PowerSync Service Analyzer Reference Manual
February 8, 2010
Sifos Technologies page
7
2. PoE and the PowerSync Analyzer
2.1.
IEEE 802.3af / 802.3at Overview
The 802.3af specification was designed to create an environment whereby Powered Devices (PD’s) from numerous
different manufacturers could be interconnected to Power Sourcing inter-networking equipment (PSE) including switches,
routers, and hubs produced by many different networking equipment manufacturers. It is envisioned that such
interoperability will lead to lower cost and higher proliferation among both the sourcing equipment and the networked
PD’s. The ongoing emergence of VoIP telephony combined with ramp-up in TCP-IP networked devices show that this is
a fast growing market. There are numerous new PD functions such as security, inventory management, environmental
management, and other applications.
The basic features of 802.3af PoE are:
•
48V DC Supply to PD’s
•
Guaranteed 15.4 Watts of Power Consumption per network connection (PD and cabling)
•
Power Sourcing from both “End-Point” switches/routers as well as “Mid-Span” power “adder” devices.
•
Safety “interlocks” to prevent powering when no PD’s are connected and to assure prompt power removal when
PD’s are disconnected as well as to limit DC current flow at all voltage levels.
•
Physical layer mechanism for PSE’s to characterize power demands of individual PD’s and thus manage power
delivered per port.
The 802.3at specification will expand upon 802.3af is several key areas:
•
Guaranteed up to 30 Watts of Power Consumption per network connection (PD and cabling)
•
Optional LLDP (MAC Layer) based protocol for negotiating power demands with a PD with granularity of 0.1
watts.
•
PoE End-Point and Mid-Span PSE’s with full gigabit Ethernet support
2.1.1.
The Power Connection
Under the 802.3af specification, DC power must be carried on 2 of the 4 pairs of a LAN (e.g. category 5) cable. Either
+48V or –48V DC is carried on one pair (common mode) and reference (zero volts) is carried on a second pair. ALT A
(alternative A) refers to the case where power is sourced on Pairs 2 and 3 (referring to EIA/TIA 568B), the data
transmission pairs for 10/100BaseT. ALT B refers to the case where power is carried on Pairs 1 and 4 which are otherwise
unused in 10/100BaseT. Mid-Span PSE’s MUST apply power on the “ALT B” pairs while End-Point PSE’s may use
either, though typically will use ALT A. This allows both End-Point and Mid-Span PSE’s to coexist on the same cable.
Power may be applied by the PSE in either an MDI or MDI-X (crossover) port configuration. This means that from the
PD’s point of view, the incoming voltage may look either like +48 VDC (MDI) or –48 VDC (MDI-X) since in the latter
configuration, pairs 2 and 3 (as well as 1 and 4) are crossed. PD’s are required to be completely insensitive to whether
power is furnished on the ALT A or ALT B pairs and whether power is positive polarity or negative polarity.
Each PSE port is responsible for managing 4 basic aspects (or phases) of PoE:
1. PD Detection
2. PD Classification
3. Power-Up
4. Power-Removal
802.3at compliant PSE’s will source at least 50 VDC and will readily differentiate between PD’s that are high power
(802.3at) versus normal power (802.3af) type PD’s. PSE’s, including Mid-Spans, can furnish power on any pair (ALT A
or ALT B) and an option
may
exist to allow powering of all 4 pairs under 802.3at.
