=Paper=
{{Paper
|id=Vol-1256/paper9
|storemode=property
|title=Applying Data Fragmentation in IEEE 802.15.4: Modeling and Analysis under Unsaturated Traffic
|pdfUrl=https://ceur-ws.org/Vol-1256/paper9.pdf
|volume=Vol-1256
|dblpUrl=https://dblp.org/rec/conf/vecos/AtmaniAA14
}}
==Applying Data Fragmentation in IEEE 802.15.4: Modeling and Analysis under Unsaturated Traffic==
https://ceur-ws.org/Vol-1256/paper9.pdf
85
Applying data fragmentation in IEEE 802.15.4:
modeling and analysis under unsaturated
traffic
Mouloud Atmani Djamil Aı̈ssani Yassine Hadjadj-Aoul
LaMOS research unit LaMOS research unit IRISA Laboratory
Faculty of Exact Sciences Faculty of Exact Sciences University of Rennes 1
University of Bejaia University of Bejaia 35042 Rennes
06000 Bejaia, Algeria 06000 Bejaia, Algeria France
atmanimouloud@yahoo.fr djamil aissani@hotmail.com yhadjadj@irisa.fr
The IEEE 802.15.4 standard, which is developed for low rate applications, offers low latency and energy
consumption for wireless sensor networks. The use of the standardized slotted Carrier Sense Multiple
Access (CSMA/CA), as a channel access mechanism, can, however, lead to a wastage of the bandwidth
utilization and an additional transmission delay. This drawback is mainly caused by deferred transmission
in the CSMA/CA algorithm at the end of the superframe, when there is not sufficient time to complete the
frame transmission. We propose in this paper to fragment a data frame into a short frame and attempt
its transmission in the current frame and transmit the remaining frame in the next superframe. The data
fragmentation mechanism is modeled using a Markov chains. A non-saturated traffic and acknowledgement
transmission are considered in our analysis. The analytical results of the normalized throughput demonstrate
the improvement of the bandwidth occupation when using the proposed data fragmentation mechanism in
the IEEE 802.15.4 slotted CSMA/CA protocol.
IEEE 802.15.4 slotted CSMA/CA, data fragmentation, Markov chains, Modeling and Analysis.
1. INTRODUCTION in the Personal Area Network (PAN). The superframe
contains an active period (for communication) and
Before the development of the IEEE 802.15.4 an inactive period (for energy conservation). The
standard, several standards offering high data rates active period includes a Contention Access Period
were proposed for local and personal wireless (CAP) and an optional Contention Free Period (CFP)
area networks (IEEE 802.11, IEEE 802.15.1, etc.). for deterministic channel accesses. During the CAP
Such standards were not, however, adapted to period, the slotted CSMA/CA algorithm is executed
miniature devices with limited energy capacities. by each node desiring to access the channel.
The IEEE 802.15.4 (IEEE std 802.15.4 (2006))
standard was developed and proposed for Low- Several researchers have modeled the slotted
Rate Wireless Personal Area Networks (LR-WPANs) CSMA/CA protocol with the Markov chains, by
with low energy resources, such as wireless referring generally to the Bianchi’s model (Bianchi
sensor networks. The IEEE 802.15.4 defines the (2000)). A simple model of the slotted CSMA-CA
specifications of the physical layer and the Medium protocol is given by (Pollin et al (2008)) using
Access Control (MAC) sublayer of the ZigBee stack. Markov chains. A generalized Markov chain of IEEE
In the MAC sublayer, the IEEE 802.15.4 standard 802.15.4 slotted CSMA/CA is given by (Park et
defines two access modes: non-beacon mode and all (2013)). The deferment of the transmission,
beacon mode. In the non-beacon mode, unslotted when there is not sufficient remaining time in the
CSMA/CA is used for attempting the channel CAP to complete the transmission, is modeled and
access. However, slotted CSMA/CA algorithm is evaluated by (Rehman et al (2011)).
used in the beacon mode. In the beacon mode,
the mode considered in this work, the coordinator In IEEE 802.11 standard, the fragmentation tech-
sends regularly beacons frames to delimitate the nique is implemented and many studies have men-
superframe and to synchronize the wireless sensors tioned that this technique improves the network
86
throughput (see, the works of IEEE Part 11 (2007),
Yazid et al (2013) and Li et al (2009)). The authors,
in Yoon, Kim and Ko (2007), have proposed the
fragmentation mechanism in IEEE 802.15.4 wireless
sensor network to improve the bandwidth utilization.
However, in this work, the risk of data collisions is
possible when a competitive node pulls a backoff
number equal to zero while the transmitter node at-
tempts to send the remaining frame, in the beginning
of the superframe.
This paper talk about the problem of the deferred
transmission that causes a significant bandwidth
loss in IEEE 802.15.4 wireless sensor networks.
For this reason, we propose to send a short frame
(equal to 18 bytes as defined by IEEE 802.15.4
standard) when a long frame can not be sent due
to an insufficient time in the current superframe.
The data fragmentation mechanism is modeled,
under non saturation traffic with transmission’s
acknowledgment, using a Markov chain. The
analytical results show that the fragmentation
mechanism clearly allows improving the network
performance in terms of throughput.
The remainder of this paper is organized as fol-
lows. Section 2 gives an overview of the slotted Figure 1: Flowshart of IEEE 802.15.4 slotted CSMA/CA
CSMA/CA. Section 3 presents our motivations for algorithm
this work and describes the applied data fragmenta-
tion mechanism in IEEE 802.15.4 slotted CSMA/CA
Once the three variables are initialized, the node
algorithm. Section 4 presents the proposed math-
waits during a period of backoff randomly chosen in
ematical model based on Markov chains of IEEE
the range [0, 2BE − 1]. If the pulled number is greater
802.15.4 standard with the proposed data fragmen-
than the remaining number of backoff periods in
tation mechanism. Section 5 gives a comprehensive
the CAP, the MAC sublayer shall pause the backoff
performances analysis of our proposal. Finally, we
countdown at the end of the CAP and resume it at
conclude in Section 6.
the start of the CAP in the next superframe.
2. OVERVIEW OF IEEE 802.15.4 SLOTTED At the expiration of the random backoff delay,
CSMA/CA the MAC sublayer ensures that the remaining
CSMA/CA operations can be undertaken and the
In this section, we describe the behavior of the IEEE entire transaction can be completed before the end
802.15.4 slotted CSMA/CA protocol. Each node of the CAP. Two cases are possible:
aiming to transmit a data frame or a control frame, Case 1: The remaining time in the CAP is
as indicated in IEEE 802.15.4 standard, initializes sufficient:
three variables (N B, BE and CW ). The variable N B The MAC sublayer requests the physical layer to
describes the number of times that the CSMA/CA perform two CCA:
algorithm is executed for attempting to access the
channel (i.e. Number of Backoff). The variable BE 1. The channel is assessed to be busy during
is used to generate a random backoff duration that one of the CCA: both N B and BE are
a node shall wait before attempting the first carrier incremented by one (BE shall be no more than
sensing (i.e. Backoff Exponent), its value depend on macM axBE), CW is reset to two. If N B is less
the value of BLE (Battery Life Extension) sent by than or equal to macM axCSM ABackof f s,
the PAN coordinator. The variable CW indicates the which is equal to 4 by default, a new BE is
number of time that a channel must be clear before pulled randomly in the range [0, 2BE − 1]. If
beginning the data transmission (i.e. Contention N B is greater than macM axCSM ABackof f s,
Window). Its value is set to two, as shown in figure 1. the CSMA/CA algorithm shall terminate with a
channel access failure status.
87
2. The channel is assessed to be idle during the frame (less than or equal to 18 bytes). In our
first CCA: CW is decremented by one and work, we propose to add the following test in the
checked whether it is equal to zero. slotted CSMA/CA algorithm: before deferring the
The same procedure is considered for the transmission of a long frame due to insufficient
second CCA, If CW is equal to zero, the data remaining time in the CAP period, slotted CSMA/CA
frame is transmitted. checks if the remaining time is sufficient to complete
the transmission of a short frame. In this case, the
Case 2: The remaining time in CAP is insuffi- rest of the frame will be transmitted in the new
cient: superframe.
The transmission will be deferred to the next super-
frame and a new slotted CSMA/CA is executed at
the beginning of the CAP, as depicted on the yellow
rectangle of the Figure 1
3. MOTIVATIONS AND PROPOSAL
In this section, we give our motivations behind
applying the data fragmentation mechanism in
IEEE 802.15.4 slotted CSMA/CA protocol. Then, we
describe our solution and we give its main interest in
the IEEE 802.15.4 wireless sensor networks. Figure 3: Applying data fragmentation in Slotted
CSMA/CA
3.1. Motivations
Given the critical nature of the applications in which
Figure 3 shows that the data fragmentation
the sensor networks are applied, it is important to
increases the bandwidth occupation and reduces the
optimize the use of all resources (bandwidth, battery,
data transmission time. To ensure the transmission
...) together with the time of communication. As
of the remaining frame without collision, at the
explained in the section 2, a node using slotted
beginning of the superframe, our technique avoids
CSMA/CA to transmit a data frame must estimate
the channel listening (CCA1 2 and CCA) for the
the remaining time in the ion, it sends the frame,
node which transmitted a short frame in the previous
otherwise it differs the transmission for the new
superframe.
superframe (see figure 2).
4. ANALYTICAL MODEL
In this section, we model and analyze the proposed
data fragmentation mechanism in the IEEE 802.15.4
with acknowledgment of the transmitted data under
unsaturated traffic conditions. We assume N nodes
connected with a PAN coordinator in a star topology.
4.1. Markov chain model
In the following, we model the behavior of a single
Figure 2: Deferred transmission in Slotted CSMA/CA node using IEEE 802.15.4 slotted CSMA/CA with
the data fragmentation mechanism, using a three
The delay caused by the deferred transmission dimensional Markov chain in order to analyze the
of a data frame increases more when the size network performances. Three stochastic processes
of a packet increases. The long frame and the are used to describe the state of the node at each
frequent deferments lead to a bandwidth misuse. time when executing slotted CSMA/CA with data
These problems can be avoided by applying the fragmentation mechanism.
data fragmentation mechanism. Thus, a rational
management of the bandwidth and a minimum time Let S(t), B(t) and T (t) be the stochastic processes
of transmission will be achieved. representing the backoff stage, the state of the
backoff counter and the packet type to transmit at
3.2. Slotted CSMA/CA with data fragmentation time t, respectively. Their values are given as follows:
S(t) = (0..m), B(t) = (−2..Wi − 1) and T (t) =
IEEE 802.15.4 describes two types of data frames:
{−1, 0, 1, 2}, where m = macM axCSM ABackof f s,
a long frame (greater than 18 bytes) and a short
88
and Wi = 2i W0 , the initial value of W0 = 2BE − The transition probabilities associated with the
1, where the value of BE is defined in figure 1. Markov chain of figure 4 are:
We note that, the backoff counter B(t) is divided
into two periods, the backoff period pulled between P (i, k, j|i, k − 1, j) = 1, k > 0 (2)
h i
{0, 2BE } and the second period represent the two
P (i, −1, j|i, 0, j) = (1 − α) (1 − Pd ) + Pd Pf ,
clear channel assessment (CCA) periods (-2 and -
1). j = {0, 1}, i 6 m (3)
P (i, −2, j|i, −1, j) = 1 − β, j = {0, 1}, i 6 m (4)
The Idle state in figure 4 indicates wether the node
has a data frame to transmit. In other words, this α + (1 − α)β
P (i, k, j|i − 1, 0, j) = (1 − Pd ) , i6m
state models the queue of a node. To satisfy the Wi
condition of non-saturated network (i.e. a node has (5)
not always a frame to transmit), we consider that the Pd
P (0, k, 0|i, 0, 0) = [(1 − Pf ) + Pf [α + (1 − α)β]],
events arrive to the nodes according to the poisson W0
process with the rate λ. Hence, the probability q that j = {0, 1}, i 6 m (6)
a data packet (event) arrives to a node during one
P (i, 0, 1|i, 0, 0) = Pd Pf , i 6 m (7)
backoff period T s is given as follows:
q
Z Ts P (0, k, 0|Idle) = ,k > 0 (8)
W0
−λt
q= λe dx (1)
0
Equation (2) defines the decrement probability of the
backoff counter. Equation (3) and (4) describes the
probability to find the channel idle for the first CCA
and second CCA, respectively. Equation (5) denotes
that the channel is busy, the node in this case selects
a new delay backoff in the new stage. Equation (6)
represent the probability to defer the transmission
to the next superframe when the remaining time in
the CAP is either insufficient to transmit a packet
or a fragment, or the fragmentation is possible
but the channel is busy. Equation (7) describes
the possibility of fragmentation when the remaining
delay of CAP is not enough to assure a successful
transmission of the original frame. The probability to
pull a data frame in the queue of a node to transmit
is given in the equation (8).
Let bi,k,j = limt→+∞ P {S(t) = i, B(t) = k, T (t) = j},
be the stationary distribution of our Markov chain.
Using equation (5), we get the probability to find a
node in any stage at the steady state:
h ii
bi,0,0 = (1−Pd ) α+(1−α)β b0,0,0 , f or 1 6 i 6 m
(9)
The probability to be in any state of the first stage is
given as follows:
W0 − k h iXm
b0,k,0 = (1 − α)(1 − β)(1 − Pd ) bi,0,0
W0 i=0
Figure 4: Markov chain model for Slotted CSMA/CA with
fragmentation
h i
+(1 − Pd ) α + (1 − α)β bm,0,0 (10)
89
The probability to be in any backoff state in any stage B) Probability β: is defined as the probability to find
is given by: the channel busy in the CCA2, given that it is free in
the CCA1 period.
Wi − k
bi,k,0 = bi,0,0 , f or1 6 i 6 m, 1 6 k 6 Wi − 1
Wi 1 − (1 − τ )N −1 + N τ (1 − τ )N −1
(11) β= . (16)
2 − (1 − τ )N + N τ (1 − τ )N −1
In the normalization conditions, the probabilities
must sum to 1. So: C) Probability of deferment (Pd ): is defined as the
X X
m Wi −1 X −1
m X X
m X
0 probability that the remaining time in the CAP is
1 = bi,k,0 + bi,k,0 + bi,k,1 not sufficient to complete the transmission of a
i=0 k=0 i=0 k=−2 i=0 k=−2 data frame and its acknowledgment. Hence, after
X
m X
LF −1 X
SF −1 the completion of the backoff decrementation, the
+ bi,0,−1 + b−1,k,0 + b−1,k,1 current time of the node must be in the interval
i=0 k=0 k=0 ]CAP − TLF + 2 ∗ Tcca + Tack wait + TACK , CAP ] to
X
RF −1 defer the transmission. This interval is illustrated by
+ b−1,k,2 + bIdle . (12) the blue stripes part as shown in the figure 5.
k=0
Therefore, the formula of b0,0,0 is given as follows :
1
ab0,0,0 = "
3
+ Pd Pf (2 − α) + (1 − α)(1 − Pd ) − x Figure 5: Insufficient remaining time in CAP for a complete
2
transmission
h
+(1 − Pc )(1 − α)(1 − β) (1 − Pd )(1 + LF )
# The probability of deferment is formulated by the
1 i 1 − xm+1 xm+1 following expression:
+Pd Pf (SF + RF + ) +
q 1−x q
TLF + 2 ∗ Tcca + Tack wait + TACK + ε
, (13) Pd = , (17)
1 − (2x)m+1 TCAP
+ w0
2(1 − 2x) where Tack wait is the time to wait before beginning
the ACK transmission. While ε is introduced in the
where, x = (1 − Pd ) α + (1 − α)β and LF , SF ,
and RF represent, respectively, a longue frame, a equation (17) to indicate that the time, in the CAP, to
short frame (fragment) and a remaining frame. The complete the frame transmission is insufficient.
probability that a node attempts to sense the channel
D) Probability of fragmentation (Pf ): is the probability
for the first time (τ ) in any stage of the Markov chain
to find, in the remaining time of the CAP,
is expressed as follows:
sufficient time to transmit a short frame and
X
m any acknowledgment. Assuming that when the
τ= bi,0,0 . (14) backoff counter of a sensor is zero its current
i=0 time is in the interval ]TCAP − TLF + 2 ∗ Tcca +
Tackw ait + TACK , TCAP − TSF + 2 ∗ Tcca + Tack wait +
To compute the performance of the network, we TACK ], the blue stripes in figure 6. In this
express all the probabilities in interaction with the case, the data fragmentation is applied. Otherwise,
probability τ . the fragmentation is not possible and the data
transmission will be deferred to the next superframe,
A) Probability α: is the probability to find the channel as depicted by the red stripes in figure 6.
busy during CCA1 due to data (or acknowledgment)
frame transmission. Similarly to Park et all (2013),
we express this probability as follows:
h N τ (1 − τ )N −1 i
α = 1−(1−τ )N −1 (1−α)(1−β) T +TACK ,
1 − (1 − τ )N
(15)
where T and TACK represent the number of backoff
Figure 6: Possibility of fragmentation in the CAP
delay required for the frame transmission and the
acknowledgment frame, respectively.
90
The probability of fragmentation is given in the 5. ANALYTICAL RESULTS
following expression:
In this section, we evaluate the performance of
TLF − TSF + ε the data fragmentation mechanism in improving
Pf = , (18)
TCAP the network throughput. The analytical parameters
taken into account in the performance analysis are
where ε is introduced in equation (18) to express the
presented in table 1.
impossibility of transmitting the original frame (LF ).
Table 1: Analytical parameters
4.2. Throughput
The unsaturation throughput (noted S), as defined Parameters Initial
in Bianchi (2000), as the fraction of time that Value
the channel is used to successfully transmit the Max packet length 127
data frame. Therefore, S depends, on the following Bytes
probabilities: Max packet transfert delay 4 ms
Radio transmission power 52.2 mw
A) Transmission probability (Ptr ): represents the Radio reception power 59.1 mw
probability that at least one node (among N nodes) Radio idle power 1.28 mw
is in the beginning of the first clear sensing (CCA1) Battery Capacity 2500
with probability τ , the channel sensed free in CCA1 mAh
and CCA2 and the transmission will not be deferred. Voltage 3.0 V
Ptr = 1 − (1 − τ )n (1 − α)(1 − β)(1 − Pd ). (19)
Figure 7 shows the results of network throughput
for 10 nodes under different traffic load. It
B) Successful transmission probability (Ps ): is the illustrates the throughput improvement using the
probability that exactly one transmission occurred data fragmentation mechanism (IEEE 802.15.4
in the channel, conditioned by the transmission Frag), comparing it with IEEE 802.15.4 standard
probability (as defined in Bianchi (2000)). (IEEE 802.15.4). We show that when the length of
the original frame is just greater than the short frame
nτ (1 − τ )n−1 (1 − α)(1 − β)(1 − Pd )
Ps = . (20) (L = 3 slots and shortf rame = 2 slots) the gain
Ptr is not large enough. However, when increasing the
frame size, the gain in throughput becomes very
Now, we can express the unsaturation throughput (S) important (see the case of L = 7 slots).
as follows:
Ptr Ps Tpload 0.11
S =
(1 − Ptr )σ + Ptr Ps Ts + (1 − Ps )Tc 0.1
. (21)
+Pd (1 − Pf )TDef 0.09
0.08
Normalized Throughput
where, Tpload is the time occupied by the packet
transmission, σ is the duration of an empty time 0.07
slot, Ts is the time of a successful transmission of 0.06
a packet, Tc is the time during which the channel is
0.05
busy due to a collision and TDef is the average time
wasted when deferring the current transmission. 0.04
IEEE 802.15.4 Frag L=3
0.03
IEEE 802.15.4 Std L=3
0.02 IEEE 802.15.4 Frag L=7
IEEE 802.15.4 Std L=7
Ts = TP HY + TM AC + Tpload + 2TCCA + TLIF S ,
0.01
+Tack + TACK . 0 0.02 0.04 0.06 0.08 0.1
Arrival Rate λ (packet/time slot)
Tc = TP HY + TM AC + Tpload + 2TCCA + TLIF S ,
+Tack .
Figure 7: Throughput versus traffic load with number of
TSF + TP HY + TM AC + TSIF S + 2Tcca
TDef = nodes = 10
2
Tack + TACK − ε
+ , Figure 8 illustrates the cases of a dense network
2
(22) (number of nodes = 50). When the frame is long
where ε indicates that there is not sufficient time to (L = 7 slots) and the traffic load (λ) is less than 0, 01,
complete the short frame transmission in the current the network throughput is better than when using
superframe. a small frame (L = 3 slots), independently from
91
considering or not the fragmentation mechanism. in term of throughput under non saturated traffic with
However, when the traffic load increases, it causes acknowledgment. The results shows the interests in
frequent collisions and deferred transmissions. That applying the data fragmentation mechanism in the
is why, the small frames give better results. In IEEE 802.15.4 standard.
all cases, we show that, the data fragmentation
mechanism offers a better throughput. 0.12
IEEE 802.15.4 Frag L=3
IEEE 802.15.4 Std L=3
IEEE 802.15.4 Frag L=7
0.1 0.1
IEEE 802.15.4 Std L=7
IEEE 802.15.4 Frag L=3
0.09 IEEE 802.15.4 Std L=3
Normalized Throughput
IEEE 802.15.4 Frag L=7 0.08
IEEE 802.15.4 Std L=7
0.08
Normalized Throughput
0.07 0.06
0.06
0.04
0.05
0.04 0.02
0.03
0
0 10 20 30 40 50
0.02 Number of Nodes
0.01
0 0.02 0.04 0.06 0.08 0.1
Arrival Rate λ (packet/time slot) Figure 10: Throughput versus number of nodes with λ =
0, 05
Figure 8: Throughput versus traffic load with number of
nodes = 50 In the figure 10, we analyzed an average traffic
(λ = 0, 05) to see the contribution of the data
fragmentation mechanism when the number of
0.1 nodes increases. The throughput decreases when
0.09
IEEE 802.15.4 Frag L=3 the number of nodes increases, due to collisions and
IEEE 802.15.4 Std L=3
IEEE 802.15.4 Frag L=7
frequent transmission deferrement.
0.08
IEEE 802.15.4 Std L=7
0.07
Normalized Throughput
6. CONCLUSION
0.06
0.05
In this paper, the data fragmentation mechanism
is proposed to be applied in IEEE 802.15.4
0.04
slotted CSMA/CA protocol. The principle of the
0.03 mechanism is simple to implement, without changing
0.02 the operating principles of IEEE 802.15.4 slotted
CSMA/CA. The data fragmentation is applied when
0.01
the transmission of a long frame is impossible due to
0
0 10 20 30 40 50 insufficient remaining time in the contention access
Number of Nodes period. Our proposal privileges the transmission of
the remaining frame in the new superframe and
Figure 9: Throughput versus number of nodes with λ = avoid its collision. In our future works, we will
0, 001 evaluate the impact of other parameters on the
overall network performance and we will analyze
Figure 9 clearly shows the contribution of the data how to improve the energy consumption using the
fragmentation mechanism, when the network traffic data fragmentation mechanism.
is low (λ = 0, 001). The difference between the
throughput of the IEEE 802.15.4 with the proposed
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