IEEE 802.3af Vs IEEE 802.3at: What Is The Difference?

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What Is Power Over Ethernet (PoE)?

PoE (Power Over Ethernet) is the name of a number of standard or adhoc systems which passes electric power along with data on twisted pair Ethernet cabling (UTP/FTP cables).  This allows a single cable to provide both data connection and electric power to devices. This is why it is possible to power devices such as IP cameras, VoIP phones, switches, wireless access points etc.

There are several common methods for transmitting power over Ethernet cabling. Three of them have been standardized by IEEE 802.3 since 2003. Power over Ethernet (PoE) as a networking feature defined by the IEEE 802.3af and 802.3at standards. PoE lets Ethernet cables supply power to network devices over the existing data connection. PoE standards have been optimized to safe voltage ranges and the device must communicate according to the established procedures.

PoE-enabled devices can be powered sourcing equipment (PSE), Power devices (PDs) or sometimes both. The device that transmits power is a PSE, while the device that is powered is a PD. Most PSEs are either network switches or PoE injectors intended for use with non-PoE switches. Common examples of PDs include VoIP phones, wireless access points and IP cameras.

Depending on the location in the transmission path where the power supply is combined with the data, the power-injection device can be divided into 2 groups:

  • PoE switches( endspam)
  • PoE adapters (midspam)

802.3af (PoE Type 1)

802.3af refers to the point at which the 802.3 committee approved clause (June 12th 2003). This clause is entitled Data Terminal Equipment (DTE) power via Media Department Interface (MDI) and defines the characteristics of the Powered Device (PD) and the Power Sourcing Equipment (PSE). 802.3af applies to the MDI interfaces 10BaseT, 100BaseT and 1000BaseT interfaces only running on 100Ω balanced cable. The MDI interface also acts as a Powered Interface (PI). What this means is that, network device (PD) may be powered, as well as communicate via the LAN interface (PI), receiving this power from a LAN switch (endpoint PSE) or inline alternative (Midspan PSE).

What You Need To Know About 802.3af

  1. A voltage range of 37-57 for PD
  2. A voltage range of 44-57 volts for PSE
  3. Provides DC power up to 15.4 W, actual power will be found 12.95 due to Ethernet cable power dissipation.
  4. Deliver current up to 350mA
  5. 20 Ohm of cable resistance for Cat 3 cable.

IEEE 802.3at (PoE Type 2)

IEEE 802.3at standard defines two types of PoE implementation:

  • Option A: power is sent together with the data on pairs ½ and 3/6.
  • Option B; power is sent on pairs 4/5 and 7/8 (unused in Fast Ethernet networks, in the case of Gigabit Ethernet the pairs are used both for data and power transmission).

Also, in regard to the transmitted power and data rates, the IEEE 802.3at standard comprises of two versions:

  • Type 1: for devices compliant with 802.3af and with power transmission employing 2 pairs of min Cat 3 cable, with max output power of 12.95 W.
  • Type 2: with maximum output power 30 W, transmitted via min Cat 5 cable.

What You Need To Know About 802.3at

  1. A voltage range of 42.7-57 for PD
  2. A voltage range of 50-57 volt for PSE
  3. Provides DC power up to 25.5 W.
  4. Delivers 51 W of power by utilizing four pairs in cat 5 cables.
  5. Delivers current up to 600mA.
  6. 5 Ohm of cable resistance for Cat cable.

Also Read: Difference Between Cell Splitting And Cell Sectoring

Difference Between 802.3af And 802.3at In Tabular Form

BASIS OF COMPARISON 802.3af 802.3at
Power for PD (Watt) 12.95   25.50
Power by PSE (Watt) 15.40   30.00
PSE Range Voltage 44-57   50-57
PD  Range Voltage 37-57   42.5-57.0
Max. Current (mA) 350   600
Max. Cable Resistance (Ohm) 20 Per Pair Cat 3 Cable 12.5 Per Pair Cat 3 Cable
Power Management Features Triple power class levels Signature negotiation. Quadruple power class levels signature negotiation.   0.1 W step negotiation by LLDP.
Operating Temperature Derating None 5 C with one mode active (i.e dual pair)
Cable Support Category 3 Cable (cat 3) Category 5 Cable (Cat 5) Category 5 Cable only (Cat 5)
Mode Support Mode A (Endspan) Mode B (Midspan) Mode A Mode B

Benefits Of PoE Technology

  • PoE enables mounting of devices in areas where it is practically impossible to install power such as drop ceilings.
  • Use PoE can reduce the number of cables and electrical outlets needed in a crowded equipment room or wiring closet.
  • PoE makes installing or expanding a network much simpler and cheaper in buildings where it is too expensive or inconvenient to install new power lines.
  • PoE saves you money on purchasing and running cable for networking equipment and VoIP phones because PoE allows you to use one cable for both power and data transmission.
  • High security and reliability- in a typical conditions, the risk of electrical shock is not high (the voltage is below 60 V) and the technology involves auto-testing procedures.
  • With PoE technology, there is protection of devices not compliant with the standard.
  • There is compatibility with older versions.

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