Digital dilemmas

Given the fact that CD is a WORM

(Write Once, Read Many) device, it tends to add credibility to the value of evidence. If your CCTV images are stored in an encrypted way on the CD then they really are tamper-proof, and should carry considerable weight – provided that the pictures are of good resolution and proper ‘maintenance’ procedures have been followed at all times. Similarly, CD boasts a single uniform standard, which makes it easier to read on a standard PC away from the recording site.

If encrypted images are saved for evidence it’s important that the viewing (or encryption) software is part of the back-up process to the CD so they travel together. Otherwise, the police or investigating body may have a wonderful collection of incriminating pictures but not the means to decode them.

A similar situation has been a problem for the police in the past where analogue systems are concerned. Here, there tend to be a large number of different multiplexers each with their own encoding method – thus no-one can decode recorded images away from site.

One major question that has often been asked is: Can digitally gathered evidence be used in British Courts of Law? The simple answer is Yes. The House of Lords gave due consideration to gathering digital evidence in the UK in 1997, prompting the formation of a special Study Group. The findings of this Study Group were published in the report ‘Select Committee on Science and Technology: Digital Images as Evidence’ (House of Lords, Session 1997-1998, 5th Report).

The Digital Imaging Procedures Document published by the Home Office Scientific Development Branch states that CCTV system managers must obtain – where necessary – the relevant authority to capture images. CCTV systems have to be regularly maintained and an audit trail kick-started as soon as possible. An audit trail for a digital system is similar to that for an analogue set-up, but in the former the audit trail must also be designed to keep records of which copy is designated the Master Copy and which is a Working Copy.

Since digital images can – in most cases, anyway – be replicated without degradation, and given the fact that the first storage medium is often integrated with the DVR (hard disk), the document lays down procedures for the creation of a Master and Working Copies for viewing and distribution to authorised parties. Master Copies must be stored securely in line with accepted evidential procedures.

To assist in this endeavour, some media manufacturers provide packs containing all of the necessary documentation, labelling and evidence bag to ‘get the end user started’ until a fully-customised system is implemented.

All about networking

The term ‘networking’ covers a wide range of possibilities. It usually implies huge IT systems, but in DVR terms this is not always the case.

If encrypted images are saved for evidence it’s important that the viewing (or encryption) software is part of the back-up process to the CD so they travel together. Otherwise, the police or investigating body may have a wonderful collection of incriminating pictures but not the means to decode them

TCP/IP (Transmission Control Protocol/Interface Protocol) is an IT standard method of communication within a network structure. Often abbreviated to IP, this means that each device has its own ‘identity’ or IP address, and can be contacted on the network by broadcasting that address through ‘pinging’. When it receives a ‘ping’, the device answers and creates a virtual link. Further communication is then possible following Password (and other) access checks.

As each device has its own address, numerous devices may simultaneously communicate using the same path to either the same or different hosts. Amounts of traffic are governed by bandwidth.

Bandwidth can be likened to a hosepipe... The bigger the bore, the more water is able to pass through in terms of volume per second. Likewise, the bandwidth of a network is described as the amount of data that can be passed in terms of ‘bits per second’ (the two terminologies often cross, and you’ll hear ‘pipe’ used as slang for bandwidth, for example: “Two Meg pipe,” etc). A ‘bit’ is the smallest unit of data in computing, with a value of 1 or 0. It is then prefixed with a kilo (1,024 bits) or a ‘mega’ (1,024 x 1,024).

In simple terms, a byte is equal to eight bits. One kilobit equates to 1,024 bytes, a megabit is equal to 1,024 kilobits and 1,024 kilobytes equate to one megabyte. A typical fast Ethernet 100/Tbase network will carry 100 Mbits of data if it’s 100% efficient. This translates into approximately 200 images per second if the picture file size is 30 kilobytes (assuming a 60% efficiency rating). Networks can vary in bandwidth from simple 10/100Base up to several megabytes.

Assuming that the DVRs have been designed with TCP/IP networking in mind, there is no reason why they should not connect to the company network to give access anywhere on that network. The application software should run on an ordinary PC so that it’s feasible for the security manager and his or her staff to access, view and review images online using their desktop PC.

At all times, consultation with the IT Department is essential. It’s likely that members of the latter will allocate a bandwidth that’s available on the system, or will determine a maximum bandwidth that the Security Department is then not allowed to exceed. In this case, it is most helpful if the selected DVR/DVRs has/have the ability to set a maximum bandwidth demand in software.

IP cameras or video on demand?

There are two schools of opposing thought on this subject. The IP camera is basically a standard CCTV camera with a Codec (network encoder/decoder) built in. The video signals are encoded (or compressed) at the camera then streamed via the network to a central server where they are viewed (or stored, or both). A continuous stream of compressed video is therefore available at the central point, or indeed anywhere on the network.

IP cameras require a lot of bandwidth (or even a dedicated network if there are many cameras). The cost of those cameras is generally very high, although it is beginning to come down. The significant factor, though, is the amount of compression required to reduce the bandwidth needs to the minimum – and the effect this may have on the final image quality or integrity.

Your system might be a single, stand-alone unit, but the opportunity to view, review and control it from your own PC on your desk can be a huge benefit. Alternatively, if you are a large corporate user with a large number of cameras on a single site (or sites) scattered all over the country, then networking has certainly become an important management tool.

IP network cameras require a lot of bandwidth (or even a dedicated network if there are many cameras in one installation). The cost of those cameras is generally very high, although it is gradually beginning to come down

TCP/IP networking allows the systems to be co-located in one place or distributed on a network. This network might be a LAN – or Local Area Network – or a Wide Area Network (or WAN, covering a large campus or even the country). It should make no difference to the speed, reliability or efficiency of the system.

For system integrity, it is desirable to monitor camera health in terms of signal presence (sync signals) and for any hint of sabotage (for example, the covering over of cameras). With camera numbers growing all the time, it’s often the case that many are not looked at except for those times when there is/has been an incident. That is too late if the cameras have failed or been sabotaged.

There are more sophisticated features that measure the camera field of view and will alarm if that field of view changes significantly. These include measuring signal presence (sync monitoring) and sabotage by covering or masking the camera (signal level monitoring). Both are very effective.

More sophisticated anti-sabotage systems have been developed in response to the problems experienced where cameras have been sabotaged by discreetly ‘moving’ the camera to change its view away from the intended crime scene prior to the event, or defocusing the camera. In these circumstances, the operator should be alerted at the earliest possible juncture.

Any system chosen should be able to grow and expand. By far the best type is, again, one with ‘open architecture’ networking, which will permit machines to be added at a later date and easily joined together into a single system.

Scaleability also means the ability to serve the requirements of two or more operators as the system grows. Via the network it should be possible for as many or as few operators as are required to view/review images from anywhere on the network.

Disk arrays, large databases

Large disk arrays are more vulnerable to disk failures. For example, it is logical to assume that a disk array with ten disks is reasonably more likely to have a disk failure than is a single disk system. If your CCTV system is large and ‘mission critical’, make sure you think long and hard about disk redundancy.

There are two types of disk array – redundant and non-redundant. A non-redundant disk array has the lowest cost because it doesn’t employ any redundancy at all and is widely offered, whereas price rather than reliability are the primary concerns. The problems appear, however, if a disk fails. The system stops working immediately... On replacement of the failed disk the whole database needs to be reformatted. Data is prone to loss if the end user’s not careful!

It is possible to create a redundant disk array by employing redundant hardware to tolerate disk failures using a RAID (Redundant Array of Inexpensive Disks) configuration.

However, there are several different RAID levels. In Level 5 RAID configurations, for example, there’s an additional redundant disk per array. The controller ‘stripes’ data and parity across multiple disks and accesses them in parallel. Thus if one disk should fail, no data is lost. The system will also keep on working.