An Efficient Self Routing Scheme by Using Parent-Child Association ...

An Efficient Self Routing Scheme by Using Parent-Child Association for WSNs Yeon-Mo Yang and IlSoo Jeon Kumoh National Inst. of Tech. (KNIT), 1 Yangho, Gumi, Gyeongbuk, 730-701, Korea {yangym,isjeon}@kumoh.ac.kr

Abstract. The IEEE standard 802.15.4 for WPAN (Wireless Personal Ares Networks) shows promise of bringing ubiquitous sensor networking (USN) into reality, at least technically. In this paper, an Association based Self Routing (ASR) scheme of a IEEE 802.15.4 beacon-enabled mode is presented to minimize the control packet overhead and surplus beacon channel power during the routing discovery process in the data transmission for the upstream direction. It also maximizes channel utilization under low duty cycles or traffic congestion. The proposed scheme, ASR routes packets from sensor routers to sink based on the parent-child association relationships established by the IEEE 802.15.4 topology formation procedure. By using the ASR, the network resources for control packets are efficiently utilized and adaptively allocated throughout different duty cycles. Extensive simulation results using TOSSIM (TinyOS mote SIMulator) and experimental results with Mica-Z show that the ASR scheme outperforms well in comparison to the conventional Ad-hoc On-Demand Distance Vector (AODV) routing scheme over the parameters such as: average throughput, number of collisions, number of network commands, and delivery ratio under the upstream data transmission. Keywords: Wireless personal area networks (WPANs), ad-hoc routing protocol, quality-of-service (QoS), self routing, super frame, beacon-enabled networks.

1 Introduction According to well known literature, in the next few years, it is highly likely that energy-efficient Ubiquitous Sensor Networks (USNs) or Wireless Sensor Networks (WSNs) will be requested for use in a wide variety of hybrid system applications such as agricultural or environmental monitoring; caring for the elderly people with emergency and disaster response; 3D (three-dimensional) entertainment and networkbased toys; industrial facility sensing and actuator control; Location Based Service (LBS) including logistics and asset tracking; and building automation [1-14]. In these applications, many embedded devices operating on small limited-power batteries are deployed in an area communicating through wireless radio channels. Compared to existing Wireless Local area Networks (WLAN) which focus on providing relatively high-throughput, low-latency for fast file transfer, video conference, and L. Kang et al. (Eds.): ISICA 2008, LNCS 5370, pp. 785–794, 2008. © Springer-Verlag Berlin Heidelberg 2008

786

Y.-M. Yang and I. Jeon

real time multimedia applications, the required data rate for body area network (BAN) or wireless personal area network (WPAN) applications is expected to be only on the order of tens of kilo bits per seconds (kbps). Similarly, the required message latency may be on the order of 100ms or more [5-7]. The typical example of WPAN topology is shown in Fig. 1. However, the key concern in these applications is that of the extremely low power consumption, since it is often infeasible or undesirable to replace or recharge batteries for the devices on a regular basis due to their remote locations and cost of deploying. PAN ID 4 : First PAN Coordinator (First ZC )

6 5

: PAN Coordinator (ZC) 4

3

PAN ID 3

2

1

: Zigbee Router or End Device (ZR, ZED)

2

0 0

4 1 3

PAN ID 2 6

5

2 0 7 1

PAN ID 1 11

4

14

20

3

4

5

12 6

8 PAN Coordinator

0

0 9 3

1

3

1

7 2

2

PAN ID 5

22

4

10

Fig. 1. The system architecture of a WPAN

With respect to previous WPAN standards such as Bluetooth [1], the IEEE 802.15.4 (referred to as 802.14.4 hereinafter) standard is characterized by ultimately lower power consumption and relative lower cost. While, in line with the family of IEEE 802 standards, the standard focuses mainly on PHYsical (PHY) and Medium Access Control (MAC) layers, upper layers of the protocol such as routing (layer 3: L3) or transportation (layer 4: L4) layers are defined by the ZigBee Alliance [15-17], which also deals with the interoperability of WPAN devices from different manufacturers. The MAC protocol plays a significant role in determining the efficiency of the wireless channel bandwidth sharing and the energy cost communication. The relationship between 802.15.4 and ZigBee is similar to that between IEEE 802.11 and the WiFi alliance. Based on 802.15.4, ZigBee has proposed a specification for a suite of high-layer communication protocols [15-18]. 802.15.4 can operate at data rates of 250 kbit/s (less than 250 kbit/s) at 2.4GHz, 40 kbit/s at 915MHz (North America), and 20 kbit/s at 868 Mhz (Europe). While the data rate is much lower than Bluetooth, the energy consumption is at least one order of magnitude lower and recent low-cost commercial versions have demonstrated the viability of this technology for low duty cycle (less than 1%) sensor applications. Significantly, the latest generations of motes from UC Berkeley with Crossbow, i.e., Mica-Z is based on the primitive 802.15.4 compliant TI Chipcon CC2420 radio [2], [3]. In this paper, motivated by the above trends, we consider an 802.15.4-based WSN and we study the relationship between the 802.15.4 topology formation mechanism

An Efficient Self Routing Scheme by Using Parent-Child Association for WSNs

787

and possible routing strategies at the network layer. Our objective is to explorer the cross-layer interdependencies between the topology formation like association information and routing mechanisms to set-up energy efficient routing paths towards a sink. To this aim, we have proposed a simple Association-based Self Routing (ASR) for the upper stack of 802.15.4 sensor networks. To verify and validate the behavior of the proposed scheme, we have compared and contrasted the performance of ASR to Ad-hoc On demand Distance Vector (AODV) [14] scheme which was designed for highly dynamic or mobile application scenarios. ASR is a kind of simple parent-child routing scheme where ASR routes packets in a upstream direction from sensors to sink, a proactive routing, which is based on the parent-child relationships established by the 802.15.4 MAC topology formation procedure. Basically ASR and AODV are inherently different, as the former is a proactive routing scheme that does not use ad hoc control messages in upstream direction, while the latter is a pure on-demand route acquisition algorithm, which broadcasts discovery packets when a routing path needs to be established. To the best of our understanding, except [10] which only considers IEEE 802.15.4 association tree in an upstream direction, none of the previous studies have addressed utilizing the parent-child relations and the BOP [16] for the routing path discovery in a WPAN under TinyOS, which triggered the motivation for this work. First, the ASR confines the number of retrial in the joining process in the aims of reducing unwanted noise which occurs during the association phase. Basically, the association process causes a beacon request message to find channels in active scan mode, it consumes a lot of resources. Thus in ASR it could be possible to limit beacon request flooding as much as possible; second, the ASR exploits information exchanged during the network formation and topology update phases, thus avoiding additional routing messages and the associated overhead.

2 Protocol Descriptions: Association Based Self Routing The proposed scheme, ASR, is explained in the directions of the upstream, the data transmission from each end node to the sink node, ZR0 (ZigBee Router 0). First, ASR has setup the routing path from the source node to the destination node without sending a flooding message, route request (RREQ). In a previous scheme, AODV, it extensively relied on sending an RREQ message to find the routing path or flooding to setup the path, causing consumption of a lot of bandwidth resources. Compared with this, we have proposed ASR to suppress the surplus flooding during the routing path discovery process. Based on the ZigBee specifications, we have proposed an Association based Self Routing (ASR) that utilizes the MAC layer associations to perform the routing function, the destination routing path. Under the ASR scheme, the data generated by the end nodes are forwarded to the sink node. Then, the data are forwarded upward to the root tree by means of the parent child relationships. The proposed ASR is a table based routing because the destination path is resolved before an end node performs frame forwarding. The established routing path is the association tree that is formed during the MAC association procedure. As a result, routing paths are relying on

788

Y.-M. Yang and I. Jeon

MAC layer parent child relationship and result in a routing path to the ZR0, e.g., the sink node in a PAN [10]. Based on this observation, to collect data generated by the end nodes at the sink node under a multi-hop network, each end node or sensor nodes shall transmit all the packets to ZR0. For ASR, some notations have been defined as following: - ZR0: the ZigBee coordinator or sink node. - ZRn: the nth ZR or end node where n id any none zero number less than 255. - Level or depth of each node: the number of hops from ZR0 to each node, ZRn.

Fig. 2. Association based Routing

Based on the above notation, every node routes all packets directed to the sink ZR0 toward its parent. We remark that the resulting routing paths do not change while synchronization between device and coordinator is maintained, and will be eventually updated after orphan scan calls, e.g., in case of loss of synchronization. Table 1. Association based routing table

The ASR routing algorithm drastically reduces the number of routing control message request since it utilizes and gets the routing information from MAC association procedures. A cross-layer consideration is thus systematically reflected

An Efficient Self Routing Scheme by Using Parent-Child Association for WSNs

789

between routing layer and MAC layer. Thereby, ASR is relatively simple, light and only consumes a small system memory for maintaining the routing tables. Specifically, these features make ASR a good candidate for a small sensor node like MICA-Z. During data communication when there is a broken in routing path, the automatic routing path recovery process is invoked by means of ad hoc path recovery. The scheme is based on continuous monitoring the number of routing failure count. When the number of routing failure has reached the predefined value, it automatically reinitiates the association process as explained the above. Fig. 2 shows the association based routing when a news packet has come and Table 1 shows an example of association based routing information.

3 Performance Evaluation We now describe our performance evaluation study and experimental results of the 802.15.4 MAC. To assess the performance of the proposed parent-child routing scheme, we implemented ASR and AODV in the TOSSIM network simulator related to Mica-Z with the SKT standard wireless extension [16]. Tiny-OS provides noncommercial the 802.15.4 physical and MAC layers from public domain and of several routing protocols which comes from CUNY [7]. Based on this, we implement a crosslayer interaction between link (layer 2: L2) and routing (L3) layers to route data packets to the coordinator (ZR0) with which each device has been successfully associated. The address of ZR0 is available at MAC layer in the association response command frame sent by ZR0 to its child and it is forwarded to the network layer which inserts it in the routing table as next-hop for packets destined to the sink. The network topology used in the simulation is a 6 hops binary tree (i.e. 64 nodes) with 20m distance between adjacent nodes. The node at the center is the ZR0. Each other node is in the radio transmission range of the others and transmission to receiving radio characteristic is symmetric for simplicity. 3.1 Model Description and Performance Metrics We consider a network consisting of N = 64 sensor nodes distributed on a 160 x 160m grid. All sensors are routers or FFD (ZRn except ZR0) devices configured in beaconenabled mode, and they are all potential coordinators in the WPAN allowing for the association of other devices. We consider a single sink (ZR0) located around the center of the area (x = 80m and y = 80m). The network topology is a binary tree, under the given architecture; all routers could generate data in a period manner that will be gathered at the sink in upstream direction. The maximum of hops from ZR0 to the end node is 6 where the total number of nodes are 2^6-1 = 63. During data transmission, any routers could participate in the multi-hop relaying process, if necessary. The traffic generated by the source nodes depends on scenarios such as increasing hops and changing duty cycles. The whole frame size is as follows: MAC header is for 8 bytes, network header is for 11 bytes, frame checker for 2 bytes, and user payload is 36

790

Y.-M. Yang and I. Jeon

bytes, so in total is 57 bytes. Packet frame frequency rages from 10 pps to 200 pps per node during experiments. Thus, the average generation data rate equals to around 4k ~ 20kbit/sec (bps). We remark that once the ZR0 starts the WPAN, network formation, assigning beacon interval and duty cycles, and initiating MAC joining process between ZigBee routes. The performance analysis presented in the next section has been carried out in 3 scenarios such as increasing hops and changing duty cycles. First, increasing the number of hops has gained by deploying more nodes in a grid. Second, changing duty cycles was done by changing BO and SO values as follows: 12.5% to 8:5, 25% to 8:6, 50% to 8:7, and 100% 8:8. We measure different performance metrics. In particular, for each source we evaluate, the performance metrics are the following: 1. Throughput (kbps): it means the amount of data received by the destination node within certain period of time. In multi-hop environment, the throughput is computed at the final destination as the intermediate nodes are responsible of relaying the packets. 2. Packet delivery ratio (%): it measures the ratio of total packets received against total packets sent in the MAC sublayer. In other words, the ratio of packets successfully received in packets sent in MAC sublayer. This metric does not differentiate transmissions and retransmissions, and therefore does not reflect what percentage of upper layer payload is successfully delivered, although they are related. 3. Collision rate: the total collision during a simulation run. Specifically, this means the total collisions that occur between hidden terminals during a simulation run. Hidden terminals prevent carrier sense from working effectively, and therefore transmissions from them are likely to collide at a third node [15]. In 802.11, the request-to-send (RTS) and clear-to-send (CTS) mechanism is used to tackle this problem [18]. 4. Number of network commands (RREQ flooding): number of route request control messages generated by sources to discover paths towards the sink (only for AODV routing). 3.2 Experimental Results and Discussions We have compared the proposed scheme, ASR and the well known AODV. We carried out an extensive experimental analysis to compare ASR and AODV. The proactive scheme, ASR, compared with a reactive mechanism, AODV, shows better performance with respect to several metrics such as average throughput, number of collisions, number of network commands, and delivery ratio. Therefore, it is able to simply set-up routing paths towards a sink (ZR0) by exploiting the cross-layer interdependencies between the topology formation and the routing mechanisms. ASR exploits information exchanged during the network formation and topology update phases, thus avoiding additional routing messages and the associated overhead. Moreover, it shows several performance enhancements with respect to AODV, such as reduced latency and number of network commands. Finally, it reduces complexity, as it is very easy to implement and does not require a specialized daemon on the host device where it runs. In general, AODV shows in general worse performance.

An Efficient Self Routing Scheme by Using Parent-Child Association for WSNs

791

However, since it does not strictly depend on the MAC layer procedures, it provides greater flexibility and can be easily extended to account for different route selection metrics. In general, AODV may be selected when the application layer requires high flexibility from the routing mechanism, on the other hand ASR, thanks to its simplicity, is a light routing protocol for simple application scenarios. Figures 3(a) and (b) illustrate the comparison of average throughput and delivery ratios with respect to (w.r.t.) ASR and AODV. Figure 4 (a) and (b) illustrate the comparison of number of networks commands and number of collisions w.r.t. ASR and AODV. The figures show the relationship between the number of hops (independent parameters); and the average throughput, delivery ratio, number of network packets, and number of collision (dependent parameters), respectively. Increasing the number of hops was gained by deploying more nodes in a grid as the network scope expended. When the hop count is a little smaller, both throughput and delivery ratio are less different, but in a larger case than 4, it is much different. Basically, for both the schemes, the delivery ratio goes down to around 50% with an increasing hop counts due to the increasing number of collisions on the channel and higher collision probability. However, when the hop count is larger than 6, with ASR the packet loss is very low (from 20% to 25%) but with respect to AODV (up to 50% or 60%). In fact, as previously discussed, ASR exploits the 802.15.4 association procedure for routing; and also it does not use RREQ control messages and suppress beacon request mechanisms during passive scan process, which are the main causes of collisions and cause surplus power consumption. Moreover, from the comparison of Fig. 3 and Fig. 4, it can be noted that the performance of BSA basically does not significantly change for different hop counts; wile in the AODV case, packet loss varies for changing hop counts. This is due to data packets collisions that occur during the whole duration of the event, and cause routing paths to fail and retry finding the route discovery mechanism. 100

250000

90

ASR

80 70

Delivery Ratio (%) .

Average Throughput (bytes) .

200000

AOD V

150000

100000

AO DV

60

ASR

50 40 30 20

50000

10 0

0 1

2

3

4

5

6

1

2

3

4

5

6

N um ber o f hops

N um be r o f ho ps

(a)

(b)

Fig. 3. Comparison between ASR and AODV for, a) average throughput and b) average delivery ratio with different number of hops

To better point out the congestion caused by the route discovery process, in Fig. 4 we report the total number of network commands as a function of for different values of hop counts. In the worst case, i.e., when the hop count is larger than 6, AODV produces about 45000 network commands, while ASR is less than 200. Moreover, the high load of routing control messages also causes route discovery failures, which result in the steep slope of the curves.

792

Y.-M. Yang and I. Jeon

50000

45000

45000

AO D V ASR

40000

AODV

35000

ASR

35000

Number of Collision .

Number of NWK CMD .

40000

30000 25000 20000 15000 10000

30000 25000 20000 15000 10000

5000

5000

0 1

2

3

4

5

6

0 1

N um be r of ho ps

2

3

4

5

6

N um be r o f ho ps

(a)

(b)

Fig. 4. Comparison between ASR and AODV for, a) average number of network commands and b) average number of collisions with different number of hops

Figures 3(a) and (b) show the comparison of average throughput and delivery ratios w.r.t. ASR and AODV. Figure 5 (a) and (b) show the comparison of number of networks commands and number of collisions w.r.t. ASR and AODV. The figures show the results of performance metrics when the duty cycles have changed. Duty cycles have changed to the value of BO and SO as by changing SO from 5 to 8. When the duty cycle is relative low, the throughput of ASR is much larger than AODV. This is well explained by Fig. 6(a) that indicates that reducing RREQ flooding reduces packet collision through multiple hops. 100

250000

90

AODV

80

ASR

200000

ASR

70

Delivery Ratio (%) .

Aggregate Throughput (bytes) .

AO D V

150000

100000

50000

60 50 40 30 20 10

0 1 2 .5 ( 5 )

25(6)

50(7)

D uty C y c le % ( S O v a lu e )

(a)

100(8)

0 1 2 .5 ( 5 )

2 5 (6 )

5 0(7)

10 0 (8 )

D uty C y c le % ( S O v a lu e )

(b)

Fig. 5. Comparison between ASR and AODV for a) average throughput and b) delivery ratio with different duty cycles

When there is congestion because of a low duty cycle such as 12.5 or 25%, in ASR the metrics show 20% less than that of AODV. As shown in Fig. 5(b), and Fig 6(b), specifically, in an upstream direction, association based routing affects to channel utilization, reducing the overheads significantly in ASR. The performance metrics are about 10% less than that of AODV scheme through out changing duty cycles.

An Efficient Self Routing Scheme by Using Parent-Child Association for WSNs

793

30000

2 0 00 0 1 8 00 0

25000

1 4 00 0

Number of Collision .

Number of NWK CMD .

1 6 00 0

AODV 1 2 00 0

ASR 1 0 00 0 8 00 0 6 00 0 4 00 0

20000

15000

AO D V ASR

10000

5000

2 00 0 0 1 2 .5 ( 5 )

2 5 (6)

50 (7 )

D uty C yc le % (S O v a lue )

(a)

1 0 0(8 )

0 1 2.5(5)

2 5 (6 )

50(7)

1 0 0 (8 )

D u ty C y c le % ( S O v a lu e )

(b)

Fig. 6. Comparison between ASR and AODV for a) of number of network commands and b) number of collision with different duty cycles

4 Conclusions We have proposed a new 802.15.4 Association-based Self Routing (ASR) scheme. Within ASR it considers utilizing the parent-child relationship for establishing a routing table in an upstream direction, and suppressing surplus beacon power during the channel association process. Practically, ASR shows a dramatic increase in network performance when traffic congestion level is severe or the duty cycle is relatively low. Furthermore, it minimizes control packet overhead during routing discovery and beach channel power in channel scanning processes. The experimental results with TOSSIM with Mica-Z and evaluation facts coincide to each other and make sure the proposed ASR outperform the previous ad-hoc routing algorithm like AODV in terms of stabilization time, control packet overhead, and channel utilization. Acknowledgments. The author would like to thank Dr. Dongkyun Kim of KNU, Dr. Young-Jin Nam, Young-Duk Kim, and Dr. Dong-Ha Lee of DGIST for their fruitful discussions and valuable comments. This paper was supported by Research Fund, Kumoh National Institute of Technology (www.kumoh.ac.kr).

References 1. Liu, Y., Lee, M., Saadawi, T.: A Bluetooth Scatternet-Route Structure for Multi-hop Ad Hoc Networks. IEEE Journal on Selected Areas in Communications 21, 229–239 (2003) 2. Levis, P., Lee, N., Welsh, M., Culler, D.: TOSSIM: Accurate and Scalable Simulation of Entire TinyOS Applications. In: SenSys 2003 (2003) 3. J. Polastre, J. Hill, and D. Culler: Versatile Low Power Media Access for Wireless Sensor Networks. In: SenSys 2004 (2004) 4. Lu, G., Krishnamachari, B., Raghavendra, C.S.: Performance Evaluation of the IEEE 802.15.4 MAC for Low-rate Low-Power Wireless Networks. In: IEEE Int. Performance Computing and Communication (2004) 5. Zheng, J., Lee, M.L.: A Comprehensive Performance Study of IEEE 802.15.4. IEEE Press Book (2004)

794

Y.-M. Yang and I. Jeon

6. Koubaa, A., Alves, M., Tovar, E.: A Comprehensive Simulation Study of Slotted CSMACA for IEEE 802.15.4 Wireless Sensor Networks. In: IEEE International Workshop on Factory Communication Systems (2006) 7. Lee, M., Zheng, J., Hu, X., Juan, H., Zhu, C., Liu, Y., Yoon, J., Saadawi, T.: A New Taxonomy of Routing Algorithms for Wireless Mobile Ad Hoc Networks: The Component Approach. IEEE Communications Magazine, 116–123 (2006) 8. Koubaa, A., Alves, M., Attia, M., Nieuwenhuyse, A.: Collision-Free Beacon Scheduling Mechanisms for IEEE 802.15.4/Zigbee Cluster-Tree Wireless Sensor Networks. Technical Report IPP-Hurray, TR-061104 (2006) 9. Suh, C., Ko, Y., Lee, C., Kim, H.: Numerical Analysis of the Idle Listening Problem in IEEE 802.15.4 Beacon-Enable Mode. In: Proc. of ACM ChinaCom 2006 (2006) 10. Cuomo, F., Della, L., Monaco, U., Melodia, F.: Routing in ZigBee: Benefits from Exploiting the IEEE 802.15.4 Association Tree. In: ICC 2007, pp. 3271–3276 (2007) 11. Mura, M.: Ultra-Low-Power Optimizations for the IEEE 802.15.4 Networking Protocol. In: IEEE MASS 2007, pp. 1–9 (2007) 12. Chen, F., Wang, N., German, R., Dressler, F.: Performance Evaluation of IEEE 802.15.4 LR-WPAN for Industrial Applications. IEEE WONS, pp. 89 – 96 (2008) 13. Pollin, S., Tan, I., Hodge, B., Chun, C., Bahai, A.: Harmful Coexistence Between 802.15.4 and 802.11. In: IEEE CrownCom, pp. 1–6 (2008) 14. Royer, E.M., Perkins, C.E., Das, S.R.: Ad hoc On-Demand Distance Vector (AODV) Routing (RFC 3561) 15. IEEE P802.15.4: Wireless Medium Access Control and Physical Layer specification for Rate Wireless Personal Area Networks. IEEE(September 2006) 16. SKT WiBEEM Specification Document (May 2007), http://www.sktelecom.com 17. ZigBee Specification Document 053474r17 (October 2007) 18. IEEE P802.11, Part 11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications. IEEE (June 2007)