IP Data Switches v Video Data Switches
The trend in CCTV to convert and integrate existing analogue CCTV systems into IP networks is accelerating, and a new trend in IP is emerging, one that demands careful consideration, as barox explains.
Today’s PoE IP cameras are demanding more and more power to be supplied via network switches. A 60W network device is now common, but network designers face limitations on the availability of switches that can cope with that demand. In addition, IEEE standards for 60W are not yet completely ratified, leaving the world of video IP networks to forge ahead independently with the requirement for its own specification of high-performance switches and components.
Power and agility
IP cameras need to stream their video frames continuously and steadily, otherwise the eye immediately perceives the change. In comparison to the needs of traditional data networks and due to the synchronous nature of high-resolution video streams, to cope with modern IP cameras, video IP network switches need to be powerful, dynamic and fast.
Megapixel cameras output large ‘jumbo’ video frames, often around 9600Bytes; that’s up to 6 times larger than normal frames. For video network installers, the problem is most network switches only support frames with a 1 Gigabit connection, but IP video cameras negotiate at 100Mbits. As a result, the switches discard the jumbo frames and data is lost, creating green/empty frames.
Video network design
Megapixels, 4K, 8K cameras … in the future we’re heading towards the need to cater for even greater data rates. A switch might be labeled 24 Gigabit but can it handle that amount of data? Conductivity is determined by the so-called switching fabrics (SF). A switch with 24Gbits has twice the SF, i.e. 48Gbits.
A comparable criterion for evaluating a switch is the forwarding rate (FR). Many access switches and Top Hat DIN rails are exempting switches, not built for Full Wire Speed on all ports, because switches in the industrial world only usually need to handle small amounts of data. When video is connected to these switches, the device is overwhelmed.
Another consideration is the bottleneck uplink. For a switch, this should also be designed for video. As an example, a 24-port 1/100/1000TX switch must be used with a minimum uplink of 10Gbits.
Why buy a general-purpose low cost 24 Gigaport switch when it is bound to fail when all 8 ports are occupied – and then a second, or even third switch to fulfill the requirement?
IP cameras behave differently compared to other PoE devices, for example, on day/night switching, current peaks are generated, or when PTZ domes move there is a short-term increase in power demand. In these situations, the network switch must cope with the demand and not fail.
Typically, 75% of the switch power is used purely for PoE, the remaining 25% is required to cope with peak currents and for the switch CPU. So, for example, a 24-Port Switch with Class 3 consumers equates to 24 x 15.4W = 370W. However, effectively the switch needs to be specified as 370W/0.75 = 500W, i.e. the remaining 130W is needed for the reserve of peak currents/switch CPU.
In the event of a power failure, a fully-featured video switch must be able to time-delay port-by-port. Otherwise all cameras could receive peak power at a stroke. Example: all domes are required to pre-set to their home position at the same time, generating enormous peak currents, impacting the power supply again and again. At the same time, modern video browsers should be able to ‘ping’ PoE devices directly and, if necessary, remove the PoE supply to successfully reboot cameras.