switching architectures for optical networks · occurs in electronic switches –solved by input...
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CSIT5600 by M. Hamdi1
Switching
Architectures for
Optical Networks
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CSIT5600 by M. Hamdi2
SONET
Data
CenterSONET
SONET
SONET
DWD
M DWD
M
AccessLong HaulAccess MetroMetro
Internet Reality
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CSIT5600 by M. Hamdi3
Hierarchies of Networks: IP / ATM /
SONET / WDM
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CSIT5600 by M. Hamdi4
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.
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CSIT5600 by M. Hamdi5
Dense Wave Division Multiplexing
(DWDM)
Multiple wavelength bands on each fiber
Transmit by combining multiple lasers @ different
frequencies
Output fibers
Long-haul fiber
1
2
3
4
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CSIT5600 by M. Hamdi
Anatomy of a DWDM System
Terminal A Terminal B
Post-
AmpPre-
AmpLine Amplifiers
M
U
X
D
E
M
U
X
Transponder
Interfaces
Transponder
Interfaces
Direct
ConnectionsDirect
Connections
Basic building blocks
• Optical amplifiers
• Optical multiplexers
• Stable optical sources
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CSIT5600 by M. Hamdi7
Core Transport Services
OC-3
OC-3
OC-12
STS-1
STS-1STS-1
• Provisioned
SONET circuits.
• Aggregated into
Lamdbas.
• Carried over
Fiber optic cables.
Circuit
Origin
Circuit
Destination
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CSIT5600 by M. Hamdi8
WDM Network: Wavelength View
WDM link
Optical Switch
Edge Router
Legacy
Interfaces
Legacy
Interfaces
Legacy
Interfaces
( e.g., PoS, Gigabit
Ethernet, IP/ATM)
Interfaces
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CSIT5600 by M. Hamdi9
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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CSIT5600 by M. Hamdi10
Typical Super POP
OXC
Core
IP
router
Interconnectio
n
Network
Large
Multi-service
Aggregation
Switch
Voice
Switch
Core
ATM
Switch
SONET
Coupler
&
Opt.amp
DWDM
+
ADM
DWDM
Metro
Ring
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CSIT5600 by M. Hamdi11
Typical POP
OXC
D
W
D
M
Voice
Switch
SONET-XC
D
W
D
M
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CSIT5600 by M. Hamdi12
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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CSIT5600 by M. Hamdi13
Wavelength Converters
• Improve utilization of available wavelengths on links
• All-optical WCs being developed
• Greatly reduce blocking probabilities
No converters
1
2 3
New request
1 3
1
2 3
New request
1 3
With converters
WC
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CSIT5600 by M. Hamdi14
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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CSIT5600 by M. Hamdi15
Wavelength Router
Wavelength Router
Control Plane:Wavelength Routing
Intelligence
Data Plane:Optical Cross
Connect Matrix
Single Channel Links to
IP Routers, SDH Muxes,
...
Unidirectional
DWDM Links to
other Wavelength
Routers
Unidirectional
DWDM Links to
other Wavelength
Routers
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CSIT5600 by M. Hamdi16
Optical Network Architecture
IP Router
Optical Cross Connect (OXC)
OXC Control unit Control Path
Data Path
UNIUNIMesh Optical Network
IP Network IP Network
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CSIT5600 by M. Hamdi17
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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CSIT5600 by M. Hamdi18
Optical Cross-connects (No
wavelength conversion)
Optical
Switch
Fabric
3
2
2
4
4
1
1
3
All Optical Cross-connect (OXC) Also known as PhotonicCross-connect (PXC)
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CSIT5600 by M. Hamdi19
Optical Cross-Connect with Full Wavelength
Conversion
• M demultiplexers at incoming side
• M multiplexers at outgoing side
• Mn x Mn optical switch has wavelength converters at switch outputs
1, 2, ... , n
1, 2, ... , n
1, 2, ... , n
1
2
M
Optical CrossBarSwitch
WavelengthConverters
WavelengthMux
WavelengthDemux
1, 2, ... , n
1, 2, ... , n
1, 2, ... , n
.
.
.
.
.
.
1
2
n
1
2
n
1
2
n
1
2
n
1
2
n
n
1
2
1
2
M
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Wavelength Router with O/E and E/O
Cross-Connect
1
3
Outgoing Interface
Outgoing Wavelength
Incoming Interface
Incoming Wavelength
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CSIT5600 by M. Hamdi21
Demux1
Incoming fibers
OE
O
Individual wavelengths
Mux
Outgoing fibers
O-E-O Crossconnect Switch (OXC)
O/EO/EO/E
O/EO/EO/E
O/EO/EO/E
N
2
E/OE/OE/O
E/OE/OE/O
E/OE/OE/O
Switches information signal on a particular wavelength on anincoming fiber to (another) wavelength on an outgoing fiber.
1
N
2WDM(many λs)
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CSIT5600 by M. Hamdi22
Optical core networkOpaque (O-E-O) and transparent (O-O) sections
E/OClientsignals
O/E
to other nodesfrom other nodes
E E O
O
Transparentoptical island
O O
OOE
OO
O O
EO
Opaque optical network
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CSIT5600 by M. Hamdi23
OEO vs. All-Optical Switches
• Capable of status monitoring
• Optical signal regenerated –
improve signal-to-noise ratio
• Traffic grooming at various levels
• Less aggregated throughput
• More expensive
• More power consumption
• Unable to monitor the contents of
the data stream
• Only optical amplification –
signal-to-noise ratio degraded
with distance
• No traffic grooming in sub-
wavelength level
• Higher aggregated throughput
• ~10X cost saving
• ~10X power saving
OEO All-Optical
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CSIT5600 by M. Hamdi24
Large customers buy “lightpaths”
A lightpath is a series of wavelength links from end to end.
cross-connect
opticalfibers
Repeater
One fiber
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CSIT5600 by M. Hamdi25
Hierarchical switching: Node with switches of different granularities
FibersOA. Entire fibers
Fibers
O O
OB. Wavelengthsubsets
O O
“Expresstrains”
OC. Individualwavelengths
E O
“Localtrains”
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CSIT5600 by M. Hamdi26
Wide Area Network (WAN)
OXC: Optical Wavelength/Waveband Cross
Connect
WAN :
Up to 200-500 wavelengths
40-160 Gbit/s/
wavebands (> 10 )
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CSIT5600 by M. Hamdi27
Packet (a) vs. Burst (b) Switching
Incoming
fibers
Fixed-length
(but unaligned) FDL’s
Synchronizer
Header
Payload
Setup
Header recognition,
processing, and generation
Switch1
B
C
DNew
headers
2
1
2 2
1
(a)
A
Switch
2
1 1
2
(b)
O/E/O
Control packet processing
(setup/bandwidth reservation)
2 2
1 1
Control
packets
Data bursts
Control
wavelengthsA
B
C
D
Data
wavelengths
Offset time
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CSIT5600 by M. Hamdi28
MAN (Country / Region)
optical
burst
formation
IP
packets
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CSIT5600 by M. Hamdi29
WDM “transparent” transmission system
Wavelengthsaggregator
multipleλs
Fibers
(O-O nodes)
Wavelengthsdisaggregator
O O OO OO
Optical switching fabric (MEMS devices, etc.)
Incoming fiberTiny mirrors
Outgoing fibers
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CSIT5600 by M. Hamdi30
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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CSIT5600 by M. Hamdi31
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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CSIT5600 by M. Hamdi32
Optical Burst Switching (OBS)
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CSIT5600 by M. Hamdi33
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 re-
combined with the header at output interface
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CSIT5600 by M. Hamdi34
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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CSIT5600 by M. Hamdi35
• 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.
payload hdr
Wavelength i
input port j
Optical
packet
hdr CPU
Optical switch
payload
payload hdr
Re-combined
Wavelength i
output port j
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CSIT5600 by M. Hamdi36
Output port contention
• Assuming a non-blocking switching matrix, more than one
packet may arrive at the same output port at the same
time.
Output ports
payloadhdr
payloadhdr
payloadhdr
.
.
.
Optical SwitchInput ports
.
.
.
.
.
.
.
.
.
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Syn
c.
•Fixed packet size
•Synchronization stages required
Slotted networks
OPS Architecture: Synchronization
Occurs in electronic switches – solved by input buffering
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CSIT5600 by M. Hamdi38
•Fixed packet size
•Synchronization stages required
Slotted networks
Syn
c.
OPS Architecture: Synchronization
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CSIT5600 by M. Hamdi39
•Fixed packet size
•Synchronization stages required
Slotted networks
OPS Architecture: Synchronization
Syn
c.
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CSIT5600 by M. Hamdi40
•Fixed packet size
•Synchronization stages required
Slotted networks
OPS Architecture: Synchronization
Syn
c.
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CSIT5600 by M. Hamdi41
•Fixed packet size
•Synchronization stages required
Slotted networks
OPS Architecture: Synchronization
Syn
c.
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CSIT5600 by M. Hamdi42
OPS Architecture: Synchronization
Syn
c.
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CSIT5600 by M. Hamdi43
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
1
1
2
3
4
1
2
3
4
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CSIT5600 by M. Hamdi45
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 re-
convert the signal back to optical domain
• Electronic memories too slow for optical networks
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•Optical buffering
OPS Architecture
Contention Resolutions
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•Optical buffering
OPS Architecture
Contention Resolutions
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•Optical buffering
OPS Architecture
Contention Resolutions
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OPS: Contention Resolution
• Wavelength multiplexing
– Resolve contention by transmitting on different
wavelengths
– Requires wavelength converters - $$$
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•Wavelength conversion
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OPS Architecture
Contention Resolutions
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•Wavelength conversion
OPS Architecture
Contention Resolutions
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•Wavelength conversion
OPS Architecture
Contention Resolutions
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•Wavelength conversion
OPS Architecture
Contention Resolutions
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•Wavelength conversion
OPS Architecture
Contention Resolutions
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Scalable Multi-Rack Switch
Architecture
Switch Core
Optical links
Line cardrack
• 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
• 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?
Crossbar
Scheduler
Switch Fabric System
Framer
Line Card
Laser Laser
Laser
LaserLocal
Buffers
Framer
Line Card
Laser LaserLocal
Buffers
Framer
Line Card
LaserLocal
Buffers
Framer
Line Card
LaserLocal
Buffers
Fiber I/O
Fiber I/O
Fiber I/O
Fiber I/O
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CSIT5600 by M. Hamdi
THANK YOU
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