Discrete Event Simulation of a large OBS
Network
by
Stein Gjessing
Simula Research Lab,
Dept. of Informatics,
University of Oslo
IEEE SMC 2005 11th Oct 2005
Arne Maus
Dept. of Informatics
University of Oslo,
1
Outline
ƒ Simulation of the COST 239 network between 11
European cities
ƒ Motivation
– Optical fiber networks and OBS - Optical Burst Switching
– A realistic simulation model
– Self similar traffic
ƒ Results
– Burst drop rate (and throughput) vs. load
– Congestion in the network
ƒ Link utilization and burst loss
– Transferring an extra stream of small bursts for ‘free’
– Throughput and drop rate as a function of number of channels
ƒ Conclusions and further work
IEEE SMC 2005 11th Oct 2005
2
The COST 239 network between 11 European cities
0, Copenhagen
3.5
9
5
5.5
1, London
2, Amsterdam
4
1.7
4, Brussels
3
3, Berlin
4.9
2.9
2.5
6.6
5, Luxemburg
6, Prague
1.5
3.7
5.2
5.1
2.3
2
2.4
2.8
4.5
9, Vienna
4.1
5.3
8, Zurich
7, Paris
6
5.4
2.9
5.6
10, Milano
Latency between routers in milliseconds
IEEE SMC 2005 11th Oct 2005
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Optical fiber networksand OBS - Optical Burst
Switching
data (burst ) app. 500 µs.
ƒ
Optical switching & conversion
initial void 200 µs
control
← time
– Each switch can simultaneously transmit data on n different
wavelengths/channels
– An incoming data stream can be switched to any outgoing connection by moving
tiny mirrors and be converted to any free channel on that connection without
being converted to electrical representation of data (wavelength conversion)
– The setup of any switching and conversion has to be done in the electrical
domain
ƒ
Electrical conversion of control packets
– For each data packet (a burst), a control packet is sent into the networkahead of
the burst to reserve its way through the network.
– A separate optical channel is reserved for such short control packets
– These control packets are converted to electrical in every switch, and this delays
the propagation of the control packet (so that it might be overtaken by the burst)
– An updated control packet is sent out (converted to optical) to the next switch for
the burst.
ƒ
A burst is dropped if:
– The control packet is overtaken by the burst
– No free channel is found out of the switch (fiber delay lines, FDLs, might be used)
IEEE SMC 2005 11th Oct 2005
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Optical Burst Switching
data (burst ) app. 500µs
ƒ Optical burst switching
CPT(void): 200 µs
control
← time
– Collects many (10-100) IP-packets in one ‘big’ burst packet
– The initial size of the void between the control packet and the burst,
the Control Packet lead Time, CPT, is typically 200µs
– The control packet delay in a switch is typically 10µs
i.e. it is overtaken by the burst after 20 switches.
– JET scheduling: The resources are scheduled just when needed
ƒ OBS is analternative to
– Optical Line switching –
ƒ fast, but too low utilization
– Optical IP packet switching –
ƒ Too slow by optical/electrical and electrical/optical conversion of data in all
switches, also moderate utilization because of ‘large’ optical switching set up
time
IEEE SMC 2005 11th Oct 2005
5
0, Copenhagen
The simulation model and parameters
ƒ Written in J-sim
ƒ Routing by fixed routing tables
– shortest path routing
ƒ Each node is:
– A switch for through traffic
– An ingress (receiving) node for bursts
– An egress (sending) node of bursts
3.5
9
5
1, London
2,
Amsterda
m
4
3
4, Brussels 1.72.9
1.5
3.7
2
5, Luxemburg
5.5
3, Berlin
4.9
2.5
6, Prague
5.1
2.3
6.6
5.2
2.4
2.8
4.5
9, Vienna
4.1
5.3
8, Zurich
7, Paris
5.4
6
2.9
5.6
10, Milano
ƒ Each connection is bidirectional
ƒ The traffic generated is all-send-to-all, and self similar (next slide)
ƒ The ingress load factor:
– The relation between the on and off periods of the IP-packet generating
sources (varied: 0.1- 0.5)
– Same load factor for all egress (sending) nodes in one simulation
ƒ Each fiber has
– n channels, each with 1 Gbit/sec
IEEE SMC 2005 11th Oct 2005
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The traffic generator and & self similar traffic
IP packets generated by N different sources (N = 5 to 35 times M), each with a Pareto
distribution that gives a self similar traffic (‘same’ variance on large and small scales).
M different OBS buffers
(M is the number of
different destinations)
Sent into the OBS
network when full or
when a timer expires
IEEE SMC 2005 11th Oct 2005
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Congestion in the network– burst being lost at
moderate ingress load factors. Why ?
0.12
Burst drop rate
0.1
0.08
0.06
0.04
0.02
0
0.0
0.1
0.2
0.3
0.4
0.5
Ingress load
IEEE SMC 2005 11th Oct 2005
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Offered load as % of capacity 5-6
Congestion on link 5-6 : Luxemburg - Prague
160
140
120
100
80
60
40
20
0
0.0
0.1
0.2
0.3
0.4
0.5
Ingress load
IEEE SMC 2005 11th Oct 2005
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Burst drop rate and link utilization
Burst drop rate on link 5-6
1
0.9
0.8
0.7
Link util.
0.6
Burst loss
0.5
0.4
0.3
0.2
0.1
0
0.00
0.10
0.20
0.30
0.40
0.50
Ingress load
IEEE SMC 2005 11th Oct 2005
Note: There is still unused capacity on the link when the
drop rate increases to unacceptable levels
10
Horizon scheduling
upcoming Horizon void
Burst data app. 500µsec.
SAT(void): 200 µsec control
Horizon void
← time
No burst can be scheduled in this void (too small)
ƒ Any free (outgoing) channel has an infinite void (the
Horizon)
ƒ If one burst is scheduled on a channel, no other burst can
be scheduled on this before most of the burst has been
transmitted (because 500 > 200)
ƒ The SAT void ( ≤ 200μs) can not be used for transmitting
bursts
ƒ Any packet sum length ≤ 200μs scheduled in the SAT or
Horizon voids can not get in the way of scheduling and
sending a burst
Small packet,
sum length < 200 μs :
IEEE SMC 2005 11th Oct 2005
data < 150 µs. SAT = 50 µs control
← time
11
Transferring an extra stream of small burst
ƒ
ƒ
ƒ
ƒ
In a congested network with a bottleneck as 5-6, one would usually
introduce extra capacity in the net
However, we wanted to study a congested network
We introduced a single data stream of small packets (2000 bytes)
going London to Vienna over the 5-6 link: Luxembourg-Prague
Four classes of priority:
1.
2.
3.
4.
Low: CPT = 50 µs
Low+FDL – but same CPT as Low, + using Fiber Delay Lines
Same: CPT = 200 µs (same CPT as regular bursts)
QoS : CPT = 200 µs + 500 µs = transmission time for regular bursts
1 and 2 will never hinder any regular burst because their total length is
less than the SAT void,
3 might conflict with bursts scheduling
4 will always have priority over any ordinary burst
(hence they might stop ‘many’ bursts)
1 and 2 might transmit extra data over a congested connection for ‘free‘
(can not hinder a single regular burst)
IEEE SMC 2005 11th Oct 2005
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Total traffic on link 5-6 when sending additional
small packets, as a function of the ingress load
1.12
Traffic rate (Base case = 1)
Low -FDL
1.10
Low
1.08
Same
QoS
1.06
1.04
1.02
1.00
0.98
0.96
0.10
0.15
0.20
0.25
0.30
0.35
Ingress load
IEEE SMC 2005 11th Oct 2005
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The drop rate on link 5-6 for the small additional data
packets
0.45
Packet drop rate
0.40
Low
0.35
Same
0.30
Low FDL
0.25
QoS
0.20
0.15
0.10
0.05
0.00
0.0
IEEE SMC 2005 11th Oct 2005
0.1
0.2
Ingress load
0.3
0.4
14
Varying the number of channels per connection:
Network throughput as a function of c, the number of
channels and the ingress load
Traffic throughput rate
1.00
0.95
c35
0.90
c20
c15
0.85
c10
c5
0.80
0.00
0.10
0.20
0.30
0.40
0.50
0.60
Ingress load
Note: Each channel operates at 1GHz and the generated traffic is proportional to c
IEEE SMC 2005 11th Oct 2005
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Conclusions and further work
ƒ Our results confirm many other investigations in OBS
ƒ Demonstrated a new way of transferring an additional
low priority data stream over a highly congested OBS
switched network
ƒ Made a realistic simulator for any large OBS network
– Self-similar instead of a neg-exp traffic arrival pattern
– Mixed size bursts (generated by the timeout)
ƒ Further studies:
– The optimal burst size
– Instead of throwing bursts that can’t be scheduled further on in a
switch, convert to electrical domain and store for later
transmission (might be better than retransmission from egress
node or from traffic source)
IEEE SMC 2005 11th Oct 2005
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