PON fundamental RAD

8/7/2019 PON fundamental RAD

http://slidepdf.com/reader/full/pon-fundamental-rad 1/94

PPassiveassive OOpticalptical NNetworketworkss

Yaakov (J) Stein May 2007

andZvika Eitan



PONs Slide 2

OutlineOutline

PON benefits

PON architecture

Fiber optic basics

PON physical layer

PON user plane

PON control plane



PONs Slide 3

PON benefitsPON benefits



PONs Slide 5

Aside – whyAside – why isis fiber better ?fiber better ?

attenuation per unit length reasons for energy loss

– copper: resistance, skin effect, radiation, coupling – fiber: internal scattering, imperfect total internal reflection

so fiber beats coax by about 2 orders of magnitude – e.g. 10 dB/km for thin coax at 50MHz, 0.15 dB/km λ =1550nm fiber

noise ingress and cross-talk copper couples to all nearby conductors no similar ingress mechanism for fiber

ground-potential, galvanic isolation, lightning protection copper can be hard to handle and dangerous no concerns for fiber



PONs Slide 6

WhyWhy not not fiber ?fiber ?

fiber beats all other technologies for speed and reach

but fiber has its own problems

harder to splice, repair, and need to handle carefully

regenerators and even amplifiers are problematic – more expensive to deploy than for copper

digital processing requires electronics – so need to convert back to electronics – we will call the converter an optical transceiver

– optical transceivers are expensive switching easier with electronics (but possible with photonics)

– so pure fiber networks are topologically limited: point-to-point rings

copper fiber


http://slidepdf.com/reader/full/pon-fundamental-rad 7/94PONs Slide 7

Access network bottleneckAccess network bottleneck

hard for end users to get high datarates because of the access bottleneck

local area networks use copper cable get high datarates over short distances

core networks use fiber optics get high datarate over long distances small number of active network elements

access networks (first/last mile)

long distances – so fiber would be the best choice many network elements and large number of endpoints

– if fiber is used then need multiple optical transceivers – so copper is the best choice – this severely limits the datarates

coreaccess

LAN



FFiber iber TToo TThehe CCurburbHybrid Fiber Coax and VDSL

switch/transceiver/miniDSLAM located at curb or in basement need only 2 optical transceivers

but not pure optical solution lower BW from transceiver to end users need complex converter in constrained environment

N end users

core

access network

feeder fiber

copper



FFiber iber TToo TThehe PPremisesremises

we can implement point-to-multipoint topology purely in optics but we need a fiber (pair) to each end user

requires 2 N optical transceivers

complex and costly to maintain

N end userscore

access network



An obvious solutionAn obvious solution

deploy intermediate switches (active) switch located at curb or in basement saves space at central office need 2 N + 2 optical transceivers

N end users

core

access network

feeder fiber

fiber



The PON solutionThe PON solution

another alternative - implement point-to-multipoint topology purely in optics avoid costly optic-electronic conversions use passive splitters – no power needed, unlimited MTBF only N+1 optical transceivers (minimum possible) !

1:2 passive splitter

1:4 passive splitter

N end users

feeder fiber

core

access network

typically N=32

max defined 128



PON advantagesPON advantages

shared infrastructure translates to lower cost per customer minimal number of optical transceivers feeder fiber and transceiver costs divided by N customers greenfield per-customer cost similar to UTP

passive splitters translate to lower cost can be installed anywhere no power needed essentially unlimited MTBF

fiber data-rates can be upgraded as technology improves

initially 155 Mbps then 622 Mbps now 1.25 Gbps soon 2.5 Gbps and higher



PONPON

architecturearchitecture



TerminologyTerminology

like every other field, PON technology has its own terminology the CO head-end is called an OLT ONUs are the CPE devices (sometimes called ONTs in ITU)

the entire fiber tree (incl. feeder, splitters, distribution fibers) is an ODN all trees emanating from the same OLT form an OAN

downstream is from OLT to ONU (upstream is the opposite direction)

downstream

O ptical Network Units

upstream

O ptical Distribution Network NNI

Terminal Equipment

UNI

coresplitter

O ptical Line Terminal

O ptical Access Network



PON typesPON types

many types of PONs have been defined

APON ATM PON

BPON Broadband PON

GPON Gigabit PON

EPON Ethernet PON

GEPON Gigabit Ethernet PON

CPON CDMA PON

WPON WDM PONin this course we will focus on GPON and EPON (including GEPON)

with a touch of BPON thrown in for the flavor



BibliographyBibliography

BPON is explained in ITU-T G.983.x

GPON is explained in ITU-T G.984.x EPON is explained in IEEE 802.3-2005 clauses 64 and 65

– (but other 802.3 clauses are also needed)

Warningdo not believe white papers from vendors

especially not with respect to GPON/EPON comparisons

EPONBPONGPON



PON principlesPON principles(almost) all PON types obey the same basic principles

OLT and ONU consist of Layer 2 (Ethernet MAC, ATM adapter, etc.) optical transceiver using different λ s for transmit and receive optionally: Wavelength Division Multiplexer

downstream transmission OLT broadcasts data downstream to all ONUs in ODN ONU captures data destined for its address, discards all other data encryption needed to ensure privacy

upstream transmission ONUs share bandwidth using T ime Division M ultiple Access OLT manages the ONU timeslots ranging is performed to determine ONU-OLT propagation time

additional functionality Physical Layer OAM Autodiscovery

Dynamic Bandwidth Allocation



Why a new protocol ?Why a new protocol ?

PON has a unique architecture

(broadcast) point-to-multipoint in DS direction

(multiple access) multipoint-to-point in US direction

contrast that with, for example

Ethernet - multipoint-to-multipoint

ATM - point-to-point

This means that existing protocols

do not provide all the needed functionality

e.g. receive filtering, ranging, security, BW allocation

downstream

upstream



(multi)point - to - (multi)point(multi)point - to - (multi)point

Multipoint-to-multipoint Ethernet avoids collisions

by CSMA/CD

This can't work for multipoint-to-point US PON

since ONUs don't see each other

And the OLT can't arbitrate without adding a roundtrip time

Point-to-point ATM can send data in the open

although trusted intermediate switches see all data

customer switches only receive their own data

This can't work for point-to-multipoint DS PONsince all ONUs see all DS data



PON encapsulationPON encapsulation

The majority of PON traffic is Ethernet

So EPON enthusiasts say

use EPON - it's just Ethernet

That's true by definition -

anything in 802.3 is Ethernet

and EPON is defined in clauses 64 and 65 of 802.3-2005

But don't be fooled - all PON methods encapsulate MAC frames

EPON and GPON differ in the contents of the header

EPON hides the new header inside the GbE preamble

GPON can also carry non-Ethernet payloads

DA SA T data FCSPON header



PONs Slide 22

EPON historyEPON history2001: IEEE 802 LMSC WG accepts

Ethernet in the First Mile Project Authorization Request

becomes EFM task force (largest 802 task force ever formed)

EFM task force had 4 tracks DSL (now in clauses 61, 62, 63)

Ethernet OAM (now clause 57) Optics (now in clauses 58, 59, 60, 65) P2MP (now clause 64)

2002 : liaison activity with ITU to agree upon wavelength allocations

2003 : WG ballot

2004 : full standard

2005: new 802.3 version with EFM clauses



PONs Slide 23

GPON historyGPON history

2001 : FSAN initiated work on extension of BPON to > 1 Gbps

Although GPON is an extension of BPON technology

and reuses much of G.983 (e.g. linecode, rates, band-plan, OAM)

decision was not to be backward compatible with BPON

2001 : GFP developed (approved 2003)

2003 : GPON became G.984

– G.984.1 : GPON general characteristics

– G.984.2 : Physical Media Dependent layer

– G.984.3 : Transmission Convergence layer – G.984.4 : management and control interface



PONs Slide 24

Fiber optics - basicsFiber optics - basics



PONs Slide 25

© = sin¯ (n2/n1)1

V =c/n

t = L·n/c

t = PropagationTimet Vacuum: n=1,

t=3.336ns/m

t Water : n=1.33,

t=4.446ns/m

Total Internal ReflectionTotal Internal Reflection

in Step-Index Multimode Fiber in Step-Index Multimode Fiber



PONs Slide 26

Multimode Graded-Index Fiber

Single-mode

Fiber

Types of Optical Fiber Types of Optical Fiber

Popular Fiber Sizes



PONs Slide 27

Click to edit Master text styles

– Second level

Third level – Fourth level

Optical Loss versus WavelengthOptical Loss versus Wavelength



PONs Slide 28

TotalDispersion

MultimodeDispersion ChromaticDispersion

MaterialDispersion

Sources of DispersionSources of Dispersion



PONs Slide 29

1 0 11

Multimode DispersionMultimode Dispersion

1 11 11 11

Dispersion limits bandwidth in opticalfiber



PONs Slide 30

1 0 11

Graded-index DispersionGraded-index Dispersion

1 10



PONs Slide 31

1 0 1 1 10

In SM the limit bandwidth is caused by chromaticdispersion.

1

Single-Mode DispersionSingle-Mode Dispersion



PONs Slide 32

How to calculatebandwidth?

T c = (20ps/nm * km) * 5nm * 15km =

1.5ns

T c = Dmat * ∆ λ *

L

T c = (20ps/nm * km) * 0.2nm * 60km = 0.24n

For Laser 1550nm Fabry Perot

For Laser 1550nm DFB

For a 1.25 Gb/s we need a BW of 0.7 BitRate =1.143ns

System Design ConsiderationSystem Design Consideration



PONs Slide 33

Material Dispersion (Dmat)Material Dispersion (Dmat)



PONs Slide 34

LASER/laser diode : Light Amplification by Stimulated Emission of Radiation. Done of the wide range of

devices that generates light by that principle. Laser light is directional, covers a narrow range of

wavelengths, and is more coherent than ordinary light. Semiconductor diode lasers are the standard light

sources in fiber optic systems. Lasers emit light by stimulated emission.

Spectral CharacteristicsSpectral Characteristics



PONs Slide 35

Laser

W

Laser Optical Power Output vs. Forward CurrentLaser Optical Power Output vs. Forward Current



PONs Slide 36

PIN DIODES (PD)

- Operation simular to LEDs, but in reverse, photon are converted to electrons

- Simple, relatively low- cost

- Limited in sensitivity and operating range

- Used for lower- speed or short distance applications

AVALANCHE PHOTODIODES (APD)

- Use more complex design and higher operating voltage than PIN diodes

to produce amplification effect- Significantly more sensitive than PIN diodes

- More complex design increases cost

- Used for long-haul/higher bit rate systems

Light DetectorsLight Detectors



PONs Slide 37

Wavelength-Division MultiplexingWavelength-Division Multiplexing



PONs Slide 39BMCDR = Burst Mode Clock Data Recovery

OLT = Optical Line Termination

ONU = Optical Network Unit

Basic Configuration of PONBasic Configuration of PON



PONs Slide 40

Typical PON Configuration and Optical PacketsTypical PON Configuration and Optical Packets

Eye diagram of ONU transceiverEye diagram of ONU transceiver



PONs Slide 41

Eye diagram of ONU transceiver Eye diagram of ONU transceiver

in burst mode operationin burst mode operation



PONs Slide 42

Burst-Mode Transmitter in ONUBurst-Mode Transmitter in ONU



PONs Slide 43

OLT Burst-Mode Receiver OLT Burst-Mode Receiver



PONs Slide 44

Burst-Mode CDRBurst-Mode CDR



PONs Slide 45

Ideal, error-free transmission

Superimposed interference

Hysteresis

Ideal sampling instant

SamplingSampling



PONs Slide 46

Transceiver Block DiagramTransceiver Block Diagram



PONs Slide 47

Optical SplittersOptical Splitters



PONs Slide 48

Optical Protection SwitchOptical Protection Switch

Optical Splitter Optical Splitter



PONs Slide 49

LB = ׀ PS ׀-׀ PO׀

LB = Link Budget

PS = Sensitivity

PO = Output Power

Example: GPON 1310nm

Power: 0dbm Single-mode fiber

Sensitivity: -23dbm } Link Budget: 23db

Budget CalculationsBudget Calculations

C



PONs Slide 50

Assume:

Optical loss = 0.35 db/km

Connector Loss = 2dB

Splitter Insertion Loss 1X32 = 17dB

Range Budget: ~11Km

Typical Range CalculationTypical Range Calculation

Relationship between transmission distanceRelationship between transmission distance



PONs Slide 51

Relationship between transmission distanceRelationship between transmission distance

and number of splitsand number of splits

GbE Fib O ti Ch t i tiGbE Fiber Optic Characteristics



PONs Slide 52

GbE Fiber Optic CharacteristicsGbE Fiber Optic Characteristics



PONs Slide 54

λ allocations - G.983.1allocations - G.983.1

Upstream and downstream directions need about the same bandwidth

US serves N customers, so it needs N times the BW of each customer

but each customer can only transmit 1/N of the time

In APON and early BPON work it was decided that 100 nm was needed

Where should these bands be placed for best results?

In the second and third windows !

Upstream 1260 - 1360 nm (1310 ± 50) second window

Downstream 1480 - 1580 nm (1530 ± 50) third window

1200 nm 1300 nm 1400 nm 1500 nm 1600 nm

US DS



PONs Slide 55

λ allocations - G.983.3allocations - G.983.3

Afterwards it became clear that there was a need for additional DS bands

Pressing needs were broadcast video and data

Where could these new DS bands be placed ?

At about the same time G.694.2 defined 20 nm CWDM bands

these were made possible because of new inexpensive hardware

(uncooled Distributed Feedback Lasers)

One of the CWDM bands was 1490 ± 10 nmsame bottom λ as the G.983.1 DS

So it was decided to use this band as the G.983.3 DS

and leave the US unchanged

1270 16301490

1200 nm 1300 nm 1400 nm 1500 nm 1600 nm

US DSavailable

guard



PONs Slide 56

λ allocations - finalallocations - final

The G.983.3 band-plan was incorporated into GPON

and via liaison activity into EPONand is now the universally accepted xPON band-plan

US 1260-1360 nm (1310 ± 50)

DS 1480-1500 nm (1490 ± 10)

enhancement bands:

– video 1550 - 1560 nm (see ITU-T J.185/J.186)

– digital 1539-1565 nm

1200 nm 1300 nm 1400 nm 1500 nm 1600 nm

US DS



PONs Slide 57

Data ratesData rates (for now …)(for now …)

PON DS (Mbps) US (Mbps)

BPON 155.52 155.52

622.08 155.52

622.08 622.08

1244.16 155.52

1244.16 622.08

1244.16 155.521244.16 622.08

1244.16 1244.16

2488.32 155.52

2488.32 622.08

2488.32 1244.162488.32 2488.32

EPON 1250* 1250*

10GEPON† 10312.5* 10312.5*

* only 1G/10G usable due to linecode† work in progress

Amd 1

Amd 2

GPON



PONs Slide 58

Reach and splitsReach and splits

Reach and the number of ONUs supported are contradictory design goals

In addition to physical reach derived from optical budget

there is logical reach limited by protocol concerns (e.g. ranging protocol)

and differential reach (distance between nearest and farthest ONUs)

The number of ONUs supported depends not only on the number of splits

but also on the addressing scheme

BPON called for 20 km and 32-64 ONUs

GPON allows 64-128 splits and the reach is usually 20 km

but there is a low-cost 10 km mode (using Fabry-Perot laser diodes in ONUs)

and a long physical reach 60 km mode with 20 km differential reach

EPON allows 16-256 splits (originally designed for link budget of 24 dB, but now 30 dB)

and has 10 km and 20 km Physical Media Dependent sublayers



PONs Slide 59

Line codesLine codes

BPON and GPON use a simple NRZ linecode (high is 1 and low is 0)An I.432-style scrambling operation is applied to payload (not to PON overhead)

Preferable to conventional scrambler because no error propagation – each standard and each direction use different LFSRs – LFSR initialized with all ones

– LFSR sequence is XOR'ed with data before transmission

EPON uses the 802.3z (1000BASE-X) line code - 8B/10B – Every 8 data bits are converted into 10 bits before transmission

– DC removal and timing recovery ensured by mapping – Special function codes (e.g. idle, start_of_packet, end_of_packet, etc)

However, 1000 Mbps is expanded to 1250 Mbps

10GbE uses a different linecode - 64B/66B



PONs Slide 60

FECFEC

G984.3 clause 13 and 802.3-2005 subclause 65.2.3

define an optional G.709-style Reed-Solomon code

Use (255,239,8) systematic RS code designed for submarine fiber (G.975)

to every 239 data bytes add 16 parity bytes to make 255 byte FEC block

Up to 8 byte errors can be corrected

Improves power budget by over 3 dB,

allowing increased reach or additional splits

Use of FEC is negotiated between OLT and ONU

Since code is systematiccan use in environment where some ONUs do not support FEC

In GPON FEC frames are aligned with PON frames

In EPON FEC frames are marked using K-codes(and need 8B10B decode - FEC - 8B10B encode)



PONs Slide 61

More physical layer problemsMore physical layer problems

Near-far problem

OLT needs to know signal strength to set decision threshold

If large distance between near/far ONUs, then very different attenuations

If radically different received signal strength can't use a single threshold

– EPON: measure received power of ONU at beginning of burst

– GPON: OLT feedback to ONUs to properly set transmit power

Burst laser problem

Spontaneous emission noise from nearby ONU lasers causes interferenceElectrically shut ONU laser off when not transmitting

But lasers have long warm-up time

and ONU lasers must stabilize quickly after being turned on



PONs Slide 62

US timing diagramUS timing diagram

How does the ONU US transmission appear to the OLT ?

grant grant

laser turn-on

laser turn-off

data

l o c

k

laser turn-on

laser turn-off

data

l o c

k

inter-ONUguard

Notes:

GPON - ONU reports turn-on and turn-off times to OLT

ONU preamble length set by OLT

EPON - long lock time as need to Automatic Gain Control and Clock/Data Recovery

long inter-ONU guard due to AGC-reset

Ethernet preamble is part of data



PONs Slide 63

PON User planePON User plane



PONs Slide 64

How does it work?How does it work?

ONU stores client data in large buffers (ingress queues)ONU sends a high-speed burst upon receiving a grant/allocation

– Ranging must be performed for ONU to transmit at the right time

– DBA - OLT allocates BW according to ONU queue levels

OLT identifies ONU traffic by label

OLT extracts traffic units and passes to network

OLT receives traffic from network and encapsulates into PON frames

OLT prefixes with ONU label and broadcastsONU receives all packets and filters according to label

ONU extracts traffic units and passes to client



PONs Slide 65

LabelsLabels

In an ODN there is 1 OLT, but many ONUs

ONUs must somehow be labeled for

– OLT to identify the destination ONU

– ONU to identify itself as the source

EPON assigns a single label Logical Link ID to each ONU(15b)

GPON has several levels of labels

– ONU_ID (1B) (1B)

– Transmission-CONTainer (AKA Alloc_ID) (12b) (can be >1 T-CONT per ONU)

For ATM mode VPI VCI

For GEM mode Port_ID (12b) (12b)

PONONU

ONU

T-CONT

T-CONT

Port

Port

VP

VP

VCVCVCVC



PONs Slide 66

DS GPON formatDS GPON format

GPON Transmission Convergence frames are always 125 µ sec long

– 19440 bytes / frame for 1244.16 rate – 38880 bytes / frame for 2488.32 rate

Each GTC frame consists of Physical Control Block downstream + payload

– PCBd contains sync, OAM, DBA info, etc.

– payload may have ATM and GEM partitions (either one or both)

payloadPCBd payloadPCBd payloadPCBd

GTC frame

PSync (4B) Ident (4B) PLOAMd (13B) BIP (1B)

PLend (4B) PLend (4B) US BW map (N*8B)

ATM

partition

GEM

partition

scrambled 125

µ sec



PONs Slide 67

GPON payloadsGPON payloads

GTC payload potentially has 2 sections:

– ATM partition (Alen * 53 bytes in length) – GEM partition (now preferred method)

ATM partitionAlen (12 bits) is specified in the PCBd

Alen specifies the number of 53B cells in the ATM partitionif Alen=0 then no ATM partition

if Alen=payload length / 53 then no GEM partition

ATM cells are aligned to GTC frame

ONUs accept ATM cells based on VPI in ATM header

GEM partitionUnlike ATM cells, GEM delineated frames may have any length

Any number of GEM frames may be contained in the GEM partition

ONUs accept GEM frames based on 12b Port-ID in GEM header

ATM cellPCBd … GEM frame GEM frame … GEM frameATM cell ATM cell

GPON E l i M d



PONs Slide 68

GGPONPON EEncapsulationncapsulation MModeode

A common complaint against BPON was inefficiency due to ATM cell tax

GEM is similar to ATM – constant-size HEC-protected header – but avoids large overhead by allowing variable length frames

GEM is generic – any packet type (and even TDM) supported

GEM supports fragmentation and reassemblyGEM is based on GFP, and the header contains the following fields:

– Payload Length Indicator - payload length in Bytes – Port ID - identifies the target ONU – Payload Type Indicator (GEM OAM, congestion/fragmentation indication) –

Header Error Correction field (BCH(39,12,2) code+ 1b even parity)The GEM header is XOR'ed with B6AB31E055 before transmission

Port ID

(12b)

PLI

(12b)

HEC

(13b)

PTI

(3b)

payload fragment

(L Bytes)

5 B

Eth t / TDM GEM



PONs Slide 69

Ethernet / TDM over GEMEthernet / TDM over GEM

When transporting Ethernet traffic over GEM:

– only MAC frame is encapsulated (no preamble, SFD, EFD) – MAC frame may be fragmented (see next slide)

When transporting TDM traffic over GEM:

– TDM input buffer polled every 125 µ sec.

– PLI bytes of TDM are inserted into payload field

– length of TDM fragment may vary by ± 1 Byte due to frequency offset

– round-trip latency bounded by 3 msec.

DA SA T data FCSPLI

Ethernet over GEM

ID PTI HEC

PLI Bytes of TDMPLI

TDM over GEM

ID PTI HEC

G fGEM f t ti



PONs Slide 70

GEM fragmentationGEM fragmentationGEM can fragment its payload

For example

GEM fragments payloads for either of two reasons:

– GEM frame may not straddle GTC frame

– GEM frame may be pre-empted for delay-sensitive data

DA SA T data FCSPLI

unfragmented Ethernet frame

ID PTI=001 HEC

DA SA T data1PLI

fragmented Ethernet frame

ID PTI=000 HEC

data2PLI ID PTI=001 HEC FCS

ATM partitionPCBd GEM frame … GEM frag 1 ATM partitionPCBd GEM frag 2 … GEM frame

ATM partitionPCBd urgent frame … large frag 1 ATM partitionPCBd urgent frame … large frag 2

PCBdPCBd



PONs Slide 71

PCBdPCBd

We saw that the PCBd is

PSync - fixed pattern used by ONU to located start of GTC frame

Ident - MSB indicates if FEC is used, 30 LSBs are superframe counter PLOAMd - carries OAM, ranging, alerts, activation messages, etc.

BIP - SONET/SDH-style Bit Interleaved Parity of all bytes since last BIP

PLend (transmitted twice for robustness) - – Blen - 12 MSB are length of BW map in units of 8 Bytes – Alen - Next 12 bits are length of ATM partition in cells – CRC - final 8 bits are CRC over Blen and Alen

US BW map - array of Blen 8B structures granting BW to US flow

will discuss later (DBA)

PSync (4B)

Ident (4B)

PLOAMd (13B)

BIP (1B)

PLend (4B)

PLend (4B)

US BW map (N*8B)

B6AB31E0

GPON US id tiGPON US id ti



PONs Slide 72

GPON US considerationsGPON US considerations

GTC fames are still 125 µ sec long, but shared amongst ONUs

Each ONU transmits a burst of data

– using timing acquired by locking onto OLT signal

– according to time allocation sent by OLT in BWmap

there may be multiple allocations to single ONU

OLT computes DBA by monitoring traffic status (buffers)

of ONUs and knowing priorities

– at power level requested by OLT (3 levels)

this enables OLT to use avalanche photodiodes which are

sensitive to high power bursts – leaving a guard time from previous ONU's transmission

– prefixing a preamble to enable OLT to acquire power and phase

– identifying itself (ONU-ID) in addition to traffic IDs (VPI, Port-ID)

– scrambling data (but not preamble/delimiter)

US GPON f tUS GPON f t



PONs Slide 73

US GPON formatUS GPON format4 different US overhead types:

Physical Layer Overhead upstream – always sent by ONU when taking over from another ONU – contains preamble and delimiter (lengths set by OLT in PLOAMd)

BIP (1B), ONU-ID (1B), and Indication of real-time status (1B)

PLOAM upstream (13B) - messaging with PLOAMd

Power Levelling Sequence upstream (120B) – used during power-set and power-change to help set ONU

power so that OLT sees similar power from all ONUs

Dynamic Bandwidth Report upstream – sends traffic status to OLT in order to enable DBA computation

PLOu PLOAMd PLSu DBRu payload

if all OH types are present:

US ll ti lUS ll ti l



PONs Slide 74

US allocation exampleUS allocation example

BWmap sent by OLT to ONUs is a list of ONU allocation IDs flags (not shown above) tell if use FEC, which US OHs to use, etc. start and stop times (16b fields, in Bytes from beginning of US frame)

payloadPCBd

DS frame

Alloc-ID SStart SStop Alloc-ID SStart Sstop Alloc-ID SStart SStopBWmap

US frame

guard

time

preamble+

delimiter

scrambled

EPON f tEPON f t



PONs Slide 75

EPON formatEPON format

EPON operation is based on the Ethernet MACand EPON frames are based on GbE frames

but extensions are needed

clause 64 - MultiPoint Control Protocol PDUs

this is the control protocol implementing the required logic clause 65 - point-to-point emulation (reconciliation)

this makes the EPON look like a point-to-point link

and EPON MACs have some special constraints instead of CSMA/CD they transmit when granted

time through MAC stack must be constant (± 16 bit durations)

accurate local time must be maintained

EPON h dEPON h d



PONs Slide 76

EPON header EPON header Standard Ethernet starts with an essentially content-free 8B preamble 7B of alternating ones and zeros 10101010 1B of SFD 10101011

In order to hide the new PON header EPON overwrites some of the preamble bytes

LLID field contains

– MODE (1b) always 0 for ONU 0 for OLT unicast, 1 for OLT multicast/broadcast

– actual Logical Link ID (15b) Identifies registered ONUs 7FFF for broadcast

CRC protects from SLD (byte 3) through LLID (byte 7)

10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011

10101010 10101010 10101011 10101010 10101010 LLID LLID CRC

MPC PDU f tMPC PDU f t



PONs Slide 77

MPC PDU formatMPC PDU format

MultiPoint Control Protocol frames are untagged MAC frameswith the same format as PAUSE frames

Ethertype = 8808

Opcodes (2B) - presently defined:

GATE/REPORT/REGISTER_REQ/REGISTER/REGISTER_ACK

Timestamp is 32b, 16 ns resolution

conveys the sender's time at time of MPCPDU transmission

Data field is needed for some messages

DA SA L/T Opcode timestamp data / RES / pad FCS



GPON tiGPON encryption



PONs Slide 79

GPON encryptionGPON encryption

OLT encrypts using AES-128 in counter modeOnly payload is encrypted (not ATM or GEM headers)

Encryption blocks aligned to GTC frame

Counter is shared by OLT and all ONUs –

46b = 16b intra-frame + 30 bits inter-frame – intra-frame counter increments every 4 data bytes reset to zero at beginning of DS GTC frame

OLT and each ONU must agree on a unique symmetric key

OLT asks ONU for a password (in PLOAMd)

ONU sends password US in the clear (in PLOAMu)

– key sent 3 times for robustness

OLT informs ONU of precise time to start using new key

QoS EPONQoS EPON



PONs Slide 80

QoS - EPONQoS - EPON

Many PON applications require high QoS (e.g. IPTV)

EPON leaves QoS to higher layers – VLAN tags – P bits or DiffServ DSCP

In addition, there is a crucial difference between LLID and Port-ID – there is always 1 LLID per ONU – there is 1 Port-ID per input port - there may be many per ONU – this makes port-based QoS simple to implement at PON layer

RT BEEF

GPON

QoS GPONQoS GPON



PONs Slide 81

QoS - GPONQoS - GPON

GPON treats QoS explicitly – constant length frames facilitate QoS for time-sensitive applications – 5 types of Transmission CONTainers

type 1 - fixed BW type 2 - assured BW

type 3 - allocated BW + non-assured BW type 4 - best effort type 5 - superset of all of the above

GEM adds several PON-layer QoS features – fragmentation enables pre-emption of large low-priority frames

– PLI - explicit packet length can be used by queuing algorithms – PTI bits carry congestion indications



PONs Slide 82

PON control planePON control plane

PrinciplesPrinciples



PONs Slide 83

PrinciplesPrinciples

GPON uses PLOAMd and PLOAMu as control channelPLOAM are incorporated in regular (data-carrying) frames

Standard ITU control mechanism

EPON uses MPCP PDUs

Standard IEEE control mechanism

EPON control model - OLT is master, ONU is slave

– OLT sends GATE PDUs DS to ONU

– ONU sends REPORT PDUs US to OLT

RangingRanging



PONs Slide 84

RangingRanging

Upstream traffic is TDMA

Were all ONUs equidistant, and were all to have a common clock

then each would simply transmit in its assigned timeslot

But otherwise the signals will overlap

To eliminate overlap guard times left between timeslots each ONU transmits with the proper delay to avoid overlap delay computed during a ranging process

Ranging backgroundRanging background



PONs Slide 85

Ranging backgroundRanging background

In order for the ONU to transmit at the correct time

the delay between ONU transmission and OLT receptionneeds to be known (explicitly or implicitly)

Need to assign an equalization-delay

The more accurately it is known

the smaller the guard time that needs to be leftand thus the higher the efficiency

Assumptions behind the ranging methods used:

can not assume US delay is equal to DS delay

delays are not constant – due to temperature changes and component aging

GPON: ONUs not time synchronized accurately enough

EPON: ONUs are accurately time synchronized (std contains jitter masks)

with time offset by OLT-ONU propagation time

GPON ranging methodGPON ranging method



PONs Slide 86

GPON ranging methodGPON ranging method

Two types of ranging

– initial ranging only performed at ONU boot-up or upon ONU discovery must be performed before ONU transmits first time

– continuous ranging

performed continuously to compensate for delay changes

OLT initiates coarse ranging by stopping allocations to all other ONUs – thus when new ONU transmits, it will be in the clear

OLT instructs the new ONU to transmit (via PLOAMd)

OLT measures phase of ONU burst in GTC frame

OLT sends equalization delay to ONU (in PLOAMd)

During normal operation OLT monitors ONU burst phase

If drift is detected OLT sends new equalization delay to ONU (in PLOAMd)

EPON ranging methodEPON ranging method



PONs Slide 87

EPON ranging methodEPON ranging method

All ONUs are synchronized to absolute time (wall-clock)

When an ONU receives an MPCPDU from OLTit sets its clock according to the OLT's timestamp

When the OLT receives an MPCPDU in response to its MPCPDU

it computes a "round-trip time" RTT (without handling times)

it informs the ONU of RTT, which is used to compute transmit delay

RTT = (T2-T0) - (T1-T0) = T2-T1

OLT compensates all grants by RTT before sending

Either ONU or OLT can detect that timestamp drift exceeds threshold

time

OLT sends MPCPDU

Timestamp = T0

ONU receives MPCPDU

Sets clock to T0

ONU sends MPCPDU

Timestamp = T1

OLT receives MPCPDU

RTT = T2 - T1

T0OLT time T2T0ONU time T1

AutodiscoveryAutodiscovery



PONs Slide 88

AutodiscoveryAutodiscovery

OLT needs to know with which ONUs it is communicatingThis can be established via NMS

– but even then need to setup physical layer parameters

PONs employ autodiscovery mechanism to automate

– discovery of existence of ONU – acquisition of identity

– allocation of identifier

– acquisition of ONU capabilities

– measure physical layer parameters

– agree on parameters (e.g. watchdog timers)

Autodiscovery procedures are complex (and uninteresting)

so we will only mention highlights

GPON autodiscoveryGPON autodiscovery



PONs Slide 89

GPON autodiscoveryGPON autodiscovery

Every ONU has an 8B serial number (4B vendor code + 4B SN)

– SN of ONUs in OAN may be configured by NMS, or – SN may be learnt from ONU in discovery phase

ONU activation may be triggered by – Operator command – Periodic polling by OLT – OLT searching for previously operational ONU

G.984.3 differentiates between three cases: – cold PON / cold ONU – warm PON / cold ONU – warm PON / warm ONU

Main steps in procedure: – ONU sets power based on DS message – OLT sends a Serial_Number request to all unregistered ONUs – ONU responds – OLT assigns 1B ONU-ID and sends to ONU – ranging is performed –

ONU is operational

EPON autodiscoveryEPON autodiscovery



PONs Slide 90

EPON autodiscoveryEPON autodiscovery

OLT periodically transmits DISCOVERY GATE messages

ONU waits for DISCOVERY GATE to be broadcast by OLT

DISCOVERY GATE message defines discovery window start time and duration

ONU transmits REGISTER_REQ PDU using random offset in window

OLT receives request registers ONU assigns LLID bonds MAC to LLID performs ranging computation

OLT sends REGISTER to ONU

OLT sends standard GATE to ONU

ONU responds with REGISTER_ACK

ONU goes into operational mode - waits for grants

Failure recoveryFailure recovery



PONs Slide 91

Failure recoveryFailure recovery

PONs must be able to handle various failure states

GPON

if ONU detects LOS or LOF it goes into POPUP state it stops sending traffic US

OLT detects LOS for ONU if there is a pre-ranged backup fiber then switch-over

EPON

during normal operation ONU REPORTs reset OLT's watchdog timer

similarly, OLT must send GATES periodically (even if empty ones)if OLT's watchdog timer for ONU times out

ONU is deregistered

Dynamic Bandwidth AllocationDynamic Bandwidth Allocation



PONs Slide 92

Dynamic Bandwidth AllocationDynamic Bandwidth Allocation

MANs and WANs have relatively stationary BW requirements

due to aggregation of large number of sources

But each ONU in a PON may serve only 1 or a small number of users

So BW required is highly variable

It would be inefficient to statically assign the same BW to each ONU

So PONs assign dynamically BW according to need

The need can be discovered – by passively observing the traffic from the ONU – by ONU sending reports as to state of its ingress queues

The goals of a Dynamic Bandwidth Allocation algorithm are – maximum fiber BW utilization – fairness and respect of priority – minimum delay introduced



EPON DBAEPON DBA



EPON DBAEPON DBA

OLT sends GATE messages to ONUs

flags include DISCOVERY and Force_Report

Force_Report tells the ONU to issue a report

Reports represent the length of each queue at time of report

OLT may use any algorithm to decide how to send the following grants

DA SA 8808 Opcode=0002 timestamp Ngrants/flags …grants

DA SA 8808 Opcode=0003 timestamp Nqueue_sets …Reports

GATE message

REPORT message

PON fundamental RAD

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