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Contents. Network Systems Network Trends Switch Fabric Type of Switches Optical Cross Connects Optical Cross Connects Architecture Large Scale Switches Optical Router Applications. Development Milestones. 2004 International Engineering Consortium. Network. Network Connectivity

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Contents
Contents

  • Network Systems

  • Network Trends

  • Switch Fabric

  • Type of Switches

  • Optical Cross Connects

  • Optical Cross Connects Architecture

  • Large Scale Switches

  • Optical Router

  • Applications


Development milestones
Development Milestones

2004 International Engineering Consortium


Network
Network

  • Network Connectivity

    • Point to Point: one to one

    • Broadcast: one to many

    • Multicast: many to many

  • Network Span

    • Local / Metro Area Network

    • Wide Area Network

    • Long Haul Network

  • Data Rates

    • Voice 64kbps

    • Video 155Mbps, etc.

  • Service Types

    • Constant or Variable bit rate

    • Messaging

    • Quality of Service


Fully connected un switched network

Ports

Ports

Fully Connected, Un-switched Network

Problem

  • limited and could not scale to thousands or millions of users

    Solution

    - switched network


Switched Network

Pervasive, high-bandwidth, reliable, transparent


Optical network issues
Optical Network - Issues

  • Capacity

    2.5 Gb/s 10 Gb/s 40 Gb/s Larger

  • Control (switching)

    • Electronics

      • 10 Gb/s (GaAs, InP) can deliver low order optical cross connects (16 x 16)

      • > 10 Gb/s ??(mainly power dissipation)

    • Optical

  • Reconfiguration:

    • Static or dynamic


Optical network elements
Optical Network Elements

  • Dense Wavelength Division Multiplexing

  • Optical Add/Drop Multiplexers (OADM)

  • Optical Gateways:

    • A critical network element.

    • A common transport structure to cater for

      • variety of bit rates and signal formats, ranging from asynchronous legacy networks to 10–Gbps SONET systems,

      • a mix of standard SONET and ATM services.


Switching electrical
Switching - Electrical

Right now, the optical switches have electrical core, where

  • Light pulses are converted back into electrical signals so that their route across the middle of the switch can be handled by conventional ASICs (application specific integrated circuits).

  • This has a number of advantages:

    • Enabling the switches to handle smaller bandwidths than whole wavelengths, which fits in with current market requirements.

    • Easier network management, because standards are in place and products are available. Optical equivalents are not, at present.

  • But, there are concerns that electrical cores won’t be able to cope with the explosion in the number of wavelengths in telecom networks (deployment of DWDM).

  • Until recently, state-of-the-art ASIC technology wouldn’t support anything more than a 512-by-512-port electrical core, and carriers demanding for at least double this capacity.


  • Optical network elements switches
    Optical Network Elements - Switches

    • Optical Bidirectional Line Switched Rings

    • Optical Cross-Connect (OXC)

      • Efficient use of existing optical fibre facilities at the optical level becomes critical as service providers started moving wavelengths around the glob. Routing and grooming are key areas, and that is where OXCs are used.

    International Engineering Consortium, 2004


    Optical switches
    Optical Switches

    • To provide high switching speed

    • To avoid the electronics speed bottleneck

    • I/O interface and switching fabric in optics

    • Switching control and switching fabric in optics

    • Switches act as routers and redirect the optical

    • signals in a specific direction.

    • It uses a simple 2x2 switch as a building block

    Main feature: Switching time (msecs - to- sub nsecs)


    All optical switches
    All Optical Switches

    • That’s the theory. But, things are turning out a little different in practice.

      • Vendors are finding ways of building larger scale electrical cores, with switch of many thousands of ports.

      • This may encourage carriers to put off decisions on moving to all-optical switches.

    • Does this mean that is the end of the idea of all-optical networks?

      • Well, not really. All that it might do is delay things.


    Electrical vs optical cross connects

    Optical

    • Electrical Limits

    • High power consumption: e.g. 1024x1024: 4 kW

    • Jitter: very large

    • Large switches

    • Need OE/EO conversion

    • Bipolar or GaAs

    1024

    512

    256

    128

    Number of ports

    64

    32

    16

    Electrical

    8

    10 MHz

    100 MHz

    1 GHz

    10 GHz

    100 GHz

    Data rate

    DS3

    OC3

    OC12

    OC48

    OC192

    Electrical vs. Optical - Cross Connects

    M C Wu


    Switching types
    Switching: Types

    • Circuit Switching: E.g. Telephone

      • Continuous streams

        • no bursts

        • no buffers

      • Connections are created and removed

        • Buffering does not exist in circuit-switches

    • Packet Switching: Uses store & forward

      • The configuration may change per packet

      • Switching/forwarding is based on the destination address mapping

      • Switching table is used to provide the mapping

      • Switching table changes according to network dynamics (e.g. congestion, failure)


    Switching fabric

    Electrical control

    Electrical control

    Optical

    input

    Optical

    output

    Optical

    input

    Optical

    output

    Switching Fabric

    • Electro-optical 2 x 2 switching elements are the key devices in the fabrication of N x N optical data path.

    • The switching elements rely on the electro-optic effect (i.e., the application of an electric field to an electro-optical material changes the refractive index of the material).

    • The result is a 2x2 optical switching element whose state is determined by an electrical control signal.

    • Can be fabricated using LiNbO3 as well as other materials.


    Switching fabric contd

    Input

    interface

    Output

    interface

    Switching

    fabric

    Switching control

    Switching Fabric – contd.


    Switching fabric contd1

    ...

    ...

    1.3 mm intra-office

    Transponders

    ...

    ...

    Optical

    Crossconnect

    (OXC)

    ...

    ...

    Optical transport system

    (1.55 mm WDM)

    Terminating equipment

    |

    SONET, ATM, IP...

    Switching Fabric – contd.


    Connectivity
    Connectivity

    • Since a switch work as a permutation that routes input to the outputs, therefore it needs to provide at least N! different configuration

    • A minimum number of Log2(N!) is needed to configure N! different permutation

    • Blocking

    • Non-Blocking


    Connectivity blocking
    Connectivity - Blocking

    • Occurs when one reduces the number of crosspoints in order to achieve low crosstalk and less complexity.

      In some switching architecture internal blocking may be reduced to zero by:

      • Improving the switching control: Wide sense non-blocking

      • Rearranging the switching configuration: Rearrangeably non-blocking


    Connectivity non blocking
    Connectivity– Non-blocking

    A new connection can always be made without disturbing the existing connections:

    • Strictly Non-blocking

      • A connection path can always be found no matter what the current switching configuration is or what switching control algorithm is used

    • Wide-Sense Non-blocking

      • A connection path can always be found regardless of the current switching configuration provided a good switching control algorithm is employed

      • No re-routing of the existing connections

    • Rearrangeably Non-blocking

      • The same as wide-sense, but requires re-routing of the existing connections to avoid blocking

      • Use fewer switches

      • Requires more complex control algorithm


    Time division switching

    1

    1

    D

    E

    M

    U

    X

    M

    U

    X

    TSI

    4 3 2 1

    N

    N

    1

    2

    3

    4

    2 4 1 3

    Time Division Switching

    • Interchanges sample (slot) position within a frame: i.e. time slot interchange (TSI)

      • when demultiplexing, position in frame determines output link

      • read and write to shared memory in different order


    Tsi properties
    TSI - Properties

    • Simple

    • Time taken to read and write to memory is the bottle-neck

    • For 120,000 telephone circuits

      • each circuit reads and writes memory once every 125 ms.

      • number of operations per second : 120,000 x 8000 x2

      • each operation takes around 0.5 ns => impossible with current technology


    Space division switching
    Space Division Switching

    • Crossbar

    • Clos

    • Benes

    • Spank - Benes

    • Spanke


    Crossbar architectures

    1

    2

    Input

    ports

    3

    4

    1

    2

    3

    4

    Output ports

    Crossbar Architectures

    • Each sample takes a different path through the switch, depending on its destination

    • Crossbar:

      • Simplest possible space-division switch

      • Wide- sense blocking: When a connection is made it can exclude the possibility of certain other connections being made

        Crosspoints

      • can be turned on or off

    Sessions: (1,4) (2,2) (3,1) (4,3)


    Crossbar architectures blocking

    Input channels

    1

    2

    Input channels

    N X N matrix S/W

    Output channels - Bars

    3

    4

    1

    2

    3

    4

    Optical

    switching

    element

    Output channels - Cross

    Crossbar Architectures - Blocking

    • M inputs x N outputs

    • Switch configuration: “set of input-output pairs simultaneously connected” that define the state of the switch

    • For X crosspoints, each point is either ON or Off, so at most 2X different configurations are supported by the switch.

    • Case 1:

      • - (3,2) ok

      • - (4,3) blocked


    Crossbar architecture wide sense non blocking

    Input channels

    1

    2

    Input channels

    3

    4

    1

    2

    3

    4

    Output channels

    Crossbar Architecture - Wide-Sense Non-blocking

    Rule: To connect ith input to the jth output, the algorithm sets the

    switch in the ith row and jth

    column at the “BAR” state and

    sets all other switches on its

    left and below at the “CROSS”

    state.

    • Case 2:

      • - (2,4) ok

      • (3,2) ok

      • (4,3) ok


    Crossbar architectures 2 layer

    2

    3x3

    5

    Crossbar Architectures – 2 Layer

    • Only uses 6 x 9 = 54 cross points rather than 9 x 9 = 81

    • Penalty is loss of connectivity


    Crossbar architectures 3 layer
    Crossbar Architectures - 3 Layer

    1

    1

    2

    2

    3

    3

    4

    4

    Output ports

    5

    Input port

    5

    6

    6

    7

    7

    8

    8

    9

    9

    Blocking still possible

    http://www.aston.ac.uk/~blowkj/index.htm


    Crossbar architectures 3 layer1

    1

    1

    2

    2

    3

    3

    *

    4

    4

    5

    5

    6

    6

    7

    *

    7

    8

    8

    9

    9

    Crossbar Architectures - 3 Layer

    Blocking

    • The first four connections have made it impossible for 3rd input to be connected to 7th output

    The 3rd input can only reach the bottom middle switch

    The 7th output line can only be reached from the top output switch.


    Crossbar architecture features
    Crossbar Architecture - Features

    Architecture: Wide Sense Non-blocking

    Switch element:N2 (based on 2 x 2)

    Switch drive: N2

    Switch loss:(2N-1).Lse +2Lfs

    SNR: XT – 10log10(N-1)

    Where XT; Crosstalk (dB),

    Lse; Loss/switch element

    Lfs; Fibre-switch loss


    Crossbar architecture properties
    Crossbar Architecture - Properties

    • Advantages:

      • simple to implement

      • simple control

      • strict sense non-blocking

      • Low crosstalk: Waveguides do not cross each other

    • Disadvantages

      • number of crosspoints = N2

      • large VLSI space

      • vulnerable to single faults

      • the overall insertion loss is different for each input-output pair: Each path goes through a different number of switches


    Time space switching arch

    time 1

    1

    MUX

    time 1

    2 1

    2 1

    2

    TSI

    TSI

    3

    MUX

    4 3

    3 4

    4

    3

    1

    2

    4

    Time-Space Switching Arch.

    • Each input trunk in a crossbar is preceded with a TSI

    • Delay samples so that they arrive at the right time for the space division switch’s schedule

    Note: No. of Crosspoints N = 4 (not 16)


    Time space switching arch1

    TSI

    TSI

    TSI

    TSI

    TSI

    TSI

    TSI

    TSI

    Time-Space Switching Arch.

    • Can flip samples both on input and output trunk

    • Gives more flexibility => lowers call blocking probability

    • Complex in terms of:

      - Number of cross points

      - Size of buffers

      -Speed of the switch bus (internal speed)


    Clos architecture

    nxp

    kxk

    pxn

    1

    1

    1

    1

    n

    n

    32

    33

    2

    2

    2

    64

    32

    32

    64

    993

    k

    p

    k

    N= 1024

    Stage1

    Stage 2

    Stage 3

    Clos Architecture

    • It is a 3-stage network

    • - 1st & 2nd stages are fully

    • connected

    • - 2nd & 3rd stages are fully

    • connected

    • - 1st & 3rd stages are not

    • directly connected

    • Defined by: (n, k, p, k, n)

      • e.g. (32, 3, 3, 3, 32)

      • (3, 3, 5, 2, 2,)

    • Widely used

    • Stage 1 (nxp)

    • Stage 2(kxk)

    • Stage 3 (pxn)


    Clos architecture1
    Clos Architecture

    In this 3-stage configuration N x N switch has:

    • 2pN + pk2 crosspoints (note N = nk)

      (compared to N2 for a 1-stage crossbar)

    • If n = k, then the total number of crosspoints = 3pN, which is < N2 if 3p < N.

      Problem:

    • Internal blocking

    • Larger number of crossovers when p is large.


    Clos architecture blocking
    Clos Architecture – Blocking

    If p < 2n-1, blocking can occur as follows:

    • Suppose input 1 want to connect to output 1 (these could be any fixed input and outputs.

    • There are n-1 other inputs at k-switch (stage 1). Suppose they each go to a different switch at stage 2.

    • Similarly, suppose the n-1 outputs in the first switch other than output 1 at the third stage are all busy again using n-1 different switches at stage 2.

    • If p <  n -1 + n -1 +1 = 2n -1 then there will be no line that input 1 can use to connect to output 1.

    • If p = 2n -1, then

      • Total Switch Element: 2kn(2n-1) + (2n -1)k2


    Clos architecture blocking1
    Clos Architecture – Blocking

    • If p = 2n -1, then

      • Total Switch Element: 2kn(2n-1) + (2n -1)k2

    • Since k = N/n, therefore

      • the number of switch elements is minimised when

        n ~(N/2) 0.5.

        Thus the number switch elements =

        4 (2)0.5N3/2 – 4N,

        which is less than N2 for the crossbar switch


    Clos architecture non blocking
    Clos Architecture – Non-blocking

    • If p 2n -1, the Clos network is strict sense non-blocking (i.e. there will free line that can be used to connect input 1 to output 1)

    • If pn, thenthe Clos network is re-arrangeably non-blocking (RNB) (i.e. reducing the number of middle stage switches)


    Clos architecture example
    Clos Architecture – Example

    • If N = 1000 and and n = 10, then the number of switches at the:

      • 1st & 3rd stages = N/n = 1000/10 = 100

      • 1st stage = 10 x p

      • 3rd stage = p x 10

      • 2nd stage = p x k x k.

    • If p = 2n -1 = 19, then the resulting switch will be non-blocking.

    • If p < 19, then blocking occurs.

    • For p = 19, the number of crosspoints are given as follow:-


    Clos architecture example contd
    Clos Architecture – Example contd.

    • In the case of a full 1000 x 1000 crossbar switch, no blocking occurs, requiring 106 crosspoints.

    • For n = 10 and p = 19, the number of crosspoints at

      • 1st and 3rd stages

        = no. of stages x (n x p) x k

        = 2 x (10 x 19) x 100 = 38,000 crosspoints

      • 2nd stage (p = 19 crossbars each of size 100 x 100, because N/n = 100.

        = p x k x k = 19 x 100 x 100 = 190000 crosspoints.

        The total no. of crosspoints = 38000 + 190000 = 228000

        Vs. the 106 points used by the complete crossbar.


    Clos architecture example contd1
    Clos Architecture – Example contd.

    Since k = N/n, the number of switch elements k is minimised when n ~(N/2)0.5 = (1000/2) 0.5 =~ 23 instead of 19.

    then k = N/n = 1000/23 =~ 44 switches in the 1st & 3rd stages, and p = 2(23)-1 = 45.

    the number of crosspoints at 1st and 3rd stages

    = no. of stages x (n x p) x k

    = 2 x (23 x 45) x 44 = 91080.

    the number of crosspoints at 2nd stage = p x k x k = 45 x 44 x 44 = 87120.

    Since n = 23 does not divide 1000 evenly, we actually have 12 extra inputs and outputs that we could switch with this configuration ( 23x44=1012 and 1012 - 1000 = 12).

    Thus the total number of crosspoints = 91090 + 87120 = 178200 best case for a non-blocking switch as compared with the:

    1,000,000 for the complete crossbar and

    about 190,000 for n = 10.

    This is a factor of over 11 less equipment needed to switch 1000 customers!


    Benes architecture

    2 2

    2 2

    N/2 N/2

    Benes

    N/2 N/2

    Benes

    N

    N

    Benes Architecture

    • NxN switch (N is power of 2) RNB built recursively from Clos network:

    • 1st step Clos(2, N/2, 2, N/2, 2)

    • Rearrangably non-blocking


    Benes architecture contd
    Benes Architecture - contd.

    • Number of stages = 2.log2N - 1

    • Number of 2x2 switches /each stage = N/2

    • Total number of crosspoints ~N.(log2N -1)/2

    • For large N, total number of crosspoint = N.log2N

    • Benes network is RNB (not SNB) and so may need re-routing:

    • Modular switch design

    • Multicast switches can be built in a modular fashion by including a copy module in front of the point-to-point switch


    Benes architecture contd1

    1

    1

    2

    2

    3

    3

    4

    4

    5

    5

    X

    6

    6

    7

    7

    8

    8

    4 to 2 Fails

    2 to 1

    1 to 5

    3 to 3

    Benes Architecture - contd.

    • e.g. Connection sequence

    Note there is no way 4 to 2 connection could be made


    Benes architecture non blocking contd

    4 to 2 OK

    2 to 1

    1 to 5

    3 to 3

    Benes Architecture –Non-blockingcontd.

    • Now use different connections

    • e.g.


    Three building blocks for oxc
    Three Building Blocks for OXC

    International Engineering Consortium, 2004


    Optical switches tow position switch

    Control Signal

    Optical Switch

    I1

    Output

    ports

    Input

    port Ii

    I2

    Optical Switches - Tow-Position Switch

    The input signal can be switched to either of the output ports without having any access to the information carried by the input optical signal

    • In the ideal case, the switching must be fast and low-loss.

    • 100% of the light should be passed to one port and 0% to

    • the other port.


    Two position switch contd

    Lens

    B

    B

    Prisem

    A

    A

    C

    C

    Fibre

    Two Position Switch - contd.

    • The two-position switch requires three fibres with collimating lenses and a prism.

    Light arriving at port A needs to be switched to port C.


    Optical switches applications
    Optical Switches - Applications

    • Provisioning: Used inside optical cross connects to reconfigure them and set-up new path. [1 - 10 msecs]

    • Protection Switching: To switch traffic from a primary fibre onto another fibre in the case of a failure. [1 to 10 usecs]

    • Packet Switching: 53 byte packet @ 10 Gb/s. [1 nsecs]

    • External Modulation: To switch on-off a laser source at a very high speed. [10 psecs << bit duration]

    • Network performance monitoring

    • Reconfiguration and restoration:Fibre networks


    Optical switching technologies
    Optical Switching - Technologies

    • Slow Switches (msecs)

      • Free space

      • Mechanical

      • Solid state

    • Fast Switches (nsecs)

      • LiNbO

      • Non-linear

      • InP


    Optical switches criteria
    Optical Switches - Criteria

    • Maximum Throughput:

      • Total number of bits/sec switched through.

      • To increase throughput:

        • Increase the number of I/O ports

        • Bit rate of each line

    • Maximum Switching Speed

      • Important:

        • Packet switched

        • Time division multiplexed

    • Minimum Number of Crosspoints

      • As the size of the switch increases, so does the number of crosspoints, thus high cost

      • Multistage switching architecture are used to reduce the number of crosspoints.


    Criteria contd
    Criteria - contd.

    • Minimum Blocking Probability: Important in circuit switching

      • External blocking:when the incoming call request an output port that is blocked.

        • Subject to external traffic conditions

      • Internal blocking:when no input port is available.

        • Subject to the switch design

    • Minimum Delay and Loss Probability

      • Important in packet switching, where buffering is used, which will introduce additional delay.

    • Scalability

      • Replacing an old switch with a new larger switch is costly.

      • Incrementally increasing the size of the existing switching as traffice grows is desirable

    • Broadcasting and Multicasting

      • To provide conferencing and multimedia applications


    Criteria contd1
    Criteria - contd.

    • Optical switches with low insertion loss and low crosstalk are needed in broadband optical networks

      • Restoration

      • Reprovisioning

      • Bandwidth on demand

    • Conventional optical switches cannot satisfy all the requirements:

      • Solid-state guided-wave switches (electro-optic, thermo-optic, SOA): limited expandability due to high crosstalk, loss, and power consumption

      • Optomechanical switches: excellent insertion loss and crosstalk, but arebulky, expensive, and suffer from poor reliability and scalability


    Optical switches types

    - Fast

    - Complex

    - Maturing

    - Lossy

    - Slow

    - Maturity

    - Reliable

    - Slow

    - Low loss & crosstalk

    - Inherently scalable

    Optical Switches - Types

    • Waveguide

    • Electro-optic effect

      • Semiconductor optical amplifier

      • LiNbO

        - InP

    • Thermo-optic effect

      - SiO2 / Si

      - Polymer

    • Free Space

      - Liquid crystal

      - Mechanical / fibre

      - Micro-optics (MEM’s)


    Optical switches thermo optic effect

    + v

    Electrodes

    Optical Switches - Thermo-Optic Effect

    • Some materials have strong thermo-optics effect that could be used to guide light in a waveguide.

    • The thermo-optic coefficient is:

      • Silica glass dn/dt = 1 x 10-5 K-1

      • Polymer dn/dt = -1 x 10-5 K-1

    • Difference thermo-optic effect results in different switch design.


    Thermo optic switch silica

    Mach – Zehnder Configuration

    Analogue

    Outputs

    Input Ii

    I1

    Heater

    I2

    Directional coupler

    Thermo-Optic Switch - Silica


    Thermo optic switch polymer

    Y – Junction Configuration

    Digital

    I1

    PH1

    Ii

    PH2

    I2

    Thermo-Optic Switch - Polymer

    • If PH1 = PH2 = 0, then I1 = I2 = Ii /2

    • If PH1 = Pon & PH2 = 0, then I1 = 0, and I2 = Ii

    • If PH1 = 0 & PH2 = Pon, then I1 = Ii, and I2 = 0


    Thermo optic switch characteristics

    Parameters

    Switch Size

    2 x 2

    Si Poly.

    8 x 8

    Si Poly.

    16 x 16

    Si

    No. of S/W

    1 1

    64 112

    256

    Insertion Loss (dB)

    2 0.6

    4 10

    18

    Crosstalk

    22 39

    18 17

    13

    S/W time (ms)

    2 1

    ~3 1.5

    ~4

    S/W power (W)

    0.6 0.005

    5 4.5

    15

    Thermo-Optic Switch - Characteristics


    Mechanical switches
    Mechanical Switches

    1st Generation – Mid. 1980’s

    • Loss Low (0.2 – 0.3 dB)

    • Speed slow (msecs)

    • Size Large

    • Reliability Has moving part

    • Applications: - Instrumentation

      - Telecom (a few)

    Size: 8 X 8

    Loss: 3 dB

    Crosstalk: 55 dB

    Switching time: 10 msecs


    Micro electro mechanical switches

    Input fibres

    Output fibres

    Lens

    Flat mirror

    Raised mirror

    Micro Electro Mechanical Switches

    Combines optomechanical structures, microactuators, and micro-optical elements on the same substrate

    • Made using micro-machining

    • Free-space: polarisation independent

    • Independent of:

      • Bit-rate

      • Wavelength

      • Protocol

    • Speed: 1 10 ms

    4 x 4 Cross point

    switch


    Micro electro mechanical switches1

    I/O Fibers

    Imaging Lenses

    Reflector

    MEMS 2-axis Tilt Mirrors

    Micro Electro Mechanical Switches

    This tiny electronically tiltable mirror

    is a building block in devices such

    as all-optical cross-connects and new types of computer data projectors.

    Lightwave


    Micro electro mechanical switches2
    Micro Electro Mechanical Switches

    • Monolithic integration --> Compact, lightweight, scalableBatch fabrication --> Low cost

    • Share the advantages of optomechanical switches without their adverse effects

    • General Characteristics:

      • Low insertion loss (~ 1 dB)

      • Small crosstalk (< - 60 dB)

      • Passive optical switch (independent of wavelength, bit rate, modulation format)

      • No standby power

      • Rugged

      • Scalable to large-scale optical crossconnect switches

      • Moderate speed ( switch time from 100 nsec to 10 msec)


    Large optical switches optical cross connects

    Control

    1

    1

    2

    2

    N

    N

    N X N Cross Connect

    Large Optical Switches - Optical Cross Connects

    • Switch sizes > 2 X 2 can be implemented by means of cascading small switches.

    • Used in all network control

    • Bit rate at which it functions depends on the applications.

      • 2.5 Gb/s are currently available

    • Different sizes are available, but not up to thousands (at the moment)



    Optical switches1
    Optical Switches

    Electrical switching and optical cabling: inputs come

    from different clock domains resulting in a switch that is generally timing-transparent.

    Optical switching and optical cabling, clocking

    and synchronization are not significant

    issues because the streams are independent.

    Inputs come from different clock domains,

    so the switch is completely timing-transparent.


    Optical switches system considerations

    For a given switch size N,

    the number of 2x2 switches should be as small as possible. When the number is large it will result in:

    high cost

    large optical power loss and crosstalk.

    A switch with reduced number of crosspoints in each configured path, can have a large internal blocking probability

    In some switching architectures, the internal blocking probability can be reduced to zero by:

    using a good switching control

    or rearranging the current switch configuration

    Optical Switches - System Considerations


    Optical routers
    Optical Routers

    • In the core large optical-switching elements have already started to appear to handle optical circuits,

    • Large, centralized IP routers are also appearing, because they're an extremely efficient solution to IP routing.

    • There are a variety of technologies and issues that influence the architecture for these types of network elements.

    • To transport Tbps, new optical technologies have emerged to enable the economic transport of incredible bandwidth over single-mode optical fibrer, including DWDM and OTDM. That means individual optical links can sustain the enormous traffic needed to support the continuing growth of IP data.


    Optical routers1
    Optical Routers

    • High-power, low-noise optical amplifiers-or erbium-doped fiber amplifiers (EDFAs)-and pulse-shaping technologies mean the high-bit-rate optical signals do not require electronic regeneration except on the very longest fiber spans.

    • New fibres with larger cross-sectional areas mean a large number of high-bit-rate signals can be wavelength-multiplexed onto a single fiber.

    • Thus, it is becoming affordable to actually construct links that can support Tbps of capacity between routing and switching centres.


    Network problems scalability
    Network Problems - Scalability

    • The bottleneck at the core of the expanding network is at the junction points of the fibre bundles: I.e the switching and routing centres. With Tbps links, a huge amount of data converges into a single central office (CO) (see Figure 1).

    • New routers emerge only to be swamped with traffic within months.


    Network problems scalability1
    Network Problems - Scalability

    Solution:

    • Use of cluster of several routers (or crossconnects).

    • However, clustering is not a good long-term solution, because:

      • a cluster of crossconnects requires interconnecting links between the crossconnects. As the number of switches in the cluster grows beyond about 4 or 5, the interconnecting links consume most of the ports. Clustered routers have the same problem.

      • the IP traffic must transit more and more devices, and the latency (the delay of IP packets) and jitter (delay variance) of the cluster grow quickly.

      • the hot-spot problem, where one of the small routers in a cluster can be overwhelmed by temporary traffic dynamics in the network that do not exceed the combined node capacity. This swamping effect also increases the delay of that saturated small router.


    Large centralized router
    Large, Centralized Router

    • Current trend in XCs is to use large micro-electromechanical systems (MEMS)-based OXCs for core node protection and grooming of DWDM traffic.

    • Similarly, large centralized routers are an efficient alternative to solving bottleneck problems:

      • by avoiding the hot-spot problems of distributed routers,

      • eliminating clustering problems, and

      • permitting global scheduling.

    • A centralized (single-hop), synchronous, large non-blocking switch fabric has the best latency and throughput performance of all router topologies. It also scales better than a clustered system-and it results in less complicated system software for the network element.


    Ip routers optical network elements

    End Customer

    Router

    Router

    Optical Network

    Router

    Router

    ONE

    ONE

    ONE

    Router

    IP Routers + Optical Network Elements

    A V Lehmen, Telecordia Tech.


    Optical layer capability reconfigurability

    IP

    Router

    IP

    Router

    IP

    Router

    IP

    Router

    Optical Layer Capability: Reconfigurability

    IP

    Router

    OXC - A

    OXC - C

    OXC - B

    OXC - D

    Crossconnects are reconfigurable:

    • Can provide restoration capability

    • Provide connectivity between any two routers

    A V Lehmen, Telecordia Tech.


    Architecture 1 large routers high capacity fibres

    Access lines

    A

    Z

    Access lines

    Architecture 1: Large Routers + High capacity Fibres

    • All traffic flows through routers

    • Optics just transports the data from one point to another

    • IP layer can handle restoration

    • Network is ‘simple’

    • But…..

      • - more hops translates into more packet delays

      • - each router has to deal with thru traffic as well as terminating traffic

    A V Lehmen, Telecordia Tech.


    Architecture 2 small routers oxc

    OXC

    OXC

    OXC

    OXC

    Architecture 2: Small Routers + OXC

    • Router interconnectivity through OXC’s

    • Only terminating traffic goes through routers

    • Thru traffic carried on optical ‘bypass’

    • Restoration can be done at the optical layer

    • Network can handle other types of traffic as well

    • But: network has more NE’s, and is more complicated

    A V Lehmen, Telecordia Tech.


    Dynamic set up of optical connection

    IP

    Router

    IP

    Router

    IP

    Router

    IP

    Router

    1. Router requests a new optical connection

    3. Path set-up message propagates through network

    4. Connection is established and routers are notified

    Dynamic Set-Up of Optical Connection

    OXC - A

    OXC - C

    OXC - B

    A V Lehmen, Telecordia Tech.

    2. OXC A makes admission and routing decisions


    Oxc router selector architecture

    1

    1

    N

    N

    1

    1

    N

    N

    OXC – Router-Selector Architecture

    • Type I - 1 x N & N x 1 optical switches

    • Type II - 1 x N passive optical splitter

    • - N x 1 Optical switches


    Oxc router feature

    Type I

    TypeII

    Architecture

    Strictly non-blocking

    Switch Element

    2N(N-1)

    N(N-1)

    Switch Drive

    2Nlog2N

    Nlog2N

    Switch Loss

    (2Nlog2N)Lse+4Lfs

    log2N(3+Lse)+2Lfs

    SNR

    2XT-10log10(log2N)

    XT-10log10(log2N)

    OXC – Router - Feature

    • Where XT; Crosstalk (dB),

    • Lse; Loss/switch element

      • Lfs; Fibre-switch loss




    Optical gateway cross connect
    Optical Gateway Cross-Connect

    Performs digital grooming, traditional multiplexing, and routing of lower-speed circuits in mesh or ring network configurations. Specifically, it brings in lower rate SONET/SDH layer OC-3/STM-1, OC-12/STM-4 and OC-48/STM-16 rates and electrical DS-3, STS-1 and STM-1e rates and grooms them into higher rate optical signals.

    Alcatel. 2001


    40 G mod

    40 G mod

    40 G mod

    40 G mod

    40 G mod

    40 G mod

    40 G mod

    40 G mod

    40 G mod

    40 G mod

    40 G mod

    40 G mod

    IP-router with Tb/s throughput can be built with

    fast tunable lasers & NxN optical mux

    Buffer

    From Input Port

    Output

    T-Tx

    40G Rx

    retiming

    T-Tx

    40G Rx

    Sche-

    duler

    T-Tx

    40G Rx

    T-Tx

    40G Rx

    Clock

    Yamada et al., 1998


    Router optical switch
    Router & Optical Switch

    CHIARO- OptIPuter Optical Switch Workshop


    The optical future tomorrow s architecture
    The Optical Future- Tomorrow's Architecture

    • Services are consolidated onto a single access line at the user site and fed into a Sonet multi-service provisioning platform at the carrier’s POP (point of presence). Several POPs feed traffic into a terabit switch capable of handling all traffic—including IP, ATM and TDM. The terabit switches sit at the edge of a three-tier network of optical switches—local, regional and long distance-each of which has a mesh topology. DWDM is used throughout the network and access lines. Where fiber is scarce, FDM (frequency division multiplexing) is used to pack as much traffic as possible into wavelengths. Light signals no longer need regeneration on long distance routes.


    • Separate access networks carry telephony and data into the carrier’s point of presence. Voice traffic runs over a TDM (time division multiplexer) network running over a Sonet (synchronous optical network) backbone. IP traffic is shunted onto an ATM backbone running over other Sonet channels. The Sonet backbone comprises three tiers of rings at the local, regional and national level, interconnected by add-drop multiplexers and cross-connects. DWDM (dense wave division multiplexing) is in use in the regional and national rings, but not the local rings. Light signals need regenerating on long distance routes.


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