Optical Fibre Communication Systems

1Prof. Z Ghassemlooy

Optical Fibre Communication Systems

Professor Z Ghassemlooy

Lecture 7 – Optical Switches

Northumbria Communications LaboratorySchool of Computing, Engineering and

Information SciencesThe University of Northumbria

U.K.http://soe.unn.ac.uk/ocr


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– 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

Problem- limited and could not scale to thousands or millions of users Solution- switched network

Ports Ports


Pervasive, high-bandwidth, reliable, transparentPervasive, high-bandwidth, reliable, transparent

Switched Network


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

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

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 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

• 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

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

Nu

mb

er o

f p

ort

s

1024

32

64

16

8

512

256

128

Optical

10 MHz

DS3

100 MHz

OC3 OC12

1 GHz

OC48 OC192

100 GHz

Electrical

10 GHz10 GHz

Data rate

Electrical Limits

• High power consumption:

e.g. 1024x1024: 4 kW

• Jitter: very large

• Large switches

• Need OE/EO conversion

• Bipolar or GaAs

M C Wu


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

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.

Opticalinput

Opticaloutput

Electrical control

Opticalinput

Opticaloutput

Electrical control


Switching Fabric – contd.

Switching control

Inputinterface

Outputinterface

Switchingfabric


Switching Fabric – contd.

...

Optical transport system(1.55 m WDM)

1.3 m intra-office...

...

...

...

OpticalCrossconnect

(OXC)

...

Transponders

Terminating equipment|

SONET, ATM, IP...


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

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

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

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

4 3 2 1 2 4 1 3

1234

TSITSIMUX

MUX

1

N

DEMUX

DEMUX

1

N


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

Crossbar

Clos

Benes

Spank - Benes

Spanke


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

Inputports

Output ports

12

3

4

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


Crossbar Architectures - Blocking

Optical switchingelement

Case 1:

- (3,2) ok

- (4,3) blocked

1

2

3

4

1 2 3 4

Inpu

t cha

nnel

s

Output channels - B

ars

Output channels - Cross

N X N matrix S/W

Input channels 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.


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.

1

2

3

4

1 2 3 4

Inpu

t cha

nnel

s

Output channels

Input channels

Case 2:

- (2,4) ok- (3,2) ok- (4,3) ok


Crossbar Architectures – 2 Layer

Only uses 6 x 9 = 54 cross points rather than 9 x 9 = 81 Penalty is loss of connectivity

3x3

2

5


Crossbar Architectures - 3 Layer

Input port

Output ports

123

456

789

123

456

789

Blocking still possiblehttp://www.aston.ac.uk/~blowkj/index.htm


Crossbar Architectures - 3 Layer

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

*

*

123

456

789

123

456

789

The 3rd input can only reach the bottom middle switch

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

Blocking


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

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.

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

MUX

MUX

MUX

MUX

1

2

3

4

2 1

3 4

2 1 TSI

4 3 TSI

time 1

time 1

31

24

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


Time-Space Switching Arch.

Can flip samples both on input and output trunk Gives more flexibility => lowers call blocking

probability

TSITSI

TSITSI

TSITSI

TSITSI

TSITSI TSITSITSITSITSITSI

Complex in terms of:- Number of cross points

- Size of buffers

-Speed of the switch bus (internal speed)


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)

11

22

kk

11

22

pp

11

22

kk

kxknxp pxn

Stage 1 Stage 3Stage 2

32

1

64

33

N= 1024

993

n n

32 64 32


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

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 – 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)0.5 N3/2 – 4N,

which is less than N2 for the crossbar switch


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, then the Clos network is re-arrangeably non-blocking (RNB) (i.e. reducing the number of middle stage switches)


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.

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 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 2N/2 N/2

Benes

N/2 N/2Benes

N N

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.

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 - contd.

•e.g. Connection sequence

2 to 1 1 to 5 3 to 3 4 to 2 Fails

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

1

2

3

4

5

6

7

8

2

3

4

5

6

7

8

1

X


• Now use different connections

• e.g.

2 to 1 1 to 5 3 to 3 4 to 2 OK

Benes Architecture –Non-blocking contd.


Three Building Blocks for OXC

International Engineering Consortium, 2004


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

Optical SwitchOptical SwitchInputport Ii

Outputports

I1

I2

Control 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.

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

B

A

C

B

A

C

Lens

Fibre

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


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

Slow Switches (msecs)– Free space– Mechanical– Solid state

Fast Switches (nsecs)– LiNbO– Non-linear– InP


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.

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 - 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 are bulky, expensive, and suffer from poor reliability and scalability


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)

- Fast- Complex- Maturing- Lossy

- Slow- Maturity- Reliable

- Slow- Low loss & crosstalk- Inherently scalable


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.

+ v

Electrodes


Thermo-Optic Switch - Silica

Directional coupler

)2/(sin 21 iI

I)2/(sin 21

iI

I)/(cos 222

iI

I)/(cos 222

iI

I

Input IiI1

I2

Outputs

Mach – Zehnder Configuration

Heater

Analogue


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

Ii

I1

I2

PH1

PH2

Y – Junction Configuration

Digital


Thermo-Optic Switch - Characteristics

155 4.50.6 0.005S/W power (W)

~4~3 1.52 1S/W time (ms)

1318 1722 39Crosstalk

184 102 0.6Insertion Loss (dB)

25664 1121 1No. of S/W

16 x 16Si

8 x 8Si Poly.

2 x 2Si Poly.

Switch SizeParameters


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 8Loss: 3 dBCrosstalk: 55 dBSwitching time: 10 msecs


Micro Electro Mechanical SwitchesIn

put f

ibre

s

Output fibres

Lens Flat mirror Raised mirror

Made using micro-machining Free-space: polarisation

independent Independent of:

– Bit-rate

– Wavelength

– Protocol

Speed: 1 10 ms

4 x 4 Cross pointswitch

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


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

I/O Fibers

Imaging Lenses

Reflector

MEMS 2-axis Tilt Mirrors


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

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)

12

N

12

NN X N Cross Connect

ControlControl


Optical Cross Connects


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.


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

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 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

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 - 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

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

Router

RouterRouterRouterRouter

ONE ONE

ONE

Router

Router

Optical Network

End Customer

A V Lehmen, Telecordia Tech.


Optical Layer Capability: Reconfigurability

IPRouter

IPRouter

IPRouter

IPRouter

IPRouter

IPRouter

OXC - AOXC - B

OXC - C

Crossconnects are reconfigurable: Can provide restoration capability Provide connectivity between any two routers

IPRouter

IPRouter

OXC - D

IPRouter

IPRouter



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

Z

Access lines

Access lines



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

OXCOXC

OXCOXC

OXCOXC

OXCOXC



Dynamic Set-Up of Optical Connection

IPRouter

IPRouter

IPRouter

IPRouter

IP Router

IP Router

OXC - AOXC - B

OXC - C

IPRouter

IPRouter

1. Router requests a new optical connection

2. OXC A makes admission and routing decisions

3. Path set-up message propagates through network

4. Connection is established and routers are notified



OXC – Router-Selector Architecture

1 1

N N1 1

NN

•Type I - 1 x N & N x 1 optical switches•Type II - 1 x N passive optical splitter - N x 1 Optical switches

•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

Where XT; Crosstalk (dB),

Lse; Loss/switch element

Lfs; Fibre-switch loss

log2N(3+Lse)+2Lfs(2Nlog2N)Lse+4LfsSwitch Loss

XT-10log10(log2N)2XT-10log10(log2N) SNR

Nlog2N2Nlog2NSwitch Drive

N(N-1)2N(N-1)Switch Element

Strictly non-blockingArchitecture

TypeIIType I


OXC + Wavelength Converters


Optical Switches: - A comparison

Characteristic Traditional Optical Switches

Next Generation Optical Switches

Switching Speed >1ms <1µsec

Multicast Not available Dynamic power partition between ports

Integrated VOA functionality

Not available High dynamic range VOA

Reliability ~10 Million cycles (Mech.dev.)

~10 Billion cycles (Opto-elect.)

Insertion loss Low Low

Cross talk High Low

Scalability Low Medium-High


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

T-Tx

T-Tx

T-Tx

T-Tx

40G Rx

40G Rx

40G Rx

40G Rx

Clock

Buffer

Sche-duler

From Input Port

retiming

Output

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

fast tunable lasers & NxN optical mux

Yamada et al., 1998

40 G mod

40 G mod

40 G mod

40 G mod

40 G mod

40 G mod

40 G mod

40 G mod


Router & Optical Switch

CHIARO- OptIPuter Optical Switch Workshop


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.

Date post:	30-Jan-2016
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Optical Fibre Communication Systems

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