Lecture 17 How do we communicate? Introduction Where are we? Power budgets budgeting segments. Transmissions capacity budgets. How is data transmitted? Standards Layers Crystal ball gazing. 1
Last time Joining and cleaving optical fibres Factors affecting splices Fusion and mechanical splices Connectors OTDR plots Power budgets System margin System deficits Power budget calculations Homework 1 : a. Calculate the system margin (deficit) for a 300km fibre link, made up of 10km fibres (Loss=0.5dB/km). Signals go through two connector pairs in a patch panel at each end. Assume splice loss=0.1db/splice. Laser power = 0.0dBm. Receiver sensitivity =-3.0dBm. b. In order to improve performance, it is decided to place amplifiers with a gain of 30dB at the 100km and 00km points. The amplifiers are placed in the link using one connector at each end (loss=0.8db/connector.) Calculate the system margin (deficit) in this case. 1. Example from Understanding Fiber Optics, Hecht
Example Part a. Mechanism Laser Power In Fibre Loss Splice Loss Connector Loss Power at Receiver Receiver Sensitivity System Deficit 0dBm 300km x- 0.5 db = -75dB 9 x -0.1dB = -.9dB 4 x 0.8dB = -3.dB -81.1dBm -3dBm -49.1dB Total Power 0dBm -75dBm -77.9dBm -81.1dBm Example Part b. Mechanism Laser Power In Fibre Loss Splice Loss Connector Loss Amplifier Gain Power at Receiver Receiver Sensitivity System Margin 0dBm 300km x- 0.5 db = -75dB 7 x -0.1dB = -.7dB 8 x 0.8dB = -6.4dB 60dB -4.1dBm -3dBm 7.9dB Total Power 0dBm -75dBm -77.7dBm -84.1dBm -4.1dBm 3
Budgeting Segments In order to check behaviour for the system, you should model each segment separately. This allows you to cope with behaviour such as gain saturation in the amplifier. Check you answer for part b. analysing each segment of the link, assuming a constant gain of 30dB for the amplifier. Segment 1 Mechanism Laser Power In Fibre Loss Splice Loss Connector Loss Power at Amplifier Amplifier Gain Output of segment 1 0dBm 100km x- 0.5 db = -5dB 9 x -0.1dB = -0.9dB 3 x 0.8dB = -.4dB -8.3dBm 30dB 1.7dBm Total Power 0dBm -5dBm -5.9dBm -8.3dBm 4
Segment Mechanism Power from Segment 1 Fibre Loss Splice Loss Connector Loss Power at Amplifier Amplifier Gain Output of Segment 1.7dBm 100km x- 0.5 db = -5dB 9 x -0.1dB = -0.9dB x 0.8dB = -1.6dB -5.8dBm 30dB 4.dBm Total Power 1.7dBm --3.3dBm -4.dBm -5.8dBm Segment 3 Mechanism Input from Segment Fibre Loss Splice Loss Connector Loss Power at Receiver Receiver Sensitivity System Margin 4.dBm 100km x- 0.5 db = -5dB 9 x -0.1dB = -0.9dB 3 x 0.8dB = -.4dB -4.1dBm -3dBm 7.9dB Total Power 4.dBm -0.8dBm -1.7dBm -4.1dBm As we found out from the composite case. 5
Other Components It is also possible to include other components in the system budget. For example you might have a splitter. The tactic here is to calculate the extra losses from the component and include them in you budget. Remember if it s a splitter to take into account the division between the individual channels. Again it s very important to consider details like amplifier saturation when performing your analysis. Getting the system design wrong could cost $ millions. Transmission Capacity Budget It is also important to work out what the maximum data rate our link can carry is. This is limited by the response time of our link. The longer the response time the less information we can carry. Can use some rules of thumb to get a quick approximation for our system capacity. For NRZ: BR max =0.7/ t where t is the total system response time. For RZ BR max =0.35/ t. 6
Calculating the transmission capacity budget t overall = t component Overall System Time Response Individual component response So: t overall = t transmitter + t receiver + t fibre Data Sheet Calculated What is t fibre? t fibre = t modal + t chromatic + t PMD So for single mode fibre: t ( D L λ) + ( D L ) fibre = chromatic fibre PMD Dispersion Coefficients 7
Transmission Capacity Budget Example Tx Rx 1550nm λ=0.1nm t rise =100ps t rise =100ps L fibre =300km D chromatic =3ps/nm/km D pmd =0.5ps/km 0.5 Have to calculate t fibre. Example continued. t ( D L λ) + ( D L ) fibre = chromatic fibre PMD = ( 3 300 0. 1) + ( 0. 5 300) = 90ps So: t = (100 +90 +100 ) = 168ps Therefore BR max =BR NRZ = 0.7 / (168x10-1 ) = 4 Gbit/s Can also use transmission capacity budgeting to design and specify components eg. The max allowable dispersion for 10GBit/s is 70ps. 8
Summary Had a very brief look at some systems design. Vitally important to plan your links before installation. Power budgeting allows us to se if our system will function. Transmission capacity budgeting allows us to check if we can shift the amount of data we want. Care must be taken to ensure that system design is based on real-world components. Eventually it will all come down to money. Will the system you design perform for the least price possible? Transporting information How does what you say in St Andrews get to San Francisco? DA POTS OTDM WDM WAN MAN OPS DWDM AD SAN Be warned this is an acronym rich environment. If you don t know it look it up! Thanks to Prof Krauss for bits in this section. 9
Global Communications Phone Calls Sound is a pressure wave. Converted by the microphone into an electronic signal. Voltage Microphone produces analogue signal. Time Need to convert to digital for our fibre network. 10
Analogue to Digital Conversion Voltage 4.9 4.3 Regular Samples. Analogue Signal Time Packet of digital data Nyquist criterion states that sampling frequency must be x highest frequency being sampled. Speech: f max =4kHz f sample > 8kHz 17 sampling levels give us a seven digit binary number. Total data rate = 7x8 kbit/s = 56 kbit/s Combining many signals Compression Combination Link capacity MUCH greater than an individual users requirements. Data processing allows packets to be compressed. Also need to put header information and parity checking into each packet. Compression is also possible. Can then combine many packets onto a single channel. This is the basis of Optical Time Division Multiplexing - OTDM 11
OTDM OTDM zips pulses of alternating channels together, doubling, quadrupling, or more, the data rate. Channel 1 in Channel in Multiplexer Multiplexed Output Optical Packets Pulse streams from individual users are then bundled together into packets and combined with packets from other users. This is only possible because transmission is much faster than communication. Packet A Packet B Packet C Thanks to Prof T. Krauss 1
Transmitting Packets Hello Pablo! How is the weather in Barcelona? Individual packets can travel by different routes through the network. Transmitting end of the network breaks the signal down. Receiving end puts the signal back together again. Allows all of the network to be used efficiently. Also allows fast rerouting of data where necessary. Header and timing information is vital Large drive to all optical packet switching. Fibre Networks 13
Standardisation Data is transmitted across may fibres run by many different countries / companies. Standardisation is vital to allow free flow of information. Most telecomms standards are based around the Open System Interconnection (OSI) model developed by the International Organisation for Standardisation. Fibre communications are normally carried out through SONET (Synchronous Optical Network) or Synchronous Data Hierarchy (SDH). SONET USA, SDH International SONET/SDH organize OTDM signals into defined blocks travelling at set data rates. This is a very complex subject and needs a course in its own right! Layers in a Telecomms System Services Voice Adaptation A to D Interchange Formats Asynchronous Transfer Mode - ATM Internet Protocol IP Transmission Formats Packaging for SONET/SDH Physical Layer SONET / SDH WDM Layer WDM if Required 14
Increasing Capacity - WDM Capacity can be further increased by transmitting signals at multiple wavelengths down the optical fibre. This is wavelength division multiplexing - WDM. Dalkeith Musselburgh Leith Edinburgh London The current record is 10.9 Tbit/s on 73 wavelength channels carrying 40 Gbit/s each. (NEC, Japan, March 001) Cable Development Figure from Uderstanding Optical Fibres - Hecht 15
The future for telecomms Recent company performance indicates that optical communications is moving back into profit. Requirement for bandwidth is increasing. Access and metro are growing rapidly. Still plenty of capacity long haul. The main driver is cost. Is your solution going to be economic? Still a requirement for future high bit rate links: WDM DWDM DWDM+OTDM???? Integrating optical functions onto a single chip Photonic Bandgap Technology? Near-disposable components Organic Semiconductors? fs-based technology new lasers and amplifiers! What will be the impact of e-healthcare? What will be the impact of security applications? BT Prediction from January 004 16
Scientific Opportunities New Science at the Limits Single photon confinement photonic bandgaps Single cycle pulse generation ultrashort pulse lasers Single photon/electron interactions quantum dots New Photonic Materials New semiconductors, polymers, crystals New Photonic Devices Environmentally tolerant lasers Versatile ultrashort-pulse lasers Broad bandwidth amplifiers Integrated photonics devices & components New System Technologies Ultrahigh bandwidth concepts TDM/WDM modulation, CDMA Optically routed systems And Finally Optical communications has revolutionised the way the world communicates and has opened up vast new areas of opportunity. Many technological hurdles have been overcome, but many more remain the is still one of the most exciting areas of interdisciplinary science and engineering. Optical fibres will be as revolutionary in the 1 st century as railways were in the 19 th. 17
Conclusions Power budgets Budgeting segments. Transmission budgets dispersion performance. Transmitting information Packets Layers in the optical network Future requirements Scientific opportunities 18