Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Passive Optical Networks


Published on

Published in: Education, Business, Technology

Passive Optical Networks

  1. 1. Passive Optical Networks Yaakov (J) Stein May 2007 and Zvika Eitan
  2. 2. Outline PON benefits PON architecture Fiber optic basics PON physical layer PON user plane PON control plane PONs Slide 2
  3. 3. PON benefits PONs Slide 3
  4. 4. Why fiber ?today’s high datarate networks are all based on optical fiberthe reason is simple (examples for demonstration sake) twisted copper pair(s) – 8 Mbps @ 3 km, 1.5 Mbps @ 5.5 km (ADSL) – 1 Gb @ 100 meters (802.3ab) microwave – 70 Mbps @ 30 km (WiMax) coax – 10 Mbps @ 3.6 km (10BROAD36) – 30 Mbps @ 30 km (cable modem) optical fiber – 10 Mbps @ 2 km (10BASE-FL) – 100 Mbps @ 400m (100BASE-FX) – 1 Gbps @ 2km (1000BASE-LX) – 10 Gbps @ 40 (80) km (10GBASE-E(Z)R) – 40 Gbps @ 700 km [Nortel] or 3000 km [Verizon] PONs Slide 4
  5. 5. Aside – why is fiber better ?attenuation per unit length reasons for energy loss – copper: resistance, skin effect, radiation, coupling – fiber: internal scattering, imperfect total internal reflection so fiber beats coax by about 2 orders of magnitude – e.g. 10 dB/km for thin coax at 50MHz, 0.15 dB/km =1550nm fibernoise ingress and cross-talk copper couples to all nearby conductors no similar ingress mechanism for fiberground-potential, galvanic isolation, lightning protection copper can be hard to handle and dangerous no concerns for fiber PONs Slide 5
  6. 6. Why not fiber ?fiber beats all other technologies for speed and reachbut fiber has its own problems harder to splice, repair, and need to handle carefully regenerators and even amplifiers are problematic – more expensive to deploy than for copper digital processing requires electronics – so need to convert back to electronics copper fiber – we will call the converter an optical transceiver – optical transceivers are expensive switching easier with electronics (but possible with photonics) – so pure fiber networks are topologically limited:  point-to-point  rings PONs Slide 6
  7. 7. Access network bottleneckhard for end users to get high datarates because of the access bottlenecklocal area networks use copper cable get high datarates over short distancescore networks use fiber optics get high datarate over long distances access core small number of active network elementsaccess networks (first/last mile) long distances LAN – so fiber would be the best choice many network elements and large number of endpoints – if fiber is used then need multiple optical transceivers – so copper is the best choice – this severely limits the datarates PONs Slide 7
  8. 8. Fiber To The CurbHybrid Fiber Coax and VDSL switch/transceiver/miniDSLAM located at curb or in basement need only 2 optical transceiversbut not pure optical solution lower BW from transceiver to end users need complex converter in constrained environment N end users core feeder fiber copper access network PONs Slide 8
  9. 9. Fiber To The Premiseswe can implement point-to-multipoint topology purely in optics but we need a fiber (pair) to each end user requires 2 N optical transceivers complex and costly to maintain N end users core access network PONs Slide 9
  10. 10. An obvious solutiondeploy intermediate switches (active) switch located at curb or in basement saves space at central office need 2 N + 2 optical transceivers N end users core feeder fiber fiber access network PONs Slide 10
  11. 11. The PON solutionanother alternative - implement point-to-multipoint topology purely in optics avoid costly optic-electronic conversions use passive splitters – no power needed, unlimited MTBF only N+1 optical transceivers (minimum possible) ! access network 1:2 passive splitter N end users core typically N=32 max defined 128 feeder fiber 1:4 passive splitter PONs Slide 11
  12. 12. PON advantagesshared infrastructure translates to lower cost per customer minimal number of optical transceivers feeder fiber and transceiver costs divided by N customers greenfield per-customer cost similar to UTPpassive splitters translate to lower cost can be installed anywhere no power needed essentially unlimited MTBFfiber data-rates can be upgraded as technology improves initially 155 Mbps then 622 Mbps now 1.25 Gbps soon 2.5 Gbps and higher PONs Slide 12
  13. 13. PONarchitecture PONs Slide 13
  14. 14. Terminologylike every other field, PON technology has its own terminology the CO head-end is called an OLT ONUs are the CPE devices (sometimes called ONTs in ITU) the entire fiber tree (incl. feeder, splitters, distribution fibers) is an ODN all trees emanating from the same OLT form an OAN downstream is from OLT to ONU (upstream is the opposite direction) downstream upstream NNI Optical Distribution Network Optical Network Unitscore splitter Optical Line Terminal UNI Optical Access Network Terminal Equipment PONs Slide 14
  15. 15. PON typesmany types of PONs have been defined APON ATM PON BPON Broadband PON GPON Gigabit PON EPON Ethernet PON GEPON Gigabit Ethernet PON CPON CDMA PON WPON WDM PONin this course we will focus on GPON and EPON (including GEPON) with a touch of BPON thrown in for the flavor PONs Slide 15
  16. 16. Bibliography BPON is explained in ITU-T G.983.x GPON is explained in ITU-T G.984.x EPON is explained in IEEE 802.3-2005 clauses 64 and 65 – (but other 802.3 clauses are also needed)Warning do not believe white papers from vendors especially not with respect to GPON/EPON comparisons GPON BPON EPON PONs Slide 16
  17. 17. PON principles(almost) all PON types obey the same basic principlesOLT and ONU consist of Layer 2 (Ethernet MAC, ATM adapter, etc.) optical transceiver using different s for transmit and receive optionally: Wavelength Division Multiplexerdownstream transmission OLT broadcasts data downstream to all ONUs in ODN ONU captures data destined for its address, discards all other data encryption needed to ensure privacyupstream transmission ONUs share bandwidth using Time Division Multiple Access OLT manages the ONU timeslots ranging is performed to determine ONU-OLT propagation timeadditional functionality Physical Layer OAM Autodiscovery Dynamic Bandwidth Allocation PONs Slide 17
  18. 18. Why a new protocol ? downstreamPON has a unique architecture upstream (broadcast) point-to-multipoint in DS direction (multiple access) multipoint-to-point in US directioncontrast that with, for example Ethernet - multipoint-to-multipoint ATM - point-to-pointThis means that existing protocols do not provide all the needed functionality e.g. receive filtering, ranging, security, BW allocation PONs Slide 18
  19. 19. (multi)point - to - (multi)pointMultipoint-to-multipoint Ethernet avoids collisions by CSMA/CDThis cant work for multipoint-to-point US PON since ONUs dont see each other And the OLT cant arbitrate without adding a roundtrip timePoint-to-point ATM can send data in the open although trusted intermediate switches see all data customer switches only receive their own dataThis cant work for point-to-multipoint DS PON since all ONUs see all DS data PONs Slide 19
  20. 20. PON encapsulationThe majority of PON traffic is EthernetSo EPON enthusiasts say use EPON - its just EthernetThats true by definition - anything in 802.3 is Ethernet and EPON is defined in clauses 64 and 65 of 802.3-2005But dont be fooled - all PON methods encapsulate MAC framesEPON and GPON differ in the contents of the header EPON hides the new header inside the GbE preamble GPON can also carry non-Ethernet payloads PON header DA SA T data FCS PONs Slide 20
  21. 21. BPON history1995 : 7 operators (BT, FT, NTT, …) and a few vendors form Full Service Access Network Initiative to provide business customers with multiservice broadband offeringObvious choices were ATM (multiservice) and PON (inexpensive) which when merged became APON1996 : name changed to BPON to avoid too close association with ATM1997 : FSAN proposed BPON to ITU SG151998 : BPON became G.983 – G.982 : PON requirements and definitions – G.983.1 : 155 Mbps BPON – G.983.2 : management and control interface – G.983.3 : WDM for additional services – G.983.4 : DBA – G.983.5 : enhanced survivability – G.983.1 amd 1 : 622 Mbps rate – G.983.1 amd 2 : 1244 Mbps rate – … PONs Slide 21
  22. 22. EPON history2001: IEEE 802 LMSC WG accepts Ethernet in the First Mile Project Authorization Request becomes EFM task force (largest 802 task force ever formed)EFM task force had 4 tracks DSL (now in clauses 61, 62, 63) Ethernet OAM (now clause 57) Optics (now in clauses 58, 59, 60, 65) P2MP (now clause 64)2002 : liaison activity with ITU to agree upon wavelength allocations2003 : WG ballot2004 : full standard2005: new 802.3 version with EFM clauses PONs Slide 22
  23. 23. GPON history2001 : FSAN initiated work on extension of BPON to > 1 GbpsAlthough GPON is an extension of BPON technology and reuses much of G.983 (e.g. linecode, rates, band-plan, OAM) decision was not to be backward compatible with BPON2001 : GFP developed (approved 2003)2003 : GPON became G.984 – G.984.1 : GPON general characteristics – G.984.2 : Physical Media Dependent layer – G.984.3 : Transmission Convergence layer – G.984.4 : management and control interface PONs Slide 23
  24. 24. Fiber optics - basics PONs Slide 24
  25. 25. Total Internal Reflection in Step-Index Multimode Fiber© = sin¯ 1(n2/n1) t = Propagation Time t Vacuum: n=1, t=3.336ns/mV =c/n t Water : n=1.33, t=4.446ns/mt = L·n/c PONs Slide 25
  26. 26. Types of Optical Fiber Popular Fiber Sizes Multimode Graded- Index Fiber Single-mode Fiber PONs Slide 26
  27. 27. Optical Loss versus Wavelength Click to edit Master text styles – Second level  Third level – Fourth level PONs Slide 27
  28. 28. Sources of Dispersion Total DispersionMultimode ChromaticDispersion Dispersion Material Dispersion PONs Slide 28
  29. 29. Multimode Dispersion1 0 1 1 1 1 1 Dispersion limits bandwidth in optical fiber PONs Slide 29
  30. 30. Graded-index Dispersion1 0 1 1 1 0 1 PONs Slide 30
  31. 31. Single-Mode Dispersion 1 0 1 1 1 0 1In SM the limit bandwidth is caused by chromatic dispersion. PONs Slide 31
  32. 32. System Design Consideration How to calculate bandwidth? For a 1.25 Gb/s we need a BW of 0.7 BitRate = 1.143ns Tc = Dmat * *L For Laser 1550nm Fabry PerotTc = (20ps/nm * km) * 5nm * 15km = 1.5ns For Laser 1550nm DFBTc = (20ps/nm * km) * 0.2nm * 60km = 0.24ns PONs Slide 32
  33. 33. Material Dispersion (Dmat) PONs Slide 33
  34. 34. Spectral CharacteristicsLASER/laser diode: Light Amplification by Stimulated Emission of Radiation. Done of the wide range ofdevices that generates light by that principle. Laser light is directional, covers a narrow range ofwavelengths, and is more coherent than ordinary light. Semiconductor diode lasers are the standard lightsources in fiber optic systems. Lasers emit light by stimulated emission. PONs Slide 34
  35. 35. Laser Optical Power Output vs. Forward Current W Laser PONs Slide 35
  36. 36. Light DetectorsPIN DIODES (PD)- Operation simular to LEDs, but in reverse, photon are converted to electrons- Simple, relatively low- cost- Limited in sensitivity and operating range- Used for lower- speed or short distance applicationsAVALANCHE PHOTODIODES (APD)- Use more complex design and higher operating voltage than PIN diodes to produce amplification effect- Significantly more sensitive than PIN diodes- More complex design increases cost- Used for long-haul/higher bit rate systems PONs Slide 36
  37. 37. Wavelength-Division Multiplexing PONs Slide 37
  38. 38. WDM Duplexing PONs Slide 38
  39. 39. Basic Configuration of PONOLT = Optical Line TerminationONU = Optical Network UnitBMCDR = Burst Mode Clock Data Recovery PONs Slide 39
  40. 40. Typical PON Configuration and Optical Packets PONs Slide 40
  41. 41. Eye diagram of ONU transceiver in burst mode operation PONs Slide 41
  42. 42. Burst-Mode Transmitter in ONU PONs Slide 42
  43. 43. OLT Burst-Mode Receiver PONs Slide 43
  44. 44. Burst-Mode CDR PONs Slide 44
  45. 45. Sampling Ideal sampling instant Hysteresis Superimposed interferenceIdeal, error-free transmission PONs Slide 45
  46. 46. Transceiver Block Diagram PONs Slide 46
  47. 47. Optical Splitters PONs Slide 47
  48. 48. Optical Protection Switch Optical Splitter PONs Slide 48
  49. 49. Budget Calculations LB = PS - POLB = Link BudgetPS = SensitivityPO = Output PowerExample: GPON 1310nmPower: 0dbm Single-modefiber } Link Budget:Sensitivity: -23dbm 23db PONs Slide 49
  50. 50. Typical Range CalculationAssume:Optical loss = 0.35 db/kmConnector Loss = 2dB Range Budget: ~11KmSplitter Insertion Loss 1X32 = 17dB PONs Slide 50
  51. 51. Relationship between transmission distance and number of splits PONs Slide 51
  52. 52. GbE Fiber Optic Characteristics PONs Slide 52
  53. 53. PON physical layer PONs Slide 53
  54. 54. allocations - G.983.1Upstream and downstream directions need about the same bandwidthUS serves N customers, so it needs N times the BW of each customer but each customer can only transmit 1/N of the timeIn APON and early BPON work it was decided that 100 nm was neededWhere should these bands be placed for best results?In the second and third windows ! Upstream 1260 - 1360 nm (1310 50) second window Downstream 1480 - 1580 nm (1530 50) third window US DS 1200 nm 1300 nm 1400 nm 1500 nm 1600 nm PONs Slide 54
  55. 55. allocations - G.983.3Afterwards it became clear that there was a need for additional DS bandsPressing needs were broadcast video and dataWhere could these new DS bands be placed ?At about the same time G.694.2 defined 20 nm CWDM bands these were made possible because of new inexpensive hardware (uncooled Distributed Feedback Lasers)One of the CWDM bands was 1490 10 nm 1270 1490 1630 same bottom as the G.983.1 DSSo it was decided to use this band as the G.983.3 DS and leave the US unchanged guard available US DS 1200 nm 1300 nm 1400 nm 1500 nm 1600 nm PONs Slide 55
  56. 56. allocations - final US DS 1200 nm 1300 nm 1400 nm 1500 nm 1600 nmThe G.983.3 band-plan was incorporated into GPON and via liaison activity into EPON and is now the universally accepted xPON band-plan US 1260-1360 nm (1310 50) DS 1480-1500 nm (1490 10) enhancement bands: – video 1550 - 1560 nm (see ITU-T J.185/J.186) – digital 1539-1565 nm PONs Slide 56
  57. 57. Data rates (for now …) PON DS (Mbps) US (Mbps) BPON 155.52 155.52 622.08 155.52 Amd 1 622.08 622.08 1244.16 155.52 Amd 2 1244.16 622.08 1244.16 155.52 1244.16 622.08 1244.16 1244.16 2488.32 155.52 GPON 2488.32 622.08 2488.32 1244.16 2488.32 2488.32 EPON 1250* 1250*10GEPON† 10312.5* 10312.5* * only 1G/10G usable due to linecode † work in progress PONs Slide 57
  58. 58. Reach and splitsReach and the number of ONUs supported are contradictory design goalsIn addition to physical reach derived from optical budget there is logical reach limited by protocol concerns (e.g. ranging protocol) and differential reach (distance between nearest and farthest ONUs)The number of ONUs supported depends not only on the number of splits but also on the addressing schemeBPON called for 20 km and 32-64 ONUsGPON allows 64-128 splits and the reach is usually 20 km but there is a low-cost 10 km mode (using Fabry-Perot laser diodes in ONUs) and a long physical reach 60 km mode with 20 km differential reachEPON allows 16-256 splits (originally designed for link budget of 24 dB, but now 30 dB) and has 10 km and 20 km Physical Media Dependent sublayers PONs Slide 58
  59. 59. Line codesBPON and GPON use a simple NRZ linecode (high is 1 and low is 0)An I.432-style scrambling operation is applied to payload (not to PON overhead)Preferable to conventional scrambler because no error propagation – each standard and each direction use different LFSRs – LFSR initialized with all ones – LFSR sequence is XORed with data before transmissionEPON uses the 802.3z (1000BASE-X) line code - 8B/10B – Every 8 data bits are converted into 10 bits before transmission – DC removal and timing recovery ensured by mapping – Special function codes (e.g. idle, start_of_packet, end_of_packet, etc)However, 1000 Mbps is expanded to 1250 Mbps10GbE uses a different linecode - 64B/66B PONs Slide 59
  60. 60. FECG984.3 clause 13 and 802.3-2005 subclause 65.2.3 define an optional G.709-style Reed-Solomon codeUse (255,239,8) systematic RS code designed for submarine fiber (G.975)to every 239 data bytes add 16 parity bytes to make 255 byte FEC blockUp to 8 byte errors can be correctedImproves power budget by over 3 dB, allowing increased reach or additional splitsUse of FEC is negotiated between OLT and ONUSince code is systematic can use in environment where some ONUs do not support FECIn GPON FEC frames are aligned with PON framesIn EPON FEC frames are marked using K-codes (and need 8B10B decode - FEC - 8B10B encode) PONs Slide 60
  61. 61. More physical layer problemsNear-far problemOLT needs to know signal strength to set decision thresholdIf large distance between near/far ONUs, then very different attenuationsIf radically different received signal strength cant use a single threshold – EPON: measure received power of ONU at beginning of burst – GPON: OLT feedback to ONUs to properly set transmit powerBurst laser problemSpontaneous emission noise from nearby ONU lasers causes interferenceElectrically shut ONU laser off when not transmittingBut lasers have long warm-up time and ONU lasers must stabilize quickly after being turned on PONs Slide 61
  62. 62. US timing diagram How does the ONU US transmission appear to the OLT ? grant grant inter-ONU guard data data lock lock laser laser laser laser turn-on turn-off turn-on turn-offNotes:GPON - ONU reports turn-on and turn-off times to OLT ONU preamble length set by OLTEPON - long lock time as need to Automatic Gain Control and Clock/Data Recovery long inter-ONU guard due to AGC-reset Ethernet preamble is part of data PONs Slide 62
  63. 63. PON User plane PONs Slide 63
  64. 64. How does it work?ONU stores client data in large buffers (ingress queues)ONU sends a high-speed burst upon receiving a grant/allocation – Ranging must be performed for ONU to transmit at the right time – DBA - OLT allocates BW according to ONU queue levelsOLT identifies ONU traffic by labelOLT extracts traffic units and passes to networkOLT receives traffic from network and encapsulates into PON framesOLT prefixes with ONU label and broadcastsONU receives all packets and filters according to labelONU extracts traffic units and passes to client PONs Slide 64
  65. 65. LabelsIn an ODN there is 1 OLT, but many ONUsONUs must somehow be labeled for – OLT to identify the destination ONU – ONU to identify itself as the sourceEPON assigns a single label Logical Link ID to each ONU (15b)GPON has several levels of labels – ONU_ID (1B) (1B) – Transmission-CONTainer (AKA Alloc_ID) (12b) (can be >1 T-CONT per ONU) For ATM mode  VPI VP VC ONU T-CONT VP VC  VCI VC VC For GEM mode PON Port  Port_ID (12b) (12b) ONU T-CONT Port PONs Slide 65
  66. 66. DS GPON formatGPON Transmission Convergence frames are always 125 sec long – 19440 bytes / frame for 1244.16 rate – 38880 bytes / frame for 2488.32 rateEach GTC frame consists of Physical Control Block downstream + payload – PCBd contains sync, OAM, DBA info, etc. – payload may have ATM and GEM partitions (either one or both) GTC frame scrambled 125 sec PCBd payload PCBd payload PCBd payload PSync (4B) Ident (4B) PLOAMd (13B) BIP (1B) ATM GEM partition partition PLend (4B) PLend (4B) US BW map (N*8B) PONs Slide 66
  67. 67. GPON payloadsGTC payload potentially has 2 sections: – ATM partition (Alen * 53 bytes in length) – GEM partition (now preferred method) PCBd ATM cell ATM cell … ATM cell GEM frame GEM frame … GEM frameATM partitionAlen (12 bits) is specified in the PCBd Alen specifies the number of 53B cells in the ATM partition if Alen=0 then no ATM partition if Alen=payload length / 53 then no GEM partitionATM cells are aligned to GTC frameONUs accept ATM cells based on VPI in ATM headerGEM partitionUnlike ATM cells, GEM delineated frames may have any lengthAny number of GEM frames may be contained in the GEM partitionONUs accept GEM frames based on 12b Port-ID in GEM header PONs Slide 67
  68. 68. GPON Encapsulation ModeA common complaint against BPON was inefficiency due to ATM cell taxGEM is similar to ATM – constant-size HEC-protected header – but avoids large overhead by allowing variable length framesGEM is generic – any packet type (and even TDM) supportedGEM supports fragmentation and reassemblyGEM is based on GFP, and the header contains the following fields: – Payload Length Indicator - payload length in Bytes – Port ID - identifies the target ONU – Payload Type Indicator (GEM OAM, congestion/fragmentation indication) – Header Error Correction field (BCH(39,12,2) code+ 1b even parity)The GEM header is XORed with B6AB31E055 before transmission PLI Port ID PTI HEC payload fragment (12b) (12b) (3b) (13b) (L Bytes) 5B PONs Slide 68
  69. 69. Ethernet / TDM over GEMWhen transporting Ethernet traffic over GEM: – only MAC frame is encapsulated (no preamble, SFD, EFD) – MAC frame may be fragmented (see next slide) Ethernet over GEM PLI ID PTI HEC DA SA T data FCSWhen transporting TDM traffic over GEM: – TDM input buffer polled every 125 sec. – PLI bytes of TDM are inserted into payload field – length of TDM fragment may vary by 1 Byte due to frequency offset – round-trip latency bounded by 3 msec. TDM over GEM PLI ID PTI HEC PLI Bytes of TDM PONs Slide 69
  70. 70. GEM fragmentationGEM can fragment its payloadFor example unfragmented Ethernet frame PLI ID PTI=001 HEC DA SA T data FCS fragmented Ethernet frame PLI ID PTI=000 HEC DA SA T data1 PLI ID PTI=001 HEC data2 FCSGEM fragments payloads for either of two reasons: – GEM frame may not straddle GTC frame PCBd ATM partition GEM frame … GEM frag 1 PCBd ATM partition GEM frag 2 … GEM frame – GEM frame may be pre-empted for delay-sensitive data PCBd ATM partition urgent frame … large frag 1 PCBd ATM partition urgent frame … large frag 2 PONs Slide 70
  71. 71. PCBdWe saw that the PCBd is PSync Ident PLOAMd BIP PLend PLend US BW map (4B) (4B) (13B) (1B) (4B) (4B) (N*8B)B6AB31E0PSync - fixed pattern used by ONU to located start of GTC frameIdent - MSB indicates if FEC is used, 30 LSBs are superframe counterPLOAMd - carries OAM, ranging, alerts, activation messages, etc.BIP - SONET/SDH-style Bit Interleaved Parity of all bytes since last BIPPLend (transmitted twice for robustness) - – Blen - 12 MSB are length of BW map in units of 8 Bytes – Alen - Next 12 bits are length of ATM partition in cells – CRC - final 8 bits are CRC over Blen and AlenUS BW map - array of Blen 8B structures granting BW to US flow will discuss later (DBA) PONs Slide 71
  72. 72. GPON US considerationsGTC fames are still 125 sec long, but shared amongst ONUsEach ONU transmits a burst of data – using timing acquired by locking onto OLT signal – according to time allocation sent by OLT in BWmap  there may be multiple allocations to single ONU  OLT computes DBA by monitoring traffic status (buffers) of ONUs and knowing priorities – at power level requested by OLT (3 levels)  this enables OLT to use avalanche photodiodes which are sensitive to high power bursts – leaving a guard time from previous ONUs transmission – prefixing a preamble to enable OLT to acquire power and phase – identifying itself (ONU-ID) in addition to traffic IDs (VPI, Port-ID) – scrambling data (but not preamble/delimiter) PONs Slide 72
  73. 73. US GPON format4 different US overhead types: Physical Layer Overhead upstream – always sent by ONU when taking over from another ONU – contains preamble and delimiter (lengths set by OLT in PLOAMd) BIP (1B), ONU-ID (1B), and Indication of real-time status (1B) PLOAM upstream (13B) - messaging with PLOAMd Power Levelling Sequence upstream (120B) – used during power-set and power-change to help set ONU power so that OLT sees similar power from all ONUs Dynamic Bandwidth Report upstream – sends traffic status to OLT in order to enable DBA computationif all OH types are present: PLOu PLOAMd PLSu DBRu payload PONs Slide 73
  74. 74. US allocation example DS frame PCBd payloadBWmap Alloc-ID SStart SStop Alloc-ID SStart Sstop Alloc-ID SStart SStop US frame preamble guard scrambled + time delimiterBWmap sent by OLT to ONUs is a list of ONU allocation IDs flags (not shown above) tell if use FEC, which US OHs to use, etc. start and stop times (16b fields, in Bytes from beginning of US frame) PONs Slide 74
  75. 75. EPON formatEPON operation is based on the Ethernet MAC and EPON frames are based on GbE framesbut extensions are needed clause 64 - MultiPoint Control Protocol PDUs this is the control protocol implementing the required logic clause 65 - point-to-point emulation (reconciliation) this makes the EPON look like a point-to-point linkand EPON MACs have some special constraints instead of CSMA/CD they transmit when granted time through MAC stack must be constant ( 16 bit durations) accurate local time must be maintained PONs Slide 75
  76. 76. EPON headerStandard Ethernet starts with an essentially content-free 8B preamble 7B of alternating ones and zeros 10101010 1B of SFD 10101011In order to hide the new PON header EPON overwrites some of the preamble bytes 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011 10101010 10101010 10101011 10101010 10101010 LLID LLID CRCLLID field contains – MODE (1b)  always 0 for ONU  0 for OLT unicast, 1 for OLT multicast/broadcast – actual Logical Link ID (15b)  Identifies registered ONUs  7FFF for broadcastCRC protects from SLD (byte 3) through LLID (byte 7) PONs Slide 76
  77. 77. MPC PDU formatMultiPoint Control Protocol frames are untagged MAC frames with the same format as PAUSE frames DA SA L/T Opcode timestamp data / RES / pad FCSEthertype = 8808Opcodes (2B) - presently defined: GATE/REPORT/REGISTER_REQ/REGISTER/REGISTER_ACKTimestamp is 32b, 16 ns resolution conveys the senders time at time of MPCPDU transmissionData field is needed for some messages PONs Slide 77
  78. 78. SecurityDS traffic is broadcast to all ONUs, so encryption is essential easy for a malicious user to reprogram ONU to capture desired framesUS traffic not seen by other ONUs, so encryption is not needed do not take fiber-tappers into accountEPON does not provide any standard encryption method – can supplement with IPsec or MACsec – many vendors have added proprietary AES-based mechanisms – in China special China Telecom encryption algorithmBPON used a mechanism called churningChurning was a low cost hardware solution (24b key) with several security flaws – engine was linear - simple known-text attack – 24b key turned out to be derivable in 512 triesSo G.983.3 added AES support - now used in GPON PONs Slide 78
  79. 79. GPON encryptionOLT encrypts using AES-128 in counter modeOnly payload is encrypted (not ATM or GEM headers)Encryption blocks aligned to GTC frameCounter is shared by OLT and all ONUs – 46b = 16b intra-frame + 30 bits inter-frame – intra-frame counter increments every 4 data bytes  reset to zero at beginning of DS GTC frameOLT and each ONU must agree on a unique symmetric keyOLT asks ONU for a password (in PLOAMd)ONU sends password US in the clear (in PLOAMu) – key sent 3 times for robustnessOLT informs ONU of precise time to start using new key PONs Slide 79
  80. 80. QoS - EPONMany PON applications require high QoS (e.g. IPTV)EPON leaves QoS to higher layers – VLAN tags – P bits or DiffServ DSCPIn addition, there is a crucial difference between LLID and Port-ID – there is always 1 LLID per ONU – there is 1 Port-ID per input port - there may be many per ONU – this makes port-based QoS simple to implement at PON layer RT EF BE GPON PONs Slide 80
  81. 81. QoS - GPONGPON treats QoS explicitly – constant length frames facilitate QoS for time-sensitive applications – 5 types of Transmission CONTainers  type 1 - fixed BW  type 2 - assured BW  type 3 - allocated BW + non-assured BW  type 4 - best effort  type 5 - superset of all of the aboveGEM adds several PON-layer QoS features – fragmentation enables pre-emption of large low-priority frames – PLI - explicit packet length can be used by queuing algorithms – PTI bits carry congestion indications PONs Slide 81
  82. 82. PON control plane PONs Slide 82
  83. 83. PrinciplesGPON uses PLOAMd and PLOAMu as control channelPLOAM are incorporated in regular (data-carrying) framesStandard ITU control mechanismEPON uses MPCP PDUsStandard IEEE control mechanismEPON control model - OLT is master, ONU is slave – OLT sends GATE PDUs DS to ONU – ONU sends REPORT PDUs US to OLT PONs Slide 83
  84. 84. RangingUpstream traffic is TDMAWere all ONUs equidistant, and were all to have a common clock then each would simply transmit in its assigned timeslotBut otherwise the signals will overlapTo eliminate overlap guard times left between timeslots each ONU transmits with the proper delay to avoid overlap delay computed during a ranging process PONs Slide 84
  85. 85. Ranging backgroundIn order for the ONU to transmit at the correct time the delay between ONU transmission and OLT reception needs to be known (explicitly or implicitly) Need to assign an equalization-delayThe more accurately it is known the smaller the guard time that needs to be left and thus the higher the efficiencyAssumptions behind the ranging methods used: can not assume US delay is equal to DS delay delays are not constant – due to temperature changes and component aging GPON: ONUs not time synchronized accurately enough EPON: ONUs are accurately time synchronized (std contains jitter masks) with time offset by OLT-ONU propagation time PONs Slide 85
  86. 86. GPON ranging methodTwo types of ranging – initial ranging  only performed at ONU boot-up or upon ONU discovery  must be performed before ONU transmits first time – continuous ranging performed continuously to compensate for delay changesOLT initiates coarse ranging by stopping allocations to all other ONUs – thus when new ONU transmits, it will be in the clearOLT instructs the new ONU to transmit (via PLOAMd)OLT measures phase of ONU burst in GTC frameOLT sends equalization delay to ONU (in PLOAMd)During normal operation OLT monitors ONU burst phaseIf drift is detected OLT sends new equalization delay to ONU (in PLOAMd) PONs Slide 86
  87. 87. EPON ranging method All ONUs are synchronized to absolute time (wall-clock) When an ONU receives an MPCPDU from OLT it sets its clock according to the OLTs timestamp When the OLT receives an MPCPDU in response to its MPCPDU it computes a "round-trip time" RTT (without handling times) it informs the ONU of RTT, which is used to compute transmit delay OLT sends MPCPDU ONU receives MPCPDU ONU sends MPCPDU OLT receives MPCPDU Timestamp = T0 Sets clock to T0 Timestamp = T1 RTT = T2 - T1 timeOLT time T0 T2ONU time T0 T1 RTT = (T2-T0) - (T1-T0) = T2-T1 OLT compensates all grants by RTT before sending Either ONU or OLT can detect that timestamp drift exceeds threshold PONs Slide 87
  88. 88. AutodiscoveryOLT needs to know with which ONUs it is communicatingThis can be established via NMS – but even then need to setup physical layer parametersPONs employ autodiscovery mechanism to automate – discovery of existence of ONU – acquisition of identity – allocation of identifier – acquisition of ONU capabilities – measure physical layer parameters – agree on parameters (e.g. watchdog timers)Autodiscovery procedures are complex (and uninteresting) so we will only mention highlights PONs Slide 88
  89. 89. GPON autodiscoveryEvery ONU has an 8B serial number (4B vendor code + 4B SN) – SN of ONUs in OAN may be configured by NMS, or – SN may be learnt from ONU in discovery phaseONU activation may be triggered by – Operator command – Periodic polling by OLT – OLT searching for previously operational ONUG.984.3 differentiates between three cases: – cold PON / cold ONU – warm PON / cold ONU – warm PON / warm ONUMain steps in procedure: – ONU sets power based on DS message – OLT sends a Serial_Number request to all unregistered ONUs – ONU responds – OLT assigns 1B ONU-ID and sends to ONU – ranging is performed – ONU is operational PONs Slide 89
  90. 90. EPON autodiscoveryOLT periodically transmits DISCOVERY GATE messagesONU waits for DISCOVERY GATE to be broadcast by OLTDISCOVERY GATE message defines discovery window  start time and durationONU transmits REGISTER_REQ PDU using random offset in windowOLT receives request  registers ONU  assigns LLID  bonds MAC to LLID  performs ranging computationOLT sends REGISTER to ONUOLT sends standard GATE to ONUONU responds with REGISTER_ACKONU goes into operational mode - waits for grants PONs Slide 90
  91. 91. Failure recoveryPONs must be able to handle various failure statesGPON if ONU detects LOS or LOF it goes into POPUP state  it stops sending traffic US  OLT detects LOS for ONU  if there is a pre-ranged backup fiber then switch-overEPON during normal operation ONU REPORTs reset OLTs watchdog timer similarly, OLT must send GATES periodically (even if empty ones) if OLTs watchdog timer for ONU times out  ONU is deregistered PONs Slide 91
  92. 92. Dynamic Bandwidth AllocationMANs and WANs have relatively stationary BW requirements due to aggregation of large number of sourcesBut each ONU in a PON may serve only 1 or a small number of usersSo BW required is highly variableIt would be inefficient to statically assign the same BW to each ONUSo PONs assign dynamically BW according to needThe need can be discovered – by passively observing the traffic from the ONU – by ONU sending reports as to state of its ingress queuesThe goals of a Dynamic Bandwidth Allocation algorithm are – maximum fiber BW utilization – fairness and respect of priority – minimum delay introduced PONs Slide 92
  93. 93. GPON DBADBA is at the T-CONT level, not port or VC/VPGPON can use traffic monitoring (passive) or status reporting (active)There are three different status reporting methods status in PLOu - one bit for each T-CONT type piggy-back reports in DBRu - 3 different formats: – quantity of data waiting in buffers, – separation of data with peak and sustained rate tokens – nonlinear coding of data according to T-CONT type and tokens ONU report in DBA payload - select T-CONT statesOLT may use any DBA algorithmOLT sends allocations in US BW map PONs Slide 93
  94. 94. EPON DBA OLT sends GATE messages to ONUsGATE messageDA SA 8808 Opcode=0002 timestamp Ngrants/flags grants … flags include DISCOVERY and Force_Report Force_Report tells the ONU to issue a reportREPORT messageDA SA 8808 Opcode=0003 timestamp Nqueue_sets Reports … Reports represent the length of each queue at time of report OLT may use any algorithm to decide how to send the following grants PONs Slide 94