as directional line-cross detection allows the external detection to be more robust, as activations would only be monitored following an analytics event at the perimeter. This ensures that an intrusion has been detected, and that the intruder is most likely a human and is exhibiting the behaviours typical of an event.

By reducing the likelihood of nuisance activations, any alarm events carry more credibility. Linking the video analytics is a simple task, as it can be achieved via a VMS. Milestone Systems is the Benchmark Smart Solutions project’s VMS partner, and the XProtect range includes a powerful Rules engine that allows ‘cause and effect’ scenarios to be created using AND/OR logic.

Subsequently, criteria can be created so that when the site is closed and the alarm armed, any video analytics detection at the perimeter will then create a pre-alarm state for the detector. Optex offers a range of external detectors which incorporate an IP-based convertor. This allows the analogue devices to be added to a VMS, simply and easily. The enhanced control and flexibility that this delivers ensures that double knock scenarios can link detection and smart video via a VMS rule.

Double knock scenario 3

A potential expansion of double knock scenario 2 is to make use of the Optex laser-based detector range.

Laser-based detectors are able to detect a variety of objects and can also discern the object’s size, speed and distance. When used with a VMS it is possible to add a plug-in that can identify the exact co-ordinates of a target.

Using this information, the detector can protect a large or open area, and when an activation occurs the location of the intrusion can be used to switch to video analytics on the most appropriate camera. If, for example, four cameras are used to protect a square perimeter, the laser detector can identify that an intruder is outside the western perimeter fence, and the camera covering this section can then be activated and its analytics monitored.

The laser-based detector range from Optex ensures that both the sensor and the video analytics combine to create a smart double knock scenario.

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