Transcript Optical Fibre Communication Systems

Optical Fibre Communication
Systems
Lecture 7 – Optical Switches
Professor Z Ghassemlooy
Northumbria Communications Laboratory
School of Computing, Engineering and
Information Sciences
The University of Northumbria
U.K.
http://soe.unn.ac.uk/ocr
Prof. Z Ghassemlooy
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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
Ports
Ports
Problem
- limited and could not scale to thousands or millions of users
Solution
- switched network
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Switched Network
Pervasive, high-bandwidth, reliable, transparent
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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 alloptical networks?
– Well, not really. All that it might do is delay things.
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Electrical vs. Optical - Cross
Connects
Optical
Electrical Limits
Number of ports
1024
512
256
• High power
consumption:
e.g. 1024x1024: 4 kW
• Jitter: very large
• Large switches
• Need OE/EO conversion
128
• Bipolar or GaAs
64
32
16
Electrical
8
10 MHz
100 MHz
1 GHz
10 GHz
100 GHz
Data rate
M C Wu
DS3
OC3
OC12
OC48
OC192
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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.
Electrical control
Optical
input
Electrical control
Optical
output
Optical
input
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Optical
output
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Switching Fabric – contd.
Input
interface
Output
interface
Switching
fabric
Switching control
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Switching Fabric – contd.
...
Optical
Crossconnect
(OXC)
Transponders
...
...
...
...
Optical transport system
(1.55 mm WDM)
...
1.3 mm intra-office
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 nonblocking
– 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
1
M
U
X
N
4 3 2 1
TSI
2 4 1 3
1
2
3
4
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D
E
M
U
X
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
1
2
Input
ports 3
4
Sessions: (1,4) (2,2) (3,1) (4,3)
1 2 3
4
Output ports
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Crossbar Architectures - Blocking
Input channels


2
N X N matrix S/W
3
4
Output channels - Bars
Input channels
1

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
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Crossbar Architecture - Wide-Sense Nonblocking
Rule: To connect ith input to
Input channels
the jth output, the algorithm
1
sets the
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.
Case 2:
4
1
2
3
4
- (2,4) ok
- (3,2) ok
- (4,3) ok
Output channels
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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
2
3x3
5
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Crossbar Architectures - 3 Layer
1
2
3
4
5
6
4
5
6
7
8
9
7
8
9
Blocking still possible
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Output ports
Input port
1
2
3
http://www.aston.ac.uk/~blowkj/index.htm
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Crossbar Architectures - 3 Layer
*
1
2
3
1
2
3
Blocking
 The first four
connections
4
have made it
5
impossible for
6
3rd input to be
connected to 7th
7 * output
4
5
6
7
8
9
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.
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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
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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 inputoutput pair: Each path goes through a different number
of switches
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Time-Space Switching Arch.
1
2
3
4
time 1
M
U
X
2 1
TSI
2 1
M
U
X
4 3
TSI
3 4
time 1
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 N = 4 (not 16)
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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
TSI
TSI
TSI
TSI
TSI
- Number of cross points
- Size of buffers
-Speed of the switch bus (internal
speed)
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Clos Architecture
1
nxp
kxk
pxn
1
1
1
n
32
33
2
2
2
64
32
64
32
993
k
p
k
Stage 1
Stage 2
Stage 3
N= 1024
Prof. Z Ghassemlooy
•It is a 3-stage network
n - 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)
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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 n1 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 nonblocking (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:Prof. Z Ghassemlooy
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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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
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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.
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
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Benes Architecture –Non-blocking
contd.
• Now use different connections
• e.g.
2 to 1
1 to 5
3 to 3
Prof. Z Ghassemlooy
4 to 2 OK
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Three Building Blocks for OXC
International Engineering Consortium, 2004
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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.
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Two Position Switch - contd.
 The two-position switch requires three fibres with
collimating lenses and a prism.
Lens
B
A
Prisem
Light arriving at port A needs to be
switched to port C.
C
Fibre
B
A
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
- Fast
- Complex
- Maturing
- Lossy
- Slow
- Maturity
- Reliable
- Slow
- Low loss & crosstalk
- Inherently scalable
- Liquid crystal
- Mechanical / fibre
- Micro-optics (MEM’s)
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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
Mach – Zehnder Configuration
Input Ii
Outputs
I1
Heater
I2
I1
 sin 2 ( / 2)
Ii
I2
 cos2 ( / 2)
Ii
Directional coupler
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Thermo-Optic Switch - Polymer
Y – Junction Configuration
PH1
I1
Ii
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
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Thermo-Optic Switch - Characteristics
Parameters
Switch Size
2x2
Si Poly.
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
Prof. Z Ghassemlooy
8x8
Poly.
16 x 16
Si
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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
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Micro Electro Mechanical Switches
Combines optomechanical structures, microactuators, and micro-optical
elements on the same substrate
Input fibres
 Made using micro-machining
 Free-space: polarisation
independent
 Independent of:
– Bit-rate
– Wavelength
– Protocol
 Speed: 1 10 ms
Output fibres
Lens
Flat mirror
4 x 4 Cross point
switch
Raised mirror
Prof. Z Ghassemlooy
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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.
I/O Fibers
Reflector
Imaging
Lenses
MEMS 2-axis
Tilt Mirrors
Lightwave
Prof. Z Ghassemlooy
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Micro Electro Mechanical Switches
 Monolithic integration --> Compact, lightweight, scalable
Batch 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)
Prof. Z Ghassemlooy
62
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)
Control
1
2
N
1
2
N X N Cross Connect
Prof. Z Ghassemlooy
N
63
Optical Cross Connects
Prof. Z Ghassemlooy
64
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.
Prof. Z Ghassemlooy
65
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
Prof. Z Ghassemlooy
66
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.
Prof. Z Ghassemlooy
67
Optical Routers
 High-power, low-noise optical amplifiers-or
erbium-doped fiber amplifiers (EDFAs)-and pulseshaping 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.
Prof. Z Ghassemlooy
68
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.
Prof. Z Ghassemlooy
69
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.
Prof. Z Ghassemlooy
70
Large, Centralized Router
 Current trend in XCs is to use large microelectromechanical 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 nonblocking 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.
Prof. Z Ghassemlooy
71
IP Routers + Optical Network Elements
End Customer
Router
Router
Router
ONE
Router
Router
ONE
ONE
Optical Network
A V Lehmen, Telecordia Tech.
Prof. Z Ghassemlooy
72
Optical Layer Capability: Reconfigurability
IP
Router
IP
Router
IP
Router
OXC - A
OXC - B
IP
Router
OXC - C
IP
Router
OXC - D
Crossconnects are reconfigurable:
 Can provide restoration capability
 Provide connectivity between any two routers
Prof. Z Ghassemlooy
A V Lehmen, Telecordia Tech.
73
Architecture 1: Large Routers + High
capacity Fibres
Access lines
A
Z
• All
traffic flows through routers
• Optics just transports the data from one point to another
• IP layer can handle restoration
• Network is ‘simple’
Access lines
A V Lehmen, Telecordia Tech.
• But…..
- more hops translates into more packet delays
- each router has to deal with thru traffic as well as terminating traffic
Prof. Z Ghassemlooy
74
Architecture 2: Small Routers + OXC
OXC
OXC
OXC
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
A V Lehmen, Telecordia Tech.
•But: network has more NE’s, and is more complicated
Prof. Z Ghassemlooy
75
Dynamic Set-Up of Optical Connection
IP
Router
IP
Router
IP
Router
OXC - A
OXC - B
IP
Router
OXC - C
A V Lehmen, Telecordia Tech.
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
Prof. Z Ghassemlooy
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OXC – Router-Selector Architecture
1
1
N
N
1
1
N
N
•Type I - 1 x N & N x 1 optical switches
•Type II - 1 x N passive optical splitter
- N x 1 Optical switches
Prof. Z Ghassemlooy
77
OXC – Router - Feature
Type I
TypeII
Strictly non-blocking
Architecture
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)
Where
XT; Crosstalk (dB),
Lse; Loss/switch element
Lfs; Fibre-switch loss
Prof. Z Ghassemlooy
78
OXC + Wavelength Converters
Prof. Z Ghassemlooy
79
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
Prof. Z Ghassemlooy
80
Optical Gateway Cross-Connect
Performs digital grooming, traditional multiplexing, and routing of lowerspeed 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 OC48/STM-16 rates and electrical DS-3, STS-1 and STM-1e rates and
grooms them into higher rate optical signals.
Alcatel. 2001
Prof. Z Ghassemlooy
81
IP-router with Tb/s throughput can be built with
fast tunable lasers & NxN optical mux
From Input Port
Scheduler
Buffer
Output
T-Tx
40 G mod
40G Rx
T-Tx
40 G mod
40G Rx
T-Tx
40 G mod
40G Rx
T-Tx
40 G mod
40G Rx
retiming
Clock
Yamada et al., 1998
Prof. Z Ghassemlooy
82
Router & Optical Switch
CHIARO- OptIPuter Optical Switch Workshop
Prof. Z Ghassemlooy
83
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.
Prof. Z Ghassemlooy
84

Prof. 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.
85