CoS-oriented Wavelength Assignment Algorithm in Burst Switching Optical Networks
Tomohiro HASHIGUCHI1, Xi WANG2, Hiroyuki MORIKAWA12, and Tomonori AOYAMA2
1School of Frontier Sciences, 2School of Engineering,
The University of Tokyo, Japan
7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656, JAPAN
Tel: +81-3-5841-6710
Fax: +81-3-5841-6776
E-mail: {gucchy, xi, mori, aoyama}@mlab.t.u-tokyo.ac.jp
Abstract
We present a CoS-oriented wavelength assignment
algorithm for burst switching optical networks with multiple
service classes. Simulation results show that CoS is realized
in terms of burst blocking probability and throughput.
1. Introduction
Optical networks employing Optical Burst Switching
(OBS) [1] as their switching paradigm are coming up as
suitable network architecture for the future Optical Internet
[2]. One of the problems of OBS is that burst blocking is
inevitable because of immaturity of photonic devices such as
optical memory [3] and all-optical wavelength converters [4].
Among various contention resolution techniques and
algorithms, a PWA (Priority-based Wavelength Assignment)
[5] algorithm is suitable for Burst Optical Networks and easy
to realize. By implementing PWA, burst blocking probability
is improved as compared with random wavelength
assignment algorithm.
However, there are also other requirements to WDM layer
besides reducing burst blocking probability. Mission- critical
and real-time applications require a high QoS (low delay,
jitter and blocking probability). Accordingly, it is necessary
to differentiate high priority services from others by
implementing CoS (Class of Service). Although there is a
research about CoS for burst switching optical networks [6],
this scheme has problems about end-to-end transmission
delay and fairness between multiple nodes. This is because
the scheme proposed in [6] uses “extra offset time” for
differentiating services in time domain. In this paper, we
present a CoS-oriented wavelength assignment (CWA)
algorithm using PWA designed for burst switching optical
networks with multiple service classes.
2. CWA Algorithm
2.1
Principle
In CWA algorithm, each sender node keeps a wavelength
priority database for every destination node. By learning
from statistical data of transmission results, each node
increases or reduces the priority of the wavelength. The
wavelength re-ranking scheme of PWA [5] is applied to the
learning process.
In order to realize CoS for each burst, the number of
assignable wavelengths is differentiated according to the
priority of each service class in wavelength assignment
procedure of sender nodes. We define the number of
assignable wavelengths as n(i), where i is the service class of
a burst. The higher the priority of the service class is, the
larger n(i) is.
Fig. 1 shows the wavelength priority database for a
sender-destination node pair and an example of wavelength
assignment procedure of CWA in networks with 2 service
classes: class 1 (high priority) and class 0 (low priority). The
height of each bar shows the priority of wavelength wk (k = 1,
2, … , n). Fig. 1 (a) shows the case of n(0)=1 and Fig.1 (b)
shows the case of n(1)=4. n(0)=1 means that the number of
assignable wavelengths for class 0 traffic is 1. If w1 which
has the highest priority among all wavelengths is idle, the
burst is sent on wavelength w1, otherwise, the burst is
blocked. On the other hand, n(1)=4 means that the number
of assignable wavelengths for class 1 traffic is 4. That is,
when w1 is busy, the sender node investigates the state of w2
with secondary highest priority. Then if w2 is idle, the burst is
sent on w2. This procedure continues until an idle
wavelength is detected from w1 to w4. The burst is blocked
only when w1-w4 are all in busy state. In other words, a burst
of class 1 can be sent out if at least one of the wavelengths of
w1-w4 is available. Accordingly, the blocking probability for
the case of Fig. 1 (b) is improved compared with that for the
case of (a).
busy
block (in case of (a))
forward
busy
idle
busy
busy
block (in case of (b))
priority
(a)
assignable (b)
wavelength
w1
w2
w3
w4
w5
…..
…..
wn
wavelength database for a sender, a destination node pair
Fig. 1 CoS by differentiating the number of assignable wavelengths: n(i).
(a) n(0) = 1, (b) n(1) = 4
In CWA algorithm, wavelength assignment is performed
in each node in a distributed manner. CoS is realized by
differentiating the number of assignable wavelengths. In
other words, we realize CoS in wavelength domain.
Accordingly, problems come from the “extra offset time”
can be solved. The advantages of CWA algorithm are:
● CoS can be realized not by offset time but by
wavelength assignment procedure. There is no influence
of burst hop counts to the performance of CoS scheme,
which is one of the problems in the scheme of [6].
● End-to-end transmission delay is not increased because
CoS is realized not in time domain but in wavelength
domain.
2.2
Wavelength assignment procedure
Wavelength assignment procedure of CWA is as follows.
Each sender node keeps a wavelength priority database for
every destination node and the number of assignable
wavelengths, n(i), for each service class. Each burst has its
3. Simulations
Assumptions of our simulations are as follows. The
network topology is a 4 × 4 mesh network. There is no
wavelength conversion and no optical buffer in burst
switches. Here in order to simplify the model, retransmission
for bursts blocked in intermediate nodes or sender nodes is
not performed. The number of wavelengths on each link is
64. The number of classes is 2: class 0 (low priority) and
class 1 (high priority). n(0)=1, n(1)=4. The rates of burst
generation for each service class are equal. The performance
metrics are the burst blocking probability and throughput. In
addition, the performance of CWA is compared with that of a
network with all bursts as class 0. We call this a “classless”
network.
Fig. 2 (a) shows the relationship between traffic load and
burst blocking probability for class 0, class 1 and classless.
As traffic load increases, blocking probability of both classes
and classless increases. However, the blocking probability of
class 1 is improved compared with those of class 0 and
classless. In addition, we found that blocking probability of
classless is slightly smaller than that of class 0 although both
of them appears to be the same in Fig. 2 (a). This indicates
bursts are classified into 2 classes: one (class 1) with higher
performance and the other (class 0) with lower performance
than a classless network. Fig. 2 (b) shows the relationship
between traffic load and throughput for class 0, class 1 and
classless. Similarly to Fig. 2 (a), the throughput of class 1 is
improved compared with that of class 0. Throughput of class
1 is improved as compared with classless, and throughput of
class 0 is lower than that of classless.
Blocking Probability
Probability
1E+00
1E-01
class 0
1E-02
class 1
classless
1E-03
0
0.2
0.4
0.6
Traffic Load
(a) Blocking Probability
1E+00
Throughput
service class, i. The detailed procedure can be classified into
wavelength assignment and wavelength re-ranking.
(a) Wavelength Assignment.
When a sender sends a burst to a certain destination, the
sender first checks the service class of the burst to determine
the number of assignable wavelengths for the burst, n(i).
Then the sender tries to assign the wavelength that has the
highest priority. If the wavelength is idle, the burst is sent out
on this wavelength. If the wavelength is busy, the sender
searches for an idle wavelength on the output link
sequentially for burst transmission based on the wavelength
priority database until either the number of search times
reaches n(i) or an idle wavelength is detected. If n(i)
wavelengths from the top of wavelength priority database
are all in use, the burst is blocked.
(b) Wavelength Re-ranking.
The scheme of wavelength re-ranking is based on that of
PWA. In burst switching optical networks, each sender
knows whether a burst sent from itself is received by the
destination node or not. If a burst is received by destination
node, an Ack packet is sent back to the sender. Conversely, a
Nack packet is sent back to the sender if a burst is blocked at
an intermediate node. When a burst sent on wavelength w is
received by the destination node, the priority of w is
increased. On the other hand, if the burst is dropped in an
intermediate node, the priority of the burst is decreased. The
difference from PWA is that the process of decreasing
priority is performed only when a burst is blocked at
intermediate nodes. This is because the blockings in sender
nodes do not affect the wavelength segregation for overall
networks.
1E-01
class 0
1E-02
class 1
classless
1E-03
0
0.2
0.4
0.6
Traffic Load
(b) Throughput
Fig. 2 Blocking probability & throughput vs. traffic load
(the number of wavelengths = 64).
4. Conclusion
In this paper, we present a CWA algorithm for burst
switching optical networks. Simulation results show that
CWA is a viable approach for providing CoS by utilizing
wavelength assignment scheme.
References
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