FREQUENTLY ASKED QUESTIONS (FAQs)

Transcription

1 FREQUENTLY ASKED QUESTIONS (FAQs) Q. What is the standard that governs Power over Ethernet? The standard governing Powered Ethernet is IEEE 802.3af Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI). This standard was released for publication in June Per 802.3af DTE powering is intended to provide a 10BASE-T, 100BASE-TX or 1000BASE-T device with a single interface to both the data it requires and the power to process these data. Q. What applications drove the development of this standard? The primary purpose for development of this standard was Voice over Internet Protocol (VoIP) phones and Wireless Access Points (WAPs). These devices operate over standard network cabling, but also require limited power for operation. By providing this power over the cabling system, the need to supply power locally is eliminated. In addition to VoIP, this type of application is also ideally suited for network attached Building Automation Services (BAS s) and remote security equipment such as video cameras, card readers, etc. where power requirements and accessibility are limited. Q. Where is the power introduced? There are two locations where power can be introduced: endpoint or midspan. Endpoint power (aka phantom power) is introduced via the active equipment as shown below in Figure 1. Figure 1: Endpoint Power W W W. S I E M O N. C O M

2 Midspan power is introduced in the Telecommunications Room (TR) via a powered patch panel as shown in Figure 2. Figure 2: Midspan Power Q. What pairs are used? There are 3 powering options as shown below in Table 1. Shaded areas have been added for reference to reflect the conductors used for 10/100 data transmission: Conducto Alternative A Alternative A Alternative B (MDI-X) (MDI) (All) 1 - VPort + VPort 2 - VPort + VPort 3 + VPort - VPort 4 + VPort 5 + VPort 6 + VPort - VPort 7 - VPort 8 - VPort Table 1: IEEE 802.3af Powering Options Endpoint power can use Alternatives A, B or both* while midspan power is limited to Alternative B and 10/100 applications only. For network managers, it is important to be aware of which powering option is utilized. It is also interesting to note that prior to the release of 802.3af, Cisco had implemented a proprietary powering implementation, but is now shipping 802.3af compliant product. * While active equipment may be capable of both Alternative A and Alternative B, they are not to operate both Alternative A and Alternative B on the same link segment simultaneously. 2

3 Q. Is there an advantage to endpoint vs. midspan implementations? Endpoint implementations benefit from significant advantages: 1. Market Acceptance: Readily available by all major equipment manufacturers such as 3Com, Avaya, Cisco, Foundry and Nortel. Midspan powered panels have limited manufacturer support and availability. 2. Gigabit Support: Capable of supporting 10/100/1000 applications as opposed to midspan implementations which are limited to 10/100 only. 3. Intrinsic Power Quality: The active equipment is already accessing power quality systems such as power conditioning and uninterruptible power supplies (UPS) allowing devices to retain power during a power outage if the active equipment is attached to the UPS. 4. Channel Model: Endpoint implementations can be configured using standard interconnect models used my most customers. Midspan implementations require introduction of a crossconnect that not only introduces additional cost, but also an additional point of failure. 5. Warranty Eligible: Introducing the power at the endpoint allows a single manufacturer cabling solution throughout the channel enabling the cabling system to be warranted. Midspan implementations do feature one distinct and unique advantage - retroactive support for existing systems. The midspan solution can introduce a powered panel into the existing cabling channel and eliminate the need to replace existing active equipment with PoE compatible equipment. The introduction of a midspan panel is not to alter the transmission performance of the permanent link and shall simply provide continuity for the signal pairs. Q. How much power is sent over the pairs? According to IEEE, there are four possible power classifications as shown below in Table 2: Class Usage Maximum power levels Maximum power levels at output of PSE 1 at input of PD 2 0 Default 15.4 Watts Watts 1 Optional 4.0 Watts Watts 2 Optional 7.0 Watts Watts 3 Optional 15.4 Watts Watts 1 PSE = Power Sourcing Equipment 2 PD = Powered Device Table 2: IEEE 802.3af Power Classifications Note: The maximum power level at the output of the PSE is 15.4 W. At the standard operating current of 350 ma, the maximum port voltage is 44 Vdc. Given a maximum roundtrip DC resistance of 20 ohms for the cabling channel, the voltage drops by 7 V to arrive at the PD as 37 V. 37 V x 350 ma = W maximum power level at input to PD. 3

4 Q. What happens if a non-poe device is plugged into a PoE port? The IEEE has identified this concern as one of the key criteria for success. As a result, they have implemented a protocol to prevent legacy equipment from being damaged. Essentially the PSE constantly polls each port prior to sending power to ensure that there is a request for power prior to sending. If power is not required, the PSE disables power to that port. Q. What specifications does this standard provide as it relates to cabling? Patch panel considerations It is possible that the current carrying capability of a cabling cross-connect may be exceeded by a PSE. The designer should consult the manufacturers specifications to ensure compliance with the appropriate requirements Cabling resistance unbalance Resistance unbalance is a measure of the difference in resistance between the two conductors in the 100 Ω balanced cabling system. The resistance unbalance shall be as specified in ISO Edition 2,Clause (reference: 3 percent). Q. Are there any other standards that relate to this application? There is currently a standard under development by TIA that provide additional cabling specifications for this application: SP-4425-AD6 (to be published as TIA/EIA-568-B.1-6) entitled Additional Cabling Guidelines for DTE Power This standard specifies additional cabling requirements in parallel to what is already specified within ISO/IEC 11801:2002, specifically: DC Loop resistance DC Resistance unbalance Current carrying capacity Q. Are there new cable and connectivity requirements for this application? IEEE 802.3af Annex 33B specifies the following cabling guidelines: "DTE power via MDI is intended to operate over a 100 ohm balanced cabling infrastructure as described in ISO/IEC " So, while there are no new requirements, more explicit parameters have been added within ISO and 4

5 TIA to specifically address this application. Even though there was little to no explicit reference to power delivery within ISO/IEC 11801:2000, significant additional parameters and associated requirements were added to support this application within ISO/IEC 11801:2002 (see excerpts below) as well as the IEC standards normatively referenced within. Connecting Hardware Connecting hardware for use in distributors and consolidation points* Parameter Input to Output Resistance Input to Output Resistance Unbalance Current carrying capacity Voltage Proof (Conductor to Conductor) Requirement 200 mω (max) 50 mω (max) 0.75 A per conductor (min) 1000 Vdc (min) *Also required for Telecommunications outlets per Cabling Channels Current carrying capacity The minimum current carrying capacity for channels of Classes D, E and F shall be 0,175 A d.c. per conductor for all temperatures at which the cabling will be used. This shall be achieved by an appropriate design Operating voltage The channels of classes D, E and F shall support an operating voltage of 72 V d.c. between any conductors for all temperatures at which the cabling is intended to be used Power capacity The channels of classes D, E and F shall support the delivery of a power of 10 W per pair for all temperatures at which the cabling is intended to be used. Q. Why doesn t the power signal interfere with the data transmission? This has already been directly addressed by the IEEE as identified within the following excerpt: The specification for power feeding ripple and noise in Table 33-5 shall be met for common-mode and/or pair-to-pair noise values for power outputs from 0.44W to 15.4W at operating V Port. The limits are meant to ensure data integrity. In short, direct current will not couple with the data signal nor cause interference. Q. Is Siemon developing a powered patch panel? We are not developing such a product as we feel that the endpoint powering option (power provided by active equipment) will become the power introduction method of choice for the majority of the market- 5

6 place (supported by 3Com, Avaya, Cisco, Foundry and Nortel). The midspan power option is limited to 10/100 Mb/s applications only whereas the phantom power approach can support up to 1000 Mb/s. There are two primary vendors in this space, Red Hawk/CDT and PowerDsine (which also OEM s switches under several other names). Q. Can TERA products be used? TERA connectors and cabling systems are fully compatible with all existing and future VoIP equipment, and with the power provisioning requirements of IEEE 802.3af. Also, because these connectors do not employ internal switching connections or printed circuitry, their ability to withstand higher steady state and transient current conditions is inherently better than those that do. From an applications perspective, the TERA outlet's ability to connect to either a single 4-pair plug or to multiple 2-pair plugs make it uniquely suited for midspan DTE power provisioning. By using a 2- pair TERA-to-"RJ" plug assembly to connect from the physical layer device (e.g., 100BASE-T hub) to the left side of a TERA outlet (pins 12/63), and another 2-pair TERA-to- RJ plug assembly to connect from the PSE to the right side of the outlet (pins 78/45), it becomes a fully compliant, cost effective means of providing midspan power to IEEE 802.3af DTE without requiring replacement or upgrade of the active equipment in the telecommunications room. No other standardized cabling interface offers this functionality. Q. Is there any new terminology or acronyms that will be associated with this application? DCPL = DC Power Load (TIA) DCPS = DC Power Source (TIA) DTE = Data Terminal Equipment (IEEE, TIA) PSE = Power Sourcing Equipment (IEEE, TIA) PD = Powered Device (IEEE) SELV = Safety Extra Low Voltage (IEEE) Q. Are there any websites that have more information on this application? There are a couple website of that may be of interest: FAQ-Powered Ethernet Rev A. 6/04 W W W. S I E M O N. C O M