Introduction to Optical Networking: From Wavelength Division Multiplexing to Passive Optical Networking Dr. Manyalibo J. Matthews Optical Data Networking Research Bell Laboratories, Lucent Technologies Murray Hill, NJ 07974 USA University of Tokyo Visit – March 22, 2004
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Introduction to Optical Networking: From Wavelength Division
Multiplexing to Passive Optical Networking
Dr. Manyalibo J. MatthewsOptical Data Networking Research
Bell Laboratories, Lucent TechnologiesMurray Hill, NJ 07974 USA
Attenuation:Due to Rayleigh scattering and chemical absorptions, the light intensity along a fiber decreases with distance. This optical loss is a function of wavelength (see plot).
Dispersion: Different colors travel at different speeds down the optical fiber. This causes the light pulses to spread in time and limits data rates.
Types of DispersionChromatic Dispersion is caused mainly by thewavelength dependence of the index of refraction (dominant in SM fibers)
Modal Dispersion arises from the differences in group velocity between the “modes” travelling down the fiber (dominant in MM fibers)
t
t t
t
launch receive
Non-Linear Effects in Fibers
Self-Phase Modulation: When the optical power of a pulse is very high, non-linear
polarization terms contribute and change the refractive index, causing pulse spreading and delay.
Four-wave Mixing: Non-linearity of fiber can cause ‘mixing’ of nearby wavelengths causing interference in WDM systems.
Stimulated Brillouin Scattering: Acoustic Phonons create sidebands that
can cause interference.
Cross-Phase Modulation: Same as SPM, except involving more than one WDM channel, causing cross-talk
between channels as well.
800 900 1000 1100 1200 1300 1400 1500 1600 1700
0.5
1.0
1.5
2.0
2.5
3.0
FirstWindow Second
Window
ThirdWindow
ATTEN
UA
TIO
N (
dB
/km
)
WAVELENGTH (nm)
1310nm 1550nm
Attenuation/Loss in Optical Fiber
• First Window @ 850nm– High loss; First-gen. semiconductor diodes (GaAs)
• Second Window @ 1310nm – Lower Loss; good dispersion; second gen. InGaAsP
• Third Window @ 1550nm– Lowest Loss; Erbium Amplification possible
850nm
First window, second window, third window correspond (roughly) to first, second and third generation optic network technology
Dispersion Characteristics*
1310nm 1550nm850nm
800 900 1000 1100 1200 1300 1400 1500 1600 1700
-120
-90
-60
-30
0
3.0
FirstWindow
SecondWindow
ThirdWindow
DIS
PER
SIO
N C
OEFF
, D
(p
s/km
-nm
)
WAVELENGTH (nm)
• Standard SMF has zero dispersion at 1310nm– Low Dispersion => Pulses don’t spread in time
• Dispersion compensation needed at 1550nm– Limits data transmission rate due to ISI (inter-
symbol interference)• Dispersion not so important at 850nm
– Loss usually dominates
* Modal dispersion not included
Characterization of System Quality
Bit Error Rate:input known pattern of ‘1’s and ‘0’s and see how manyare correctly recongnized at output.
Eye Diagram: Measure ‘openness’ of transmitted 1/0 pattern usingscope triggered on each bit.
‘Eye opening’
Effect of Dispersion and Attenuation on Bit Rate
30
10
1
Bit rate (Mb/s)
Dis
tan
ce (
km)
0.1 10 100 1000 10,0001
1550nm
1310nm850nm
Dispersion limitedAttenuation limited
single-mode fiber
multi-m
ode fiberCoaxialcable
• For short reaches (1-2 km), all optics are “Gigabit capable”• For longer reaches (~10 km), only 1310/1550 nm optics are “Gigabit capable”
20
x x
Cat 3 limit
Cat 7 limit
Cat 5 limit
x
Twisted Pair
Technology Trends
850nm & 1310nm Preferred by high-volume, moderate performancedata comm manufacturers
1310nm & 1550nm Preferred by high performancebut lower volume (today)telecomm manufacturers
Reason? You need lots of them, they don’t need to go far, and you’re not using enough fiber ($) to justify wavelengthdivision multiplexing (WDM), I.e. low-quality lasers are OK.
Reason? You don’t need lots, but they have to be good enough to transmit over long distances… cost of fiber (and TDM) justifies WDM… 1550nm is better for WDM
DFB vs. FP laser
Simple FP
mirror
gain
cleave
+
- mirror
gain
AR coating
+
-Etchedgrating
DFB
FP: • Multi-longitudinal Mode operation
• Large spectral width
• high output power
• Cheap
DFB: • Single-longitudinal Mode operation
• Narrow spectral width
• lower output power
• expensive
Fiber Bragg Grating External Cavity Laser for Access/Metro Networks
• SHOW PLOTS OF FBG-ECL DATA• SHOW PICTURE OF XPONENT’S EXTENDED REACH FP
CWDM:• Low channel count, large channel spacing• Uncooled DFBs can be used• Filters can be made athermal
xWDM?:• Moderate channel count, moderate channel spacing• FBG-ECL or Temp-stablized DFBs required• Filters can be made athermal• suitable for athermal WDM PON!
1260nm 1610nm
1480nm 1610nm
1480nm 1610nm
Example 1: 10Gbps Coarse WDM
-Used currently in Metro systems (rings, linear, mesh)-Spacing of CWDM ‘grid’ determined by DFB wavelength drift-Current systems limited to 2.5Gbps due to cheaper optics-Possible upgrade to 10Gbps?
Note on Lasers:-Use DFB at headend (shared)-Use FP at Homes (not shared)
DFB
FP
ONU Design
ReportGenerator
Packet Memory
TX
RX
ControlParser
Dem
ux
watchdog0
watchdog1
discoveryPeriodicReport
generatorEPON driver
EPON core
RX
TX
EPON MAC
Mux Timesta
mpCRC LLIDMemory
managerQueue
manager
GMII
SERDES&
Optics
CPUFPGA
Serial Port
GigE uplink
Packet memory
1.25G BM BiDi Xcvr
Flash (CPU)memory
10/100bTdiagnosticport
SERDES(w/CDR)
PON
FPGA w/EmbeddedProcessor
“CHILD” BOARD
“PARENT”BOARD
ONU
GrantList
GateGenerator
Packet Memory
RTT table
TX
RX
ControlParser
Dem
ux
watchdog0
watchdog1
discovery Keepalive scheduler
EPON driver MPCP driver
EPON core MPCP core
RX
TX
EPON MAC
Mux Timesta
mpCRC LLIDMemory
managerQueue
manager
RTT Processor
Report processor
GMII
SERDES&
Optics
Report table CPUFPGA
OLT Design
Serial Port
GigE uplink
Packet memory
1.25G BM BiDi Xcvr
Flash (CPU)memory
10/100bTdiagnosticport
SERDES(w/CDR)
PON
FPGA w/EmbeddedProcessor
• Downstream: continuous, MAC addressed– Uses Ethernet Framing and Line Coding– Packets selected by MAC address– QOS / Multicast support provided by Edge Router
• Upstream: Some form of TDMA– ONU sends Ethernet Frames in timeslots– Must avoid timeslot collisions– Must operate in burst-mode– BW allocation easily mapped to timeslots
EPON downstream/upstream traffic
1 2 3 2
1
2 2
3
1 2 3 21
2 2
3
12
32
1 2 3 2
12
32
1
2
2
OLT
OLT
3
3
3 3
ONU
ONUO
NU
ONU
ONUO
NU
Edge Router
ONU: Optical Network UnitOLT: Optical Line Termination
Edge Router
Control “Gates”
Control “Reports”
PON TDMA BURSTMODE OPTICS
• Because upstream transmissions must avoid collisions, each ONU must transmit only during allowed timeslot
• Transmitting “0”s during quiet time is not allowed!– Average “0” power ~ -10 to –5 dBm – Summing over 16 ONUs would result in a ~1dBm noise floor
• Distinct from “Bursty” nature of Ethernet TRAFFIC – Ethernet transmitters never stop transmitting (Idle characters)– CDR circuit at receiver stays locked even when no data is transmitted
• Besides PONs, other systems use burstmode– Wireless– Shared buses/backplanes– Optical burst switched (OBS) systems