Day and Night Access (DNA) Scheme for Low Power IEEE 802.15.4 WPANs

Shiann-Tsong Sheu†, Yun-Yen Shih* and Lu-Wei Chen† †Department of Communication Engineering, National Central University, Taiwan, R.O.C.

*Department of Electrical Engineering, Tamkang University, Taiwan, R.O.C. E-mail: †stsheu@ce.ncu.edu.tw

Abstract— In the IEEE 802.15.4 MAC protocol, peer-to-peer transmission between a pair of neighboring nodes is not easy due to nodes frequently staying at sleep-ing mode. However, in star topology, data exchanging be-tween two non-hidden nodes are relayed by a network co-ordinator which spends double power consumptions for one valid frame transmission from sender to destination. This paper proposes a day and night access (DNA) scheme to enhance transmission efficiency and to minimize power consumption simultaneously. Simulation results demon-strate that DNA scheme outperforms IEEE 802.15.4 MAC standard in terms of data update success rate, data update latency and energy dissipation per update.

I. INTRODUCTION

Recently, many attractive sensor-based applications are developed over growingly matured wireless networks [1]. Users expect the wireless sensor network (WSN) to have the features of low power consumption, low data rate, low cost, short transmission distance, moderate response time, good reliability and survivability. These features become the major challenges on designing WSNs. Typically, the WSN consists of one sink and a number of source nodes with sensors. Source sensor nodes combining wireless transceiver and various sensors are able to execute specific missions such as ecological detecting, health monitoring, digital home automation [2], and so on. Sensor nodes are allocated in the sensing field randomly or artificially for retrieving useful environmental information and return re-sults via wireless communication to the sink(s).

The IEEE 802.15.4 low-rate wireless personal area net-work (LR-WPAN) [3-4] is one of candidates of serving as WSNs. This standard specifies two physical (PHY) layers: an 868/915 MHz direct sequence spread spectrum (DSSS) PHY and a 2450 MHz DSSS PHY. The 2450 MHz PHY supports data rate of 250 kb/s and the 868/915 MHz PHY supports data rates of 20 kb/s and 40 kb/s respectively. De-pending on the transmitting power level, the transmission distance varies from 10 to 100 meters.

Regarding to the computation capability of devices, the IEEE 802.15.4 devices are categorized as full-function de-vices (FFD) and reduced-function devices (RFD). In star mode, the first active FFD will establish the WPAN and naturally becomes the WPAN coordinator. The coordinator periodically broadcasts beacon frames not only to inform

This work was supported by National Science Council under con-tract NSC 94-2213-E-032-006.

neighbor devices its existence but also to synchronize tim-ings of periods of each superframe. Devices belonging to a WPAN can be either the FFD or RFD and they are denoted as ‘nodes’ in this paper. The coordinator and nodes have two kinds of transmission manners. One is to contend channel in the contention access period (CAP), and the other is to access channel in a collision-free manner during a contention-free period (CFP).

In CAP, data exchanges between any pair of nodes is based on a modified CSMA/CA protocol, which is evolved from the IEEE 802.11 DCF protocol and is particularly suitable for low-power WPANs. When a node desires to transmit data, it first selects a random backoff value from the initial contention window (CW) and then enters the backoff process. Differing from standard CSMA/CA, this node actually starts sensing channel only when the backoff countdown process is completed, and the carrier sensing period takes two consecutive unit backoff periods (UBPs), where UBP is the basic time unit in the backoff process. In other words, a node transmits frames only when the chan-nel is detected idle for two consecutive UBPs after random backoff. Contrarily, if the channel is busy during carrier sensing period, it stops listening and then reselects a ran-dom backoff value from doubled CW for the next attempt. Then the node turns off its transceiver and starts to count-down the new chosen backoff time. The contention scheme in star or peer-to-peer topology suffers from hid-den-node/terminal problems [6-7] in the IEEE 802.15.4 standard protocol. Opposite to the CAP, the CFP is com-posed of several guaranteed time slots (GTSs) for providing nodes to have their own transmission periods. There are at most seven FFD nodes to demand the coordinator to allot GTS. The GTS allocation information is attached in beacon frame. For each GTS period, only the owner can trans-mit/receive. By eliminating contention overheads, CFP can provide a better channel utilization and shorter transmission latency than CAP.

WPAN coordinator performs synchronization by using the superframe structure. A superframe is bounded by bea-con frames, and has an active portion and an inactive por-tion. Devices could turn off their transceivers to save power during inactive period. Active portion consists of CAP and CFP, and begins at the beacon broadcasting. System pa-rameter superframe_order (SO) and beacon_order (BO) respectively decides the interval of active period and the interval of beacon frames (which equals to superframe in-terval). Both active period and superframe interval consist

1-4244-0063-5/06/$20.00 ©2006 IEEE

of 16 equal size time slots. The intervals of an active period

and an inactive period are equal to 48 2SO UBPs and

48 (2BO-2SO) UBPs respectively. The values of SO and BO,

denoted as SO and BO respectively, are set as in the range

[0, 14]. Notably, the BO≥SO and the inactive period does not exist when BO=SO. Values of parameters SO and BO are carried on the beacon frame and they will affect the system performance [8-9].

In WSN, any node could be sink or source node. When a sink desires to retrieve data from sources, data are required to pass through the coordinator. As the coordinator receives the data destining to another node, it first buffers this frame and then it utilizes the beacon frame of the next superframe to notify the destination node it has to wake up to retrieve data from coordinator. After then, notified nodes will be awake to send Data_Request control frame to coordinator in contention manner and to wait the data replied from co-ordinator (also in contention manner) later. Such data flow is denoted as indirect transmission in standard. Obviously, the channel bandwidth is consumed twice per data frame in active period.

To accomplish transmission efficiency improvement and power consumption minimization, this paper proposes a simple direct link transmission solution named as Day and Night Access (DNA) scheme. DNA scheme schedules spe-cific pairs of nodes to exchange data directly and schedules source nodes to broadcast data to its sinks during inactive periods. It gets rid of blind contentions in crowded active periods and it reduces transmission times by direct trans-mission and data broadcasting.

The rest of the paper is organized as follows. Section II introduces the concept of DNA scheme. Section III de-scribes simulation model and results for evaluating pro-posed scheme. Conclusions are given in Section IV.

II. DAY AND NIGHT ACCESS (DNA) SCHEME

A. Overview

The proposed DNA scheme amends the standard to be adaptable to monitoring applications over WSNs, in which, data information sensed from actuator at source node is re-ported to sink node(s) periodically. In other words, any sink node can request the designated data source node(s) to de-liver data to it. For simplicity, the data source node and sink node are denoted as ‘source’ and ‘sink’. The DNA scheme peculates inactive periods for dir ect links (i.e. source-to-sink), as the link from source B to sink A illus-trated in Figure 1, to enhance update efficiency and to alle-viate unnecessary contentions. When source B is visible to sink A and sink A desires to obtain the data from source B, the DNA scheme will allot appropriately exclusive subpe-riod(s) of inactive period(s) for sink A to receive data

Figure 1. Illustrating the direct and indirect links in WPAN where sink A receives data from source B and source D via direct and indirect link respectively.

from source B in peer-to-peer mode. Contrarily, when hid-den-node situation exists between source, say D, and sink A, data frame must be first transmitted from source D to coor-dinator (node C) during active period and then the data frame could be forwarded to sink A during inactive periods in collision-free manner. (The reason why the former and latter transmissions of indirect link are respectively ar-ranged at active period and inactive period will be ex-plained in subsection D). Due to the data is transmitted once during active period, the transmission efficiency of indirect link in DNA scheme is better than conventional IEEE 802.15.4 protocol.

In DNA scheme, active and inactive periods in super-frames appear alternatively just like day time and night time respectively in our daily life. This is the reason this scheme is named as Day and Night Access scheme. Notably, standard command frames and additional command frames used in DNA scheme are still transmitted in contention manner at day time, and only data frames of direct links or specified indirect links are sent at night time.

B. Command Frames

There are five kinds of command frames used in DNA scheme: Beacon, ACK, Query, LSetup (Link Setup) and LStop (Link Stop) frames. The Beacon and ACK frames are the same as standard, but the Query, LSetup and LStop frames are additional command frames that are needed in DNA scheme for establishing and terminating the di-rect/indirect link. The Query command frame sent from sink is used to request source to set up a link back to the sink. It contains three major fields: source_address field, sink_address field and data_update_rate field. The source_address is the address of source node which has the sensed data, sink_address is the address of sink node which sends the Query command frame, and the data_update_rate is the desired number of data updates per superframe. The Confirm field in LSetup command frame includes the indi-cation that the request is accepted or rejected and the agreed data update rate if any. The LStop command frame

is simply used to notify the sink and/or source the link ter-mination request.

C. Timing Diagrams

Data link set up as direct link or indirect link is depend-ing on the hidden relationship between the nodes of link. Figure 2 illustrates the timing diagrams of DNA scheme establishing the direct and indirect links between source and sink. Data transmissions over direct links are always arranged at night time, and data transmissions over indirect links could be arranged in two phases: transmissions from sources to coordinator at day time and transmissions from coordinator to sinks at night time.

In Figure 1, as sink A desires to monitor the sensed data of source node B, it triggers the reporting process by issu-ing the Query command frame with requested data update rate to source node B. This is the first step of link estab-lishment as shown in Figure 2(a). As mentioned before, the Query frame will be buffered in the coordinator C and then coordinator adds the MAC address of source B into address pending list in Beacon frame. In the next superframe, source B will pull back the Query command frame from coordinator by issuing the Data_Request command frame. So, in the second step, the indirect transmission is per-formed to pass this command. After then, source B sends the LSetup command frame with the agreed data update rate for creating a reporting link back to sink A. This is the third step. At this stage, if sink A detects the LSetup frame successfully, then it directly replies the ACK frame. The ACK frame is going to reach both coordinator C and source B and direct link is established successfully (see step 4). The transmission schedule for night time is planned by the coordinator depending on data-update-rate agreement. Once the resource in night time is exclusively allocated and declared by the Beacon frame, source B will send data to sink A at night time (see step 5). Notably, the fourth step determines whether direct link is possible or not. In other words, LSetup frame is not only used to setup data link but also used for investigation of hidden-node relationship be-tween sink and source. If sink A wants data information of source D, following link establishment steps, it will fail to receive the LSetup command frame sent from source D at the fourth step due to hidden-node situation. In this case, coordinator also fails to receive ACK command frame sent from sink A. Thus, the coordinator should automatically send the ACK frame to source D and generate the LSetup frame for sink A after timeout. Sink A will reply the ACK frame when it receives the LSetup frame from coordinator. The detailed timing diagram is illustrated in Figure 2(b). We can see that, no matter data link is setup as direct link or indirect link, the DNA scheme always completes the link establishment within two superframes if frame transmis-sions are successful.

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+1

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Figure 2. Timing diagrams of establishing direct and indi-rect links in DNA scheme.

After sending the Query command frame, the sink should keep awake to check whether it can hear the LSetup frame or not. If yes, the sink replies an ACK frame to the source immediately. As coordinator gets this ACK frame, it perceives that direct link between source and sink is possi-ble. Otherwise, coordinator relays LSetup frame to sink to setup an indirect link when ACK timeout occurs.

D. Resource Allocation Strategy (RAS)

In the direct link case, if the source already has any di-rect link with other sink(s), the coordinator should assign the demanding sink to receive data from existing subpe-

riods that are previously allocated to source. In this case, source could broadcast sensing data to update sinks to avoid bandwidth exhaustion. By resource sharing strategy, the wireless channel capacity can be maximized. On the other hand, in indirect link case, we need to cope with two separate links: one link from source to coordinator (denoted as front-link) and the other link from coordinator to sink (denoted as rear-link). The transmissions of front-link and rear-link could be arranged at either day time or night time because coordinator is not hidden node to sink and source. Similarly, if the source already has any direct link with other sink at night time, the coordinator should reuse it as night-time front-link to receive data without expensing ex-tra resources; otherwise, the coordinator lets source send data at day time as standard does for accommodating more direct links (at night time) in the future.

When a new direct link is going to be established from source, which already has a day-time front-link, to sink, this front-link could be migrated to night time due to there are two or more sinks sharing the data. Therefore, the coor-dinator and the newly correspondent sink must stay awake during the assigned subperiod(s) at night time for receiving data from this new night-time front-link. In a word, when coordinator detects that there are two or more recipients could share the data transmitted over front-link or rear-link at day time, it will migrate the link(s) into night-time to maximize channel utilization. Contrarily, the termination of data sharing might make the front-link and/or rear-link be migrated back to day time. In our opinion, for the new coming indirect link, if we assign the front-link to use the day time and the rear-link to use the night time, then the channel resource in active period is only consumed once as standard does. Based on this concept, the channel utiliza-tion and data update latency are definitely better than stan-dard protocol. No matter the existing link is either direct or indirect link, the number of exclusive subperiods in night time for existing links might be changed by the new request. Moreover, no interference will be caused in night time. As soon as link setup has done, the coordinator will generate a proper schedule to scatter traffic load of all night-time links. The algorithm of RAS is listed as follows:

Algorithm RAS (source, coordinator, sink) Begin If (direct link is suitable for the request) then {

If (source already has direct link) then { Sink joins to the existing direct link;

} Else { /* source does not have direct link yet */ Create direct link with appropriate subperiods; }

} Else { /* indirect link is necessary for request */ If (source already has direct link) then {

Use the existing direct link as front-link; } Else { /* source dose not have direct link yet */

If (source already has day-time front-link) { Share it and migrate it to night time;

} Else { /* setup the first front-link of source */ Let source send data to coordinator over front-link at day time; Create rear-link with appropriate subperiods at night time; } /* reduce forwarding delay */

} Adjust subperiods of the used links according to data-update-rate of request if necessary; End

III. SIMULTAION MODEL AND RESULTS

In this section, we compare the performance of DNA scheme with that of IEEE 802.15.4 protocol with GTS (Standard-GTS) and without GTS (Standard-noGTS). The simulation model assumes the WPAN contains one coordi-nator and twenty nodes. Nodes randomly distributed in network may play the roles of source, sink, or both, and any two of them have 41% probability unheard each other [9]. Parameters such as data payload mean length (L), beacon order (BO), and superframe order (SO) are set based on hypothetical monitoring applications. The data arrival rate of each source node is 5 frames per superframe and we

assume L=30 bytes, BO=6 (≈ one superframe per second), and SO=4 (one-fourth active period). Each simulation takes 1000 superframes and every result is derived by averaging 100 times simulations.

Four metrics Goodput, Update Success Rate, Delay per Update, and Energy Dissipation per Update are used to evaluate the improvements of DNA scheme. Goodput is the ratio of the amount of successfully transferred data pay-loads (in bytes) to the channel capacity (in bytes) during simulation period. Update Success Rate is the ratio of the number of data frames successfully received by sinks to the number of data frames produced by sources. Delay per Update is the mean latency of all successful updates. Delay of one update is measured from the time of data arriving at source to the time of data received by sink. Energy Dissipa-tion per Update is derived by dividing the summation of time intervals of transceivers staying on TX or RX state by the total number of successful updates. The value of Energy Dissipation per Update displays the average time summa-tion of all transceivers in the network working for one data transmission. For the sake of revealing the power saving capability of proposed DNA scheme, in our simulations, we adopt the power consumption parameters of the Chipcon CC2420 transceiver [10] to observe how long the battery longevity can be extended. Note that CC2420 RF trans-ceiver operates at 2.4GHz and it provides 250 Kb/s. Power consumptions in reception mode, transmission mode, and idle mode under 3V are 59.1mW, 52.2mA, and 0.06mW, respectively.

Figure 3 shows performance comparisons of the pro-posed DNA scheme, standard with GTS (Standard-GTS), and standard without GTS (Standard-noGTS) under differ-

ent number of sinks in WPAN. The number of sinks is con-sidered from 1 to 10 in simulations and there are 5 corre-spondent source nodes of sink randomly selected from the other 19 nodes. In the simulation, we assume that the buffer size of each node is 50 (in frames), and the situation of dropping data frame may be either buffer overflow or the number of failed transmissions of one data frame reaching the maximum retry count (=3, in our simulations). Accord-ing to the parameters set in the simulation, the maximal goodput equals 1.2% when every node plays a source node (a node can be source and sink simultaneously) and the whole data transmissions are successful. We note that the maximal goodputs could be roughly derived by

λ×L×min(m×n,M)/R,

where denotes the data arrival rate, L denotes the mean frame length, m denotes the number of correspondent sources of each sink, n denotes the number of sinks, M in-dicates the maximum number of nodes in the network and R indicates the data rate. In this simulation, equals 5 frames per second, L equals 30 bytes, m equals 5, n is from 1 to 10 for simulating different situations, M equals 20 nodes and R equals 250 kbps while the band operating on 2.4GHz in IEEE 802.15.4. From the simulation results shown in Figure 3, we can easily find that the DNA scheme has the best performance on network goodput, update la-tency and power conservation. The standard IEEE 802.15.4 with GTS function (i.e. Standard-GTS) has the second good performance. Although it provides guaranteed periods for data transmissions, the network still wastes twice band-width for forwarding data frames via the coordinator. The standard protocol without GTS (i.e. Standard-noGTS) ob-tains the worst performance because it suffers from the severe competitions and collisions caused by hidden-node situation [7]. Note that the curve of Energy Dissipation per Update of DNA scheme diminishes as the number of sinks increasing, as shown in Figure 3(d). It is because that more sinks requiring information update from the same source(s), the enhancement of RAS becomes more obvious. From simulation results, we conclude that the standard protocol without GTS is only suitable for serving applications with light traffic load.

IV. CONCLUSIONS

This paper proposed the Day and Night Access (DNA) scheme which was designed to provide the direct link be-tween two neighboring nodes in star-based IEEE 802.15.4 networks. The concept of DNA scheme is to move data exchanges between source(s) and sink from active period to inactive period. By stealing inactive period, DNA scheme can significantly increase the transmission efficiency, minimize the power consumption, and decrease the degree of contention as well as update latency.

REFERENCES

[1] B. Sinopoli, C. Sharp, L. Schenato, S. Schaffert and S.S. Sas-try, “Distributed control applications within sensor net-works,” in Proceedings of IEEE, Sensor Networks and Ap-plications, vol. 91, Issue 8, pp. 1235-1246, Aug. 2003.

[2] J.A. Gutierrez and D.B. Durocher, “On the Use of IEEE 802.15.4 to Enable Wireless Sensor Networks in Building Automation,” in Proceedings of Pulp and Paper Industry Technical Conference, Jun., 2005.

[3] IEEE 802 Working Group, “Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low Rate Wireless Personal Area Networks (LR-WPANs),” ANSI/IEEE 802.15.4, Oct. 2003.

[4] J. Zheng and M.J. Lee, “Will IEEE 802.15.4 Make Ubiqui-tous Networking a Reality? A Discussion on a Potential Low Power, Low Bit Rate Standard”, IEEE Comm. Magazine, vol. 42, pp. 140-146, Jun. 2004.

[5] Hanada E, Hoshino Y, Kudou T., “Safe Introduction of In-hospital Wireless LAN,” in Proceedings of International Medical Informatics 2004 (Medinfo. 2004), pp. 1426-1429, Sep. 2004.

[6] L.C. Huang, S.T. Sheu, Y.Y. Shih and Y.C. Cheng, “Grouping Strategy for Solving Hidden Node Problem in IEEE 802.15.4 LR-WPAN,” in Proc. of IEEE WICON’2005, Jul. 2005.

[7] S.T. Sheu and Y.Y. Shih, “P-Frozen Contention Strategy (PFCS) for Solving Collision Chain Problem in IEEE 802.15.4 WPANs,” in Proceedings of IEEE VTC’2006 Spring, May 2006.

[8] S.T. Sheu, Y.Y. Shih, and L.W. Chen, “An Adaptive Inter-leaving Access Scheme (IAS) for IEEE 802.15.4 WPANs,” in Proceedings of IEEE VTC’2005 Spring, May 2005.

[9] Y.C. Tseng, S.Y. Ni and E.Y. Shih, “Adaptive Approaches to Relieving Broadcast Storms in a Wireless Multihop Mobile Ad Hoc Network,” IEEE Trans. on Computers, vol. 52, no. 5, pp. 545-557, May 2003.

[10] Data Sheet for CC2420 2.4GHz IEEE 802.15.4/Zigbee RF Transceiver, available online at http://www.chipcon. com/files/CC2420_Data_Sheet_1_2.pdf.

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Figure. 3. Performance Comparisons among DNA scheme, Standard-GTS and Standard-noGTS.