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ONS MSTP DWDM Networking Primer October 2003

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Presentation on theme: "ONS MSTP DWDM Networking Primer October 2003"— Presentation transcript:

1 ONS 15454 MSTP DWDM Networking Primer October 2003

2 Agenda Introduction Optical Fundamentals
Dense Wavelength Division Multiplexing (DWDM)

3 Optical Fundamentals

4 Some terminology Decibels (dB): unit of level (relative measure)
X dB is 10-X/10 in linear dimension e.g. 3 dB Attenuation = = 0.501 Standard logarithmic unit for the ratio of two quantities. In optical fibers, the ratio is power and represents loss or gain. Decibels-milliwatt (dBm) : Decibel referenced to a milliwatt X mW is 10log10(X) in dBm, Y dBm is 10Y/10 in mW. 0dBm=1mW, 17dBm = 50mW Wavelength (): length of a wave in a particular medium. Common unit: nanometers, 10-9m (nm) 300nm (blue) to 700nm (red) is visible. In fiber optics primarily use 850, 1310, & 1550nm Frequency (): the number of times that a wave is produced within a particular time period. Common unit: TeraHertz, 1012 cycles per second (Thz) Wavelength x frequency = Speed of light   x  = C speed of light is 299,792,458 m/s (meters per second - that is equal to miles per second ), but to be correct we must point out that this speed light has in vacuum.

5 Some more terminology Attenuation = Loss of power in dB/km
The extent to which lighting intensity from the source is diminished as it passes through a given length of fiber-optic (FO) cable, tubing or light pipe. This specification determines how well a product transmits light and how much cable can be properly illuminated by a given light source. Chromatic Dispersion = Spread of light pulse in ps/nm-km The separation of light into its different coloured rays. ITU Grid = Standard set of wavelengths to be used in Fibre Optic communications. Unit Ghz, e.g. 400Ghz, 200Ghz, 100Ghz Optical Signal to Noise Ration (OSNR) = Ratio of optical signal power to noise power for the receiver Lambda = Name of Greek Letter used as Wavelength symbol () Optical Supervisory Channel (OSC) = Management channel

6 dB versus dBm dBm used for output power and receive sensitivity (Absolute Value) dB used for power gain or loss (Relative Value)

7 Bit Error Rate ( BER) BER is a key objective of the Optical System Design Goal is to get from Tx to Rx with a BER < BER threshold of the Rx BER thresholds are on Data sheets Typical minimum acceptable rate is bit error ratio (BER): The number of erroneous bits divided by the total number of bits transmitted, received, or processed over some stipulated period. Note 1: Examples of bit error ratio are (a) transmission BER, i.e., the number of erroneous bits received divided by the total number of bits transmitted; and (b) information BER, i.e., the number of erroneous decoded (corrected) bits divided by the total number of decoded (corrected) bits. Note 2: The BER is usually expressed as a coefficient and a power of 10; for example, 2.5 erroneous bits out of 100,000 bits transmitted would be 2.5 out of 105 or 2.5 × 10-5. 10-12 = 1 bit error in 10 Trillion bits received !

8 Basic Optical Budget = Output Power – Input Sensitivity
Pout = +6 dBm R = -30 dBm Budget = 36 dB Optical Budget is affected by: Fiber attenuation Splices Patch Panels/Connectors Optical components (filters, amplifiers, etc) Bends in fiber Contamination (dirt/oil on connectors)

9 Fiber Optics Requires Very High Purity Glass
Glass Purity Fiber Optics Requires Very High Purity Glass Window Glass 1 inch (~3 cm) Optical Quality Glass 10 feet (~3 m) Fiber Optics 9 miles (~14 km) Propagation Distance Need to Reduce the Transmitted Light Power by 50% (3 dB)

10 Fiber Fundamentals Attenuation Dispersion Nonlinearity Distortion
It May Be a Digital Signal, but It’s Analog Transmission Transmitted Data Waveform Waveform After 1000 Km

11 Analog Transmission Effects
Attenuation: Reduces power level with distance Dispersion and Nonlinearities: Erodes clarity with distance and speed Signal detection and recovery is an analog problem

12 Fiber Geometry An optical fiber is made of three sections: Core
Cladding An optical fiber is made of three sections: The core carries the light signals The cladding keeps the light in the core The coating protects the glass Coating

13 Propagation in Fiber n2 Cladding
q1 q0 n1 Core Intensity Profile Light propagates by total internal reflections at the core-cladding interface Total internal reflections are lossless Each allowed ray is a mode

14 Different Types of Fiber
Cladding Multimode fiber Core diameter varies 50 mm for step index 62.5 mm for graded index Bit rate-distance product >500 MHz-km Single-mode fiber Core diameter is about 9 mm Bit rate-distance product >100 THz-km n1 Core n2 Cladding n1 Core

15 Wavelength: l (nanometers)
Optical Spectrum UV IR 125 GHz/nm l Visible Light Ultraviolet (UV) Visible Infrared (IR) Communication wavelengths 850, 1310, 1550 nm Low-loss wavelengths Specialty wavelengths 980, 1480, 1625 nm 850 nm 980 nm 1310 nm 1480 nm 1550 nm 1625 nm Wavelength: l (nanometers) Frequency: ¦ (terahertz) C =¦ x l

16 Optical Attenuation Specified in loss per kilometer (dB/km)
0.40 dB/km at 1310 nm 0.25 dB/km at 1550 nm Loss due to absorption by impurities 1400 nm peak due to OH ions EDFA optical amplifiers available in 1550 window 1550 Window 1310 Window

17 Optical Attenuation Pulse amplitude reduction limits “how far”
Attenuation in dB Power is measured in dBm: Examples 10dBm 10 mW 0 dBM 1 mW -3 dBm 500 uW -10 dBm 100 uW -30 dBm 1 uW ) P i P T T

18 Types of Dispersion Chromatic Dispersion
Different wavelengths travel at different speeds Causes spreading of the light pulse Polarization Mode Dispersion (PMD) Single-mode fiber supports two polarization states Fast and slow axes have different group velocities Causes spreading of the light pulse

19 A Snapshot on Chromatic Dispersion
Interference Affects single channel and DWDM systems A pulse spreads as it travels down the fiber Inter-symbol Interference (ISI) leads to performance impairments Degradation depends on: laser used (spectral width) bit-rate (temporal pulse separation) Different SM types

20 Limitations From Chromatic Dispersion
Dispersion causes pulse distortion, pulse "smearing" effects Higher bit-rates and shorter pulses are less robust to Chromatic Dispersion Limits "how fast“ and “how far” 10 Gbps t 60 Km SMF-28 40 Gbps t 4 Km SMF-28 basica di prova - autore Andreoli 1

21 Combating Chromatic Dispersion
Use DSF and NZDSF fibers (G.653 & G.655) Dispersion Compensating Fiber Transmitters with narrow spectral width

22 Dispersion Compensating Fiber
By joining fibers with CD of opposite signs (polarity) and suitable lengths an average dispersion close to zero can be obtained; the compensating fiber can be several kilometers and the reel can be inserted at any point in the link, at the receiver or at the transmitter

23 Dispersion Compensation
Total Dispersion Controlled +100 -100 -200 -300 -400 -500 Cumulative Dispersion (ps/nm) No Compensation With Compensation Distance from Transmitter (km) Dispersion Shifted Fiber Cable Transmitter Dispersion Compensators

24 How Far Can I Go Without Dispersion?
Specification of Transponder (ps/nm) Distance (Km) = Coefficient of Dispersion of Fiber (ps/nm*km) A laser signal with dispersion tolerance of 3400 ps/nm is sent across a standard SMF fiber which has a Coefficient of Dispersion of 17 ps/nm*km. It will reach 200 Km at maximum bandwidth. Note that lower speeds will travel farther.

25 Polarization Mode Dispersion
Caused by ovality of core due to: Manufacturing process Internal stress (cabling) External stress (trucks) Only discovered in the 90s Most older fiber not characterized for PMD Most fiber prior to 1996 not characterized for PMD. Recommend any fiber being considered for 10G or 40G transmission to be characterized for PMD, as values can be effected by installation methods (too much pulling tension or other stresses), and is subject to change over time. (vs Chromatic Dispersion is relatively constant, and can be compensated for with non-active devices such as DCUs). PMD mitigators or under development/review, but have limited installed base.

26 Polarization Mode Dispersion (PMD)
Ex Ey Pulse As It Enters the Fiber Spreaded Pulse As It Leaves the Fiber nx ny The optical pulse tends to broaden as it travels down the fiber; this is a much weaker phenomenon than chromatic dispersion and it is of little relevance at bit rates of 10Gb/s or less

27 Combating Polarization Mode Dispersion
Factors contributing to PMD Bit Rate Fiber core symmetry Environmental factors Bends/stress in fiber Imperfections in fiber Solutions for PMD Improved fibers Regeneration Follow manufacturer’s recommended installation techniques for the fiber cable

28 Types of Single-Mode Fiber
SMF-28(e) (standard, 1310 nm optimized, G.652) Most widely deployed so far, introduced in 1986, cheapest DSF (Dispersion Shifted, G.653) Intended for single channel operation at 1550 nm NZDSF (Non-Zero Dispersion Shifted, G.655) For WDM operation, optimized for 1550 nm region TrueWave, FreeLight, LEAF, TeraLight… Latest generation fibers developed in mid 90’s For better performance with high capacity DWDM systems MetroCor, WideLight… Low PMD ULH fibers

29 Different Solutions for Different Fiber Types
SMF (G.652) Good for TDM at 1310 nm OK for TDM at 1550 OK for DWDM (With Dispersion Mgmt) DSF (G.653) OK for TDM at 1310 nm Good for TDM at 1550 nm Bad for DWDM (C-Band) NZDSF (G.655) Good for DWDM (C + L Bands) Extended Band (G.652.C) (suppressed attenuation in the traditional water peak region) OK for TDM at nm OK for DWDM (With Dispersion Mgmt Good for CWDM (>8 wavelengths) The primary Difference is in the Chromatic Dispersion Characteristics

30 The 3 “R”s of Optical Networking
A Light Pulse Propagating in a Fiber Experiences 3 Type of Degradations: Pulse as It Enters the Fiber Pulse as It Exits the Fiber Loss of Energy Shape Distortion Loss of Timing (Jitter) (From Various Sources) t ts Optimum Sampling Time Phase Variation

31 The 3 “R”s of Optical Networking (Cont.)
The Options to Recover the Signal from Attenuation/Dispersion/Jitter Degradation Are: Pulse as It Enters the Fiber Pulse as It Exits the Fiber Amplify to Boost the Power Re-Shape DCU Re-Generate O-E-O Re-gen, Re-shape and Remove Optical Noise t ts Optimum Sampling Time Phase Re-Alignment Phase Variation t ts Optimum Sampling Time t ts Optimum Sampling Time


33 Agenda Introduction Components Forward Error Correction DWDM Design Summary

34 Increasing Network Capacity Options
Same bit rate, more fibers Slow Time to Market Expensive Engineering Limited Rights of Way Duct Exhaust More Fibers (SDM) Same fiber & bit rate, more ls Fiber Compatibility Fiber Capacity Release Fast Time to Market Lower Cost of Ownership Utilizes existing TDM Equipment WDM As the need for more capacity increased over the years, first for voice traffic, today mostly for internet traffic, different solutions have been adopted. The simplest one is Space Division Multiplexing: it simply means to deploy and use more links of the same type. This approach is very expensive since it uses up all available resources and asks for infrastructure upgrades A more efficient solution came in with TDM technologies. In this case, we keep the same transmission medium but we increase the bit rate over it. If you go back a few years, SDH/SONET equipment, and routers as well, was transmitting at 155 Mbps, then 622 Mbps, finally 2.5 Gbps and just recently 10 Gbps. It is true that the transmission medium is always the same, but the transmission equipment is getting more and more complicated and expensive. Additionally, the maximum transported capability over a fiber pair is iin the range of a few Gbps. The way to scale to higher transported capacity is WDM. This technology keeps the same fiber, the same bit rate, but uses multiple colours to multiply transported capacity. Faster Electronics (TDM) Higher bit rate, same fiber Electronics more expensive

35 Single Fiber (One Wavelength) (Multiple Wavelengths)
Fiber Networks Time division multiplexing Single wavelength per fiber Multiple channels per fiber 4 OC-3 channels in OC-12 4 OC-12 channels in OC-48 16 OC-3 channels in OC-48 Wave division multiplexing Multiple wavelengths per fiber 4, 16, 32, 64 channels per system Channel 1 Single Fiber (One Wavelength) Channel n l1 l2 Single Fiber (Multiple Wavelengths) ln

36 TDM and DWDM Comparison
TDM (SONET/SDH) Takes sync and async signals and multiplexes them to a single higher optical bit rate E/O or O/E/O conversion (D)WDM Takes multiple optical signals and multiplexes onto a single fiber No signal format conversion DS-1 DS-3 OC-1 OC-3 OC-12 OC-48 SONET ADM Fiber OC-12c OC-48c OC-192c DWDM OADM Fiber

37 DWDM History Early WDM (late 80s) “Second generation” WDM (early 90s)
Two widely separated wavelengths (1310, 1550nm) “Second generation” WDM (early 90s) Two to eight channels in 1550 nm window 400+ GHz spacing DWDM systems (mid 90s) 16 to 40 channels in 1550 nm window 100 to 200 GHz spacing Next generation DWDM systems 64 to 160 channels in 1550 nm window 50 and 25 GHz spacing

38 Why DWDM—The Business Case
Conventional TDM Transmission—10 Gbps 40km 40km 40km 40km 40km 40km 40km 40km 40km TERM 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR TERM TERM 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR TERM TERM 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR TERM TERM 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR 1310 RPTR TERM DWDM Transmission—10 Gbps OC-48 OC-48 OC-48 OC-48 120 km OC-48 120 km 120 km OC-48 OC-48 OC-48 OA OA OA OA 4 Fibers Pairs 32 Regenerators 1 Fiber Pair 4 Optical Amplifiers

39 Drivers of WDM Economics
Fiber underground/undersea Existing fiber Conduit rights-of-way Lease or purchase Digging Time-consuming, labor intensive, license $15,000 to $90,000 per Km 3R regenerators Space, power, OPS in POP Re-shape, re-time and re-amplify Simpler network management Delayering, less complexity, less elements The are several drivers to WDM economics. The main point is that the fiber is out there in quantity. Over the past 15 years service providers (at those times all incumbent service providers), deployed fiber everywhere, underground, undersea, within aerial links, in order to provide a low loss medium for their transmission equipment. Today, fiber deployment has not stopped. However, there are additional constraints since you need right of way permissions before you install new fiber. The good opportunities to deploy fiber are already gone. In some city it is even prohibited to deploy new fiber since citizens are bored with unavailable curbs. That’s why fiber is now something you can lease or buy Apart from permissions, deploying new fiber is time consuming and expensive. Main reason is the cost for digging, accounting for over 80% of the total cost of a new fiber installation For these reason WDM systems are much appreciated since they can leverage the existing infrastructure to transport more capacity. In addition the use of WDM systems leads to additional advantages compared to standard TDM technologies. You need less electronic regenerators, with significant savings in term of space, power and operational expenses Last, the modern approach to transport networks includes de-layering, that is direct interconnection of IP services over WDM systems. When traditional TDM equipment is moved out from the equation, you get less complexity, less elements and a simpler network to manage

40 Characteristics of a WDM Network Wavelength Characteristics
Transparency Can carry multiple protocols on same fiber Monitoring can be aware of multiple protocols Wavelength spacing 50GHz, 100GHz, 200GHz Defines how many and which wavelengths can be used Wavelength capacity Example: 1.25Gb/s, 2.5Gb/s, 10Gb/s 50 100 150 200 250 300 350 400

41 Optical Transmission Bands
Wavelength (nm) 1260 – 1360 “New Band” 1360 – 1460 S-Band 1460 – 1530 C-Band 1530 – 1565 L-Band 1565 – 1625 U-Band 1625 – 1675 The majority of DWDM systems today operate in the C-Band. Moving into L next, then potentially S. CWDM operates primarily across S-C-L, with Extended Band fibers (like Allwave and SMF-28e) opening up the window to support additional CWDM wavelengths.

42 ITU Wavelength Grid l  ITU-T l grid is based on 191.7 THz + 100 GHz
nm nm 0.80 nm 195.9 THz 193.0 THz 100 GHz ITU-T l grid is based on THz GHz It is a standard for laser in DWDM systems Saw this in the Optical Fundamentals course as well.

43 Fiber Attenuation Characteristics
Attenuation vs. Wavelength S-Band:1460–1530nm L-Band:1565–1625nm 2.0 dB/Km C-Band:1530–1565nm Fibre Attenuation Curve 0.5 dB/Km The spectral attenuation curve shows the wavelength-dependence of many of the loss factors. This diagram shows the span of wavelengths used in telecom fibre, from 600nm to 1600nm. The normal operating “bands” of wavelengths, centered around 850nm, 1300nm, and 1550nm, are highlighted. They are areas of the lowest loss (or low-cost sources in the case of the 850nm band). The effects of Rayleigh Scattering are decreased with longer wavelengths. UV absorption also decreases with wavelength. However, IR absorption picks up at the longer wavelengths and effectively limits the higher wavelength operations. The “OH” peaks are caused by absorption of light due to the OH ions created in the manufacturing process . They are also called “water peaks” because the OH ion is a component of water -- H2O. 0.2 dB/Km 800 900 1000 1100 1200 1300 1400 1500 1600 Wavelength in Nanometers (nm)

44 Ability to put multiple services onto a single wavelength
Characteristics of a WDM Network Sub-wavelength Multiplexing or MuxPonding Ability to put multiple services onto a single wavelength

45 Why DWDM? The Technical Argument
DWDM provides enormous amounts of scaleable transmission capacity Unconstrained by speed of available electronics Subject to relaxed dispersion and nonlinearity tolerances Capable of graceful capacity growth

46 Agenda Introduction Components Forward Error Correction DWDM Design

47 Optical Add/Drop Multiplexer
DWDM Components l1 850/1310 15xx l1...n l2 l3 Transponder Optical Multiplexer Optical Add/Drop Multiplexer (OADM) l1 l1 l1...n l2 l2 l3 l3 Optical De-multiplexer

48 Variable Optical Attenuator Dispersion Compensator (DCM / DCU)
More DWDM Components Optical Amplifier (EDFA) Optical Attenuator Variable Optical Attenuator Dispersion Compensator (DCM / DCU)

49 Typical DWDM Network Architecture
DWDM SYSTEM DWDM SYSTEM VOA EDFA DCM DCM EDFA VOA Service Mux (Muxponder) Service Mux (Muxponder)

50 Transponders Converts broadband optical signals to a specific wavelength via optical to electrical to optical conversion (O-E-O) Used when Optical LTE (Line Termination Equipment) does not have tight tolerance ITU optics Performs 2R or 3R regeneration function Receive Transponders perform reverse function OEO l2 ln l1 From Optical OLTE To DWDM Mux Low Cost IR/SR Optics Wavelengths Converted

51 Performance Monitoring
Performance monitoring performed on a per wavelength basis through transponder No modification of overhead Data transparency is preserved

52 Laser Characteristics
Non DWDM Laser Fabry Perot DWDM Laser Distributed Feedback (DFB) l Power lc l lc Power Spectrally broad Unstable center/peak wavelength Dominant single laser line Tighter wavelength control So, any transponder acts as a converter between an optical incoming signal and an optical output signal. This means we have two light sources on any transponder, one facing the client and one facing the WDM side Due to different requirements, this light sources are generally different. The one facing the client is a fabry perot laser. This is a spectrally broad laser, with a non well stabilized center wavelength, but it is low cost and that’s the reason why it is used The WDM side asks for a better performance and here we find DFB laser. A DFB laser is much more than a fabry perot laser since a periodic corrugation of the waveguide has been added (a grating). The result is an expensive component with improved optical performances. First of all we have a narrow llinewidth (50 MHz), a more stable wavelength and higher power Active medium Mirror Partially transmitting Amplified light

53 DWDM Receiver Requirements
Receivers Common to all Transponders Not Specific to wavelength (Broadband)

54 Optical Amplifier G EDFA amplifiers
Pin Pout = GPin G EDFA amplifiers Separate amplifiers for C-band and L-band Source of optical noise Simple

55 OA Gain and Fiber Loss OA gain is centered in 1550 window
Typical Fiber Loss 25 THz 4 THz OA Gain OA gain is centered in 1550 window OA bandwidth is less than fiber bandwidth

56 Erbium Doped Fiber Amplifier
Isolator Coupler Coupler Isolator Erbium-Doped Fiber (10–50m) Pump Laser Pump Laser Two basic types of amplifiers: Co-directional (pumping) and Counter-directional. Co-directional pumping results in good SNR performance (pre-amp), where as Counter-directional results in better gain performance (Post/Power amp). One amplifier does not necessary fit all applications!, especially in LH networks. “Simple” device consisting of four parts: Erbium-doped fiber An optical pump (to invert the population). A coupler An isolator to cut off backpropagating noise

57 Optical Signal-to Noise Ratio (OSNR)
Signal Level X dB Noise Level Depends on : Optical Amplifier Noise Figure: (OSNR)in = (OSNR)outNF Target : Large Value for X EDFA Schematic (OSNR)out (OSNR)in NF Pin OSNR Calculations For a single optical amplifier between transmitter and receiver For multi-stage optical amplifiers use the formula : Where: SNROUT is the OSNR at the output power SNRIN is the OSNR of the previous amplifier f is the noise figure (ratio) h is Planck’s constant v is the light frequency B is the BW measuring the noise figure PIN is the input power to the amplifier

58 Loss Management: Limitations Erbium Doped Fiber Amplifier
Each EDFA at the Output Cuts at Least in a Half (3dB) the OSNR Received at the Input Noise Figure > 3 dB Typically between 4 and 6 Each amplifier adds noise, thus the optical SNR decreases gradually along the chain; we can have only have a finite number of amplifiers and spans and eventually electrical regeneration will be necessary Gain flatness is another key parameter mainly for long amplifier chains Gain flatness can be managed with Gain Flattening Filters (GFFs) and/or pre-emphasis.

59 Optical Filter Technology
Dielectric Filter l1,l2,l3,...ln l1, ,l3,...ln l2 Well established technology, up to 200 layers

60 Multiplexer / Demultiplexer
DWDM Mux DWDM Demux Wavelength Multiplexed Signals Wavelength Multiplexed Signals Wavelengths Converted via Transponders Wavelengths separated into individual ITU Specific lambdas Loss of power for each Lambda

61 Optical Add/Drop Filters (OADMs)
OADMs allow flexible add/drop of channels Drop Channel Drop & Insert Add Channel Pass Through loss and Add/Drop loss

62 Agenda Introduction Components Forward Error Correction DWDM Design Summary

63 Transmission Errors Errors happen!
A old problem of our era (PCs, wireless…) Bursty appearance rather than distributed Noisy medium (ASE, distortion, PMD…) TX/RX instability (spikes, current surges…) Detect is good, correct is better Whatever transmission technology we choose to send bits, there is a finite probability to have errors during the transmission That’s why techniques to identify and possible correct errors are not at all new (think of memory, disks, wireless…) (make a digression on PC RAM affected by Cosmic rays in Madrid more than in Milan due to altitude, hint to pkzip). The new thing in optical transmission is how errors appear. They are bursty rather than distributed. Among the reasons, we can mention amplifiers noise, signal distortion, the statistics of PMD and other bursty impairments such as TX/RX spikes and current surges due to interference or power supply instability Wherever they are from, errors need to be detected and even better corrected. Transmitter Receiver Transmission Channel Information Noise

64 Error Correction Error correcting codes both detect errors and correct them Forward Error Correction (FEC) is a system adds additional information to the data stream corrects eventual errors that are caused by the transmission system. Low BER achievable on noisy medium Read… As an analogy, try to consider a noisy restaurant where everyone is speaking loudly. You will not be able to catch all words from your host since for example every 10th word from the person speaking is inaudible. The listener still understands what the speaker is trying to say because information in their speech offers clues to the missing or garbled words. In a digital-communications system, FEC works the same way, allowing the receiver to "fix" errors based on additional information transmitted within the original data

65 FEC Performance, Theoretical
FEC gain  BER Received Optical power (dBm) Bit Error Rate 10 -30 -10 -46 -44 -42 -40 -38 1 -20 -36 -34 -32 BER without FEC BER with FEC Coding Gain BER floor From the practical point of view, the most important parameter characterizing a FEC code is its coding gain. This plot shows the bit error rate (BER) of a digital transmission system with and without FEC. On the y-axis we have the BER in a logarithmic scale. On the x- axis we have the received optical power that, for a given noise, is directly proportional to the OSNR. The trace without FEC is the blue one. The BER decreases as we increase the received optical power. The slope of the curve depends on the system noise (electronic or optical) and receiver characteristics. Typically it is in the range of 1 dB of OSNR per decade of BER. In practical systems we also have noise floors. This means that we can’t improve the BER even if we continue to increase the optical power on the receiver (ma a cosa e’ dovuto il noise floor? Inoltre lo vedo se ho un grafico contro Pin o anche contro OSNR????) The red trace is obtained with FEC. Note that this trace is much steeper than the other one and noise floors can’t be seen. If we compare the amount of power required to achieve a given BER (let’s say 10E-12), we immediately see that with FEC we need less power than without FEC. This power difference (or OSNR difference) is known as coding gain and it is measured in dB. Note that if we make the same comparison at a different BER we have a different coding gain: the lower the BER, the higher the coding gain. So, be careful when listening to advertisement from system vendors. In order to compare FEC performances, you have to specify the BER at which you make the comparison! In order to fully appreciate the performance improvement with FEC, let’ see the theoretical results with RS(255,239) code. With an input BER of 8 x 10E-5 we already have an output BER of 10E-15. The coding gain with this algorithm is about 6.3 dB at 10E-15 BER (fatti spiegare bene la differenza fra OSNR e potenza ricevuta!)

66 FEC in DWDM Systems FEC implemented on transponders (TX, RX, 3R)
9.58 G 10.66 G 10.66 G 9.58 G IP IP FEC FEC SDH FEC FEC SDH . . ATM FEC FEC ATM This is how a commercial DWDM system with FEC would look like. As you see, the difference with a system without FEC is only in the transponders: the rest is unaffected Since people installed DWDM first at 2.5 Gbps, FEC is actually used to keep the system working also when upgraded to 10 Gbps, with no change to the network layout. Many service providers have been using 2.5 Gbps transponders with no FEC and now they are installing on the same system 10 Gbps transponders with FEC in order to keep the same BER with a four times higher transport capability 2.48 G 2.66 G 2.66 G 2.48 G FEC implemented on transponders (TX, RX, 3R) No change on the rest of the system

67 Agenda Introduction Components Forward Error Correction DWDM Design Summary

68 DWDM Design Topics DWDM Challenges Unidirectional vs. Bidirectional Protection Capacity Distance

69 Transmission Effects Attenuation: Dispersion and nonlinear effects:
Reduces power level with distance Dispersion and nonlinear effects: Erodes clarity with distance and speed So in summary we have attenuation that reduced power level over distance, and we have dispersion and non linear effects eroding signal clarity with distance and speed. Additionally we have noise and jitter leading to a blurred image Noise and Jitter: Leading to a blurred image

70 Solution for Attenuation
Optical Amplification Loss How do we take care of attenuation in optical fibers? What is the countermeasure? Very easy: optical amplification. We will have more on this later on. The idea is to take a weak optical signal and simply amplify it, without affecting the shape or the clock. This is also known as 1R regeneration OA

71 Solution For Chromatic Dispersion
Saw Tooth Compensation Dispersion Dispersion DCU DCU Fiber spool Fiber spool Total dispersion averages to ~ zero And what about chromatic dispersion? We need to compensate for it. This is done by inserting into the WDM system special elements, called dispersion compensating units, capable to introduce in the system an amount of dispersion equal and opposed to the dispersion in the transmission fiber. DCU are typically inserted in every amplifying site. So, if we follow accumulated dispersion along the fiber we have a linear increase in the first span, then we go down to zero (actually near zero, need residual dispersion to counter FWM!) with the compensating unit, next we start again accumulating dispersion in the second span and then back again to zero with the second DCU and so on…The goal is to keep the total link dispersion close to zero. From the practical point of view, with state of the art technology, this can be achieved for a specific wavelength, whereas the others will experience a small amount of residual dispersion, but this is not a big deal if it is within the receiver tolerance Blue line delimits the maximum range of accumulated dispersion +D -D Length

72 Uni Versus Bi-directional DWDM
DWDM systems can be implemented in two different ways Uni -directional l 1 l 3 l 5 l 7 Fiber l 2 l 4 l 6 l 8 Uni-directional: wavelengths for one direction travel within one fiber two fibers needed for full-duplex system Bi-directional: a group of wavelengths for each direction single fiber operation for full-duplex system There is no single architecture for WDM systems. Over the years, vendors have developed different solutions to provide WDM capability for addressing specific requirements. A first big distinction is based on the number of fibers required to connect two nodes. There are two different implementation schemes: unidirectaional and bidirectional systems (…just read the slide…) Unidirectional systems using two fibers are by far the most popular ones and we will see why in the next few slides Bi -directional l 5 l 6 l 7 l 8 Fiber l 1 l 2 l 3 l 4

73 Uni Versus Bi-directional DWDM (cont.)
Uni-directional 32 channels system 32 ch full duplex Bi-directional 32 channels system In order to better understand the principle of operation, let’s compare the two implementations based on a 32 channels example. In a unidirectional DWDM system, this means we have 32 colors travelling down the fiber in the same direction (from node A to node B). Eventually a chain of amplifiers will be found on the link to extend distances. At the same time we have another 32 channels, with the same colors as before, travelling back for node B to node A on a second fiber strand. Thus a 32 channel unidirectional WDM system provides a 32 channel full duplex connectivity On the contrary, in a bidirectional DWDM system we have 32 different colors travelling down the single fiber, but 16 in one direction (TX) and 16 in the other (RX). In the end, a 32 channel bidirectional WDM system provides only a 16 channel full duplex connectivity. 16 ch full duplex

74 DWDM Protection Review
Y-Cable and Line Card Protected Client Protected Unprotected Splitter Protected

75 Unprotected 1 Client Interface 1 Transponder 1 client & 1 trunk laser (one transponder) needed, only 1 path available No protection in case of fiber cut, transponder failure, client failure, etc..

76 Client Protected Mode 2 Transponders 2 Client interfaces 2 client & 2 trunk lasers (two transponders) needed, two optically unprotected paths Protection via higher layer protocol

77 Optical Splitter Protection
Working lambda Optical Splitter Switch protected lambda Only 1 client & 1 trunk laser (single transponder) needed Protects against Fiber Breaks

78 Line Card / Y- Cable Protection
working lambda Only one TX active 2 Transponders “Y” cable protected lambda 2 client & 2 trunk lasers (two transponders) needed Increased cost & availability

79 Designing for Capacity
Distance Bit Rate Solution Space Wavelengths Goal is to maximize transmission capacity and system reach Figure of merit is Gbps • Km Long-haul systems push the envelope Metro systems are considerably simpler

80 Designing for Distance
L = Fiber Loss in a Span Pin Pout Pnoise S G = Gain of Amplifier Amplifier Spacing D = Link Distance Link distance (D) is limited by the minimum acceptable electrical SNR at the receiver Dispersion, Jitter, or optical SNR can be limit Amplifier spacing (S) is set by span loss (L) Closer spacing maximizes link distance (D) Economics dictates maximum hut spacing

81 Link Distance vs. OA Spacing
Amp Spacing 20 60 km 10 80 km Wavelength Capacity (Gb/s) 100 km 5 120 km 140 km 2.5 2000 4000 6000 8000 Total System Length (km) System cost and and link distance both depend strongly on OA spacing

82 OEO Regeneration in DWDM Networks
Long Haul OA noise and fiber dispersion limit total distance before regeneration Optical-Electrical-Optical conversion Full 3R functionality: Reamplify, Reshape, Retime Longer spans can be supported using back to back systems

83 3R with Optical Multiplexor and OADM
Back-to-back DWDM 1 1 Express channels must be regenerated Two complete DWDM terminals needed 2 2 3 3 4 4 N N 7 7 Optical add/drop multiplexer Provides drop-and- continue functionality Express channels only amplified, not regenerated Reduces size, power and cost 1 1 2 2 OADM 3 3 4 4 N N 7 7

84 Agenda Introduction Components Forward Error Correction DWDM Design Summary

85 DWDM Benefits DWDM provides hundreds of Gbps of scalable transmission capacity today Provides capacity beyond TDM’s capability Supports incremental, modular growth Transport foundation for next generation networks

86 Metro DWDM Metro DWDM is an emerging market for next generation DWDM equipment The value proposition is very different from the long haul Rapid-service provisioning Protocol/bitrate transparency Carrier Class Optical Protection Metro DWDM is not yet as widely deployed


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