node architectures for optical packet and burst...
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Node architectures foroptical packet and burst
switching
Chris Develder,Jan Cheyns, Erik Van Breusegem, Elise Baert,
Ann Ackaert, Mario Pickavet, Piet Demeester
Dept. of Information Technology (INTEC)Ghent University, Belgium
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 2
Outline
• Introduction– OPS and OBS concepts– packet & header format
• Node architecture– functionality– optics and/or electronics
• Switch matrix– alternatives– scalability
• Contention resolution– problem & solution– buffer architectures
Introduction
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 4
Optical switches
• Optical switching:• direct light from an
input port to an output port• possibly wavelength conversion
• circuit-switching:• continuous bit-stream• pre-established light-paths• set-up: “manually” or dynamic
• packet/burst switching• chunks of bits, encapsulated in packets• packet header determines forwarding• e.g. label switching, GMPLS based f f
cc ba
d
b
e
c
f
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 5
Packet format
• fixed/variable duration:– pro variable = no fragmentation/
reassembly, no padding, less header overhead
– contra = long packets can block many short ones
• slotted/unslotted operation:– pro slotted = easier packet scheduling
(synchronous switching)– contra = cost of synchronisation
components
• OPS = fixed, slotted packets• OBS = variable, unslotted packets
λ1
λ2
unslotted, variable length
λ1
λ2
slotted, variable length
λ1
λ2
slotted, fixed length
padding
single packet
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 6
Header format
–
– out of band: orthogonal channel (e.g. DPSK)
–
– out of band: dedicated wavelength; also multi-wavelength headers have been proposed (see e.g. PS.TuC1)
• position of header:– in band: header and payload are sent sequentially,
separated in time
λ1
λ2
λ3
λ4
λ1
λ2
λ3
λ4
λ1
λ2
λ3
λ4
phase
intensity
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 7
Operation of OPS
• fixed-length packets, slotted operation• header accompanies payload
• contains necessary information to make forwarding decision
• each timeslot:• inspect packets at input ports• decide which packets can be forwarded without collisions
• switch is “memory-less”• no knowledge of packets scheduled in past is necessary
OPSnode
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 8
Operation of OBS (1)
• variable packet lengths, unslotted operation• header is sent Toffset before payload
• contains necessary information to make forwarding decision• functions as one-way reservation (allows timely config. of switch fabric)• offset decreases by header processing time per hop
• on arrival of header:• decide whether burst can be forwarded without collisions• make necessary resource reservations if burst is accepted
• switch needs “memory”:• keep track of reservations made in past
OBSnode
Toffset - δ
Toffset
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 9
Operation of OBS (2)
• Note: WR-OBS = wavelength-routed OBS– two-way reservation
• “header” is sent from source to destination within OBS network• if all goes well, acknowledgement is sent back to source
– this is more like wavelength switching at very short timescales – proposed by P. Bayvel et al. (Univ. College London, UK)
Node architectures
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 11
Functionality of an OPS/OBS node
• input interface:• header extraction (straightforward if out-of-band)• synchronisation: detect beginning of packet/burst• in OPS: align packets
• switching matrix (see further)• output interface
• e.g. regeneration of optical signal; header re-writing…
synchr.control
switchcontrol
headerrewriting
inputinterface
switching matrix
outputinterface
payload
header
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 12
Combine the best ofelectronics and Optics
• optical packet switching “today”:• header is processed electronically• payload is switched optically
⇒ optics for capacity & switching,electronics for routing & forwarding
• note:• all-optical header processing is “under study” (e.g. PS2001, PThD5)
synchr.control
switchcontrol
headerrewriting
inputinterface
switching matrix
outputinterface
optical processing
electronicprocessing
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 13
Oh-oh-oh*
: optical : electrical• O-O-O:
– optical input interface, optical switch fabric, optical output interface
– payload is switched transparently, without leaving optical domain
– pro = bitrate-transparent– con = still emerging technologies, BER can not be monitored
• O-E-O:– optical inputs, converted to el. for switching, back to optics at
outputs– “opaque”: no more all-optical; but straightforward grooming– pro = well established techno, 3R regen. “for free”– con = no bit-rate transparency, scalability ~ Moore’s law
• OEO-O-OEO– (some) inputs and outputs: electronic 3R regen.– pro = scalability of optical switch fabric with 3R regen.– con = bit more complex than O-O-O
*: G.Bennet, at www.lightreading.com
Switch fabric
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 15
Overview of dominant architectures
• dominant approaches to optical switch fabrics:• MEMS (=micro-electro-mechanical systems)• broadcast-and-select (e.g. SOA-based)• AWG and tuneable lasers
• MEMS:– principle:
– main components:• tiny mirrors (2D pop-up, or 3D tilting)
– characteristics• low loss, good scalability (=high port counts)• but… too slow for packet switching (ns timescale not feasible)
© Lucent, e.g. OFC2002
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 16
Broadcast-and-select switch
• principle: (e.g. David, http://david.com.dtu.dk; ECOC’01)
• main components:– splitters– selectors
• characteristics:– split losses: calls for regeneration– inherent multicast capability
l1lm
l1lm
. . .
. . .
1:(m.n) n:m
lj
lk
lj
lk
lm
l1
lm
l1
complete fibre is broadcast to all output ports
first array of SOAs selects 1 input fibre
second array of SOAs is used to select 1 wavelength
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 17
AWG based switch
• principle: (e.g. Stolas, http://www.ist-stolas.org)
• main components:– tuneable wavelength converters– AWG
• characteristics:– passive component, no split losses– multicast quite complex
TWC
TWC
TWC
TWC
TWC
TWC
TWC
TWCAWGAWG
1 2 3 4 5 6 7 8
2345678
1 λ1 λ2 λ3 λ4 λ5 λ6 λ7 λ8
λ1
λ1
λ1
λ1
λ1
λ1
λ1
λ2
λ2
λ2
λ2
λ2
λ2
λ2
λ3
λ3
λ3
λ3
λ3
λ4
λ4
λ4
λ4
λ4
λ4
λ4
λ3
λ3
λ5
λ5
λ5
λ5
λ5
λ5
λ5
λ6
λ6
λ6
λ6
λ6
λ6
λ6
λ7
λ7
λ7
λ7
λ7
λ7
λ7
λ8
λ8
λ8
λ8
λ8
λ8
λ8
in-port
out-p
ort
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 18
Scalability
• switch dimensions limited by• B&S: split losses• AWG: tuneability range of TWCs
• Possible solution:• multi-stage switches, e.g. Clos-networks
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 19
A three-stage Clos-network (1)
• slotted OPS• re-arrangeable non-blocking• second-stage switches: k ≥ n
• unslotted OBS• fully non-blocking• second-stage switches: k ≥ 2n-1
N/nswitches
kswitches
...
...
...
......
...
...
n
n
n
n
3-stage Clos switch
Noutput ports
A
B
N/nswitches
... ...
...
...
n n
n
n
N in
put p
orts
Noutput ports
k x nN/nx
N/n
n x k
k x nN/nx
N/nn x k
n x k
k x nN/nx
N/n
⇒slotted OPS needs only half as much second stage switches
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 20
A three-stage Clos-network (2)
• note: if wavelength conversion is allowed, third switching stage can be eliminated
• e.g. slotted OPS:• F fibres, W wavelengths per fibre• make 1st and 3rd stage per input/output fibre
......
...WxW
WxW
WxW
FxF
FxF
FxF
Fswitches
Wswitches
WFconvertors
WF input ports
WF
output ports
Contention resolution
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 22
Problem and possible solutions
• Problem:• two or more packets contend for same resource: destined for same
outgoing port at the same time
• Solutions:• deflection routing• wavelength conversion• buffering: optical buffer = Fibre Delay Lines (FDLs)
contention
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 23
What solution to choose?figures © Yao et al., Opticomm’00
netw
ork
thro
ughp
utpa
cket
loss
rate
load (packet arrival rate, pkt/s)
load (packet arrival rate, pkt/s)
• Deflection:• packets are “stored” in network:• increases load, increases delay• only works for low loads
• Wavelength conversion:• no packet storage• allows high network throughput, no
increased delay
• Buffering:• local packet storage at nodes• small delay penalty
⇒ Use combination of wavelength conversion and buffers
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 24
Buffer architectures
feed-forward vs feed-back• feed-forward vs feed-back
• feed-forward: input or output buffering
• feed-back: shared, recirculatingFDLs
• single stage vs multiple stage• multiple stages separated by
switching elements• e.g.: each stage different delay
resolution (“units”, “tens”, “hundreds”…)
• choice of FDL lengths• multiple FDLs: multiples of “unit”,
resolution D• uniform/non-uniform (D,2D,3D or
something else)
single vs multiple stages
D-1
10
...
D-1
10
D-1
10
... ...
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 25
OPS/OBS: fixed vs increasing FDLs
1
B
…1
1
1.E-07
1.E-05
1.E-03
1.E-01
0 8 16 24 32 40 48 56 64nr. buffer ports (B)
ParetoOnOff, incr ParetoOnOff, fix
GeoOnoff, incr GeoOnoff, fix
Poisson, incr Poisson, fix
see COIN.TuD1 for details
1
B
…
…
1
B
• sample results for fixed-length, slotted OPS
• Increasing FDL lengths give far lower PLRs (order of magnitude or more)
• “penalty”: reordering of packets, higher delays
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 26
OBS: choice of granularity
sample results for slotted switch with output buffering, single wavelength per fibre, geometric distr. packet lengths, bernouilliarrivals, 20 FDL lengths
• Optimal granularity D:⇒Non-trivial choice!
• Tradeoff between resolution and buffering capacity
• small D: small gaps, but limited buffer depth
• large D: large buffer depth, but large gaps between packets
• Function of• load• traffic profile (packet size
distribution, burstiness…)
D-1
10
...
pack
et lo
ss p
roba
bilit
y
granularity D
1E+0
1E-2
1E-4
1E-6
0 50 100 150
ρ=0.6
ρ=0.5
ρ=0.4
optimal D depends on load
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 27
OBS: void filling
• Void creation in case of variable packet-lengths (OBS):• FDL buffer offers discrete set of delays
(in contrast to e.g. electronic RAM)• gaps between successive packets: “voids”
⇒Need for intelligent buffer scheduling if we want to improve link throughput (i.e. void filling)
OBS node+ FDL buffer
d D-d
example: we need delay d, but FDL only offers D ⇒ gap of D-d is created;void filling will attempt to insert packet in this gap
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 28
Contention resolution
• wavelength conversion greatly reduces need for buffering
• feed-back architecture allows sharing of FDL resources among all output ports
• when using different delay lengths, this calls for more intelligent buffer scheduling; for variable-length packets (OBS) the issue of void creation arises
• OBS: choice of delay lengths, i.e. the FDL granularity, is non-trivial issue
Conclusions
PS 2002 Node architectures for optical packet and burst switching - C. Develder, et al. 30
Summary
• reviewed OPS/OBS concepts– OBS as possible 1st step– OPS: fully exploit fast switching technologies
• switch architectures:– MEMS: too slow for OPS– broadcast & select: allows multicast, but splitting losses– AWG: passive core component, but no multicast– multi-stage architecture to increase scalability
(OPS fewer switching elements than OBS)
• buffering:– push buffers to network edges– use FDLs to lower PLR in core– non-trivial choice of FDL granularity for OBS– quite complex scheduling for OBS
That’s all, folks!
… thanks for your attention
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