Transcript Optical Switch

ENG735 – COMUNICAÇÕES ÓPTICAS
CHAVEADORES ÓPTICOS
http://soe.northumbria.ac.uk/ocr/teaching/fibre/pp/Components-L2.ppt
http://soe.northumbria.ac.uk/ocr/teaching/fibre/pp/opticalsw.ppt
Prof. Dr. Vitaly F. Rodríguez-Esquerre
Switching is the process by which the destination of a individual optical
information signal is controlled
Types of Optical
Switching
Space Division
Switching
Wavelength
Division
Switching
Time Division
Switching
Hybrid of Space,
Wavelength and
Time
Switch control may be:
 Purely electronic (present situation)
 Hybrid of optical and electronic (in development)
 Purely optical (awaits development of optical logic, memory etc.)
Optical Switching Element Technologies
High Loss
Not Scalable
Polarization Dependent
Gel/oil based
LiNbO
Liquid
Crystal
3
Poor Reliability
Mechanical
Indium
Phosphide
Optical
Switching
Element
Technologies
SiO2 /Si
SOA
Micro-Optic
(MEMS)
Fibre
(acousto -optic)
Thermooptic
Waveguide
Free Space
Bubble
Can be configured in two or three
dimensional architectures
– Electro-optic Switch
• Use a directional coupler
• Its coupling ratio is changed by varying the refractive
index
–
–
–
–
Thermo-optic Switch
Liquid-Crystal Switch
Bubble Switch
Acousto-optic Switch
Two axis motion
Micro mirror
2D MEMS based Optical Switch Matrix
Output fibre
Input fibre
 Mirrors have only two possible positions
 Light is routed in a 2D plane
 For N inputs and N outputs we need N2
mirrors
 Loss increases rapidly with N
SEM photo of 2D MEMS mirrors
3D MEMS based Optical Switch Matrix
 Mirrors require complex closed-loop analog control
 But loss increases only as a function of N1/2
 Higher port counts possible
SEM photo of 3D MEMS mirrors
Total Internal Reflection LC Switch
Liquid crystal (Total internal
Reflection)
2
3
1
Input beam
Output beam
(transmittive state)
1
3
Output beam
(reflective state)
The glass and
nematic liquid crystal
refractive indices are
chosen to be equal in
the transmittive state
and to satisfy the total
reflection condition in
the reflective state
Schematic diagram of the total reflection
switch: 1- glass prisms; 2- liquid crystal layer;
3-spacers
Optomechanical
Switch
MEMS
Electro-optic
Milliseconds
Milliseconds
Nanoseconds
Insertion loss
Low
Low
High
PDL
Low
Low
high
Scalability
Bad
Good
Bad
Switching time
Optical
Number of ports
Electrical Limits
1024
• High power consumption:
e.g. 1024x1024: 4 kW
512
• Jitter: very large
• Provide fast switching speed
• No bottleneck due to electronics
speed
• I/O interface and switching fabric
in optics
• Switching control and switching
fabric in optics
•Uses a simple 2x2 switch as a
building block
• Large switches
256
128
• Need OE/EO conversion
• Bipolar or GaAs
64
32
16
Electrical
8
10 MHz
100 MHz
1 GHz
10 GHz
100 GHz
Data rate
DS3
OC3
OC12
OC48
OC192
Electrical control
Optical
input
Electrical control
Optical
output
Optical
input
Input
interface
Optical
output
Output
interface
Switching
fabric
Switching control
Optical Switches: - A comparison
Characteristic
Traditional Optical
Switches
Next Generation
Optical Switches
>1ms
<1µsec
Multicast
Not available
Dynamic power partition
between ports
Integrated VOA
functionality
Not available
High dynamic range VOA
~10 Million cycles (Mech.dev.)
~10 Billion cycles (Optoelect.)
Insertion loss
Low
Low
Cross talk
High
Low
Scalability
Low
Medium-High
Switching Speed
Reliability
14
Optical Switches - Tow-Position Switch
Control Signal
Input
port Ii
Optical Switch
I1 Output
ports
I2
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.
15
Two Position Switch - contd.
• The two-position switch requires three
fibres with collimating lenses and a prism.
Lens
B
Prism
A
Light arriving at port A needs to be
switched to port C.
C
Fibre
B
A
C
16
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
17
Optical Switching - Technologies
 Slow Switches (msecs)
– Free space
– Mechanical
– Solid state
 Fast Switches (nsecs)
– LiNbO
– Non-linear
– InP
18
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.
19
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
20
Optical Switches - Types
 Waveguide


Electro-optic effect
- Semiconductor optical amplifier
- LiNbO
- InP
Thermo-optic effect
- SiO2 / Si
- Polymer
- Fast
- Complex
- Maturing
- Lossy
- Slow
- Maturity
- Reliable
 Free Space
- Liquid crystal
- Mechanical / fibre
- Micro-optics (MEM’s)
- Slow
- Low loss & crosstalk
- Inherently scalable
21
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
22
Thermo-Optic Switch - Silica
Mach – Zehnder Configuration
Input Ii
Heater
Outputs
I1
I2
I1
 sin 2 ( / 2)
Ii
I2
 cos 2 ( / 2)
Ii
Directional coupler
23
Thermo-Optic Switch - Polymer
Y – Junction Configuration
PH1
Ii
I1
PH2
I2
• 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
24
Thermo-Optic Switch - Characteristics
Parameters
Switch Size
Si
2x2
Poly.
Si
8x8
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
25
Mechanical Switches
1st Generation – Mid. 1980’s
•
•
•
•
•
Loss
Speed
Size
Reliability
Applications:
Low (0.2 – 0.3 dB)
slow (msecs)
Large
Has moving part
- Instrumentation
- Telecom (a few)
Size:
Loss:
Crosstalk:
Switching time:
8X8
3 dB
55 dB
10 msecs
26
Micro Electro Mechanical Switches
Input fibres




Output fibres
Lens
Flat mirror
Made using micro-machining
Free-space: polarisation
independent
Independent of:
– Bit-rate
– Wavelength
– Protocol
Speed: 1 10 ms
4 x 4 Cross point
switch
Raised mirror
27
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
28
Switching Fabric – contd.
Terminating equipment
|
SONET, ATM, IP...
...
Optical
Crossconnect
(OXC)
Transponders
...
...
...
29
...
Optical transport system
(1.55 mm WDM)
...
1.3 mm intra-office
Space Division Switching
• Crossbar
• Clos
• Benes
• Spank - Benes
• Spanke
30
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
Input
ports
1
2
3
4
Sessions: (1,4) (2,2) (3,1) (4,3)
1 2 3 4
Output ports
31
Crossbar Architectures - Blocking
Input channels


Output channels - Bars
Input channels
1
2
N X N matrix S/W
3
4

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
Optical
switching
element
1
2
3
4
- (4,3) blocked
Output channels - Cross
32
Crossbar Architecture - Wide-Sense Nonblocking
Input channels
Rule: To connect ith input to the jth
output, the algorithm sets the
1
Input channels
switch in the ith row and jth
column at the “BAR” state and
2
sets all other switches on its
left and below at the “CROSS”
3
state.
4
Case 2:
1
2
3
4
- (2,4) ok
- (3,2) ok
- (4,3) ok
Output channels
33
Crossbar Architectures – 2 Layer
• Only uses 6 x 9 = 54 cross points rather than 9 x 9 = 81
• Penalty is loss of connectivity
1
1
4
4
2
Inputs
3x3
N
5
Outputs
N
34
Crossbar Architectures - 3 Layer
1
2
3
1
2
3
4
5
6
7
8
9
7
8
9
Blocking still possible
Output
ports
Input port
4
5
6
http://www.aston.ac.uk/~blowkj/index.htm
35
Crossbar Architectures - 3 Layer
*
1
2
3
4
5
6
7
8
9
Blocking
1
2
3
• The first four
connections have
4
made it impossible
for 3rd input to be
5
connected to 7th
6
output
7
8
9
*
The 3rd input can only reach the bottom middle switch
The 7th output line can only be reached from the top output switch.
36
Crossbar Architecture - Features
Architecture:
Switch element:
Switch drive:
Switch loss:
SNR:
Wide Sense Non-blocking
N2 (based on 2 x 2)
N2
(2N-1).Lse +2Lfs
XT – 10log10(N-1)
Where XT: Crosstalk (dB),
Lse: Loss/switch element
Lfs: Fibre-switch loss
37
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 inputoutput pair: Each path goes through a different
number of switches
38
Time-Space Switching Arch.
1
time 1
M
UX
2 1
TSI
2 1
M
UX
4 3
TSI
3 4
time 1
2
3
4
3
1
2
4
• 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: 4 (not 16)
39
Time-Space Switching Arch.
• Can flip samples both on input and output
trunk
• Gives more flexibility => lowers call
blocking probability
TSI
 Complex in terms of:
TSI
TSI
- Number of cross points
- Size of buffers
-Speed of the switch bus (internal speed)
TSI
TSI
TSI
TSI
TSI
40
Clos Architecture
1
nxp
kxk
pxn
1
1
1
2
2
2
n
32
33
64
32
64
32
n
•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
993
k
p
Stage 1
Stage 2
k
N= 1024
Stage 3
• Stage 1 (nxp)
• Stage 2(kxk)
• Stage 3 (pxn)
41
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.
Problems:
• Internal blocking
• Larger number of crossovers when p is large.
42
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.
43
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
44
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 nonblocking (RNB) (i.e. reducing the number of
middle stage switches)
45
Clos Architecture – Example
• If N and n are equal to 100 and 10, respectively,
then
– the number of switches at the 1st & 3rd stages are N/n
= 1000/10 = 100.
– at the 1st stage, they are 10 x p switches
– at the 3rd stage they are p x 10 switches.
– the 2nd stage will have p switches of size 100 x 100.
• 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:46
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,
– each switch at stage 1 is a 10 x 19 crossbar requiring
190 crosspoints and there are 100 such switches.
Same for the third stage. So the 1st & 3rd stages use
2x190x100 = 38,000 crosspoints altogether.
– The 2nd stage consists of p = 19 crossbars each of
size 100 x 100, because N/n = 100. So stage 2 uses
190,000 crosspoints.
• Altogether, the Clos construction uses 228,000
crosspoints Vs. the 106 points used by the
complete crossbar.
47
Clos Architecture – Example
contd.
Since k = N/n, therefore the number of switch elements is minimised
when = n ~(N/2)0.5 = (500) 0.5 =~ 32
We would then use 44 switches in the 1st & 3rd stages and
p = 2n-1= 2x23 – 1 = 45.
Since n = 23 does not divide 1000 evenly, thus we actually have 12
extra inputs and outputs that we could switch with this configuration
( 23x44=1012 and 1012 - 1000 = 12).
So we use
2x23x44x45=91,080 crosspoints in the 1st & 3rd stages
and an additional 44x44x45=87,120 crosspoints in the 2nd stage.
Thus the total number of crosspoints in the best Clos construction
involves fewer than 180,000 crosspoints 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!
48
Benes Architecture
22
22
N/2  N/2
Benes
N
N
N/2  N/2
Benes
• 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 (RNB)
49
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
50
Benes Architecture - contd.
1
1
2
2
3
3
4
4
5
5
6
6
X
7
7
8
8
•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
51
Benes Architecture –Non-blocking
contd.
• Now use different connections
• e.g.
2 to 1
1 to 5
3 to 3
4 to 2 OK
52