Future Internet Chapter 2: Optical Networking Holger Karl Computer - PowerPoint PPT Presentation

Future Internet Chapter 2: Optical Networking Holger Karl Computer Networks Group Universität Paderborn

Goals of this chapter • Optical networks realize today’s Internet’s immense transport capacity • This chapter • Looks into some of the physical properties that make optical networks possible • Identifies some of the resulting challenges (in particular, no fast switches) • Discuss the algorithmic challenges ensuing from physical properties/limitations of optical networks SS 19, v 1.1.1 FI - Ch 2: Optical networking 2

Content • Optical devices • Optical networks • Optimization problems SS 19, v 1.1.1 FI - Ch 2: Optical networking 3

Transmitters • Light-emitting diodes (LED, cheap) • Simple lasers • Work well, but downside: multiplexing 100 wavelengths needs 100 different simple lasers • Standard operation: 10s to 100s of wavelengths carried by a fibre • Tunable lasers (!) • Simplify practical logistics compared to simple lasers • Make reconfiguration of optical networks practical • Put n tunable lasers at a node for n lightpaths; pick wavelength • But: complex, expensive, only starting to be commercially available SS 19, v 1.1.1 FI - Ch 2: Optical networking 4

Directional Coupler • Combines and splits signals arriving on input towards output • Splitting ratio: Percentage of power going from one input to opposite output • E.g., 3dB coupler: 50:50 power distribution; useful for switches • 95:5 couplers for tapping, monitoring, … • Can be frequency-flat or frequency-selective • Phase is shifted by ¼ /2 when crossing over to other arm Input 1 Output 1 Fibers or waveguides l (coupling length) Input 2 Output 2 Figure 3.1 A directional coupler. The coupler is typically built by fusing two fibers together. It can also be built using waveguides in integrated optics. From [1] SS 19, v 1.1.1 FI - Ch 2: Optical networking 5

Isolators and Circulators • Isolator: only one-directional (unlike most optical devices) • Circulators: clever combination of isolators 2 2 1 1 3 3 4 (a) (b) Figure 3.3 Functional representation of circulators: (a) three-port and (b) four-port. The arrows represent the direction of signal flow. From [1] SS 19, v 1.1.1 FI - Ch 2: Optical networking 6

Multiplexers and Filters � ��� ��� ��� � � � � Wavelength � � filter � ��� ��� � � � • Multiplexers and filters: (a) In frequency domain � � � ��� ��� ��� � � � � � � Wavelength � � multiplexer � � (b) � � � � � � � � � � � � � ��� ��� ��� � ��� ��� ��� � � � � � � � � � � � � Demultiplexer Multiplexer � � � � � � � � � ��� ��� ��� � ��� ��� ��� � � � � � � � � Figure 3.7 A static wavelength crossconnect. The device routes signals from an input port to an output port based on the wavelength. From [1] SS 19, v 1.1.1 FI - Ch 2: Optical networking 7

Gratings: Mach-Zehnder Interferometer (MZI) Input 1 Output 1 • Couplers combined with Path difference, � L delay • Acts as filters Input 2 Output 2 or (a) (de)multiplexer Input 1 Output 1 MZI s ( � L ) Input 2 Output 2 (b) Input 1 Output 1 MZI MZI MZI MZI ( ) (2 ) (4 ) (8 ) � L � L � L � L Input 2 Output 2 (c) Figure 3.21 (a) An MZI constructed by interconnecting two 3 dB directional couplers. (b) A block diagram representation of the MZI in (a). � L denotes the path difference between the two arms. (c) A block diagram of a four-stage Mach-Zehnder interferometer, From [1] which uses different path length differences in each stage. SS 19, v 1.1.1 FI - Ch 2: Optical networking 8

Gratings: Arrayed Waveguide Grating (AWG) • Think of AWG as an MZI generalized to multiple ports /--01,2 304,5'62,7 $%&'( .'(&'( 304,5'62,7 304,5'62,7 $%&'( .'(&'( )*'&+,- )*'&+,- Figure 3.24 An arrayed waveguide grating. � � � � � � � � � ��� ��� ��� � ��� ��� ��� � � � � � � � � � � � � � � � � � ��� ��� ��� Arrayed � ��� ��� ��� � � � � � � � � waveguide � � � � � � � � � ��� ��� ��� � ��� ��� ��� grating � � � � � � � � � � � � � � � � � ��� ��� ��� � ��� ��� ��� � � � � � � � � Figure 3.25 The crossconnect pattern of a static wavelength crossconnect constructed from an arrayed waveguide grating. The device routes signals from an input to an output based on their wavelength. From [1] SS 19, v 1.1.1 FI - Ch 2: Optical networking 9

Acousto-Optical Tunable Filter • Make an AWG controllable by external input (of sound waves) From [1] SS 19, v 1.1.1 FI - Ch 2: Optical networking 10

Optical switches • Goals: • Provision new lightpaths • Switch to backup in case of failure (“protection switching”) • Ideally: switch on individual packets Time BETWEEN two packets From [1] SS 19, v 1.1.1 FI - Ch 2: Optical networking 11

Example technology: MEMS-based switch From [1] SS 19, v 1.1.1 FI - Ch 2: Optical networking 12

Wavelength converters • Device to convert one wavelength into another wavelength • Needed to adapt between different optical technologies • … or to increase switching flexibility • Variations: • Fixed input, fixed output • Variable input, fixed output: all incoming wavelengths (in range) converted to one specific one on output • Fixed input, variable output: Convert a specific incoming wavelength into a selectable output one • Variable input, variable output: … SS 19, v 1.1.1 FI - Ch 2: Optical networking 13

Content • Optical devices • Optical networks • Optimization problems SS 19, v 1.1.1 FI - Ch 2: Optical networking 14

Optical networks – Client layers • The large times necessary to modify a switch make an optical network essentially circuit-switched • How to transport packets (e.g., IP) over such a network? • “Client layers” introduce intermediate functions • SONET/SDH: Historical from telco carriers; complicated time division multiplexing hierarchy based on a fixed basic rate (51.84 Mbit/s); framing structure (125 µ s, irrespective of line rate) • Optical Transport Network (OTN, G.709): designed to bridge gap between optics and IP; partly similar in spirit to SONET; adds FEC, management, protocol transparency (carry any kind of payload packet, e.g., IP or Ethernet frames) • Ethernet: conceive of optical circuit as a link between (electrical) Ethernet switches • MPLS SS 19, v 1.1.1 FI - Ch 2: Optical networking 15

Optical wavelength division networks • Goal: provide optical channels / lightpaths (LPs) between client nodes across network nodes • Lightpath: a direct flow of light from source to destination without need to convert to electrical signals • Network nodes may switch and convert wavelengths • Effectively routing wavelengths (not packets) • Questions: • Which elements are needed? • How to configure switches, wavelength converters to fulfil given demand? SS 19, v 1.1.1 FI - Ch 2: Optical networking 16

WDM elements • The usual: terminals, amplifiers, wavelength conversion, … • Also: Optical Add/Drop Multiplexer (OADM) • Ideally: configurable; choose wavelength to add/drop Node A Node B Node C OLT Add/Drop Transponder (a) Node A Node B Node C Node B OADM Optical passthrough Add/Drop (b) Figure 7.4 A three-node linear network example to illustrate the role of optical add/drop multi- plexers. Three wavelengths are needed between nodes A and C, and one wavelength each between nodes A and B and between nodes B and C. (a) A solution using point-to-point WDM systems. (b) From [1] A solution using an optical add/drop multiplexer at node B. SS 19, v 1.1.1 FI - Ch 2: Optical networking 17

WDM Optical Crossconnect (OXC) • Generalizes and combines switches and OADM, terminals • May internally be electrical, pure optics, hybrid OLT OXC IP SONET ATM SDH Figure 7.10 Using an OXC in the network. The OXC sits between the client equipment of the optical layer and the optical layer OLTs. From [1] SS 19, v 1.1.1 FI - Ch 2: Optical networking 18

WDM main elements OLT A IP Lightpath � 2 router OXC � 1 � 1 C D IP SONET X � 2 � 1 terminal router B E OADM F SONET IP � 1 IP terminal router router � 2 Figure 7.1 A wavelength-routing mesh network showing optical line terminals (OLTs), optical add/drop multiplexers (OADMs), and optical crossconnects (OXCs). The network provides lightpaths to its users, such as SONET boxes and IP routers. A lightpath is carried on a wavelength between its source and destination but may get converted from From [1] one wavelength to another along the way. SS 19, v 1.1.1 FI - Ch 2: Optical networking 19