Access Passive Optical Networks

PoliComFondazione POLITECNICO DI MILANO

Paola Parolari

pparolari@elet.polimi.it

Optical Access Network

Access networks: traditionally called last-mile networksLast segment connection from service providers central office (CO) to end usersOptical fibers widely used in backbone networks

huge available bandwidthvery low loss

Recently importance as the technology of last-mile connection for next-generation access

“Killer” applications:how much bandwidth is enough?

Carriers are merging their video and data delivery platform into a unified platform basedon IP technologies

reduce capital and operational expenditures VOD has become the killer application for broadband access network development

requirement of at least 100 Mbps per broadband household

Capacity vs distance

For short reaches (1-2 km), all optics are “Gigabit capable”For longer reaches (~10 km), only 1310/1550 nm optics are “Gigabit capable”

30

10

1

Bit rate (Mb/s)

Dis

tanc

e (k

m)

0.1 10 100 1000 10,0001

1550nm

1310nm850nm

Dispersion limitedAttenuation limited

single-mode fiber

multi-mode fiberCoaxialcable

20

x x

Cat 3 limit

Cat 7 limit

Cat 5 limit

x

Twisted Pair

Fiber To The x (FFTx)

Service Node

ONU

FTTH

FTTB

FTTC

FTTCab

Optical Fiber

xDSL

OLTONU NT

NT

Passive Optical Splitter

FTTx

FTTC:Fiber To The CurbFTTCab :Fiber To The Cabinet

FTTH :Fiber To The HomeFTTB :Fiber To The Building

Internet

Leased Line

Frame/CellRelay

Telephone

Interactive Video

Twisted Pair

ONT

ONT

Optical Distribution Network (ODN)

hybrid

Fiber Optical Network

Fiber structure to reach the different customers Active optical networks (AONs) Passive optical networks (PONs)

1:x PassiveOptical SplittersOC3OC3

OC12OC12IP In-BandIP In-BandRF Overlay RF Overlay

downstream

upstream

Central Office 20 km / 10-25 dB loss

OLTOLT

Video

Data

PSTNOC3OC3OC1OC122

)ONU

ONU

ONU

Central Office node (CO)

Service provider endpoint of a PON: placed at the centraloffice or head end in systemsOptical line terminal (OLT) sends and receives messagesor data to/from optical network units (ONUs) connectedvia ODN

OC3OC3OC12OC12

IP In-BandIP In-BandRF Overlay RF Overlay

Central Office

OLTOLT

Video

Data

PSTNOC3OC3OC1OC122

Optical Network Terminal (ONT)

User nodesONUs in subscriber neighborhoodterminating the optical fiber transmission lineproviding electrical signals over metallic linesONUs receives data from OLT by PONS and converts theoptical signal into electrical

Optical Distribution Network (ODN)

Fiber links (10- 40 km)Remote nodes (RN)

Power splitters (1:16, 1:32)WDM splitter (Arrayed Waveguide Grating)

RN

RN

TDM-PON WDM-PON

Topology

Tree and branch (P2MP) Star (P2P)

BusRing

Advantages

Full exploitation of a single fiber to serve up to N (e.g.) 32subscribersOptical Fibers has huge bandwith and low loss providing greaterflexibility for adding future servicesLow cost of equipment per subscriberPassive components require little maintenance and have a highMTBF (Mean Time Between Failure)Additional buildings can be added to the network easilySupports a broad range of applications including triple play (voice,data, video)Flexible and scalable bandwidth assignment

Standardization

ITU-T G.983 (BPON) standardITU-T G.984 (GPON) standardIEEE 802.3 Ethernet PON (EPON or GEPON) standardITU-T G.652 standard for WDM fibers

• IEEE - Institute of Electrical & Electronic Engineers (http://www.ieee.org)

• ITU - International Telecommunication Union (http://www.itu.int)

In early ‘90 Full Service Access Network (FSAN)

Two-Fiber vs One-Fiber

Two separate fibers for bidirectional communications (space divisionduplex)

No separation of US (ONU to OLT) and DS (OLT to ONU) signals in time, frequency, or wavelength domainsSimple to implement1.3-µm for both US and DS with low-cost Fabry-Perot (FP) lasersExpensive from both the capital and operational standpoints

One-Fiber Single-Wavelength Full DuplexBudget loss (3-dB coupler)Near-end cross talk (NEXT)

Time vs wavelength duplex

Time division duplex approach: OLT and ONU turns to use the fiber in aping-pong fashion for upstream and downstream transmissions

Use directional couplersNEXT is avoided, but reduced system throughput by about 50%

OLT coordinates the time slots assigned for US and DSBurst mode receivers both the OLT and ONU

Wavelength Division Duplex (CWDM)reduced the connector reflectivity requirements at the RN ONU: US low cost 1.3-µm F-P LDOLT: DS 1.55-µm DFB (cost shared by the multiple ONUs)

Splitting ratio (SR)

OLTPassivesplitter

Commercial PON systems: splitting ratio of 1:16 or 1:32A higher SR means that the cost of the PON OLT is better shared among ONUsSR affects system power budget: high-power transmitters, high sensitivity receivers, and low-loss optical componentsHigh SR: OLT bandwidth is shared among more ONUs thus less bandwidth per user

Commercial TDM PON

OLT

FiberBPO

NG

PON

Max 32 way split (may becascaded)

OLT implementations may not necessarily support all PON technologies indicated

•Typically: 622 Mbps/155 Mbps (down/up)

•ATM-based transport

LU #1

LU #N, N ≤32

Fiber

Max 64 way split [constrainedby PMD attenuation limits]

• Typically: 2488/1244 Mbps

• GFP-like transports (Ethernet, and/orTDM)

LU #1

LU #N, N ≤64

EPO

N

Fiber

• 1250 Mbps/1250 Mbps [~850 Mbpseffective payload rate]

• Ethernet-based transport

LU #1

LU #N, N ≤32

20 km Maximum Reach20 km ONU differential range

B-PON

G-PON

E-PON

Max 32 way split (16-wayspecified in standard)

splitter

splitter

splitter

ITU-T G.983.x

ITU-T G.984.x

1000BASE-PX20per IEEE 802.3ah

Network optical transceiver (TXR)

TXR

TXR

TXRLU #N, N ≤32

ONT

ONT

ONT

Commercial TDM PON

Multiple OLTs in the CO are interconnected with a backbone switch or XCPON section: signals encoded and multiplexed in different formats and schemes

depending on the PON standard implementedStandard format used for client interface for hand-off, switching, and cross-connectSignals from and to different ONUs are frame interleaved:

each frame is identified with a unique ONU ID in the frame headerDownstream link: one-to many broadcast connectionUpstream: many-to-one connection

communications between ONUs need to be forwarded to the CO and relayed

OLT and ONU

Physical medium dependent (PMD) layer defines the optical transceiver and the wavelengthdiplexer at an OLT or ONU

Medium access control (MAC) layer schedules the right to use the PM avoiding contentionMAC OLT is master and MAC ONU serves as client

OLT service adaptation layer provides the translation between the backbone signal formatsand PON section signals

The interface from an OLT to the backbone is the service network interface (SNI)ONU service adaptation layer provides the translation between client equipment signal format

and PON signal formatThe interface from an ONU to client network equipment is the user network interface (UNI)

Ranging

DS: OLT interleaves the frames as a continuous stream and broadcast to all ONUsEach ONU extracts its own frame based on the header addressUS: ONUs need to take turns to send their data to the OLTif ONU is not sending upstream data, it has to turn off to avoid interferingONU transmits in burst mode: first sends a preamble sequence to the OLTThe OLT uses the preamble as a training sequence: adjust decision threshold, perform

synchronizationTo avoid collision between bursts: scheduling upstream transmission by the OLT MAC layerNecessary to establish a timing reference between the OLT and ONUs:RANGING process

I. OLT sends out a ranging request to ONU(s) to be rangedII. An ONU participating in ranging replies with a ranging responseIII. OLT measures the round-trip time (RTT) from the ranging responseIV. OLT updates the ONU with measured RTT

Ranging is usually done at the time an ONU joins a PONThe OLT periodically broadcasts ranging requests for ONU discovery

Security

In power-splitting PON the DS channel broadcast nature favorseavesdropping

The biggest security exposure is in the ranging processOLT broadcasts the ID of the ranged ONUinformation can be used for spoofingavoided through authentication process

Churning procedure to scramble the data for downstream connectionswith an encryption key

B- PON

ONT

ONT

ONT

A

B

C

A B C + GRANT A B C +GRANT

A B C + GRANT

A B C + GRANT

ONT

ONT

ONT

A

B

C

A

Upstream: Time Division Multiple AccessDownstream: Time Division Multiplex

Downstream: 622 Mbps @ 1490nm

Upstream: 155 Mbps @ 1310nm

Outside Plant: Optical Distribution Network Customer Premise: ONTCO, Feeder: OLTOLT distribute the 8-kHzclock timing to ONUs

ATM switch,PSTN,Internet

Services to user:POTS,Internet Access

Passive Optical Splitter

G- PON

Downstream (single -fiber systems): 1490 nm Upstream: 1310 nm RF video (if present) 1555 nm

TDM Time Division Multiplex TDMA Time Division Multiple Access CC Cross Connect NB Narrow Band BB Broadband OLT Optical Line Termination ONT Optical Network Termination

TDM ONT2

ONT32

1:32 Optical splitter (or 1:64 for shorter reaches or

with Reach Extender)

OLT

Access Node

NB

BB CC Video

Data

E1/T1/ Telephony

Data

E1/DS1

GbE STMn/OCn

ONT1

E1/DS1/ Telephony

POTS

VOIP

(and/or)

TDMA

Up to 60 km* physical reach(* with G.984.6 Reach Extender)

Multilongitudinal-mode (MLM) lasers cannot be used at ONU to avoid excessive dispersion penalty.Loss budget requirements: use of APDsG-PON transmission convergence (GTC) layer functions:Transport multiplexing between the OLT and ONUs Adaptation of client layer signal protocolsPhysical layer OAM (PLOAM) functions Interface for dynamic bandwidth allocation (DBA)ONU ranging and registration Forward error correction (optional)Downstream data encryption (optional) Communication channel for the OMCI

155.52 Mb/s622.08 Mb/s1244.16 Mb/s2488.32 Mbps

1244.16 Mb/s2488.32 Mb/s

G- PON

1260 1280 1300 1320 1340 1360 1380 1400 1420 1460 1480 1500 1520 1540 1560 1580 1600 1620 1640 16601440 1680

O-band E-band S-band C-band L-band U-band

G.984.2

Legend: GPON Up

GPON Dn

RF Overlay Present

Future

* Requires the use of reduce water peak fiber (G.652.C/D)** the upper-limit value is determined as an operator choice from 1580 to 1625 nm

G.984.5

or

or

NG-PON

Regular (FP)

Reduce (DFB)

Narrow (CWDM)

NG-PON (G.9xx)

A BNG-PON Option 1 *

CNG-PON Option 2**

D

E- PON

1.0 Gbps both in the upstream and downstream directions8B/10B line coding ( IEEE802.3z gigabit Ethernet standard)

The downstream physical link maintains continuous signal stream and clock synchronizationCircuit emulation is needed to implement fixed-bandwidth TDM circuits

Convergence

1.0

WDM-PON

Access node

OLT

SNI

wavelengthsplitter

1 to N λs on single fiber

Wavelength selection here

dedicated λ1 pair

dedicated λ2 pair

ONTBitrate 1

ONTBitrate 2

Feeder Fiber

Colorless ONTs: Transmitter and Receiver front-endfilter characteristics are wavelength adaptable

TDMAONT

(Fixed Optics)power

splitter

Hybrid WDM-PON

AW

G

ONTBitrate N

dedicated λΝ pair

WDM-PON

Advantages:Passive optical distribution plant: low maintenance and high-reliability of PS-PONEach user receives its own wavelength: excellent privacyP2P connections between OLT and ONUs are realized in wavelength domain:

simplifies the MAC layerEasy pay-as-you-grow upgrade: λ channels are independent

Challenges :High costs of WDM componentsTemperature control: athermal WDM components Colorless ONU operation

PS-networks: components

Fabry-PerotLaser

mirror

gain

cleave

+

- λ mirror

gain

AR coating

+

- Etchedgrating

λ

Distributed Feed BackLaser

FP: Multi-longitudinal mode operationLarge spectral widthHigh output powerCheap

DFB: Single-longitudinal Mode operationNarrow spectral widthLower output powerExpensive

Sources

Optical Power SplitterSignificant improvements by the introduction of planar lightwave circuit (PLC) technology

High reliabilityLow cost per portLow insertion lossHigh splitting ratio uniformity

WDM-PON: components

Arrayed waveguide grating (AWG)Passive WDM mux/demux WDM routing component at RNCyclic propertyCentral λ of a conventional silica-based AWG shifts ( 0.0125 nm/°C)

Athermal operation Usage of guiding materials with negative thermo-optic coefficientIncorporating a mechanically movable compensation plate in the AWG structure

WDM-PON: components

Colorless ONUThe emission λ is nonspecific and determined by external factors

RN AWG filtering properties injection/seeding light λ

identical ONUs can be mass-produced and deployed across the network

I. Upstream data modulated broadband source (LED) spectrally sliced at the RN AWGII. Injection-locked FP-LD or a wavelength-seeded RSOAIII. Source-free ONUs. US data modulated onto the DS carrier and sent back to CO

Acronyms

Acronyms

Acronyms

Acronyms

Acronyms