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Optical access networksAs part of the course: “TTM1” by Steinar Bjørnstad 10-2014

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Content Optical Access Networks

Motivation Main characteristics FTTC, FTTB, FTTH WDM-PON

– WDMA– Statistical Multiplexing– WDM light-sources for access networks– Systemarchitecture– Protocol-stack

“PON in adolescence, from TDMA to WDM-PON”

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Expected characteristics of future access-networks Need for real-time services Evolve from text-based web to image and video-based web. Convergence among broadcast services and Internet-

sevices – Everything in one fiber

Symmetrical traffic-pattern?

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Bandwidth and access-networks Triple-play supports

– HDTV broadcast

– Standard definition TV channels (multitude)

– Voice (over Internet Protocol VoIP)

– Plain old telephony service (POTS)

– Video on demand

– Video conference

Red selection: is offerd in current Triple-Play networks

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Expected bandwidth growth

Perhaps too optimistic? Remember data-

compression!

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Access-networks and cost

Costs have to be shared among several customers Components cost is more important here than for metro and

core networks Laying fiber and digging ditches for the fiber may represent

~50 % of the total costs– Will represent a lower limit to the costs of FTTx installation

– If copper wires are already laid in tubes in the ground, then digging and laying of the fiber is substantially cheaper

Equipment costs represent ~25 %– As the technology improves and the volume increases the costs are

continously reduced

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Access technologies properties: xDSL Typically asymmetric, downlink 1/4-1/8 of uplink Twisted pair copper cable, fundamental physical limit is

close, Shannon theorem Bandwidth/distance tradeoff

VDSL required for high capacity triple play

ADSL/RealADSL2

CapacityMbit/s

Distance (Km)1

6

25

6

31.5

15

ADSL

VDSL Shannon

52

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DSLAM

ADSL plant

CPE

typically 300m – 3 km

DSLAM: DSL Access Multiplexer CPE: Customer Premises Equipment

optical fibre

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DSLAM

BAP

Upgrading to VDSL

CPE

VDSL-26 Mbit asym: < 1km

VDSL-52 Mbit asym: < 300m

optical fibreDSLAM

BAP: Broadband Access Point

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Fibre cable: Fiber to the Curb (FTTC)

OLT

FTTx

Fibre cable: Fiber to the Home (FTTH)

Fibre cable: Fiber to the Building (FTTB)

Another access technologyin the building

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Fiber to the Home (FttH) variants

•Consentrator => less fibers, needs power

•Many Fibers => no external power is needed

•Passive =>Higher power loss Do not need power

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multi-fibre cable

OLT

ONU

several 10s of kms

Schematic of Physical Plant

FttH architecture comparison

pros:

the ultimate performance

cons

use of many fibres

Point-to-Point Optical Network

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multi-fibre cable

OLT

Active (AON) versus passive (PON) Optical Network

ONU

several kms

Schematic of Physical Plant

RemoteNode (RN)Active = needs power!Passive = passive splitting(No need for power)

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PON: SCMA, TDMA, WDMA

Sub Carrier Multiple Access (SCMA)– Unique RF frekquency to each subscriber. Share wavelengths

Time Division Multiple Access (TDMA)– Collision avoidance with access protocols

– ATM-PON (B-PON), Gigabit PON (G-PON), Ethernet-PON (E-PON), Gigabit Ethernet PON (GE-PON)

Wavelength Division Multiple Access (WDMA)– no collisions

– higher capacity

– more expensive

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Passive Optical Network (TDMA)

OLT

ONU

up to 20km

OLT: Optical Line Terminal ONU: Optical Network Unit

downstream

passive splitter

Limitation on power budget

Time-sharing offiber resources

Burst mode transmissionDifferent power from each subscriberMakes capacity upgrades difficult

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passive splitter

upstream

Passive Optical Network (TDMA)

OLT

ONU

up to 20km

OLT: Optical Line Terminal ONU: Optical Network Unit

FttH architecture comparison

pros:

passive fibre plant

low OpEx

one connection at OLT

cons:

broadcast centric

less scalable

less upgradeable

complex customer differentiation

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TDMA PON’s – Two variants

EPON – Ethernet PON– Japan and Korea

– Low interface cost

– Integrated with Ethernet OAM

GPON – Gigabit PON– Widely deployed in US and Europe

– Higher bandwidth and bandwidth efficiency than EPON

– Native support of legacy services

– Longer reach

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Downstream Ethernet-PON ATM is expensive, Ethernet sells in high volume and is

therefore cheap– QoS og VLAN

Fiber resources in E-PON is shared and Point-to-Point Ethernet broadcast downstream (as in CSMA/CD)

– All frames are received by all subcribers

– Upstream the ONUs must share capacity and resources

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Upstream and multiple access Collisions must be avoided

– Too long distances implies a too long collision domain

Time-sharing is therefore preferred, timeslots to each ONU All ONUs are synchronized to a common time-reference

– Buffer in ONU assembles packets and sends in time-slot

– Allocation of resources is an issue

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GPON/EPON characteristics

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WDM PON for the future

GPON/EPON may not handle future requirements on bitrate 10GPON – 10 Gb/s

– Power budget imposes severe limitations on distances and splitting ratio

WDM-PONs solves the limitations of TDMA-PON– Dedicated wavelength to each subscriber

– May be combined with TDMA-PON in a hybrid, allowing 1:1000 splitting ratio.

– Many variants of WDM-PON

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WDM, One wavelength to each subscriber

OLT

ONT

WDM-PON (WDMA)

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Basic WDM-PON architectures

B&S architecture– Passive splitter– Unique filter in ONU– Individual wavelength

upstream– Broadcast security issues

AWG based– Low insertion loss, 5 dB– Universal Rx– Wavelength specific Tx– Periodic routing behavior

AWG + Identical ONU’s– Single shared wavelength

upstream (TDMA)– Broadband LEDs and

spectral slicing give poor power budget

– Bidirectional OLT using a circulator

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Colourless identical ONU’s SOA broadband modulators + seed lasers: Laser adjust to

Seed wavelength– Separate upstream and downstream fibre required– Reflective SOA

Re-use OLT Tx wavelength– Seed signal achieved using FSK downstream– FSK removed in RSOA and replaced by OOK upstream

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Most Cost effective: CWDM-PON

16 CWDM wavelengths on SFW supports 8 ONU’s– 1270 nm to 1610, ITU-T standard

High power budget but potential problems with old fibers (OH peak)

Employs standard low-cost pluggable SFP modules– Capex is low, Opex moderate (higher than colourless)

DWDM much more expensive than CWDM, why?

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Power budget CWDM What is a power budget? What is it useful for? What causes the greatest loss? Why is the power budget higher for DWDM compared to CWDM

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CAPEX Cost on different PON-solutions

CWDM most cost-effective, but lowest splitting ratio Amplified TDMA highest splitting ratio

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Unified infrastructure: core to access

PON not only to residentials Mobile back-haul ADSL back-haul Enterprise networks Combine with WDM Metro

rings Combine with ROADM

nodes Cost optimization

– Common management and control plane required

– Common protocols required (Not SDH and Ethernet and…)

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Summary

GPON and Point-to-point is presently being deployed – In Europe

GPON does not handle the future needs for bandwidth WDM-PON and point-to-point scales Hybrid GPON and WDM-PON allows a gradual migration

towards WDM-PON PON’s may be used for more than access to residentials

– Business customers

– Mobile base-station back-haul

– DSL back-haul