IEEE 802.15.4 employs carrier sense multiple access with collision avoidance (CSMA/CA), which is known to have difficulty for supporting quality of service (QoS). In this paper, a new priority scheme called priority jamming (PJ) is proposed for service differentiation for IEEE 802.15.4. The main idea of the proposed scheme is deferring the low priority packet transmission for the high priority packet. Clear channel assessment of IEEE 802.15.4 is modified to support the proposed PJ. The efficiency of the proposed scheme is validated by comparing the delay, throughput, and energy-per-bit with those of standard CSMA/CA. Simulations results showed that PJ improves the delay and throughput simultaneously while maintaining marginal difference in energy efficiency

Shin, Soo Young

IEEE 802.15.4 employs carrier sense multiple access with collision avoidanceÂ (CSMA/CA), which is known to have difficulty for supporting quality of serviceÂ (QoS). In this paper, a new priority scheme called priority jamming (PJ) is proposedÂ for service differentiation for IEEE 802.15.4. The main idea of the proposed schemeÂ is deferring the low priority packet transmission for the high priority packet. ClearÂ channel assessment of IEEE 802.15.4 is modified to support the proposed PJ. The efficiencyÂ of the proposed scheme is validated by comparing the delay, throughput, andÂ energy-per-bit with those of standard CSMA/CA. Simulations results showed thatÂ PJ improves the delay and throughput simultaneously while maintaining marginalÂ difference in energy efficiency

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INT J COMPUT COMMUN, ISSN 1841-98368(1):127-135, February, 2013.A Novel Method for Service Differentiation in IEEE 802.15.4 :Priority JammingS.Y. ShinSoo Young ShinSchool of Electronic Engineering,Kumoh National Institute of Technology,Room 112, Digital Building, Yanghodong, Gumi, Gyeongbuk, KoreaE-mail: wdragon@kumoh.ac.krAbstract: IEEE 802.15.4 employs carrier sense multiple access with collision avoid-ance (CSMA/CA), which is known to have difficulty for supporting quality of service(QoS). In this paper, a new priority scheme called priority jamming (PJ) is proposedfor service differentiation for IEEE 802.15.4. The main idea of the proposed schemeis deferring the low priority packet transmission for the high priority packet. Clearchannel assessment of IEEE 802.15.4 is modified to support the proposed PJ. The ef-ficiency of the proposed scheme is validated by comparing the delay, throughput, andenergy-per-bit with those of standard CSMA/CA. Simulations results showed thatPJ improves the delay and throughput simultaneously while maintaining marginaldifference in energy efficiency.Keywords: CSMA/CA, QoS, priority jamming, service differentiation, IEEE802.15.41 IntroductionIEEE 802.15.4 is originally designed for low duty cycle and low rate applications such aswireless personal area network (PAN) [1]. However, it can be adopted for some delay-sensitiveapplications such as emergency detecting, intruder alarming, health care, and so on. For sup-porting these applications, QoS (quality of service) requirements need to be supported.IEEE 802.15.4 adopts carrier-sense multiple access with collision avoidance (CSMA/CA)mechanism. Therefore, every node has a statistically equal chance to use the wireless mediumand to transmit packets. Because of unpredictable and undeterministic nature of CSMA/CAcased by collisions and random backoff algorithm, to provide QoS in CSMA/CA is generallyknown to be difficult.Some works such as [2], [3], [4] studied the QoS support in IEEE 802.15.4. In [2], guaranteedtime slot (GTS) mechanism is exploited for the real-time service when enabling beacon mode.The GTS uses the contention-free period (CFP), which is optional feature in IEEE 802.15.4standard. In addition, if the available resources are not sufficient, i.e., the number of nodesrequires real-time service is large, the GTS may not be sufficient because of limited numberof GTS slots. In [3], differnet backoff exponent(BE) and contention window(CW) are used toprovide service differentiation in IEEE 802.15.4. However, nodes with different priorities can havethe same value of BE and/or CW based on slotted carrier-sense multiple-access with collisionavoidance (CSMA/CA) algorithm of IEEE 802.15.4, which leads to collisions among the nodeswith different priorities and increased delay for successful packet transmission. In [4], the frametailoring (FRT) and the priority toning (PRT) are proposed to support QoS. Using the FRT, i.e.padding zeroes, IEEE 802.15.4 nodes with high priority performs one clear channel assessment(CCA) only. However, the FRT causes a packet overhead and the acknowledgement packet ofthe normal priority node and the data packet of the high priority node can collide. To preventcollisions among high priority and normal priority packets, the priority toning (PRT) is usedto defer the normal priority packets by allocating some active portion of active period for theCopyright c© 2006-2013 by CCC Publications128 S.Y. ShinTable 1: Parameters of IEEE 802.15.4Parameters ValuesaMinBE 3 (default)aMaxBE 5 (default)CW 2 (default)macMaxCSMABackoffs 5 (default)Slot duration (UnitbackoffPeriod) 20 symbolsCCA duration of low priority packet 20 symbolsCCA duration of high priority packet 8 symbolsJamming signal duration 8 symbolshigh priority packet transmissions. However, if the number of the high priority packets is largeand network load is high, collisions may frequently occur among the high priority packets inthe allocated portion by the PRT. Moreover, those methods are designed for the beacon-enabledIEEE 802.15.4 network only.To support service differentiation, a novel method for supporting priority called priorityjamming (PJ) is proposed in this paper. The core idea of the PJ is deferring the normal prioritypacket transmissions using a jamming signal transmitted for high priority nodes who have packetsready to transmit. Because the proposed method exploit channel sensing part of IEEE 802.15.4,i.e., clear channel assessment (CCA), it can be applied both beacon and non-beacon enablednetwork. Although the proposed method considers slotted CSMA/CA in this paper, it can beused both slotted and unslotted versions of IEEE 802.15.4 CSMA/CA.The paper is organized asfollows. The suggested PJ is described in the next section. Section 3 evaluates the performanceof PJ and compares with standard CSMA/CA. Finally, we draw our conclusions in Section 4.2 Priority JammingIn this paper, non-beacon enabled network with slotted CSMA/CA is assumed. Let’s examinethe operation of standard CSMA/CA of IEEE 802.15.4 first. IEEE 802.15.4 CSMA/CA worksas follows. Three variables are maintained at each device for a channel access: NB, CW and BE.NB is the number of times the CSMA/CA backoffs while attempting the current transmission,and is reset to 0 for each new data transmission. CW is the contention window length, whichis reset to 2 either for a new data transmission or when the channel is found to be busy. BEis the backoff exponent, related to the backoff periods a device should wait before attemptingcarrier sensing. When a device needs to transmit, it delays for a random number of backoffperiods (up to 2BE − 1 periods) and then determines if the channel is clear. If the channel isbusy, the MAC increases both NB and BE by one, and resets CW to 2. If NB is less than orequal tomacMaxCSMABackoffs, the CSMA/CA delays for a random time again, otherwise itterminates with a failure. If the channel is assessed to be idle, it must ensure that the contentionwindow is expired before starting transmission. For this, the MAC sublayer first decrements CWby one. If CW is not equal to 0, it must go to another channel sensing step. Otherwise, it startstransmission on the boundary of the next slot period [1].In an IEEE 802.15.4 network, it is assumed that there are two data categories, namely, highand normal priority packets. The main idea of PJ is to provide high priority packets with greaterpossibility to access the channel compared to normal priority packets. Figure 1 describes theoperation of the proposed scheme and some parameters related to both PJ and standard MACA Novel Method for Service Differentiation in IEEE 802.15.4 : Priority Jamming 129BE=aMinBE, NB=0, CW=2Locate Backoff period boundaryDelay for random [0,2BE-1] unitBackoffPeriodsHigh priority packets?Ready for transmitPerform CCAChannel idle?CW=2, NB=NB+1, BE=min(BE+1, aMaxBE)Transmission failureCW=CW-1NB=macMaxCSMABackoffs? CW=0?Transmission successTransmit priority jamming signalYNNNNYYYFigure 1: Operation of Priority Jamming in IEEE 802.15.4130 S.Y. Shinoperation are summarized at Table 1. To transmit either a high or normal priority packet,nodes will contend to access channel using CSMA/CA of IEEE 802.15.4 as described earlier.To introduce PJ into IEEE 802.15.4, grey-colored portion is changed or modified compared tostandard CSMA/CA of IEEE 802.15.4 as shown in Figure 1. When a node is ready to transmithigh priority packet, it not only performs CCA to see whether the channel is busy or idle, butalso set aside some slot time to send a jamming signal to notify other nodes that a high prioritypacket is ready to be transmitted. In other words, a node with high priority packets, performsCCA during 8 symbol times as defined in IEEE 802.15.4 standard and transmits a jammingsignal. For jamming signal, any signal whose duration is less than (slot duration-8) symbols canbe used. In this paper, we used the preamble of IEEE 802.15.4 packet for the jamming signal.With PJ, a node with normal priority packets performs CCA for entire slot time (20 symboltime), not for just 8 symbol times. Because a node with normal priority packets listens entireslot time and then the channel will be assessed to be busy due to jamming signal by the nodewith high priority packets ready to be transmitted. Then, a node with normal priority packetswill defer its transmission and perform another random backoff procedure. By deferring thetransmissions of normal priority packets, the collision probability among nodes with high andnormal priority packets will be reduced compared to standard CSMA/CA. In other words, theeffective node numbers which contend for the channel will be reduced by adopting PJ.Note that if all the packets have the same priority level, i.e., under either 100% high priorityor normal priority traffic, the behavior of PJ nodes will be identical to standard CSMA/CA.Also, when nodes with standard CSMA/CA coexist with nodes using PJ, they are considered asnodes with high priority packets. This means that PJ guarantees backward compatibility withlegacy IEEE 802.15.4 standard.Data ACKChannelNode with normalpriorityBackoff BackoffCCA1CCA1CCA1CCA0TransmissionPriority Jamming iCCA0DataBackoffNode with highpriority(i+2)(i+1)(i-1)Figure 2: An Example of Priority Jamming in slotted CSMA/CA of IEEE 802.15.4Figure 2 shows an example of PJ operation with one node with a high priority packet andone node with normal priority packet. As illustrated, a node with normal priority packet finishesbackoff at (i−1)-th slot and performs the first CCA, CCA1. At this time, the channel is assessedto be idle and (CW = CW −1). At i-th slot, a node with high priority packet finishes its backoffand performs CCA1 with PJ. At the same time, the node with normal priority starts the secondCCA, CCA0. Now the channel is assessed to be busy because of the jamming signal transmittedA Novel Method for Service Differentiation in IEEE 802.15.4 : Priority Jamming 131by the node with high priority packet. Then, at (i+1)-th slot, another random backoff and CCA0are performed by the node with normal priority and the node with high priority, respectively.Finally, the node with high priority has a chance to transmit packet at (i+ 2)-th slot.3 Performance EvaluationTo evaluate the performance, we developed an OPNET simulation model of the slottedCSMA/CA and priority jamming of IEEE 802.15.4. Star topology network with one coordinatorand 20 end devices is used for the simulations as illustrated in Figure 3.Figure 3: Simulation scenario of IEEE 802.15.4 CSMA/CA with Priority Jamming132 S.Y. ShinAll end devices send data packets to Coordinator, which responds with corresponding ACKpackets. All end devices generate 102 bytes long packets based on exponential distribution withmean 1/λ. Two simulation parameters are inter-arrival time, i.e. λ, and ratio of high prioritypackets in the total traffic. The λ varies from 0.15 to 0.4, which means the traffic introducedto the network varies from 108.8 to 40.8 kbps. The ratio of high priority in the traffic, is oneof 10, 30, and 50 %. As the measure of performance, delay, throughput, and energy efficiencyare used in this paper. Then, the performances of PJ are compared to those of standard slottedCSMA/CA.0.15 0.2 0.25 0.3 0.35 0.40.010.0150.020.0250.030.0350.040.0450.05packet inter−arrival time (sec)delay (sec)stdhigh (50%)normal(50%)high(30%)normal(30%)high(10%)normal(10%)Figure 4: Delay vs. inter-arrival timeFigure 4 shows packet delays as the inter-arrival time increases. Here, (x %) in the legendof Figure 4 means that x percentage of the total traffic introduced to IEEE 802.15.4 network ishigh priority packets. The parallel lines to y-axis means the delay doesn’t converge. In otherwords, there are so many collisions that the packet delay increases. For example, with λ < 0.2,the delay of standard CSMA/CA doesn’t converge.However, when PJ is applied, the delays of high priority packets are bounded even for λ =0.15. This is because the collision probability of high priority packets decreases due to servicedifferentiation provided by PJ. For example, at λ = 0.2, the collision probabilities of the highand normal priority packets are 0.007, 0.026, 0.049, and 0.111, 0.089, 0.060 for 10, 30, 50 %cases, while that of the standard is 0.129. As the ratio of high priority packets increases, thecollision probabilities of high priority packets and those of normal priority packets increase anddecrease because the node numbers of high priority and normal priority increases and decreases,respectively. Note that the delays of normal priority packets with 10% high priority packages atA Novel Method for Service Differentiation in IEEE 802.15.4 : Priority Jamming 133λ = 0.18 are about 5 second.For λ ≥ 0.18, the delays of the high priority packets increases at the ratios of the highpriority packet increases. As λ decreases, the channel will be more crowded, even for the CCA ofthe high priority nodes. Then, channel could be assessed as "busy" and may cause many randomback-offs. 1As the packet inter-arrival time decreases, the channel occupation by packet transmissions ismore dominant than collisions. For larger λ, the channel is not frequently occupied, the delay ismore dependent on the collision probability. Hence, the delay of high priority packet decreasesas the ratio of high priority decreases.Note that the delay experienced by both high and normal priority packets in PJ is less thanthat of standard slotted CSMA/CA (std).0.15 0.2 0.25 0.3 0.35 0.4020406080100120packet inter−arrival time (sec)throughput (kbps)stdPJ total(50%)high (50%)normal(50%)PJ total(30%)high(30%)normal(30%)PJ total(10%)high(10%)normal(10%)Figure 5: Throughput vs. inter-arrival timeFig. 5 illustrates throughput of PJ with high priority, low priority, and total (sum of highand normal priority throughput) and compares to throughput of standard IEEE 802.15.4. Thethroughput for high priority packets are monotonically increases as the inter-arrival time de-creases. This is because the high priority packets really have priority to access the channel bypriority jamming. The total throughput with PJ outperforms that of standard IEEE 802.15.4due to decreased collision probability as mentioned earlier. Why the delay with the standardCSMA/CA diverges when λ = 0.18? The reason is that the throughput is 81.2 kbps, smallerthan the generated traffic, 90.7 kbps. Contrarily, throughput with PJ are 91.8 kbps (50% of high1In standard CSMA/CA, channel is already saturated at λ = 0.2.134 S.Y. Shinpriority), 92.6 kbps (30% of high priority), 92.9 kbps (10% of high priority), which are higer thatthe generated traffic, so delays with PJ are still bounded.0.15 0.2 0.25 0.3 0.35 0.400.511.522.533.5x 10−4inter arrival time (sec)Energy per bit (µJ/bit)stdhigh (50%)normal(50%)high(30%)normal(30%)high(10%)normal(10%)Figure 6: Energy-per-bit vs. inter-arrival timeFig. 6 compares the energy efficiency of PJ and standard CSMA/CA using the concept ofenergy consumption for transmitting a bit. The power consumption of idle, transmit and receivestates are set to Pidle=712 µW , Ptx= 31.32 mW , and Prx=35.28 mW , respectively [5]. BecausePJ scheme has energy consuming elements such as jamming signal (node with high prioritypackets) and longer CCA time (node with normal priority packets), the energy consumption ofPJ is higher than that of standard CSMA/CA. However, the µJ/bit differences are relativelysmall because throughput with PJ are increased and energy-per-bit values are the same order ofmagnitude.4 ConclusionIn this paper, a new service differentiation scheme which is called priority jamming (PJ) isproposed for IEEE 802.15.4. The main idea of the proposed priority jamming is deferring thetransmissions of normal priority packets using a jamming signal transmitted for high prioritynodes who have packets ready to transmit. By using the proposed scheme, the average delay ofhigh priority packets is reduced. In addition, the net throughput of IEEE 802.15.4 is increasedand delay of normal priority packets is decreased because of lowering the collision probabilitiesamong IEEE 802.15.4 nodes.A Novel Method for Service Differentiation in IEEE 802.15.4 : Priority Jamming 135Although PJ consumes more energy for a packet transmission because energy consumingfactors such as jamming signal transmission and long CCA time, PJ shows better performanceboth in the delay and the throughput. Therefore, the differences in PJ and standard IEEE802.15.4 are relatively small.The proposed algorithm can be used both beacon-enabled and non-beacon enabled mode ofIEEE 802.15.4 and guarantee backward compatibility with legacy IEEE 802.15.4 standard.By providing the service differentiation using PJ, this paper may contribute to enlarge thedelay-sensitive application area of IEEE 802.15.4 such as as emergent alarms and intruder de-tections.AcknowledgmentsThis research was supported by Basic Science Research Program through the National Re-search Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technol-ogy(2012R1A1A1009442).Bibliography[1] IEEE 802.15 TG, IEEE Std.802.15.4: IEEE Standard for Wireless Medium Access Control(MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Net-works (LR-WPANs), IEEE Standard, 2006[2] A. Koubaa, M. Alves, E. Tovar, Guaranteeing Real-Time Services for Industrial Wireless Sen-sor Networks With IEEE 802.15.4, IEEE Transactions on Industrial Electronics, 57(11):3868-2876, 2010.[3] M. J. Kim, C. H., Priority-Based Service-Differentiation Scheme for IEEE 802.15.4 Sen-sor Networks in Nonsaturation Environments, IEEE Transactions on Vehicular Technology,59(7):3524-3535, 2010.[4] T. H. Kim, S. Choi, Priority-Based Delay Mitigation for Event Monitoring IEEE 802.15.4LR-WPANs, IEEE Communications Letters, 7(3):213-215, 2006.[5] B. Bougard, F. Catthoor, D. Daly, A. Chandrakasan, W. Dehaene, Energy efficiency of theIEEE 802.15.4 standard in dense wireless microsensor networks: Modeling and improvementperspectives, IEEE Proceedings of DATE, 196-201, 2005

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