Priority Toning Strategy for Fast Emergency Notification in IEEE 802.15.4 LR-WPAN Tae Hyun KimO, Doo Hwan Lee, Jae-hyun Ahn, and Sunghyun Choi School of Electrical & Computer Engineering, Seoul National University
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[email protected] Abstract IEEE 802.15.4 Low Rate Wireless Personal Area Network (LR-WPAN) is developed for emerging applications for industrial automation, home networking, wireless sensor networks, etc. In this paper, we propose a new scheme to enhance IEEE 802.15.4 in order to enable the fast delivery of packets, which announce the emergent situations or events of the network. With the parameter tuning of the legacy IEEE 802.15.4 slotted Carrier-Sense Multiple Access with Collision Avoidance (CSMA-CA), we enhance the current CSMA-CA algorithm by adopting a Priority Toning strategy. Through the simulations, we validate that our scheme is very efficient in the context of energy consumption as well as delay without system throughput degradation.
1. Introduction Recently, a lot of efforts have been made in the community to develop networking protocols for practical applications of low rate wireless networks. As a result, IEEE 802.15.4 for Low Rate Wireless Personal Area Networks (LR-WPANs) [1] was published in December 2003. As its name indicates, this standard specification defines both physical and MAC layer for the low-rate wireless network, which can be used for home automation, cable replacement, wireless sensor networks, etc. However, in the view of wireless sensor networks, it has several limitations because its application coverage is quite large, and it is not optimized for one purpose. In this paper, we consider the MAC layer of an application of wireless sensor networks, which monitor the environment in order to detect an indication of emergency. To do this job effectively, the packet indicating the emergency should be conveyed to the sink node as fast as possible. We select IEEE 802.15.4 as the base MAC protocol since it is the only standardization related to wireless sensor networks today. Actually, remarkable work has been done already to reduce the latency in wireless sensor networks. Wei Ye et al. improve their original SMAC scheme in [6] with a coordinated adaptive sleeping method to lessen the latency [2]. Nevertheless, in [3], the authors point out Data Forward Interruption (DFI) problem in existing wireless sensor MAC protocols including SMAC and that proposed in [4]. DFI problem is the phenomenon that packet delivery experiences large amount of delay due to the sleep period originally designed for energy saving. Even if DFI is solved by certain mechanisms such as DMAC proposed in [3], there still exists latency caused by This research was supported by University IT Research Center Project.
contentions between nodes having emergent packets and normal packets to grab the channel, especially, around the sink node. Commonly, in wireless sensor networks, the closer packet approaches to the sink node, the harder a device would get a chance to transmit a packet to the next hop. Note that packet relay is usually delayed by heavy contentions on the way to the sink node even though total traffic in the network is low. However, existing MAC protocols assume a very low traffic load at every node across the network. To overcome this latency rooted from medium access contention, prioritized channel access is needed. To our best knowledge, prioritized channel access scheme has hardly ever been discussed yet for the latency reduction of important packet transmission. In [5], the authors assert the demands of prioritized packet transmission, but the way of prioritizing is provided by multi-path forwarding to increase reaching probability. To minimize the end-to-end delivery delay, this kind of approach is not enough to solve the fundamental problem. Certainly, it should be taken into account at the MAC layer. We propose a novel scheme for IEEE 802.15.4 to provide a delivery of the efficient, fast emergency notification. The scheme is composed of two parts. One is a prioritized CSMA/CA, which basically differentiate the access parameters of the legacy IEEE 802.15.4. It supports differentiated channel access to shorten the latency of high priority packet transmission. Another is “Priority Toning” strategy which decreases probability of burst collisions inherited from IEEE 802.15.4 slotted CSMA-CA while higher priority packet acquire a privilege to be transmitted before other packets. Through the computer simulation, our
scheme works better than the legacy one in the context of important packet’s delivery latency as well as energy efficiency of each node without any loss of system throughput. The rest of the paper is organized as follows. In Section 2, we give the overview of IEEE 802.15.4 with its problems. In Section 3, our new scheme Prioritized CSMA/CA with Priority Toning is presented. We then evaluate the performance of the proposed schemes in Section 4, and finally conclude the paper in Section 5. 2. Overview of IEEE 802.15.4 Basically, IEEE 802.15.4 has two different operating modes. One is beacon-enabled mode, and the other is nonbeacon enabled mode. The non-beacon enabled mode is designed to support mesh networking. However, it yields longer delay and more energy consumption since data exchange is done by indirect transmissions when devices operate in the non-beacon enabled mode. In fact, it gives flexibility of network topology at the cost of degraded performance. Additionally, implementers recommend that data relaying devices in a mesh network would be better to be AC powered. Therefore, we select the beacon enabled mode as a basic operating mode in consideration. In the beacon enabled mode, devices are connected to each other with certain relationship such that coordinator and child device by exchanging association message and its response. The coordinator is in charge of transmitting beacon frames 1 , and supports indirect transmission between two devices attached to itself as children, and relaying frames from children to its coordinator. Meanwhile, a device has no charge for other coordinators or devices. It just sends frames to its coordinator to convey them to the final destination. Note that this feature makes a tree topology of whole network devices.
Fig. 1. Superframe structure in beacon-enabled mode The superframe structure of the beacon enabled mode is depicted in Fig. 1. All channel access operation should be done according to the time slot basis. Superframe is composed of beacon, contention access period (CAP), contention free period (CFP), and inactive period. The length of a superframe is controlled by superframe order (SO) while the beacon interval (BI) is determined by beacon order (BO) as illustrated in Fig. 1. Because of its poor performance [7], Guaranteed Time Slot (GTS) mechanism for CFP is not 1 We use the term ‘frame’ to indicate that it is the packet managed at MAC layer.
considered in our scheme. Therefore, we assume that an active period comprises a beacon and a CAP only. Beacons are transmitted periodically at the exact time after BI symbols from the preceding beacon transmission. CSMA-CA scheme in the beacon enabled mode is designed to save as much energy as possible when the traffic load of the operating network is low. When a device has a frame to send, it initiates the random backoff mechanism, which works as follows. First, it chooses a random number from 0 to 2^BE-1, where BE is backoff exponent, and if macRxOnWhenIdle in MAC PAN information base (PIB) is set, the RF radio is turned on to observe the channel to check whether the frame for itself is coming or not. Otherwise a device is able to change its state idle to save the energy during the random backoff. At the end of the random backoff the device calculates the time needed for transmission and two times of clear channel accesses (CCA). If this transmission cannot be completed in the current superframe, the device defers its CCAs and transmission to the subsequent superframe. The last backoff in Fig. 2 shows this situation. Once CCAs are performed and each of them reports that channel is idle, the device decides to transmit a frame into the channel. If the channel is determined busy, BE and the number of backoffs (NB) are increased by one and then, a new backoff count is drawn randomly. The maximum BE value is defined by aMaxBE as a MAC layer constant.
Fig. 2. Legacy IEEE 802.15.4 CSMA/CA in beacon enabled mode Frame transmissions of a coordinator in IEEE 802.15.4 could be done directly or indirectly according to the macRxOnWhenIdle of each device. During the association process, a coordinator is notified the macRxOnWhenIdle of an association requesting device by the Capability Information bitmap. By storing each device’s macRxOnWhenIdle, a coordinator can determine a transmission method for each device. In other words, if the macRxOnWhenIdle of a target child device is set, a coordinator chooses a direct transmission for the device. Reversely, if a macRxOnWhenIdle is clear, it transmits frames by an indirect manner. IEEE 802.15.4 defines one more option for its flexibility. That is, a device, which transmits a frame, can decide whether acknowledgement (ACK) frame is expected or not per frame basis. Without ACK frame, a device can save more energy, but transmission reliability is compromised. For example, if frequent reports to the sink node are scheduled, ACK frames can be skipped to lengthen the lifetime of the device. To save energy, the standard offers battery life extension mode (BLE mode). Once it is activated for the beacon
enabled mode, backoff counting down and channel access shall be done during the first 6 backoff periods only. If a device acquires the channel during this period, it can transmit a frame as long as the length of active period. On the contrary, the other devices, which fail to grab the channel during 6 backoff periods, go to the sleeping mode to save energy. Using the BLE mode, IEEE 802.15.4 saves large amount of energy. However, the energy gain from this mode is achieved by suppressing its frame transmission relatively long time, and it results in more delay of each frame. However, in wireless sensor networks, the majority of frames flow to the same device, i.e., the sink node. Since traffic load of the network is not balanced well, it causes a large delay. That is, the BLE mode is a good alternative in networks with small depth, but it is likely to yield an additional long delay in large multihop networks. As mentioned in the previous section, IEEE 802.15.4 has many problems for the environment monitoring application. First, most devices except ones placed at the edge of the network should turn their radio on in active period to receive frames from child nodes; macRxOnWhenIdle should be set. Since the energy consumption for receiving a frame could be larger than that for transmission 2 [8], it is very critical in terms of device’s lifetime. Second, heavy contention and burst collisions are expected at the beginning of a superframe. All frames received from the upper layer during inactive period and deferred from the previous superframe contend to grab the channel at that time. In the case of deferred frames from the previous active period, burst collisions occur because they have already done the random backoff. Therefore, after beacon transmission is followed by two times of CCAs, all devices containing such a frame start transmission, which causes a collision. 3. Proposed Scheme In this section, the proposed prioritized CSMA-CA and Priority Toning strategy are presented. Both schemes cooperate to minimize the frame delay of emergent packets while conserving the energy. Basically, we classify frames according to the priority, namely, emergent and normal frames. In the current IEEE 802.15.4, the scheduling algorithm for the beacon transmission time is required for multihop communication in the beacon-enabled mode, which is done by the network layer. We in this paper limit ourselves into the one-hop communication’s performance only. 3.1. Prioritized CSMA-CA There are 2 types of access parameters, which can be exploited for prioritization: BE for each NB and the number of CCAs before transmission. The specification defines that the number of CCAs is 2 for the purpose of protecting ACK frame, and giving enough time for a receiving device to process the frame. When an ACK frame is required by the transmitter, the receiver should send it after tACK time which 2
Which is very different from other types of wireless devices
varies from 14 to 34 symbols (one backoff period is 20 symbols). Therefore, one time of CCA could yield a collision between a newly transmitting frame and the ACK. However, one time of CCA can give a strong priority to obtain the channel. If ACK collision is not so severe, it should be employed for high priority transmission. To give a strong priority for the emergent frame, BE is set to a proper constant regardless of NB. This could yield frequent CCAs when the number of contending devices is more than the constant BE value. However, emergency in the monitoring environment is not likely to happen very frequently. Therefore, we assume that the constant BE value does not cause unnecessarily frequent CCAs. In multihop communication, it is obvious that the farther the frame comes from, the longer the frame’s experienced delay is. From this fact, we are able to measure the priority among emergent frames. An emergent frame with the earliest generation time could be considered the most urgent frame to transmit. This can be easily implemented with generated timestamp and sorting function. 3.2. Priority Toning Strategy This scheme is aimed to avoid burst collisions at the beginning of a superframe, and guarantee the transmission of emergent frames. Previous toning schemes [9][10] are not proper for wireless sensor networks in terms of energy consumption since they are designed to use it frequently for priority ordering. To reduce the energy consumption, we should use it as less frequently as we can. In Priority Toning scheme, a tone signal is transmitted by devices, which have emergent frames right before the beacon transmission. The coordinator device which transmits a beacon has to wake up one backoff period earlier and listen to the channel to observe whether a tone signal is transmitted by its child devices. If it detects a tone signal, which could be from multiple devices, an emergency notification is conveyed in the beacon frame, which makes all other devices having no emergent frame defer their transmission to some amount of time in order not to contend with the devices with emergent frames. Unlike the standard CSMA-CA in the beacon-enabled mode, a deferred transmission is restarted not from the CCAs, but a random backoff again. This is to avoid a burst collision as pointed out in the previous section. This scheme can be adopted only for the PAN in which there is an inactive period in every beacon interval. P r io r it y T o n in g
beacon
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D e fe r m e n t o f C S M A /C A b y p r io r ity to n in g
Fig. 3. Priority Toning strategy
RX power
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TX power
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0.036 mW
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0.0018 mW
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31.32 mW
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Normal, BkOffRange=4 Normal, BkOffRange=8 Normal, BkOffRange=12 Normal, BkOffRange=16 Emergent, BkOffRange=4 Emergent, BkOffRange=8 Emergent, BkOffRange=12 Emergent, BkOffRange=16
0.45
Delay (sec)
We perform the computer simulation to validate our scheme using ns-2 simulator. The IEEE 802.15.4 radio characteristics [8] are summarized in Table 1. When device is in idle mode, it turns off the radio. In sleeping mode, device is allowed to turn off some processing units to save more energy. Note that the receiving power is larger than transmitting power. An ACK frame exchange is always activated to detect collisions. In terms of the network topology, devices are placed 5m apart from their children or coordinator. All devices except a coordinator generate UDP/CBR traffic with 50-byte frames. All simulations are run independently during 1000 seconds and their results are averaged using 10 different seeds. BO and SO are selected to 5 and 1 which are drawn for realistic environment monitoring networks; beacon interval is 0.49152 seconds and active period is 0.03072 seconds, which results 6.25% of duty cycle. Lastly, the emergent frames represent 5% of total traffic. First, appropriate values for the backoff range 3 and the length of deferment when emergency is notified by piggybacked beacon is needed. Fig. 4 shows that the delay of emergent frames is reduced as the deferment length is increased, but it yields longer delay for normal frames. Backoff ranges 4 and 8 with the deferment lengths 4 and 8 respectively are chosen for the following simulations. The number of devices in the one-hop range affects these values. In Fig. 5, backoff range 8 with Priority Toning performs closely to backoff range 4 with Priority Toning in terms of the emergent frame’s delay. In addition, after the saturation of the system (over 4.3 kb/s), the emergent frame still could be relayed to a coordinator. Generally, the fact that the neighbor nodes of the sink node are heavily loaded with the frames to relay is acceptable. Therefore, the proposed scheme guarantees the transmission of the emergent frame in a reasonable time even if the device is in a saturation situation. Through Fig. 6 and 7, it is confirmed that the delay of the normal frames is also shorter than the one in the legacy. Additionally, Fig. 7 shows that the Priority Toning encourages the reduction of the normal frame’s delay due to the smaller number of devices to contend with.
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Fig. 4. Length of deferment for Priority Toning and frame delay 1.5 Legacy BkOffRange=4, PTOff BkOffRange=4, PTOn
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Fig. 6. Delay of normal frames in large scale We use the term ‘backoff range’ (BkOffRange in figures) as the range to draw a random backoff counter. Note that it is slightly different from the BE.
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4. Performance Evaluation
Table 1. Radio parameters [8] Transmission rate
Delay (sec)
In addition, this scheme helps additional energy saving even though it needs a tone signal transmission for child devices and listening to the channel for every one backoff period per beacon interval for coordinator devices. When a tone signal is transmitted and piggybacked beacon is received by all child devices, child devices having no emergent frame can turn their radio off during the deferment to guarantee the transmission of emergent frames of others.
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5. Conclusion
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Fig. 7. Delay of normal frames under low traffic load Priority Toning Sleep Idle Tx Rx
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Fig. 8 Power consumption with macRxOnWhenIdle set 4000
System Throughput (b/s)
We have proposed a new scheme for the applications demanding fast emergency notification in IEEE 802.15.4based LR-WPANs. To reduce the delay of emergent frames, we propose prioritized CSMA-CA and Priority Toning strategy. In prioritized CSMA-CA, we investigate the parameters for prioritizing and find proper values for them through the simulation. Priority Toning scheme is proposed to avoid burst collisions inherited from IEEE 802.15.4’s CSMA-CA in the beacon enabled mode and to guarantee the emergent frame’s delivery. From ns-2 simulations, it is shown that our scheme outperforms the legacy one in terms of energy consumption as well as delay. It is because the proposed Priority Toning strategy allows devices to take more time for which they can turn their radio off while emergent frames are being transmitted by other devices.
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Fig. 9. System throughput The power consumption under 2 kb/s traffic load is depicted in Fig. 8. Using Priority Toning strategy helps each device take more time in idle state. As a result, the proposed scheme is more efficient than the legacy one in terms of energy as well as delay; under the given parameters, 17% of energy consumption is reduced. Moreover, it is shown that additional energy for Priority Toning is negligible. Note that when backoff range is 8, devices consume less energy, especially Rx energy, than backoff range of 4. Frequent use of CCAs when backoff range is 4 is the main reason for it. Finally, it is found that this scheme does not cause any throughput degradation in Fig. 9.
[1] 802.15.4-2003 IEEE Standard for Information Technology-Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) specifications for Low Rate Wireless Personal Area Networks (LRWPANS), 2003. [2] W. Ye, J. Heidemann, and D. Estrin, “Medium Access Control with Coordinated Adaptive Sleeping for Wireless Sensor Networks,” IEEE/ACM Transactions on Networking, June 2004. [3] G. Lu, B. Krishnamachari, and C. R. Raghavendra, “An Adaptive Energy-Efficient and Low-Latency MAC for Data Gathering in Wireless Sensor Networks,” in Proc. IEEE IPDPS, April 2004. [4] T. van Dam and K. Langendoen, “An adaptive energyefficient MAC protocol for wireless sensor networks,” in Proc. ACM SenSys, Nov. 2003. [5] S. Bhatnagar, B. Deb, and B. Nath, "Service Differentiation in Sensor Network,” in Proc. Fourth International Symposium on Wireless Personal Multimedia Communications, September 2001. [6] W. Ye, J. Heidemann, and D. Estrin, “An energyefficient MAC protocol for wireless sensor networks,” in Proc. IEEE INFOCOM, pp. 1567–1576, June 2002. [7] J. Zheng and M. J. Lee, "A Comprehensive Performance Study of IEEE 802.15.4," IEEE Press Book, 2004. [8] Data sheet, CC2420 2.4GHz IEEE 802.154/Zigbee RF Transceiver, available online at http://www.chipcon. com/files/CC2420_Data_Sheet_1_2.pdf. [9] V. Kanodia et al., “Distributed Priority Scheduling and Medium Access in Ad Hoc Networks,” Wireless Networks, vol. 8, 455-466, Sept. 2002. [10] Xue Yang and Nitin H. Vaidya, "Priority Scheduling in Wireless Ad Hoc Networks," in Proc. ACM MobiHoc, pp. 71-79, June 2002.
