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					                                   NTNU




Optical access networks
As part of the course: “TTM1” by
Steinar Bjørnstad 15/10-2009
                                                  NTNU
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


           52
                  VDSL                 Shannon
           25
           15
Capacity
 Mbit/s                ADSL
           6                    ADSL/RealADSL2
           1
                1.5   3         6 Distance (Km)
        VDSL required for high capacity triple play
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ADSL plant




                                              CPE

          DSLAM
optical
fibre




                               typically 300m – 3 km



                  DSLAM: DSL Access Multiplexer   CPE: Customer Premises Equipment

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Upgrading to VDSL


                  DSLAM




                                                   CPE

   BAP                                     DSLAM

                optical fibre




                                                   VDSL-26 Mbit asym:   < 1km
                                                   VDSL-52 Mbit asym:   < 300m

         BAP: Broadband Access Point
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FTTx




                                     Fibre cable: Fiber to
                                     the Home (FTTH)

   OLT
                                 Fibre cable: Fiber to
                                 the Curb (FTTC)




                                                             Another access technology
                                                             in the building

         Fibre cable: Fiber to the
         Building (FTTB)
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Fiber to the Home (FttH) variants

                                •Many Fibers =>
                                 no external power
                                  is needed


                                    •Consentrator =>
                                     less fibers,
                                     needs power


                                •Passive =>
                                Higher power loss
                                Do not need power
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Point-to-Point Optical Network




                                               ONU

                                FttH architecture comparison
     OLT
                                pros:
            multi-fibre cable
                                the ultimate performance
                                cons
                                use of many fibres
                                        several 10s of kms


                                                 Schematic of Physical Plant


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Active (AON) versus passive (PON)
Optical Network



                                  ONU


    OLT             several kms

           Remote multi-fibre cable
           Node (RN)
           Active = needs power!
           Passive = passive splitting
           (No need for power)
                                   Schematic of Physical Plant


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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)

                                                               Time-sharing of
                                                               fiber resources

                                                     ONU

                                     downstream
     OLT
                  passive splitter
                  Limitation on power budget




  Burst mode transmission      up to 20km
  Different power from each subscriber
  Makes capacity upgrades difficult
                                       OLT: Optical Line Terminal   ONU: Optical Network Unit
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Passive Optical Network (TDMA)



          FttH architecture comparison
          pros:                                     ONU
          passive fibre plant
                                     upstream
    OLT   low OpEx
                  passive splitter
          one connection at OLT
          cons:
          broadcast centric
          less scalable
                                          up to 20km
          less upgradeable
          complex customer differentiation

                                      OLT: Optical Line Terminal   ONU: Optical Network Unit

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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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                            NTNU

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




                                     21
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WDM-PON (WDMA)




                                 ONT


   OLT
            WDM, One
            wavelength to each
            subscriber




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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
                  23           circulator
                                                              NTNU
     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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                                                        NTNU
    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




                             26
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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




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posted:8/29/2011
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