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

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”

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?

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

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

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

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

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

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

FTTx OLT Another access technology in the building 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)

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

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

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

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

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

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

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

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

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

GPON/EPON characteristics

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

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

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

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

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?

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

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

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…)

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