Switching
Architectures for
Optical Networks
CSIT5600 by M. Hamdi
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Internet Reality
Data
Center
SONET
SONET
DWD
M
DWD
M
SONET
SONET
Access
Metro
Long Haul
Metro
Access
CSIT5600 by M. Hamdi
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Hierarchies of Networks: IP / ATM /
SONET / WDM
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Why Optical?
• Enormous bandwidth made available
– DWDM makes ~160 channels/ possible in a fiber
– Each wavelength “potentially” carries about 40 Gbps
– Hence Tbps speeds become a reality
• Low bit error rates
– 10-9 as compared to 10-5 for copper wires
• Very large distance transmissions with very little
amplification.
CSIT5600 by M. Hamdi
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Dense Wave Division Multiplexing
(DWDM)
1
2
3
4
Long-haul fiber
Output fibers
Multiple wavelength bands on each fiber
 Transmit by combining multiple lasers @ different
frequencies
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Anatomy of a DWDM System
Terminal B
Terminal A
Transponder
Interfaces
M
U
X
PostAmp
Line Amplifiers
Direct
Connections
PreAmp
D
E
M
U
X
Transponder
Interfaces
Direct
Connections
Basic building blocks
• Optical amplifiers
• Optical multiplexers
• Stable optical sources
CSIT5600 by M. Hamdi
• Provisioned
SONET circuits.
• Aggregated into
Lamdbas.
Core Transport Services
Circuit
Origin
• Carried over
Fiber optic cables.
Circuit
Destination
OC-3
OC-3
OC-12
STS-1
STS-1
STS-1
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WDM Network: Wavelength View
WDM link
Edge Router
Legacy
Interfaces
Legacy
( e.g.,
PoS, Gigabit
Interfaces
Ethernet, IP/ATM)
Interfaces
Legacy
Interfaces
Optical Switch
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Relationship of IP and Optical
• Optical brings
–Bandwidth multiplication
–Network simplicity (removal
of redundant layers)
• IP brings
–Scalable, mature control
plane
–Universal OS and
application support
–Global Internet
• Collectively IP and Optical
(IP+Optical) introduces a set
of service-enabling
technologies
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Typical Super POP
Interconnectio
n
Network
Core
IP
router
DWDM
DWDM
+
Metro
Ring ADM
Large
Multi-service
Aggregation
Switch
Voice
Switch
Core
ATM
Switch
OXC
SONET
Coupler
&
Opt.amp
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Typical POP
Voice
Switch
D
W
D
M
OXC
D
W
D
M
SONET-XC
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What are the Challenges with Optical
Networks?
• Processing: Needs to be done with electronics
– Network configuration and management
– Packet processing and scheduling
– Resource allocation, etc.
• Traffic Buffering
– Optics still not mature for this (use Delay Fiber Lines)
– 1 pkt = 12 kbits @ 10 Gbps requires 1.2 s of delay =>
360 m of fiber)
• Switch configuration
– Relatively slow
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Wavelength Converters
• Improve utilization of available wavelengths on links
• All-optical WCs being developed
• Greatly reduce blocking probabilities
3
2
3
2
WC
No  converters
1
New request
1 3
With  converters
1
New request
1 3
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Wavelength Cross-Connects (WXCs)
• A WDM network consists of wavelength cross-connects (WXCs)
(OXC) interconnected by fiber links.
• 2 Types of WXCs
– Wavelength selective cross-connect (WSXC)
• Route a message arriving at an incoming fiber on some
wavelength to an outgoing fiber on the same wavelength.
• Wavelength continuity constraint
– Wavelength interchanging cross-connect (WIXC)
• Wavelength conversion employed
• Yield better performance
• Expensive
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Wavelength Router
Wavelength Router
Control Plane:
Wavelength Routing
Intelligence
Data Plane:
Optical Cross
Connect Matrix
Unidirectional
DWDM Links to
other Wavelength
Routers
Single Channel Links to
IP Routers, SDH Muxes,
...
Unidirectional
DWDM Links to
other Wavelength
Routers
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Optical Network Architecture
UNI
Mesh Optical Network
UNI
IP Network
IP Network
IP Router
OXC Control unit
Optical Cross
Connect (OXC)
Control Path
Data Path
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OXC Control Unit
• Each OXC has a control unit
• Responsible for switch configuration
• Communicates with adjacent OXCs or the client
network through single-hop light paths
– These are Control light paths
– Use standard signaling protocol like GMPLS for control
functions
• Data light paths carry the data flow
– Originate and terminate at client networks/edge routers
and transparently traverse the core
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Optical Cross-connects (No
wavelength conversion)
2
4
All Optical Cross-connect (OXC)
Also known as Photonic
Cross-connect (PXC)
1
3
Optical
Switch
Fabric
3
4
1
2
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Optical Cross-Connect with Full Wavelength
Conversion
Wavelength
Converters
2
1
2
n
1,2, ... ,n
1
1
2
n
1,2, ... ,n
2
.
.
.
1,2, ... ,n
M
Wavelength
Demux
1,2, ... ,n
1
n
1
1
2
n
1
2
n
n
1
2
Optical CrossBar
Switch
1,2, ... ,n
2
.
.
.
1,2, ... ,n
M
Wavelength
Mux
• M demultiplexers at incoming side
• M multiplexers at outgoing side
• Mn x Mn optical switch has wavelength converters at switch
outputs
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Wavelength Router with O/E and E/O
Cross-Connect
Incoming Interface
Incoming Wavelength
Outgoing Interface
Outgoing Wavelength
1
3
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O-E-O Crossconnect Switch (OXC)
Incoming
fibers
Demux
1
2
N
WDM
(many λs)
Individual wavelengths
O
O
O/E
E/O
E
O/E
E/O
O/E
O/E
O/E
O/E
E/O
E/O
E/O
E/O
O/E
O/E
O/E
E/O
E/O
E/O
Outgoing
fibers
Mux
1
2
N
Switches information signal on a particular wavelength on an
incoming fiber to (another) wavelength on an outgoing fiber.
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Optical core network
Opaque (O-E-O) and transparent (O-O) sections
E/O
Client
signals
Transparent
optical island
O/E
O
O
from other nodes
O
O
O
E
O
O
E
E
O
O
to other nodes
E
O
O
Opaque optical network
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OEO vs. All-Optical Switches
OEO
• Capable of status monitoring
• Optical signal regenerated –
improve signal-to-noise ratio
• Traffic grooming at various levels
All-Optical
• Unable to monitor the contents of
the data stream
• Only optical amplification –
signal-to-noise ratio degraded
with distance
• No traffic grooming in subwavelength level
• Less aggregated throughput
• More expensive
• Higher aggregated throughput
• More power consumption
• ~10X cost saving
• ~10X power saving
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Large customers buy “lightpaths”
A lightpath is a series of wavelength links from end to end.
optical
fibers
One fiber
Repeater
cross-connect
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Hierarchical switching: Node with switches
of different granularities
A. Entire fibers
O
O
Fibers
O
Fibers
B. Wavelength
subsets
O
O
O
C. Individual
wavelengths
O
E
O
“Express
trains”
“Local
trains”
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Wide Area Network (WAN)
WAN :
Up to 200-500 wavelengths
40-160 Gbit/s/
wavebands (> 10 )
OXC: Optical Wavelength/Waveband Cross
Connect
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Packet (a) vs. Burst (b) Switching
Header recognition,
processing, and generation
Payload
C
Header
A
Setup
Synchronizer
1
2
A
New
headers
(a)
Control
wavelengths
Control
packets
D
C
2
2
O/E/O
1
Control packet processing
(setup/bandwidth reservation)
Offset time
2
B
2
FDL’s
1
Data
wavelengths
1
2
Fixed-length
(but unaligned)
B
Switch
1
Incoming
fibers
2
Switch
1
1
Data bursts
(b)
D
CSIT5600 by M. Hamdi
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MAN (Country / Region)
IP
packets
optical
burst
formation
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Optical Switching Technologies
•
•
•
•
•
•
•
•
MEMs – MicroElectroMechanical
Liquid Crystal
Opto-Mechanical
Bubble Technology
Thermo-optic (Silica, Polymer)
Electro-optic (LiNb03, SOA, InP)
Acousto-optic
Others…
Maturity of technology, Switching speed, Scalability, Cost,
Relaiability (moving components or not), etc.
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MEMS Switches for Optical CrossConnect
Moveable Micromirror
Proven technology, switching time (10 to 25 msec), moving mirrors is a
reliability problem.
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WDM “transparent” transmission system
(O-O nodes)
Wavelengths
disaggregator
O
Fibers
Wavelengths
aggregator
O
O
O
multiple
λs
O
O
Optical switching fabric (MEMS devices, etc.)
Incoming fiber
Outgoing fibers
Tiny mirrors
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Upcoming Optical Technologies
• WDM routing is circuit switched
– Resources are wasted if enough data is not sent
– Wastage more prominent in optical networks
• Techniques for eliminating resource wastage
– Burst Switching
– Packet Switching
• Optical burst switching (OBS) is a new method to transmit data
• A burst has an intermediate characteristics compared to the
basic switching units in circuit and packet switching, which are a
session and a packet, respectively
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Optical Burst Switching (OBS)
• Group of packets a grouped in to ‘bursts’, which is
the transmission unit
• Before the transmission, a control packet is sent
out
– The control packet contains the information of burst
arrival time, burst duration, and destination address
• Resources are reserved for this burst along the
switches along the way
• The burst is then transmitted
• Reservations are torn down after the burst
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Optical Burst Switching (OBS)
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Optical Packet Switching
• Fully utilizes the advantages of statistical
multiplexing
• Optical switching and buffering
• Packet has Header + Payload
– Separated at an optical switch
• Header sent to the electronic control unit, which
configures the switch for packet forwarding
• Payload remains in optical domain, and is recombined with the header at output interface
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Optical Packet Switch
• Has
– Input interface, Switching fabric, Output interface and
control unit
• Input interface separates payload and header
• Control unit operates in electronic domain and
configures the switch fabric
• Output interface regenerates optical signals and
inserts packet headers
• Issues in optical packet switches
– Synchronization
– Contention resolution
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• Main operation in a switch:
– The header and the payload are separated.
– Header is processed electronically.
– Payload remains as an optical signal throughout the switch.
– Payload and header are re-combined at the output interface.
hdr
payload
CPU
hdr
payload
hdr
payload
Wavelength i
input port j
Re-combined
Wavelength i
output port j
Optical
packet
Optical switch
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Output port contention
• Assuming a non-blocking switching matrix, more than one
packet may arrive at the same output port at the same
time.
Input ports
Optical Switch
Output ports
payloadhdr
.
.
.
payloadhdr
.
.
.
payloadhdr
.
.
.
.
.
.
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OPS Architecture: Synchronization
Occurs in electronic switches – solved by input buffering
Slotted networks
•Fixed packet size
•Synchronization stages required
Sync.
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OPS Architecture: Synchronization
Slotted networks
•Fixed packet size
•Synchronization stages required
Sync.
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OPS Architecture: Synchronization
Slotted networks
•Fixed packet size
•Synchronization stages required
Sync.
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OPS Architecture: Synchronization
Slotted networks
•Fixed packet size
•Synchronization stages required
Sync.
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OPS Architecture: Synchronization
Slotted networks
•Fixed packet size
•Synchronization stages required
Sync.
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OPS Architecture: Synchronization
Sync.
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OPS: Contention Resolution
• More than one packet trying to go out of the same
output port at the same time
– Occurs in electronic switches too and is resolved by
buffering the packets at the output
– Optical buffering ?
• Solutions for contention
– Optical Buffering
– Wavelength multiplexing
– Deflection routing
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OPS Architecture
Contention Resolutions
1
2
3
1
1
2
1
3
4
4
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OPS: Contention Resolution
• Optical Buffering
– Should hold an optical signal
• How? By delaying it using Optical Delay Lines (ODL)
– ODLs are acceptable in prototypes, but not commercially
viable
– Can convert the signal to electronic domain, store, and reconvert the signal back to optical domain
• Electronic memories too slow for optical networks
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OPS Architecture
Contention Resolutions
•Optical buffering
1
1
2
3
1
2
1
3
4
4
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OPS Architecture
Contention Resolutions
•Optical buffering
1
1
2
2
3
3
4
4
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OPS Architecture
Contention Resolutions
•Optical buffering
1
1
1
2
2
3
3
4
4
1
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OPS: Contention Resolution
• Wavelength multiplexing
– Resolve contention by transmitting on different
wavelengths
– Requires wavelength converters - $$$
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OPS Architecture
Contention Resolutions
•Wavelength conversion
1
1
1
1
2
2
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OPS Architecture
Contention Resolutions
•Wavelength conversion
1
1
2
2
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OPS Architecture
Contention Resolutions
•Wavelength conversion
1
1
1
1
2
2
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OPS Architecture
Contention Resolutions
•Wavelength conversion
1
1
2
2
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OPS Architecture
Contention Resolutions
•Wavelength conversion
1
1
1
1
2
2
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Deflection routing
• When there is a conflict between two optical packets, one will
be routed to the correct output port, and the other will be
routed to any other available output port.
• A deflected optical packet may follow a longer path to its
destination. In view of this:
– The end-to-end delay for an optical packet may be
unacceptably high.
– Optical packets may have to be re-ordered at the
destination
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Electronic Switches
Using Optical Crossbars
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Scalable Multi-Rack Switch
Architecture
Optical links
Line card
rack
Switch Core
• Number of linecards is limited in a single rack
– Limited power supplement, i.e. 10KW
– Physical consideration, i.e. temperature, humidity
• Scaling to multiple racks
– Fiber links and central fabrics
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Logical Architecture of Multi-rack Switches
Scheduler
Line Card
Local
Fiber I/O
Framer
Buffers
Laser
Laser
Line Card
Laser
Laser
Local
Buffers
Framer
Fiber I/O
Crossbar
Line Card
Local
Fiber I/O
Framer
Buffers
Line Card
Laser
Laser
Laser
Laser
Local
Buffers
Framer
Fiber I/O
Switch Fabric System
• Optical I/O interfaces connected to WDM fibers
• Electronic packet processing and buffering
– Optical buffering, i.e. fiber delay lines, is costly and not mature
• Optical interconnect
– Higher bandwidth, lower latency and extended link length than
copper twisted lines
• Switch fabric: electronic? Optical?
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Optical Switch Fabric
Scheduler
Line Card
Local
Fiber I/O
Framer
Buffers
Laser
Laser
Line Card
Laser
Laser
Local
Buffers
Framer
Fiber I/O
Crossbar
Line Card
Local
Fiber I/O
Framer
Buffers
Line Card
Laser
Laser
Laser
Laser
Local
Buffers
Framer
Fiber I/O
Switch Fabric System
• Less optical-to-electrical conversion inside switch
– Cheaper, physically smaller
• Compare to electronic fabric, optical fabric brings advantages in
– Low power requirement, Scalability, Port density, High capacity
• Technologies that can be used
– 2D/3D MEMS, liquid crystal, bubbles, thermo-optic, etc.
CSIT5600 by M. Hamdi
• Hybrid architecture takes advantage
of the strengths of both
electronics and optics
61
Electronic Vs. Optical Fabric
Electronic
Trans. Buffer InterLine
connection
Inter- Buffer Trans.
connection
Line
Switching
Fabric
Optical
Electronic
E/O or O/E
Conversion
Optical
Trans. Buffer InterLine
connection
Inter- Buffer Trans.
connection
Line
Switching
Fabric
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Multi-rack Hybrid Packet Switch
Rack
Buf f er
E/O
O/E
Buf f er
Buf f er
E/O
O/E
Buf f er
Optical Optical
Fiber Crossbar
Buf f er
E/O
Optical
Fiber
O/E
Buf f er
O/E
Buf f er
Linecard
Buf f er
E/O
Switch Core
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Features of Optical Fabric
• Less E/O or O/E conversion
• High capacity
• Low power consumption
• Less cost
However,
• Reconfiguration overhead (50-100ns)
– Tuning of lasers (20-30ns)
– System clock synchronization (10-20ns or higher)
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Scheduling Under Reconfiguration
Overhead
• Traditional slot-by-slot approach
Scheduler
Schedule Reconfigure Transfer
Time Line
• Low bandwidth usage
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Reduced Rate Scheduling
Fabric setup (reconfigure)
Traffic transfer
Time slot
Slot-by-slot Scheduling, zero fabric setup time
Slot-by-slot Scheduling with reconfigure delay
Reduced rate Scheduling, each schedule is held for some time
•
Challenge: fabric reconfiguration delay
–
•
Traditional slot-by-slot scheduling brings lots of overhead
Solution: slow down the scheduling frequency to compensate
–
•
Each schedule will be held for some time
Scheduling task
1.
2.
Find out the matching
Determine the holding time
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Scheduling Under Reconfiguration
Overhead
• Reduce the scheduling rate
– Bandwidth Usage = Transfer/(Reconfigure+Transfer)
Constant
• Approaches
– Batch scheduling: TSA-based
– Single scheduling: Schedule + Hold
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Single Scheduling
• Schedule + Hold
– One schedule is generated each time
– Each schedule is held for some time (holding time)
– Holding time can be fixed or variable
– Example: LQF+Hold
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Routing and
Wavelength
Assignment
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Optical Circuit Switching
• An optical path established between two nodes
• Created by allocation of a wavelength throughout the path.
• Provides a ‘circuit switched’ interconnection between two
nodes.
– Path setup takes at least one RTT
– No optical buffers since path is pre-set
Desirable to establish light paths between every pair of nodes.
• Limitations in WDM routing networks,
– Number of wavelengths is limited.
– Physical constraints:
• limited number of optical transceivers limit the number of
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channels.
70
Routing and Wavelength Assignment (RWA)
• Light path establishment involves
– Selecting a physical path between source and
destination edge nodes
– Assigning a wavelength for the light path
• RWA is more complex than normal routing
because
– Wavelength continuity constraint
• A light path must have same wavelength along all the links
in the path
– Distinct Wavelength Constraint
• Light paths using the same link must have different
wavelengths
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No Wavelength Converters
WSXC
Access Fiber
Wavelength 1
POP
POP
Wavelength 2
Wavelength 3
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With Wavelength Converters
WIXC
Wavelength 1
Access Fiber
POP
POP
Wavelength 2
Wavelength 3
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Routing and Wavelength
Assignment (RWA)
• RWA algorithms based on traffic assumptions:
• Static Traffic
– Set of connections for source and destination pairs are given
• Dynamic Traffic
– Connection requests arrive to and depart from network one
by one in a random manner.
– Performance metrics used fall under one of the following
three categories:
• Number of wavelengths required
• Connection blocking probability: Ratio between number of
blocked connections and total number of connections arrived
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Static and Dynamic RWA
• Static RWA
– Light path assignment when traffic is known well in
advance
– Arises in capacity planning and design of optical networks
• Dynamic RWA
– Light path assignment to be done when requests arrive in
random fashion
– Encountered during real-time network operation
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Static RWA
• RWA is usually solved as an optimization
problem with Integer Programming (IP)
formulations
• Objective functions
– Minimize average weighted number of hops
– Minimize average packet delay
– Minimize the maximum congestion level
– Minimize number of Wavelenghts
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Static RWA
• Methodologies for solving Static RWA
– Heuristics for solving the overall ILP sub-optimally
– Algorithms that decompose the static RWA problem
problem into
into
a set of individual sub-problems, and solve a sub-set
–http://www.tct.hut.fi/~esa/java/wdm/
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Solving Dynamic RWA
• During network operation, requests for new lightpaths come randomly
• These requests will have to be serviced based on
the network state at that instant
• As the problem is in real-time, dynamic RWA
algorithms should be simple
• The problem is broken down into two sub-problems
– Routing problem
– Wavelength assignment problem
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