IP NETWORK DESIGN, PROVISION & SUPPORT FOR CCTV SYSTEMS
A Keyfort Briefing:
Internet Protocol (IP) networks are now the de facto
standard for communication between computers both within the
workplace and globally across the Internet. This networking
technology has been combined with the bourgeoning growth of
CCTV to provide remote control, central alarm, number plate
recognition, recording and remote surveillance systems.
Whilst the convergence of these technologies is providing
substantial benefits for both businesses and local
governments, it is important to consider the particular
demands placed upon the IP network in order to, rapidly
realize the benefits of the capable CCTV systems which are
now available. It is for this purpose that this briefing has
been provided.
Glossary of terms
BANDWIDTH: Although derived from
the band of wavelengths available for transmitting data,
it’s now commonly used to describe the volume of data a
network may accept in bits per second (bps). A single
alphabetical character is stored in eight bits of data known
as a byte.
IP: Internet Protocol – an
agreed standard for transferring information between
computers connected to the Internet, WAN or LAN.
LAN: Local area network - a
network using agreed communication standards to facilitate
the transfer of information between computers within an
office using one or more of the following copper, fibre
and/or wireless links.
JITTER: Where video or voice
transmissions freeze, slow or speed up resulting in
distortion of the picture or voice.
Latency: The time, usually
measured in milli seconds, to transit between two end points
on the network. Latency should not be confused with the
bandwidth.
NPR: Number plate recognition –
automatic reading and logging of vehicle registration
plates.
TCP: Transport control protocol
– an agreed standard for reassembling, at the receiving
computer, packets of data in the order in which they were
sent including requesting resending any packets of data that
have been ‘lost’ in transit.
VPN: Virtual private network –
provides a secure, encrypted link across the Internet
between two or more sites.
WAN: Wide area network – a
network using agreed communication standards to facilitate
the transfer of information beyond the office to other
computers using one or more of the following copper, fibre,
wireless and satellite links.
Overview
The IP network was originally designed to provide resilient
links between sites for the transfer of data. By splitting
the data flow into packets of data such resilience is
provided; each individual package is sent by the best route
at the time of sending. This means that if a particular link
is broken or busy the subsequent packets of data will use
different links, if available, to reach their destination.
When a particular node in the network is busy it will buffer
the data packets if possible or discard them if not.
Consequently the data packets may reach their destination
computer out of the order in which they were sent or
possibly not at all. This is dealt with by a network
transport control protocol (TCP) that ensures that the
packets of data are reconstituted at the receiving computer,
in the order that they were sent, including requesting data
packet resends where the originals were ‘lost’ in transit.
This means of transmitting data across the IP network
ensures the integrity of data transfer, but at the expense
of time synchronization. This works well for file transfer
and data access but is not best suited for video and voice
transfer. For video and voice traffic some loss of packets
may be sacrificed provided that the transit time is kept
short and constant to avoid information corruption and
jitter. Thus general considerations to be met when
implementing an IP network for CCTV use include: bandwidth
allocation, traffic prioritisation, latency and resilience.
Additionally, all types of IP network traffic (data, voice
or video), require administrators to ensure the security of
both LAN resources and information transfer across the
Internet or WANs. The basic steps to meeting this
requirement are the provision of suitable physical security,
authentication, data encryption, VPNs, firewalls between
network segments, network antivirus, intrusion prevention
and alerts, network traffic logging, reporting and
monitoring.
Specific IP network for CCTV
considerations:
Packet loss
Since even well-engineered IP networks tend to have a small
residual packet loss rate caused by low-probability
statistical congestion events and amplification of bit
errors in the underlying hardware, most professional
solutions use some kind of forward error correction to
ensure that the encoded video stream can be reconstructed
even if a few packets are lost. This is typically applied at
the packet level, since the encapsulated video bitstream is
typically only designed to tolerate low levels of bit or
burst errors, rather than the loss of whole packets.
Resending packets is not an option because of the sequential
nature of the underlying video signal. For live video, a
resent packet would arrive well after the arrival of the
next frame of video.
Network delay variation
Network delay variation can be kept to a minimum by using a high-speed network backbone, and ensuring that video traffic does not encounter excessive queue delays. This is typically done by either ensuring that the network is not too close to its full capacity, or that video traffic is prioritized using traffic engineering techniques.
The remaining delay variation can be removed by buffering, at the expense of added latency. If forward error correction is used, a small proportion of packets arriving after the deadline can be tolerated, since they can be dealt with by being discarded on receipt, and then treated in the same way as lost packets. Added time delay is particularly unwelcome in cameras as it makes operator control difficult at values over 250ms.
Timing reconstruction
The other problem presented by latency variation is that it makes synchronization more complex by making the recovery of the underlying timing of the video signal far more difficult. This is typically solved by genlocking both ends of the system to external station sync signals, typically generated from sources such as GPS or atomic clocks, thus only requiring the extraction of coarse timing information at the receiving end in order to achieve high-quality video synchronization. The extraction of coarse timing data is typically done using a phase locked loop with a long time constant.
Adequate bandwidth
Even with packet loss mitigation, video over IP will only work if the network is capable of carrying the content with some reasonable maximum packet loss rate. In practice, this means that video over IP will not work on overloaded networks. Since IP does not in itself offer any traffic guarantees, this must be applied at the network engineering level. One approach to this is the "quantity of service" approach which simply allocates sufficient bandwidth to video-carrying traffic that it will not congest under any possible load pattern. Other approaches include dynamic reduction in frame rate or resolution, network admission control, bandwidth reservation, traffic shaping, and traffic prioritization techniques, which require more complex network engineering, but will work when the simple approach of building a non-blocking network is not possible.
Use by Security Industry
Within the security products industry, IP-based Closed Circuit Television (CCTV) is making gains on the analog market. Key components of IP-based CCTV remain consistent with analog technologies: image capture, with a combination of IP-based cameras or analog cameras using IP-based encoders; image transmission; storage and retrieval, which uses techologies such as RAID arrays and iSCSI for recorded and indexed video; and video management, which affords web browser-enabled management and control of IP-based CCTV systems. One key advantage of IP-based CCTV is the ability to use secure network infrastructure, providing adequate bandwidth and availability of switching and routing, rather than coaxial cabling. However, running bandwidth-intensive surveillance video over corporate data networks without ensuring adequate bandwidth provision will result in network congestion.
Example - IP network provision for a number plate recognition (NPR) system
Keyfort provides for a waste management company (WMC) a secure IP network that links a Dallmeier NPR system, previously installed by a third party, to the WMC’s offices and those of the county council whose community amenity sites are being monitored.
The Dallmeier International - Detector NPR, an Intelligent Video Analysis System, uses smart sensor technology and progressive hardware and software technologies to detect and identify the number plates of stationary and moving vehicles (up to 100 MPH), even in bad weather or darkness.
The secure IP network provided, monitored and supported by Keyfort, enables remote management of the DI-Detector NPR system. It also ensures that the number plate information is encrypted for transfer from the community amenity sites to the county council’s central database. From this information various actions can be performed including the opening of gates, switching of traffic lights and triggering of alarms.
The applications of such a system include:
- Gate control for car parks and company car parks
- Automatic recognition of driving direction
- Visitors and associates are managed in two separate lists
- Unknown vehicles (number plates) can be reported
- For every recognised number plate a picture is saved
- Recording of video sequences by software triggering a Dallmeier recorder with possibility of searching number plates with PView in the data base
- Control of gate and/or traffic light using relay contact
- Operation of system possible by using browser interface of any working station in the network
- System access is password protected (separate user rights for user and service)
For further information please do
not hesitate to contact Keyfort tel. 08707 273535 email
information@keyfort.co.uk
Your feed back on this briefing document would be
appreciated and dealt with promptly, thank you.
References.
Professional video over IP - http://en.wikipedia.org/wiki/Professional_video_over_IP
See RSVP for one approach to IP network traffic engineering.
The Pro-MPEG Wide Area Network group has done much recent
work on creating a draft standard for interoperable
professional video over IP.
3. Dallmeier International -
http://www.dallmeier-electronic.com