Download Welcome to ENTC 415

Switching
Architectures for
Optical Networks
CSIT5600 by M. Hamdi
1
Internet Reality
Data
Center
SONET
SONET
DWD
M
DWD
M
SONET
SONET
Access
Metro
Long Haul
Metro
Access
CSIT5600 by M. Hamdi
2
Hierarchies of Networks: IP / ATM /
SONET / WDM
CSIT5600 by M. Hamdi
3
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
4
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
CSIT5600 by M. Hamdi
5
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
CSIT5600 by M. Hamdi
7
WDM Network: Wavelength View
WDM link
Edge Router
Legacy
Interfaces
Legacy
( e.g.,
PoS, Gigabit
Interfaces
Ethernet, IP/ATM)
Interfaces
Legacy
Interfaces
Optical Switch
CSIT5600 by M. Hamdi
8
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
CSIT5600 by M. Hamdi
9
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
CSIT5600 by M. Hamdi
10
Typical POP
Voice
Switch
D
W
D
M
OXC
D
W
D
M
SONET-XC
CSIT5600 by M. Hamdi
11
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
CSIT5600 by M. Hamdi
12
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
CSIT5600 by M. Hamdi
13
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
CSIT5600 by M. Hamdi
14
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
CSIT5600 by M. Hamdi
15
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
CSIT5600 by M. Hamdi
16
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
CSIT5600 by M. Hamdi
17
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
CSIT5600 by M. Hamdi
18
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
CSIT5600 by M. Hamdi
19
Wavelength Router with O/E and E/O
Cross-Connect
Incoming Interface
Incoming Wavelength
Outgoing Interface
Outgoing Wavelength
1
3
CSIT5600 by M. Hamdi
20
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.
CSIT5600 by M. Hamdi
21
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
CSIT5600 by M. Hamdi
22
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
CSIT5600 by M. Hamdi
23
Large customers buy “lightpaths”
A lightpath is a series of wavelength links from end to end.
optical
fibers
One fiber
Repeater
cross-connect
CSIT5600 by M. Hamdi
24
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”
CSIT5600 by M. Hamdi
25
Wide Area Network (WAN)
WAN :
Up to 200-500 wavelengths
40-160 Gbit/s/
wavebands (> 10 )
OXC: Optical Wavelength/Waveband Cross
Connect
CSIT5600 by M. Hamdi
26
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
27
MAN (Country / Region)
IP
packets
optical
burst
formation
CSIT5600 by M. Hamdi
28
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.
CSIT5600 by M. Hamdi
29
MEMS Switches for Optical CrossConnect
Moveable Micromirror
Proven technology, switching time (10 to 25 msec), moving mirrors is a
reliability problem.
CSIT5600 by M. Hamdi
30
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
CSIT5600 by M. Hamdi
31
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
CSIT5600 by M. Hamdi
32
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
CSIT5600 by M. Hamdi
33
Optical Burst Switching (OBS)
CSIT5600 by M. Hamdi
34
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
CSIT5600 by M. Hamdi
35
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
CSIT5600 by M. Hamdi
36
• 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
CSIT5600 by M. Hamdi
37
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
.
.
.
.
.
.
CSIT5600 by M. Hamdi
38
OPS Architecture: Synchronization
Occurs in electronic switches – solved by input buffering
Slotted networks
•Fixed packet size
•Synchronization stages required
Sync.
CSIT5600 by M. Hamdi
39
OPS Architecture: Synchronization
Slotted networks
•Fixed packet size
•Synchronization stages required
Sync.
CSIT5600 by M. Hamdi
40
OPS Architecture: Synchronization
Slotted networks
•Fixed packet size
•Synchronization stages required
Sync.
CSIT5600 by M. Hamdi
41
OPS Architecture: Synchronization
Slotted networks
•Fixed packet size
•Synchronization stages required
Sync.
CSIT5600 by M. Hamdi
42
OPS Architecture: Synchronization
Slotted networks
•Fixed packet size
•Synchronization stages required
Sync.
CSIT5600 by M. Hamdi
43
OPS Architecture: Synchronization
Sync.
CSIT5600 by M. Hamdi
44
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
CSIT5600 by M. Hamdi
45
OPS Architecture
Contention Resolutions
1
2
3
1
1
2
1
3
4
4
CSIT5600 by M. Hamdi
46
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
CSIT5600 by M. Hamdi
47
OPS Architecture
Contention Resolutions
•Optical buffering
1
1
2
3
1
2
1
3
4
4
CSIT5600 by M. Hamdi
48
OPS Architecture
Contention Resolutions
•Optical buffering
1
1
2
2
3
3
4
4
CSIT5600 by M. Hamdi
49
OPS Architecture
Contention Resolutions
•Optical buffering
1
1
1
2
2
3
3
4
4
1
CSIT5600 by M. Hamdi
50
OPS: Contention Resolution
• Wavelength multiplexing
– Resolve contention by transmitting on different
wavelengths
– Requires wavelength converters - $$$
CSIT5600 by M. Hamdi
51
OPS Architecture
Contention Resolutions
•Wavelength conversion
1
1
1
1
2
2
CSIT5600 by M. Hamdi
52
OPS Architecture
Contention Resolutions
•Wavelength conversion
1
1
2
2
CSIT5600 by M. Hamdi
53
OPS Architecture
Contention Resolutions
•Wavelength conversion
1
1
1
1
2
2
CSIT5600 by M. Hamdi
54
OPS Architecture
Contention Resolutions
•Wavelength conversion
1
1
2
2
CSIT5600 by M. Hamdi
55
OPS Architecture
Contention Resolutions
•Wavelength conversion
1
1
1
1
2
2
CSIT5600 by M. Hamdi
56
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
CSIT5600 by M. Hamdi
57
Electronic Switches
Using Optical Crossbars
CSIT5600 by M. Hamdi
58
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
CSIT5600 by M. Hamdi
59
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?
CSIT5600 by M. Hamdi
60
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
CSIT5600 by M. Hamdi
62
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
CSIT5600 by M. Hamdi
63
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)
CSIT5600 by M. Hamdi
64
Scheduling Under Reconfiguration
Overhead
• Traditional slot-by-slot approach
Scheduler
Schedule Reconfigure Transfer
Time Line
• Low bandwidth usage
CSIT5600 by M. Hamdi
65
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
CSIT5600 by M. Hamdi
66
Scheduling Under Reconfiguration
Overhead
• Reduce the scheduling rate
– Bandwidth Usage = Transfer/(Reconfigure+Transfer)
Constant
• Approaches
– Batch scheduling: TSA-based
– Single scheduling: Schedule + Hold
CSIT5600 by M. Hamdi
67
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
CSIT5600 by M. Hamdi
68
Routing and
Wavelength
Assignment
CSIT5600 by M. Hamdi
69
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
CSIT5600 by M. Hamdi
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
CSIT5600 by M. Hamdi
71
No Wavelength Converters
WSXC
Access Fiber
Wavelength 1
POP
POP
Wavelength 2
Wavelength 3
CSIT5600 by M. Hamdi
72
With Wavelength Converters
WIXC
Wavelength 1
Access Fiber
POP
POP
Wavelength 2
Wavelength 3
CSIT5600 by M. Hamdi
73
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
CSIT5600 by M. Hamdi
74
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
CSIT5600 by M. Hamdi
75
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
CSIT5600 by M. Hamdi
76
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/
CSIT5600 by M. Hamdi
77
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
CSIT5600 by M. Hamdi
78