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[8023-POEP] 100BASE-T normative specification of 350uH inductance



All,

On a number of occasions it has been mentioned that the normative 
requirement in 100BASE-T for a 350uH inductance, combined with the 
increase in DC current imbalance that results from the increase PoE+ 
current, will result in a increase the cost and physical size of PoE+ 
capable magnetics compared to POE capable magnetics. It has also been 
suggested that there are now other ways to address the problem that the 
inductance specification is there to address, baseline wander.

A while back I had drafted a maintenance request to change the IEEE 802.3 
standard to allow an implementation to either meet the 350uH inductance 
requirement or, alternatively, not meet an inductance requirement if the 
PHY can still achieve the target BER under worse case conditions through 
some other implementation. I have attached the draft below. I suggested 
this approach as it is the BER that is really what is of concern, not the 
way it is achieved. Havening the specified inductance is indeed one 
approach but other implementation approaches should be allowed by the 
standard so long as they achieve the same BER.

I think that due to the issues with current imbalance IEEE P802.3at should 
consider including the proposed change below as part of its draft. I will 
request presentation time at next weeks IEEE P802.3at meeting to allow 
consideration of this proposal.

Regards,
  David



Rational for revision
=====================

IEEE Std 802.3-2005 mandates, through a reference to ANSI X3.263-1995 
(TP-PMD), a requirement to meet a specific measurable inductance on 
100BASE-T ports. The reference to ANSI X3.263-1995 (TP-PMD) is contained 
in subclause 25.2, the text in ANSI X3.263-1995 (TP-PMD) can be found in 
subclause 9.1.7 'Worst case droop of transformer'. This text reads:

---

Baseline Wander tracking by the receiver is dependent on the worst case 
droop that can be produced by a transmitter. Droop is directly related to 
the Open Circuit Inductance (OCL) which varies with temperature, 
manufacturing tolerance, and bias current.

Worst case Baseline Wander Frames vary the transformer bias which causes 
the droop to change with data content. This variation must be accounted 
for by the receiver to track the Baseline Wander over long frames. 
Variation in inductance caused by bias of the transformer can be on the 
order of 2:1.

The minimum inductance measured at the transmit pins of the AOI shall be 
greater than or equal to 350 uH with any DC bias current between 0 mA and 
+8 mA injected as shown in figure 13.

---

I believe that this requirement for a specific inductance in IEEE Std 
802.3-2005, through it reference to the TP-PMD standard, is an over 
constraint. The fundamental requirement is for the link to operate at an 
acceptable Bit Error Ratio (BER) under worse case conditions, including 
worse case baseline wander. I believe that the inductance requirement 
found in TP-PMD subclause 9.1.7 was added to specifically overcome the 
problem of baseline wander but since the development of the TP-PMD 
specification other techniques can be successfully and economically be 
used to overcome the baseline wander.

The requirement is therefore an unnecessary additional mandatory 
requirement for many equipment vendors that has to be met simply to 
conform to IEEE 802.3. In these cases the magnetics are physically larger 
than would be otherwise required, making the construction of todays 
smaller form factor equipment more difficult, as well as being more 
costly.

To address this an additional exception should be included in Clause 25 
that allow implementers to either meet the inductance defined in TP-PMD 
subclause 9.1.7 or to meet a BER requirement. Unfortunately neither the 
TP-PMD standard nor Clause 25 provide a BER target for 100BASE-TX however 
10BASE-T has a BER target of 10^8 and 1000BASE-T has a BER target of 10^10 
therefore a target of 10^9 for 100BASE-T seems reasonable.


Proposed change
===============

Insert a new subclause 25.4.4 as follows, renumber the following subclause 
as required.

25.4.4 Change to 9.1.7 "Worst case droop of transformer"

The Worst case droop of transformer specified does not apply if the 
following specification is met.

While receiving data from the transmitter specified in subclause 7.1 of 
TP-PMD through a link segment specified in 25.4.6, a receiver shall send 
the proper PMA_UNITDATA.indication message to the PMA at a symbol error 
ratio of less than one part in 10^10 when the receiver is connected to a 
noise source as described in 25.4.6.3.


Impact on existing networks
===========================

None. It is additional option to conform to the BER requirement rather 
than the inductance requirement. Implementations that in the past have met 
the inductance requirement will still comply with the standard. 
Implementations that choose to meet the inductance requirement in the 
future will still comply with the standard.