Passive Optical NetworksYaakov (J) Stein May 2007 andZvika Eitan

PONs Slide *OutlinePON benefitsPON architectureFiber optic basicsPON physical layerPON user planePON control plane

PONs Slide *PON benefits

PONs Slide *Why fiber ?todays 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 fiber10 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 *Aside why is fiber better ?attenuation per unit length reasons for energy losscopper: resistance, skin effect, radiation, couplingfiber: internal scattering, imperfect total internal reflectionso fiber beats coax by about 2 orders of magnitude e.g. 10 dB/km for thin coax at 50MHz, 0.15 dB/km l =1550nm fibernoise ingress and cross-talkcopper couples to all nearby conductorsno similar ingress mechanism for fiberground-potential, galvanic isolation, lightning protectioncopper can be hard to handle and dangerousno concerns for fiber

PONs Slide *Why not fiber ?fiber beats all other technologies for speed and reachbut fiber has its own problemsharder to splice, repair, and need to handle carefullyregenerators and even amplifiers are problematicmore expensive to deploy than for copper digital processing requires electronicsso need to convert back to electronicswe will call the converter an optical transceiveroptical transceivers are expensiveswitching easier with electronics (but possible with photonics)so pure fiber networks are topologically limited:point-to-pointrings

PONs Slide *Access network bottleneckhard for end users to get high datarates because of the access bottlenecklocal area networksuse copper cableget high datarates over short distances core networksuse fiber opticsget high datarate over long distancessmall number of active network elementsaccess networks (first/last mile) long distancesso fiber would be the best choicemany network elements and large number of endpoints if fiber is used then need multiple optical transceiversso copper is the best choicethis severely limits the datarates

PONs Slide *Fiber To The CurbHybrid Fiber Coax and VDSLswitch/transceiver/miniDSLAM located at curb or in basementneed only 2 optical transceivers but not pure optical solutionlower BW from transceiver to end usersneed complex converter in constrained environment

N end userscoreaccess networkfeeder fiber copper

PONs Slide *Fiber To The Premiseswe can implement point-to-multipoint topology purely in opticsbut we need a fiber (pair) to each end userrequires 2 N optical transceiverscomplex and costly to maintain

PONs Slide *An obvious solutiondeploy intermediate switches (active) switch located at curb or in basementsaves space at central officeneed 2 N + 2 optical transceivers

N end userscoreaccess networkfeeder fiberfiber

PONs Slide *The PON solutionanother alternative - implement point-to-multipoint topology purely in opticsavoid costly optic-electronic conversions use passive splitters no power needed, unlimited MTBFonly N+1 optical transceivers (minimum possible) !

PONs Slide *PON advantagesshared infrastructure translates to lower cost per customerminimal number of optical transceiversfeeder fiber and transceiver costs divided by N customersgreenfield per-customer cost similar to UTPpassive splitters translate to lower costcan be installed anywhereno power neededessentially unlimited MTBFfiber data-rates can be upgraded as technology improvesinitially 155 Mbpsthen 622 Mbpsnow 1.25 Gbpssoon 2.5 Gbps and higher

PONs Slide *PONarchitecture

PONs Slide *Terminologylike every other field, PON technology has its own terminologythe CO head-end is called an OLTONUs are the CPE devices (sometimes called ONTs in ITU)the entire fiber tree (incl. feeder, splitters, distribution fibers) is an ODNall trees emanating from the same OLT form an OANdownstream is from OLT to ONU (upstream is the opposite direction)

PONs Slide *PON typesmany types of PONs have been defined APONATM PON BPONBroadband PON GPONGigabit PON EPONEthernet PON GEPONGigabit Ethernet PON CPONCDMA PON WPONWDM PONin this course we will focus on GPON and EPON (including GEPON)with a touch of BPON thrown in for the flavor

PONs Slide *BibliographyBPON is explained in ITU-T G.983.xGPON is explained in ITU-T G.984.xEPON is explained in IEEE 802.3-2005 clauses 64 and 65(but other 802.3 clauses are also needed)Warningdo not believe white papers from vendorsespecially not with respect to GPON/EPON comparisons

EPONBPONGPON

PONs Slide *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 ls for transmit and receiveoptionally: Wavelength Division Multiplexerdownstream transmissionOLT broadcasts data downstream to all ONUs in ODNONU captures data destined for its address, discards all other dataencryption needed to ensure privacyupstream transmissionONUs share bandwidth using Time Division Multiple AccessOLT manages the ONU timeslotsranging is performed to determine ONU-OLT propagation timeadditional functionalityPhysical Layer OAMAutodiscoveryDynamic Bandwidth Allocation

PONs Slide *Why a new protocol ?PON has a unique architecture(broadcast) point-to-multipoint in DS direction(multiple access) multipoint-to-point in US directioncontrast that with, for example Ethernet - multipoint-to-multipointATM - point-to-pointThis means that existing protocolsdo not provide all the needed functionalitye.g. receive filtering, ranging, security, BW allocation

PONs Slide *(multi)point - to - (multi)pointMultipoint-to-multipoint Ethernet avoids collisionsby CSMA/CDThis can't work for multipoint-to-point US PONsince ONUs don't see each otherAnd the OLT can't arbitrate without adding a roundtrip time

Point-to-point ATM can send data in the openalthough trusted intermediate switches see all datacustomer switches only receive their own dataThis can't work for point-to-multipoint DS PONsince all ONUs see all DS data

PONs Slide *PON encapsulationThe majority of PON traffic is EthernetSo EPON enthusiasts say use EPON - it's just EthernetThat's true by definition - anything in 802.3 is Ethernetand EPON is defined in clauses 64 and 65 of 802.3-2005But don't be fooled - all PON methods encapsulate MAC framesEPON and GPON differ in the contents of the headerEPON hides the new header inside the GbE preambleGPON can also carry non-Ethernet payloads

PONs Slide *BPON history1995 : 7 operators (BT, FT, NTT, ) and a few vendors form Full Service Access Network Initiativeto 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.983G.982 : PON requirements and definitionsG.983.1 : 155 Mbps BPONG.983.2 : management and control interfaceG.983.3 : WDM for additional servicesG.983.4 : DBAG.983.5 : enhanced survivability G.983.1 amd 1 : 622 Mbps rateG.983.1 amd 2 : 1244 Mbps rate

PONs Slide *EPON history2001: IEEE 802 LMSC WG accepts Ethernet in the First Mile Project Authorization Requestbecomes EFM task force (largest 802 task force ever formed)EFM task force had 4 tracksDSL (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 standard 2005: new 802.3 version with EFM clauses

PONs Slide *GPON history2001 : FSAN initiated work on extension of BPON to > 1 GbpsAlthough GPON is an extension of BPON technologyand 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.984G.984.1 : GPON general characteristicsG.984.2 : Physical Media Dependent layerG.984.3 : Transmission Convergence layerG.984.4 : management and control interface

PONs Slide *

Fiber optics - basics

PONs Slide * = sin (n2/n1)1V =c/nt = Ln/ct = Propagation Timet Vacuum: n=1, t=3.336ns/mt Water : n=1.33, t=4.446ns/mTotal Internal Reflection in Step-Index Multimode Fiber

PONs Slide *Multimode Graded-Index FiberSingle-mode FiberTypes of Optical FiberPopular Fiber Sizes

PONs Slide *Click to edit Master text stylesSecond levelThird levelFourth levelOptical Loss versus Wavelength

PONs Slide *Total Dispersion Multimode DispersionChromatic DispersionMaterial DispersionSources of Dispersion

PONs Slide *1011Multimode DispersionDispersion limits bandwidth in optical fiber

PONs Slide *1011Graded-index Dispersion

PONs Slide *101110 In SM the limit bandwidth is caused by chromatic dispersion.1Single-Mode Dispersion

PONs Slide *How to calculate bandwidth?Tc = (20ps/nm * km) * 5nm * 15km = 1.5nsTc = Dmat * * LTc = (20ps/nm * km) * 0.2nm * 60km = 0.24nsFor Laser 1550nm Fabry Perot For Laser 1550nm DFBFor a 1.25 Gb/s we need a BW of 0.7 BitRate = 1.143nsSystem Design Consideration

PONs Slide *Material Dispersion (Dmat)

PONs Slide *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.

Spectral Characteristics

PONs Slide *LaserWLaser Optical Power Output vs. Forward Current

PONs Slide *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

Light Detectors

PONs Slide *Wavelength-Division Multiplexing

PONs Slide *WDM Duplexing

PONs Slide *BMCDR = Burst Mode Clock Data RecoveryOLT = Optical Line TerminationONU = Optical Network UnitBasic Configuration of PON

PONs Slide *Typical PON Configuration and Optical Packets

PONs Slide *Eye diagram of ONU transceiver in burst mode operation

PONs Slide *Burst-Mode Transmitter in ONU

PONs Slide *OLT Burst-Mode Receiver

PONs Slide *Burst-Mode CDR

PONs Slide *Ideal, error-free transmissionSuperimposed interferenceHysteresisIdeal sampling instantSampling

PONs Slide *Transceiver Block Diagram

PONs Slide *Optical Splitters

PONs Slide *Optical Protection SwitchOptical Splitter

PONs Slide *LB = PS - PO LB = Link BudgetPS = SensitivityPO = Output Power Example: GPON 1310nm Power: 0dbm Single-mode fiberSensitivity: -23dbm}Link Budget: 23dbBudget Calculations

PONs Slide *Assume:Optical loss = 0.35 db/kmConnector Loss = 2dBSplitter Insertion Loss 1X32 = 17dBRange Budget: ~11KmTypical Range Calculation

PONs Slide *Relationship between transmission distance and number of splits

PONs Slide *Gb Ethernet Fiber Optic Characteristics

Table 1 Giga Ethernet Fiber Optic characteristics

GbE Fiber Optic Characteristics

PONs Slide *PON physical layer

PONs Slide *l allocations - G.983.1Upstream and downstream directions need about the same bandwidthUS serves N customers, so it needs N times the BW of each customerbut 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 windowDownstream 1480 - 1580 nm (1530 50) third window

PONs Slide *l 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 bandsthese were made possible because of new inexpensive hardware (uncooled Distributed Feedback Lasers)One of the CWDM bands was 1490 10 nm same bottom l as the G.983.1 DSSo it was decided to use this band as the G.983.3 DSand leave the US unchanged

PONs Slide *l allocations - finalThe G.983.3 band-plan was incorporated into GPONand via liaison activity into EPON and is now the universally accepted xPON band-planUS 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 *Data rates (for now ) * only 1G/10G usable due to linecode work in progressAmd 1Amd 2

GPON

PONDS (Mbps)US (Mbps)BPON155.52155.52622.08155.52622.08622.081244.16155.521244.16622.081244.16155.521244.16622.081244.161244.162488.32155.522488.32622.082488.321244.162488.322488.32EPON1250*1250*10GEPON10312.5*10312.5*

PONs Slide *Reach and splitsReach and the number of ONUs supported are contradictory design goalsIn addition to physical reach derived from optical budgetthere 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 splitsbut also on the addressing schemeBPON called for 20 km and 32-64 ONUsGPON allows 64-128 splits and the reach is usually 20 kmbut 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 *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 propagationeach standard and each direction use different LFSRsLFSR initialized with all onesLFSR sequence is XOR'ed with data before transmission

EPON uses the 802.3z (1000BASE-X) line code - 8B/10BEvery 8 data bits are converted into 10 bits before transmissionDC removal and timing recovery ensured by mappingSpecial 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 *FECG984.3 clause 13 and 802.3-2005 subclause 65.2.3define 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 *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 can't use a single thresholdEPON: measure received power of ONU at beginning of burstGPON: OLT feedback to ONUs to properly set transmit power Burst laser problemSpontaneous emission noise from nearby ONU lasers causes interference Electrically shut ONU laser off when not transmittingBut lasers have long warm-up timeand ONU lasers must stabilize quickly after being turned on

PONs Slide *US timing diagramHow does the ONU US transmission appear to the OLT ?Notes: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 *PON User plane

PONs Slide *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 timeDBA - OLT allocates BW according to ONU queue levelsOLT identifies ONU traffic by labelOLT extracts traffic units and passes to network

OLT 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 *LabelsIn an ODN there is 1 OLT, but many ONUsONUs must somehow be labeled forOLT to identify the destination ONUONU to identify itself as the sourceEPON assigns a single label Logical Link ID to each ONU (15b)GPON has several levels of labelsONU_ID (1B) (1B)Transmission-CONTainer (AKA Alloc_ID) (12b) (can be >1 T-CONT per ONU)For ATM modeVPIVCIFor GEM modePort_ID (12b) (12b)

PONs Slide *DS GPON formatGPON Transmission Convergence frames are always 125 msec long19440 bytes / frame for 1244.16 rate38880 bytes / frame for 2488.32 rateEach GTC frame consists of Physical Control Block downstream + payloadPCBd contains sync, OAM, DBA info, etc.payload may have ATM and GEM partitions (either one or both)

PONs Slide *GPON payloadsGTC payload potentially has 2 sections:ATM partition (Alen * 53 bytes in length)GEM partition (now preferred method)ATM partitionAlen (12 bits) is specified in the PCBdAlen specifies the number of 53B cells in the ATM partitionif Alen=0 then no ATM partitionif 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 *cntatasemblyGPON Encapsulation ModeA common complaint against BPON was inefficiency due to ATM cell taxGEM is similar to ATMconstant-size HEC-protected header but avoids large overhead by allowing variable length frames GEM 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 BytesPort ID - identifies the target ONUPayload 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 transmission5 B

PONs Slide *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)

When transporting TDM traffic over GEM:TDM input buffer polled every 125 msec.PLI bytes of TDM are inserted into payload fieldlength of TDM fragment may vary by 1 Byte due to frequency offset round-trip latency bounded by 3 msec.

PONs Slide *GEM fragmentationGEM can fragment its payloadFor example

GEM fragments payloads for either of two reasons:GEM frame may not straddle GTC frame

GEM frame may be pre-empted for delay-sensitive data

PONs Slide *PCBdWe saw that the PCBd is

PSync - 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 BytesAlen - Next 12 bits are length of ATM partition in cellsCRC - 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)B6AB31E0

PONs Slide *GPON US considerationsGTC fames are still 125 msec long, but shared amongst ONUsEach ONU transmits a burst of datausing timing acquired by locking onto OLT signalaccording to time allocation sent by OLT in BWmapthere may be multiple allocations to single ONUOLT 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 burstsleaving a guard time from previous ONU's transmissionprefixing a preamble to enable OLT to acquire power and phaseidentifying itself (ONU-ID) in addition to traffic IDs (VPI, Port-ID)scrambling data (but not preamble/delimiter)

PONs Slide *US GPON format4 different US overhead types:Physical Layer Overhead upstreamalways sent by ONU when taking over from another ONUcontains 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 ONUsDynamic Bandwidth Report upstreamsends traffic status to OLT in order to enable DBA computation

PONs Slide *US allocation example BWmap sent by OLT to ONUs is a list of ONU allocation IDsflags (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 *EPON formatEPON operation is based on the Ethernet MACand EPON frames are based on GbE framesbut extensions are neededclause 64 - MultiPoint Control Protocol PDUs this is the control protocol implementing the required logicclause 65 - point-to-point emulation (reconciliation) this makes the EPON look like a point-to-point link

and EPON MACs have some special constraintsinstead of CSMA/CD they transmit when grantedtime through MAC stack must be constant ( 16 bit durations)accurate local time must be maintained

PONs Slide *EPON headerStandard Ethernet starts with an essentially content-free 8B preamble7B of alternating ones and zeros 101010101B of SFD 10101011In order to hide the new PON headerEPON overwrites some of the preamble bytes

LLID field containsMODE (1b) always 0 for ONU0 for OLT unicast, 1 for OLT multicast/broadcastactual Logical Link ID (15b)Identifies registered ONUs7FFF for broadcastCRC protects from SLD (byte 3) through LLID (byte 7)

PONs Slide *MPC PDU formatMultiPoint Control Protocol frames are untagged MAC frames with the same format as PAUSE frames

Ethertype = 8808Opcodes (2B) - presently defined:GATE/REPORT/REGISTER_REQ/REGISTER/REGISTER_ACKTimestamp is 32b, 16 ns resolutionconveys the sender's time at time of MPCPDU transmissionData field is needed for some messages

PONs Slide *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 neededdo not take fiber-tappers into accountEPON does not provide any standard encryption methodcan supplement with IPsec or MACsecmany vendors have added proprietary AES-based mechanismsin China special China Telecom encryption algorithmBPON used a mechanism called churningChurning was a low cost hardware solution (24b key)with several security flawsengine was linear - simple known-text attack24b key turned out to be derivable in 512 triesSo G.983.3 added AES support - now used in GPON

PONs Slide *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 ONUs46b = 16b intra-frame + 30 bits inter-frameintra-frame counter increments every 4 data bytesreset 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 *QoS - EPONMany PON applications require high QoS (e.g. IPTV)EPON leaves QoS to higher layersVLAN tagsP bits or DiffServ DSCPIn addition, there is a crucial difference between LLID and Port-IDthere is always 1 LLID per ONUthere is 1 Port-ID per input port - there may be many per ONUthis makes port-based QoS simple to implement at PON layer

PONs Slide *QoS - GPONGPON treats QoS explicitlyconstant length frames facilitate QoS for time-sensitive applications5 types of Transmission CONTainerstype 1 - fixed BWtype 2 - assured BWtype 3 - allocated BW + non-assured BWtype 4 - best efforttype 5 - superset of all of the aboveGEM adds several PON-layer QoS featuresfragmentation enables pre-emption of large low-priority framesPLI - explicit packet length can be used by queuing algorithmsPTI bits carry congestion indications

PONs Slide *PON control plane

PONs Slide *PrinciplesGPON uses PLOAMd and PLOAMu as control channelPLOAM are incorporated in regular (data-carrying) framesStandard ITU control mechanism

EPON uses MPCP PDUsStandard IEEE control mechanismEPON control model - OLT is master, ONU is slaveOLT sends GATE PDUs DS to ONU ONU sends REPORT PDUs US to OLT

PONs Slide *RangingUpstream traffic is TDMAWere all ONUs equidistant, and were all to have a common clockthen each would simply transmit in its assigned timeslotBut otherwise the signals will overlapTo eliminate overlapguard times left between timeslotseach ONU transmits with the proper delay to avoid overlapdelay computed during a ranging process

PONs Slide *Ranging backgroundIn order for the ONU to transmit at the correct timethe delay between ONU transmission and OLT reception needs to be known (explicitly or implicitly)Need to assign an equalization-delayThe more accurately it is knownthe smaller the guard time that needs to be leftand thus the higher the efficiencyAssumptions behind the ranging methods used:can not assume US delay is equal to DS delaydelays are not constant due to temperature changes and component agingGPON: ONUs not time synchronized accurately enoughEPON: ONUs are accurately time synchronized (std contains jitter masks) with time offset by OLT-ONU propagation time

PONs Slide *GPON ranging methodTwo types of ranginginitial rangingonly performed at ONU boot-up or upon ONU discoverymust be performed before ONU transmits first timecontinuous ranging performed continuously to compensate for delay changesOLT initiates coarse ranging by stopping allocations to all other ONUsthus 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 *EPON ranging methodAll ONUs are synchronized to absolute time (wall-clock)When an ONU receives an MPCPDU from OLTit sets its clock according to the OLT's timestampWhen the OLT receives an MPCPDU in response to its MPCPDUit computes a "round-trip time" RTT (without handling times)it informs the ONU of RTT, which is used to compute transmit delay

RTT = (T2-T0) - (T1-T0) = T2-T1OLT compensates all grants by RTT before sendingEither ONU or OLT can detect that timestamp drift exceeds thresholdtimeOLT sends MPCPDUTimestamp = T0ONU receives MPCPDUSets clock to T0ONU sends MPCPDUTimestamp = T1OLT receives MPCPDURTT = T2 - T1T0OLT timeT2T0ONU timeT1

PONs Slide *AutodiscoveryOLT needs to know with which ONUs it is communicatingThis can be established via NMSbut even then need to setup physical layer parametersPONs employ autodiscovery mechanism to automatediscovery of existence of ONUacquisition of identityallocation of identifieracquisition of ONU capabilitiesmeasure physical layer parametersagree on parameters (e.g. watchdog timers)Autodiscovery procedures are complex (and uninteresting)so we will only mention highlights

PONs Slide *GPON autodiscoveryEvery ONU has an 8B serial number (4B vendor code + 4B SN)SN of ONUs in OAN may be configured by NMS, orSN may be learnt from ONU in discovery phaseONU activation may be triggered byOperator commandPeriodic polling by OLTOLT searching for previously operational ONUG.984.3 differentiates between three cases:cold PON / cold ONUwarm PON / cold ONUwarm PON / warm ONUMain steps in procedure:ONU sets power based on DS messageOLT sends a Serial_Number request to all unregistered ONUsONU responds OLT assigns 1B ONU-ID and sends to ONUranging is performedONU is operational

PONs Slide *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 requestregisters ONUassigns LLIDbonds MAC to LLIDperforms ranging computationOLT sends REGISTER to ONUOLT sends standard GATE to ONUONU responds with REGISTER_ACKONU goes into operational mode - waits for grants

PONs Slide *Failure recoveryPONs must be able to handle various failure statesGPONif ONU detects LOS or LOF it goes into POPUP state it stops sending traffic USOLT detects LOS for ONUif there is a pre-ranged backup fiber then switch-overEPONduring normal operation ONU REPORTs reset OLT's watchdog timersimilarly, OLT must send GATES periodically (even if empty ones)if OLT's watchdog timer for ONU times outONU is deregistered

PONs Slide *Dynamic Bandwidth AllocationMANs and WANs have relatively stationary BW requirementsdue 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 discoveredby passively observing the traffic from the ONUby ONU sending reports as to state of its ingress queuesThe goals of a Dynamic Bandwidth Allocation algorithm are maximum fiber BW utilizationfairness and respect of priorityminimum delay introduced

PONs Slide *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 methodsstatus in PLOu - one bit for each T-CONT typepiggy-back reports in DBRu - 3 different formats:quantity of data waiting in buffers,separation of data with peak and sustained rate tokensnonlinear coding of data according to T-CONT type and tokensONU report in DBA payload - select T-CONT states OLT may use any DBA algorithmOLT sends allocations in US BW map

PONs Slide *EPON DBAOLT sends GATE messages to ONUs

flags include DISCOVERY and Force_ReportForce_Report tells the ONU to issue a report

Reports represent the length of each queue at time of reportOLT may use any algorithm to decide how to send the following grantsGATE messageREPORT message