IP
Video Security Surveillance - Understanding the Storage Issues
With
many users now requiring CCTV recordings for 90+ days the focus has been
placed on the storage costs of IP Video systems. Barry Keepence, IndigoVision
CTO, explains that with the very best compression technology and the right
storage architecture IP-CCTV systems can easily accommodate this requirement.
Many organisations are now requiring that CCTV video images are recorded
and archived continuously from all cameras for 90 days or more. In large
systems this can create a significant storage requirement. This is compounded
by users wanting to store video at the highest quality and maximum framerate,
particularly in homeland security and law enforcement applications where
CCTV footage is required for evidential purposes.
The storage element of a large IP-CCTV system can therefore dominate the
overall cost of the system, both up front costs and lifecycle costs as
hard drives will often need to be replaced over the life of the project.
With potentially large amounts of data being stored, the technology behind
the compression employed and the architecture of the storage solution
are critical to ensure these extended recording times can be accommodated.
Similarly, the security of the stored data is important so that individual
disk and equipment failures do not result in data loss.
The Scale of the Problem
The factors that affect how much disc storage is required are video resolution,
frame-rate, number of cameras and the quality of the compression technology
deployed. So when comparing systems the main variable is the compression
technology, as all other factors are constant. The way in which video
is compressed can make a significant difference to the storage requirements.
The following worked example demonstrates the storage requirements for
a typical system, comparing equipment that has an average compression
with the very best that is available.
The amount of digital data generated by a single camera:
Best compression: 20 Gigabytes/day
Average compression: 40 Gigabytes/day
This doesn't at first seem unmanageable since hard disk storage is inexpensive
and 750 Gigabyte disks are readily available. However, assume a system
with 100 cameras and the storage becomes:
Best compression: 2 Terabytes/day
Average compression: 4 Terabytes/day
Archiving recordings for 90 days gives:
Best compression: 180 Terabytes
Average compression: 360 Terabytes
The difference between an average compression and the best is 180 Terabytes
of extra storage and that's just for a relatively small system of 100
cameras.
NVR Performance
When it comes to storage, CCTV systems are very different to IT systems.
The amount of data coming from the cameras to the NVRs is huge and continuous.
The amount of data coming from the NVR to the users is very low and periodic.
The workload is constant, i.e. the rate of writing data to the disk is
constantly high, not in bursts as with typical IT applications.
The processing overhead for writing and reading the video streams to disk
is therefore an important factor in the overall performance of the NVR.
There can be a considerable difference in this overhead between different
vendors of NVR software. Software which can minimise this processing will
be able to handle many more camera streams per NVR. The best NVR server
software on the market has such a low CPU loading that 200 camera streams
can be recorded on the lowest specification server PC.
Therefore, in a large system with a 90 day recording requirement, the
limitation on the server is its storage, not its processing power.
Virtual Servers
Virtualisation is a proven software technology that is rapidly transforming
the IT landscape. Today's powerful PC hardware was originally designed
to run only a single operating system and a single application. Virtualisation
breaks that bond, making it possible to run multiple operating systems
and multiple applications on the same computer at the same time, increasing
the utilisation and flexibility of hardware.
In essence, virtualisation transforms hardware into software. Software
such as VMware transforms or ‘virtualises’ the hardware resources
of a PC - including the CPU, RAM, hard disk and network controller - to
create a fully functional virtual machine that can run its own operating
system and applications just like a ‘real’ computer.
Multiple virtual machines share hardware resources without interfering
with each other so that several operating systems and applications can
safely be run at the same time on a single computer. Dedicated standalone
NVRs can provide higher levels of reliability and redundancy compared
to PC NVR servers.
Storage
Architectures
The storage architecture for CCTV systems can be typically categorised
as either centralised or distributed.
Centralised Storage Architectures
A typical example of such as system would be a Casino, where a high density
of cameras are located in one building. The NVRs would be located in one
central IT facility, with a central network switch. In this situation
all of the video could be recorded on one server, however, this would
almost certainly be storage limited, not CPU limited.
Using the worked example above with a 100 camera system, one option would
be to use 10 NVR servers each fitted with an 18 Terabyte disk array, assuming
the system has the best compression technology available. This is still
a lot of hardware, but because the CPU utilisation is low, virtual servers
can be deployed.
Leading NVR vendors can run their software under applications such as
VMware allowing 10 virtual servers to be configured on one physical server,
considerably reducing equipment costs. In reality the typical casino uses
between 500 and 1000 cameras, so therefore this approach is an important
factor in keeping down costs.
Distributed Storage Architectures
A typical example of such a system would be a rail network that could
have 200 cameras located across 25 stations with on average 8 cameras
per station, with some stations only having 4 cameras. In this situation
what is needed is a small and flexible storage architecture which can
use the right amount of storage for the right amount of cameras. For this
example assume 500 Gigabytes/camera is required for 90 day recording.
An NVR with two Terabytes could be deployed at each station, one NVR per
four cameras. The smaller stations would have 1 NVR with the larger ones
having 5, a total of 50 NVRs for all 200 cameras. This would significantly
reduce the bandwidth required from the network. The flexibility of the
virtual matrix created by the IP Video system means any user with the
right permissions could view and analyse recorded video from any NVR,
no matter where on the network they are located.
If PC based servers were used in the example above, then 50 separate servers
would be required each with their own local attached storage, which comes
at a significant cost. An alternative would be to use dedicated, standalone
NVR units which have the processing hardware and storage in a single compact
unit. These are considerably less expensive than a PC server and equivalent
storage.
In order to satisfy different types of applications a vendor needs to
be able to provide a flexible and scalable recording solution that can
use both PC server configurations and dedicated NVR units as appropriate.
Data Security
With so much valuable data being recorded it's important to consider NVR
security and reliability. Most PC based NVR servers will deploy RAID disk
arrays. A Redundant Array of Independent Drives (RAID) is an umbrella
term for computer data storage schemes that divide and/or replicate data
among multiple hard drives.
There are different RAID levels, giving different levels of protection.
In a RAID 5 configuration for example the data is striped across 3 separate
disks. If any disk fails no data is lost and the computer can continue
without interruption.
To take NVR security a level further dedicated NVR hardware can be deployed.
These standalone robust hardware units can have redundant power supplies
and network connections, RAID configurations and hot-swappable drives.
An IP Video system that deploys standalone NVR units should also have
a flexible NVR backup strategy. For example if an NVR failure is detected
by the system software then recordings can be automatically switched to
a backup NVR or distributed amongst other NVRs in the system. Like RAID
configurations in PCs, NVRs can be mirrored, with the same video being
recorded on two NVRs simultaneously, providing the highest level of security.
IP Video systems based on H.264 can reduce the amount of storage required
by between 25% and 50% compared to MPEG-4 based systems.
DVR
vs NVR
So far we have discussed the storage issues with IP Video systems and
NVRs, but what about traditional analogue CCTV systems that use Digital
Video Recorders (DVRs)? Firstly however, it is important to differentiate
between DVRs and NVRs, as both are often termed 'digital'. A DVR digitally
compresses analogue video feeds and stores them on a hard-drive, the term
'digital' referring to the compression and storage technology, not the
transmitted video images. The DVR therefore has to be located near the
analogue feeds and is only really used in centralised architectures. In
contrast an NVR stores digital images directly from the IP-network and
can be located anywhere on the network.
It is therefore typical for a DVR to be located centrally, close to the
analogue matrix and control room equipment. As the size of the system
increases and the number of recording days increases there is no other
option but to keep adding additional DVRs. This in itself is not a problem
other than the cost overhead associated with the DVR. A high-end DVR would
typically be able to record 16 cameras on to a 2 Terabyte disk. Using
our worked example for a Casino and assuming the very best compression
available this would give just over 6 days of recording for all 16 cameras
or 90 days for just one camera.
Clearly DVR technology is simply too expensive for large scale digital
recording applications.
Reducing the Amount of Video
There are a number of compression standards currently employed in IP Video
systems. H.264 is the latest official video compression standard, which
follows on from the highly successful MPEG-2 and MPEG-4 video standards
and offers improvements in both video quality and compression. The most
significant benefit for IP Video systems is the ability to deliver the
same high-quality, low latency, digital video with savings of between
25% and 50% on bandwidth and therefore on the storage requirements. By
selecting a system based on H.264 further savings on storage can be achieved.
Even though H.264 is more efficient than MPEG-4, there are still differences
between vendors' implementation of the standard and hence the amount of
storage required.
There is no point in recording video from a camera at full frame rate
if there is nothing in the scene to record. By using applications such
as IndigoVision's unique Actively Controlled Framerate (ACF), the amount
of video and hence storage can be significantly reduced. When a scene
is inactive the video can be streamed at a much lower frame rate.
As soon as the motion analysis software detects movement the video is
streamed at full frame rate. Similarly analytics such as virtual tripwire
can detect an object crossing a line and raise an alarm. This alarm can
start recording or increase the frame-rate from that particular camera.
Summary
The requirement to record CCTV images for 90+ days will have a large impact
on the storage requirement of the project and hence the cost. With larger
systems the inflexibility and costs involved with an analogue/DVR solution
means IP Video is the only way forward. It is therefore paramount that
the end user chooses an IP Video solution that can deploy the very best
compression technology and flexible and distributed NVR architectures,
in order to minimise the additional costs due to the storage overhead.
About the Author
Before joining IndigoVision as Chief Technical Officer (CTO) in 1999,
Barry Keepence worked for over 10 years in the space industry and at 3Com
as a System Architect.
Dr Barry Keepence, CTO,
IndigoVision Group plc,
The Edinburgh Technopole
Bush Loan Edinburgh EH26 0PJ
Tel: 0131 475 7200
Fax: 0131 475 7201
Web: http://www.indigovision.com
