Let's network

Increasingly ,the workforce is becoming more mobile … people are either working from home or out on the road. It's in situations like these that remote access plays such an important role in providing connectivity to the corporate network. Within security, the ability to transmit digital images via networking helps increase levels of security considerably and maximises the potential of any digital recording system.

A network is simply a group of two or more computers linked together. Many types of network exist, but the most common types are Local Area Networks (LANs) and Wide Area Networks (WANs). Networking enables the user to access images from any location – a clear advantage in security applications as a site can be monitored from anywhere at any time. Transmitting digitally compressed video over telephone lines is not new. What is innovative, is the use of digital compression technologies that provide video transmission at a standard appropriate for high security applications.

Many high security sites will not only have their captured CCTV images monitored at the actual site itself, but also at a separate control room and out of hours. Often, due to the nature of the premises being surveyed, the police or other authorities will want to maintain the ability to monitor activity at a remote location.

Remote control

In situations such as these the need to transmit video signals over long distances via networking is essential. In the security market, the ability to access data remotely helps to increase overall security by cutting reaction time. Most importantly, it means that resources are not wasted on nuisance alarms. Products that allow users to take control of a remote location improve security by allowing other users to view information as if they were physically there.

Transmission of images from sites on the other side of the world is now possible over wide area networks. Similarly, local images can also be transmitted over local area networks, and viewed on everyday computers. At the click of a mouse, a site in London can check up on and view images from a site in New York. Images can be e-mailed directly to other interested parties or saved as single bitmap images and printed out. Soon, images being relayed over the Internet and other exciting transmission technology will become everyday options for those with digitised images.

The two most important decisions to make when opting for network servers are:

1. The method of encoding the images and data to ensure speed and quality;

2. The physical medium through which the image is transmitted.

All transmission methods are comparatively slow and could represent real bottlenecks in the system unless the image data is converted to a format optimised for transmission. This will almost certainly be the same as the format used in storage as the requirements for stored images such as high quality, legal admissibility etc are not the same as those for remote viewing, e.g. fast update.

The image will be compressed to reduce the amount of data needed to be transferred and then reformatted and packaged according to the protocol best suited to the transmission medium.

A protocol is a set of rules that control the communications between computers on a particular network. These guidelines help regulate the following characteristics of a network: access method, types of cabling, allowed physical topologies and speed of data transfer.

Protocol for security

One of the most popular protocols in security applications today is Transmission Control Protocol/ Internet Protocol TCP/IP), as it is reliable, low cost and widely accepted. TCP/IP provides the foundation and framework for many computer applications, including the world's largest, the Internet, and is particularly suited to security as it is fast, effective and reliable. TCP/IP is used by many digital recording manufacturers.

Once a protocol has been selected, the image then needs to be compressed in order to speed up the transfer time. Compression is the task of removing non-essential information from an image, in order to reduce the overall size of the data. It works by eliminating similar groups of pixels and information within the image or within successive frames.

All compression systems operate by selecting areas of nearly the same colour and representing this block with a short code defining its colour, area and position. Some systems compress further by separating the information in the frame that has changed from that which has stayed the same. Only the changing information is transmitted, while the decoder replaces the missing information at the receiving end with the aim of disturbing the image as little as possible.

In the security market, the ability to access data remotely helps to increase overall security by cutting reaction time. Most importantly, it means resources are not wasted on nuisance alarms

There are many different types of digital compression methods, the most commonly used in the security industry being JPEG and WAVELET for storage as these store a series of individual pictures which can be very high quality and good legal evidence. MPEG or H263, which are not suitable for quality recording, are well suited to high compression for streaming data over networks.

Video compression makes use of the innate redundancy within real-life images. Compression removes the pixels that relate to these redundancies. As a result, a condensed packet of digital video data, which requires less bandwidth for transmission, can be utilised.

Transmission methods

The four methods of transmission in use are: telephone-based systems, allowing national and international remote monitoring; fibre optics, and infrared or microwave links for shorter ranges.

Telephone transmission is the most common method currently used to transmit images for high security applications because it uses an existing, readily available communications infrastructure, providing a relatively cost-effective installation. Various techniques are used to process images and transmit them over telephone lines. The three most common are slow-scan, conditional refresh and digital signal processing.

Slow-Scan systems, as far as security applications are concerned, are unable to meet the basic requirements. It can take as long as 20 seconds for a slow-scan transmitted frame to appear on a monitor for viewing. A delay of this time scale would mean that an operator could easily miss important events.

Conditional Refresh systems scan and transmit a full image and then refresh only the segments that change. The transmission of the first frame can take roughly 20 seconds. The system then updates only the pixels, which correlate to the changes in the scene. This clearly decreases the time taken to display scenes including motion, but it also means that you are not seeing a 'true 'picture. Conditional Refresh systems, by the very way in which they work, are not suitable for particular types of applications, such as those where a large portion of the image is likely to change frequently,such as busy, spacious scenes.

Optical Fibre offers an effective solution to CCTV transmission. Fibre has a wider bandwidth and lower transmission loss attribute than coaxial cable, enabling high definition images to be transmitted over very long distances without the need for in-line amplifiers or repeaters. Also, the system is impervious to electrical interference.

Microwave has not been as prominent in the transmission of digital CCTV images. It is often used when fibres or cables are too expensive or simply not possible. It is generally cheaper to transmit microwave when crossing rivers or roads.

LANs and WANs

Once the data has been encoded and the method of transmission selected, the image is ready to be transmitted. The decision then has to be made as to which type of network you require e.g. LAN or WAN. Some digital recording systems offer both, allowing images to be transmitted either within the same complex or further afield.

A LAN is confined to a relatively small area, such as a site complex or Head Office complex. Computers networked via LAN are rarely more than a mile apart.

In a classic LAN configuration, one computer is nominated as the file server. It stores all of the software that controls the network, including the software that can be shared by the computers attached to the network. Computers connected to the file server are referred to as workstations. On most LANs, cables are used to connect the network interface cards in each computer.

The most common methods are: Ethernet, Local Talk, Token Ring, and ATM. Ethernet, however, is by far the most widely used standard in security.

Ethernet uses an access method called CSMA/CD (Carrier Sense Multiple Access/ Collision Detection). The computer listens to the cable before it transmits information through the network. If the network is clear, the system will continue to transmit. If some other node is already transmitting on the cable, the computer will wait and continue the process when the line is free.