Communications and Network, 2013, 5, 444-447
http://dx.doi.org/10.4236/cn.2013.53B2082 Published Online September 2013 (http://www.scirp.org/journal/cn)
Limited Delay Preem ption Based Priority in D i fferen tiated
Optical Burst Switching Network s with S m al l Buffers
Jiuru Yang, Hong’an Ye
Key Lab of Electronics Engineering, College of Heilongjiang Province, Heilongjiang University, Harbin, China
Received June 2013
ABSTRACT
The composite scheme based on preemption and small buffers is an efficient method for contention resolution. To sup-
port services differentiation, it is the first time that the analytical model of delay preemption based priority is built. Fu r-
ther, in order to guarantee the low-loss requirement for high priority bursts, an improved scheme is proposed and inves-
tigated by limiting the b uffered right of low priority bursts within the specific traffic states. The simulation results show
that, without the deterioration of blocking performance, there is more than 40% reduction on burst loss being achieved
under the condition ρ = 1.0 fo r high priority bursts.
Keywords: Optical Burst Switching; Preemption; Small Buffers; Blocking
1. Introduction
Since the concept of optical burst switching (OBS) is
proposed by Qiao in 1999, it has been regarded as one of
promising solutions for future optical Internet [1]. In OBS,
due to one-way reservation behavior, the contention among
bursts is inevitable, and much works have been dedicated
to contention resolution by means of tunable wavelength
converters (TWCs) and optical buffers, i.e., fiber delay
lines (FDLs) [2-4]. However, it is determinate that the
abundant usage of TWCs and FDLs at core nodes will
bring the problems of not only the huge complexity in
configuration, but also the large energy consumption be-
cause of the astonishing traffic increase recently [5].
Under the wave of green networks, some practical and
applic able sch emes bas ed small buffers (the leng th is about
tens of packets) are concerned. In [6,7], the cost-effi-
ciency based feedback buffer configuration is studied.
Centering real-time and TCP traffic, the effects of size of
small buffers on data loss are analyzed by Rouskas [8].
And to avoid burst-discarding, preemption is introduced
through pre-reserving or post-reserving channel for the
collided and buffered bursts [9,10]. Alternatively, [11]
mitigates the performance degradation resulting from small
buffers by smoothing traffic burstiness at edge nodes.
Also, combine pre-reservation and segmentation, the au-
thors proposed delay preemption to further reduce the
packets loss probability of OBS [12]. In this paper, to
support services differentiation, it is the first time that the
analytical model on delay preemption based pr iority (DPP)
is built. Moreover, on the basis of DPP, an improved
scheme called Limited Delay Preemption Based Priority
(LDPP) is proposed to guarantee the quantity of service
(QoS) requirements of both mission-critical and delay-
sensitive services by constraining the right of low priori-
ty bursts to enter FDLs within the specific traffic states.
2. Analytical Model of Delay Preemption
Based Priority
Assuming the mean arrival rate and service time of bursts
are λ and 1/μ, respectively. And the incoming packets
with the common destinations are aggregated into a burst
at edge nodes. For simplicity, the aggregated threshold in
length Lb for all bursts is the same and fixed. In delay
preemption with single-class, when more than two bursts
transmit and switch in the same wavelength channel si-
multaneously, a channel contention occurs. Given the
FDLs is idle, the contended burst can be buffered and
preempt the channel in advance [12].
We then define ρ = λ/Wμ is offered load and PD de-
notes the total burst loss probability, where W is the
number of wavelength in each fiber link. Thus, the prob-
ability of channel usage at core nodes can be represented
as
(1 )
D
uP ρ = −
(1)
Likewise, in two-class system, we define both mis-
sion-critical and delay-sensitive s ervices with high pr ior-
ity, denote d by Bhig, and other bursts are with low priority,

J.-R. YANG, H.-A. YE
445
denoted by Blow. Their offered loads are ρ1 and ρ2 respec-
tively, and ρ = ρ1 + ρ2. Moreover, the probabilities of
channel usage of Bhig and Blow are denoted by u1 and u2,
and u = u1 + u2. Further, define λ1 = c1·λ and λ2 = c2·λ,
where c1 = ρ1/ρ and c2 = ρ2/ρ are the load ratios of Bhig
and Blow in networks. Then, owing to c1 + c2 = 1, the
probabilities of channel usage in two-class system are
written as
11 1
(1 )
D
ucu cP ρ
= =−
and
22 2
(1 )
D
ucu cP ρ = =−
Next, the basics of delay preemption based priority
(DPP) are shown in Figure 1, where b is the FDLs length
and r is blocking length. Apparently, b should be larger
than r to avoid burst congestion in buffer. According to
[13], the expectation of r can be presented as
2
[] 22
b
L
b
b
L
Er L
δ
= +
where
b
L
is the mean of Lb, and
2
b
L
δ
is its variance.
Because Lb is fixed in this paper, namely
bb
LL=
, it
means
2
0
b
L
δ
→
, and E[r] ≈ Lb/2. Besides, based the
result in [8], we design b = Lb. Thus, E[r] = b/2.
Furthermore, in Figure 1(a), we see a chann el conten-
tion occurs between the high priority burst Bhig, i and low
priority burst Blow, j. Under FIFO rule (first in first out),
Blow, j cuts through the channel directly. In this case, if the
FDL is in the idle/free state, the blocked burst Bhig, i can
be buffered and preempt the channel b − r + Lb for itself
in advance. Otherwise, Bhig, i would be simply dropped
and retransmitted. Hence, for high priority, its loss prob-
ability PD, 1 can be written as
Figure 1. Principles of delay preemption based priority, (a)
blocking of high priority burst; (b) blocking of low priority
bursts.
,1 1
1
Pr.{ }
D
FDL
Pcchannel busybuffer busy
c uP
=
= ⋅

(2)
where PFDL = P1, FDL + P2, FDL represents the probability
of buff er usage, P1, FDL and P2, FDL are the probabilities of
buffer usage from Bhig and Blow respectively. Based on
[14], the idle probability within the inter-arrival time t
can be represented as e−λt. Then,
1
1,
1
b
FDL
Pe
λ
−
= −
and
2
2,
1
b
FDL
Pe
λ −
= −
. In other hand, see Figure 1(b), only
the right to be buffered is given to low priority. That
means, though it has been delayed b in FDLs, Blow, j
would be discarded if a burst arrived within the void b-r.
In addition, because of no priority to enter the buffer, an
extra burst loss for Blow can be brought when the buffer is
idle but pre-reserved by high priority bursts. Therefore,
the burst loss probability of low priority can be calcu-
lated as
,2 2
1
21
121 1,2,
Pr.{ }
Pr.{ }
(1 )
[ ()()]
D
FDL FDL
FDL FDL
Pcchannelbusy bufferbusy
channelbusybuffer freec
c uPuPc
uccc PP
=
+⋅
=⋅+− ⋅
=⋅+ −+


(3)
where (1 − PFDL) is the free probability of buffer. Be-
cause PD = PD, 1 + PD, 2 and c1 + c2 = 1, the total loss
probability in DPP will be
11211,1,
12 1,2,
[ ()()]
[ ()]
DFDLFDL FDL
FDL FDL
Pc uPucccPP
uc cPP
= ⋅+⋅+−+
=⋅+ +
(4)
Combine Equations (1) and (4), we ge t
2 1,2,1
2 1,2,1
[() ]
[() ]1
FDL FDL
DFDL FDL
cP Pc
PcP Pc
ρ
ρ
++
=+ ++
(5)
Then, with the various load ratios, the blocking per-
formance of DPP is investigated from a typical simula-
tion scenario used in [12], and b is designed as 80 μs
specially. In Figure 2, there are two apparent natures on
data loss in DPP. First, compared to the result of sin-
gle-class, it is certain that there are the obvious reduc-
tions on loss probability existing in all traffic states, es-
pecially when c1 is a smaller value. Secondly, we find the
value of PD is positive proportional to the variant c1. For
instance, when c1 = 0.2 or 0.3, a very low PD, less than
10−3, is obtained in light load (ρ < 0.3). Instead, for c1 =
0.5, the burst loss probability is increased quickly, and
reaches 2 × 10−1 at ρ = 0.7, which is almost equal to the
result of single-class. Although as the minority of net-
works, the load ratio of high priority is general less than
30% [15], we should strongly notice that, being in heavy
load, a higher PD is also demonstrated when c1 is a small
value (e.g. when c1 = 0.2, PD is equal to 10−2 at ρ = 0.7).
In one word, DPP is suboptimal due to the fact a higher
loss probability still exists in high traffic states.
r
FDL length=b
L b
burst
dropped
Blow,j
Bhig,i
preemption
Bhig,i
t
(a)
burst
r
B
low,j
B
hig,i
t
FDL length=b
B
low,j
dropped
L b
(b)

J.-R. YANG, H.-A. YE
446
Figure 2. Total loss of DPP.
Based on Equations (2)-(4), we take a further analysis
for high and low priority on blocking performance, and a
typical value 0.3 is selected for c1. The numerical results
in Figure 3 indicate that Blow is the main contributor of
the total burst loss probability. And for high priority
bursts, two apparent natures on PD, 1 are exhibited in dif-
ferent traffic states. In light load, its loss probability is
just about 1/10 of PD, 2. But, with the increase of offered
load, PD, 1 is soared dramatically and reaches 0.33 PD
under the condition ρ = 1.0. From Equation (2), the loss
of high priority is mainly dependant on the parameters u
and PFDL. Considering the majority nature, we infer that
such higher PD, 1 in heavy load is resulted from the
over-occupancy of the huge low priority bursts on chan-
nel and buffer.
3. Limited Delay Preemption Based Priority
According to the above analysis, the proposed DPP scheme
cannot guarantee the low-loss requirements of high prior-
ity bursts because the buffer is unfairly over-occupied by
low priority. To overcome the weakness of DPP, it is ne-
cessary to take a limitation on buffer usage of low prior-
ity within high traffic states. Therewith, an improved
scheme called Limited Delay Preemption Based Priority
(LDPP) is proposed. In LDPP, the buffered right for low
priority bursts is deprived when offered load reaches the
designed threshold ρth. Thereby, given the channel con-
tention takes place, the burst with low priority will be
discarded, whether the buffer is in the idle state or not.
We then get PD, 2 = c2u when ρ > ρth. In addition, owing
to P2, FDL = 0, the loss probability of high priority bursts
PD, 1 is reduced as
,1 1 1,DFDL th
PcuP ρρ =⋅>
(6)
Therefore, in LDPP, the total loss will be improved
and Equation (5) is substituted by
2 1,2,1
2 1,2,1
1 1,2
1 1,2
[() ]
1 [()]
()
1( )
FDL FDLth
FDL FDL
DFDL th
FDL
cP Pc
cP Pc
PcP c
cP c
ρρρ
ρ
ρρρ
ρ
++
≤
+ ++

=+
>
++

(7)
From Equation (7), the performance evaluation of LDPP
on blocking is developed and the simulation results are
shown in Figures 4 and 5. Compared to DPP, in the
range of ρth = 0.3 - 0.7, LDPP obtained a blocking im-
provement, at least 20%, under the condition ρ = 1.0.
Besides, we find that, the lower threshold ρth is selected,
the smaller loss probability is gained (see Figure 4). Whe-
reas, considering the loss requirements of Blow and the
buffer usage efficiency in light load, the value of load
threshold shoul d not be too small.
Here, an eclectic value ρth = 0.5 is chosen, and the
blocking performance of high and low priority is again
investigated. Comparatively, on one hand, the loss prob-
ability of Blow in LDPP is not deteriorated in all traffic
states though the buffered right has been deprived par-
tially. On the other hand, the blocking performance of
high priority bursts is improved, and more than 40% re-
duction on PD, 1 under the condition ρ = 1.0 is shown in
Figure 5. In consequence, the total loss probability PD is
Figure 3. Blocking of high priority and low priority in DPP.
Figure 4. Total loss of LDPP.
Figure 5. Blocking comparison between DPP and LDPP.
0.2 0.4 0.6 0.8 1.0
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
Offered load per channel
Burst loss probability
single class
c 1=0.2,c2=0.8
c 1=0.3,c2=0.7
c 1=0.5,c2=0.5
0.2 0.40.6 0.8 1.0
10
-5
10
-4
10
-3
10
-2
10
-1
Offered load per channel
Burst loss probability
c
1
=0.3, c
2
=0. 7
high priority
low priority
total loss
0.0 0.2 0.4 0.6 0.8 1.0
10
-4
10
-3
10
-2
10
-1
0.4 0.6 0.8 1.0
0.02
0.04
0.06
Burst loss probability
Offered load per channel
DPP
LDPP( ρ
th
=0.7)
LDPP( ρ
th
=0.5)
LDPP( ρ
th
=0.3)
c
1
=0.3, c
2
=0. 7
0.0 0.2 0.4 0.6 0.8 1.0
10
-5
10
-4
10
-3
10
-2
10
-1
c
1
=0.3, c
2
=0. 7
ρ
th
=0. 5
Offered load per channel
Burst loss probability
high priority(LDPP)
high priority(DPP )
low priority (LDPP)
low priority (DPP)

J.-R. YANG, H.-A. YE
447
also reduced about 20% in heavy load, which has been
proved in Figure 4.
4. Summary
In this paper, to support services differentiation, the ana-
lytical model of delay preemption based priority is de-
veloped, and an improved scheme LDPP is proposed to
alleviate the over-occupancy of buffer from low priority
in heavy load. The simulation results show that, through
taking a tradeoff on the buffer usage of bursts, more than
40% reduction of high priority on blocking is obtained in
LDPP, without the deterioration of total blocking. And
our future work will focus on the evaluation of LDPP in
terms of fairness and energy consumption.
5. Acknowledgements
This work is supported by the Heilongjiang Province
Natural Science Foundation (No. 201009).
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