Optical access networks

1. Optical access networks As part of the course: “TTM1” by Steinar Bjørnstad 10-2012

2. 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”

3. 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?

4. 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

5. Expected bandwidth growth • Perhaps too optimistic? • Remember data- compression!

6. 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

7. 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

8. ADSL plant CPE DSLAM optical fibre typically 300m – 3 km DSLAM: DSL Access Multiplexer CPE: Customer Premises Equipment

9. DSLAM DSLAM Upgrading to VDSL CPE BAP optical fibre VDSL-26 Mbit asym: < 1km VDSL-52 Mbit asym: < 300m BAP: Broadband Access Point

10. OLT FTTx Fibre cable: Fiber to the Home (FTTH) Fibre cable: Fiber to the Curb (FTTC) Another access technology in the building Fibre cable: Fiber to the Building (FTTB)

11. 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

12. OLT FttH architecture comparison pros: the ultimate performance cons use of many fibres Point-to-Point Optical Network ONU multi-fibre cable several 10s of kms Schematic of Physical Plant

13. OLT Active (AON) versus passive (PON) Optical Network ONU several kms Remote Node (RN) Active = needs power! Passive = passive splitting (No need for power) multi-fibre cable Schematic of Physical Plant

14. 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

15. Passive Optical Network (TDMA) Time-sharing of fiber resources ONU downstream OLT passive splitter Limitation on power budget up to 20km Burst mode transmission Different power from each subscriber Makes capacity upgrades difficult OLT: Optical Line Terminal ONU: Optical Network Unit

16. FttH architecture comparison pros: passive fibre plant low OpEx one connection at OLT cons: broadcast centric less scalable less upgradeable complex customer differentiation Passive Optical Network (TDMA) ONU upstream OLT passive splitter up to 20km OLT: Optical Line Terminal ONU: Optical Network Unit

17. 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

18. 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

19. 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

20. GPON/EPON characteristics

21. 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

22. OLT WDM-PON (WDMA) ONT WDM, One wavelength to each subscriber

23. 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

24. 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

25. 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?

26. 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

27. CAPEX Cost on different PON-solutions • CWDM most cost-effective, but lowest splitting ratio • Amplified TDMA highest splitting ratio

28. 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…)

29. 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