802.3bt - High power ( 90W ) Power over ethernet standard Just aproved. - Page 1
Forum section: electronics / projects

author: mrpackethead
author_rank: Super Contributor
timestamp: September 29, 2018, 03:44:21 am
content: Horrah!Its now an offical standard as of Thursday. 90W down the wire!

author: FotatoPotato
author_rank: Regular Contributor
timestamp: September 29, 2018, 03:56:00 am
content: Wow... Just wow!That's incredible! Imagine having a NUC or other small PC run off of one cable that does data and power. This is gonna be cool.

author: mrpackethead
author_rank: Super Contributor
timestamp: September 29, 2018, 04:08:41 am
content: Quote from: FotatoPotato on September 29, 2018, 03:56:00 amyes, it opens up all sorts of applicaitons.

author: Hydron
author_rank: Super Contributor
timestamp: September 29, 2018, 08:46:59 am
content: It's about time, this has taken WAY too long, previous standards (802.3af, at) only gave up to 30W - 25.5W with voltage drop on the maximum length cable - unless you used some hacky semi-standardised techniques to double it.The old "30W" 802.3at PoE+ standard also _required_ support for both requesting power at the physical layer (as done in .af, and uses the circuitry already in place for detection/classification of PoE powered devices) but also in the layer 2 ethernet stack with special frames sent to the PoE power provider (thank Cisco for getting that bollocks put into the standard). Made it impossible to meet the standard for a PoE device power supply that needed >15W and didn't have access to the ethernet software stack (e.g. an add-on PoE power supply for an existing device).If you look around there are PoE solutions for higher power already out there - they either use the draft 802.3bt standard, bits from other standards (e.g. the powsection from HDBaseT) or various proprietary stuff like LT's LTPoE++ or Microsemi's equivalent. Having dealt with a bunch of this stuff it's a complete mess and desperately needed a proper standard to follow.Unfortunately the standards process seems to have resulted in some annoying compromises - e.g. the hacky dual-signature stuff (pushed by a few of the parties to support stuff they already have in the market) has made it into the standard. Having talked to someone on the drafting committee it sounds like the worst stuff has been avoided (hello, Cisco layer 2 requirement!) but there's still a lot of politics and deviation from the most technically elegant solution involved. If the follow-on standard from 802.3at had appeared sooner I think a lot of complication could have been avoided, but the standards vacuum above the 30W power level meant that everyone came up with their own proprietary solution in the meantime and now wants it accommodated in 802.3bt.EDIT: It looks like I might be wrong about not needing to support LLDP (Layer 2) for 802.3bt. If this is the case then it's a major cost and/or inconvenience for many applications - you can't just let the hardware ask for the power it needs, you have to get it to ask for a smaller amount then add extra software to ask for more once the device has booted. In this case I think we'll see a lot of devices which will simply not support the standard fully (as it would not be feasible to), and won't work with PSE vendors who force use of LLDP power negotiation.

author: BrianHG
author_rank: Super Contributor
timestamp: September 29, 2018, 09:09:56 am
content: Topic: 802.3bt - High power ( 90W ) Power over ethernet standard Just aproved.Quote from: mrpackethead on September 29, 2018, 03:44:21 amI can now power and run my BitTorrent seedboxs laptops at the end of my network cable.

author: LapTop006
author_rank: Unknown
timestamp: October 01, 2018, 07:50:28 am
content: Microsemi have adecent summary write-up.

author: Hydron
author_rank: Super Contributor
timestamp: October 01, 2018, 11:05:15 am
content: Does anyone have a copy of (or access to) the standard yet? I haven't yet found conclusive answers to some crucial details (see update to my earlier post).

author: xani
author_rank: Frequent Contributor
timestamp: October 01, 2018, 12:06:06 pm
content: Quote from: Hydron on September 29, 2018, 08:46:59 amHow you imagine supporting it without any kind of negotiation ? On the switch side negotiation is required because not every device can provide full power on every port connected, so negotiation is there to "reserve" required power.That way you do not need to have 5 kW (!!!) for single 48 port switch if you just need few high power devices and a bunch of low or medium power onesAnd you also need to be compatible with current devices, incl. ones that do not care about PoE so you can't just send garbage (from non-poe device POV) to wire and hope it will not cause problems.

author: Hydron
author_rank: Super Contributor
timestamp: October 01, 2018, 02:04:33 pm
content: Quote from: xani on October 01, 2018, 12:06:06 pmThere is _always_ a detection/negotiation process involved - when a PoE "Powered Device" aka PD connects it presents a specified impedance between two of the pairs, then there is a negotiation/classification sequence where the PD informs the PoE "Power Sourcing Equipment" aka PSE (normally a switch) how much power it wants, and the PSE replies with how much it will give. This is a low-level process agnostic of what's on the pairs used (doesn't even need data to be present), and is used for all three PoE standards (802.3af, at and bt).For a LLDP (layer 2) requiring PSE, after detection/negotiation the PD gets up to the max power supported by 802.3af (~13 watts), boots up with this limited amount of power, then the software stack negotiates a SECOND time with the switch using LLDP packets for more power, then finally fully starts up with the (higher) new power limit. For a PSE that does _not_ require LLDP the PSE and PD negotiate to the required power level directly, ONCE, using an extension of the existing negotiation/classification method that's already present and used for lower 802.3af power levels.This is why LLDP as a mandatory requirement is crazy - you MUST write a bunch of software to do the second negotiation, and also design your hardware to operate in a low power mode to get the ethernet/processor running, then move to a higher power mode later after negotiation. This also eliminates the possibility of building an in-line PD power supply (to supply data/power to a separate non-PoE device, passing the data untouched and just doing the PD stuff to get power), as you must have higher level protocol access to the ethernet interface, not just connections to the centre-tap of the ethernet magnetics, which is all the non-LLDP negotiation needs.The only possible benefit I can see (other than helping Cisco who happen to make switches using LLDP) is to allow for re-negotiation at a later time for less power in applications where this would make sense. Even this shouldn't need LLDP support to be mandatory - it should be optional for use in these specific cases./rant

author: mrpackethead
author_rank: Super Contributor
timestamp: October 01, 2018, 07:32:42 pm
content: LLDP has never been a requirement of POE. Its there so you can get 'better' control of your power requirments, but it all of the exisiting standards have worked just fine without out.

author: Hydron
author_rank: Super Contributor
timestamp: October 01, 2018, 08:27:23 pm
content: Quote from: mrpackethead on October 01, 2018, 07:32:42 pmUnfortunately this is incorrect - Layer 2 (LLDP) support is a requirement for any PDs using 802.3at at above 13W - the standard says they must support _both_ Layer 1 (2-event physical layer signalling) and Layer 2 classification. The power source can pick to support either option, but it is mandatory for the device to support both.I don't have the standard in-front of me, but I assure you that's what is required (honestly I wish you were right!), and unfortunately it sounds like the insanity has spread to 802.3bt as well. Type-1 devices (<13W) are covered by the earlier hardware-only 802.3af standard so there isn't an issue there.I am working on a project which will need >13W but where there isn't access to the software stack running on the ethernet end-point, so unfortunately will have to break the rules in this area and release something that doesn't work with LLDP-only PSEs. Hopefully it doesn't bite us in the ass, but there's really no other option, and I suspect we won't be the only ones in this position.

author: mrpackethead
author_rank: Super Contributor
timestamp: October 01, 2018, 08:42:10 pm
content: Quote from: Hydron on October 01, 2018, 08:27:23 pmYou have misread the standard. To be compliant a PSE requires both L2 and 2 event. A PD only requires 2 event classification.Consider what happens at startup.. You dont' have any power on your microprocessor, or ethernet phy. How could you reply to or send a LLDP message? You cna't. The intial start up has to be done using the 2 event classification. After its powered up, it MAY then start communicating via LLDP, and then the ports can be set up for lower power levels..

author: Hydron
author_rank: Super Contributor
timestamp: October 02, 2018, 07:15:57 am
content: Quote from: mrpackethead on October 01, 2018, 08:42:10 pmA couple of excerpts from the 802.3at-2009 standard:"Type 2 PDs implement both 2-Event class signature (see 33.3.5.2) and Data Link Layer classification (see 33.6).""Type 2 PDs that require more than 13.0 W support Data Link Layer classification (see 33.3.5). Data Link Layer classification is optional for all other devices."From Microsemi white paper:"Type 2 PDs requiring more than 13.0 W must support data link-layer classification but the method is optional for all other devices, making allocation of power solely based on classification problematic."https://www.microsemi.com/document-portal/doc_view/24-understanding-ieee802-3at-poeplusSo I don't think I misread. You can get up to 13W (limit of Type 1 power) using standard classification method, but after that you have to be able to boot enough to send LLDP messages asking for more.

author: mrpackethead
author_rank: Super Contributor
timestamp: October 02, 2018, 07:57:58 am
content: Quote from: Hydron on October 02, 2018, 07:15:57 amI've implemetned 1000's of PD's using NCP1093 PD controllers with NO LLDP at 25W.I've also just implemented 802.3bt using TI's chip sets and dont' need any Layer 2.THe devices have been connected to many differnet types of switches.When the PSE detects a connected PD, it will send a short voltage pulse to determine how much power the PD needs. Depending on how much current is drawn during this pulse, the PSE will classify the PD into one of classes 0-4 (or 0-3 for type 1 PSEs and PDs).If a type 2 PSE classifies the connected PD into class 4 (PoE+ power), a second handshake is required. This is both to allow the PD to know that full 802.3at power is available, as well as to allow the PSE to confirm that a type 2 PD is actually connected to the port for safety reasons. Prior to this second handshake taking place, the PD and PSE shoujld restrict power to 802.3af limits.This second handshake can take one of two forms: a hardware-based Two-Event Classification or a software-based LLDP classification. Per the 802.3at standard, PSEs must support both of these mechanisms. However, PDs can support one or both.In the hardware-based two-event classification, the initial voltage pulse classification event is repeated. Once the PD passes the second classification event as a class 4 PD, it will be provided full class 4 802.3at power. This method was initially intended to be used by mid-span PoE injectors, as they generally will not support LLDP required for software-based classification.For the software-based LLDP classification, LLDP must be enabled on both the PSE and PD, with the 802.3at TLVs enabled. This will allow the PSE and PD to negotiate power requirements via software, rather than the second classification pulse used in the hardware based method. This also allows power requirements to be dynamically negotiated, allowing a PSE's power budget to be better managed.

author: Hydron
author_rank: Super Contributor
timestamp: October 02, 2018, 08:56:47 am
content: Quote from: mrpackethead on October 02, 2018, 07:57:58 amThis is the bit that is at odds with the standard (and every commentary I've read on it). The way you interpret it is how it seems it _should_ have been written, but it actually says the opposite. Also, if a PSE needed to support both mechanisms, a midspan injector would be impossible to build, as it could not support the LLDP method.I understand all the handshaking stuff (both in theory and having gotten a 25W PoE PD up and running on the bench), and am very happy to hear of your experience with non-LLDP PDs working in practice - this suggests that lack of LLDP support won't be a severe problem in practice.I've attached a table (presented in a very non-intuitive form it must be said) taken from the 802.3at standard showing what each end is required to support - note that the only allowed permutation for a type-2 PD is to support both 2-event and Link-Layer classification, while a PSE can do either or both.

author: xani
author_rank: Frequent Contributor
timestamp: October 02, 2018, 10:32:24 am
content: Quote from: Hydron on October 01, 2018, 02:04:33 pmAbout LLDP. Not Cisco. Cisco had CDP, Foundry had FDP, Nortel had NDP. LLDP is actually result of standardisation of those, supported now by pretty much every vendor of enterprise hardware and it is VERY useful outside of just power negotiation