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Abstract and Figures

IEEE standard 802.15.4 is widely studied due to its huge applicability. This vary study is based upon analyzing techniques and methodology discussed in IEEE 802.15.4 standard in context of contention access period. According to standard specifications, there are two minor differences in CSMA/CA algorithm used in CAP along with different frequency ranges. These frequency ranges are accepted in different geographical regions of this world. Considering medium access algorithm used, one flavor supports ACK frame after successful transmission and other do not. ACK mode of CSMA/CA is widely analyzed analytically and simulated in earlier conducted researches,however, there is no study discussing behaviour of non-ACK mode. This non-ACK mode is prescribed as a different flavor in IEEE 802.15.4 for applications that do not focus on taking ACK packet after every transmission. In this work, we modify a markov chain model for non-ACK mode and compare both modes analytically followed by extensive simulations and discussions. For health care applications ACK mode shows its worth however, in streaming data or playing games, non-ACK mode can be preferred due to its lower delay, higher throughput and lower control load. c ⃝ 2014 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of Elhadi M. Shakshuki.
Content may be subject to copyright.
Procedia Computer Science 34 ( 2014 ) 204 211
1877-0509 © 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Selection and peer-review under responsibility of Conference Program Chairs
doi: 10.1016/j.procs.2014.07.011
ScienceDirect
Available online at www.sciencedirect.com
The 9th International Conference on Future Networks and Communications (FNC-2014)
Analyzing and Evaluating Contention Access Period of Slotted
CSMA/CA for IEEE802.15.4
D. Mahmooda, Z. A. Khanb, U. Qasimc, M. Umair Narua, S. Mukhtard,
M. I. Akrama,N.Javaid
a
aCOMSATS Institute of Information Technology, Islamabad, Pakistan
bInternetworking Program, FE, Dalhousie University, Halifax, Canada
cUniversity of Alberta, Alberta, Canada
dBahria Univerisity Islamabad, Pakistan
Abstract
IEEE standard 802.15.4 is widely studied due to its huge applicability. This vary study is based upon analyzing techniques and
methodology discussed in IEEE 802.15.4 standard in context of contention access period. According to standard specifications,
there are two minor dierences in CSMA/CA algorithm used in CAP along with dierent frequency ranges. These frequency ranges
are accepted in dierent geographical regions of this world. Considering medium access algorithm used, one flavor supports ACK
frame after successful transmission and other do not. ACK mode of CSMA/CA is widely analyzed analytically and simulated in
earlier conducted researches,however, there is no study discussing behaviour of non-ACK mode. This non-ACK mode is prescribed
a
sadierent flavor in IEEE 802.15.4 for applications that do not focus on taking ACK packet after every transmission. In this work,
we modify a markov chain model for non-ACK mode and compare both modes analytically followed by extensive simulations and
discussions. For health care applications ACK mode shows its worth however, in streaming data or playing games, non-ACK mode
can be preferred due to its lower delay, higher throughput and lower control load.
c
2014 The Authors. Published by Elsevier B.V.
Selection and peer-review under responsibility of Elhadi M. Shakshuki.
Keywords: 802.15.4; Slotted CSMA/CA; MAC; Physical; Contention Access Period.
1. Introduction
World is changing, automation in every aspect of life is the current need. For that automation, we humans are now
surrounded with processors to process dierent attributes. Here comes the domain of communication where these
processings normally called as information is sent, stored and utilized in a beneficial way. Combining processing,
transmitting, storing and utilizing that information gives birth to networks; initially wired and then wireless. Entering
Nadeem Javaid, www.njavaid.com, Tel.: +92-300-579-2728
E-mail address: nadeemjavaid@comsats.edu.pk, nadeem.javaid@univ-paris12.fr
© 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Selection and peer-review under responsibility of Conference Program Chairs
205
D. Mahmood et al. / Procedia Computer Science 34 ( 2014 ) 204 – 211
wireless domain, there are numerous networks proposed for dierent applications. Amongst these, wireless sensor
networks (WSN’s) are emerging field that has enormous potential for applications in it. These networks consists of
tiny sensors having limited energy and low computational power.
IEEE 802.15 working group is specified for wireless personal area networks (WPAN)and is further divided into
dierent extensions. Within these extensions, IEEE 802.15.4 [1] is designed for low rate and longer battery life. Due
to simplicity, longer life time and wide range of applications, IEEE802.15.4 standard has got focus light in research
as well as industrial aspect. This standard contrary to other methodologies (WiFi) stresses on least possible cost for
communication between devices with minimal underlying resources. This standard is designed for a communication
range of 10 meters along with data rate of 250kbps.
The said standard stipulates MAC and Physical Layers and provides basis to dierent technologies i.e. ZigBee,
ISA100, WirelessHART and MiWi alliances. For MAC sublayer, a device can operate in contention free period or
contention access period. In contention free period, a node may reserve guaranteed time slots while in contention
access period, carrier sense multiple access with collision avoidance is used with some modifications from earlier
standards. In physical layer, IEEE802.15.4 is operable on three dierent frequency assignments. This work as the
title indicates, focuses on workability of contention access period of IEEE802.15.4 under three dierent frequency
assignments.
2. Related Work
In IEEE802.11 [2] if a node that has a packet to transmit, gets channel access, decrements backocounter by
one other wise, it is blocked. Considering IEEE802.15.4, backocounter decrements whether channel is accessed
or found busy. Due to such variations, Researchers were interested in establishing mathematical models regarding
IEEE802.15.4. Initial researches [4-7] were conducting using markov chain models without taking superframe struc-
ture into account. [8] enhanced the research by considering super frame structure as well. Modeling of said standard
is further enhanced by [9] that proposed a model for slotted CSMA/CA. Authors of [10] calculated probabilistic val-
ues of reliability, energy consumption and delay which was further modified by [11] and [12]. The study regarding
performance analysis of all frequency assignments that are mentioned in standard text [1] was presented by [13].
Considering medium access techniques of contention access period, [14-16] give comparison of dierent schemes
to be applied on body area networks. Access schemes like time and frequency division multiple access, pure and
slotted Aloha along with CSMA/CA were observed using key metrics of path loss, delay and good put. CSMA/CA
performance in IEEE802.15.4 is widely studied however, for the applications where ACK is not utterly needed after
transmission of a packet, there is specified a flavor in standard text that is non-ACK mode. Functioning of CSMA/CA
remains same but with absence of this control packet. In this study, we compare both of these modes i.e. ACK and
non-ACK using markov transitional model (enhanced version of [10] and [12]) under all three frequency assignments
that are mentioned in [1] for the said standard. For comparison analysis, we calculate transmission, delay and channel
accessing probabilities using Markov model which are the key features for any protocol and simulate them using
MATLAB.
Rest of the paper is organized as: Overview of IEEE 802.15.4 (MAC and Physical Layer aspect) in section 3, Proba-
bilistic Modeling regarding ACK and non-ACK mode of CSMA/CA is presented in section 4 while simulated results
and logical reasoning is provided in section 5 followed by conclusion of this paper.
3. An Overview: IEEE 802.15.4
3.1. MAC SUBLAYER
MAC layer provides data as well as management services. Management services tends to interface between up-
per layers with MAC layer while data services are concerned with seamless functioning of transmission/reception
(MPDU’s) procedures in accordance with physical layer. Considering IEEE 802.15.4 standard, the major role of this
sub layer do not change however, there are some modifications regarding MAC of previous standards [11]. Super
frame structure of MAC as illustrated in fig. 1 is classified into two classes; active and inactive. Within Active part of
super frame structure, all operations regarding data transmissions and receptions are undertaken. In slotted version,
206 D. Mahmood et al. / Procedia Computer Science 34 ( 2014 ) 204 – 211
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Fig. 1. Superframe Structure
this active period is divided into equally distributed time slots. Every transmission begins at the start of one of these
time slots to ensure minimum wastage of resources due to packet collision if it happens. Besides such functionalities,
active period further has two domains. Contention Access Period (CAP) and Contention Free Period (CFP). In con-
tention free period, a point coordinator takes control of channel allocation and every node that opt for CFPis given
specified time slots for their transmissions, these nodes do not have to content for channel accessing. Whereas on
other hand, CAP is based on best eort procedures. All nodes that opt for CAP mode of active period have to content
with each other to get channel access. IEEE 802.15.4 specified CSMA/CA algorithm for ecient accession of chan-
nel. Though this protocol is already widely accepted and applied however, in this standard it has some slight changes
in accordance with its application scenario and limitations (power and computational limitations). In IEEE 802.15.4
[1] CSMA/CA performs two clear channel assessments convectively to get access of channel which is not in the case
of [2] where only one clear channel assessment is under taken.We, in this work focus on contention access period
for further analyzing and evaluating. Like any other protocol, the said protocol has dierent stages. When a network
underlying this protocol initiates, a node being part of this network stages it self in idle state. In this state, it actually
is waiting for any data packet from upper layers to transmit. Besides three counters are generated as well, i.e. back o
counter, contention window counter and retransmission counter. At this stage, MAC layer has to reset the following
parameters as:Number of backos(NB =0), Contention window(CW =2), Backoexponent (BE =macMinBE)
and Retransmission times (RT =0).
When a packet from network layer reaches MAC layer, the idle state ends and now, the node is in channel sensing state.
In between channel sensing and idle state, there is a random back operiod within the range of o- to- 2BE -1. Channel
access is granted after channel sensing state that comprises of two consecutive successful clear channel assessments
(CCA). After completion of 1st CC A, The contention window counter is decremented by one and after successful
CCA2, CW counter reaches to 0. This means that channel is accessed and data packet is to be transmitted ending the
state of channel sensing and now it is in transmission state. If CCAfails, back ocounter is incremented and packet
again has to wait a random back otime till maximum value of macMaxCSMABackosand macMaxBE. When value
reaches the said threshold, packet is discarded. In transmission state, either packet is successfully transmitted or a
Table 1. MAC parameters for Slotted CSMA/CA (IEEE 802.15.4).
Parameters Value
macMinBE 3
macMaxBE 5
macMa xFrameRetries 3
ACK reception Time aT urnaroundT ime +aU nitBacko f f Period
LIFS (for Larger packet) 40 Symbols
IFS(for smaller packet) 12 Symbols
collision occurs. If packet is transmitted successfully, node again gets into idle state waiting for next packet to come
from network layer. However, if collision occurs retransmission counter is incremented and data packet is set into
wait for random back otime. After completion back otime, contention window counter and back ocounter are
set as default (CW =2, BE =macMinBE). Afterwards, again channel is to be sensed , accessed and then transmit
according to described procedures. There is also a threshold for retransmission retries. If number of retries reaches
207
D. Mahmood et al. / Procedia Computer Science 34 ( 2014 ) 204 – 211
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Fig. 2. Packet Transmission in IEEE 802.15.4
the value of macMa xFrameRetrie s, packet is discarded throwing node into idle state. Within all procedure, there is a
variation in protocol. AC K mode or non AC K mode. In ACKmode, after successful reception of a packet, receiver
issue this control packet ensuring reception of data packet. If sender fails to receive ACKpacket due to any reason,
retransmission counter is incremented and packet is to be transmitted again as per protocol. Transmission of ACK
packet do not follow any protocol as it is just issued on successful reception of data packet[1]. Considering non AC K
mode, the protocol functioning remain same with absence of this control packet. In this mode, data is transmitted and
if no collision occurs, it is understood as successful transmission. Having two clear channel assessment already has
eliminated major chances of collision. This statement is discussed briefly in later sections of this study.
During communication process, MAC layer specified dierent vacant spaces between transmission of two frames.
These vacant spaces are termed as inter frame space and are placed to avoid any chance of collision due to propaga-
tion delay and other factors. There are certain types of inter frame spaces prescribed, however, there are two major
types in which we are interested i.e. short inter frame space (SIFS) and large inter frame space (LIFS ). (SIFS) are
used between very small data/control frames however, for frames having larger chunks of data (LIFS ) are recom-
mended. Fig. 2 expresses this concept in terms of IEEE 802.15.4 specifically.
3.2. PHYSICAL LAYER
According to standard specifications, IEEE 802.15.4 can use any of three assigned frequency bands i.e. 868MHz,
915MHz and 2.4GHz. Each oering 20Kbps,40Kbps and 250Kbps respectively. Within these bands, there are 27
channels. Channel 0 is specified for 868MHz band, channels ranging 1-10 are reserved for 915MHz band while 10 to
26 are nominated for 2.4GHz band.
In 868MHz and 915MHz bands, BPS K is used as modulation scheme where as in 2.4GHz band, OQPS K is
used along with symbol rates of 20KSymbols/sec, 40KSymbols/sec and 62.5KSymbols/sec respectively. Slotted
CSMA/CA is used in Contention Access Period (CAP) which is essence of this study.
IEEE 802.15.4 specifies, certain modes are described for physical layer that should be capable of channel assessment.
Any one of these modes can be implemented in network to get desired results. Mode1 states that physical layer
reports channel as busy if it finds energy level above a certain threshold. Considering Mode2 if physical layer finds
a signal with same modulation /spreading scheme, channel will be reported as busy. In Mode3, physical layer not
only sense the energy level but also matches the modulation scheme. If that scheme matches, it declares channel
busy.[1]. Considering data transmission and reception, data reception is given priority. A node that is receiving data,
will attempt for channel accessing only after it has no more packet to receive. In this work, we focus on IEEE 802.15.4
Table 2. Frequency Assignments for IEEE 802.15.4
Frequency As-
signment
Number of
Channels
Channel Band-
width
Symbol Rate Data Rate Modulation
Scheme
868-868.6 MHz 1 600KHz 20KSymbols/Sec 20Kbits/sec BPSK
902-928 MHz 10 2MHz 40KSymbols/Sec 40Kbits/sec BPSK
2.4-2.4835 GHz 16 5MHz 62.5KSymbols/Sec250Kbits/sec O-QPSK
208 D. Mahmood et al. / Procedia Computer Science 34 ( 2014 ) 204 – 211
by comparing various parameters using three prescribed frequency ranges in standard. Moreover, we implement both
flavors of CSMA/CA for IEEE 802.15.4 i.e. acknowledged and non-acknowledged with all prescribed frequency
bands. We use Markov model presented by Park et.al [10] for slotted 802.15.4 and modified it for non AC K mode
to simulate (using MATLAB) and validate our discussions. The values are set as in accordance with standard text.
4. IEEE 802.15.4 Operational Modes
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Fig. 3. Comparison, ACK and Non-ACK mode of CSMA/CA - a Node’s Prospective
According to specifications, CSMA/CA is operable in non-ACK and ACK modes (fig. 3). If we look closely on
eect of this tiny packet over network performance we will be astonished. As discussed in [4] probability of getting
CCA1 busy is dependent on two main features, i.e. data packet and ACK packet. Taking the logical reasoning further
ahead, we can conclude that, ACK packet is one reason of not finding free channel during clear channel assessment 1
or absence of this packet may raise chances of getting CCA1 successful. Considering impact of CCA1 over network is
enormous. Once CCA1 is successfully done, there is a high chance of getting CCA2 free as well. Now, if we look into
details of this algorithm, we can say that, having high chances of CCA1 gives higher probability of gainings access
of channel, afterwards lower chances of collision resulting in minimizing probability of going to back ostages, and
retrying retransmission attempts. Hence, we achieve better good put, lower delay and increased reliability.
Numerous authors have placed their analytical and experimental work considering network performance by taking
dierent parameters. This study focus on both flavors of CSMA/CA (with and without ACK) over prescribed three
dierent frequency ranges. For that we taking further ahead the work of [10] and [3], modified gernalized markov
chain for non-ACK mode and simulate it using MATLAB. Reliability, Delay and Throughput are taken as performance
metrics for both modes over all three dierent frequency bands for a network comprising of 10 nodes.
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D. Mahmood et al. / Procedia Computer Science 34 ( 2014 ) 204 – 211
5. Analysis and Results
5.1. Channel Accessing
Once MAC layer receives a packet to transmit, according to protocol standards, it will go on for a random back
operiod. On completing of this period, it will start sensing transmission medium so that packet can be transmitted.
As discussed earlier and in standard that a node will carry forward two consecutive clear channel assessments to
transmit. 1st CCA plays a vital role in channel accessing. If CCA1 is acquired, probability of getting CCA2 will
increase. In non-acknowledged mode of CSMA/CA, probability of getting CCA1 increases. Here we simulate the
said protocol on three dierent frequency ranges i.e. 868MHz, 915MHz and 2.4GHz to compare which range suits
best for IEEE 802.15.4. The impact of AC Kpacket at transmission range of 858MHz is highest in comparison with
rest of two higher ranges. The reason is simple, as range of communication increases, the less will be time taken by
load/trac to pass on. Considering Trac load of 1000 bits per application/node, at 858MHZ, probability of getting
CCA1 busy is highest and it gradually increase with respect to increase in trac load. Same pattern can be observed
with the rest of two higher frequency ranges however, probability of getting CCA1 busy is minimized. Comparing
1000 2000 3000 4000 5000 6000 7000
Traffic (bits/application/node)
Frequency band = 868MHz (non−ACK)
Frequency band = 915MHz (non−ACK)
Frequency band= 2400MHz (non−ACK)
Frequency band= 915MHz (ACK)
Frequency band= 868MHz (ACK)
Frequency band= 2400MHz (ACK)
(a)
1000 2000 3000 4000 5000 6000 7000
Traffic (bits/application/node)
(b)
1000 2000 3000 4000 5000 6000 7000
Traffic (bits/application/node)
(c)
1000 2000 3000 4000 5000 6000 7000
Offered Load (bits/application/node)
Frequency band = 868MHz (non−ACK)
Frequency band = 915MHz (non−ACK)
Frequency band = 2400MHz (non−ACK)
Frequency band = 2400MHz (ACK)
Frequency band = 915MHz (ACK)
Frequency band = 868MHz (ACK)
(d)
Fig. 4. (a)Busy Channel Probability during CCA1, (b) Busy Channel Probability during CCA2, (c) Discarded Packets Probability due to Channel
Access Failure, (d) Discarded Packets Probability due to Retry Limits
results of CCA1 and CCA2, Fig 4a and Fig 4b, we come to know an interesting fact that, probability of getting CCA1
busy is higher than probability of getting CCA2 busy. Logically it should be like that because in CCA2, there is
minimal almost negligible chance of getting CCA2 busy due to ACK packet. Length of data packet with respect to
ACK packet is longer, hence if CCA1 is done successfully, there is a greater chance of getting CCA2 successfully
as well. Considering frequency ranges, results are same, higher the range is, better is the performance. There is a
procedure described in protocol as, a node will try to get channel for a finite number of times. When that number is
over, packet is discarded. According to simulated results, as probability of getting CCA1 and CCA2 successfully is
higher at 2.4GHz. This is also depicted when we simulate probability of discarded packets due to failure in channel
accession. Fig 4c illustrates probability of packet drop ratio is maximum in 858MHz, this ratio declines at 915MHz
and minimal at 2.4GHz.
5.2. Packet Transmission
As a node gets two CCAs successfully, node immediately transmits packet. Although, the algorithm almost van-
ishes the chances of collision however, there still are chances of collisions. As in accordance with protocol specifica-
tions, if collision occurs, node will try to resend it again for a finite number of iterations. Once that counter reaches its
210 D. Mahmood et al. / Procedia Computer Science 34 ( 2014 ) 204 – 211
limit, packet than is discarded for further processing or forwarding. Considering plots as in Fig 4d, there is a dierent
behavior between ACK and non-ACK mode of protocol. Here, ACK mode of CSMA/CA supersedes non-ACK mode
in higher frequencies however, both stands same at 858MHz band. At this stage, either a successful transmission
happens or packet is to be discarded. Either way, protocol running on each node refresh itself and same procedure is
repeated.
6. Performance Metrics
6.1. Reliability
Probability of Successful transmission inversely depends upon‘packet drop due to channel access failure and ex-
ceeding retry limits. Reliability is considered as number of successful transmissions. In above sections we simulated
the probabilities of packet drop. To calculate reliability we have to add these packet drop probabilities and plot them
inversely. Considering the simulated result, we come to know that for 915MHz band, initially reliability is very high
almost near to 1 but then it decrease dramatically. Reliability for 858MHz band declines in same fashion however, it
is less reliable with respect to rest two. As the trac load increases, reliability decreases gradually for all frequency
bands however, for 2.4GHz band, the degree of declination is not that steep showing much better performance for
IEEE 802.15.4 (slotted non-beacon enabled CSMA/CA). If we compare the modes non-ACK mode supersedes ACK
mode as it bears minimal control overhead along with higher probability of accessing a channel for transmission. Fig
5a expresses the probabilistic curve of reliability as discussed earlier.
1000 2000 3000 4000 5000 6000 7000
Traffic (bits/application/node)
Frequency band = 868MHz(non−ACK)
Frequency band = 915MHz(non−ACK)
Frequency band = 2400MHz(non−ACK)
Frequency band = 2400MHz(ACK)
Frequency band = 915MHz(ACK)
Frequency band = 868MHz(ACK)
(a)
1000 2000 3000 4000 5000 6000 7000
Traffic (bits/application/node)
Frequency band = 868MHz (non−ACK)
Frequency band = 915MHz (non−ACK)
Frequency band = 2400MHz (non−ACK)
Frequency band = 868MHz (ACK)
Frequency band = 915MHz (ACK)
Frequency band = 2400MHz (ACK)
(b)
1000 2000 3000 4000 5000 6000 7000
Traffic (bits/application/node)
Frequency band = 868MHz(without ACK)
Frequency band = 915MHz(without ACK)
Frequency band = 2400MHz(without ACK)
Frequency band = 2400MHz(with ACK)
Frequency band = 915MHz(with ACK)
Frequency band = 868MHz(with ACK)
(c)
Fig. 5. (a) Reliability Factor (Probability) Vs Trac Load,(b) Delay(Probability) Vs Trac Load, (c)Throughput(Probability) Vs Trac Load
6.2. Delay
Delay is one of the most critical performance criteria of any protocol. At times, for specific applications, delay
is not tolerated however, In this study we compare this performance metric for dierent variations of IEEE 802.15.4
standard. Basically, delay comprises of two parts. One is delay due to protocol algorithm that includes back os and
collisions etc., while the second part is based upon length of packet to be transmitted, turnaround time, and inter frame
spaces. Simulations show that, delay is lower at 2.4GHz band as back oslot duration is minimal there with respect
to rest of two frequency bands. Considering non-ACK mode, here again it has upper hand due to its simplicity and
almost negligible control overhead as shown in fig.5b.
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D. Mahmood et al. / Procedia Computer Science 34 ( 2014 ) 204 – 211
6.3. Throughput
Throughput is another very important performance metric for any network. In under consideration flavors of IEEE
802.15.4,as can be depicted from Fig. 5c, non-ACK mode in 2.4GHz band proves itself best amongst rest of all flavors.
Reason is very obvious that at greater spectrum, higher will be throughput. non-ACK mode performs better as there is
no control overhead and have high chances of getting clear channel with respect to ACK mode. 915MHz band stands
second however, in this band also, non-ACK mode performs better. However, there is left no big dierence between
non-ACK and ACK mode at 858MHzband that has lowest throughput amongst all.
7. Conclusion
In this study, we focused on contention access period of IEEE 802.15.4. ACK mode and non-ACK mode as
prescribed in standard text are analyzed analytically and than simulated to verify impact of ACK packet on algorithm
using all three frequency assignments. Our study suggests that for critical health monitoring applications where
sensors may or may not be implanted in body, ACK mode show its worth due to low packet drop ration. However, for
streaming data or applications that do not need ACK of every transmitted packet, non-ACK mode performs better due
to its higher throughput, good put and lower delay, control load leading to longer battery life. Using higher frequency
assignments may apparently lead to excessive power usage however, higher the frequency is, lower is the packet drop
ratio, delay and easy channel access.
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