Passive Optical Networks Yaakov J Stein May 2007

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

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

PON benefits 3

Why fiber ? today’s high datarate networks are all based on optical fiber the reason is simple (examples for demonstration sake) n twisted copper pair(s) – 8 Mbps @ 3 km, 1. 5 Mbps @ 5. 5 km (ADSL) – 1 Gb @ 100 meters (802. 3 ab) n microwave – 70 Mbps @ 30 km (Wi. Max) n coax – 10 Mbps @ 3. 6 km (10 BROAD 36) – 30 Mbps @ 30 km (cable modem) n optical fiber – 10 Mbps @ 2 km (10 BASE-FL) – 100 Mbps @ 400 m (100 BASE-FX) – 1 Gbps @ 2 km (1000 BASE-LX) – 10 Gbps @ 40 (80) km (10 GBASE-E(Z)R) – 40 Gbps @ 700 km [Nortel] or 3000 km [Verizon] 4

Aside – why is fiber better ? attenuation per unit length n reasons for energy loss – copper: resistance, skin effect, radiation, coupling – fiber: internal scattering, imperfect total internal reflection n so fiber beats coax by about 2 orders of magnitude – e. g. 10 d. B/km for thin coax at 50 MHz, 0. 15 d. B/km l =1550 nm fiber noise ingress and cross-talk n copper couples to all nearby conductors n no similar ingress mechanism for fiber ground-potential, galvanic isolation, lightning protection n copper can be hard to handle and dangerous n no concerns for fiber 5

Why not fiber ? fiber beats all other technologies for speed and reach but fiber has its own problems n harder to splice, repair, and need to handle carefully n regenerators and even amplifiers are problematic – more expensive to deploy than for copper n digital processing requires electronics – so need to convert back to electronics – we will call the converter an optical transceiver – optical transceivers are expensive n copper fiber switching easier with electronics (but possible with photonics) – so pure fiber networks are topologically limited: l point-to-point l rings 6

Access network bottleneck hard for end users to get high datarates because of the access bottleneck local area networks n use copper cable n get high datarates over short distances core networks n use fiber optics n get high datarate over long distances n small number of active network elements access core access networks (first/last mile) LAN n long distances – so fiber would be the best choice n 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 7

Fiber To The Curb Hybrid Fiber Coax and VDSL n switch/transceiver/mini. DSLAM located at curb or in basement n need only 2 optical transceivers but not pure optical solution n lower BW from transceiver to end users n need complex converter in constrained environment core N end users feeder fiber copper access network 8

Fiber To The Premises we can implement point-to-multipoint topology purely in optics n but we need a fiber (pair) to each end user n requires 2 N optical transceivers n complex and costly to maintain core N end users access network 9

An obvious solution deploy intermediate switches n (active) switch located at curb or in basement n saves space at central office n need 2 N + 2 optical transceivers core N end users feeder fiber access network 10

The PON solution another alternative - implement point-to-multipoint topology purely in optics n avoid costly optic-electronic conversions n use passive splitters – no power needed, unlimited MTBF n only N+1 optical transceivers (minimum possible) ! access network 1: 2 passive splitter core N end users typically N=32 feeder fiber 1: 4 passive splitter max defined 128 11

PON advantages shared infrastructure translates to lower cost per customer n minimal number of optical transceivers n feeder fiber and transceiver costs divided by N customers n greenfield per-customer cost similar to UTP passive splitters translate to lower cost n can be installed anywhere n no power needed n essentially unlimited MTBF fiber data-rates can be upgraded as technology improves n initially 155 Mbps n then 622 Mbps n now 1. 25 Gbps n soon 2. 5 Gbps and higher 12

PON architecture 13

Terminology like every other field, PON technology has its own terminology n the CO head-end is called an OLT n ONUs are the CPE devices (sometimes called ONTs in ITU) n the entire fiber tree (incl. feeder, splitters, distribution fibers) is an ODN n all trees emanating from the same OLT form an OAN n downstream is from OLT to ONU (upstream is the opposite direction) downstream upstream NNI core Optical Distribution Network Optical Line Terminal Optical Network Units splitter Optical Access Network UNI Terminal Equipment 14

PON types many types of PONs have been defined APON ATM PON Broadband PON Gigabit PON Ethernet PON GEPON Gigabit Ethernet PON CDMA PON WDM PON in this course we will focus on GPON and EPON (including GEPON) with a touch of BPON thrown in for the flavor 15

Bibliography n n n 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 16

PON principles (almost) all PON types obey the same basic principles OLT and ONU consist of n Layer 2 (Ethernet MAC, ATM adapter, etc. ) n optical transceiver using different ls for transmit and receive n optionally: Wavelength Division Multiplexer downstream transmission n OLT broadcasts data downstream to all ONUs in ODN n ONU captures data destined for its address, discards all other data n encryption needed to ensure privacy upstream transmission n ONUs share bandwidth using Time Division Multiple Access n OLT manages the ONU timeslots n ranging is performed to determine ONU-OLT propagation time additional functionality n Physical Layer OAM n Autodiscovery n Dynamic Bandwidth Allocation 17

Why a new protocol ? downstream upstream PON has a unique architecture n (broadcast) point-to-multipoint in DS direction n (multiple access) multipoint-to-point in US direction contrast that with, for example n n 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 18

(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 19

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 Gb. E preamble GPON can also carry non-Ethernet payloads PON header DA SA T data FCS 20

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 15 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 – … 21

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 n DSL (now in clauses 61, 62, 63) n Ethernet OAM (now clause 57) n Optics (now in clauses 58, 59, 60, 65) n P 2 MP (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 22

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 23

Fiber optics - basics 24

Total Internal Reflection in Step-Index Multimode Fiber © = sin¯ 1(n 2/n 1) V =c/n t = Propagation Time t Vacuum: n=1, t=3. 336 ns/m t Water : n=1. 33, t=4. 446 ns/m t = L·n/c 25

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

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

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

Multimode Dispersion 1 0 1 1 1 Dispersion limits bandwidth in optical fiber 29 11

Graded-index Dispersion 1 0 11 1 0 1 30

Single-Mode Dispersion 1 0 11 1 0 1 In SM the limit bandwidth is caused by chromatic dispersion. 31

System Design Consideration How to calculate bandwidth? For a 1. 25 Gb/s we need a BW of 0. 7 Bit. Rate = 1. 143 ns Tc = Dmat * * L For Laser 1550 nm Fabry Perot Tc = (20 ps/nm * km) * 5 nm * 15 km = 1. 5 ns For Laser 1550 nm DFB Tc = (20 ps/nm * km) * 0. 2 nm * 60 km = 0. 24 ns 32

Material Dispersion (Dmat) 33

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. 34

Laser Optical Power Output vs. Forward Current W Laser 35

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 36

Wavelength-Division Multiplexing 37

WDM Duplexing 38

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

Typical PON Configuration and Optical Packets 40

Eye diagram of ONU transceiver in burst mode operation 41

Burst-Mode Transmitter in ONU 42

OLT Burst-Mode Receiver 43

Burst-Mode CDR 44

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

Transceiver Block Diagram 46

Optical Splitters 47

Optical Protection Switch Optical Splitter 48

Budget Calculations LB = ׀ P S ׀ - ׀ P O ׀ = Link Budget PS = Sensitivity PO = Output Power LB Example: GPON 1310 nm Power: 0 dbm Single-mode fiber Sensitivity: -23 dbm } Link Budget: 23 db 49

Typical Range Calculation Assume: Optical loss = 0. 35 db/km Connector Loss = 2 d. B Range Budget: ~11 Km Splitter Insertion Loss 1 X 32 = 17 d. B 50

Relationship between transmission distance and number of splits 51

Gb. E Fiber Optic Characteristics 52

PON physical layer 53

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 ! n Upstream 1260 - 1360 nm (1310 ± 50) second window n Downstream 1480 - 1580 nm (1530 ± 50) third window US 1200 nm 1300 nm DS 1400 nm 1500 nm 1600 nm 54

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 same bottom l as the G. 983. 1 DS 1270 1630 1490 So it was decided to use this band as the G. 983. 3 DS and leave the US unchanged guard available US 1200 nm 1300 nm DS 1400 nm 1500 nm 1600 nm 55

allocations - final US 1200 nm 1300 nm DS 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 x. PON band-plan n US 1260 -1360 nm (1310 ± 50) n DS 1480 -1500 nm (1490 ± 10) n enhancement bands: – video 1550 - 1560 nm (see ITU-T J. 185/J. 186) – digital 1539 -1565 nm 56

Data rates (for now …) PON BPON Amd 1 Amd 2 GPON EPON 10 GEPON† DS (Mbps) 155. 52 622. 08 1244. 16 2488. 32 1250* 10312. 5* US (Mbps) 155. 52 622. 08 1244. 16 155. 52 622. 08 1244. 16 2488. 32 1250* 10312. 5* * only 1 G/10 G usable due to linecode † work in progress 57

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 d. B, but now 30 d. B) and has 10 km and 20 km Physical Media Dependent sublayers 58

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. 3 z (1000 BASE-X) line code - 8 B/10 B – 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 10 Gb. E uses a different linecode - 64 B/66 B 59

FEC G 984. 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 d. B, 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 8 B 10 B decode - FEC - 8 B 10 B encode) 60

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 61

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

PON User plane 63

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 64

Labels In an ODN there is 1 OLT, but many 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 (15 b) GPON has several levels of labels – ONU_ID (1 B) – Transmission-CONTainer (AKA Alloc_ID) (12 b) (can be >1 T-CONT per ONU) For ATM mode l VPI VC VP VC ONU T-CONT VP l VCI VC VC For GEM mode PON Port l Port_ID (12 b) ONU T-CONT Port 65

DS GPON format GPON Transmission Convergence frames are always 125 msec 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 PCBd payload PSync (4 B) Ident (4 B) 125 msec scrambled PCBd payload PLOAMd (13 B) PLend (4 B) BIP (1 B) PCBd payload ATM partition GEM partition US BW map (N*8 B) 66

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 GEM frame … GEM frame ATM partition Alen (12 bits) is specified in the PCBd Alen specifies the number of 53 B 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 12 b Port-ID in GEM header 67

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+ 1 b even parity) The GEM header is XOR'ed with B 6 AB 31 E 055 before transmission PLI (12 b) Port ID (12 b) 5 B PTI (3 b) HEC (13 b) payload fragment (L Bytes) 68

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 msec. – 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 69

GEM fragmentation GEM can fragment its payload For example unfragmented Ethernet frame PLI ID PTI=001 HEC DA SA T T data FCS fragmented Ethernet frame PLI ID PTI=000 HEC DA SA PLI ID PTI=001 HEC data 2 data 1 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 may be pre-empted for delay-sensitive data PCBd ATM partition urgent frame … large frag 1 PCBd ATM partition urgent frame … GEM frame large frag 2 70

PCBd We saw that the PCBd is PSync (4 B) B 6 AB 31 E 0 Ident (4 B) PLOAMd BIP PLend US BW map (13 B) (1 B) (4 B) (N*8 B) 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 8 B structures granting BW to US flow will discuss later (DBA) 71

GPON US considerations GTC fames are still 125 msec 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 l there may be multiple allocations to single ONU l OLT computes DBA by monitoring traffic status (buffers) of ONUs and knowing priorities – at power level requested by OLT (3 levels) l 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) 72

US GPON format 4 different US overhead types: n 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 (1 B), ONU-ID (1 B), and Indication of real-time status (1 B) n PLOAM upstream (13 B) - messaging with PLOAMd n Power Levelling Sequence upstream (120 B) – used during power-set and power-change to help set ONU power so that OLT sees similar power from all ONUs n 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 73

US allocation example DS frame PCBd BWmap payload Alloc-ID SStart SStop Alloc-ID SStart Sstop Alloc-ID SStart SStop US frame preamble + delimiter guard time scrambled BWmap sent by OLT to ONUs is a list of n ONU allocation IDs n flags (not shown above) tell if use FEC, which US OHs to use, etc. n start and stop times (16 b fields, in Bytes from beginning of US frame) 74

EPON format EPON operation is based on the Ethernet MAC and EPON frames are based on Gb. E frames but extensions are needed clause 64 - Multi. Point Control Protocol PDUs this is the control protocol implementing the required logic n clause 65 - point-to-point emulation (reconciliation) this makes the EPON look like a point-to-point link n and EPON MACs have some special constraints n instead of CSMA/CD they transmit when granted n time through MAC stack must be constant (± 16 bit durations) n accurate local time must be maintained 75

EPON header Standard Ethernet starts with an essentially content-free 8 B preamble n 7 B of alternating ones and zeros 1010 n 1 B of SFD 10101011 In order to hide the new PON header EPON overwrites some of the preamble bytes 10101010 10101010 10101011 10101010 LLID CRC LLID field contains – MODE (1 b) l always 0 for ONU l 0 for OLT unicast, 1 for OLT multicast/broadcast – actual Logical Link ID (15 b) l Identifies registered ONUs l 7 FFF for broadcast CRC protects from SLD (byte 3) through LLID (byte 7) 76

MPC PDU format Multi. Point 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 (2 B) - presently defined: GATE/REPORT/REGISTER_REQ/REGISTER_ACK Timestamp is 32 b, 16 ns resolution conveys the sender's time at time of MPCPDU transmission Data field is needed for some messages 77

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 (24 b key) with several security flaws – engine was linear - simple known-text attack – 24 b key turned out to be derivable in 512 tries So G. 983. 3 added AES support - now used in GPON 78

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 – 46 b = 16 b intra-frame + 30 bits inter-frame – intra-frame counter increments every 4 data bytes l 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 79

Qo. S - EPON Many PON applications require high Qo. S (e. g. IPTV) EPON leaves Qo. S to higher layers – VLAN tags – P bits or Diff. Serv 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 Qo. S simple to implement at PON layer RT EF BE GPON 80

Qo. S - GPON treats Qo. S explicitly – constant length frames facilitate Qo. S for time-sensitive applications – 5 types of Transmission CONTainers l type 1 - fixed BW l type 2 - assured BW l type 3 - allocated BW + non-assured BW l type 4 - best effort l type 5 - superset of all of the above GEM adds several PON-layer Qo. S 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 81

PON control plane 82

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 83

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 n guard times left between timeslots n each ONU transmits with the proper delay to avoid overlap n delay computed during a ranging process 84

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 n delays are not constant – due to temperature changes and component aging n GPON: ONUs not time synchronized accurately enough n EPON: ONUs are accurately time synchronized (std contains jitter masks) with time offset by OLT-ONU propagation time n 85

GPON ranging method Two types of ranging – initial ranging l only performed at ONU boot-up or upon ONU discovery l 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) 86

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 Timestamp = T 0 Sets clock to T 0 ONU sends MPCPDU Timestamp = T 1 OLT receives MPCPDU RTT = T 2 - T 1 time OLT time ONU time T 0 T 1 T 2 RTT = (T 2 -T 0) - (T 1 -T 0) = T 2 -T 1 OLT compensates all grants by RTT before sending Either ONU or OLT can detect that timestamp drift exceeds threshold 87

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 88

GPON autodiscovery Every ONU has an 8 B serial number (4 B vendor code + 4 B 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 / 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 1 B ONU-ID and sends to ONU – ranging is performed – ONU is operational 89

EPON autodiscovery OLT periodically transmits DISCOVERY GATE messages ONU waits for DISCOVERY GATE to be broadcast by OLT DISCOVERY GATE message defines discovery window l start time and duration ONU transmits REGISTER_REQ PDU using random offset in window OLT receives request l registers ONU l assigns LLID l bonds MAC to LLID l performs ranging computation OLT sends REGISTER to ONU OLT sends standard GATE to ONU responds with REGISTER_ACK ONU goes into operational mode - waits for grants 90

Failure recovery PONs must be able to handle various failure states GPON if ONU detects LOS or LOF it goes into POPUP state l it stops sending traffic US l OLT detects LOS for ONU l 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 l ONU is deregistered 91

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 92

GPON 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 n status in PLOu - one bit for each T-CONT type n 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 n ONU report in DBA payload - select T-CONT states OLT may use any DBA algorithm OLT sends allocations in US BW map 93

EPON DBA OLT sends GATE messages to ONUs GATE message DA SA 8808 Opcode=0002 timestamp Ngrants/flags grants … Reports … flags include DISCOVERY and Force_Report tells the ONU to issue a report REPORT message DA SA 8808 Opcode=0003 timestamp Nqueue_sets Reports represent the length of each queue at time of report OLT may use any algorithm to decide how to send the following grants 94