NETGEAR GS108PEv3 8 Port PoE Gigabit Ethernet Plus Switch User Guide
- June 14, 2024
- NETGEAR
Table of Contents
NETGEAR GS108PEv3 8 Port PoE Gigabit Ethernet Plus Switch
PoE Basics
Power-Over-Ethernet (PoE) is a feature whereby a networked device, for
example, a VoIP phone or Wireless Access Point, can receive it’s electrical
power over the same Ethernet cable that connects it to the network. This
eliminates the need for this device to have a local power supply connected to
the mains.
The power can be supplied from an Ethernet switch that has PoE capability, or
it can be supplied by a power supply connected to a PoE “injector” connected
at any point along the Ethernet cable:
PoE Standards and Compatibility
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The current standard for PoE, supported by all current NETGEAR and Cisco PoE equipment, is 802.11af.
However, before this standard was agreed upon, Cisco designed its own, proprietary, PoE standard. The two standards are not compatible, and there is lots of equipment still in use that only supports the “Cisco pre-standard” PoE standard, so compatibility issues often arise. -
In general, NETGEAR PoE switches only support 802.11af, and therefore will not power devices that require Cisco prestandard PoE. The one known exception to this is the FSM7326P, which does seem to power both 802.3af and Cisco Legacy devices, however as only 802.3af is specified in it’s specifications, continued support for Cisco pre-standard devices cannot be guaranteed.
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NETGEAR devices that can be powered by PoE (currently this is only the ProSAFE wireless access points) only accept power from 802.3af-compliant sources, and therefore cannot be powered from Cisco prestandard PoE switches.
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NETGEAR does not currently sell any PoE injectors, and cannot recommend any third-party products. However, any 802.3af-compliant PoE injector should be able to power NETGEAR PoE devices. PoE Injectors that only support Cisco prestandard PoE will not be able to power NETGEAR PoE devices.
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The most common Cisco pre-standard PoE devices that NETGEAR gets calls about are older Cisco PoE phones. All the newer Cisco IP phones are designed to be compatible with both 802.3af AND Cisco pre-standard PoE, but the following Cisco IP phones only support Cisco prestandard PoE: 7960, 7940, 7910, 7912, 7905, 7902
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The Cisco Catalyst range of switches only supports Cisco prestandard PoE, and therefore cannot be used to power NETGEAR PoE devices.
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802.3af and Cisco prestandard PoE both aim to supply 48-volt DC to PoE devices. Based on this superficial similarity, many people have tried to make up specially wired cables that will convert one PoE standard into the other. Occasionally this might work, but since both standards are quite complex and different in many ways, it is not recommended and is certainly not supported by NETGEAR.
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The IEEE is working on a new PoE standard, 802.3at, also known as “PoE Plus”, which will deliver 30 watts down an Ethernet cable, twice as much as 802.3af. The standard will probably be ratified in 2008, although some manufacturers are already offering Draft PoE Plus products. NETGEAR does not currently offer any PoE Plus products.
Summary of the 802.3af PoE standard
Power delivery
The standard refers to two types of devices, Power Sourcing Equipment (PSE)
and Powered Devices (PD). Power Sourcing Equipment provides power to the
Powered Devices.
- A PSE can support power delivery Type A, or Type B, or both.
- Type A involves sending power over 2 unused pairs or wires on a CAT3 or CAT 5 or CAT5E cable. This works ok for 10/100 links, but Gigabit Ethernet uses all the pairs in a CAT5E cable, so Type B uses a “phantom power” technique to send power and data over the same pairs.
- When an Ethernet device is connected to a PSE, the PSE initially applies a low voltage (2-10 volts) to sense if the device is a PoE PD. If it is a PD, 48 volts will be supplied. If it is not a PD, no power will be supplied.
- The maximum power that a PSE will supply down an Ethernet cable is 15 watts.
- However, due to possible losses on a long cable, the maximum power that a PD can receive down an Ethernet cable is 12.95 watts.
Power Classification (optional)
- A PSE has a PoE “power budget”, which is simply the total power it can supply down Ethernet cables. Each time a PD is plugged into the PSE, the PSE subtracts the PD’s maximum power usage from it’s power budget. If the power budget is all used up, and another PD is connected, that PD will not receive any power.
- By default, a PSE subtracts 15 watts from it’s power budget for each PD that is attached to it. At this rate, most PoE switches will run out of power budget long before they run out of PoE ports. For example, the NETGEAR GS724TP has 24 PoE ports, but a power budget of only 192 watts. At 15 watts per PD, only the first 12 PDs that connect will get power.
- To avoid this problem, 802.3af specifies a method whereby a PD can indicate to the PSE what it’s maximum power usage will be, by indicating that it complies with one of 4 “power classes” specified in the standard. For example, if a PD indicates that it complies with power class 2, the PSE knows that it only has to subtract 3.84watts from it’s power budget for that PD, thus leaving more power budget for other PDs.
- Compliance with this Power Classification scheme is optional for 802.3af-compliant devices. All NETGEAR PoE devices implement this scheme, but other manufacturer’s equipment may not. This could cause unexpected results, for example, a NETGEAR 192watt PoE switch might be able to supply power to 24 IP phones, but another manufacturer 192watt PoE switch which does not recognize Power Classification would only be able to power 12 IP phones, as it would subtract 15watts from it’s power budget for each IP phone
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