ntnu optical access networks as part of the course: “ttm1” by steinar bjørnstad 10-2014
TRANSCRIPT
NTNU
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