Bach Seat

A client recently asked what happened to the network design rule of thumb which said do not install data cables anywhere near electrical cables? The fear of cross-talk, interference, and corruption of the data traffic seems to have disappeared with Power over Ethernet (PoE). He rightly pointed out that now it seems OK to mix data and power in the same cable going to a networked device. 

Read part 2 here.

PoE is similar in principle to the way that the copper wire pair that carries your POTS (Plain Old Telephone System) telephone signals into your house also carries enough electricity (48v DC) from the telco Central Office to power the phone’s core elements of the headset, dial, and ringer. Power over Ethernet’s development started with early implementations of Voice over Internet Protocol VoIP)phone systems. VoIP pioneers did not have a telco CO to power the phones and powering the VoIP phones with wall warts proved unreliable. The phones stopped working when unplugged from the wall or if the building lost power.

In 2000 Cisco (CSCO) developed the first successful technique of putting 48v DC on the LAN data cable along with the data traffic. This proprietary system allowed Cisco to overcome customer objections to wall warts and sell a lot of VoIP systems.

Cisco’s original PoE equipment was capable of delivering up to 10W per port. The endpoint and the Cisco switch negotiated the amount of power to be delivered based on a power value in the proprietary Cisco Discovery Protocol (CDP). The Power Sourcing Equipment (PSE) will send a Fast Link Pulse (FLP) on the transmit pair. The Powered Device (PD) connects the transmit line to the receiving line via a low pass filter. And thus the PSE gets the FLP in return. Cisco’s original PoE implementation is not software upgradeable to the IEEE 802.3af standard. Cisco manufactured many IP phones and WLAN access points devices that were not compliant with the IEEE 802.3-2005 Clause 33 including:

Throughout 2001 and 2002, other VoIP and Wireless Access Point (WAP) vendors saw Cisco’s success and developed their own proprietary (and often non-interoperable) powering systems. As more proprietary systems were developed the Institute of Electrical and Electronics Engineers (IEEE) recognized the need to standardize PoE. The IEEE 802.3 Ethernet Committee assigned PoE technology to a new working subcommittee called 802.3af. The IEEE working group’s charge was to create a standardized version of the Power Over Ethernet so that any manufacturer who wanted to could make their products PoE ready. The IEEE working group took commentary from 2001 to 2003 and released the ratified IEEE 802.3af-2003 Power over Ethernet standard in June 2003 which added clause 33 to the IEEE 802.3 standard

The 802.3af document describes how PoE systems should work.  The standard defines two types of PoE equipment, Power Sourcing Equipment (PSE) and the Powered Device (PD). Power Sourcing Equipment sends the power out over the LAN cabling system to the Powered Device. The PSE would send out a maximum of 15.4 watts DC per link to each device, (limited to standard Ethernet distances). 12.95 watts are assumed to be available at the PD because some power is lost in the cable.

The nominal voltage is 48 V, over two of the four available pairs on a Cat. 3/Cat. 5e cable. “Phantom power” is used to allow the powered pairs to also carry data. This permits PoE to be used with 10BASE-T and 100BASE-TX, which use only two of the four pairs in the cable, but also with 1000BASE-T (gigabit Ethernet), which uses all four pairs for data transmission. This is possible because all versions of Ethernet over twisted pair cable specify differential data transmission over each pair with transformer coupling; the DC supply and load connections can be made to the transformer center-taps at each end. Each pair thus operates in “common mode” as one side of the DC supply, so two pairs are required to complete the circuit. The polarity of the DC supply may be inverted by cross cables; the powered device must work with either pair: spare pairs 4-5 and 7-8 or data pairs 1-2 and 3-6. Polarity is required on data pairs and ambiguously implemented for spare pairs, with the use of a bridge rectifier. (Source)

Power Sourcing Equipment can be in two form factors. A PSE can be implemented as an endspan which is an Ethernet switch with powered ports (a PoE enabled switch) or midspan which is a power hub that is used along with a non-powered switch the end-user already has in place. PD’s can receive PoE equally well from either type of PSE per the standard. The decision to use an endspan or a midspan is left up to the end-user.  The end device can use either powering technique.

The Powered Device (PD) is a network device like VoIP phones, Wireless Access Points, and IP cameras. which are capable of taking the power off the LAN cable, through the RJ-45 (8P8C) connector and using it to power itself. Some pre-standard PoE devices are incompatible with 802.3af equipment. More PoE ready PD’s are available every year  PoE ready end devices can reduce installation costs by as much as 90% over traditional powering techniques.  Among the newer PoE PD’s devices on the market or coming soon are IP Paging, Speaker Systems, POS Terminals, Door and Gate Security hardware, Public Information signs, Building Access, Temperature Control Systems, Stage Lighting, and Computers. These newer PD’s were pushing 802.3af to its limits and the IEEE began work to evolve the standard. This power limitation prevented “high power” devices that required up to 30W to be supported via the industry-standard PoE solution.

Related articles

• Bypass Electrical Headaches with Power over Ethernet (blogs.cisco.com)
• Cisco UPOE Ramps to One Million Ports (blogs.cisco.com)

Ralph Bach has been in IT long enough to know better and has blogged from his Bach Seat about IT, careers, and anything else that catches his attention since 2005. You can follow him on LinkedIn, Facebook, and Twitter. Email the Bach Seat here.