Post on 27-Nov-2015
description
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GPON Technologies
Outline
PON benefits PON architecture Fiber optic basics PON physical layer PON user plane PON control plane
PON benefits
Why fiber ?
today’s high data rate 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 fiber10 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]
Aside – why is fiber better ?
attenuation per unit length reasons for energy loss
copper: resistance, skin effect, radiation, couplingfiber: 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-talkcopper couples to all nearby conductorsno similar ingress mechanism for fiber
ground-potential, galvanic isolation, lightning protectioncopper can be hard to handle and dangerousno concerns for fiber
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 problematicmore expensive to deploy than for copper
digital processing requires electronicsso need to convert back to electronicswe will call the converter an optical transceiveroptical transceivers are expensive
switching easier with electronics (but possible with photonics)so pure fiber networks are topologically limited:
point-to-pointrings
copper fiber
Access network bottleneck
hard for end users to get high datarates because of the access bottleneck
local area networksuse copper cableget high datarates over short distances
core networksuse fiber opticsget high datarate over long distancessmall 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 transceiversso copper is the best choicethis severely limits the datarates
coreaccess
LAN
Fiber To The Curb
N end userscore
access network
feeder fiber
copper
Hybrid Fiber Coax and VDSLswitch/transceiver/miniDSLAM located at curb or in basementneed only 2 optical transceivers
but not pure optical solutionlower BW from transceiver to end usersneed complex converter in constrained environment
Fiber To The Premises
we can implement point-to-multipoint topology purely in optics
but we need a fiber (pair) to each end user
requires 2 N optical transceivers
complex and costly to maintain
N end userscore
access network
An obvious solution
deploy intermediate switches (active) switch located at curb or in basementsaves space at central officeneed 2 N + 2 optical transceivers
N end userscore
access network
feeder fiber
fiber
The PON solution
another alternative - implement point-to-multipoint topology purely in opticsavoid costly optic-electronic conversions use passive splitters – no power needed, unlimited MTBFonly 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
PON advantages
shared infrastructure translates to lower cost per customerminimal number of optical transceiversfeeder fiber and transceiver costs divided by N customersgreenfield per-customer cost similar to UTP
passive splitters translate to lower costcan be installed anywhereno power neededessentially unlimited MTBF
fiber data-rates can be upgraded as technology improvesinitially 155 Mbpsthen 622 Mbpsnow 1.25 Gbpssoon 2.5 Gbps and higher
PON
architecture
Terminology
like every other field, PON technology has its own terminologythe CO head-end is called an OLTONUs are the CPE devices (sometimes called ONTs in ITU)
the entire fiber tree (incl. feeder, splitters, distribution fibers) is an ODNall trees emanating from the same OLT form an OANdownstream 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
PON types
many types of PONs have been defined
APON ATM PON
BPON Broadband PON
GPON Gigabit PON
EPON Ethernet PON
GEPONGigabit 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
Bibliography
BPON is explained in ITU-T G.983.xGPON is explained in ITU-T G.984.xEPON 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
PON principles
(almost) all PON types obey the same basic principles
OLT and ONU consist of Layer 2 (Ethernet MAC, ATM adapter, etc.)optical transceiver using different s for transmit and receiveoptionally: Wavelength Division Multiplexer
downstream transmissionOLT broadcasts data downstream to all ONUs in ODNONU captures data destined for its address, discards all other dataencryption needed to ensure privacy
upstream transmissionONUs share bandwidth using Time Division Multiple AccessOLT manages the ONU timeslotsranging is performed to determine ONU-OLT propagation time
additional functionalityPhysical Layer OAMAutodiscoveryDynamic Bandwidth Allocation
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
(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
PON encapsulation
The majority of PON traffic is Ethernet
So EPON enthusiasts say use EPON - it's just Ethernet
That's true by definition - anything in 802.3 is Ethernet
and EPON is defined in clauses 64 and 65 of 802.3-2005
But don't be fooled - all PON methods encapsulate MAC frames
EPON and GPON differ in the contents of the headerEPON hides the new header inside the GbE preamble
GPON can also carry non-Ethernet payloads
DA SA T data FCSPON header
GPON 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.984G.984.1 : GPON general characteristicsG.984.2 : Physical Media Dependent layerG.984.3 : Transmission Convergence layerG.984.4 : management and control interface
Fiber optics - basics
© = 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
Multimode Graded-Index Fiber
Single-mode Fiber
Types of Optical FiberTypes of Optical Fiber
Popular Fiber Sizes
Click to edit Master text stylesSecond level
Third levelFourth level
Optical Loss versus WavelengthOptical Loss versus Wavelength
Total Dispersion
Multimode Dispersion
Chromatic Dispersion
Material Dispersion
Sources of DispersionSources of Dispersion
1 0 11
Multimode DispersionMultimode Dispersion
1 11 11 11
Dispersion limits bandwidth in optical fiber
1 0 11
Graded-index DispersionGraded-index Dispersion
1 10
1 0 1 1 10
In SM the limit bandwidth is caused by chromatic dispersion.
1
Single-Mode DispersionSingle-Mode Dispersion
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
Material Dispersion (Dmat)Material Dispersion (Dmat)
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
Laser
Laser Optical Power Output vs. Laser Optical Power Output vs. Forward CurrentForward Current
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
Wavelength-Division MultiplexingWavelength-Division Multiplexing
WDM DuplexingWDM Duplexing
BMCDR = Burst Mode Clock Data Recovery
OLT = Optical Line Termination
ONU = Optical Network Unit
Basic Configuration of PONBasic Configuration of PON
Typical PON Configuration and Typical PON Configuration and Optical PacketsOptical Packets
Eye diagram of ONU transceiver Eye diagram of ONU transceiver in burst mode operationin burst mode operation
Burst-Mode Transmitter in ONUBurst-Mode Transmitter in ONU
OLT Burst-Mode Receiver OLT Burst-Mode Receiver
Burst-Mode CDRBurst-Mode CDR
Ideal, error-free transmission
Superimposed interference
Hysteresis
Ideal sampling instant
SamplingSampling
Transceiver Block DiagramTransceiver Block Diagram
Optical SplittersOptical Splitters
Optical Protection SwitchOptical Protection Switch
Optical SplitterOptical Splitter
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
Assume:
Optical loss = 0.35 db/km
Connector Loss = 2dB
Splitter Insertion Loss 1X32 = 17dB
Range Budget: ~11Km
Typical Range Calculation Typical Range Calculation
Relationship between transmission distance Relationship between transmission distance and number of splitsand number of splits
GbE Fiber Optic CharacteristicsGbE Fiber Optic Characteristics
PON physical layer
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 customerbut 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
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 bandsthese 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
allocations - 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
Data rates (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
Reach 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
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 propagationeach standard and each direction use different LFSRsLFSR initialized with all onesLFSR sequence is XOR'ed with data before transmission
EPON uses the 802.3z (1000BASE-X) line code - 8B/10BEvery 8 data bits are converted into 10 bits before transmissionDC removal and timing recovery ensured by mappingSpecial 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
FEC
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)
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 thresholdEPON: 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 timeand ONU lasers must stabilize quickly after being turned on
US 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
PON User plane
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 timeDBA - 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
Labels
In an ODN there is 1 OLT, but many ONUs
ONUs must somehow be labeled forOLT to identify the destination ONUONU to identify itself as the source
GPON has several levels of labelsONU_ID (1B) (1B)
Transmission-CONTainer (AKA Alloc_ID) (12b) (can be >1 T-CONT per ONU)
For ATM modeVPIVCI
For GEM modePort_ID (12b) (12b)
PON
ONU
ONU
T-CONT
T-CONT
Port
Port
VP
VP VCVCVCVC
DS GPON format
GPON Transmission Convergence frames are always 125 sec long19440 bytes / frame for 1244.16 rate38880 bytes / frame for 2488.32 rate
Each GTC frame consists of Physical Control Block downstream + payloadPCBd 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
GPON payloads
GTC 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
GPON Encapsulation Mode
A common complaint against BPON was inefficiency due to ATM cell tax
GEM is similar to ATMconstant-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 BytesPort ID - identifies the target ONUPayload 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
Ethernet / TDM over GEM
When transporting Ethernet traffic over GEM:only MAC frame is encapsulated (no preamble, SFD, EFD)MAC frame may be fragmented (see next slide)
When transporting TDM traffic over GEM:TDM input buffer polled every 125 sec.PLI bytes of TDM are inserted into payload fieldlength 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 fragmentation
GEM can fragment its payload
For example
GEM fragments payloads for either of two reasons:GEM frame may not straddle GTC frame
GEM frame may be pre-empted for delay-sensitive data
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
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 BytesAlen - Next 12 bits are length of ATM partition in cellsCRC - 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 considerations
GTC fames are still 125 sec long, but shared amongst ONUs
Each ONU transmits a burst of datausing timing acquired by locking onto OLT signal
according to time allocation sent by OLT in BWmapthere 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 format
4 different US overhead types:
Physical Layer Overhead upstreamalways sent by ONU when taking over from another ONUcontains 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 upstreamsends traffic status to OLT in order to enable DBA computation
PLOu PLOAMd PLSu DBRu payload
if all OH types are present:
US allocation example
BWmap sent by OLT to ONUs is a list of ONU allocation IDsflags (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
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 neededdo not take fiber-tappers into account
EPON does not provide any standard encryption methodcan supplement with IPsec or MACsecmany vendors have added proprietary AES-based mechanismsin 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 attack24b key turned out to be derivable in 512 tries
So G.983.3 added AES support - now used in GPON
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 ONUs46b = 16b intra-frame + 30 bits inter-frameintra-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
QoS
GPON treats QoS explicitlyconstant length frames facilitate QoS for time-sensitive applications5 types of Transmission CONTainers
type 1 - fixed BWtype 2 - assured BWtype 3 - allocated BW + non-assured BWtype 4 - best efforttype 5 - superset of all of the above
GEM adds several PON-layer QoS featuresfragmentation enables pre-emption of large low-priority framesPLI - explicit packet length can be used by queuing algorithmsPTI bits carry congestion indications
PON control plane
Principles
GPON uses PLOAMd and PLOAMu as control channel
PLOAM are incorporated in regular (data-carrying) frames
Standard ITU control mechanism
Ranging
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 overlapguard times left between timeslotseach ONU transmits with the proper delay to avoid overlapdelay computed during a ranging process
Ranging 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 enoughEPON: ONUs are accurately time synchronized (std contains jitter masks)
with time offset by OLT-ONU propagation time
GPON ranging method
Two types of ranginginitial ranging
only performed at ONU boot-up or upon ONU discoverymust 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 ONUsthus 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)
Autodiscovery
OLT needs to know with which ONUs it is communicating
This can be established via NMSbut even then need to setup physical layer parameters
PONs employ autodiscovery mechanism to automatediscovery of existence of ONUacquisition of identityallocation of identifieracquisition of ONU capabilitiesmeasure physical layer parametersagree on parameters (e.g. watchdog timers)
Autodiscovery procedures are complex (and uninteresting)so we will only mention highlights
GPON autodiscovery
Every ONU has an 8B serial number (4B vendor code + 4B SN)SN of ONUs in OAN may be configured by NMS, orSN may be learnt from ONU in discovery phase
ONU activation may be triggered byOperator commandPeriodic polling by OLTOLT searching for previously operational ONU
G.984.3 differentiates between three cases:cold PON / cold ONUwarm PON / cold ONUwarm PON / warm ONU
Main steps in procedure:ONU sets power based on DS messageOLT sends a Serial_Number request to all unregistered ONUsONU responds OLT assigns 1B ONU-ID and sends to ONUranging is performedONU is operational
Failure recovery
PONs must be able to handle various failure states
GPONif ONU detects LOS or LOF it goes into POPUP state
it stops sending traffic USOLT detects LOS for ONUif there is a pre-ranged backup fiber then switch-over
EPONduring 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 outONU is deregistered
Dynamic 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 discoveredby passively observing the traffic from the ONUby ONU sending reports as to state of its ingress queues
The goals of a Dynamic Bandwidth Allocation algorithm are maximum fiber BW utilizationfairness and respect of priorityminimum delay introduced
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 tokensnonlinear 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