Neighbor Table Based Shortcut Tree Routingin ZigBee Wireless Networks

Taehong Kim, Seong Hoon Kim, Jinyoung Yang, Seong-eun Yoo, Member, IEEE, and

Daeyoung Kim, Member, IEEE

AbstractThe ZigBee tree routing is widely used in many resource-limited devices and applications, since it does not require any

routing table and route discovery overhead to send a packet to the destination. However, the ZigBee tree routing has the fundamental

limitation that a packet follows the tree topology; thus, it cannot provide the optimal routing path. In this paper, we propose the shortcut

tree routing (STR) protocol that provides the near optimal routing path as well as maintains the advantages of the ZigBee tree routing

such as no route discovery overhead and low memory consumption. The main idea of the shortcut tree routing is to calculate remaining

hops from an arbitrary source to the destination using the hierarchical addressing scheme in ZigBee, and each source or intermediate

node forwards a packet to the neighbor node with the smallest remaining hops in its neighbor table. The shortcut tree routing is fully

distributed and compatible with ZigBee standard in that it only utilizes addressing scheme and neighbor table without any changes of the

specification. The mathematical analysis proves that the 1-hop neighbor information improves overall network performances by

providing an efficient routing path and distributing the traffic load concentrated on the tree links. In the performance evaluation, we show

that the shortcut tree routing achieves the comparable performance to AODV with limited overhead of neighbor table maintenance as

well as overwhelms the ZigBee tree routing in all the network conditions such as network density, network configurations, traffic type, and

the network traffic.

Index TermsZigBee, tree routing, shortcut tree routing (STR), neighbor table, MANET, WSN, IEEE 802.15.4

1 INTRODUCTION

ZIGBEE is a worldwide standard of wireless personal areanetwork targeted to low-power, cost-effective, reliable,and scalable products and applications. Different from the

other personal area network standards such as Bluetooth,

UWB, and Wireless USB, ZigBee provides the low power

wireless mesh networking and supports up to thousands of

devices in a network. Based on these characteristics, ZigBeeAlliance has extended the applications to the diverse areas

such as smart home, building automation [1], health care

[2], smart energy [3], telecommunication, and retail services.The ZigBee network layer [4], which is the core of the

standard, provides dynamic network formation, addres-sing, routing, and network management functions. ZigBeesupports up to 64,000 devices in a network with themultihop tree and mesh topologies as well as star topology.Every node is assigned a unique 16-bit short addressdynamically using either distributed addressing or stochas-tic addressing scheme. The routing protocols of ZigBee arediverse so that a system or users can choose the optimalrouting strategy according to the applications.

The reactive routing protocol in ZigBee is derived fromAODVjr (AODV junior) [5], which is one of the representa-tive routing protocols in mobile ad hoc networks (MANET).Similar with other MANET routing protocols [6], [7], [8], [9],[10], ZigBee reactive routing protocol provides the optimalrouting path for the arbitrary source and destination pairthrough the on-demand route discovery. It requires theroute discovery process for each communication pair, so theroute discovery overhead and the memory consumptionproportionally increases with the number of traffic sessions.Moreover, route discovery packets are flooded to theoverall network, which interfere with transmission of otherpackets even in the spatially uncorrelated area with theroute discovery.

On the other hand, ZigBee tree routing (ZTR) [4]prevents the route discovery overhead in both memoryand bandwidth using the distributed block addressingscheme. In ZTR, since each node is assigned a hierarchicaladdress, a source or an intermediate node only decideswhether to forward a packet to the parent or one of thechildren by comparing its address with the destinationaddress. The most benefit of ZTR is that any source nodecan transmit a packet to an arbitrary destination in anetwork without any route discovery overheads. Due tothis efficiency, ZTR is considered as a promising protocolfor resource constrained devices in diverse applicationssuch as smart grid project and Internet of Things (IoT).However, in ZTR, packets are forwarded along the treetopology to the destination even if the destination is locatednearby. Thus, ZTR cannot provide the optimal routing path,while it does not require any route discovery overhead.

Our objective is to provide the near optimal routing pathlike the reactive routing protocol as well as to maintain the

706 IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS, VOL. 25, NO. 3, MARCH 2014

. T. Kim, S. Kim, J. Yang, and D. Kim are with the Department of ComputerScience, Korea Advanced Institute of Science and Technology, 373-1Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea.E-mail: {damiano, shkim08, jyyang0308, kimd}@kaist.ac.kr

. S. Yoo is with the School of Computer and Communication Engineering,Daegu University, 7313A, Jilyang-up, Gyeongsan-si, Gyeongbuk 712-714,Republic of Korea. E-mail: seyoo@daegu.ac.kr

Manuscript received 29 Mar. 2012; revised 15 Jan. 2013; accepted 17 Jan.2013; published online 15 Feb. 2013.Recommended for acceptance by Y.C. Hu.For information on obtaining reprints of this article, please send e-mail to:tpds@computer.org, and reference IEEECS Log Number TPDS-2012-03-0326.Digital Object Identifier no. 10.1109/TPDS.2014.9.

1045-9219/14/$31.00 2014 IEEE Published by the IEEE Computer Society

advantages of ZTR such as no route discovery overheadand little memory consumption for the routing table.We propose the shortcut tree routing (STR) that signifi-cantly enhances the path efficiency of ZTR by only addingthe 1-hop neighbor information. Whereas ZTR only usestree links connecting the parent and child nodes, STRexploits the neighbor nodes by focusing that there exist theneighbor nodes shortcutting the tree routing path in themesh topology. In other words, in STR, a source or anintermediate node selects the next hop node having thesmallest remaining tree hops to the destination regardless ofwhether it is a parent, one of children, or neighboring node.The routing path selection in STR is decided by individualnode in a distributed manner, and STR is fully compatiblewith the ZigBee standard that applies the different routingstrategies according to each nodes status. Also, it requiresneither any additional cost nor change of the ZigBeestandard including the creation and maintenance mechan-ism of 1-hop neighbor information.

The main contributions of this paper are as follows: First,we propose STR to resolve the main reasons of overallnetwork performance degradation of ZTR, which are thedetour path problem and the traffic concentration problem.Second, we prove that the 1-hop neighbor nodes used bySTR improve the routing path efficiency and alleviate thetraffic load concentrated on tree links in ZTR. Third, weanalyze the performance of ZTR, STR, and AODV bydifferentiating the network conditions such as networkdensity, ZigBee network constraints, traffic types, and thenetwork traffic.

This paper is organized as follows: Section 2 summarizesthe related works on the routing protocols, and Section 3describes ZTR and its problems. Section 4 presents theshortcut tree routing algorithm and analyzes the propertiesof STR in a mathematical way. The diverse performancesare evaluated in Section 5, and conclude this paper inSection 6.

2 RELATED WORKS

MANET [11] routing protocols can be classified intoproactive and reactive routing protocols. The proactiverouting protocol periodically updates the topology infor-mation, so it always has an up-to-date optimal routingpath. The representative examples of proactive routingprotocols are OLSR [6] and DSDV [7]. In contrast, thereactive routing protocol invokes the route discoveryprocedure only when an application requests transmissionof data. Thus, it does not generate the control packetoverhead if there is no data packet to transmit, while itcauses long delay to find a routing path. AODV [8], DSR[9], and TORA [10] are examples of the reactive routingprotocol. Regardless of whether it is proactive routing orreactive routing, these MANET routing protocols providethe optimal routing path for the given source anddestination pair. However, the required routing table sizeof these protocols is too big to store all the routing paths inthe resource-limited devices [12].

Moreover, they need to exchange control packets tomaintain and discover the routing path, and the inter-ference of these control packets on the other transmissions

of the packets may be severe in the low rate and narrowbandwidth channels.

Here, before explaining the other routing protocols, wecategorize communication traffic patterns into any-to-any,many-to-one, and one-to-many traffic pattern [13]. In theany-to-any traffic pattern, all the nodes can be a source or adestination of the packets. The many-to-one traffic patterndesignates one destination and this destination collects theinformation from all the other devices in a network.Conversely, the one-to-many traffic pattern is used for thedesignated one source node to transmit the packets to theother devices. CTP [14] and RPL [15] are the representativewireless personal area network protocols mainly support-ing many-to-one and one-to-many traffic pattern.

Collection tree protocol (CTP) in TinyOS [16] is therepresentative tree routing protocol. In CTP, the base stationas a root of the tree builds a collection tree and every sensornode selects its parent node. The routing metric of CTP isthe expected transmissions count (ETX), and a root has anETX of 0. Each node calculates its ETX by sum of the ETX ofits parent and ETX of its link to its parent. The CTPmaintains the ETX of its neighboring device and selects thenode with the lowest ETX as the parent. When a sensornode has data to send, it simply sends a data packet to itsparent. This forwarding process is repeated until the basestation receives.

RPL (IPv6 Routing Protocol for Low Power and LossyNetworks) is the IETF standard protocol based on CTP. RPLconstructs a destination oriented directed acyclic graph(DODAG) to optimize the many-to-one traffic pattern.Every device in the DODAG establishes the optimal routingpath to the destination using a single route request from thedestination, which may be the gateway of a network. TheDODAG significantly reduces the route discovery overheadand routing table size, because it requires only one time ofroute discovery from the destination comparing withMANET routing protocols requiring all the individualsources to invoke route discovery to the same destination.

The main advantage of these protocols is that theysignificantly reduce the route discovery overhead byconcentrating on the many-to-one and one-to-many traffic.Even though they can support the any-to-any traffic pattern,a routing path is inefficient by traversing along the treetopology and they also suffer from detour path and trafficconcentration problems like ZigBee tree routing.

For the ZigBee standard, there have been researches onimproving the path efficiency of the ZigBee tree routing.The preliminary version of our paper [17] suggests utilizingthe 1-hop neighbor table to reduce the routing cost ofZTR. The proposed STR algorithm selects the neighbornode if it can reduce the routing cost to the destination. Itshowed that the proposed algorithm saves more than 30percent of hop count compared with ZTR without any routediscovery overhead. However, it is limited on evaluatingthe saved hop count comparing with ZTR. In this paper, inadditional to the inefficient routing path of ZTR [17], wehave identified that ZTR suffers from performance degra-dation when all the packets are concentrated on the treelinks. We demonstrate these problems of the ZTR by thenetwork simulation, and prove that STR significantly

KIM ET AL.: NEIGHBOR TABLE BASED SHORTCUT TREE ROUTING IN ZIGBEE WIRELESS NETWORKS 707

enhances overall network performances such as packetdelivery ratio, end-to-end latency, path stretch and so on.The mathematical analyses are also provided in this paperto prove that STR alleviates the traffic load concentrated onthe tree links as well as provides an efficient routing pathwithout loop.

3 ZIGBEE TREE ROUTING

ZTR is designed for resource constrained ZigBee devices tochoose multihop routing path without any route discoveryprocedure, and it works based on hierarchical blockaddressing scheme described in (1) and (2).

Fig. 1 shows an example of the ZigBee address assign-ment scheme and the address hierarchy when Cm(nwkMaxChildren), Rm (nwkMaxRouters), and Lm (nwkMaxDepth) are given with 3, 2, and 3 respectively. Cm, Rm,and Lm are defined as the maximum number of children aparent may have, the maximum number of routers a parentmay have as children, and the maximum tree level of anetwork in ZigBee standard, respectively,

Cskipd 1 Cm Lm d 1; if Rm 1;1 Cm-Rm-Cm RmLmd1

1Rm ; otherwise;

8