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Bronze

poe 802.3af standard questions

Hi every body!

I need some detailed link about how the PSE detrmine the class of PD

The following link was forwarded to me on net pro did not address the following issues.

Here is some excerpt from the link.

" During classification,PSE presents a voltage between 14.5 to 20.5 v. At this time PD presents a load current as determined by the resistance on the Rclass pin."

The questions are:

1) PD is not "ON" yet, how can it offer load current?

2) what is Rclass pin? just dropped it in the link without any explantion?

Another excerpt"

"After PSE measures the load current and determines it can deliver the requested power, it moves into "ON" state by raising the voltage above 42 V"

the question is does switch has to allocate this much voltage (above 42 v) per port, then it can add up as switch could many poE ports.

I really appreciate if someone help me understand this.

thanks a lot!

.http://www.akrossilicon.com/products/docs/poe_overview.pdf

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Silver

Re: poe 802.3af standard questions

Hello Sarah,

1) to measure a resistance the PD device doesn't need to be "ON" is enough to apply a voltage and to measure the current that flows.

This comes from electrical theory and practice.

A resistance is a passive component

For example in CIPT6.0 vol. I says:

The PSE (Cisco Catalyst switch) detects a powered device by applying a voltage in the range of

-2.8V to -10V on the cable and then looks for a 25kOhm signature resistor. Compliant

powered devices must support this resistance method. If the appropriate resistance is found, the

Cisco Catalyst switch delivers power

2) rclass pin means the resistance value.

when the switch decides to power on the port it applies the full voltage 42V.

With an higher voltage a lower current is needed to provide the same power

see the following show from one of our access switch

sh power inline

Available:1400(w) Used:1013(w) Remaining:387(w)

Interface Admin Oper Power(Watts) Device Class

From PS To Device

--------- ------ ---------- ---------- ---------- ------------------- -----

Gi2/1 auto on 5.6 5.0 IP Phone 7911 n/a

Gi2/2 auto off 0.0 0.0 n/a n/a

Gi2/3 auto on 16.7 14.9 IP Phone 7971 3

Gi2/4 auto on 14.5 12.9 IP Phone 7961 3

Gi2/5 auto on 5.6 5.0 IP Phone 7911 n/a

Gi2/6 auto on 14.5 12.9 IP Phone 7961 3

notice 7911 doesn't support the signature (n/a)

the more powerful the phone the more power it requires

That high voltage may come from the telephony world (-48V in central office)

Hope to help

Giuseppe

Silver

Re: poe 802.3af standard questions

How this rclass resistance helps PSE determine the class of PD?. Impedance of PSE= impedance of PD, otherwise , the power will not be transferred fully

This resistance has value of

Class Open

Class1 130Ω

Class2 71.5Ω

Class3 46.4Ω

When the classification option is not required, simply leave the RCLASS pin open to set the PD to the default Class 0 state.

During the classification mode, the voltage is between 12V and 23.5V. In this voltage

range, the class resistor RCLASS is engaged.

When volatege rises above 23.5V, RCLASS resistor will disengage.

4 REPLIES
Hall of Fame Super Silver

Re: poe 802.3af standard questions

Hello Sarah,

1) to measure a resistance the PD device doesn't need to be "ON" is enough to apply a voltage and to measure the current that flows.

This comes from electrical theory and practice.

A resistance is a passive component

For example in CIPT6.0 vol. I says:

The PSE (Cisco Catalyst switch) detects a powered device by applying a voltage in the range of

-2.8V to -10V on the cable and then looks for a 25kOhm signature resistor. Compliant

powered devices must support this resistance method. If the appropriate resistance is found, the

Cisco Catalyst switch delivers power

2) rclass pin means the resistance value.

when the switch decides to power on the port it applies the full voltage 42V.

With an higher voltage a lower current is needed to provide the same power

see the following show from one of our access switch

sh power inline

Available:1400(w) Used:1013(w) Remaining:387(w)

Interface Admin Oper Power(Watts) Device Class

From PS To Device

--------- ------ ---------- ---------- ---------- ------------------- -----

Gi2/1 auto on 5.6 5.0 IP Phone 7911 n/a

Gi2/2 auto off 0.0 0.0 n/a n/a

Gi2/3 auto on 16.7 14.9 IP Phone 7971 3

Gi2/4 auto on 14.5 12.9 IP Phone 7961 3

Gi2/5 auto on 5.6 5.0 IP Phone 7911 n/a

Gi2/6 auto on 14.5 12.9 IP Phone 7961 3

notice 7911 doesn't support the signature (n/a)

the more powerful the phone the more power it requires

That high voltage may come from the telephony world (-48V in central office)

Hope to help

Giuseppe

Bronze

Re: poe 802.3af standard questions

Thanks a lot Giuseppe!

I understand PSE detects the PD by:

V=IR

So R=V/I

If " R" turns out to be 25 ohms, PSE concludes indeed PD is connected. Now the 2nd stage comes"Classification of PD"

During classification, PSE offers 14.5v-20.5 volt( PD is still in off-state), PSE responds by inserting "rclass" resistance.

1)How this rclass resistance helps PSE determine the class of PD?.

It appears to me there is some sort of mapping between current and Power needed by the PD.

My logic is If PSE needs to find the current flowing in the circuit to determine the class of PSE, then we may deduce that there must be some mapping between the current and class(power requirement by PD).

How does PSE determine that current?

Form your reply:

"when the switch decides to power on the port it applies the full voltage 42V.

With an higher voltage a lower current is needed to provide the same power"

Let say PSE apply power 42 watts

p=(42)I

For PD, to use that power fully( same power) (42 watts)( ignoring the power loss on the line)

following must be true:

Impedance of PSE= impedance of PD, otherwise , the power will not be transferred fully.

Am i correct?

thanks a lot!

Silver

Re: poe 802.3af standard questions

How this rclass resistance helps PSE determine the class of PD?. Impedance of PSE= impedance of PD, otherwise , the power will not be transferred fully

This resistance has value of

Class Open

Class1 130Ω

Class2 71.5Ω

Class3 46.4Ω

When the classification option is not required, simply leave the RCLASS pin open to set the PD to the default Class 0 state.

During the classification mode, the voltage is between 12V and 23.5V. In this voltage

range, the class resistor RCLASS is engaged.

When volatege rises above 23.5V, RCLASS resistor will disengage.

Silver

Re: poe 802.3af standard questions

1) PD is not "ON" yet, how can it offer load current?

There are 2 options with PD like cisco IP phone,

First you connect them with external power like your PC.Then you can connect them to switches to which you connect ur PC etc.

Second they are not connecetd to any power source you just connect a RJ 45 cable like you connect ur PC. PSE detect them and provide power and they are ON. Like a switch detects a signal from end device and u see a green light on switch. Difference here the detection protocol is detecting from a non powered device

what is Rclass pin? just dropped it in the link without any explantion?

Not an expert on this topic ,I am also reading about PoE and didn't try to dig the theory behind the process. Maybe someone else can help you on this.

Few links on google from companies who makes that chip,

www.national.com/ds/LM/LM5073.pdf

See page 2 of this where they given diagram

focus.ti.com/lit/an/slva223/slva223.pdf

the question is does switch has to allocate this much voltage (above 42 v) per port, then it can add up as switch could many poE ports.

PoE equipments have power modules with higher power rating than a switch without PoE.

The Cisco Catalyst 3560-E Series access switches have one power supply slot and support the following power supplies. PoE switches require a PoE power supply. Data-only switches can operate with any of the power supplies:

• C3K-PWR-1150WAC: 1150WAC power supply with 740W PoE

• C3K-PWR-750WAC: 750WAC power supply for 24-port switch with 370W PoE

• C3K-PWR-265WAC: 265WAC power supply for 48- or 24-port switch without PoE

• C3K-PWR-265WDC: 265WDC power supply for 48- or 24-port switch without PoE

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps7078/product_data_sheet0900aecd805bac22.html

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