Optical Fibre Communication Systems
description
Transcript of 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
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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
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Development Milestones
2004 International Engineering Consortium
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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
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Fully Connected, Un-switched Network
Problem- limited and could not scale to thousands or millions of users Solution- switched network
Ports Ports
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Pervasive, high-bandwidth, reliable, transparentPervasive, high-bandwidth, reliable, transparent
Switched Network
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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
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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.
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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.
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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
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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)
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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.
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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
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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)
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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
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Switching Fabric – contd.
Switching control
Inputinterface
Outputinterface
Switchingfabric
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Switching Fabric – contd.
...
Optical transport system(1.55 m WDM)
1.3 m intra-office...
...
...
...
OpticalCrossconnect
(OXC)
...
Transponders
Terminating equipment|
SONET, ATM, IP...
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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
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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
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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
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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
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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
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Space Division Switching
Crossbar
Clos
Benes
Spank - Benes
Spanke
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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)
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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.
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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
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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
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Crossbar Architectures - 3 Layer
Input port
Output ports
123
456
789
123
456
789
Blocking still possiblehttp://www.aston.ac.uk/~blowkj/index.htm
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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
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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
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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
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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
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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)
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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
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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.
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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
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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
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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)
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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:-
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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.
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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!
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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
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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
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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
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• Now use different connections
• e.g.
2 to 1 1 to 5 3 to 3 4 to 2 OK
Benes Architecture –Non-blocking contd.
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Three Building Blocks for OXC
International Engineering Consortium, 2004
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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.
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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.
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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
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Optical Switching - Technologies
Slow Switches (msecs)– Free space– Mechanical– Solid state
Fast Switches (nsecs)– LiNbO– Non-linear– InP
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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Optical Cross Connects
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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.
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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
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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.
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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.
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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.
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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.
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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.
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IP Routers + Optical Network Elements
Router
RouterRouterRouterRouter
ONE ONE
ONE
Router
Router
Optical Network
End Customer
A V Lehmen, Telecordia Tech.
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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
A V Lehmen, Telecordia Tech.
74Prof. Z Ghassemlooy
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
A V Lehmen, Telecordia Tech.
75Prof. Z Ghassemlooy
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
A V Lehmen, Telecordia Tech.
76Prof. Z Ghassemlooy
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
A V Lehmen, Telecordia Tech.
77Prof. Z Ghassemlooy
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
78Prof. Z Ghassemlooy
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
79Prof. Z Ghassemlooy
OXC + Wavelength Converters
80Prof. Z Ghassemlooy
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
81Prof. Z Ghassemlooy
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
82Prof. Z Ghassemlooy
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
83Prof. Z Ghassemlooy
Router & Optical Switch
CHIARO- OptIPuter Optical Switch Workshop
84Prof. Z Ghassemlooy
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.
85Prof. Z Ghassemlooy
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.