pon
TRANSCRIPT
PPassiveassive O Opticalptical N Networketworkss
Yaakov (J) Stein May 2007 andZvika Eitan
PONs Slide 2
OutlineOutline
PON benefits PON architecture Fiber optic basics PON physical layer PON user plane PON control plane
PONs Slide 3
PON benefitsPON benefits
PONs Slide 4
Why fiber ?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 5
Aside – why Aside – why isis fiber better ? fiber better ?
attenuation per unit length reasons for energy loss
– copper: resistance, skin effect, radiation, coupling– fiber: internal scattering, imperfect total internal reflection
so fiber beats coax by about 2 orders of magnitude – e.g. 10 dB/km for thin coax at 50MHz, 0.15 dB/km =1550nm fiber
noise ingress and cross-talk copper couples to all nearby conductors no similar ingress mechanism for fiber
ground-potential, galvanic isolation, lightning protection copper can be hard to handle and dangerous no concerns for fiber
PONs Slide 6
Why Why notnot fiber ? fiber ?
fiber beats all other technologies for speed and reach
but fiber has its own problems
harder to splice, repair, and need to handle carefully
regenerators and even amplifiers are problematic– more expensive to deploy than for copper
digital processing requires electronics– so need to convert back to electronics– we will call the converter an optical transceiver– optical transceivers are expensive
switching easier with electronics (but possible with photonics)– so pure fiber networks are topologically limited:
point-to-point rings
copper fiber
PONs Slide 7
Access network bottleneckAccess network bottleneck
hard for end users to get high datarates because of the access bottleneck
local area networks use copper cable get high datarates over short distances
core networks use fiber optics get high datarate over long distances small number of active network elements
access networks (first/last mile) long distances
– so fiber would be the best choice many network elements and large number of endpoints
– if fiber is used then need multiple optical transceivers– so copper is the best choice– this severely limits the datarates
coreaccess
LAN
PONs Slide 8
FFiber iber TTo o TThe he CCurburbHybrid Fiber Coax and VDSL switch/transceiver/miniDSLAM located at curb or in basement need only 2 optical transceivers but not pure optical solution lower BW from transceiver to end users need complex converter in constrained environment
N end userscore
access network
feeder fiber
copper
PONs Slide 9
FFiber iber TTo o TThe he PPremisesremises
we can implement point-to-multipoint topology purely in optics but we need a fiber (pair) to each end user requires 2 N optical transceivers complex and costly to maintain
N end userscore
access network
PONs Slide 10
An obvious solutionAn 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 userscore
access network
feeder fiber
fiber
PONs Slide 11
The PON solutionThe PON solution
another alternative - implement point-to-multipoint topology purely in optics avoid costly optic-electronic conversions use passive splitters – no power needed, unlimited MTBF only N+1 optical transceivers (minimum possible) !
1:2 passive splitter
1:4 passive splitter
N end users
feeder fiber
core
access network
typically N=32
max defined 128
PONs Slide 12
PON advantagesPON 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 13
PONPON
architecturearchitecture
PONs Slide 14
TerminologyTerminology
like every other field, PON technology has its own terminology the CO head-end is called an OLT ONUs are the CPE devices (sometimes called ONTs in ITU) the entire fiber tree (incl. feeder, splitters, distribution fibers) is an ODN all trees emanating from the same OLT form an OAN downstream is from OLT to ONU (upstream is the opposite direction)
downstream
Optical Network Units
upstream
Optical Distribution NetworkNNI
Terminal Equipment
UNI
coresplitter
Optical Line Terminal
Optical Access Network
PONs Slide 15
PON typesPON 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 GPON and EPON (including GEPON)
with a touch of BPON thrown in for the flavor
PONs Slide 16
BibliographyBibliography 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)
Warningdo not believe white papers from vendors
especially not with respect to GPON/EPON comparisons
EPONBPONGPON
PONs Slide 17
PON principlesPON principles(almost) all PON types obey the same basic principles
OLT and ONU consist of Layer 2 (Ethernet MAC, ATM adapter, etc.) optical transceiver using different s for transmit and receive optionally: Wavelength Division Multiplexer
downstream transmission OLT broadcasts data downstream to all ONUs in ODN ONU captures data destined for its address, discards all other data encryption needed to ensure privacy
upstream transmission ONUs share bandwidth using Time Division Multiple Access OLT manages the ONU timeslots ranging is performed to determine ONU-OLT propagation time
additional functionality Physical Layer OAM Autodiscovery Dynamic Bandwidth Allocation
PONs Slide 18
Why a new protocol ?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 protocolsdo not provide all the needed functionalitye.g. receive filtering, ranging, security, BW allocation
downstream
upstream
PONs Slide 19
(multi)point - to - (multi)point(multi)point - to - (multi)point
Multipoint-to-multipoint Ethernet avoids collisionsby CSMA/CD
This can't work for multipoint-to-point US PONsince ONUs don't see each otherAnd the OLT can't arbitrate without adding a roundtrip time
Point-to-point ATM can send data in the openalthough trusted intermediate switches see all datacustomer switches only receive their own data
This can't work for point-to-multipoint DS PONsince all ONUs see all DS data
PONs Slide 20
PON encapsulationPON 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 GbE preamble
GPON can also carry non-Ethernet payloads
DA SA T data FCSPON header
PONs Slide 21
BPON historyBPON history1995 : 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– …
PONs Slide 22
EPON historyEPON history2001: IEEE 802 LMSC WG accepts
Ethernet in the First Mile Project Authorization Request
becomes EFM task force (largest 802 task force ever formed)
EFM task force had 4 tracks DSL (now in clauses 61, 62, 63) Ethernet OAM (now clause 57) Optics (now in clauses 58, 59, 60, 65) P2MP (now clause 64)
2002 : liaison activity with ITU to agree upon wavelength allocations
2003 : WG ballot
2004 : full standard
2005: new 802.3 version with EFM clauses
PONs Slide 23
GPON historyGPON history
2001 : FSAN initiated work on extension of BPON to > 1 Gbps
Although GPON is an extension of BPON technologyand reuses much of G.983 (e.g. linecode, rates, band-plan, OAM)decision was not to be backward compatible with 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
PONs Slide 24
Fiber optics - basicsFiber optics - basics
PONs Slide 25
© = sin¯ (n2/n1)1
V =c/n
t = L·n/c
t = Propagation Timet Vacuum: n=1, t=3.336ns/mt Water : n=1.33, t=4.446ns/m
Total Internal Reflection Total Internal Reflection in Step-Index Multimode Fiberin Step-Index Multimode Fiber
PONs Slide 26
Multimode Graded-Index Fiber
Single-mode Fiber
Types of Optical FiberTypes of Optical Fiber
Popular Fiber Sizes
PONs Slide 27
Click to edit Master text styles
– Second level
Third level
– Fourth level
Optical Loss versus WavelengthOptical Loss versus Wavelength
PONs Slide 28
Total Dispersion
Multimode Dispersion
Chromatic Dispersion
Material Dispersion
Sources of DispersionSources of Dispersion
PONs Slide 29
1 0 11
Multimode DispersionMultimode Dispersion
1 11 11 11
Dispersion limits bandwidth in optical fiber
PONs Slide 30
1 0 11
Graded-index DispersionGraded-index Dispersion
1 10
PONs Slide 31
1 0 1 1 10
In SM the limit bandwidth is caused by chromatic dispersion.
1
Single-Mode DispersionSingle-Mode Dispersion
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 ConsiderationSystem Design Consideration
PONs Slide 33
Material Dispersion (Dmat)Material Dispersion (Dmat)
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 CharacteristicsSpectral Characteristics
PONs Slide 35
Laser
W
Laser Optical Power Output vs. Forward CurrentLaser Optical Power Output vs. Forward Current
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 DetectorsLight Detectors
PONs Slide 37
Wavelength-Division MultiplexingWavelength-Division Multiplexing
PONs Slide 38
WDM DuplexingWDM Duplexing
PONs Slide 39
BMCDR = Burst Mode Clock Data Recovery
OLT = Optical Line Termination
ONU = Optical Network Unit
Basic Configuration of PONBasic Configuration of PON
PONs Slide 40
Typical PON Configuration and Optical PacketsTypical PON Configuration and Optical Packets
PONs Slide 41
Eye diagram of ONU transceiver Eye diagram of ONU transceiver in burst mode operationin burst mode operation
PONs Slide 42
Burst-Mode Transmitter in ONUBurst-Mode Transmitter in ONU
PONs Slide 43
OLT Burst-Mode Receiver OLT Burst-Mode Receiver
PONs Slide 44
Burst-Mode CDRBurst-Mode CDR
PONs Slide 45
Ideal, error-free transmission
Superimposed interference
Hysteresis
Ideal sampling instant
SamplingSampling
PONs Slide 46
Transceiver Block DiagramTransceiver Block Diagram
PONs Slide 47
Optical SplittersOptical Splitters
PONs Slide 48
Optical Protection SwitchOptical Protection Switch
Optical SplitterOptical Splitter
PONs Slide 49
LB = ׀ PS ׀ - ׀
PO׀ LB = Link Budget
PS = Sensitivity
PO = Output Power
Example: GPON 1310nm
Power: 0dbm Single-mode fiberSensitivity: -23dbm } Link Budget: 23db
Budget Calculations Budget Calculations
PONs Slide 50
Assume:
Optical loss = 0.35 db/km
Connector Loss = 2dB
Splitter Insertion Loss 1X32 = 17dB
Range Budget: ~11Km
Typical Range Calculation Typical Range Calculation
PONs Slide 51
Relationship between transmission distance Relationship between transmission distance and number of splitsand number of splits
PONs Slide 52
GbE Fiber Optic CharacteristicsGbE Fiber Optic Characteristics
PONs Slide 53
PON physical layerPON physical layer
PONs Slide 54
allocations - G.983.1allocations - 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
1200 nm 1300 nm 1400 nm 1500 nm 1600 nm
US DS
PONs Slide 55
allocations - G.983.3allocations - 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 as the G.983.1 DS
So it was decided to use this band as the G.983.3 DSand leave the US unchanged
1270 16301490
1200 nm 1300 nm 1400 nm 1500 nm 1600 nm
US DSavailable
guard
PONs Slide 56
allocations - finalallocations - final
The G.983.3 band-plan was incorporated into GPONand 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
1200 nm 1300 nm 1400 nm 1500 nm 1600 nm
US DS
PONs Slide 57
Data rates Data rates (for now …)(for now …)
PON DS (Mbps) US (Mbps)
BPON 155.52 155.52622.08 155.52
622.08 622.08
1244.16 155.52
1244.16 622.08
1244.16 155.52
1244.16 622.08
1244.16 1244.16
2488.32 155.52
2488.32 622.08
2488.32 1244.16
2488.32 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
PONs Slide 58
Reach and splitsReach and splits
Reach and the number of ONUs supported are contradictory design goals
In addition to physical reach derived from optical budgetthere is logical reach limited by protocol concerns (e.g. ranging protocol)and differential reach (distance between nearest and farthest ONUs)
The number of ONUs supported depends not only on the number of splitsbut also on the addressing scheme
BPON called for 20 km and 32-64 ONUs
GPON allows 64-128 splits and the reach is usually 20 kmbut there is a low-cost 10 km mode (using Fabry-Perot laser diodes in ONUs)and a long physical reach 60 km mode with 20 km differential 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
PONs Slide 59
Line codesLine 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 60
FECFEC
G984.3 clause 13 and 802.3-2005 subclause 65.2.3define 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 framesIn EPON FEC frames are marked using K-codes
(and need 8B10B decode - FEC - 8B10B encode)
PONs Slide 61
More physical layer problemsMore 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
PONs Slide 62
US timing diagramUS timing diagram
How does the ONU US transmission appear to the OLT ?
grant grant
laser turn-on
laser turn-off
data
lock
laser turn-on
laser turn-off
data
lock
inter-ONUguard
Notes:GPON - ONU reports turn-on and turn-off times to OLT ONU preamble length set by OLTEPON - long lock time as need to Automatic Gain Control and Clock/Data Recovery long inter-ONU guard due to AGC-reset Ethernet preamble is part of data
PONs Slide 63
PON User planePON User plane
PONs Slide 64
How does it work?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 65
LabelsLabels
In an ODN there is 1 OLT, but many ONUs
ONUs must somehow be labeled for– OLT to identify the destination ONU– ONU to identify itself as the source
EPON assigns a single label Logical Link ID to each ONU (15b)
GPON has several levels of labels– ONU_ID (1B) (1B)
– Transmission-CONTainer (AKA Alloc_ID) (12b) (can be >1 T-CONT per ONU)
For ATM mode VPI VCIFor GEM mode Port_ID (12b) (12b)
PON
ONU
ONU
T-CONT
T-CONT
Port
Port
VP
VP VCVCVCVC
PONs Slide 66
DS GPON formatDS GPON format
GPON Transmission Convergence frames are always 125 sec 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)
payloadPCBd payloadPCBd payloadPCBd
GTC frame
PSync (4B) Ident (4B) PLOAMd (13B) BIP (1B)
PLend (4B) PLend (4B) US BW map (N*8B)
ATM partition
GEM partition
scrambled 125 sec
PONs Slide 67
GPON payloadsGPON payloadsGTC payload potentially has 2 sections:
– ATM partition (Alen * 53 bytes in length)– GEM partition (now preferred method)
ATM partitionAlen (12 bits) is specified in the PCBd
Alen specifies the number of 53B cells in the ATM partitionif Alen=0 then no ATM partitionif 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 partitionUnlike 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
PONs Slide 68
GGPON PON EEncapsulation ncapsulation MModeode
A common complaint against BPON was inefficiency due to ATM cell tax
GEM is similar to ATM– constant-size HEC-protected header – but avoids large overhead by allowing variable length frames
GEM is generic – any packet type (and even TDM) supported
GEM supports fragmentation and reassembly
GEM is based on GFP, and the header contains the following fields:– Payload Length Indicator - payload length in Bytes– Port ID - identifies the target ONU– Payload Type Indicator (GEM OAM, congestion/fragmentation indication)– Header Error Correction field (BCH(39,12,2) code+ 1b even parity)
The GEM header is XOR'ed with B6AB31E055 before transmission
Port ID
(12b)
PLI
(12b)
HEC
(13b)
PTI
(3b)
payload fragment
(L Bytes)
5 B
PONs Slide 69
Ethernet / TDM over GEMEthernet / TDM over GEM
When transporting Ethernet traffic over GEM:– only MAC frame is encapsulated (no preamble, SFD, EFD)– MAC frame may be fragmented (see next slide)
When transporting TDM traffic over GEM:– TDM input buffer polled every 125 sec.– PLI bytes of TDM are inserted into payload field– length of TDM fragment may vary by ± 1 Byte due to frequency offset – round-trip latency bounded by 3 msec.
DA SA T data FCSPLI
Ethernet over GEM
ID PTI HEC
PLI Bytes of TDMPLI
TDM over GEM
ID PTI HEC
PONs Slide 70
GEM fragmentationGEM fragmentationGEM can fragment its payload
For example
GEM fragments payloads for either of two reasons:– GEM frame may not straddle GTC frame
– GEM frame may be pre-empted for delay-sensitive data
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
PONs Slide 71
PCBdPCBd
We saw that the PCBd is
PSync - fixed pattern used by ONU to located start of GTC frame
Ident - MSB indicates if FEC is used, 30 LSBs are superframe counter
PLOAMd - carries OAM, ranging, alerts, activation messages, etc.
BIP - SONET/SDH-style Bit Interleaved Parity of all bytes since last BIP
PLend (transmitted twice for robustness) -– Blen - 12 MSB are length of BW map in units of 8 Bytes– Alen - Next 12 bits are length of ATM partition in cells– CRC - final 8 bits are CRC over Blen and Alen
US BW map - array of Blen 8B structures granting BW to US flow will discuss later (DBA)
PSync (4B)
Ident (4B)
PLOAMd (13B)
BIP (1B)
PLend (4B)
PLend (4B)
US BW map (N*8B)
B6AB31E0
PONs Slide 72
GPON US considerationsGPON US considerations
GTC fames are still 125 sec long, but shared amongst ONUs
Each ONU transmits a burst of data– using timing acquired by locking onto OLT signal– according to time allocation sent by OLT in BWmap
there may be multiple allocations to single ONU OLT computes DBA by monitoring traffic status (buffers)
of ONUs and knowing priorities – at power level requested by OLT (3 levels)
this enables OLT to use avalanche photodiodes which are sensitive to high power bursts
– leaving a guard time from previous ONU's transmission– prefixing a preamble to enable OLT to acquire power and phase– identifying itself (ONU-ID) in addition to traffic IDs (VPI, Port-ID)– scrambling data (but not preamble/delimiter)
PONs Slide 73
US GPON formatUS GPON format4 different US overhead types:
Physical Layer Overhead upstream– always sent by ONU when taking over from another ONU– contains preamble and delimiter (lengths set by OLT in PLOAMd) BIP (1B), ONU-ID (1B), and Indication of real-time status (1B)
PLOAM upstream (13B) - messaging with PLOAMd
Power Levelling Sequence upstream (120B)– used during power-set and power-change to help set ONU
power so that OLT sees similar power from all ONUs
Dynamic Bandwidth Report upstream– sends traffic status to OLT in order to enable DBA computation
PLOu PLOAMd PLSu DBRu payload
if all OH types are present:
PONs Slide 74
US allocation example 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
guardtime
preamble+
delimiter
scrambled
PONs Slide 75
EPON formatEPON format
EPON operation is based on the Ethernet MAC
and EPON frames are based on GbE frames
but extensions are needed
clause 64 - MultiPoint Control Protocol PDUs
this is the control protocol implementing the required logic
clause 65 - point-to-point emulation (reconciliation)
this makes the EPON look like a point-to-point link
and EPON MACs have some special constraints instead of CSMA/CD they transmit when granted time through MAC stack must be constant (± 16 bit durations) accurate local time must be maintained
PONs Slide 76
EPON headerEPON 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 headerEPON overwrites some of the preamble bytes
LLID field contains– MODE (1b)
always 0 for ONU 0 for OLT unicast, 1 for OLT multicast/broadcast
– actual Logical Link ID (15b) Identifies registered ONUs 7FFF for broadcast
CRC protects from SLD (byte 3) through LLID (byte 7)
10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011
10101010 10101010 10101011 10101010 10101010 LLID LLID CRC
PONs Slide 77
MPC PDU formatMPC PDU format
MultiPoint Control Protocol frames are untagged MAC frames with the same format as PAUSE frames
Ethertype = 8808
Opcodes (2B) - presently defined:GATE/REPORT/REGISTER_REQ/REGISTER/REGISTER_ACK
Timestamp is 32b, 16 ns resolutionconveys 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
PONs Slide 78
SecuritySecurity
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 neededdo 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 79
GPON encryptionGPON 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 80
QoS - EPONQoS - 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 BEEFGPON
PONs Slide 81
QoS - GPONQoS - 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
PONs Slide 82
PON control planePON control plane
PONs Slide 83
PrinciplesPrinciples
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 84
RangingRanging
Upstream traffic is TDMA
Were all ONUs equidistant, and were all to have a common clockthen 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 85
Ranging backgroundRanging background
In order for the ONU to transmit at the correct timethe delay between ONU transmission and OLT reception
needs to be known (explicitly or implicitly)Need to assign an equalization-delay
The more accurately it is knownthe smaller the guard time that needs to be leftand 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 86
GPON ranging methodGPON 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 87
EPON ranging methodEPON ranging methodAll ONUs are synchronized to absolute time (wall-clock)
When an ONU receives an MPCPDU from OLTit sets its clock according to the OLT's timestamp
When the OLT receives an MPCPDU in response to its MPCPDUit computes a "round-trip time" RTT (without handling times)it informs the ONU of RTT, which is used to compute transmit delay
RTT = (T2-T0) - (T1-T0) = T2-T1
OLT compensates all grants by RTT before sending
Either ONU or OLT can detect that timestamp drift exceeds threshold
time
OLT sends MPCPDUTimestamp = T0
ONU receives MPCPDUSets clock to T0
ONU sends MPCPDUTimestamp = T1
OLT receives MPCPDURTT = T2 - T1
T0OLT time T2T0ONU time T1
PONs Slide 88
AutodiscoveryAutodiscovery
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 89
GPON autodiscoveryGPON autodiscoveryEvery 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 and sends to ONU– ranging is performed– ONU is operational
PONs Slide 90
EPON autodiscoveryEPON 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 91
Failure recoveryFailure 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 92
Dynamic Bandwidth AllocationDynamic Bandwidth Allocation
MANs and WANs have relatively stationary BW requirementsdue 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
PONs Slide 93
GPON DBAGPON 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 94
EPON DBAEPON DBA
OLT sends GATE messages to ONUs
flags include DISCOVERY and Force_Report
Force_Report tells the ONU to issue a report
Reports represent the length of each queue at time of report
OLT may use any algorithm to decide how to send the following grants
DA SA 8808 Opcode=0002 timestamp Ngrants/flags …grants
DA SA 8808 Opcode=0003 timestamp Nqueue_sets …Reports
GATE message
REPORT message