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What is PoE?
The seemingly universal network connection, twisted-pair Ethernet cable, has another role to play, providing electrical power to low-wattage electrical devices. Power over Ethernet (PoE) was ratified by the... more/see it nowInstitute of Electrical and Electronic Engineers (IEEE) in June 2000 as the 802.3af-2003 standard. It defines the specifications for low-level power delivery—roughly 13 watts at 48 VDC—over twisted-pair Ethernet cable to PoE-enabled devices such as IP telephones, wireless access points, Web cameras, and audio speakers.
Recently, the basic 802.3af standard was joined by the IEEE 802.3at PoE standard (also called PoE+ or PoE plus), ratified on September 11, 2009, which supplies up to 25 watts to larger, more power-hungry devices. 802.3at is backwards compatible with 802.3af.
How does PoE work?
The way it works is simple. Ethernet cable that meets CAT5 (or better) standards consists of four twisted pairs of cable, and PoE sends power over these pairs to PoE-enabled devices. In one method, two wire pairs are used to transmit data, and the remaining two pairs are used for power. In the other method, power and data are sent over the same pair.
When the same pair is used for both power and data, the power and data transmissions don’t interfere with each other. Because electricity and data function at opposite ends of the frequency spectrum, they can travel over the same cable. Electricity has a low frequency of 60 Hz or less, and data transmissions have frequencies that can range from 10 million to 100 million Hz.
There are two types of devices involved in PoE configurations: Power Sourcing Equipment (PSE) and Powered Devices (PD).
PSEs, which include end-span and mid-span devices, provide power to PDs over the Ethernet cable. An end-span device is often a PoE-enabled network switch that’s designed to supply power directly to the cable from each port. The setup would look something like this:
End-span device → Ethernet with power
A mid-span device is inserted between a non-PoE device and the network, and it supplies power from that juncture. Here is a rough schematic of that setup:
Non-PoE switch → Ethernet without PoE → Mid-span device → Ethernet with power
Power injectors, a third type of PSE, supply power to a specific point on the network while the other network segments remain without power.
PDs are pieces of equipment like surveillance cameras, sensors, wireless access points, and any other devices that operate on PoE.
PoE applications and benefits.
• Use one set of twisted-pair wires for both data and low-wattage appliances.
• In addition to the applications noted above, PoE also works well for video surveillance, building management, retail video kiosks, smart signs, vending machines, and retail point-of-information systems.
• Save money by eliminating the need to run electrical wiring.
• Easily move an appliance with minimal disruption.
• If your LAN is protected from power failure by a UPS, the PoE devices connected to your LAN are also protected from power failure.
The Threat — A sag is a decline in the voltage level. Also known as “brownouts,” sags are the most common power problem.
The Cause — Sags can be caused... more/see it nowlocally by the start-up demands of electrical devices such as motors, compressors, and elevators. Sags may also happen during periods of high electrical use, such as during a heat wave.
The Effect — Sags are often the cause of “unexplained” computer glitches such as system crashes, frozen keyboards, and data loss. Sags can also reduce the efficiency and lifespan of electrical motors.
The Threat — A blackout is a total loss of power.
The Cause — Blackouts are caused by excessive demand on the power grid, an act of nature such as lightning or an earthquake, or a human accident such as a car hitting a power pole or a backhoe digging in the wrong place.
The Effect — Of course a blackout brings everything to a complete stop. You also lose any unsaved data stored in RAM and may even lose the total contents of your hard drive.
The Threat — A spike, also called an impulse, is an instantaneous, dramatic increase in voltage.
The Cause — A spike is usually caused by a nearby lightning strike but may also occur when power is restored after a blackout.
The Effect — A spike can damage or completely destroy electrical components and also cause data loss.
The Threat — A surge is an increase in voltage lasting at least 1/120 of a second.
The Cause — When high-powered equipment such as an air conditioner is powered off, the excess voltage is dissipated though the power line causing a surge.
The Effect — Surges stress delicate electronic components causing them to wear out before their time.
The Threat — Electrical noise, more technically called electromagnetic interference (EMI) and radio frequency interference (RFI), interrupts the smooth sine wave expected from electrical power.
The Cause — Noise has many causes including nearby lightning, load switching, industrial equipment, and radio transmitters. It may be intermittent or chronic.
The Effect — Noise introduces errors into programs and data files. collapse
Optical isolation protects your equipment from dangerous ground loops. A ground loop is a current across a conductor, created by a difference in potential between two grounded points, as in... more/see it nowequipment in two buildings connected by a run of RS-232 or other data line. When two devices are connected and their potentials are different, voltage flows from high to low by traveling through the data cable. If the voltage potential is large enough, your equipment wont be able to handle the excess voltage and one of your ports will be damaged. Ground loops can also exist in industrial environments. They can be created when power is supplied to your equipment from different transformers or when someone simply turns equipment on and off. Ground loops can also occur when there is a nearby lightning strike. During an electrical storm, the ground at one location can be charged differently than the other location, causing a heavy current flow through the serial communication lines that damage components.You cant test for ground loops. You dont know you have one until a vital component fails. Only prevention works. For data communication involving copper cable, optical isolation is key.With optical isolation, electrical data is converted to an optical beam, then back to an electrical pulse. Because there is no electrical connection between the DTE and DCE sides, an optical isolator unlike a surge suppressorwill not pass large sustained power surges through to your equipment. Since data only passes through the optical isolator, your equipment is protected against ground loops and other power surges. collapse
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