access passive optical networks -...
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Access Passive Optical Networks
PoliComFondazione POLITECNICO DI MILANO
Paola Parolari
Optical Access Network
Access networks: traditionally called last-mile networksLast segment connection from service providers central office (CO) to end usersOptical fibers widely used in backbone networks
huge available bandwidthvery low loss
Recently importance as the technology of last-mile connection for next-generation access
“Killer” applications:how much bandwidth is enough?
Carriers are merging their video and data delivery platform into a unified platform basedon IP technologies
reduce capital and operational expenditures VOD has become the killer application for broadband access network development
requirement of at least 100 Mbps per broadband household
Capacity vs distance
For short reaches (1-2 km), all optics are “Gigabit capable”For longer reaches (~10 km), only 1310/1550 nm optics are “Gigabit capable”
30
10
1
Bit rate (Mb/s)
Dis
tanc
e (k
m)
0.1 10 100 1000 10,0001
1550nm
1310nm850nm
Dispersion limitedAttenuation limited
single-mode fiber
multi-mode fiberCoaxialcable
20
x x
Cat 3 limit
Cat 7 limit
Cat 5 limit
x
Twisted Pair
Fiber To The x (FFTx)
Service Node
ONU
FTTH
FTTB
FTTC
FTTCab
Optical Fiber
xDSL
OLTONU NT
NT
Passive Optical Splitter
FTTx
FTTC:Fiber To The CurbFTTCab :Fiber To The Cabinet
FTTH :Fiber To The HomeFTTB :Fiber To The Building
Internet
Leased Line
Frame/CellRelay
Telephone
Interactive Video
Twisted Pair
ONT
ONT
Optical Distribution Network (ODN)
hybrid
Fiber Optical Network
Fiber structure to reach the different customers Active optical networks (AONs) Passive optical networks (PONs)
1:x PassiveOptical SplittersOC3OC3
OC12OC12IP In-BandIP In-BandRF Overlay RF Overlay
downstream
upstream
Central Office 20 km / 10-25 dB loss
OLTOLT
Video
Data
PSTNOC3OC3OC1OC122
)ONU
ONU
ONU
Central Office node (CO)
Service provider endpoint of a PON: placed at the centraloffice or head end in systemsOptical line terminal (OLT) sends and receives messagesor data to/from optical network units (ONUs) connectedvia ODN
OC3OC3OC12OC12
IP In-BandIP In-BandRF Overlay RF Overlay
Central Office
OLTOLT
Video
Data
PSTNOC3OC3OC1OC122
Optical Network Terminal (ONT)
User nodesONUs in subscriber neighborhoodterminating the optical fiber transmission lineproviding electrical signals over metallic linesONUs receives data from OLT by PONS and converts theoptical signal into electrical
Optical Distribution Network (ODN)
Fiber links (10- 40 km)Remote nodes (RN)
Power splitters (1:16, 1:32)WDM splitter (Arrayed Waveguide Grating)
RN
RN
TDM-PON WDM-PON
Topology
Tree and branch (P2MP) Star (P2P)
BusRing
Advantages
Full exploitation of a single fiber to serve up to N (e.g.) 32subscribersOptical Fibers has huge bandwith and low loss providing greaterflexibility for adding future servicesLow cost of equipment per subscriberPassive components require little maintenance and have a highMTBF (Mean Time Between Failure)Additional buildings can be added to the network easilySupports a broad range of applications including triple play (voice,data, video)Flexible and scalable bandwidth assignment
Standardization
ITU-T G.983 (BPON) standardITU-T G.984 (GPON) standardIEEE 802.3 Ethernet PON (EPON or GEPON) standardITU-T G.652 standard for WDM fibers
• IEEE - Institute of Electrical & Electronic Engineers (http://www.ieee.org)
• ITU - International Telecommunication Union (http://www.itu.int)
In early ‘90 Full Service Access Network (FSAN)
Two-Fiber vs One-Fiber
Two separate fibers for bidirectional communications (space divisionduplex)
No separation of US (ONU to OLT) and DS (OLT to ONU) signals in time, frequency, or wavelength domainsSimple to implement1.3-µm for both US and DS with low-cost Fabry-Perot (FP) lasersExpensive from both the capital and operational standpoints
One-Fiber Single-Wavelength Full DuplexBudget loss (3-dB coupler)Near-end cross talk (NEXT)
Time vs wavelength duplex
Time division duplex approach: OLT and ONU turns to use the fiber in aping-pong fashion for upstream and downstream transmissions
Use directional couplersNEXT is avoided, but reduced system throughput by about 50%
OLT coordinates the time slots assigned for US and DSBurst mode receivers both the OLT and ONU
Wavelength Division Duplex (CWDM)reduced the connector reflectivity requirements at the RN ONU: US low cost 1.3-µm F-P LDOLT: DS 1.55-µm DFB (cost shared by the multiple ONUs)
Splitting ratio (SR)
OLTPassivesplitter
Commercial PON systems: splitting ratio of 1:16 or 1:32A higher SR means that the cost of the PON OLT is better shared among ONUsSR affects system power budget: high-power transmitters, high sensitivity receivers, and low-loss optical componentsHigh SR: OLT bandwidth is shared among more ONUs thus less bandwidth per user
Commercial TDM PON
OLT
FiberBPO
NG
PON
Max 32 way split (may becascaded)
OLT implementations may not necessarily support all PON technologies indicated
•Typically: 622 Mbps/155 Mbps (down/up)
•ATM-based transport
LU #1
LU #N, N ≤32
Fiber
Max 64 way split [constrainedby PMD attenuation limits]
• Typically: 2488/1244 Mbps
• GFP-like transports (Ethernet, and/orTDM)
LU #1
LU #N, N ≤64
EPO
N
Fiber
• 1250 Mbps/1250 Mbps [~850 Mbpseffective payload rate]
• Ethernet-based transport
LU #1
LU #N, N ≤32
20 km Maximum Reach20 km ONU differential range
B-PON
G-PON
E-PON
Max 32 way split (16-wayspecified in standard)
splitter
splitter
splitter
ITU-T G.983.x
ITU-T G.984.x
1000BASE-PX20per IEEE 802.3ah
Network optical transceiver (TXR)
TXR
TXR
TXRLU #N, N ≤32
ONT
ONT
ONT
Commercial TDM PON
Multiple OLTs in the CO are interconnected with a backbone switch or XCPON section: signals encoded and multiplexed in different formats and schemes
depending on the PON standard implementedStandard format used for client interface for hand-off, switching, and cross-connectSignals from and to different ONUs are frame interleaved:
each frame is identified with a unique ONU ID in the frame headerDownstream link: one-to many broadcast connectionUpstream: many-to-one connection
communications between ONUs need to be forwarded to the CO and relayed
OLT and ONU
Physical medium dependent (PMD) layer defines the optical transceiver and the wavelengthdiplexer at an OLT or ONU
Medium access control (MAC) layer schedules the right to use the PM avoiding contentionMAC OLT is master and MAC ONU serves as client
OLT service adaptation layer provides the translation between the backbone signal formatsand PON section signals
The interface from an OLT to the backbone is the service network interface (SNI)ONU service adaptation layer provides the translation between client equipment signal format
and PON signal formatThe interface from an ONU to client network equipment is the user network interface (UNI)
Ranging
DS: OLT interleaves the frames as a continuous stream and broadcast to all ONUsEach ONU extracts its own frame based on the header addressUS: ONUs need to take turns to send their data to the OLTif ONU is not sending upstream data, it has to turn off to avoid interferingONU transmits in burst mode: first sends a preamble sequence to the OLTThe OLT uses the preamble as a training sequence: adjust decision threshold, perform
synchronizationTo avoid collision between bursts: scheduling upstream transmission by the OLT MAC layerNecessary to establish a timing reference between the OLT and ONUs:RANGING process
I. OLT sends out a ranging request to ONU(s) to be rangedII. An ONU participating in ranging replies with a ranging responseIII. OLT measures the round-trip time (RTT) from the ranging responseIV. OLT updates the ONU with measured RTT
Ranging is usually done at the time an ONU joins a PONThe OLT periodically broadcasts ranging requests for ONU discovery
Security
In power-splitting PON the DS channel broadcast nature favorseavesdropping
The biggest security exposure is in the ranging processOLT broadcasts the ID of the ranged ONUinformation can be used for spoofingavoided through authentication process
Churning procedure to scramble the data for downstream connectionswith an encryption key
B- PON
ONT
ONT
ONT
A
B
C
A B C + GRANT A B C +GRANT
A B C + GRANT
A B C + GRANT
ONT
ONT
ONT
A
B
C
A
Upstream: Time Division Multiple AccessDownstream: Time Division Multiplex
Downstream: 622 Mbps @ 1490nm
Upstream: 155 Mbps @ 1310nm
Outside Plant: Optical Distribution Network Customer Premise: ONTCO, Feeder: OLTOLT distribute the 8-kHzclock timing to ONUs
ATM switch,PSTN,Internet
Services to user:POTS,Internet Access
Passive Optical Splitter
G- PON
Downstream (single -fiber systems): 1490 nm Upstream: 1310 nm RF video (if present) 1555 nm
TDM Time Division Multiplex TDMA Time Division Multiple Access CC Cross Connect NB Narrow Band BB Broadband OLT Optical Line Termination ONT Optical Network Termination
TDM ONT2
ONT32
1:32 Optical splitter (or 1:64 for shorter reaches or
with Reach Extender)
OLT
Access Node
NB
BB CC Video
Data
E1/T1/ Telephony
Data
E1/DS1
GbE STMn/OCn
ONT1
E1/DS1/ Telephony
POTS
VOIP
(and/or)
TDMA
Up to 60 km* physical reach(* with G.984.6 Reach Extender)
Multilongitudinal-mode (MLM) lasers cannot be used at ONU to avoid excessive dispersion penalty.Loss budget requirements: use of APDsG-PON transmission convergence (GTC) layer functions:Transport multiplexing between the OLT and ONUs Adaptation of client layer signal protocolsPhysical layer OAM (PLOAM) functions Interface for dynamic bandwidth allocation (DBA)ONU ranging and registration Forward error correction (optional)Downstream data encryption (optional) Communication channel for the OMCI
155.52 Mb/s622.08 Mb/s1244.16 Mb/s2488.32 Mbps
1244.16 Mb/s2488.32 Mb/s
G- PON
1260 1280 1300 1320 1340 1360 1380 1400 1420 1460 1480 1500 1520 1540 1560 1580 1600 1620 1640 16601440 1680
O-band E-band S-band C-band L-band U-band
G.984.2
Legend: GPON Up
GPON Dn
RF Overlay Present
Future
* Requires the use of reduce water peak fiber (G.652.C/D)** the upper-limit value is determined as an operator choice from 1580 to 1625 nm
G.984.5
or
or
NG-PON
Regular (FP)
Reduce (DFB)
Narrow (CWDM)
NG-PON (G.9xx)
A BNG-PON Option 1 *
CNG-PON Option 2**
D
E- PON
1.0 Gbps both in the upstream and downstream directions8B/10B line coding ( IEEE802.3z gigabit Ethernet standard)
The downstream physical link maintains continuous signal stream and clock synchronizationCircuit emulation is needed to implement fixed-bandwidth TDM circuits
Convergence
1.0
WDM-PON
Access node
OLT
SNI
wavelengthsplitter
1 to N λs on single fiber
Wavelength selection here
dedicated λ1 pair
dedicated λ2 pair
ONTBitrate 1
ONTBitrate 2
Feeder Fiber
Colorless ONTs: Transmitter and Receiver front-endfilter characteristics are wavelength adaptable
TDMAONT
(Fixed Optics)power
splitter
Hybrid WDM-PON
AW
G
ONTBitrate N
dedicated λΝ pair
WDM-PON
Advantages:Passive optical distribution plant: low maintenance and high-reliability of PS-PONEach user receives its own wavelength: excellent privacyP2P connections between OLT and ONUs are realized in wavelength domain:
simplifies the MAC layerEasy pay-as-you-grow upgrade: λ channels are independent
Challenges :High costs of WDM componentsTemperature control: athermal WDM components Colorless ONU operation
PS-networks: components
Fabry-PerotLaser
mirror
gain
cleave
+
- λ mirror
gain
AR coating
+
- Etchedgrating
λ
Distributed Feed BackLaser
FP: Multi-longitudinal mode operationLarge spectral widthHigh output powerCheap
DFB: Single-longitudinal Mode operationNarrow spectral widthLower output powerExpensive
Sources
Optical Power SplitterSignificant improvements by the introduction of planar lightwave circuit (PLC) technology
High reliabilityLow cost per portLow insertion lossHigh splitting ratio uniformity
WDM-PON: components
Arrayed waveguide grating (AWG)Passive WDM mux/demux WDM routing component at RNCyclic propertyCentral λ of a conventional silica-based AWG shifts ( 0.0125 nm/°C)
Athermal operation Usage of guiding materials with negative thermo-optic coefficientIncorporating a mechanically movable compensation plate in the AWG structure
WDM-PON: components
Colorless ONUThe emission λ is nonspecific and determined by external factors
RN AWG filtering properties injection/seeding light λ
identical ONUs can be mass-produced and deployed across the network
I. Upstream data modulated broadband source (LED) spectrally sliced at the RN AWGII. Injection-locked FP-LD or a wavelength-seeded RSOAIII. Source-free ONUs. US data modulated onto the DS carrier and sent back to CO
Acronyms
Acronyms
Acronyms
Acronyms
Acronyms