Transmission: How to determine topology and power budget?

Transmission: How to determine topology and power budget?
Transmission plays a significant role in a security system for large sites. Not only should the transmission equipment be hardened and reliable, the way transmission is set up to ensure optimal data and power delivery is also key. That said, choosing a network topology and calculating the power budget at a particular site needs to be factored in during the design process.

Network topology

There are various network topologies, or how devices and transmission equipment are arranged in a security system. The three primary topologies are bus, star and ring, each with strengths and weaknesses.
In a bus topology, all nodes or devices are connected to a main cable through which data gets transmitted from one point to another. Since only one cable is involved, It’s relatively easy and inexpensive to install. But then, since there’s only one cable, a failure on one device causes the whole system to fail.
A star topology, meanwhile, features a central node – either a hub or switch – that connects to various devices in a point-to-point fashion. All data goes through the switch, which then passes the data to the intended recipient. It is easy to install and expand as the user needs to add just one cable to connect an additional camera to the system. However the star topology can be expensive, as a separate cable is required for each device. Meanwhile, since all data goes through the switch, it is the single point of failure. If it goes down, so does the system.
The ring topology is by far the most reliable. All devices are connected in a ring. If one device in the ring fails, the data gets rerouted to a redundant ring and gets to the right place.
Selecting which topology to deploy is dependent on various factors, including the application itself as well as the user’s budget and reliability requirement. “For the bus technology, the devices are connected in a sequence; examples include highways. The star topology is point-to-point connection, so they will be suitable for devices connected to a single-point location such as the control room,” said Kelvin Chan, Senior Product Manager at OT Systems.
“It also depends on the system reliability requirement,” he added. “The bus is simple and costs less, but there is a reliability issue, as the failure of one node causes the whole system to fail. The star topology provides better system reliability. However, if the central point fails, the system still fails. Ring costs less than star, which requires a cable for each IP device in the system. Ring also offers the best system reliability, because if one device fails, another will pick up the drop.”

Power budget

Since most modern IP cameras get their power from Ethernet instead of a separate power source, the power budget or the total power needed at a particular site needs to be considered during the system design process.
Basically, calculating the power budget boils down to the number of cameras at the site and the power consumption of each camera. As an example, if there are eight 15.4W cameras and two 30W cameras on site, then the total budget will be 183.2W. The user therefore needs to get a 10-port PoE switch capable of supplying 183.2W of power.
“It's really determined by the installer,” said Kirby Han, Art Director at Altronix. “If they call us, they tell us they have one or two cameras on a pole and how much does that device draw, we can help them walk through the system.”
The current standards, IEEE 802.3af and IEEE 802.3at, specify output of 15.4 watts and 30 watts, respectively, from each port of the PoE switch. But more and more, there are cameras that consume more power than 30W.
“Some cameras are high PoE which is 60 watts,” said Han. “Each camera is different. Some cameras might have heaters/blowers. If the heater is not running, then the camera doesn’t even draw 15 watts. When that heater is on, it's drawing x amount of current. It's the heather/blower devices inside them to keep them either cool or warm.”
Amid this trend, more and more PoE switches offer 60W ports to accommodate. “That's why we made certain ports on our units to supply the 60 watts. We also have 30-watt ports, also. Of course it'll also supply the 15 watts,” Han said. “We have that amount of power. We pump out 120 watts from our power spot, so you can divide that power however you want.”

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Digital Transmission
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