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Download Unit 1.7 Optical networking and processing
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Optical Networking Introduction: What and why.... Implementing Optical Networking Developments in l conversion Future developments.... Evolution of Optical Networking Optical Networks DWDM + OADMs + OXCs DWDM + Optical ADMs Dense Wavelength Division Multiplexing Point-to-Point Fibre Transmission Optical Networking In most existing networks optical technology is used on links to transport signals Most processing is carried out electrically, so called node-by-node electrical processing Moving toward all-optical networks, where transport and processing is optical. Nodes Links Source: Master 7_7 Conventional Electronic & Optical Networking In most existing networks optical technology is used to transport signals across links Most processing is carried out electrically, so called node-by-node electrical processing Demands many expensive Optical to Electronic to Optical (OEO) conversions Not bit rate transparent Classic example is an SDH (Synchronous Digital Hierarchy) ring Drop Traffic Out Add Traffic in ADM based SDH Ring Fibre Links Optical Networking Optical technology is used to transport and process signals Reduces need for OEO conversions, replaced by OOO conversions, bit rate transparent But control of processing remains electronic. Optical processing is taking place in a time frame >> bit interval Signals are ultimately converted to an electronic form at the network edges "Photonic" networks is often used as alternative title Optical Network OXC: Optical Cross Connect OADM: Optical Add-drop multiplexer Source: Master 7_7 Disadvantages of Hybrid Electronic/Optical Networking Demands Optical to Electronic to Optical (OEO) conversions at each node Significant cost implication Not bit rate transparent Power dissipation and physical space are also issues But does allow very fine bit level control SDH Optical in SDH Optical Out SDH R/X SDH T/X Electrical levels SDH T/X SDH Proc. & Switching Electrical levels SDH Node SDH R/X SDH Optical Out SDH Optical in Taking stock of where we are: The Fourth Quadrant Electronic Photonic Analog FDM: manipulation of modulated carriers 1950s DWDM: manipulation of modulated carriers 1990s - 2000s Digital Electronic digital processing: direct manipulation of bits 1960s to 1990s Optical digital processing: direct manipulation of bits 2000s A. J. Poustie, R. J. Manning and A. Kelley @ BT Labs Why Optical Networking? Driving forces: The "electronic bit-rate bottleneck" The complexity/inflexibility/cost of hybrid electronic/optical systems The demand for more bandwidth Development of simple optical processing elements Widespread deployment of Dense Wavelength Division Multiplexing Potential for exploitation of the full capacity of singlemode fibre Driving Forces Driving forces toward optical networking: The demand for bandwidth The "electronic bottleneck" The complexity/inflexibility of hybrid electronic/optical systems The availability of technologies such as Dense Wavelength Division Multiplexing 300 Gb/s Bit rate 200 Gb/s needed on largest 100 Gb/s routes New broadband traffic Existing traffic 40 Gb/s 1998 2002 Source: Master 7_7 The Demand for Bandwidth…..? Predicted World Telephone and IP Traffic Growth (Source: Analysys) Millions of Terabytes 25 20 IP Traffic 15 10 Telephone Traffic 5 0 1996 1998 2000 2002 2004 Year 1999: What we thought would happen..... 2006 Looking back: Slowdown in Demand for Bandwidth & Equipment Germany United Kingdom Italy France Spain 1000's of people online in Europe Capital expenditure by North-American telecoms carrier companies (RHK Feb 2002) Demand for bandwidth does continue to grow, bandwidth costs much less Broadband to the home (eg. xDSL) technologies will raise demand in the core network Even worst case growth rates for internet traffic are 50%+ per year to 2008 Optical Functions Required Ultra-fast and ultra-short optical pulse generation High speed modulation and detection High capacity multiplexing Wavelength division multiplexing Optical time division multiplexing Wideband optical amplification Optical switching and routing Optical clock extraction and regeneration Ultra-low dispersion and low non-linearity fibre Greater Availability of Optical Processing Elements Tunable l Transmitters Receivers Fibre joints Optical Amplifiers AWG Mux/Demux Controllable attenuators Dispersion Compensation Simple Optical Switching Optical Filters Equalizers Isolators Couplers & splitters Fixed l Transmitter Receiver Fibre joints 1992 1998 2008 Optical Add/Drop Multiplexers Optical Add-Drop Multiplexer An Optical Add-Drop Multiplexer allow access to individual DWDM signals without conversion back to an electronic domain In the example below visible colours are used to mimic DWDM wavelengths Wavelengths 1,3 and 4 enter the OADM Wavelengths 1 and 4 pass through Wavelength 3 (blue) is dropped to a customer Wavelengths 2 (green) and a new signal on 3 (blue) are added Downstream signal has wavelengths 1,2,3 and 4 Wavelengths 1 2 3 4 Wavelengths 1 2 3 4 OADM 1234 Dropped Wavelength(s) 1234 Added Wavelength(s) First Generation Optical Add-Drop Multiplexer Simple OADM structure ODU demultiplexes all wavelengths and drops off wavelengths as required OMU multiplexes added wavelengths as well as those that pass through Disadvantages: Unnecessary demultiplexing and multiplexing of pass-through wavelengths Typical number of drop channels is limited to 25-50% of total payload ODU DWDM in Added Wavelengths OMU Pass-through wavelengths Dropped Wavelengths DWDM out ODU: Optical Demultiplex Unit OMU: Optical Multiplex Unit Simple Optical Network Example OADM: Optical Add-Drop Multiplexer Wavelengths 1 2 3 4 N o d e OADM OADM A N o d e B 1234 1234 N o d e D 1234 OADM OADM N o d e C Note wavelength reuse of "blue" wavelength (no. 3), links Node A and B as well as Node C and A Source: Master 7_7 Optical Cross-Connect (OXC) Need reconfigurable OADM, allows change to the added and dropped wavelengths OADM becomes an OXC (Optical Cross-Connect) Large number of DWDM wavelengths possible means a large number of ports Needs to be remotely configurable, intelligent Should be non-blocking, any combination of dropped/added possible In addition, insertion loss, physical size, polarization effects, and switching times are critical considerations. Incoming DWDM signal Outgoing DWDM signal OXC Dropped Wavelength Fibre Ports Added Wavelength Fibre Ports Source: Master 7_7 OADM Requirements In general an OADM should be able to: Add/Drop wavelengths in any order Be locally and remotely configurable Pass-through wavelengths should not be demultiplexed Provide a low loss and low noise path for pass-through wavelengths Reducing disturbance to pass-through wavelengths reduces need for OEO regeneration Wavelengths 1 2 3 4 Wavelengths 1 2 3 4 OADM 1234 1234 Components for a 2nd Generation OADM Optical Circulator Tunable Fibre Gratings l1....ln 1 2 li & lj 3 Light in port 1 2 3 Light out port 2 3 1 l1....ln less li & lj Gratings etched in fibre Can pass or reflect selected wavelengths Wavelength selection is tunable (thermal or piezoelectric strain) Diagram shows a series of gratings reflecting two wavelengths li & lj Lucent 2nd Generation Programmable OADM Input Circulator (IC) 1 Output Circulator (OC) 1 2 2 DWDM out DWDM in 3 Input Fibre Gratings (IFG) Output Fibre Gratings (OFG) 3 Demux Mux Drop Wavelengths Add Wavelengths All incoming wavelengths pass through the IC from port 1 to 2. At the IFG pass-through wavelengths continue toward the OC. The tuning of the IFG selects drop wavelengths that are reflected back to the IC to port 2 and are passed to port 3 to be demultiplexed. Add wavelengths are sent to port 3 of the OC and are passed to port 1 of the OC backward to the OFG. The OFG selects which, if any, of the add wavelengths are reflected forward to port 1 of the OC along with the pass-through wavelengths. At port 1 of the OC the selected add wavelengths and pass-through wavelengths are passed to the output at port 2. Sample Optical Networks Source: Master 7_7 Ciena MultiWave Metro System (I) MultiWave™ Metro is a Dense Wave Division Multiplexing (DWDM) system for use in metropolitan ring applications. The MultiWave Metro system consists of Optical Add/Drop Multiplexer (OADM) nodes connected together on a two fiber ring. It provides up to 24 duplex channels over a single fibre pair, allowing this single fibre pair to carry up to 120 Gb/s. ITU Compliant Channel Plan: MultiWave Metro uses 200 GHz channel spacing as specified in ITU G.692. Uses Standard fibre: MultiWave Metro operates on standard single mode fibre. Ciena MultiWave Metro System (II) The multi-colored columns in the figure represent MultiWave Metro nodes which are interconnected by a two-fiber ring. The different colors represent the different wavelengths available at each node. The colored lines interconnecting the nodes represent logical connections between nodes The connection formed by dropping wavelength 1 (red) at all nodes shows a typical SONET ring application where traffic is added and dropped at each node. The connection formed by dropping wavelength 2 (pink) at nodes 2, 3, 4, and 5 shows how a private SONET ring might be implemented Protection and Restoration in Optical Networks Optical Networks and Noise Amplifier Noise Protection Switching in Rings (I) Could use a classic 4 fibre bidirectional ring for protection Two fibres in each direction, one working, one backup or protection. In the event of a Cable fault signal is routed over an alternative fibre path Normal working linking nodes #1 and #2 #2 #1 Working fibre Protection fibre OAD M Ring #4 #3 Protection Switching in Rings (II) Assume a cable fault between OADM #1 and #2 Signal is now re-routed over protection fibre via nodes #4 and #3 #2 #1 Working fibre Protection fibre OADM Ring #4 #3 Protection Switching Issues in Optical Networks Three important issues arise in ring switching OADMs for protection: Wavelength reuse may mean a protection path is unavailable under certain conditions Signal quality may mean long protection paths are unworkable Mechanical switching reliability? Wavelength Reuse Issues (I) Same wavelength is used twice: Signal A uses node #1 to #2 Signal A Signal B uses node #3 to #4 No problem in the no fault state #2 Signal A #1 OAD M Ring #3 #4 Working fibre Protection fibre Signal B Signal B Wavelength Reuse Issues (II) Assume a cable fault between OADM #1 and #2 Signal is re-routed via OADM #4 and #3 as before Signal A But now Signal B has no protection path as the protection fibre between OADM #3 and #1 is using its wavelength Signal A #2 #1 OAD M Ring #3 #4 Working fibre Protection fibre Signal B Signal B Signal Quality Issues in OADM Rings Maintaining signal quality is a key issue, even without protection switching Protection switching makes signal quality even more difficult In SDH rings full regeneration takes place at ADMs but in an OADM this not so An optical wavelength transporting an SDH STM-N may pass through a number of amplifiers, fibre segments etc.. Signal may progressively deteriorate due to amplifier noise, crosstalk, dispersion etc.. Signal Quality on Conventional SDH ADM Rings In SDH rings full regeneration takes place Noise is completely removed at each ADM Quality of signal is independent of number of ADMs traversed #2 Electrical output Electrical input #1 ADM based SDH Ring #4 #3 Signal Quality on All-Optical OADM Rings Noise is added by amplifiers etc., both in-line and at OADMs Optical signal-to-noise ratio (OSNR) degrades as does the signal shape l1 Must be taken account of in ring design Different wavelengths may pass through different numbers of OADMs #2 Optical input l1 #1 l2 Simple OADM Ring #4 #3 l2 Protection Switching and Noise Say we use a 4 fibre bidirectional ring for protection In the event of a Cable fault signal is routed over a larger number of OADMs than expected Signal degradation increases: may mean that some protection paths are unworkable as OSNR degrades too much #2 #1 OAD M Ring #4 #3 Optical networks are close to realisation Routing and switching technologies need to be improved Several WDM based optical network topologies proposed By 2010 it is estimated that most Telecoms networks will have moved to an optical core Greater Availability of Optical Processing Elements Tunable l Transmitters Receivers Fibre joints Optical Amplifiers AWG Mux/Demux Controllable attenuators Dispersion Compensation Simple Optical Switching Optical Filters Equalizers Isolators Couplers & splitters Fixed l Transmitter Receiver Fibre joints 1992 1998 2004 All-Optical Wavelength Conversion Current DWDM Wavelength Convertors (Transponders) 1550 nm SDH SDH R/X 1550 nm SDH SDH T/X Electrical levels Electrical levels C band T/X C band signal l C Band R/X C band signal l Transponders are frequently formed by two transceivers back-to-back So called Optical-Electrical-Optical (OEO) transponders Expensive solution at present True all-optical transponders without OEO needed Approaches to Wavelength Conversion Requirements: Must preserve digital content, low BER penalty Must be agile, tunable, (band or individual wavelength possible) Highly non-linear fibre with a four wave mixer (Fujitsu) 32 x 10 Gb/s DWDM Converted band: 1535.8 -1560.6 nm to 1599.4 - 1573.4 nm Cross-polarization modulation in a semiconductor optical amplifier (COBRA, NL) 1 x 10 Gb/s DWDM Converted single l 1550.92 nm to 1552.52 nm Downsides of Present forms of Optical Processing Optical processing is taking place in a time frame >> a bit interval. This has several disadvantages: No access to digital overhead as in SDH/Sonet ADM Difficult to detect and isolate signal degradation May involve separate out-of-band signaling channel Finally as we have no OEO this means no signal regeneration Build-up of noise and crosstalk "Analog transmission" problems l1 #2 Optical input l1 #1 l2 Simple OADM Ring #3 l2 #4 Source: Master 7_9 All-Optical Regeneration All-Optical Regeneration We will need to get beyond the speed limitations of electronics, eg. 160 Gbits/sec + This mean all-optical solutions operating at speeds comparable to bit level time Basic function is regeneration and optical clock extraction Allows us to avoid OEO, while still regenerating bits Regeneration actually involves three sub-functions (so called 3R) All-optical clock extraction Decision/threshold Regeneration using simple optical logic Regen All-Optical Clock Extraction with Division (TCD/DIT) Active research topic in many labs All-optical clock extraction demonstrated with Nortel Uses twin section self-pulsating laser 500 ps 678 MHz Clock Component /div Extracted clock 678 MHz 1 ns /div SPAN 1.00 GHz Center 0.50 GHz Spectrum of 678 Mbit/s data Demonstrated with Northern Telecom Extracted and divided clock at 339 MHz Threshold System One current approach uses a Semiconductor Optical Amplifier (SOA) 40 Gbits/Sec demonstrated to date Other approaches involved fibre lasers, very fast Input Amplitude restoration of pulses with a threshold (BT Labs and Aston University) Threshold Output