Switching Architectures for Optical Networks by m4S53q8c

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									Switching Architectures
 for Optical Networks



         CSIT560 by M. Hamdi   1
                             Internet Reality

 Data
                                                                    SONET
Center
                  SONET




                              DWD
                               M                          DWD
                                                           M




             SONET
                                                                    SONET




         Access                        Long Haul            Metro       Access
                          Metro

                                    CSIT560 by M. Hamdi                     2
Hierarchies of Networks: IP / ATM /
          SONET / WDM




             CSIT560 by M. Hamdi      3
                    Why Optical?
• Enormous bandwidth made available
   – 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.




                         CSIT560 by M. Hamdi               4
    Dense Wave Division Multiplexing
              (DWDM)


                  1
                  2
                  3
                                 Long-haul fiber
                  4
  Output fibers

Multiple wavelength bands on each fiber
   Transmit by combining multiple lasers @ different
     frequencies

                       CSIT560 by M. Hamdi          5
              Anatomy of a DWDM System

     Terminal A                                              Terminal B



                                                               D
                                                                   Transponder
Transponder   M                                                E    Interfaces
 Interfaces
              U                                                M
              X                                                U
                  Post-         Line Amplifiers       Pre-     X
                  Amp                                 Amp

  Direct                                                             Direct
Connections                                                        Connections


                          Basic building blocks
                           • Optical amplifiers
                           • Optical multiplexers
                           • Stable optical sources

                               CSIT560 by M. Hamdi
       User Services & Core Transport

                EDGE                              CORE


Frame Relay              Frame    OC-3

                         Relay    OC-3
         IP
                 IP
               Router             OC-12
                          ATM
      ATM                Switch
                                  STS-1
                         Sonet    STS-1
Lease Lines                       STS-1
                TDM      ADM
               Switch



Users         Service Provider            Transport Provider
Services      Networks                    Networks




                        CSIT560 by M. Hamdi                    7
• Provisioned           Core Transport Services
SONET circuits.

• Aggregated into     Circuit
Lamdbas.
                      Origin

• Carried over
Fiber optic cables.




             OC-3                                     Circuit
             OC-3                                     Destination
            OC-12


            STS-1
            STS-1
            STS-1




                                CSIT560 by M. Hamdi         8
              WDM Network: Wavelength View

                               WDM link

                                                     Edge Router

          Legacy
          Interfaces
                                                         Legacy
( e.g.,   PoS, Gigabit                                   Interfaces
  Ethernet, IP/ATM)
 Interfaces


                                                      Legacy
                                                      Interfaces

                                    Optical Switch




                         CSIT560 by M. Hamdi                       9
              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


                                 CSIT560 by M. Hamdi   10
                  Typical Super POP
                   Interconnectio
                          n
                      Network


                                            Core    SONET
     Core        Large
       IP     Multi-service    Voice        ATM
     router   Aggregation      Switch      Switch
                Switch



       DWDM
                              OXC                     Coupler
DWDM                                                    &
 Metro   +                                            Opt.amp
 Ring ADM
                          CSIT560 by M. Hamdi               11
    Typical POP

                            Voice
                            Switch



                                     D
D
                                     W
W    OXC                             D
D
                                     M
M


                           SONET-XC
     CSIT560 by M. Hamdi                 12
     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 = 12 kbits @ 10 Gbps requires 1.2 s of delay => 360
     m of fiber)
• Switch configuration
   – Relatively slow


                         CSIT560 by M. Hamdi               13
                      Optical Hardware
• Optical Add-Drop Multiplexer (OADM)
   – Allows transit traffic to bypass node optically



             1                                        1
             2                  OADM                  2
             3                                        ’3



                                  3   ’3


          Add and Drop



                                                                  DCS




                                CSIT560 by M. Hamdi          14
                      Wavelength Converters
• Improve utilization of available wavelengths on links
• All-optical WCs being developed
• Greatly reduce blocking probabilities
        2                  3                                      3
                                              2

                                                  WC

                 No  converters                        With  converters

    1   New request                       1   New request
          1 3                                  1 3




                               CSIT560 by M. Hamdi                          15
                    Late 90s: Backbone Nodes

                              ADM

                                        ADM

                                                    ADM

                                                                ADM



                                       Digital Crossconnect


          DWDM
Multiplexer & Demultiplexer     IP                             ATM
                              Router                          Switch




                                       CSIT560 by M. Hamdi             16
                       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


                        CSIT560 by M. Hamdi           17
          Today: Optical Cross Connect (OXC)


                                              Optical

                                           Crossconnect




                                ATM           Digital        Terabit
                              Backbone        Cross            IP
                               Switch        Connect         Router
       DWDM

Multiplexer & Demultiplexer
                                           IP        ATM
                                         Router     Switch




                                                                       Source: JPMS
                                         CSIT560 by M. Hamdi               18
    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

                             CSIT560 by M. Hamdi                   19
                      Wavelength Router
                                                       Wavelength Router




 Control Plane:
Wavelength Routing
   Intelligence

   Data Plane:
  Optical Cross
  Connect Matrix


           Unidirectional
          DWDM Links to                                   Unidirectional
         other Wavelength                                DWDM Links to
                             Single Channel Links to    other Wavelength
             Routers
                             IP Routers, SDH Muxes,         Routers
                                        ...
                            CSIT560 by M. Hamdi                      20
                  Optical Network Architecture

                        Mesh Optical Network
              UNI                                  UNI
IP Network                                               IP Network




      IP Router
                         OXC Control unit           Control Path

      Optical Cross                                 Data Path
     Connect (OXC)

                             CSIT560 by M. Hamdi                      21
                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

                        CSIT560 by M. Hamdi                    22
         Optical Cross-connects (No wavelength
                      conversion)
                           2                          4

All Optical Cross-connect (OXC)
Also known as Photonic
Cross-connect (PXC)

              1
                                                             3

                                          Optical
                                          Switch
                                          Fabric



                                                              3
                4
                                  1
                                                        2
                                       CSIT560 by M. Hamdi         23
    Optical Cross-Connect with Full Wavelength
                    Conversion
                                                        Wavelength
                                                        Converters
                                1                             2
          1,2, ... ,n                                                     1,2, ... ,n
                                2                            1
      1                                                                                       1
                                n                            n

                                1                            1
          1,2, ... ,n                                                     1,2, ... ,n
                                2                            2
      2                                                                                       2
                                n                            n
                           .                                             .
                           .                                             .
                           .                                             .
                                1                            n
          1,2, ... ,n                                                     1,2, ... ,n
                                2                            1
      M
                                n                            2                              M

                   Wavelength        Optical CrossBar                Wavelength
                    Demux                 Switch                       Mux

• M demultiplexers at incoming side
• M multiplexers at outgoing side
• Mn x Mn optical switch has wavelength converters at switch
  outputs
                                     CSIT560 by M. Hamdi                                          24
        Wavelength Router with O/E and E/O

                       Cross-Connect
  Incoming Interface                          Outgoing Interface
Incoming Wavelength                          Outgoing Wavelength




          1




                                                  3




                       CSIT560 by M. Hamdi                     25
      O-E-O Crossconnect Switch (OXC)

Incoming                                               Outgoing
                     Individual wavelengths             fibers
  fibers
        Demux          O              O             Mux
                       O/E     E      E/O
 1                     O/E            E/O                      1
                        O/E                  E/O
                        O/E                  E/O                    2
 2                      O/E                  E/O
     WDM                O/E                  E/O
     (many λs)

 N                      O/E                  E/O                    N
                        O/E                  E/O
                        O/E                  E/O

Switches information signal on a particular wavelength on an
incoming fiber to (another) wavelength on an outgoing fiber.
                              CSIT560 by M. Hamdi              26
   Optical core network
   Opaque (O-E-O) and transparent (O-O) sections
                                                       Transparent
                                                       optical island
Client             E/O        O/E
signals
                               O             O     O            O
           E                          E                   O



          to other nodes
                           from other nodes
                         O     O            O      O            O
               E                       E                  O




                              network
               Opaque opticalCSIT560 by M. Hamdi                  27
                OEO vs. All-Optical Switches
           OEO                                         All-Optical

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




                                 CSIT560 by M. Hamdi                     28
        Large customers buy “lightpaths”
A lightpath is a series of wavelength links from end to end.



                         optical
                         fibers

                             One fiber
                                                      Repeater
                                      cross-connect




                        CSIT560 by M. Hamdi                  29
  Hierarchical switching: Node with switches
            of different granularities
A. Entire fibers            O          O              O
                   Fibers                         Fibers


                                                           “Express
B. Wavelength                           O             O     trains”
   subsets
                            O




C. Individual               O           E             O
   wavelengths
                                                           “Local
                                                            trains”
                                CSIT560 by M. Hamdi           30
               Wide Area Network (WAN)




WAN :
Up to 200-500 wavelengths
40-160 Gbit/s/
wavebands (> 10 )               OXC: Optical Wavelength/Waveband Cross
                                 Connect
                            CSIT560 by M. Hamdi                           31
                          Packet (a) vs. Burst (b) Switching
                                                                                      Header recognition,
        Payload                                                                    processing, and generation
                            Header
                                                                                                                            C
    A
                                                           Synchronizer                                 Setup

                  1                                                                            Switch
Incoming                                                                      1                                     1
fibers
                      2
                                                                              2                                     2
               Fixed-length
              (but unaligned)                          FDL’s
    B                                                                                                        New            D
                                                                     (a)                                    headers
        A         Control                                                                                                   C
                wavelengths
                                                   2                                                            2
                                                               1   O/E/O                                                1
                      Control
                                                                            Control packet processing
                      packets                Offset time                    (setup/bandwidth reservation)
                                2                                                         2
   Data                                                            Switch
wavelengths
                                1                                                                1

        B                            Data bursts
                                                                      (b)                                                   D

                                                            CSIT560 by M. Hamdi                                                 32
            MAN (Country / Region)
    IP
 packets
  optical
   burst
formation




                   CSIT560 by M. Hamdi   33
           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,
Relaiability (moving components or not), etc.


                        CSIT560 by M. Hamdi                   34
  MEMS Switches for Optical Cross-Connect
                                         Moveable Micromirror




Proven technology, switching time (10 to 25 msec), moving mirrors is a
reliability problem.

                             CSIT560 by M. Hamdi                         35
WDM “transparent” transmission system
         (O-O nodes)

Wavelengths     Wavelengths
disaggregator   aggregator
    O           O         O           O                O
Fibers                        multiple
                                               O
                              λs


                Optical switching fabric (MEMS devices, etc.)

                               Tiny mirrors
         Incoming fiber


                         CSIT560 by M. Hamdi                36
             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

                           CSIT560 by M. Hamdi                37
        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


                        CSIT560 by M. Hamdi                  38
Optical Burst Switching (OBS)




          CSIT560 by M. Hamdi   39
              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


                         CSIT560 by M. Hamdi          40
               Optical Packet Switch
• Has
   – 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
                        CSIT560 by M. Hamdi                    41
• 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
                    Optical
                                                                    Wavelength i
  Wavelength i      packet                                          output port j
  input port j
                                                 Optical switch




                                          CSIT560 by M. Hamdi                       42
              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         .
                                payloadhdr
                                                    .
               .                   .                .
                                   .
               .                   .                .
               .                payloadhdr
                                                    .
                                                    .




                          CSIT560 by M. Hamdi                  43
    OPS Architecture: Synchronization
Occurs in electronic switches – solved by input buffering

                Slotted networks
 •Fixed packet size
 •Synchronization stages required
                   Sync.




                           CSIT560 by M. Hamdi         44
  OPS Architecture: Synchronization

           Slotted networks
•Fixed packet size
•Synchronization stages required
              Sync.




                      CSIT560 by M. Hamdi   45
 OPS Architecture: Synchronization

           Slotted networks
•Fixed packet size
•Synchronization stages required
              Sync.




                      CSIT560 by M. Hamdi   46
 OPS Architecture: Synchronization

           Slotted networks
•Fixed packet size
•Synchronization stages required
              Sync.




                      CSIT560 by M. Hamdi   47
 OPS Architecture: Synchronization

           Slotted networks
•Fixed packet size
•Synchronization stages required
              Sync.




                      CSIT560 by M. Hamdi   48
OPS Architecture: Synchronization




        Sync.




                CSIT560 by M. Hamdi   49
            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



                         CSIT560 by M. Hamdi                50
           OPS Architecture
Contention Resolutions

 1                                      1




              1
 2                                      2




                  1
 3                                      3




 4                                      4




                  CSIT560 by M. Hamdi   51
          OPS: Contention Resolution

• 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




                          CSIT560 by M. Hamdi                 52
              OPS Architecture
Contention Resolutions
  •Optical buffering

          1                                    1

          2          1                         2

          3      1                             3

          4                                    4




                         CSIT560 by M. Hamdi       53
              OPS Architecture
Contention Resolutions
  •Optical buffering

          1                                  1

          2                                  2

          3                                  3

          4                                  4




                       CSIT560 by M. Hamdi       54
              OPS Architecture
Contention Resolutions
  •Optical buffering

          1                                  1   1

          2                                      2

          3                                      3

          4                                      4




                                   1




                       CSIT560 by M. Hamdi           55
         OPS: Contention Resolution
• Wavelength multiplexing
  – Resolve contention by transmitting on different
    wavelengths
  – Requires wavelength converters - $$$




                       CSIT560 by M. Hamdi            56
                    OPS Architecture
Contention Resolutions
     •Wavelength conversion


                1
 1                                             1




            1
 2                                             2




                        CSIT560 by M. Hamdi   57
               OPS Architecture
Contention Resolutions
     •Wavelength conversion


 1                                          1




 2                                          2




                     CSIT560 by M. Hamdi   58
               OPS Architecture
Contention Resolutions
     •Wavelength conversion


                                    1
 1                                          1
                                1




 2                                          2




                     CSIT560 by M. Hamdi   59
               OPS Architecture
Contention Resolutions
     •Wavelength conversion


 1                                          1




 2                                          2




                     CSIT560 by M. Hamdi   60
               OPS Architecture
Contention Resolutions
     •Wavelength conversion


                                               1
 1                                                  1
                                           1




 2                                                  2




                     CSIT560 by M. Hamdi           61
                   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




                          CSIT560 by M. Hamdi                62
Electronic Switches Using
    Optical Crossbars



         CSIT560 by M. Hamdi   63
 Scalable Multi-Rack Switch Architecture
                                       Optical links




 Line card
    rack                      Switch Core
• 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

                            CSIT560 by M. Hamdi           64
            Logical Architecture of Multi-rack Switches

                                                      Scheduler
                  Line Card                                                              Line Card

                        Local                                                                  Local
Fiber I/O                        Laser   Laser                           Laser   Laser                           Fiber I/O
             Framer    Buffers                                                                Buffers   Framer




                                                      Crossbar
                  Line Card                                                              Line Card

                        Local                                                                 Local
                                 Laser   Laser                           Laser   Laser                           Fiber I/O
Fiber I/O    Framer    Buffers                                                               Buffers    Framer


                                                  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?
                                                 CSIT560 by M. Hamdi                                             65
                                  Optical Switch Fabric
                                                      Scheduler
                   Line Card                                                             Line Card

                         Local                                                                 Local
Fiber I/O                         Laser   Laser                          Laser   Laser                           Fiber I/O
              Framer    Buffers                                                               Buffers   Framer




                                                      Crossbar
                   Line Card                                                             Line Card

                         Local                                                                Local
                                  Laser   Laser                          Laser   Laser                           Fiber I/O
Fiber I/O     Framer    Buffers                                                              Buffers    Framer


                                                  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
                               CSIT560 by M. Hamdi                                                               66
       electronics and optics
             Electronic Vs. Optical Fabric


Electronic

              Trans. Buffer Inter-                    Inter- Buffer Trans.
               Line        connection               connection       Line
                                        Switching
                                         Fabric                              Optical

                                                                             Electronic

                                                                             E/O or O/E
                                                                             Conversion
Optical

              Trans. Buffer Inter-                    Inter- Buffer Trans.
               Line        connection               connection       Line
                                        Switching
                                         Fabric




                                CSIT560 by M. Hamdi                             67
Multi-rack Hybrid Packet Switch

        Rack

   Buf f er    E/O                         O/E   Buf f er


   Buf f er    E/O                         O/E   Buf f er

                 Optical Optical     Optical
                  Fiber Crossbar      Fiber

   Buf f er    E/O                        O/E    Buf f er

     Linecard
   Buf f er    E/O                        O/E    Buf f er


                       Switch Core




                     CSIT560 by M. Hamdi                    68
            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)




                           CSIT560 by M. Hamdi          69
  Scheduling Under Reconfiguration Overhead

• Traditional slot-by-slot approach


                 Scheduler

                                 Schedule Reconfigure Transfer




                                                                 Time Line



• Low bandwidth usage




                             CSIT560 by M. Hamdi                         70
                                     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
    1.        Find out the matching
    2.        Determine the holding time
                                                          CSIT560 by M. Hamdi                                      71
    Scheduling Under Reconfiguration
                Overhead

• Reduce the scheduling rate
   – Bandwidth Usage = Transfer/(Reconfigure+Transfer)
                                          Constant

• Approaches
   – Batch scheduling: TSA-based
   – Single scheduling: Schedule + Hold




                        CSIT560 by M. Hamdi              72
                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




                      CSIT560 by M. Hamdi                73
Routing and
Wavelength
Assignment


   CSIT560 by M. Hamdi   74
                 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.

                                CSIT560 by M. Hamdi                      75
   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


                           CSIT560 by M. Hamdi                          76
               No Wavelength Converters

                                             WSXC


Access Fiber
                         Wavelength 1



                                                    POP
  POP
                         Wavelength 2




                        Wavelength 3
                       CSIT560 by M. Hamdi           77
         With Wavelength Converters

                                           WIXC




                            Wavelength 1
Access Fiber


                                             POP
POP
                    Wavelength 2




                   Wavelength 3
                  CSIT560 by M. Hamdi         78
                 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


                              CSIT560 by M. Hamdi                    79
             Static and Dynamic RWA
• Static 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



                         CSIT560 by M. Hamdi                   80
                   Static RWA

• 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


                     CSIT560 by M. Hamdi        81
                    Static RWA

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


  – http://www.tct.hut.fi/~esa/java/wdm/



                     CSIT560 by M. Hamdi                82
              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


                       CSIT560 by M. Hamdi           83
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
                       CSIT560 by M. Hamdi               84
        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)




                              CSIT560 by M. Hamdi                    85
How the Internet Really Looks Like Today


                         SONET/SDH
                           DWDM




               CSIT560 by M. Hamdi     86
How the Internet Really Looks Like Today


              Modems, DSL




               CSIT560 by M. Hamdi     87
Why Is the Internet Packet Switched in the First
                    Place?

                    Gallager:
• PS is bandwidth     “Circuit switching is rarely used
  efficient           for data networks, ... because of
  “Statistical        very inefficient use of the links”
  Multiplexing”


                    Tanenbaum:
                      ”For high reliability, ... [the
• PS networks are     Internet] was to be a datagram
  robust              subnet, so if some lines and
                      [routers] were destroyed,
                      messages could be ... rerouted”
                      CSIT560 by M. Hamdi            88
    Are These Assumptions Valid Today?
                          •    10-15% average link
                               utilization in the
• PS is bandwidth              backbone today.
  efficient               •    Similar story for
                               access networks


                             Routers/Switches are
• PS networks are
                              designed for <5s down-time
  robust
                              per year.
                             They take >1min to recover
                              when they do (circuit
                              switches must recover in
                              <50ms).
                    CSIT560 by M. Hamdi              89
How Can Circuit Switching Help the Internet?


 •       Simple switches/routers:
     •     No buffering                            Higher
     •     No per-packet processing (just          capacity
           per connection processing)
                                                   switches
     •     Possible all-optical data path


 •       Peak allocation of BW
                                                   Simple but
     •     No delay jitter
                                                   strict QoS
                             CSIT560 by M. Hamdi              90
     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.




                    CSIT560 by M. Hamdi           91
                                 Packet Switch Capacity
Instructions per arriving byte




                                                     What we’d like: (more features)
                                                     QoS, Multicast, Security, …




                                                 What will happen: (fewer features)
                                                 Or perhaps we’re doing something wrong?




                                                             time
                                       CSIT560 by M. Hamdi                        92
  What Is the Performance of Circuit Switching?
                   End-to-End
                                                   File = 10Mbit
         100 clients
                                             1 server
                          1 Gb/s
                                                             x 100



              Circuit sw Packet sw                      99% of
    Flow BW 1 Gb/s       10 Mb/s                        Circuits
  Avg latency 0.505 s    1s                              Finish
Worst latency 1 s            1s                         Earlier
                       CSIT560 by M. Hamdi                   93
What Is the Performance of Circuit Switching?
                                         File = 10Gbit/10Mbit
          100 clients
                                              1 server
                           1 Gb/s
                                                            x 99



               Circuit sw Packet sw                   A big file
     Flow BW 1 Gb/s       10Mb/s+1Gb/s               can kill CS
  Avg latency 10.495 s     1.099 sec                 if it blocks
 Worst latency 10.990 s 10.990 sec                     the link
                        CSIT560 by M. Hamdi                 94
What Is the Performance of Circuit Switching?
                                         File = 10Gbit/10Mbit
          100 clients
                                              1 server
                           1 Gb/s
                                                           x 99
             1 Mb/s

                                                         No
               Circuit sw Packet sw
                                                     difference
     Flow BW 1 Mb/s       1 Mb/s
                                                      between
  Avg latency     109.9s     109.9sec
                                                     CS and PS
 Worst latency 10,000 s 10,000 sec
                                                       in core
                        CSIT560 by M. Hamdi                95
       Possible Implementation


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




               CSIT560 by M. Hamdi            96
TCP Switching Exposes Circuits to IP

                                   IP routers




            TCP Switches

             CSIT560 by M. Hamdi                97
           TCP “Creates” a Connection


Source      Router    Router          Router   Destina-
                                                 tion
     SYN



                                           SYN+ACK


     DATA



    Packets     Packets      Packets       Packets
                     CSIT560 by M. Hamdi             98
        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.




                       CSIT560 by M. Hamdi          99
      Software Implementation Results

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




                      CSIT560 by M. Hamdi   100

								
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