PoliCom Fondazione POLITECNICO DI MILANO Access Passive Optical Networks Paola Parolari pparolari@elet.polimi.it
Optical Access Network Access networks: traditionally called last-mile networks Last segment connection from service providers central office (CO) to end users Optical fibers widely used in backbone networks huge available bandwidth very low loss Recently importance as the technology of last-mile connection for next-generation access
Killer applications: how much bandwidth is enough? Carriers are merging their video and data delivery platform into a unified platform based on IP technologies reduce capital and operational expenditures VOD has become the killer application for broadband access network development requirement of at least 100 Mbps per broadband household
Capacity vs distance Distance (km) 30 20 10 1 Attenuation limited Coaxial cable Twisted Pair 850nm Cat 3 limit x Dispersion limited multi-mode fiber Cat 5 limit single-mode fiber Cat 7 limit 1310nm 1550nm 0.1 1 10 100 1000 10,000 Bit rate (Mb/s) x x For short reaches (1-2 km), all optics are Gigabit capable For longer reaches (~10 km), only 1310/1550 nm optics are Gigabit capable
Fiber To The x (FFTx) Service Node Internet Leased Line Frame/Cell Relay Telephone Interactive Video Optical Distribution Network (ODN) ONT Optical Fiber ONT OLT Passive Optical Splitter ONU NT Twisted Pair ONU NT xdsl FTTx FTTH FTTB FTTC FTTCab hybrid FTTH :Fiber To The Home FTTB :Fiber To The Building FTTC:Fiber To The Curb FTTCab :Fiber To The Cabinet
Fiber Optical Network Fiber structure to reach the different customers Active optical networks (AONs) Passive optical networks (PONs) Central Office 20 ) km / 10-25 db loss ONU PSTN Data Video OC3 OC1 2 OLT IP In-Band OC3 RF Overlay OC12 downstream upstream 1:x Passive Optical Splitters ONU ONU
Central Office node (CO) Service provider endpoint of a PON: placed at the central office or head end in systems Optical line terminal (OLT) sends and receives messages or data to/from optical network units (ONUs) connected via ODN Central Office PSTN Data OC3 OC1 2 OLT Video IP In-Band OC3 RF Overlay OC12
Optical Network Terminal (ONT) User nodes ONUs in subscriber neighborhood terminating the optical fiber transmission line providing electrical signals over metallic lines ONUs receives data from OLT by PONS and converts the optical signal into electrical
Optical Distribution Network (ODN) Fiber links (10-40 km) Remote nodes (RN) Power splitters (1:16, 1:32) WDM splitter (Arrayed Waveguide Grating) RN RN TDM-PON WDM-PON
Topology Tree and branch (P2MP) Star (P2P) Ring Bus
Advantages Full exploitation of a single fiber to serve up to N (e.g.) 32 subscribers Optical Fibers has huge bandwith and low loss providing greater flexibility for adding future services Low cost of equipment per subscriber Passive components require little maintenance and have a high MTBF (Mean Time Between Failure) Additional buildings can be added to the network easily Supports a broad range of applications including triple play (voice, data, video) Flexible and scalable bandwidth assignment
Standardization In early 90 Full Service Access Network (FSAN) ITU-T G.983 (BPON) standard ITU-T G.984 (GPON) standard IEEE 802.3 Ethernet PON (EPON or GEPON) standard ITU-T G.652 standard for WDM fibers IEEE - Institute of Electrical & Electronic Engineers (http://www.ieee.org) ITU - International Telecommunication Union (http://www.itu.int)
Two-Fiber vs One-Fiber Two separate fibers for bidirectional communications (space division duplex) No separation of US (ONU to OLT) and DS (OLT to ONU) signals in time, frequency, or wavelength domains Simple to implement 1.3-µm for both US and DS with low-cost Fabry-Perot (FP) lasers Expensive from both the capital and operational standpoints One-Fiber Single-Wavelength Full Duplex Budget loss (3-dB coupler) Near-end cross talk (NEXT)
Time vs wavelength duplex Time division duplex approach: OLT and ONU turns to use the fiber in a ping-pong fashion for upstream and downstream transmissions Use directional couplers NEXT is avoided, but reduced system throughput by about 50% OLT coordinates the time slots assigned for US and DS Burst mode receivers both the OLT and ONU Wavelength Division Duplex (CWDM) reduced the connector reflectivity requirements at the RN ONU: US low cost 1.3-µm F-P LD OLT: DS 1.55-µm DFB (cost shared by the multiple ONUs)
Splitting ratio (SR) Commercial PON systems: splitting ratio of 1:16 or 1:32 A higher SR means that the cost of the PON OLT is better shared among ONUs SR affects system power budget: high-power transmitters, high sensitivity receivers, and low-loss optical components High SR: OLT bandwidth is shared among more ONUs thus less bandwidth per user OLT Passive splitter
Commercial TDM PON OLT TXR BPON Typically: 622 Mbps/155 Mbps (down/up) ATM-based transport B-PON ITU-T G.983.x 20 km Maximum Reach 20 km ONU differential range Max 32 way split (may be cascaded) Fiber splitter LU #1 LU #N, N 32 ONT TXR GPON Typically: 2488/1244 Mbps GFP-like transports (Ethernet, and/or TDM) G-PON ITU-T G.984.x Max 64 way split [constrained by PMD attenuation limits] Fiber splitter LU #1 LU #N, N 64 ONT TXR EPON 1250 Mbps/1250 Mbps [~850 Mbps effective payload rate] Ethernet-based transport E-PON 1000BASE-PX20 per IEEE 802.3ah Max 32 way split (16-way specified in standard) Fiber splitter LU #1 ONT LU #N, N 32 Network optical transceiver (TXR) OLT implementations may not necessarily support all PON technologies indicated
Commercial TDM PON Multiple OLTs in the CO are interconnected with a backbone switch or XC PON section: signals encoded and multiplexed in different formats and schemes depending on the PON standard implemented Standard format used for client interface for hand-off, switching, and cross-connect Signals from and to different ONUs are frame interleaved: each frame is identified with a unique ONU ID in the frame header Downstream link: one-to many broadcast connection Upstream: many-to-one connection communications between ONUs need to be forwarded to the CO and relayed
OLT and ONU Physical medium dependent (PMD) layer defines the optical transceiver and the wavelength diplexer at an OLT or ONU Medium access control (MAC) layer schedules the right to use the PM avoiding contention MAC OLT is master and MAC ONU serves as client OLT service adaptation layer provides the translation between the backbone signal formats and PON section signals The interface from an OLT to the backbone is the service network interface (SNI) ONU service adaptation layer provides the translation between client equipment signal format and PON signal format The interface from an ONU to client network equipment is the user network interface (UNI)
Ranging DS: OLT interleaves the frames as a continuous stream and broadcast to all ONUs Each ONU extracts its own frame based on the header address US: ONUs need to take turns to send their data to the OLT if ONU is not sending upstream data, it has to turn off to avoid interfering ONU transmits in burst mode: first sends a preamble sequence to the OLT The OLT uses the preamble as a training sequence: adjust decision threshold, perform synchronization To avoid collision between bursts: scheduling upstream transmission by the OLT MAC layer Necessary to establish a timing reference between the OLT and ONUs: RANGING process I. OLT sends out a ranging request to ONU(s) to be ranged II. An ONU participating in ranging replies with a ranging response III. OLT measures the round-trip time (RTT) from the ranging response IV. OLT updates the ONU with measured RTT Ranging is usually done at the time an ONU joins a PON The OLT periodically broadcasts ranging requests for ONU discovery
Security In power-splitting PON the DS channel broadcast nature favors eavesdropping The biggest security exposure is in the ranging process OLT broadcasts the ID of the ranged ONU information can be used for spoofing avoided through authentication process Churning procedure to scramble the data for downstream connections with an encryption key
B- PON CO, Feeder: OLT OLT distribute the 8-kHz clock timing to ONUs ATM switch, PSTN, Internet Outside Plant: Optical Distribution Network Downstream: 622 Mbps @ 1490nm Passive Optical Splitter Upstream: 155 Mbps @ 1310nm Customer Premise: ONT Services to user: POTS, Internet Access Downstream: Time Division Multiplex ONT A B C + GRANT A B C + GRANT ONT A B C + GRANT A B Upstream: Time Division Multiple Access ONT A A ONT B A B C + GRANT ONT C ONT C
G- PON Downstream (single -fiber systems): 1490 nm Upstream: 1310 nm RF video (if present) 1555 nm 1244.16 Mb/s 2488.32 Mb/s E1/DS1 (and/or) GbE STMn/OCn TDM TDMA CC NB BB OLT ONT NB BB Access Node CC OLT Time Division Multiplex Time Division Multiple Access Cross Connect Narrow Band Broadband Optical Line Termination Optical Network Termination TDM TDMA 155.52 Mb/s 622.08 Mb/s 1244.16 Mb/s 2488.32 Mbps Up to 60 km* physical reach (* with G.984.6 Reach Extender) ONT1 ONT2 1:32 Optical splitter (or 1:64 for shorter reaches or with Reach Extender) ONT32 E1/DS1/ Telephony Data VOIP E1/T1/ Telephony Video POTS Data Multilongitudinal-mode (MLM) lasers cannot be used at ONU to avoid excessive dispersion penalty. Loss budget requirements: use of APDs G-PON transmission convergence (GTC) layer functions: Transport multiplexing between the OLT and ONUs Adaptation of client layer signal protocols Physical layer OAM (PLOAM) functions Interface for dynamic bandwidth allocation (DBA) ONU ranging and registration Forward error correction (optional) Downstream data encryption (optional) Communication channel for the OMCI
G- PON O-band E-band S-band C-band L-band U-band 1260 1280 1300 1320 1340 1360 1380 1400 1420 1440 1460 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 G.984.2 G.984.5 Regular (FP) Reduce (DFB) Narrow (CWDM) or or NG-PON (G.9xx) A B NG-PON Option 1 * C NG-PON Option 2** D Legend: GPON Up RF Overlay Present GPON Dn NG-PON Future * Requires the use of reduce water peak fiber (G.652.C/D) ** the upper-limit value is determined as an operator choice from 1580 to 1625 nm
E- PON 1.0 Gbps both in the upstream and downstream directions 8B/10B line coding ( IEEE802.3z gigabit Ethernet standard) The downstream physical link maintains continuous signal stream and clock synchronization Circuit emulation is needed to implement fixed-bandwidth TDM circuits
Convergence 1.0
WDM-PON Access node wavelength splitter Hybrid WDM-PON TDMA power splitter ONT (Fixed Optics) SNI OLT Feeder Fiber 1 to N λs on single fiber AWG dedicated λ 1 pair dedicated λ 2 pair ONT Bitrate 1 ONT Bitrate 2 Wavelength selection here dedicated λ Ν pair ONT Bitrate N Colorless ONTs: Transmitter and Receiver front-end filter characteristics are wavelength adaptable
WDM-PON Advantages: Passive optical distribution plant: low maintenance and high-reliability of PS-PON Each user receives its own wavelength: excellent privacy P2P connections between OLT and ONUs are realized in wavelength domain: simplifies the MAC layer Easy pay-as-you-grow upgrade: λ channels are independent Challenges : High costs of WDM components Temperature control: athermal WDM components Colorless ONU operation
PS-networks: components Sources + gain Fabry-Perot Laser Distributed Feed Back Laser + gain mirror cleave - λ mirror FP: Multi-longitudinal mode operation Large spectral width High output power Cheap Optical Power Splitter - AR coating Etched grating DFB: Single-longitudinal Mode operation Narrow spectral width Lower output power Expensive Significant improvements by the introduction of planar lightwave circuit (PLC) technology High reliability Low cost per port Low insertion loss High splitting ratio uniformity λ
WDM-PON: components Athermal operation Arrayed waveguide grating (AWG) Passive WDM mux/demux WDM routing component at RN Cyclic property Central λ of a conventional silica-based AWG shifts ( 0.0125 nm/ C) Usage of guiding materials with negative thermo-optic coefficient Incorporating a mechanically movable compensation plate in the AWG structure
WDM-PON: components Colorless ONU The emission λ is nonspecific and determined by external factors RN AWG filtering properties injection/seeding light λ identical ONUs can be mass-produced and deployed across the network I. Upstream data modulated broadband source (LED) spectrally sliced at the RN AWG II. Injection-locked FP-LD or a wavelength-seeded RSOA III. Source-free ONUs. US data modulated onto the DS carrier and sent back to CO
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