Passive Optical Network (PON) re

8/12/2019 Passive Optical Network (PON) re

1/94

PassiveOpticalNetworks

Yaakov (J) Stein May 2007and

Zvika Eitan


2/94

PONs Slide 2

Outline

PON benefits

PON architecture

Fiber optic basics

PON physical layer

PON user plane

PON control plane


3/94

PONs Slide 3

PON benefits


4/94

PONs Slide 4

Why fiber ?todays 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]


5/94

PONs Slide 5

Asidewhy isfiber better ?

attenuation per unit length

reasons for energy loss

copper: resistance, skin effect, radiation, coupling

fiber: internal scattering, imperfect totalinternal 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


6/94

PONs Slide 6

Why no tfiber ?

fiber beats all other technologies for speed and reachbut 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


7/94PONs Slide 7

Access network bottleneck

hard for end users to get high datarates because of the access bottlenecklocal 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


8/94PONs Slide 8

Fiber To The CurbHybrid Fiber Coax and VDSL

switch/transceiver/miniDSLAM located at curb or in basement need only 2 optical transceivers

but no tpure optical solution

lower BW from transceiver to end users

need complex converter in constrained environment

N end users

core

access network

feeder fiber

copper


9/94PONs Slide 9

Fiber To The Premises

we canimplement 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 userscore

access network


10/94PONs 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


11/94PONs Slide 11

The PON solution

another alternative - implement point-to-multipoint topology purely in optics

avoid costly optic-electronic conversions

usepassive splittersno 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


12/94PONs 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


13/94PONs Slide 13

PON

architecture


14/94PONs 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

downstreamis from OLT to ONU (upstreamis the opposite direction)

downstream

Optical Network Units

upstream

Optical Distribution Network

NNI

Terminal Equipment

UNI

coresplitter

Optical Line Terminal

Optical Access Network


15/94PONs Slide 15

PON types

many types of PONs have been defined

APON ATM PON

BPON Broadband PON

GPON Gigabit PON

EPON Ethernet PON

GEPON Gigabit Ethernet PON

CPON CDMA PON

WPON WDM PON

in this course we will focus on GPONand EPON (including GEPON)

with a touch of BPONthrown in for the flavor


16/94PONs 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 802.3-2005 clauses 64 and 65 (but other 802.3 clauses are also needed)

Warning

do not believe white papers from vendorsespecially not with respect to GPON/EPON comparisons

EPONBPONGPON


17/94PONs 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 ls for transmit and receive optionally: Wavelength Division Multiplexer

downstream transmission

OLT broadcastsdata downstream to all ONUs in ODN ONU captures data destined for its address, discards all other data encryptionneeded to ensure privacy

upstream transmission

ONUs share bandwidth using Time Division Multiple Access OLT manages the ONU timeslots rangingis performed to determine ONU-OLT propagation time

additional functionality

Physical Layer OAM Autodiscovery

Dynamic Bandwidth Allocation


18/94PONs 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


19/94PONs 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 PONsince all ONUs see all DS data


20/94PONs Slide 20

PON encapsulation

The majority of PON traffic is Ethernet

So EPON enthusiasts say

use EPON - it'sjus tEthernet

That's true by definition -

anything in 802.3 isEthernet

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

But don't be fooled- all PON methods encapsulateMAC frames

EPON and GPON differ in the contentsof the header

EPON hides the new header inside the GbE preamble

GPON can also carry non-Ethernetpayloads

DA SA T data FCSPON header


21/94PONs Slide 21

BPON history

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

Full Service Access Network Initiativeto 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


22/94

PONs Slide 22

EPON history2001: IEEE 802 LMSC WG accepts

Ethernet in the First Mile ProjectAuthorization Request

becomes EFM task force (largest 802 task force ever formed)

EFM task force had 4 tracks

DSL (now in clauses 61, 62, 63)

Ethernet OAM (now clause 57) Optics (now in clauses 58, 59, 60, 65)

P2MP (now clause 64)

2002 : liaison activity with ITU to agree upon wavelength allocations

2003 : WG ballot

2004 : full standard

2005: new 802.3 version with EFM clauses


23/94

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 notto 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


24/94

PONs Slide 24

Fiber optics - basics


25/94

PONs Slide 25

= sin (n2/n1)1

V =c/n

t = Ln/c

t = Propagation Timet Vacuum: n=1, t=3.336ns/m

t Water : n=1.33, t=4.446ns/m

Total Internal Reflection

in Step-Index Multimode Fiber


26/94

PONs Slide 26

Multimode Graded-Index Fiber

Single-mode

Fiber

Types of Optical Fiber

Popular FiberSizes


27/94

PONs Slide 27

Click to edit Master text styles

Second level

Third level Fourth level

Optical Loss versus Wavelength


28/94

PONs Slide 28

Total Dispersion

MultimodeDispersion ChromaticDispersion

MaterialDispersion

Sources of Dispersion


29/94

PONs Slide 29

1 0 11

Multimode Dispersion

1 11 11 11

Dispersion limits bandwidth in optical fiber


30/94

PONs Slide 30

1 0 11

Graded-index Dispersion

1 10


31/94

PONs Slide 31

1 0 1 1 10

In SMthe limit bandwidth is caused by chromatic dispersion.

1

Single-Mode Dispersion


32/94

PONs Slide 32

How to calculate bandwidth?

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

Tc = Dmat* * 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


33/94

PONs Slide 33

Material Dispersion (Dmat)


34/94

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


35/94

PONs Slide 35

Laser

W

Laser Optical Power Output vs. Forward Current


36/94

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


37/94

PONs Slide 37

Wavelength-Division Multiplexing


38/94

PONs Slide 38

WDM Duplexing


39/94

PONs Slide 39BMCDR = Burst Mode Clock Data Recovery

OLT = Optical Line Termination

ONU = Optical Network Unit

Basic Configuration of PON


40/94

Eye diagram of ONU transceiver


41/94

PONs Slide 41

Eye diagram of ONU transceiver

in burst mode operation


42/94

PONs Slide 42

Burst-Mode Transmitter in ONU


43/94

PONs Slide 43

OLT Burst-Mode Receiver


44/94


45/94

PONs Slide 45

Ideal, error-free transmission

Superimposed interference

Hysteresis

Ideal sampling instant

Sampling


46/94

PONs Slide 46

Transceiver Block Diagram


47/94

PONs Slide 47

Optical Splitters


48/94

PONs Slide 48

Optical Protection Switch

Optical Splitter

C


49/94

PONs Slide 49

LB = PS - PO

LB =Link Budget

PS = Sensitivity

PO=Output Power

Example: GPON 1310nm

Power: 0dbm Single-mode

fiber

Sensitivity: -23dbm

} Link Budget:23db

Budget Calculations

T i l R C l l ti


50/94

PONs Slide 50

Assume:

Optical loss = 0.35 db/km

Connector Loss = 2dBSplitter Insertion Loss 1X32 = 17dB

Range Budget: ~11Km

Typical Range Calculation

Relationship between transmission distance


51/94

PONs Slide 51

Relationship between transmission distance

and number of splits

GbE Fiber Optic Characteristics


52/94

PONs Slide 52

GbE Fiber Optic Characteristics


53/94

PONs Slide 53

PON physical layer


54/94

PONs Slide 54

allocations - G.983.1

Upstream and downstream directions need about the same bandwidth

US serves N customers, so it needs N times the BW of each customer

but each customer can only transmit 1/N of the time

In APON and early BPON work it was decided that 100 nm was needed

Where should these bands be placed for best results?

In the secondand thirdwindows!

Upstream 1260 - 1360 nm (1310 50) second window

Downstream 1480 - 1580 nm (1530 50) th i rd window

1200 nm 1300 nm 1400 nm 1500 nm 1600 nm

US DS


55/94


56/94

PONs Slide 56

allocations - final

The G.983.3 band-plan was incorporated into GPON

and via liaison activity into EPONand 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

1200 nm 1300 nm 1400 nm 1500 nm 1600 nm

US DS


57/94

PONs Slide 57

Data rates (for now )

PON DS (Mbps) US (Mbps)

BPON 155.52 155.52

622.08 155.52

622.08 622.08

1244.16 155.52

1244.16 622.08

1244.16 155.521244.16 622.08

1244.16 1244.16

2488.32 155.52

2488.32 622.08

2488.32 1244.162488.32 2488.32

EPON 1250* 1250*

10GEPON 10312.5* 10312.5*

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

Amd 1

Amd 2

GPON


58/94

PONs Slide 58

Reach and splits

Reach and the number of ONUs supported are contradictory design goals

In addition tophysical reachderived from optical budget

there is logical reachlimited by protocol concerns (e.g. ranging protocol)

and differential reach(distance between nearest and farthest ONUs)

The number of ONUs supported depends not only on the number of splits

but also on the addressing scheme

BPON called for 20 km and 32-64 ONUs

GPON allows 64-128 splits and the reach is usually 20 km

but there is a low-cost 10 km mode (using Fabry-Perot laser diodes in ONUs)

and a long physical reach 60 km mode with 20 km differential reach

EPON allows 16-256 splits (originally designed for link budget of 24 dB, but now 30 dB)

and has 10 km and 20 km Physical Media Dependent sublayers


59/94

PONs Slide 59

Line codes

BPON and GPON use a simple NRZlinecode (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


60/94

PONs Slide 60

FEC

G984.3 clause 13 and802.3-2005 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)


61/94

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 interferenceElectrically shut ONU laser off when not transmitting

But lasers have long warm-up time

and ONU lasers must stabilize quickly after being turned on


62/94

PONs Slide 62

US timing diagram

How does the ONU US transmission appear to the OLT ?

grant grant

laserturn-on

laserturn-off

data

lock

laserturn-on

laserturn-off

data

lock

inter-ONUguard

Notes:

GPON - ONU reports turn-on and turn-off times to OLT

ONU preamble length set by OLT

EPON - long lock time as need toAutomatic Gain Control and Clock/Data Recovery

long inter-ONU guard due to AGC-reset

Ethernet preamble is part of data


63/94

PONs Slide 63

PON User plane


64/94


65/94

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 IDto 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)

PONONU

ONU

T-CONT

T-CONT

Port

Port

VP

VP

VCVCVCVC


66/94

GPON l d


67/94

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 partitionif 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 GEM frame GEM frameATM cell ATM cell

GPON E l ti M d


68/94

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 genericany packet type (and even TDM) supported

GEM supports fragmentation and reassemblyGEM is basedon 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

Eth t / TDM GEM


69/94

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 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.

DA SA T data FCSPLI

Ethernet over GEM

ID PTI HEC

PLI Bytes of TDMPLI

TDM over GEM

ID PTI HEC

GEM f t ti


70/94

PONs Slide 70

GEM fragmentationGEM can fragmentits 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

DA SA T data FCSPLI

unfragmented Ethernet frame

ID PTI=001 HEC

DA SA T data1PLI

fragmented Ethernet frame

ID PTI=000 HEC

data2PLI ID PTI=001 HEC FCS

ATM partitionPCBd GEM frame

GEM frag 1 ATM partitionPCBd GEM frag 2

GEM frame

ATM partitionPCBd urgent frame large frag 1 ATM partitionPCBd urgent frame large frag 2

PCBd


71/94

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 counterPLOAMd- 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)

PLend(4B)

US BW map(N*8B)

B6AB31E0

GPON US id ti


72/94

PONs Slide 72

GPON US considerations

GTC fames are still 125 msec long, but shared amongst ONUs

Each ONU transmits a burstof 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)

US GPON f t


73/94

PONs Slide 73

US GPON format4 different US overhead types:

Physical Layer Overhead upstream always sent by ONU when taking over from another ONU

contains preambleand 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 ONUpower so that OLT sees similar power from all ONUs

Dynamic Bandwidth Report upstream

sends traffic status to OLT in order to enable DBA computation

PLOu PLOAMd PLSu DBRu payload

if all OH types are present:

US ll ti l


74/94

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 SStopBWmap

US frame

guard

time

preamble+

delimiter

scrambled


75/94

EPON h d


76/94

PONs Slide 76

EPON headerStandard 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

LLIDfield 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)

10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011

10101010 10101010 10101011 10101010 10101010 LLID LLID CRC

MPC PDU format


77/94

PONs Slide 77

MPC PDU format

MultiPoint Control Protocol frames are untagged MAC frameswith 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

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

Security


78/94

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.983.3 added AES support - now used in GPON

GPON encryption


79/94

PONs Slide 79

GPON encryption

OLT encrypts using AES-128 in counter modeOnly 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


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QoS GPON


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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


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PONs Slide 82

PON control plane

Principles


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PONs Slide 83

Principles

GPON uses PLOAMdand PLOAMuas 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

Ranging


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PONs Slide 84

Ranging

Upstream traffic is TDMA

Were all ONUs equidistant, and were all to have a common clock

then each would simply transmit in its assigned timeslot

But otherwise the signals will overlap

To eliminate overlap guard times left between timeslots

each ONU transmits with the proper delay to avoid overlap

delay computed during a ranging process


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GPON ranging method


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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)

EPON ranging method


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PONs Slide 87

EPON ranging method

All ONUs are synchronized to absolute time (wall-clock)

When an ONU receives an MPCPDU from OLTit 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

T0OLT time T2T0ONU time T1

Autodiscovery


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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

GPON autodiscovery


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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.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_Numberrequest to all unregistered ONUs ONU responds OLT assigns 1B ONU-IDand sends to ONU ranging is performed ONU is operational

EPON autodiscovery


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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 offsetin 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

Failure recovery


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PONs Slide 91

Failure recovery

PONs must be able to handle various failure states

GPON

if ONU detects LOS or LOF it goes into POPUPstate

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



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PONs Slide 92


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 aDynamic Bandwidth Allocation algorithm are maximum fiber BW utilization

fairness and respect of priority

minimum delay introduced

GPON DBA


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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

EPON DBA


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EPON DBA

OLT sends GATE messages to ONUs

flagsinclude DISCOVERYand Force_Report

Force_Reporttells the ONU to issue a report

Reportsrepresent the length of each queue at time of report

OLT may use any algorithm to decide how to send the following grants

DA SA 8808 Opcode=0002 timestamp Ngrants/flags grants

DA SA 8808 Opcode=0003 timestamp Nqueue_sets Reports

GATE message

REPORT message

Passive Optical Network (PON) re

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