Power saving in (X)GPONs

Frank EffenbergerRapporteur Q2/15

VP Access R&D, Futurewei

Joint ITU/IEEE Workshop on Ethernet - Emerging Applications and Technologies

(Geneva, Switzerland, 22 September2012)

Introduction

Power saving in the ITUSupplement G.45Actual power consumption

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Initial activities

First contributions were made by semiconductor vendors, to consider signaling methodsThe Q2/15 group thought these were premature

The requirements for power saving were not clearThe impact on existing systems was not clearThere was a concern it would degrade the user experience

It was agreed that a survey would be made of the operators to learn their requirements

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Power saving survey #1

Survey gathered information on Basic power supply designsWho pays, Who changes the battery?Overall requirements and interest in power saving

The most telling result was: Which is a higher priority, service availability or power savings?

The answer was overwhelmingly “Service quality is much more important that saving power” Apparently, the “Green Revolution” had not yet happened

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Power saving surveys #2 and #3

Survey #2 focused on the case of power failure

What UNI’s are in common use?How do they get powered down if the main power fails?Who can control this powering down process?

Survey #3 focused on regulations about lifeline

Are their regulations that force the maintenance of service during a power failure?How do those regulations vary from service to service (POTS vs. video vs. Internet)?

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Outcome of the surveys

It was clear that there was interest in power savingFor the most part, power saving was seen as a way to lengthen the life of the battery during an outageOperators were not willing to compromise much in the name of power saving

Can’t cost any moreServices can’t be effected

This set the stage for the G.sup45 document

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Introduction

Power saving in the ITUSupplement G.sup45Access power consumption

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Outline of G.sup45

RequirementsClassification of techniquesSignaling of ONU operationsComparative analysis Conclusions

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Requirements

Surveys were used as a primary requirementPower saving mode should be triggered by power failure, and NOT low traffic or unused ports

EU CoCThe current state of these power targets was reviewed

The “low power” mode was noted to be only half of full power mode

There are two main requirementsTo maintain service during a power failure

To save power at all times

No operator consensus on the balance between these two requirements

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Classification of techniques

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Power Shedding

When triggered, the ONU turns off the UNI’sTurn off in this context means fully powering down the circuit (not just deactivating service)

Each UNI type can have a different shut-down period (e.g., video can turn off after 30 minutes, but POTS is maintained for several hours)

When to trigger is a questionDuring power failure, some UNI’s can be turned off safely (e.g., video, because the TV sets will not have power in most cases)

During normal times, it is difficult to judge if UNI is busy

This is the least service effecting methodONU maintains contact with OLT at all times

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DozingWhen triggered, the ONU should stop transmitting in the upstream, even if it is given BW allocations

This allows transmitter circuitry to go into low power mode

Lower than normal “off” in between bursts

May take longer to recover (10’s of milliseconds)

Trigger would be the inactivity of the ONUDifficulty is that data services (and VoIP) tend to “chatter” all the time

Downstream receiver and signal chain remains onONU can be signaled by the OLT

Incoming calls can be received without delay

This impacts services slightlyOutgoing communications might suffer a delay, as normal bidirectional communication is reestablished

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Fast Sleep

When triggered, the ONU shuts off entire PON interface for a short period of time

ONU periodically wakes up to see if OLT has anything to say

During the shutdown, the ONU could conceivably have nearly zero power drain (only the wake-up timer would be powered)

Key issue is how fast can you wake up the opticsNormal transceiver designs are not optimized to turn on fast

However, proper optimization could get times down to ~1 ms

Side note: Recent result have shown circa 60% reductions

This method can have relatively low service impact

ONU maintains contact with OLT (albeit transiently)

Interactions with higher layer protocols must be considered

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Deep Sleep

When triggered, the ONU completely shuts off

Services are definitely impacted, no apologies for that

Power drain is zero, or nearly so

Challenge: How to wake up?Snow White method: A prince (the user) kisses the deep sleeper (presses a button on the ONU)

Rip Van Winkle method: Deep sleeper wakes up after a preset time, and sees if anything has changed

This method only appropriate for long outagesIt seems that the usual obligations are excused if power is out for a long time, and users and regulators understand that

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Signaling of ONU operations

Dying gasp: Enhancing the existing messageFor G-PON, not accepted, because it changes the TC-layer

PLOAM-based: Signaling for fast sleep methodFor G-PON, not accepted , because it changes the TC-layer

OMCI-based: Configuration of power featuresFor G-PON, OMCI additions have been made

Extended Power shedding: Detailed controlFor G-PON, fine-grain control of shedding has be standardized

Implicit signaling: OLT suppresses alarmsNo standards impact, so OLT vendors are free to implement

Security aspect: Impostor attackWhen the ONU is asleep, impostor can more easily jump in

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Comparative analysis

Model of ONU power consumption is given, and used to evaluate the savings for each type power saving

This model is only an example, based on a particular ONU design and circuit power values (these change over time)

Key findings of this evaluationPower shedding accomplishes a lot (70%) of power saving The other methods have increasing implementation difficulty and declining efficacy of power saving

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Conclusions of G.sup45

Power saving is an important topicMain object is to improve handling of power failuresRecommendations to improve power usage

Continuous improvement of design (ASIC, optics, power conv. Etc.) Power shedding should be supported and activatedDozing can be implemented with little cost“Aggressive” sleeping modes are of lowest priority

Final note: G-PON saves power in ICT field and other industries, so some credit should be given for that

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Introduction

Power saving in the ITUSupplement G.45Access power consumption

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System architecture of a VDSL system

DSLAM CPE

CPE

CPE

LT

LT

LT

LT

LT

WAN

WAN SW

SW

Typical VDSL linecard consumption today is 2W per line (i.e., per user)

Typical VDSL HG CPE consumption is 10W per user

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System architecture of G-PON system

OLT ONU

ONU

ONU

LT

LT

LT

LT

LT

WAN

WAN SW

SW

Typical OLT linecard consumption today is 7W per PON port (@ 28 users/PON = 250mW/user!)

Typical GPON HG CPE consumption is 10W per user 20

Central office / Node “power crunch”

Central office dissipation is dictated by NEBSTypical US number: 2000W per bay, 3 racks per bay

Typical DSLAM has 16x24 lines = 768W per rackThis barely fits in the 2000W number

Typical OLT has 16x8 PONs = 896 W per rackHave to leave 1 rack-space empty!

Is PON hitting the “crunch”? NO! One OLT serves 3584 users, while a DSLAM serves only 384 usersWe need 9 times fewer OLTs than DSLAMs

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Trend in broadband access CO equipment

The power per chassis is increasing marginallyPerhaps a 30% increase generation-over-generation

The capability per chassis is increasing incrediblyAggregate bandwidth increases 4~10x per generationUsers per chassis increased ~10x from copper to fiber

Total access power per user is already decliningDriven by the acceptance of fiber access

Power density is increasingSuggests a rethinking of the CO power design guidelinesPerhaps even a redesign of the cooling method entirely

Diffused air cooling (typical in today’s CO) is inappropriate for intense point heat loads

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The CPE power issue - Functional blocks

Typical Single family home gateway CPE consumption is 10W

EthernetInterf (x2)

MACWAN

interface

Mem

POTSInterf (x2)

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Breakdown of ONU (VDSL is similar)

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Observations on baseline consumption

Power consumption is reasonably balanced amongst functions – there is not one “bad actor”The majority of power (60%) lies in functions that are not particularly related to PON

You find them in any access systemMany are legacy dictates (ringing a bell)

They are designed for reliability and performanceE.g. Power converters consuming 20% of the power… why? To handle the stress environment that Telco requirements give us

Flexible hardware (e.g., CP instead of ASIC) is usedThe flexibility is a meta requirement of the ever changing marketBut, this is never the most power efficient way to build equipment

If the true power cost of all the requirements was rationalized, just imagine what we might save!

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Power saving technologies

Most natural path is intrinsic improvementCurrent designs were not designed with power as a key requirementTime to market, performance, and simplicity were always more important to the designer

Example: burst mode laser driverThe average ONU duty cycle is ~3% (32 ONUs per PON) But, the typical laser driver consumes current 100% of the timeWhy? Because it was easier that way

This is straightforward to fixThe designers only need to be guided that power consumption is an important goal that has value

This process is underway already!

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“Always on” means “always polluting”

Recall the original telephone networkYou only used power when off hook – very efficient, and natural behavior to the user

Data separated the “session” from userIdeally, users interact with their computer, and the computer establishes the (logical) sessions automatically User involvement in session control (dial up) was slow and painful This quickly drove the “always on” model

Power consumption was not considered!

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Sleep modes for access equipmentProtocols for sleeping and dozing are standardized in the XG-PON system

ONU state diagram OLT state diagram

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Future possibilities

OLT power consumption could be reduced in future PON systemsOLT “shedding”: If a port is not used, it should be powered down

As deep into the card as possible

OLT “sleeping”: If a TWDM-PON is underused, reduce the active waves

ONUs would be concentrated onto fewer channels

This could improve the load-dynamic power consumption of the CO

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Getting a good night’s sleep

Standardization is only the beginningThe hardware must be designed to use it

Optoelectronics must have fast turn-on/off Logic devices must support the protocolsSwitching must recognize that link is transient

The operators must be motivated to use it Operators respond to competitors and usersIn a choice between performance and power-saving, which wins? Example: ADSL has power saving for some years now – almost never used

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Conclusions

Current access power consumption is trending in the right direction, considering the incredible BW improvements The CO-side solution is in our hands: deploy PON, and you cut your CO power by an order of magnitudeThe CPE-side is much larger problemLegacy interface requirements are an issue

If we could only redesign POTS… If only Telco’s could agree on a service profile and stick to it…

Power saving modes have good potentialChanging “always on” into “always available”Already standardized – we just have to do it

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