IEEE standard for Power over Ethernet Cables (802.3af)
Criteria for Evaluating Detection Schemes

October 9, 2000

This table contains criteria by which the competing detection concepts can be compared. PROs/CONs discuss concepts that pertain to each criterion. Past reported measurement results are sometimes included as Notes to substantiate concepts. It is assumed that measurements or “numbers” will be provided by or at the November 2000 meeting to address key PRO/CON concepts.
Non-Interfering 

I-1	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Does not interfere with 10/100 Ethernet (via crosstalk)	•	Uses DC for detection (no repetitive pulses with dv/dt to cross-talk). Monitors current for one and if necessary a second DC voltage (very rare transitions)
•	Detect state lowers the ac noise on the cable compared to the “power state”, since PSE has high source impedance (75K ohms) during detect phases. Any 48 volt source noise is greatly attenuated by this high impedance before it sees the cable (~100 ohms at AC). 	.	•	Pulse shaping and random patterns constrain pulse energy to < 120KHz,  aiding compatibility with Ethernet 
	•	During detection, continuously issues pulses that raise the noise level in the < 120KHz region
•	Pulses require shaping to control cross-talk and EMI.	
Notes:			Ethernet performance tests for a single disturber model show no measurable effect on 10/100.		

I-2	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Minimizes cross-talk interference potential to Gigabit Ethernet 	•	PROs of I-1 still apply.
•	Places no limitations on how one filters the 48v DC source		•	PROs of I-1 still apply
•	In theory, could apply pulses by a paralleled method, avoiding need for low impedance path through power supply and giving additional filtering flexibility.	•	CONs of I-1 still apply
•	Prototyped approach requires a low impedance path through the power supply. This requires coordination with power supply filter needed to control the supply’s cross talk interference.	
Notes:					

I-3	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Minimizes EMI (conducted and radiated)	•	PROs of I-2 still apply; leads to lowest EMI.		In theory, could apply pulses by a paralleled method, avoiding need for low impedance path through power supply and giving additional filtering flexibility.	Prototyped approach requires a low impedance path through the power supply. This requires coordination with power supply filter needed to control the supply’s EMI interference.	
Notes:			EMI data shows the risk of a problem has been demonstrated to be extremely low, but not zero.		

Detection Robustness and Flexibility 

R-1	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Avoids “false positive” detections (quiet and noisy environments)	Only well defined non-linear V-I characteristic will “pass”; pure resistance will not.		Signature is highly unique. PSE can force a change in the response from the PD, verifying a valid PD.		
Notes:					

R-2	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Reliably detects proper PDs (quiet and noisy environments)	Detection depends on dc current flow from a high impedance source. It is insensitive to loop length, translating into margin in a quiet environment. Controller can average over many samples at each DC voltage to mitigate fluctuations.	Sensitivity to component tolerances (time, temperature) and board surface contamination have not been shown.		•	Requires reliable pair-to-pair mode transmission path for 120 KHz signal energy. Needs “verifying” in several cable types.
•	Sensitivity to component tolerances (time, temperature) and board surface contamination have not been shown.	
Notes:	Tested to 1200 meters without a problem (quiet environment). Results for noisy environment needed.		Detection well beyond 100m is not a requirement.	One prototype had a false detection against a diode at 270-290m. Algorithm refinement could probably avoid this. 	

R-3	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Avoids “false detecting” and hence powering paralleled devices 	Two valid PEs on legacy paralleled cables would not give a valid signature. A valid PD paralleled with most anything except an open circuit would not give a valid signature.			No inherent characteristic to help discriminate against paralleled devices.	
Notes:	TIA TR41.4 liaison (Bell, May 19,2000) cautioned that we should guard against the potential that there are paralleled devices attached. E.G., an office complex wherein an ISDN system is being replaced with an 802.3
system.  The ISDN system might legitimately have multiple S-Bus devices on
the same 4-pair cable.				

R-4	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Has potential for detecting multiple classes of PE	A different resistance could signify an additional power class, with no additional hardware. 		Different R and C values could signify additional classes. Current two-stage detection could be extended to at 3rd or nth stage, each using higher duty cycle pulses. One would look for the duty cycle that transforms the signature into an “open”.	Introducing higher duty cycles will increase the energy in the 10-30 KHz frequency range, somewhat. Would need to validate that this causes no interference.	
Notes:					

R-5	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Detection technique yield useful information on channel integrity 	●	Positive detection verifies there’s a proper channel for delivering DC power to a valid PE
●	Mis-wired or split cable scenarios still allow a positive detection and proper PSE-PD power operation, but no data transmission. Would therefore troubleshoot the transmission line.
●	Negative detection requires trouble shooting only PSE and PE (not transmission channel).		Positive detection verifies there’s a proper transmission channel for < 120 KHz (pair-to-pair mode) to a valid PE; i.e. is in a single cable with pairs intact.	Mis-wired or split cable scenarios yield a negative detection, no power delivery, and no data transmission. Would therefore require troubleshooting all three:  PSE, PE, and transmission line.	
Notes:					

R-6	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Detection technique is amenable to a broad set of uses (e.g., ISDN, non PHY-centric devices) that potentially involve >100m cable lengths.	DC current propagation is relatively insensitive to cable length.			Requires reliable pair-to-pair mode transmission path for 120 KHz signal energy. Needs “verifying” in several cable types.	
Notes:	Been tested to 1200 meters with no problem (quiet cable)			One prototype had a false detection against a diode at 270-290m. Algorithm refinement could probably avoid this. Detection well beyond 100m is not a requirement.	

Avoids Damage to DTE

D-1	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Low risk of equipment damage IF power is ever inappropriately provided	Positive detection verifies the DTE’s DC resistance is >= 25Kohms), limiting the current 48v would cause. The 12 and 24 volt stages give an opportunity to break-down or change a false termination before full power is applied.			•	 Positive detection reveals no useful information about the DTE’s DC characteristics before applying 48 volts.
•	Detection is done at one low voltage, giving no opportunity to “break down” or change a false device before giving full power.	
Notes:			A separate function can be provided as part of the DC voltage ramp-up, to offer protection. 		

Efficiency, Maximum Power Delivery 

E-1	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Facilitates delivering maximum, usable power to the PE	Detection scheme inherently has the ability to precisely measure DC current level. This is reused to reliably shut off current at the maximum allowed continuous current, guaranteeing highest continuous power to PE at low cost.			Precisely measuring DC current level is not used by the detection scheme, making precise current measurement an add-on function. 	
Notes:	Prototype included the hardware to do this function.		Add-on hardware can be provided to do this.		
Cost  

C-1	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Leverage the necessary function of precisely monitoring current levels, to inexpensively perform detection.	Same as under E-1.			Same as under E-1.	
Notes:	Same as under E-1.		Same as under E-1.		

C-2	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Amenable to integrating into the PHY		Not possible.	Is possible.		
Notes:					

C-3	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Compact, allowing high density PSE implementations	Easy to integrate and achieve small space in Mid-span or LAN switch implementations. 		•	Where PHY magnetics can be used for pulse insertion, can achieve very compact space.
•	In theory, non-magnetic implementations should be feasible, and amenable to integration and compact size.	•	Only magnetic pulse insertion has been prototyped. This would yield a bulkier Mid-span implementation compared to silicon-based.
•	Two 0.3 to 1uF capacitors are used in the PSE; will remain bulky.
	
Notes:	Evidence has been provided that a “dime sized” circuit can do the four required PSE functions 1 , only one of which is signature detection.		It has been reported small caps, 0.1 uF, will also work.		

C-4	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Compact for PD implementations	•	Easy to integrate. Very small integrated signature with current limiting/isolation has been shown as feasible.
•	Only one signature network is required for the PD to be detected on either set of pairs.			•	The 0.3 to 1 uF capacitor for > 54 volts will remain big and expensive.
•	Two of these large capacitors (in fact two signature networks) will be required for the PD to be detected on either set of pairs.	
Notes:			It has been reported small caps, 0.1 uF, will also work.		

C-5	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Amenable to low cost, Mid Span implementations – Env A (including four main necessary elements)	Non-magnetic implementation has been prototyped. How to leverage integration has been demonstrated.		In theory, non-magnetic implementations should be feasible, and amenable to integration.	Only a magnetic implementation has been Prototyped.	
Notes:	Per ports costs of from $1.99 (single port) to $0.53 (24 ports sharing) have been shown.				

C-6	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Amenable to low cost LAN Switch integrated implementations - Env. A (including the four necessary functions1)	•	Involves no magnetics. Can leverage integration.
•	Feasible configuration and cost has been presented, including all four main functions1.		In theory, non-magnetic implementations should be feasible, and amenable to integration.	Only a magnetic implementation has been Prototyped.	
Notes:	Per ports costs of from $1.99 (single port) to $0.53 (24 ports sharing) have been shown.				

C-7	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Amenable to Mid Span Env B implementations 	•	Involves no magnetics. Can leverage integration.
•	Feasible configuration and cost has been presented, including all four main functions1.		In theory, non-magnetic implementations should be feasible, and amenable to integration.	Only a magnetic implementation has been Prototyped.	
Notes:	DC/DC converter per port costs overwhelm other costs				

C-8	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Amenable to LAN Switch integrated Env B implementations 	•	Involves no magnetics. Can leverage integration.
•	Feasible configuration and cost has been presented, including all four main functions1.		In theory, non-magnetic implementations should be feasible, and amenable to integration.	Only a magnetic implementation has been Prototyped.	
Notes:	DC/DC converter per port costs overwhelm other costs				

Survivability 

S-1	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Ability to survive ESD	PSE has high source impedance (75K ohms) during detect/idle phases. The current seen by the PSE logic circuitry due to high differential voltage surges on the cable are greatly attenuated by this high impedance, improving survivability.				
Notes:	PSE Prototype survived 9KV contact discharge and 10KV air discharge.				

S-2	“Resistive + Diodes” 	“Coupled Diode”	
Criteria	PRO	CON	PRO	CON	
Ability to survive Cable discharge					
Notes: