Passive Optical Network

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Transcript Passive Optical Network

assive

ptical

etwork

Yaakov (J) Stein May 2007 and Zvika Eitan

     

PON benefits PON architecture Fiber optic basics PON physical layer PON user plane PON control plane

Outline

PONs Slide 2

PON benefits

PONs Slide 3

Why fiber ?

today’s high datarate networks are all based on optical fiber the reason is simple (examples for demonstration sake)     – twisted copper pair(s) 8 Mbps @ 3 km, 1.5 Mbps @ 5.5 km (ADSL) – 1 Gb @ 100 meters (802.3ab) – microwave 70 Mbps @ 30 km (WiMax) – coax 10 Mbps @ 3.6 km (10BROAD36) – 30 Mbps @ 30 km (cable modem) optical fiber – 10 Mbps @ 2 km (10BASE-FL) – – 100 Mbps @ 400m (100BASE-FX) 1 Gbps @ 2km (1000BASE-LX) – – 10 Gbps @ 40 (80) km (10GBASE-E(Z)R) 40 Gbps @ 700 km [Nortel] or 3000 km [Verizon]

PONs Slide 4

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 l =1550nm fiber

noise ingress and cross-talk

  copper couples to all nearby conductors no similar ingress mechanism for fiber

ground-potential, galvanic isolation, lightning protection

  copper can be hard to handle and dangerous no concerns for fiber

PONs Slide 5

**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 copper  switching easier with electronics (but possible with photonics) – so pure fiber networks are topologically limited:   point-to-point rings fiber

PONs Slide 6

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 core access networks (first/last mile)  long distances – so fiber would be the best choice  LAN many network elements and large number of endpoints – if fiber is used then need multiple optical transceivers – so copper is the best choice – this severely limits the datarates

PONs Slide 7

iber

o

he

urb

ybrid

iber

oax 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 core N end users feeder fiber copper access network

PONs Slide 8

iber

o

he

remises

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 core N end users access network

PONs Slide 9

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 core feeder fiber fiber access network N end users

PONs Slide 10

The PON solution

another alternative - implement point-to-multipoint topology purely in optics    avoid costly optic-electronic conversions use

passive splitters

– no power needed, unlimited MTBF only N+1 optical transceivers (minimum possible) !

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

PONs Slide 11

PON advantages

shared infrastructure translates to lower cost per customer    minimal number of optical transceivers feeder fiber and transceiver costs divided by N customers greenfield per-customer cost similar to UTP    passive splitters translate to lower cost can be installed anywhere no power needed essentially unlimited MTBF     fiber data-rates can be upgraded as technology improves initially 155 Mbps then 622 Mbps now 1.25 Gbps soon 2.5 Gbps and higher

PONs Slide 12

PON architecture

PONs Slide 13

Terminology

like every other field, PON technology has its own terminology      the CO head-end is called an OLT ONU s are the CPE devices (sometimes called ONT s 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) core downstream upstream NNI

ptical

istribution

etwork splitter

ptical

ine

erminal

ptical

etwork

nits UNI

ptical

ccess

etwork

erminal

quipment

PONs Slide 14

PON types

many types of PONs have been defined

APON

ATM PON

BPON GPON

Broadband PON Gigabit PON

EPON GEPON CPON WPON

Ethernet PON 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

PONs Slide 15

Bibliography

   BPON is explained in ITU-T G.983.x

GPON is explained in ITU-T G.984.x

EPON is explained in IEEE 802.3-2005 clauses 64 and 65 – (but other 802.3 clauses are also needed) Warning do not believe white papers from vendors especially not with respect to GPON/EPON comparisons GPON BPON EPON

PONs Slide 16

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 l s for transmit and receive optionally:

avelength

ivision

ultiplexer    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

ime

ivision

ultiple

ccess

OLT manages the ONU timeslots

ranging

is performed to determine ONU-OLT propagation time    additional functionality

hysical

ayer

OAM

Autodiscovery

ynamic

andwidth

llocation

PONs Slide 17

Why a new protocol ?

PON has a unique architecture   downstream upstream (broadcast) point-to-multipoint in DS direction (multiple access) multipoint-to-point in US direction contrast that with, for example   Ethernet - multipoint-to-multipoint ATM - point-to-point This means that existing protocols do not provide all the needed functionality e.g. receive filtering, ranging, security, BW allocation

PONs Slide 18

(multi)point - to - (multi)point

Multipoint-to-multipoint Ethernet avoids collisions by CSMA/CD This can't work for multipoint-to-point US PON since ONUs don't see each other And the OLT can't arbitrate without adding a roundtrip time Point-to-point ATM can send data in the open although trusted intermediate switches see all data customer switches only receive their own data This can't work for point-to-multipoint DS PON since all ONUs see all DS data

PONs Slide 19

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

Ethernet and EPON is defined in clauses 64 and 65 of 802.3-2005 But

don't be fooled

- all PON methods

encapsulate

MAC frames EPON and GPON differ in the

contents

of the header EPON hides the new header inside the GbE preamble GPON can also carry

non-Ethernet

payloads PON header DA SA T data FCS

PONs Slide 20

BPON history

1995 : 7 operators (BT, FT, NTT, …) and a few vendors form

ull

ervice

ccess

etwork Initiative to provide business customers with multiservice broadband offering Obvious choices were ATM (multiservice) and PON (inexpensive) which when merged became APON 1996 : name changed to BPON to avoid too close association with ATM 1997 : FSAN proposed BPON to ITU SG15 1998 : BPON became G.983

– G.982 : PON requirements and definitions – G.983.1 : 155 Mbps BPON – – – – G.983.2 : management and control interface G.983.3 : WDM for additional services G.983.4 : DBA G.983.5 : enhanced survivability – – – G.983.1 amd 1 : 622 Mbps rate G.983.1 amd 2 : 1244 Mbps rate

… PONs Slide 21

EPON history

2001: IEEE 802 LMSC WG accepts

thernet in the

irst

ile P roject A uthorization R equest becomes EFM task force (largest 802 task force ever formed)     EFM task force had 4 tracks DSL (now in clauses 61, 62, 63) Ethernet OAM (now clause 57) Optics (now in clauses 58, 59, 60, 65 ) P2MP (now clause 64) 2002 : liaison activity with ITU to agree upon wavelength allocations 2003 : WG ballot 2004 : full standard 2005: new 802.3 version with EFM clauses

PONs Slide 22

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 : G.984.3 : P T hysical M edia ransmission C D ependent layer onvergence layer G.984.4 : management and control interface

PONs Slide 23

Fiber optics - basics

PONs Slide 24

Total Internal Reflection in Step-Index Multimode Fiber 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 PONs Slide 25

Types of Optical Fiber

Popular Fiber Sizes Multimode Graded Index Fiber Single-mode Fiber

PONs Slide 26

Optical Loss versus Wavelength

 Click to edit Master text styles – Second level  Third level – Fourth level

PONs Slide 27

Sources of Dispersion Total Dispersion Multimode Dispersion Chromatic Dispersion Material Dispersion

PONs Slide 28

Multimode Dispersion

1 0 1

Dispersion limits bandwidth in optical fiber

PONs Slide 29

1 0

Graded-index Dispersion

1 0 1

PONs Slide 30

1 0

Single-Mode Dispersion

1 0 1

In SM the limit bandwidth is caused by chromatic dispersion.

PONs Slide 31

System Design Consideration How to calculate bandwidth?

For a 1.25 Gb/s we need a BW of 0.7 BitRate = 1.143ns

c = D

mat

*

l

* L

For Laser 1550nm Fabry Perot

**c = (20ps/nm * km) * 5nm * 15km = 1.5ns**

For Laser 1550nm DFB

**c = (20ps/nm * km) * 0.2nm * 60km = 0.24ns**

PONs Slide 32

Material Dispersion (Dmat)

PONs Slide 33

Spectral Characteristics

LASER/laser diode

: L ight A mplification by S timulated E mission of R adiation. Done of the wide range of devices that generates light by that principle. Laser light is directional, covers a narrow range of wavelengths, and is more coherent than ordinary light. Semiconductor diode lasers are the standard light sources in fiber optic systems. Lasers emit light by stimulated emission .

PONs Slide 34

Laser Optical Power Output vs. Forward Current

Laser

PONs Slide 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

PONs Slide 36

Wavelength-Division Multiplexing

PONs Slide 37

WDM Duplexing

PONs Slide 38

Basic Configuration of PON

OLT = Optical Line Termination ONU = Optical Network Unit BMCDR = Burst Mode Clock Data Recovery

PONs Slide 39

Typical PON Configuration and Optical Packets

PONs Slide 40

Eye diagram of ONU transceiver in burst mode operation

PONs Slide 41

Burst-Mode Transmitter in ONU

PONs Slide 42

OLT Burst-Mode Receiver

PONs Slide 43

Burst-Mode CDR

PONs Slide 44

Sampling

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

PONs Slide 45

Transceiver Block Diagram

PONs Slide 46

Optical Splitters

PONs Slide 47

Optical Protection Switch Optical Splitter

PONs Slide 48

Budget Calculations L

B =

Link Budget P

S =

Sensitivity P

O =

Output Power

L B = ׀ P S ׀ ׀ P O ׀

Example: GPON 1310nm Power: 0dbm Single-mode fiber Sensitivity: -23dbm

}

Link Budget: 23db

PONs Slide 49

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

PONs Slide 50

Relationship between transmission distance and number of splits

PONs Slide 51

GbE Fiber Optic Characteristics

PONs Slide 52

PON physical layer

PONs Slide 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

  Upstream 1260 - 1360 nm (1310 ± 50)

second window

Downstream 1480 - 1580 nm (1530 ± 50)

third window

US DS 1200 nm 1300 nm 1400 nm 1500 nm 1600 nm

PONs Slide 54

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 1490 1630 So it was decided to use this band as the G.983.3 DS and leave the US unchanged US guard available DS 1200 nm 1300 nm 1400 nm 1500 nm 1600 nm

PONs Slide 55

allocations - final

US DS 1200 nm 1300 nm 1400 nm 1500 nm 1600 nm The G.983.3 band-plan was incorporated into GPON and via liaison activity into EPON and is now the universally accepted xPON band-plan    US 1260-1360 nm (1310 ± 50) DS 1480-1500 nm (1490 ± 10) enhancement bands: – – video 1550 - 1560 nm (see ITU-T J.185/J.186) digital 1539-1565 nm

PONs Slide 56

Data rates (for now …)

PON BPON Amd 1 Amd 2 GPON EPON 10GEPON † DS (Mbps) 155.52

622.08

1244.16

2488.32

1250

10312.5

US (Mbps) 155.52

155.52

622.08

155.52

622.08

155.52

622.08

1244.16

155.52

622.08

1244.16

2488.32

1250

10312.5

† * only 1G/10G usable due to linecode work in progress

PONs Slide 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 dB, but now 30 dB) and has 10 km and 20 km P hysical M edia D ependent sublayers

PONs Slide 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 802.3z (1000BASE-X) line code – – – 8B/10B Every 8 data bits are converted into 10 bits before transmission DC removal and timing recovery ensured by mapping Special function codes (e.g. idle, start_of_packet, end_of_packet, etc) However, 1000 Mbps is expanded to 1250 Mbps 10GbE uses a different linecode - 64B/66B

PONs Slide 59

FEC

G984.3 clause 13

and

802.3-2005 subclause 65.2.3

define an optional G.709-style Reed-Solomon code Use (255,239,8) systematic RS code designed for submarine fiber (G.975) to every 239 data bytes add 16 parity bytes to make 255 byte FEC block Up to 8 byte errors can be corrected Improves power budget by over 3 dB, allowing increased reach or additional splits Use of FEC is negotiated between OLT and ONU Since code is systematic can use in environment where some ONUs do not support FEC In GPON FEC frames are aligned with PON frames In EPON FEC frames are marked using K-codes (and need 8B10B decode - FEC - 8B10B encode)

PONs Slide 60

US timing diagram

How does the ONU US transmission appear to the OLT ?

grant grant

inter-ONU guard

data data

laser turn-on laser turn-off 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 A utomatic G ain C ontrol and C lock/ D ata R ecovery long inter-ONU guard due to AGC-reset Ethernet preamble is part of data

PONs Slide 62

PON User plane

PONs Slide 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

PONs Slide 64

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

ogical

ink

to each ONU

(15b)

GPON has several levels of labels – – ONU_ID (1B)

(1B) T

ransmission-

CONT

ainer (AKA

Alloc_ID

)

(12b) (can be >1 T-CONT per ONU)

  For ATM mode

VPI VCI

ONU

T-CONT VP VP VC VC VC VC

 For GEM mode

Port_ID (12b) (12b)

PON ONU

T-CONT Port Port PONs Slide 65

DS GPON format

PON

ransmission

onvergence frames are always 125 m sec long – – 19440 bytes / frame for 1244.16 rate 38880 bytes / frame for 2488.32 rate Each GTC frame consists of

hysical

ontrol

lock

ownstream + payload – – PCBd contains sync, OAM, DBA info, etc.

payload may have ATM and GEM partitions (either one or both)

PCBd

GTC frame

payload PCBd

scrambled

payload PCBd

125 m sec

payload

PSync (4B) Ident (4B) PLOAMd (13B) BIP (1B) PLend (4B) PLend (4B) US BW map (N*8B)

ATM partition GEM partition PONs Slide 66

GPON payloads

GTC payload potentially has 2 sections: – –

PCBd

ATM partition (Alen * 53 bytes in length) GEM partition (now preferred method)

ATM cell ATM cell … ATM cell GEM frame GEM frame ATM partition …

Alen (12 bits) is specified in the PCBd Alen specifies the number of 53B cells in the ATM partition if Alen=0 then no ATM partition if Alen=payload length / 53 then no GEM partition ATM cells are aligned to GTC frame ONUs accept ATM cells based on VPI in ATM header

GEM frame GEM partition

Unlike ATM cells, GEM delineated frames may have any length Any number of GEM frames may be contained in the GEM partition ONUs accept GEM frames based on 12b Port-ID in GEM header

PONs Slide 67

PON

ncapsulation

ode

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

– – – – P Port ID P H ayload ayload eader on GFP, and the header contains the following fields: - identifies the target ONU E L T ength ype rror I C I ndicator - payload length in Bytes ndicator ( GEM OAM, congestion/fragmentation indication orrection field ( BCH(39,12,2) code+ 1b even parity ) ) The GEM header is XOR'ed with B6AB31E055 before transmission

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

PLI bytes of TDM are inserted into payload field length of TDM fragment may vary by ± 1 Byte due to frequency offset round-trip latency bounded by 3 msec.

TDM over GEM PLI ID PTI HEC PLI Bytes of TDM

PONs Slide 69

GEM fragmentation

GEM can

fragment

its payload For example unfragmented Ethernet frame PLI ID PTI=001 HEC DA SA T data fragmented Ethernet frame PLI ID PTI=000 HEC DA SA T data

PLI ID PTI=001 HEC data

FCS FCS GEM fragments payloads for either of two reasons: –

PCBd

GEM frame may not straddle GTC frame

ATM partition GEM frame … GEM frag 1 PCBd ATM partition GEM frag 2 … GEM frame

–

PCBd

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

ATM partition urgent frame … large frag 1 PCBd ATM partition urgent frame … large frag 2 PONs Slide 70

PCBd

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

B6AB31E0

PSync - fixed pattern used by ONU to located start of GTC frame Ident - MSB indicates if FEC is used, 30 LSBs are superframe counter PLOAMd - carries OAM, ranging, alerts, activation messages, etc.

BIP - SONET/SDH-style Bit Interleaved Parity of all bytes since last BIP PLend – – – (transmitted twice for robustness) Blen Alen CRC - 12 MSB are length of BW map in units of 8 Bytes - Next 12 bits are length of ATM partition in cells - final 8 bits are CRC over Blen and Alen US BW map - array of Blen 8B structures granting BW to US flow will discuss later (DBA)

PONs Slide 71

GPON US considerations

GTC fames are still 125 m sec long, but shared amongst ONUs Each ONU transmits a

burst

of data – – – – – – – using timing acquired by locking onto OLT signal according to time allocation sent by OLT in BWmap  there may be multiple allocations to single ONU  OLT computes DBA by monitoring traffic status (buffers) of ONUs and knowing priorities at power level requested by OLT (3 levels)  this enables OLT to use avalanche photodiodes which are sensitive to high power bursts leaving a guard time from previous ONU's transmission prefixing a preamble to enable OLT to acquire power and phase identifying itself (ONU-ID) in addition to traffic IDs (VPI, Port-ID) scrambling data (but not preamble/delimiter)

PONs Slide 72

US GPON format

4 different US overhead types:   P hysical L ayer O verhead u pstream – – 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 Ind ication of real-time status (1B) PLOAM u pstream (13B) - messaging with PLOAMd   P ower L evelling S equence u pstream (120B) – used during power-set and power-change to help set ONU power so that OLT sees similar power from all ONUs D ynamic B andwidth R eport u pstream – sends traffic status to OLT in order to enable DBA computation if all OH types are present: PLOu PLOAMd PLSu DBRu payload

PONs Slide 73

US allocation example

DS frame

payload PCBd

BWmap 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    ONU allocation IDs flags (not shown above) tell if use FEC, which US OHs to use, etc.

start and stop times (16b fields, in Bytes from beginning of US frame)

PONs Slide 74

EPON format

EPON operation is based on the Ethernet MAC and EPON frames are based on GbE frames but extensions are needed   clause 64 -

ulti

oint

ontrol

rotocol PDUs this is the control protocol implementing the required logic clause 65 - point-to-point emulation (reconciliation) this makes the EPON look like a point-to-point link and EPON MACs have some special constraints    instead of CSMA/CD they transmit when granted time through MAC stack must be constant ( ± 16 bit durations) accurate local time must be maintained

PONs Slide 75

EPON header

Standard Ethernet starts with an essentially content-free 8B preamble   7B of alternating ones and zeros 10101010 1B of SFD 10101011 In order to hide the new PON header EPON overwrites some of the preamble bytes 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011 10101010 10101010

10101011

10101010 10101010

LLID

LLID field contains CRC – – MODE (1b)  always 0 for ONU  0 for OLT unicast, 1 for OLT multicast/broadcast actual L ogical L ink ID   (15b) Identifies registered ONUs 7FFF for broadcast protects from SLD (byte 3) through LLID (byte 7)

LLID CRC PONs Slide 76

MPC PDU format

MultiPoint Control Protocol frames are untagged MAC frames with the same format as PAUSE frames

DA SA L/T Opcode timestamp data / RES / pad FCS

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

PONs Slide 77

Security

DS traffic is broadcast to all ONUs, so encryption is essential easy for a malicious user to reprogram ONU to capture desired frames US traffic not seen by other ONUs, so encryption is not needed do not take fiber-tappers into account EPON does not provide any standard encryption method – – – can supplement with IPsec or MACsec many vendors have added proprietary AES-based mechanisms in China special China Telecom encryption algorithm BPON used a mechanism called churning Churning was a low cost hardware solution (24b key) with several security flaws – engine was linear - simple known-text attack – 24b key turned out to be derivable in 512 tries So G.983.3 added AES support - now used in GPON

PONs Slide 78

GPON encryption

OLT encrypts using AES-128 in counter mode Only payload is encrypted (not ATM or GEM headers) Encryption blocks aligned to GTC frame Counter is shared by OLT and all ONUs – – 46b = 16b intra-frame + 30 bits inter-frame intra-frame counter increments every 4 data bytes  reset to zero at beginning of DS GTC frame OLT and each ONU must agree on a unique symmetric key OLT asks ONU for a password (in PLOAMd) ONU sends password US in the clear (in PLOAMu) – key sent 3 times for robustness OLT informs ONU of precise time to start using new key

PONs Slide 79

QoS - EPON

Many PON applications require high QoS (e.g. IPTV) EPON leaves QoS to higher layers – – VLAN tags P bits or DiffServ DSCP In addition, there is a crucial difference between LLID and Port-ID – – – there is always 1 LLID per ONU there is 1 Port-ID per input port - there may be many per ONU this makes port-based QoS simple to implement at PON layer RT EF BE GPON

PONs Slide 80

QoS - GPON

GPON treats QoS explicitly – – constant length frames facilitate QoS for time-sensitive applications 5 types of

ransmission

CONT

ainers   type 1 - fixed BW type 2 - assured BW    type 3 - allocated BW + non-assured BW type 4 - best effort type 5 - superset of all of the above GEM adds several PON-layer QoS features – – – fragmentation enables pre-emption of large low-priority frames PLI - explicit packet length can be used by queuing algorithms PTI bits carry congestion indications

PONs Slide 81

PON control plane

PONs Slide 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

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

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

PONs Slide 85

GPON ranging method

Two types of ranging – initial ranging   only performed at ONU boot-up or upon ONU discovery must be performed before ONU transmits first time – continuous ranging performed continuously to compensate for delay changes OLT initiates coarse ranging by stopping allocations to all other ONUs – thus when new ONU transmits, it will be in the clear OLT instructs the new ONU to transmit (via PLOAMd) OLT measures phase of ONU burst in GTC frame OLT sends equalization delay to ONU (in PLOAMd) During normal operation OLT monitors ONU burst phase If drift is detected OLT sends new equalization delay to ONU (in PLOAMd)

PONs Slide 86

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 Timestamp = T0 ONU receives MPCPDU Sets clock to T0 ONU sends MPCPDU Timestamp = T1 OLT receives MPCPDU RTT = T2 - T1 time OLT time ONU time T0 T0 RTT = (T2-T0) - (T1-T0) = T2-T1 T1 OLT compensates all grants by RTT before sending T2 Either ONU or OLT can detect that timestamp drift exceeds threshold

PONs Slide 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

PONs Slide 88

GPON autodiscovery

Every ONU has an 8B serial number (4B vendor code + 4B SN) – SN of ONUs in OAN may be configured by NMS, or – SN may be learnt from ONU in discovery phase ONU activation may be triggered by – Operator command – Periodic polling by OLT – OLT searching for previously operational ONU G.984.3 differentiates between three cases: – cold PON / cold ONU – warm PON / cold ONU – warm PON / warm ONU Main steps in procedure: – ONU sets power based on DS message – OLT sends a

Serial_Number

request to all unregistered ONUs – – – – ONU responds OLT assigns 1B

ONU-ID

ranging is performed ONU is operational and sends to ONU

PONs Slide 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  start time and duration ONU transmits REGISTER_REQ PDU using

random offset

in window OLT receives request  registers ONU    assigns LLID bonds MAC to LLID performs ranging computation OLT sends REGISTER to ONU OLT sends standard GATE to ONU ONU responds with REGISTER_ACK ONU goes into operational mode - waits for grants

PONs Slide 90

Failure recovery

PONs must be able to handle various failure states

GPON

if ONU detects LOS or LOF it goes into

POPUP

state    it stops sending traffic US OLT detects LOS for ONU if there is a pre-ranged backup fiber then switch-over

EPON

during normal operation ONU REPORTs reset OLT's watchdog timer similarly, OLT must send GATES periodically (even if empty ones) if OLT's watchdog timer for ONU times out  ONU is deregistered

PONs Slide 91

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 – – –

ynamic

andwidth maximum fiber BW utilization fairness and respect of priority minimum delay introduced

llocation algorithm are

PONs Slide 92

GPON DBA

DBA is at the T-CONT level, not port or VC/VP GPON can use traffic monitoring (passive) or status reporting (active) There are three different status reporting methods    status in PLOu - one bit for each T-CONT type piggy-back reports in DBRu - 3 different formats: – – – quantity of data waiting in buffers, separation of data with peak and sustained rate tokens nonlinear coding of data according to T-CONT type and tokens ONU report in DBA payload - select T-CONT states OLT may use any DBA algorithm OLT sends allocations in US BW map

PONs Slide 93

EPON DBA

OLT sends GATE messages to ONUs GATE message

DA SA 8808 Opcode=0002 timestamp Ngrants/flags

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

grants …

REPORT message

DA SA 8808 Opcode=0003 timestamp Nqueue_sets Reports

Reports represent the length of each queue at time of report OLT may use any algorithm to decide how to send the following grants

… PONs Slide 94