GPON Technologies

Outline

PON benefits PON architecture Fiber optic basics PON physical layer PON user plane PON control plane

PON benefits

Why fiber ?

today’s high data rate networks are all based on optical fiber

the reason is simple (examples for demonstration sake) twisted copper pair(s)

8 Mbps @ 3 km, 1.5 Mbps @ 5.5 km (ADSL)1 Gb @ 100 meters (802.3ab)

microwave 70 Mbps @ 30 km (WiMax)

coax 10 Mbps @ 3.6 km (10BROAD36)30 Mbps @ 30 km (cable modem)

optical fiber10 Mbps @ 2 km (10BASE-FL)100 Mbps @ 400m (100BASE-FX)1 Gbps @ 2km (1000BASE-LX)10 Gbps @ 40 (80) km (10GBASE-E(Z)R)40 Gbps @ 700 km [Nortel] or 3000 km [Verizon]

Aside – why is fiber better ?

attenuation per unit length reasons for energy loss

copper: resistance, skin effect, radiation, couplingfiber: 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 =1550nm fiber

noise ingress and cross-talkcopper couples to all nearby conductorsno similar ingress mechanism for fiber

ground-potential, galvanic isolation, lightning protectioncopper can be hard to handle and dangerousno concerns for fiber

Why not 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 problematicmore expensive to deploy than for copper

digital processing requires electronicsso need to convert back to electronicswe will call the converter an optical transceiveroptical transceivers are expensive

switching easier with electronics (but possible with photonics)so pure fiber networks are topologically limited:

point-to-pointrings

copper fiber

Access network bottleneck

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

local area networksuse copper cableget high datarates over short distances

core networksuse fiber opticsget high datarate over long distancessmall 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 transceiversso copper is the best choicethis severely limits the datarates

coreaccess

LAN

Fiber To The Curb

N end userscore

access network

feeder fiber

copper

Hybrid Fiber Coax and VDSLswitch/transceiver/miniDSLAM located at curb or in basementneed only 2 optical transceivers

but not pure optical solutionlower BW from transceiver to end usersneed complex converter in constrained environment

Fiber To The Premises

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 solution

deploy intermediate switches (active) switch located at curb or in basementsaves space at central officeneed 2 N + 2 optical transceivers

N end userscore

access network

feeder fiber

fiber

The PON solution

another alternative - implement point-to-multipoint topology purely in opticsavoid costly optic-electronic conversions use passive splitters – no power needed, unlimited MTBFonly 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 advantages

shared infrastructure translates to lower cost per customerminimal number of optical transceiversfeeder fiber and transceiver costs divided by N customersgreenfield per-customer cost similar to UTP

passive splitters translate to lower costcan be installed anywhereno power neededessentially unlimited MTBF

fiber data-rates can be upgraded as technology improvesinitially 155 Mbpsthen 622 Mbpsnow 1.25 Gbpssoon 2.5 Gbps and higher

PON

architecture

Terminology

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

the entire fiber tree (incl. feeder, splitters, distribution fibers) is an ODNall trees emanating from the same OLT form an OANdownstream is from OLT to ONU (upstream is the opposite direction)

downstream

Optical Network Units

upstream

Optical Distribution NetworkNNI

Terminal Equipment

UNI

coresplitter

Optical Line Terminal

Optical Access Network

PON types

many types of PONs have been defined

APON ATM PON

BPON Broadband PON

GPON Gigabit PON

EPON Ethernet PON

GEPONGigabit Ethernet PON

CPON CDMA PON

WPON WDM PON

in this course we will focus on GPON and EPON (including GEPON)

with a touch of BPON thrown in for the flavor

Bibliography

BPON is explained in ITU-T G.983.xGPON is explained in ITU-T G.984.xEPON 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 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 s for transmit and receiveoptionally: Wavelength Division Multiplexer

downstream transmissionOLT broadcasts data downstream to all ONUs in ODNONU captures data destined for its address, discards all other dataencryption needed to ensure privacy

upstream transmissionONUs share bandwidth using Time Division Multiple AccessOLT manages the ONU timeslotsranging is performed to determine ONU-OLT propagation time

additional functionalityPhysical Layer OAMAutodiscoveryDynamic Bandwidth Allocation

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 protocolsdo not provide all the needed functionalitye.g. receive filtering, ranging, security, BW allocation

downstream

upstream

(multi)point - to - (multi)point

Multipoint-to-multipoint Ethernet avoids collisionsby CSMA/CD

This can't work for multipoint-to-point US PONsince ONUs don't see each otherAnd the OLT can't arbitrate without adding a roundtrip time

Point-to-point ATM can send data in the openalthough trusted intermediate switches see all datacustomer switches only receive their own data

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

PON 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 headerEPON hides the new header inside the GbE preamble

GPON can also carry non-Ethernet payloads

DA SA T data FCSPON header

GPON history

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

Although GPON is an extension of BPON technologyand 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.984G.984.1 : GPON general characteristicsG.984.2 : Physical Media Dependent layerG.984.3 : Transmission Convergence layerG.984.4 : management and control interface

Fiber optics - basics

© = sin¯ (n2/n1)1

V =c/n

t = L·n/c

t = Propagation Timet Vacuum: n=1, t=3.336ns/mt Water : n=1.33, t=4.446ns/m

Total Internal Reflection Total Internal Reflection in Step-Index Multimode Fiberin Step-Index Multimode Fiber

Multimode Graded-Index Fiber

Single-mode Fiber

Types of Optical FiberTypes of Optical Fiber

Popular Fiber Sizes

Click to edit Master text stylesSecond level

Third levelFourth level

Optical Loss versus WavelengthOptical Loss versus Wavelength

Total Dispersion

Multimode Dispersion

Chromatic Dispersion

Material Dispersion

Sources of DispersionSources of Dispersion

1 0 11

Multimode DispersionMultimode Dispersion

1 11 11 11

Dispersion limits bandwidth in optical fiber

1 0 11

Graded-index DispersionGraded-index Dispersion

1 10

1 0 1 1 10

In SM the limit bandwidth is caused by chromatic dispersion.

1

Single-Mode DispersionSingle-Mode Dispersion

How to calculate bandwidth?

Tc = (20ps/nm * km) * 5nm * 15km = 1.5ns

Tc = Dmat * * L

Tc = (20ps/nm * km) * 0.2nm * 60km = 0.24ns

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

Material Dispersion (Dmat)Material Dispersion (Dmat)

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

Laser

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

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

Wavelength-Division MultiplexingWavelength-Division Multiplexing

WDM DuplexingWDM Duplexing

BMCDR = Burst Mode Clock Data Recovery

OLT = Optical Line Termination

ONU = Optical Network Unit

Basic Configuration of PONBasic Configuration of PON

Typical PON Configuration and Typical PON Configuration and Optical PacketsOptical Packets

Eye diagram of ONU transceiver Eye diagram of ONU transceiver in burst mode operationin burst mode operation

Burst-Mode Transmitter in ONUBurst-Mode Transmitter in ONU

OLT Burst-Mode Receiver OLT Burst-Mode Receiver

Burst-Mode CDRBurst-Mode CDR

Ideal, error-free transmission

Superimposed interference

Hysteresis

Ideal sampling instant

SamplingSampling

Transceiver Block DiagramTransceiver Block Diagram

Optical SplittersOptical Splitters

Optical Protection SwitchOptical Protection Switch

Optical SplitterOptical Splitter

LB = ׀ PS ׀ - ׀

PO׀ LB = Link Budget

PS = Sensitivity

PO = Output Power

Example: GPON 1310nm

Power: 0dbm Single-mode fiberSensitivity: -23dbm } Link Budget: 23db

Budget Calculations Budget Calculations

Assume:

Optical loss = 0.35 db/km

Connector Loss = 2dB

Splitter Insertion Loss 1X32 = 17dB

Range Budget: ~11Km

Typical Range Calculation Typical Range Calculation

Relationship between transmission distance Relationship between transmission distance and number of splitsand number of splits

GbE Fiber Optic CharacteristicsGbE Fiber Optic Characteristics

PON physical layer

allocations - 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 customerbut 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

allocations - 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 bandsthese were made possible because of new inexpensive hardware (uncooled Distributed Feedback Lasers)

One of the CWDM bands was 1490 ± 10 nm same bottom as the G.983.1 DS

So it was decided to use this band as the G.983.3 DSand leave the US unchanged

1270 16301490

1200 nm 1300 nm 1400 nm 1500 nm 1600 nm

US DSavailable

guard

allocations - final

The G.983.3 band-plan was incorporated into GPONand via liaison activity into EPON and 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

Data rates (for now …)

PON DS (Mbps) US (Mbps)

BPON 155.52 155.52622.08 155.52

622.08 622.08

1244.16 155.52

1244.16 622.08

1244.16 155.52

1244.16 622.08

1244.16 1244.16

2488.32 155.52

2488.32 622.08

2488.32 1244.16

2488.32 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

Reach and splits

Reach and the number of ONUs supported are contradictory design goals

In addition to physical reach derived from optical budgetthere 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 splitsbut 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 kmbut 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

Line 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 propagationeach standard and each direction use different LFSRsLFSR initialized with all onesLFSR sequence is XOR'ed with data before transmission

EPON uses the 802.3z (1000BASE-X) line code - 8B/10BEvery 8 data bits are converted into 10 bits before transmissionDC removal and timing recovery ensured by mappingSpecial 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

FEC

G984.3 clause 13 and 802.3-2005 subclause 65.2.3define 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 systematic can use in environment where some ONUs do not support FEC

In GPON FEC frames are aligned with PON framesIn EPON FEC frames are marked using K-codes

(and need 8B10B decode - FEC - 8B10B encode)

More 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 thresholdEPON: 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 interference

Electrically shut ONU laser off when not transmitting

But lasers have long warm-up timeand ONU lasers must stabilize quickly after being turned on

US timing diagram

How does the ONU US transmission appear to the OLT ?

grant grant

laser turn-on

laser turn-off

data

lock

laser turn-on

laser turn-off

data

lock

inter-ONUguard

Notes:GPON - ONU reports turn-on and turn-off times to OLT ONU preamble length set by OLTEPON - 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

PON User plane

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 timeDBA - 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 broadcasts

ONU receives all packets and filters according to label

ONU extracts traffic units and passes to client

Labels

In an ODN there is 1 OLT, but many ONUs

ONUs must somehow be labeled forOLT to identify the destination ONUONU to identify itself as the source

GPON has several levels of labelsONU_ID (1B) (1B)

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

For ATM modeVPIVCI

For GEM modePort_ID (12b) (12b)

PON

ONU

ONU

T-CONT

T-CONT

Port

Port

VP

VP VCVCVCVC

DS GPON format

GPON Transmission Convergence frames are always 125 sec long19440 bytes / frame for 1244.16 rate38880 bytes / frame for 2488.32 rate

Each GTC frame consists of Physical Control Block downstream + payloadPCBd 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 sec

GPON 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 partitionif 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 Encapsulation Mode

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

GEM is similar to ATMconstant-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 reassembly

GEM is based on GFP, and the header contains the following fields:Payload Length Indicator - payload length in BytesPort ID - identifies the target ONUPayload 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

Ethernet / 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 sec.PLI bytes of TDM are inserted into payload fieldlength 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

GEM fragmentation

GEM 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

PCBd

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 BytesAlen - Next 12 bits are length of ATM partition in cellsCRC - 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 considerations

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

Each ONU transmits a burst of datausing timing acquired by locking onto OLT signal

according to time allocation sent by OLT in BWmapthere 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 format

4 different US overhead types:

Physical Layer Overhead upstreamalways sent by ONU when taking over from another ONUcontains 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 upstreamsends traffic status to OLT in order to enable DBA computation

PLOu PLOAMd PLSu DBRu payload

if all OH types are present:

US allocation example

BWmap sent by OLT to ONUs is a list of ONU allocation IDsflags (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

guardtime

preamble+

delimiter

scrambled

Security

DS traffic is broadcast to all ONUs, so encryption is essential easy for a malicious user to reprogram ONU to capture desired frames

US traffic not seen by other ONUs, so encryption is not neededdo not take fiber-tappers into account

EPON does not provide any standard encryption methodcan supplement with IPsec or MACsecmany vendors have added proprietary AES-based mechanismsin China special China Telecom encryption algorithm

BPON used a mechanism called churning

Churning was a low cost hardware solution (24b key)with several security flaws

engine was linear - simple known-text attack24b key turned out to be derivable in 512 tries

So G.983.3 added AES support - now used in GPON

GPON encryption

OLT encrypts using AES-128 in counter mode

Only payload is encrypted (not ATM or GEM headers)

Encryption blocks aligned to GTC frame

Counter is shared by OLT and all ONUs46b = 16b intra-frame + 30 bits inter-frameintra-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

GPON treats QoS explicitlyconstant length frames facilitate QoS for time-sensitive applications5 types of Transmission CONTainers

type 1 - fixed BWtype 2 - assured BWtype 3 - allocated BW + non-assured BWtype 4 - best efforttype 5 - superset of all of the above

GEM adds several PON-layer QoS featuresfragmentation enables pre-emption of large low-priority framesPLI - explicit packet length can be used by queuing algorithmsPTI bits carry congestion indications

PON control plane

Principles

GPON uses PLOAMd and PLOAMu as control channel

PLOAM are incorporated in regular (data-carrying) frames

Standard ITU control mechanism

Ranging

Upstream traffic is TDMA

Were all ONUs equidistant, and were all to have a common clockthen each would simply transmit in its assigned timeslot

But otherwise the signals will overlap

To eliminate overlapguard times left between timeslotseach ONU transmits with the proper delay to avoid overlapdelay computed during a ranging process

Ranging background

In order for the ONU to transmit at the correct timethe delay between ONU transmission and OLT reception

needs to be known (explicitly or implicitly)Need to assign an equalization-delay

The more accurately it is knownthe 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 enoughEPON: ONUs are accurately time synchronized (std contains jitter masks)

with time offset by OLT-ONU propagation time

GPON ranging method

Two types of ranginginitial ranging

only performed at ONU boot-up or upon ONU discoverymust 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 ONUsthus 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)

Autodiscovery

OLT needs to know with which ONUs it is communicating

This can be established via NMSbut even then need to setup physical layer parameters

PONs employ autodiscovery mechanism to automatediscovery of existence of ONUacquisition of identityallocation of identifieracquisition of ONU capabilitiesmeasure physical layer parametersagree on parameters (e.g. watchdog timers)

Autodiscovery procedures are complex (and uninteresting)so we will only mention highlights

GPON autodiscovery

Every ONU has an 8B serial number (4B vendor code + 4B SN)SN of ONUs in OAN may be configured by NMS, orSN may be learnt from ONU in discovery phase

ONU activation may be triggered byOperator commandPeriodic polling by OLTOLT searching for previously operational ONU

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

Main steps in procedure:ONU sets power based on DS messageOLT sends a Serial_Number request to all unregistered ONUsONU responds OLT assigns 1B ONU-ID and sends to ONUranging is performedONU is operational

Failure recovery

PONs must be able to handle various failure states

GPONif ONU detects LOS or LOF it goes into POPUP state

it stops sending traffic USOLT detects LOS for ONUif there is a pre-ranged backup fiber then switch-over

EPONduring 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 outONU is deregistered

Dynamic Bandwidth Allocation

MANs and WANs have relatively stationary BW requirementsdue 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 discoveredby passively observing the traffic from the ONUby ONU sending reports as to state of its ingress queues

The goals of a Dynamic Bandwidth Allocation algorithm are maximum fiber BW utilizationfairness and respect of priorityminimum delay introduced

GPON DBA

DBA is at the T-CONT level, not port or VC/VP

GPON can use traffic monitoring (passive) or status reporting (active)

There are three different status reporting methods

status in PLOu - one bit for each T-CONT type

piggy-back reports in DBRu - 3 different formats:quantity of data waiting in buffers,separation of data with peak and sustained rate tokensnonlinear coding of data according to T-CONT type and tokens

ONU report in DBA payload - select T-CONT states

OLT may use any DBA algorithm

OLT sends allocations in US BW map