Switching Architectures for Optical Networks

Switching Architectures for Optical Networks

Internet Reality Access Access Long Haul Metro Metro Data Center SONET
DWDM SONET Data Center DWDM SONET SONET SONET Access Access Long Haul Metro Metro

Hierarchies of Networks: IP / ATM / SONET / WDM

Why Optical? Enormous bandwidth made available Low bit error rates
DWDM makes ~160 channels/ possible in a fiber Each wavelength “potentially” carries about 40 Gbps Hence Tbps speeds become a reality Low bit error rates 10-9 as compared to 10-5 for copper wires Very large distance transmissions with very little amplification.

Dense Wave Division Multiplexing (DWDM)
1 2 3 Long-haul fiber 4 Output fibers Multiple wavelength bands on each fiber Transmit by combining multiple different frequencies

Anatomy of a DWDM System
Terminal A Terminal B D E M U X Transponder Interfaces M U X Transponder Interfaces Post- Amp Line Amplifiers Pre- Amp Direct Connections Direct Connections Basic building blocks Optical amplifiers Optical multiplexers Stable optical sources

User Services & Core Transport
EDGE CORE OC-3 OC-12 STS-1 Frame Relay Frame Relay IP Router IP ATM Switch ATM Sonet ADM Lease Lines TDM Switch Users Services Service Provider Networks Transport Provider Networks

Core Transport Services
Provisioned SONET circuits. Aggregated into Lamdbas. Circuit Origin Carried over Fiber optic cables. Circuit Destination OC-3 OC-3 OC-12 STS-1 STS-1 STS-1

WDM Network: Wavelength View
WDM link Optical Switch Edge Router Legacy Interfaces ( e.g., PoS, Gigabit Ethernet, IP/ATM)

Relationship of IP and Optical
Optical brings Bandwidth multiplication Network simplicity (removal of redundant layers) IP brings Scalable, mature control plane Universal OS and application support Global Internet Collectively IP and Optical (IP+Optical) introduces a set of service-enabling technologies

OXC Typical Super POP SONET Core Core ATM Large Voice IP Switch
Interconnection Network SONET Core ATM Switch Voice Switch Core IP router Large Multi-service Aggregation Switch Coupler & Opt.amp DWDM + ADM OXC DWDM Metro Ring

Typical POP Voice Switch OXC D W M D W M SONET-XC

What are the Challenges with Optical Networks?
Processing: Needs to be done with electronics Network configuration and management Packet processing and scheduling Resource allocation, etc. Traffic Buffering Optics still not mature for this (use Delay Fiber Lines) 1 pkt = Gbps requires 1.2 s of delay => 360 m of fiber) Switch configuration Relatively slow

Optical Hardware Optical Add-Drop Multiplexer (OADM)
Allows transit traffic to bypass node optically OADM 1 2 3 ’3 DCS Add and Drop

Wavelength Converters
Improve utilization of available wavelengths on links All-optical WCs being developed Greatly reduce blocking probabilities No  converters 1 2 3 New request 1 3 With  converters WC

Multiplexer & Demultiplexer
Late 90s: Backbone Nodes ADM Digital Crossconnect IP Router ATM Switch DWDM Multiplexer & Demultiplexer

Problems About 80% traffic through each node is “pass-through”
No need to electronically process such traffic 80-channel DWDM requires 80 ADMs Speed upgrade requires replacing all the ADMs in the node

Today: Optical Cross Connect (OXC)
ATM Digital Terabit Backbone Cross IP Switch Connect Router DWDM Multiplexer & Demultiplexer IP ATM Router Switch Source: JPMS

Wavelength Cross-Connects (WXCs)
A WDM network consists of wavelength cross-connects (WXCs) (OXC) interconnected by fiber links. 2 Types of WXCs Wavelength selective cross-connect (WSXC) Route a message arriving at an incoming fiber on some wavelength to an outgoing fiber on the same wavelength. Wavelength continuity constraint Wavelength interchanging cross-connect (WIXC) Wavelength conversion employed Yield better performance Expensive

Wavelength Router Control Plane: Data Plane: Wavelength Router
Wavelength Routing Intelligence Data Plane: Optical Cross Connect Matrix Unidirectional DWDM Links to other Wavelength Routers Unidirectional DWDM Links to other Wavelength Routers Single Channel Links to IP Routers, SDH Muxes, ...

Optical Network Architecture
Mesh Optical Network UNI UNI IP Network IP Network IP Router Control Path OXC Control unit Optical Cross Connect (OXC) Data Path

OXC Control Unit Each OXC has a control unit
Responsible for switch configuration Communicates with adjacent OXCs or the client network through single-hop light paths These are Control light paths Use standard signaling protocol like GMPLS for control functions Data light paths carry the data flow Originate and terminate at client networks/edge routers and transparently traverse the core

Optical Cross-connects (No wavelength conversion)
All Optical Cross-connect (OXC) Also known as Photonic Cross-connect (PXC) l1 l3 Optical Switch Fabric l3 As this graphic shows, the current breed of devices switch lambda’s between ports through an optical switch matrix. They do not convert between different speeds, for example a 2.5Gbps port to a 10Gbps port, nor do they convert electrical based ports (eg DS3) to optical ports and vice versa. OXC’s are very good at port switching large quantities of bandwidth such as lambda’s, but they are not currently as useful as a DCS or ADM – this should change in the future as better technology is developed. Let’s take a look at the methods used in OXC’s to switch lambda’s…….

Optical Cross-Connect with Full Wavelength Conversion
Converters l 1 l 2 l 1, l 2, ... , l n l 1, l 2, ... , l n l 2 l 1 1 l l 1 n n l 1 l 1 l 1, l 2, ... , l n l 1, l 2, ... , l n l 2 l 2 2 l n l 2 n . . . . . . l 1 l n l 1, l 2, ... , l n l 1, l 2, ... , l n l 2 l 1 M l n l 2 M A MxM OXC with wavelength conversion shown in figure 24 also consists of M demultiplexers at the incoming side and M multiplexers at the outgoing side. The difference is the MWxMW optical switch connected to the demultiplexers with wavelength converters at the switch outputs. This configuration is capable of taking an incoming wavelength and switching it to any of the M output ports on any of theW wavelengths but note the complexity of that implementation. Wavelength Wavelength Optical CrossBar Demux Mux Switch M demultiplexers at incoming side M multiplexers at outgoing side Mn x Mn optical switch has wavelength converters at switch outputs

Wavelength Router with O/E and E/O
Cross-Connect Incoming Interface Incoming Wavelength Outgoing Interface Outgoing Wavelength l1 l3

Individual wavelengths
O-E-O Crossconnect Switch (OXC) Outgoing fibers Incoming fibers Individual wavelengths O O Demux Mux O/E 1 E E/O 1 E/O E/O 2 O/E E/O 2 E/O WDM (many λs) E/O N O/E E/O N E/O E/O Switches information signal on a particular wavelength on an incoming fiber to (another) wavelength on an outgoing fiber.

Optical core network Opaque (O-E-O) and transparent (O-O) sections
optical island E/O O/E Client signals O O O O E E O to other nodes from other nodes O O O O O E E O Opaque optical network

OEO vs. All-Optical Switches
Capable of status monitoring Optical signal regenerated – improve signal-to-noise ratio Traffic grooming at various levels Less aggregated throughput More expensive More power consumption Unable to monitor the contents of the data stream Only optical amplification – signal-to-noise ratio degraded with distance No traffic grooming in sub-wavelength level Higher aggregated throughput ~10X cost saving ~10X power saving

Large customers buy “lightpaths”
A lightpath is a series of wavelength links from end to end. optical fibers One fiber Repeater cross-connect

Hierarchical switching: Node with switches of different granularities
A. Entire fibers O Fibers Fibers “Express trains” O O B. Wavelength subsets O E O C. Individual wavelengths O “Local trains”

Wide Area Network (WAN)
GAN links A typical Wide Area Network (WAN) contains spans typically ranging from hundreds to several thousands of kilometres. It interconnects networks of national size and may range over a whole continent like Europe. The nodes also serve as entry points to the MANs and the GAN. It is assumed that all signals will be fully regenerated at these entry point. The switching in the WAN will be mainly performed on wavelength and waveband level, due to the highly aggregated traffic. A waveband may consist of more than 10 wavelengths. For this kind of switching MEMS will be the technology of the choice, which also will be used for fast restoration in the meshed network. A possible evolution scenario is that the WAN will evolve into a small number of nodes interconnected by very high capacity links; most of the processing (e.g. burst or packet routing) takes place in the MAN. The figures are supported by the long term scenario. WAN : Up to wavelengths Gbit/s/l wavebands (> 10 l) OXC: Optical Wavelength/Waveband Cross Connect

Packet (a) vs. Burst (b) Switching

MAN (Country / Region) IP packets optical burst formation
This diagram shows the possible introduction of Optical Burst Switching (OBS) and Optical Packet Switching (OPS). OBS transmission and switching technology that lies between optical channel (wavelength) switching and packet switching. Essentially a burst is a collection of packets assembled at the ingress to the MAN and transmitted into the MAN when resources are available. The bursts are routed through the network by fast switch fabrics. Bursts can be multiplexed, and the technique enables better use to be made of the network bandwidth. Burst switching is less demanding on technology than packet switching eg ideally no buffers are required and the switch reconfiguration time is in the microsecond regime. In optical packet switching (10-20 year timeframe) optical packets similar to electronic packets (with headers) are formed at the network edge. These packets are routed by fast (ns) switches within the network. Optical buffers are required, which poses technology challenges.

Optical Switching Technologies
MEMs – MicroElectroMechanical Liquid Crystal Opto-Mechanical Bubble Technology Thermo-optic (Silica, Polymer) Electro-optic (LiNb03, SOA, InP) Acousto-optic Others… Maturity of technology, Switching speed, Scalability, Cost, Reliability (moving components or not), etc.

MEMS Switches for Optical Cross-Connect
Proven technology, switching time (10 to 25 msec), moving mirrors is a reliability problem.

WDM “transparent” transmission system
(O-O nodes) Wavelengths disaggregator Wavelengths aggregator O O O O O O Fibers multiple λs Optical switching fabric (MEMS devices, etc.) Tiny mirrors Incoming fiber Outgoing fibers

Upcoming Optical Technologies
WDM routing is circuit switched Resources are wasted if enough data is not sent Wastage more prominent in optical networks Techniques for eliminating resource wastage Burst Switching Packet Switching Optical burst switching (OBS) is a new method to transmit data A burst has an intermediate characteristics compared to the basic switching units in circuit and packet switching, which are a session and a packet, respectively

Optical Burst Switching (OBS)
Group of packets a grouped in to ‘bursts’, which is the transmission unit Before the transmission, a control packet is sent out The control packet contains the information of burst arrival time, burst duration, and destination address Resources are reserved for this burst along the switches along the way The burst is then transmitted Reservations are torn down after the burst

Has intermediate characteristics compared circuit switching and packet switching If two bursts collide, the later burst will be dropped because of zero buffering Bandwidth is reserved in a one-way process, without a ACK, whereas in circuit switching is a two-way process A burst will cut through intermediate nodes without being buffered In packet switching, a packet is stored and forwarded at each intermediate node

Optical Packet Switching
Fully utilizes the advantages of statistical multiplexing Optical switching and buffering Packet has Header + Payload Separated at an optical switch Header sent to the electronic control unit, which configures the switch for packet forwarding Payload remains in optical domain, and is re-combined with the header at output interface

Optical Packet Switch Has Input interface separates payload and header
Input interface, Switching fabric, Output interface and control unit Input interface separates payload and header Control unit operates in electronic domain and configures the switch fabric Output interface regenerates optical signals and inserts packet headers Issues in optical packet switches Synchronization Contention resolution

Main operation in a switch:
The header and the payload are separated. Header is processed electronically. Payload remains as an optical signal throughout the switch. Payload and header are re-combined at the output interface. hdr CPU payload hdr payload hdr payload Re-combined Wavelength i output port j Optical packet Wavelength i input port j Optical switch

Output port contention
Assuming a non-blocking switching matrix, more than one packet may arrive at the same output port at the same time. Input ports Optical Switch Output ports payload hdr . . . . . . payload . . . hdr . . . payload hdr

OPS Architecture: Synchronization
Occurs in electronic switches – solved by input buffering Slotted networks Fixed packet size Synchronization stages required Sync.

Slotted networks Fixed packet size Synchronization stages required Sync.

OPS: Contention Resolution
More than one packet trying to go out of the same output port at the same time Occurs in electronic switches too and is resolved by buffering the packets at the output Optical buffering ? Solutions for contention Optical Buffering Wavelength multiplexing Deflection routing

Contention Resolutions
OPS Architecture Contention Resolutions 1 1 1 2 2 1 3 3 4 4

Optical Buffering Should hold an optical signal How? By delaying it using Optical Delay Lines (ODL) ODLs are acceptable in prototypes, but not commercially viable Can convert the signal to electronic domain, store, and re-convert the signal back to optical domain Electronic memories too slow for optical networks

OPS Architecture Contention Resolutions Optical buffering 1 1 2 1 2 3 1 3 4 4

OPS Architecture Contention Resolutions Optical buffering 1 1 2 2 3 3 4 4

OPS Architecture Contention Resolutions Optical buffering 1 1 1 2 2 3 3 4 4 1

Wavelength multiplexing Resolve contention by transmitting on different wavelengths Requires wavelength converters - $$$

OPS Architecture Contention Resolutions Wavelength conversion 1 1 1 1 2 2

OPS Architecture Contention Resolutions Wavelength conversion 1 1 2 2

Deflection routing When there is a conflict between two optical packets, one will be routed to the correct output port, and the other will be routed to any other available output port. A deflected optical packet may follow a longer path to its destination. In view of this: The end-to-end delay for an optical packet may be unacceptably high. Optical packets may have to be re-ordered at the destination

Electronic Switches Using Optical Crossbars

Scalable Multi-Rack Switch Architecture
Optical links Line card rack Switch Core network operators can supply and dissipate about 10KW per rack; each rack can only accommodate limited number of line cards to guarantee temperature, humidity, etc. These indicate that terabit system with high power consumption and large number of ports can no longer be built in a compact, single-rack fashion [2]. Most high-capacity switches currently under development employ multi-rack system architecture, with switching fabric in one rack and line cards spreading around several racks. Racks are connected with each other via cables. Number of linecards is limited in a single rack Limited power supplement, i.e. 10KW Physical consideration, i.e. temperature, humidity Scaling to multiple racks Fiber links and central fabrics

Logical Architecture of Multi-rack Switches
Scheduler Line Card Line Card Local Buffers Crossbar Local Buffers Fiber I/O Framer Laser Laser Laser Laser Framer Fiber I/O Line Card Line Card Local Buffers Local Buffers Fiber I/O Framer Laser Laser Laser Laser Framer Fiber I/O Switch Fabric System Optical I/O interfaces connected to WDM fibers Electronic packet processing and buffering Optical buffering, i.e. fiber delay lines, is costly and not mature Optical interconnect Higher bandwidth, lower latency and extended link length than copper twisted lines Switch fabric: electronic? Optical?

Optical Switch Fabric Scheduler Line Card Line Card Local Buffers Crossbar Local Buffers Fiber I/O Framer Laser Laser Laser Laser Framer Fiber I/O Line Card Line Card Local Buffers Local Buffers Fiber I/O Framer Laser Laser Laser Laser Framer Fiber I/O Switch Fabric System Less optical-to-electrical conversion inside switch Cheaper, physically smaller Compare to electronic fabric, optical fabric brings advantages in Low power requirement Scalability Port density High capacity Technologies that can be used 2D/3D MEMS, liquid crystal, bubbles, thermo-optic, etc. Hybrid architecture takes advantage of the strengths of both electronics and optics

Electronic Vs. Optical Fabric
Trans. Line Buffer Inter- connection Inter- connection Buffer Trans. Line Switching Fabric Optical Electronic E/O or O/E Conversion Optical favorred Trans. Line Buffer Inter- connection Inter- connection Buffer Trans. Line Switching Fabric

Multi-rack Hybrid Packet Switch

Features of Optical Fabric
Less E/O or O/E conversion High capacity Low power consumption Less cost However, Reconfiguration overhead (50-100ns) Tuning of lasers (20-30ns) System clock synchronization (10-20ns or higher)

Scheduling Under Reconfiguration Overhead
Traditional slot-by-slot approach Scheduler Transfer Schedule Reconfigure Time Line Low bandwidth usage

Reduced Rate Scheduling
Fabric setup (reconfigure) Traffic transfer Time slot Slot-by-slot Scheduling, zero fabric setup time Slot-by-slot Scheduling with reconfigure delay Reduced rate Scheduling, each schedule is held for some time Challenge: fabric reconfiguration delay Traditional slot-by-slot scheduling brings lots of overhead Solution: slow down the scheduling frequency to compensate Each schedule will be held for some time Scheduling task Find out the matching Determine the holding time

Scheduling Under Reconfiguration Overhead
Reduce the scheduling rate Bandwidth Usage = Transfer/(Reconfigure+Transfer) Constant Approaches Batch scheduling: TSA-based Single scheduling: Schedule + Hold

Single Scheduling Schedule + Hold One schedule is generated each time
Each schedule is held for some time (holding time) Holding time can be fixed or variable Example: LQF+Hold

Routing and Wavelength Assignment

Optical Circuit Switching
An optical path established between two nodes Created by allocation of a wavelength throughout the path. Provides a ‘circuit switched’ interconnection between two nodes. Path setup takes at least one RTT No optical buffers since path is pre-set Desirable to establish light paths between every pair of nodes. Limitations in WDM routing networks, Number of wavelengths is limited. Physical constraints: limited number of optical transceivers limit the number of channels.

Routing and Wavelength Assignment (RWA)
Light path establishment involves Selecting a physical path between source and destination edge nodes Assigning a wavelength for the light path RWA is more complex than normal routing because Wavelength continuity constraint A light path must have same wavelength along all the links in the path Distinct Wavelength Constraint Light paths using the same link must have different wavelengths

No Wavelength Converters
WSXC Access Fiber Wavelength 1 POP POP Wavelength 2 Wavelength 3

Wavelength Conversion
Process of converting the wavelength of an incoming channel to another wavelength at the outgoing channel. Assume that two packets are destined to go out of the same output port at the same time. Both packets can be still be transmitted, but on two different wavelengths. Different categories of wavelength conversion are: Full conversion: Convert an incoming wavelength to any outgoing wavelength. Limited conversion: Convert an incoming wavelength to a subset of the outgoing wavelengths. Fixed conversion: Convert an incoming wavelength to a fixed outgoing wavelength (e.g., from λ1 to λ3 and λ7). Sparse wavelength conversion: Networks are comprised of a mix of wavelength converters.

Wavelength Converters
Input Output Full Wavelength conversion Limited Wavelength conversion Fixed Wavelength conversion

With Wavelength Converters
WIXC Wavelength 1 Access Fiber POP POP Wavelength 2 Wavelength 3

Routing and Wavelength Assignment (RWA)
RWA algorithms based on traffic assumptions: Static Traffic Set of connections for source and destination pairs are given Dynamic Traffic Connection requests arrive to and depart from network one by one in a random manner. Performance metrics used fall under one of the following three categories: Number of wavelengths required Connection blocking probability: Ratio between number of blocked connections and total number of connections arrived

Static and Dynamic RWA Static RWA Dynamic RWA
Light path assignment when traffic is known well in advance Arises in capacity planning and design of optical networks Dynamic RWA Light path assignment to be done when requests arrive in random fashion Encountered during real-time network operation

Static RWA – Virtual Topology Design
Problem Given physical topology, and traffic demands, set up long-lived light paths among the edge nodes such that the RWA constraints are satisfied Light paths create a logical or virtual topology and hence the name A simple solution Given N edge nodes, create a completely connected N(N-1) virtual topology Will work great, provided So many wavelengths can be supported in a fiber Each node (OXC) can be built with so many Rcv and Xmt

RWA is usually solved as an optimization problem with Integer Programming (IP) formulations Objective functions Minimize average weighted number of hops Minimize average packet delay Minimize the maximum congestion level Minimize number of Wavelenghts

Methodologies for solving Static RWA Heuristics for solving the overall ILP sub-optimally Algorithms that decompose the static RWA problem into a set of individual sub-problems, and solve a sub-set Methodologies for solving Static RWA Heuristics for solving the overall ILP sub-optimally Algorithms that decompose the static RWA problem into a set of individual sub-problems, and solve a sub-set Methodologies for solving Static RWA Heuristics for solving the overall ILP sub-optimally Algorithms that decompose the static RWA problem into a set of individual sub-problems, and solve a sub-set

Virtual Topology An example Physical Topology Virtual Topology B B A C
Lightpath B B A C A C D D Physical Topology Virtual Topology

Solving Dynamic RWA During network operation, requests for new light-paths come randomly These requests will have to be serviced based on the network state at that instant As the problem is in real-time, dynamic RWA algorithms should be simple The problem is broken down into two sub-problems Routing problem Wavelength assignment problem

Optical Circuit Switching all the Way: End-to-End !!!
Why might this be possible: Huge CS bandwidth (large # of wavelength) – BW efficiency is not very crucial Circuit switches have a much higher capacity than Packet switches, and QoS is trivial Optical Technology is suited for CS

How the Internet Looks Like Today
The core of the Internet is already “predominantly” CS. Even a “large” portion of the access networks use CS (Modem, DSLs)

How the Internet Really Looks Like Today
SONET/SDH DWDM

How the Internet Really Looks Like Today
Modems, DSL

Why Is the Internet Packet Switched in the First Place?
Gallager: “Circuit switching is rarely used for data networks, ... because of very inefficient use of the links” PS is bandwidth efficient “Statistical Multiplexing” PS networks are robust Tanenbaum: ”For high reliability, ... [the Internet] was to be a datagram subnet, so if some lines and [routers] were destroyed, messages could be ... rerouted”

Are These Assumptions Valid Today?
10-15% average link utilization in the backbone today. Similar story for access networks PS is bandwidth efficient PS networks are robust Routers/Switches are designed for <5s down-time per year. They take >1min to recover when they do (circuit switches must recover in <50ms).

How Can Circuit Switching Help the Internet?
Simple switches/routers: No buffering No per-packet processing (just per connection processing) Possible all-optical data path Peak allocation of BW No delay jitter Higher capacity switches Simple but strict QoS

Myth: Packet switching is simpler
A typical Internet router contains over 500M gates, 32 CPUs and 10Gbytes of memory. A circuit switch of the same generation could run ten times faster with 1/10th the gates and no memory.

Packet Switch Capacity
What will happen: (fewer features) Or perhaps we’re doing something wrong? What we’d like: (more features) QoS, Multicast, Security, … Instructions per arriving byte time

What Is the Performance of Circuit Switching? End-to-End
File = 10Mbit 100 clients 1 server 1 Gb/s x 100 1 s Worst latency 99% of Circuits Finish Earlier Packet sw Circuit sw 10 Mb/s 1 Gb/s Flow BW 1 s 0.505 s Avg latency

What Is the Performance of Circuit Switching?
File = 10Gbit/10Mbit 100 clients 1 server 1 Gb/s x 99 A big file can kill CS if it blocks the link Packet sw Circuit sw 10Mb/s+1Gb/s 1 Gb/s Flow BW 1.099 sec s Avg latency sec s Worst latency

What Is the Performance of Circuit Switching?
File = 10Gbit/10Mbit 100 clients 1 server 1 Gb/s x 99 1 Mb/s 10,000 sec 10,000 s Worst latency 109.9sec 109.9s Avg latency No difference between CS and PS in core Packet sw Circuit sw 1 Mb/s Flow BW

Possible Implementation
Create a separate circuit for each flow IP controls circuits Optimize for the most common case TCP (85-95% of traffic) Data (8-9 out of 10 pkts) TCP Switching

TCP Switching Exposes Circuits to IP
IP routers TCP Switches

TCP “Creates” a Connection
Router Destina-tion Source SYN SYN+ACK DATA Packets Packets Packets

State Management Feasibility
Amount of state Minimum circuit = 64 kb/s. 156,000 circuits for OC-192. Update rate About 50,000 new entries per sec for OC-192. Readily implemented in hardware or software.

Software Implementation Results
TCP Switching boundary router: Kernel module in Linux 2.4 1GHz PC Forwarding latency Forward one packet: 21ms. Compare to: 17ms for IP. Compare to: 95ms for IP + QoS. Time to create new circuit: 57ms.

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