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Photonic slot routing
By Alisa Javadi
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Photonic slot routing
• PSR
– Wavelength-routing AONs interconnect pairs of source &
destination nodes via all-optical point-to-point lightpaths
– Due to cost & efficiency reasons, it is impossible to
interconnect each pair of nodes by a dedicated lightpath
– Possible solutions
• Use of multiple lightpaths => loss of transparency
• Electronic traffic grooming at each source node (grooming:
many small=>large nits)
– Alternative solution: Photonic slot routing (PSR)
• Avoids loss of transparency & need for electronic traffic
grooming
• Intermediate nodes switch entire slots, each carrying multiple
packets on distinct wavelength channels, all-optically &
individually without OEO conversion
• Allows traffic aggregation to be done optically without electronic 2
traffic grooming
Photonic slot routing
• Photonic slot
– In PSR networks, time is divided into fixed-size slots
– Each slot spans all W wavelengths => photonic slot
– Photonic slot
• May contain a single data packet on each wavelength
• All packets in a given photonic slot are required to be destined
for the same node
• Each photonic slot may be destined for a different node
• Routed as a single entity
– Advantages
• No wavelength-sensitive components needed at intermediate
nodes => lower costs & avoidance of interchannel switching
crosstalk
• Reduced complexity of switching operation & electronic control
by factor W (number of wavelengths)
• Cost-effective realization by using simple optical components
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Photonic slot routing
• PSR functions
– (a) photonic slot switching
• Contention resolution
– (b) photonic slot copying
• Multicasting
– (c) photonic slot merging
Photonic slot routing
• Synchronization
– To achieve PSR functions, photonic slots must arrive
synchronized at PSR nodes
– Dispersion compensation is a must
– Synchronization approaches
• Optical synchronizer (local)
– Use of fiber delay lines (FDLs) at input ports of PSR nodes
to delay arriving photonic slots
• Network-wide photonic slot synchronization
– Briefly having a signal to sync the slot all over network
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Photonic slot routing
• Access control
– Each photonic slot has
its own destination
address
– Access control
• Source node is allowed
to send packet on any
free wavelength of
arriving photonic slot if
its destination address
matches the packet’s
destination address
Photonic slot routing
• Sorting access protocol
– Used by each source node to ensure collision-free wavelength channel access & organize packet transmission
– Operation
• Destination address of first packet transmitted in a given photonic
slot determines its destination address
• Each source node stores packets in separate transmission buffers
according to their destination address
• Packet selection at source node
– If arriving photonic slot is assigned a destination
» Head-of-line packet of buffer associated with destination
is selected for transmission, provided not all wavelengths
are used
– If arriving photonic slot is not assigned a destination
» Oldest packet among head-of-line packets is selected for
transmission
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Photonic slot routing
• PSR node
– Coupler taps part of incoming signal off input fiber link
– Slot detector finds out
• Whether arriving photonic slot is destined for local node
• Which wavelengths in arriving photonic slot carry packets
– Based on this information
• Switch is set by electronic control
• Locally generated packet is inserted using another coupler
Photonic slot routing
• PSR bridge
– Interconnects different PSR network segments
– Example: 2x2 PSR bridge
• Equipped with photonic 2x2 cross-bar switch
• Two pairs of control receiver RXC and control transmitter TXC
• RXC inform bridge control unit of destination of photonic slots arriving
on each network segment
• Electronic control sets switch in bar or cross state
Photonic slot routing
• PSR bridge: Contention
– In PSR networks, photonic slots may experience
contention at intermediate PSR bridges
– Contention
• Occurs when more than one photonic slot simultaneously arriving at a
given PSR bridge need to be switched to the same output port
– Possible solutions
• Only one photonic slot is switched while remaining ones are dropped
by PSR bridge & retransmitted by source PSR nodes
• Alternatively, PSR bridge resolves contention (e.g. SDL)
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Photonic slot routing
• SDL bridge
– Contention can be mitigated by adding switched delay line
(SDL) to PSR bridge => SDL bridge (as buffer)
Photonic slot routing
• Multiport SDL bridge/PSR node
Photonic slot routing
• Contention resolution
– Schemes to resolve contention in mesh PSR
networks
• Retransmission of dropped photonic slots by source PSR nodes
• Buffering contending photonic slots at SDL bridges
• Deflection routing
– One photonic slot is routed through output fiber link specified
by routing algorithm whereas other contending slots are routed
through any of remaining free output fiber links
– Deflection counter in photonic slots prevents slots from being
deflected too often
– Benefits heavily depend on topology & routing algorithm
– Results for shortest path routing
• Under low load: Buffering & deflection routing achieve similar
throughput-delay performance
• With increasing load: Deflection routing may outperform buffering by
means of load balancing
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Photonic slot routing
• Pros & cons of PSR
– PSR networks have pros & cons
• Pros
– Can be realized using inexpensive wavelength-insensitive
devices & cross-connects based on proven technologies
• Cons
– PSR nodes ready to send packets to destinations other
than that of photonic slot are prevented from using it,
even though most wavelengths may be free
– This inefficiency can be avoided by neglecting the
requirement that all wavelengths need to be destined for
same node
– As a consequence, each individual wavelength can be
accessed independently from each other => individual
wavelength switching (IWS)
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Photonic slot routing
• IWS
– Cost of optical components & switches expected to decrease
significantly in the long run
– With optical technology advances, cost-effective wavelengthsensitive optical packet switches become feasible .
– Resultant wavelength-sensitive IWS switch able to switch optical
fixed-size packets on each individual wavelength
– Results
• Network capacity can be increased significantly by carefully replacing a
relatively small percentage of conventional PSR switches with IWS
switches according to given traffic demands and/or cost constraints
• Thus, IWS enables cautious upgrade & smooth migration paths from
PSR networks to synchronous fixed-size optical packet switching
(OPS) networks
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Photonic slot routing
• Implementation
– To date, PSR networks were experimentally
investigated only to a limited extent
• Wavelength stacking
– Enables PSR nodes to send/receive multiple packets in
each photonic slot using only one tunable transceiver
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Photonic slot routing
• Wavelength stacking
Optical flow switching
By Alisa Javadi
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Optical flow switching
• Electro-optical bottleneck
– Unlike individual wavelength switching (IWS) electronic
IP packet switching networks provide several benefits
• Network-wide synchronization is not required
• Support of variable-size IP packets
• Simpler & more efficient contention resolution by using electronic
random access memory (RAM)
– However, due to steadily growing line rates & amount
of traffic electronic routers may become bottleneck in
high-speed optical networks => electro-optical
bottleneck
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Optical flow switching
• OFS
– One of the main bottlenecks in today’s Internet is (electronic)
routing at IP layer
– Methods to alleviate routing bottleneck
• Switching long-duration flows at lower layers ,routers are
offloaded & electro-optical bottleneck is alleviated
– Concept of lower-layer switching can be extended to
switching large transactions and/or long-duration
flows at optical layer => optical flow switching
(OFS)
– Definition of flow
• Unidirectional sequence of IP packets between given pair of
source & destination IP routers
• Both source & destination IP addresses, possibly together with
additional IP header information such as port numbers and/or
type of service (ToS), used to identify flow
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Optical flow switching
• OFS
– In OFS, a lightpath is established for the transfer of
large data files or long-duration & high-bandwidth
streams
– Forms of OFS
• Use of entire wavelength for a single transaction
• Flows with similar characteristics may be aggregated & switched
together by means of grooming in order to improve lightpath
utilization
– Issues of OFS
• How to recognize start & end of flows
• Size of flow should be in the order of the product of roundtrip propagation delay & line rate of set-up lightpath
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Optical flow switching
• OFS vs. electronic
routing
– In OFS, data is routed
all-optically in order to
bypass & offload routers
– Set-up lightpath eliminates
need for packet buffering
& processing at
intermediate routers
– OFS can be
• End-user initiated
• IP-router initiated
Optical flow switching
• Advantages
– Mitigation of electro-optical bottleneck by optically
bypassing & thus offloading electronic IP routers
– OFS represents highest-grade QoS
• Established lightpath provides dedicated connection not impaired by
presence of other users
• Issues
– Set-up of lightpaths must be carefully determined
since wavelengths are typically a scarce resource
– Without use of wavelength converters, wavelength
continuity constraint further restricts number of
available wavelengths
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Optical flow switching
• Integrated OFS approaches
Dynamic lightpath set-up in OFS networks
involves three steps
• Routing
• Wavelength assignment
• Signaling
– Integrated OFS approaches for end-user
initiated lightpath set-up
• Tell-and-go (TG) reservation
• Reverse reservation (RR)
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Optical flow switching
• Tell-and-go (TG) reservation
– Distributed algorithm with no wavelength conversion based
on link state updates
– Updates processed at each network node to acquire &
maintain global network state
– Given the network state, TG uses combined routing &
wavelength assignment strategy
• K shortest path routing with first-fit wavelength assignment
• Optical flow is dropped if no route with available wavelength
can be found
– Connection set-up achieved using tell-and-go signaling
• One-way reservation
• Control packet precedes optical flow along chosen route in
order to establish lightpath for trailing optical flow
• Control packet & optical flow are terminated if not sufficient
resources available at any intermediate node
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Optical flow switching
• Reverse reservation (RR)
– Unlike TG, RR does not require (periodic or event-driven)
updates to acquire & maintain global network state
– Initiator of optical flow sends information-gathering packets, socalled info-packets, to destination node on K shortest paths
– Info-packets record link state information at each hop
– After receiving all K info-packets, destination node performs
routing & first-fit wavelength assignment
– Connection established via reverse reservation
• Destination node sends reservation control packet along chosen
route in reverse
• Control packet configures intermediate switches & finally informs
initiator about lightpath set-up
• Otherwise, reservation is terminated & all resources held by
reservation are released by sending additional control packets if
control packet does not find sufficient resources
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Optical flow switching
• Implementation
– OFS experimentally investigated in Next Generation
Internet Optical Network for Regional Access using
Multiwavelength Protocols (NGI ONRAMP) testbed
• Bidirectional feeder WDM ring (8 wavelengths in each direction)
connecting 10-20 access nodes (ANs) & backbone network
• ANs serve as gateways to attached distribution networks of variable
topologies, each accommodating 20-100 users
• AN
– Consists of IP router & ROADM(reconfigurable optical add
drop MUX)
– Routes optical wavelength channels & IP packets inside
wavelength channels between feeder ring, IP router, and
distribution network
• Services
– IP service
» Involves electronic routing
– Optical service
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» OFS with all-optical end-to-end connection
Optical flow switching
• NGI ONRAMP
Optical flow switching
• Flow detection
– Flow detection that triggers the dynamic setup of lightpaths is critical in OFS networks
– Example of flow detection
• x/y classifier
– x denotes number of passing packets belonging to a
given flow
– y denotes prespecified period of time
– Depending on whether value of classifier is above or
below predefined threshold, flow is considered active or
inactive, respectively
– Node detects beginning of flow if value exceeds
threshold
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– Node assumes end of flow if value falls below threshold
Thanks
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