![Why fiber ?
today’s high datarate networks are all based on optical fiber
the 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](https://image.slidesharecdn.com/pon-120326095629-phpapp01/85/Passive-Optical-Networks-4-320.jpg)
Passive Optical Networks
- 1. Passive Optical Networks Yaakov (J) Stein May 2007 and Zvika Eitan
- 2. Outline PON benefits PON architecture Fiber optic basics PON physical layer PON user plane PON control plane PONs Slide 2
- 3. PON benefits PONs Slide 3
- 4. Why fiber ? today’s high datarate networks are all based on optical fiber the 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. 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 fiber noise ingress and cross-talk copper couples to all nearby conductors no similar ingress mechanism for fiber ground-potential, galvanic isolation, lightning protection copper can be hard to handle and dangerous no concerns for fiber PONs Slide 5
- 6. Why not fiber ? fiber beats all other technologies for speed and reach but 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. Access network bottleneck hard for end users to get high datarates because of the access bottleneck local area networks use copper cable get high datarates over short distances core networks use fiber optics get high datarate over long distances access core small number of active network elements access 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. Fiber To The Curb Hybrid Fiber Coax and VDSL switch/transceiver/miniDSLAM located at curb or in basement need only 2 optical transceivers but 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. Fiber To The Premises we 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. An obvious solution deploy 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. The PON solution another 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. PON advantages shared 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 UTP passive splitters translate to lower cost can be installed anywhere no power needed essentially unlimited MTBF fiber 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. PON architecture PONs Slide 13
- 14. Terminology like 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 Units core splitter Optical Line Terminal UNI Optical Access Network Terminal Equipment PONs Slide 14
- 15. PON types many 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 PON in 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. 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. PON principles (almost) all PON types obey the same basic principles OLT and ONU consist of Layer 2 (Ethernet MAC, ATM adapter, etc.) optical transceiver using different s for transmit and receive optionally: Wavelength Division Multiplexer downstream 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 privacy upstream transmission ONUs share bandwidth using Time Division Multiple Access OLT manages the ONU timeslots ranging is performed to determine ONU-OLT propagation time additional functionality Physical Layer OAM Autodiscovery Dynamic Bandwidth Allocation PONs Slide 17
- 18. Why a new protocol ? downstream PON has a unique architecture upstream (broadcast) point-to-multipoint in DS direction (multiple access) multipoint-to-point in US direction contrast that with, for example Ethernet - multipoint-to-multipoint ATM - point-to-point This means that existing protocols do not provide all the needed functionality e.g. receive filtering, ranging, security, BW allocation PONs Slide 18
- 19. (multi)point - to - (multi)point Multipoint-to-multipoint Ethernet avoids collisions by CSMA/CD This can't work for multipoint-to-point US PON since ONUs don't see each other And the OLT can't arbitrate without adding a roundtrip time Point-to-point ATM can send data in the open although trusted intermediate switches see all data customer switches only receive their own data This can't work for point-to-multipoint DS PON since all ONUs see all DS data PONs Slide 19
- 20. PON encapsulation The majority of PON traffic is Ethernet So EPON enthusiasts say use EPON - it's just Ethernet That's true by definition - anything in 802.3 is Ethernet and EPON is defined in clauses 64 and 65 of 802.3-2005 But don't be fooled - all PON methods encapsulate MAC frames EPON 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. BPON history 1995 : 7 operators (BT, FT, NTT, …) and a few vendors form Full Service Access Network Initiative to provide business customers with multiservice broadband offering Obvious choices were ATM (multiservice) and PON (inexpensive) which when merged became APON 1996 : name changed to BPON to avoid too close association with ATM 1997 : FSAN proposed BPON to ITU SG15 1998 : 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. EPON history 2001: 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 allocations 2003 : WG ballot 2004 : full standard 2005: new 802.3 version with EFM clauses PONs Slide 22
- 23. GPON history 2001 : FSAN initiated work on extension of BPON to > 1 Gbps Although 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 BPON 2001 : 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. Fiber optics - basics PONs Slide 24
- 25. Total Internal Reflection in Step-Index Multimode Fiber © = sin¯ 1(n2/n1) t = Propagation Time t Vacuum: n=1, t=3.336ns/m V =c/n t Water : n=1.33, t=4.446ns/m t = L·n/c PONs Slide 25
- 26. Types of Optical Fiber Popular Fiber Sizes Multimode Graded- Index Fiber Single-mode Fiber PONs Slide 26
- 27. Optical Loss versus Wavelength Click to edit Master text styles – Second level Third level – Fourth level PONs Slide 27
- 28. Sources of Dispersion Total Dispersion Multimode Chromatic Dispersion Dispersion Material Dispersion PONs Slide 28
- 29. Multimode Dispersion 1 0 1 1 1 1 1 Dispersion limits bandwidth in optical fiber PONs Slide 29
- 30. Graded-index Dispersion 1 0 1 1 1 0 1 PONs Slide 30
- 31. Single-Mode Dispersion 1 0 1 1 1 0 1 In SM the limit bandwidth is caused by chromatic dispersion. PONs Slide 31
- 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 Perot Tc = (20ps/nm * km) * 5nm * 15km = 1.5ns For Laser 1550nm DFB Tc = (20ps/nm * km) * 0.2nm * 60km = 0.24ns PONs Slide 32
- 33. Material Dispersion (Dmat) PONs Slide 33
- 34. Spectral Characteristics LASER/laser diode: Light Amplification by Stimulated Emission of Radiation. Done of the wide range of devices that generates light by that principle. Laser light is directional, covers a narrow range of wavelengths, and is more coherent than ordinary light. Semiconductor diode lasers are the standard light sources in fiber optic systems. Lasers emit light by stimulated emission. PONs Slide 34
- 35. Laser Optical Power Output vs. Forward Current W Laser PONs Slide 35
- 36. Light Detectors PIN 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 applications AVALANCHE 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. Wavelength-Division Multiplexing PONs Slide 37
- 38. WDM Duplexing PONs Slide 38
- 39. Basic Configuration of PON OLT = Optical Line Termination ONU = Optical Network Unit BMCDR = Burst Mode Clock Data Recovery PONs Slide 39
- 40. Typical PON Configuration and Optical Packets PONs Slide 40
- 41. Eye diagram of ONU transceiver in burst mode operation PONs Slide 41
- 42. Burst-Mode Transmitter in ONU PONs Slide 42
- 43. OLT Burst-Mode Receiver PONs Slide 43
- 44. Burst-Mode CDR PONs Slide 44
- 45. Sampling Ideal sampling instant Hysteresis Superimposed interference Ideal, error-free transmission PONs Slide 45
- 46. Transceiver Block Diagram PONs Slide 46
- 47. Optical Splitters PONs Slide 47
- 48. Optical Protection Switch Optical Splitter PONs Slide 48
- 49. Budget Calculations LB = PS - PO LB = Link Budget PS = Sensitivity PO = Output Power Example: GPON 1310nm Power: 0dbm Single-mode fiber } Link Budget: Sensitivity: -23dbm 23db PONs Slide 49
- 50. Typical Range Calculation Assume: Optical loss = 0.35 db/km Connector Loss = 2dB Range Budget: ~11Km Splitter Insertion Loss 1X32 = 17dB PONs Slide 50
- 51. Relationship between transmission distance and number of splits PONs Slide 51
- 52. GbE Fiber Optic Characteristics PONs Slide 52
- 53. PON physical layer PONs Slide 53
- 54. allocations - G.983.1 Upstream and downstream directions need about the same bandwidth US serves N customers, so it needs N times the BW of each customer but each customer can only transmit 1/N of the time In APON and early BPON work it was decided that 100 nm was needed Where 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. allocations - G.983.3 Afterwards it became clear that there was a need for additional DS bands Pressing needs were broadcast video and data Where 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 DS So 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. allocations - final US DS 1200 nm 1300 nm 1400 nm 1500 nm 1600 nm The 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. 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. Reach and splits Reach and the number of ONUs supported are contradictory design goals In 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 scheme BPON called for 20 km and 32-64 ONUs GPON 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 reach EPON 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. Line codes BPON 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 XOR'ed with data before transmission EPON 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 Mbps 10GbE uses a different linecode - 64B/66B PONs Slide 59
- 60. FEC G984.3 clause 13 and 802.3-2005 subclause 65.2.3 define an optional G.709-style Reed-Solomon code Use (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 block Up to 8 byte errors can be corrected Improves power budget by over 3 dB, allowing increased reach or additional splits Use of FEC is negotiated between OLT and ONU Since code is systematic can use in environment where some ONUs do not support FEC In GPON FEC frames are aligned with PON frames In EPON FEC frames are marked using K-codes (and need 8B10B decode - FEC - 8B10B encode) PONs Slide 60
- 61. More physical layer problems Near-far problem OLT needs to know signal strength to set decision threshold If large distance between near/far ONUs, then very different attenuations If radically different received signal strength can't use a single threshold – EPON: measure received power of ONU at beginning of burst – GPON: OLT feedback to ONUs to properly set transmit power Burst laser problem Spontaneous emission noise from nearby ONU lasers causes interference Electrically shut ONU laser off when not transmitting But lasers have long warm-up time and ONU lasers must stabilize quickly after being turned on PONs Slide 61
- 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-off Notes: GPON - ONU reports turn-on and turn-off times to OLT ONU preamble length set by OLT EPON - 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. PON User plane PONs Slide 63
- 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 levels OLT identifies ONU traffic by label OLT extracts traffic units and passes to network OLT receives traffic from network and encapsulates into PON frames OLT prefixes with ONU label and broadcasts ONU receives all packets and filters according to label ONU extracts traffic units and passes to client PONs Slide 64
- 65. Labels In an ODN there is 1 OLT, but many ONUs ONUs must somehow be labeled for – OLT to identify the destination ONU – ONU to identify itself as the source EPON 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. DS GPON format GPON Transmission Convergence frames are always 125 sec long – 19440 bytes / frame for 1244.16 rate – 38880 bytes / frame for 2488.32 rate Each 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. GPON payloads GTC 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 frame ATM partition Alen (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 partition ATM cells are aligned to GTC frame ONUs accept ATM cells based on VPI in ATM header GEM partition Unlike ATM cells, GEM delineated frames may have any length Any number of GEM frames may be contained in the GEM partition ONUs accept GEM frames based on 12b Port-ID in GEM header PONs Slide 67
- 68. GPON Encapsulation Mode A common complaint against BPON was inefficiency due to ATM cell tax GEM is similar to ATM – constant-size HEC-protected header – but avoids large overhead by allowing variable length frames GEM is generic – any packet type (and even TDM) supported GEM supports fragmentation and reassembly GEM 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 XOR'ed with B6AB31E055 before transmission PLI Port ID PTI HEC payload fragment (12b) (12b) (3b) (13b) (L Bytes) 5B PONs Slide 68
- 69. Ethernet / TDM over GEM When 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 FCS When 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. GEM fragmentation GEM can fragment its payload For 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 FCS GEM 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. PCBd We saw that the PCBd is PSync Ident PLOAMd BIP PLend PLend US BW map (4B) (4B) (13B) (1B) (4B) (4B) (N*8B) B6AB31E0 PSync - fixed pattern used by ONU to located start of GTC frame Ident - MSB indicates if FEC is used, 30 LSBs are superframe counter PLOAMd - carries OAM, ranging, alerts, activation messages, etc. BIP - SONET/SDH-style Bit Interleaved Parity of all bytes since last BIP PLend (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 Alen US BW map - array of Blen 8B structures granting BW to US flow will discuss later (DBA) PONs Slide 71
- 72. GPON US considerations GTC fames are still 125 sec long, but shared amongst ONUs Each 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 ONU's 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. US GPON format 4 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 computation if all OH types are present: PLOu PLOAMd PLSu DBRu payload PONs Slide 73
- 74. US allocation example DS frame PCBd payload BWmap Alloc-ID SStart SStop Alloc-ID SStart Sstop Alloc-ID SStart SStop US frame preamble guard scrambled + time delimiter BWmap 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. EPON format EPON operation is based on the Ethernet MAC and EPON frames are based on GbE frames but 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 link and 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. EPON header Standard Ethernet starts with an essentially content-free 8B preamble 7B of alternating ones and zeros 10101010 1B of SFD 10101011 In 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 CRC LLID 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 broadcast CRC protects from SLD (byte 3) through LLID (byte 7) PONs Slide 76
- 77. MPC PDU format MultiPoint Control Protocol frames are untagged MAC frames with the same format as PAUSE frames DA SA L/T Opcode timestamp data / RES / pad FCS Ethertype = 8808 Opcodes (2B) - presently defined: GATE/REPORT/REGISTER_REQ/REGISTER/REGISTER_ACK Timestamp is 32b, 16 ns resolution conveys the sender's time at time of MPCPDU transmission Data field is needed for some messages PONs Slide 77
- 78. Security DS traffic is broadcast to all ONUs, so encryption is essential easy for a malicious user to reprogram ONU to capture desired frames US traffic not seen by other ONUs, so encryption is not needed do not take fiber-tappers into account EPON 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 algorithm BPON used a mechanism called churning Churning 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 tries So G.983.3 added AES support - now used in GPON PONs Slide 78
- 79. GPON encryption OLT encrypts using AES-128 in counter mode Only payload is encrypted (not ATM or GEM headers) Encryption blocks aligned to GTC frame Counter 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 frame OLT and each ONU must agree on a unique symmetric key OLT asks ONU for a password (in PLOAMd) ONU sends password US in the clear (in PLOAMu) – key sent 3 times for robustness OLT informs ONU of precise time to start using new key PONs Slide 79
- 80. QoS - EPON Many PON applications require high QoS (e.g. IPTV) EPON leaves QoS to higher layers – VLAN tags – P bits or DiffServ DSCP In 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. QoS - GPON GPON 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 above GEM 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. PON control plane PONs Slide 82
- 83. Principles GPON uses PLOAMd and PLOAMu as control channel PLOAM are incorporated in regular (data-carrying) frames Standard ITU control mechanism EPON uses MPCP PDUs Standard IEEE control mechanism EPON 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. Ranging Upstream traffic is TDMA Were all ONUs equidistant, and were all to have a common clock then each would simply transmit in its assigned timeslot But otherwise the signals will overlap To 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. Ranging background In 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-delay The more accurately it is known the smaller the guard time that needs to be left and thus the higher the efficiency Assumptions 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. GPON ranging method Two 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 changes OLT initiates coarse ranging by stopping allocations to all other ONUs – thus when new ONU transmits, it will be in the clear OLT instructs the new ONU to transmit (via PLOAMd) OLT measures phase of ONU burst in GTC frame OLT sends equalization delay to ONU (in PLOAMd) During normal operation OLT monitors ONU burst phase If drift is detected OLT sends new equalization delay to ONU (in PLOAMd) PONs Slide 86
- 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 OLT's 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 time OLT time T0 T2 ONU 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. Autodiscovery OLT needs to know with which ONUs it is communicating This can be established via NMS – but even then need to setup physical layer parameters PONs 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. GPON autodiscovery Every 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 phase ONU activation may be triggered by – Operator command – Periodic polling by OLT – OLT searching for previously operational ONU G.984.3 differentiates between three cases: – cold PON / cold ONU – warm PON / cold ONU – warm PON / warm ONU Main 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. EPON autodiscovery OLT periodically transmits DISCOVERY GATE messages ONU waits for DISCOVERY GATE to be broadcast by OLT DISCOVERY GATE message defines discovery window start time and duration ONU transmits REGISTER_REQ PDU using random offset in window OLT receives request registers ONU assigns LLID bonds MAC to LLID performs ranging computation OLT sends REGISTER to ONU OLT sends standard GATE to ONU ONU responds with REGISTER_ACK ONU goes into operational mode - waits for grants PONs Slide 90
- 91. Failure recovery PONs must be able to handle various failure states GPON 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-over EPON during normal operation ONU REPORTs reset OLT's watchdog timer similarly, OLT must send GATES periodically (even if empty ones) if OLT's watchdog timer for ONU times out ONU is deregistered PONs Slide 91
- 92. Dynamic Bandwidth Allocation MANs and WANs have relatively stationary BW requirements due to aggregation of large number of sources But each ONU in a PON may serve only 1 or a small number of users So BW required is highly variable It would be inefficient to statically assign the same BW to each ONU So PONs assign dynamically BW according to need The need can be discovered – by passively observing the traffic from the ONU – by ONU sending reports as to state of its ingress queues The goals of a Dynamic Bandwidth Allocation algorithm are – maximum fiber BW utilization – fairness and respect of priority – minimum delay introduced PONs Slide 92
- 93. GPON DBA DBA is at the T-CONT level, not port or VC/VP GPON 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 states OLT may use any DBA algorithm OLT sends allocations in US BW map PONs Slide 93
- 94. EPON DBA OLT sends GATE messages to ONUs GATE message DA SA 8808 Opcode=0002 timestamp Ngrants/flags grants … flags include DISCOVERY and Force_Report Force_Report tells the ONU to issue a report REPORT message DA 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