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Passive Optical Networks Yaakov (J) Stein May 2007 and Zvika Eitan.

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Presentation on theme: "Passive Optical Networks Yaakov (J) Stein May 2007 and Zvika Eitan."— Presentation transcript:

1 Passive Optical Networks Yaakov (J) Stein May 2007 and Zvika Eitan

2 PONs Slide 2 Outline PON benefits PON architecture Fiber optic basics PON physical layer PON user plane PON control plane

3 PONs Slide 3 PON benefits

4 PONs Slide 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 3 km, km (ADSL) –1 100 meters (802.3ab) microwave –70 30 km (WiMax) coax – km (10BROAD36) –30 30 km (cable modem) optical fiber –10 2 km (10BASE-FL) – m (100BASE-FX) –1 2km (1000BASE-LX) –10 40 (80) km (10GBASE-E(Z)R) – km [Nortel] or 3000 km [Verizon]

5 PONs Slide 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

6 PONs Slide 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 –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 copper fiber

7 PONs Slide 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 small number of active network elements access networks (first/last mile) long distances –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 coreaccess LAN

8 PONs Slide 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 access network feeder fiber copper

9 PONs Slide 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

10 PONs Slide 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 access network feeder fiber fiber

11 PONs Slide 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) ! 1:2 passive splitter 1:4 passive splitter N end users feeder fiber core access network typically N=32 max defined 128

12 PONs Slide 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

13 PONs Slide 13 PON architecture

14 PONs Slide 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 Optical Network Units upstream Optical Distribution Network NNI Terminal Equipment UNI core splitter Optical Line Terminal Optical Access Network

15 PONs Slide 15 PON types many types of PONs have been defined APONATM PON BPONBroadband PON GPONGigabit PON EPONEthernet PON GEPONGigabit Ethernet PON CPONCDMA PON WPONWDM PON in this course we will focus on GPON and EPON (including GEPON) with a touch of BPON thrown in for the flavor

16 PONs Slide 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 clauses 64 and 65 –(but other clauses are also needed) Warning do not believe white papers from vendors especially not with respect to GPON/EPON comparisons EPON BPON GPON

17 PONs Slide 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

18 PONs Slide 18 Why a new protocol ? PON has a unique architecture (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 downstream upstream

19 PONs Slide 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

20 PONs Slide 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 is Ethernet and EPON is defined in clauses 64 and 65 of 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 DASATdataFCSPON header

21 PONs Slide 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 SG : BPON became G.983 –G.982 : PON requirements and definitions –G : 155 Mbps BPON –G : management and control interface –G : WDM for additional services –G : DBA –G : enhanced survivability –G amd 1 : 622 Mbps rate –G amd 2 : 1244 Mbps rate –…

22 PONs Slide 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 version with EFM clauses

23 PONs Slide 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 : GPON general characteristics –G : Physical Media Dependent layer –G : Transmission Convergence layer –G : management and control interface

24 PONs Slide 24 Fiber optics - basics

25 PONs Slide 25 © = sin¯ (n2/n1) 1 V =c/n t = L·n/c t = Propagation Time t Vacuum: n=1, t=3.336ns/m t Water : n=1.33, t=4.446ns/m Total Internal Reflection in Step-Index Multimode Fiber

26 PONs Slide 26 Multimode Graded- Index Fiber Single-mode Fiber Types of Optical Fiber Popular Fiber Sizes

27 PONs Slide 27 Click to edit Master text styles –Second level Third level – Fourth level Optical Loss versus Wavelength

28 PONs Slide 28 Total Dispersion Multimode Dispersion Chromatic Dispersion Material Dispersion Sources of Dispersion

29 PONs Slide Multimode Dispersion Dispersion limits bandwidth in optical fiber

30 PONs Slide Graded-index Dispersion 110

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

32 PONs Slide 32 How to calculate bandwidth? Tc = (20ps/nm * km) * 5nm * 15km = 1.5ns T c = D mat *  * L Tc = (20ps/nm * km) * 0.2nm * 60km = 0.24ns For Laser 1550nm Fabry Perot For Laser 1550nm DFB For a 1.25 Gb/s we need a BW of 0.7 BitRate = 1.143ns System Design Consideration

33 PONs Slide 33 Material Dispersion (Dmat)

34 PONs Slide 34 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

35 PONs Slide 35 Laser W Laser Optical Power Output vs. Forward Current

36 PONs Slide 36 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

37 PONs Slide 37 Wavelength-Division Multiplexing

38 PONs Slide 38 WDM Duplexing

39 PONs Slide 39 BMCDR = Burst Mode Clock Data Recovery OLT = Optical Line Termination ONU = Optical Network Unit Basic Configuration of PON

40 PONs Slide 40 Typical PON Configuration and Optical Packets

41 PONs Slide 41 Eye diagram of ONU transceiver in burst mode operation

42 PONs Slide 42 Burst-Mode Transmitter in ONU

43 PONs Slide 43 OLT Burst-Mode Receiver

44 PONs Slide 44 Burst-Mode CDR

45 PONs Slide 45 Ideal, error-free transmission Superimposed interference Hysteresis Ideal sampling instant Sampling

46 PONs Slide 46 Transceiver Block Diagram

47 PONs Slide 47 Optical Splitters

48 PONs Slide 48 Optical Protection Switch Optical Splitter

49 PONs Slide 49 L B = ׀ P S ׀ - ׀ P O ׀ L B = Link Budget P S = Sensitivity P O = Output Power Example: GPON 1310nm Power: 0dbm Single-mode fiber Sensitivity: -23dbm } Link Budget: 23db Budget Calculations

50 PONs Slide 50 Assume: Optical loss = 0.35 db/km Connector Loss = 2dB Splitter Insertion Loss 1X32 = 17dB Range Budget: ~11Km Typical Range Calculation

51 PONs Slide 51 Relationship between transmission distance and number of splits

52 PONs Slide 52 Gb Ethernet Fiber Optic Characteristics Table 1 Giga Ethernet Fiber Optic characteristics GbE Fiber Optic Characteristics

53 PONs Slide 53 PON physical layer

54 PONs Slide 54 allocations - G allocations - G 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 nm (1310 ± 50) second window Downstream nm (1530 ± 50) third window 1200 nm1300 nm1400 nm1500 nm1600 nm US DS

55 PONs Slide 55 allocations - G allocations - G 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 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 same bottom as the G DS So it was decided to use this band as the G DS and leave the US unchanged nm1300 nm1400 nm1500 nm1600 nm US DS available guard

56 PONs Slide 56 allocations - final allocations - final The G band-plan was incorporated into GPON and via liaison activity into EPON and is now the universally accepted xPON band-plan US nm (1310 ± 50) DS nm (1490 ± 10) enhancement bands: –video nm (see ITU-T J.185/J.186) –digital nm 1200 nm1300 nm1400 nm1500 nm1600 nm US DS

57 PONs Slide 57 Data rates (for now …) PONDS (Mbps) US (Mbps) BPON EPON1250* 10GEPON † * * only 1G/10G usable due to linecode † work in progress Amd 1 Amd 2 GPON

58 PONs Slide 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 ONUs GPON allows 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 splits (originally designed for link budget of 24 dB, but now 30 dB) and has 10 km and 20 km Physical Media Dependent sublayers

59 PONs Slide 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

60 PONs Slide 60 FEC G984.3 clause 13 and subclause 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)

61 PONs Slide 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

62 PONs Slide 62 US timing diagram How does the ONU US transmission appear to the OLT ? grant laser turn-on laser turn-off data lock laser turn-on laser turn-off data lock inter-ONU guard 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

63 PONs Slide 63 PON User plane

64 PONs Slide 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

65 PONs Slide 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 VCI For GEM mode Port_ID (12b) (12b) PON ONU T-CONT Port VP VC

66 PONs Slide 66 DS GPON format GPON Transmission Convergence frames are always 125  sec long –19440 bytes / frame for rate –38880 bytes / frame for 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) payloadPCBdpayloadPCBdpayloadPCBd GTC frame PSync (4B) Ident (4B) PLOAMd (13B) BIP (1B) PLend (4B) US BW map (N*8B) ATM partition GEM partition scrambled 125  sec

67 PONs Slide 67 GPON payloads GTC payload potentially has 2 sections: –ATM partition (Alen * 53 bytes in length) –GEM partition (now preferred method) 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 ATM cellPCBd … GEM frame … ATM cell

68 PONs Slide 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 Port ID (12b) PLI (12b) HEC (13b) PTI (3b) payload fragment (L Bytes) 5 B

69 PONs Slide 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) 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. DASATdataFCSPLI Ethernet over GEM IDPTIHECPLI Bytes of TDMPLI TDM over GEM IDPTIHEC

70 PONs Slide 70 GEM fragmentation GEM can fragment its payload For 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 DASATdataFCSPLI unfragmented Ethernet frame IDPTI=001HEC DASATdata 1 PLI fragmented Ethernet frame IDPTI=000HEC data 2 PLIIDPTI=001HECFCS ATM partitionPCBdGEM frame … GEM frag 1ATM partitionPCBdGEM frag 2 … GEM frameATM partitionPCBdurgent frame … large frag 1ATM partitionPCBdurgent frame … large frag 2

71 PONs Slide 71 PCBd We saw that the PCBd is 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) PSync (4B) Ident (4B) PLOAMd (13B) BIP (1B) PLend (4B) US BW map (N*8B) B6AB31E0

72 PONs Slide 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)

73 PONs Slide 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 PLOuPLOAMdPLSuDBRupayload if all OH types are present:

74 PONs Slide 74 US allocation example 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) payloadPCBd DS frame Alloc-ID SStart SStop Alloc-ID SStart Sstop Alloc-ID SStart SStop BWmap US frame guard time preamble + delimiter scrambled

75 PONs Slide 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

76 PONs Slide 76 EPON header Standard Ethernet starts with an essentially content-free 8B preamble 7B of alternating ones and zeros B of SFD In order to hide the new PON header EPON overwrites some of the preamble bytes 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) LLID CRC

77 PONs Slide 77 MPC PDU format MultiPoint Control Protocol frames are untagged MAC frames with the same format as PAUSE frames 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 DASAL/TOpcodetimestampdata / RES / padFCS

78 PONs Slide 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 added AES support - now used in GPON

79 PONs Slide 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

80 PONs Slide 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 RTBEEF GPON

81 PONs Slide 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

82 PONs Slide 82 PON control plane

83 PONs Slide 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

84 PONs Slide 84Ranging 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

85 PONs Slide 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

86 PONs Slide 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)

87 PONs Slide 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 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 time OLT sends MPCPDU Timestamp = T0 ONU receives MPCPDU Sets clock to T0 ONU sends MPCPDU Timestamp = T1 OLT receives MPCPDU RTT = T2 - T1 T0 OLT time T2 T0 ONU time T1

88 PONs Slide 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

89 PONs Slide 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 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

90 PONs Slide 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

91 PONs Slide 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

92 PONs Slide 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

93 PONs Slide 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

94 PONs Slide 94 EPON DBA OLT sends GATE messages to ONUs flags include DISCOVERY and Force_Report Force_Report tells the ONU to issue a report Reports represent the length of each queue at time of report OLT may use any algorithm to decide how to send the following grants DASA8808Opcode=0002timestampNgrants/flags…grantsDASA8808Opcode=0003timestampNqueue_sets…Reports GATE message REPORT message


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