effect of node mobility and network aimension to be the zigbee routing method

Effect of node mobility and network dimension to theZigbee routing methodJiasong Mu/Kaihua LiuSchool of Electronic Information EngineeringTianjin UniversityTianjin Chinamujiasong@Abstract—Both the hierarchical tree routing and the table-driven routing algorithm are available in Zigbee specification. However, the routing selection method is not included in the standard. In this article, we analyzed the effect of the node mobility and the change of the network dimension in Zigbee networks. We used OPNET modeler to simulate the Zigbee networks. The simulation results showed the performances of different routing algorithms in the same conditions. We also simulate the performances of different routing methods with the change of the node mobility and network dimension. We analyzed the reasons and made a conclusion. To the networks, of which, the structure is stable or the change of the structure is not much, we figured out the routing selection strategy based on the scale of the networks. We also find the critical boundary for the different routing method. Tree routing may have better performance when the nodes move less than 3 times on average in 100 seconds, the routing discovery method should be used when nodes move more than 4 times.Zigbee; routing selection methond; node mobilityI.I NTRODUCTIONThe ZigBee Alliance published the ZigBee specification version 1.0 (ZigBee v.1.0) to enable reliable, low-cost and low-power wireless monitoring and control in December 2004. The ZigBee v.1.0 defines network and application layers on top of medium access control (MAC) and physical (PHY) layers of IEEE 802.15.4[1]. The ZigBee v.1.0 is one of the first wireless ad-hoc network standards. Since ZigBee devices are designed for low cost and low data rates, it is expected their use in home and building automation with significantly small costs. Moreover, ZigBee networks support star and mesh topology, self-forming and self-healing as well as more than 65000 address spaces; thus, network can be easily extended in terms of size and coverage area.One of the most important research topics on wireless ad-hoc networks has been the routing protocol. There have been many studies on the routing protocols for wireless ad-hoc networks. ZigBee combines the hierarchical routing protocol (HRP) and the well-known ad-hoc on-demand distance vector (AODV) routing protocol in order to meet the objectives such as cost-effectiveness and path robustness. However, since the ZigBee specification and its applications is getting more and more developed recently, the application configuration, performance analysis and optimization remain further discussed[2]. There is still no ZigBee module in the simulation tool extensively used for the network simulation and so there is no relative analysis about the performance of the ZigBee routing [3]. And the routing strategy and its optimization of ZigBee must also be taken into consideration.The rest of this paper is organized as follows. In section II, the ZigBee protocol will be introduced. Section III will explain the routing algorithm used in ZigBee network. In section IV, the simulation results and analysis will be shown. Finally in section V, there will be the conclusion and the further work.II.O VERVIEW OF THE Z IG B EE P ROTOCOL The protocol stack of ZigBee networks is detailed in Fig.1. Based on the IEEE 802.15.4 protocol, the ZigBee specification defines the standard of higher layers. At the PHY layer, IEEE 802.15.4 defines a total of 27 channels: 16 channels at a maximum rate of 250 kbps in the ISM 2.4-2.4835 GHz band, 10 channels at 40 kbps in the ISM 902 - 928 MHz band, and one channel at 20 kbps in the 868.0 - 868.6 MHz band. At the MAC layer, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) or the optional slotted CSMA/CA mechanism are used, and respectively utilize by the beaconless and beaconed modes.Two device types are specified within the IEEE 802.15.4 framework: full function device (FFD) and reduced function device (RFD). An FFD generally has more responsibilities in that they must maintain routing tables, participate in route discovery and repair, maintain beaconing framework, and handle node joins. Moreover, an FFD has the capability of communicating with any other devices within its transmissionrange. On the other hand, RFD simply maintains the minimumFigure 1. ZigBee protocol stack978-1-4244-3709-2/10/$25.00 ©2010 IEEEamount of knowledge to stay on the network, and it does not participate in routing. RFDs can only associate and communicate with FFDs. Three device types are defined in ZigBee: ZigBee coordinator, ZigBee routers, and ZigBee end devices. An RFD can only be a ZigBee end device; whereas an FFD can be either a ZigBee coordinator or ZigBee router. The ZigBee coordinator is responsible for starting a new network. ZigBee coordinator and routers are "routing capable", while the ZigBee end devices can't participate in routing and have to rely on their corresponding ZigBee parent routers for that functionality [4].III. Z IG B EE R OUTING A LGORITHMSA. Address Allocation Mechanism and Tree RoutingThe address of device in a ZigBee network is assigned by its parent node following the tree structure. The parameter nwkmaxChildren (m C ) represents the largest number of children nodes which can associate with a router or a coordinator. The parameter nwkmaxRouters (m R ) means the number of children nodes which can be a router. The parameter nwkMaxDepth (m L ) decides the most depth in the network. And for the same network, different nodes usually have the constant m C and m R .Every potential parent is provided with a finite sub-block of the address space, which is used to assign network addresses to its children. Given nwkmaxChildren (m C ), nwkMaxDepth (m L ), and nwkmaxRouters (m R ), we can compute the functiond Cskip as the size of the address sub-block distributed by eachparent at depth d as follows:10,0,1(1),1,1.1,1m m d m m m L d m m m m m m R Cskip C L d R R C R C R R −−⎧⎪=⎪⎪=+⋅−−=⎨⎪>+−−⋅⎪⎪−⎩(1)Network addresses 1,d rn A + and 1,d el A + shall be assigned to the n-th router child and l-th end device child at depth d+1 in a sequential manner, respectively, according to （2）：()1,1,11d rn parent d d el parent d m A A Cskip n A A Cskip R l++=+−+=+⋅+(2)Where 1m n R ≤≤, and parent A represents the address of the parent.The tree routing algorithm supports simple but reliable routing for any destination address. In ZigBee, network addresses are assigned using the distributed addressing scheme mentioned above. Due to such addressing scheme, the networkconstructs a tree topology; each device can manage the address space of its descendant. If the destination address is in the address space that a node is managing, the node forwards the packet to one of its child nodes. Otherwise, it forwards the packet to its parent node. The parent or child node which receives the packet selects the next hop node according to the destination address in the same manner. Although the tree routing is efficient in the view point of memory usage, we can see that the tree routing requires no memory cache. However, because the tree topology cannot offer the shortest paths, the routing cost is sometimes inefficient.B. Table-driven RoutingTable-driven routing is basically similar to the Ad-hoc On-demand Distance Vector (AODV) routing protocol for general multi-hop ad-hoc network in ZigBee networks. Although the ZigBee node cannot maintain a 2-hop routing table due to the limit of memory and energy like other AODV networks. The ZigBee specification defines the table-driven routing method in the ZigBee network the AODV [5].In ZigBee mesh routing, route requests (RREQ) are broadcasted on-demand when data is to be transmitted to a destination of an unknown path. Routes are constructed based on the route replies (RRPL from intermediate nodes and destination node), and a route error (RERR) message is transmitted to the user when a path can't be found. The route repair mechanism repairs invalid routes when a previous route cannot be found. Since only coordinators/routers (FFDs) can actively participate in mesh routing, the end devices (RFDs) have to rely exclusively on their parent nodes to perform mesh routing on their behalves.C. Routing Selection in ZigBeeAccording to the standard in the network layer, the DiscoverRoute field of the data frame head is defined as the routing approach for data frames [2]. And it can have three values as followed: •Suppress route discovery (SRD): the network only uses the routing tables that exist already. The network disables the routing discovery progress when there is no corresponding address of the destination node •Enable route discovery (ERD): if there is the routing address in the routing table, the routing will follow this routing table. Otherwise, the router will initiate the routing discovery. When this node has no ability to initiate the routing discovery, it will exploit the Tree Routing.•Force route discovery (FRD): The node has to initiate the routing discovery constrainedly no matter whether there is the corresponding routing table or not.In the ZigBee specification, the ERD is the initial value of this field, and there is no relative information about the selection of routing method and how these parameters are configured in order to choose the routing strategy.IV. S IMULATION AND A NALYSISWe used the OPNET modeler to simulate the ZigBee network. The parameters were shown in TABLE I and II. To identify the performance in different situations, we both simulated the stable and unstable net structure for 100 seconds. We defined the stable network as follows: the node had a probability to stay following the exponential distribution which the expectation was 150 seconds, the moving speed of the node was a uniform distribution from 10 to 30 m/s, and the distance the node moved was a uniform distribution from 10 to 50 meters. As the parameter set like this, more than half of the nodes would not move during the simulation and only a few nodes would changed the position more than 1 time.To the unstable network, the probability of the node to stay following the exponential distribution which the expectation was 20 seconds, the moving speed set to the uniform between 10 to 50 m/s, and the distance was the same. In an unstable network, almost all the node moved. The structure changed frequently.We also simulated 2 scales of network. In a small network, the simulation range was 100m*100m, and there were 10 nodes deployed randomly. A large network had a range of 300m*300m and 30 nodes, in which the initial position of the nodes was random.To see the performance of each type of routing method, the statistic Network Load was shown. For every single node had the same parameter on sending packets, which was 100bit long for each packet and the packet interval time, followed the exponential distribution which the expectation was 1 second and the destination of the packet was random, the network load indicated the efficiency of the routing method. And to reduce the effect caused by the network initialization, the data started from 10 seconds. And all the nodes in the network were ZigBee FFDs. We simulated 10 times for each condition and calculated the average.The performance of 3 kinds of routing method in the small stable network was shown in Fig.2, from which, we could see the FRD had the worst performance because of its non-stop routing discovery. The SRD and ERD had similar results. The ERD had more bits because it may have routing request, and it had a better performance later due to the shortest path it may find.TABLE I. P ARAMETERS FOR DIFFERENT DIMENSIONSParameters Small Network Large Network Network Size 100m*100m 300m * 300mNumber of Nodes 10 30Simulation Duration100 seconds Packet Size100 bit TABLE II.P ARAMETERS FOR DIFFERENT MOBILITYParameters Unstable StableProbability to stay exponential distributionexpectation 20 seconds exponential distribution expectation 150 seconds Moving SpeedUniform (10m/s,30m/s) Uniform (10m/s,50m/s) Moving DistanceUniform (10m,50m) Uniform (10m,50m) Fig.3 illustrated the small unstable network. While the network load of FRD didn’t change much compare to the stable one, it still was the most inefficiency. The SRD had much more bits because the node would rejoin the network more frequently caused by the variety of the network structure. The large stable network performance was shown in Fig.4. FRD had most network load. Although ERD might find the shortest path, the routing discovery occupied the network transmission a lot due to the limit of the memory cache of the nodes. The stable structure made the SRD the most efficient and stable.As indicated in Fig.5, the ERD had the best performance while the FRD and SRD had a similar high inefficiency. The FRD tried to make a routing discovery no matter whether it was needed or not. And SRD had to maintain a tree topology when the structure changed at any moment, which made it inefficient and unstable.Time (s)N e t w o r k L o a d (b i t /s )Figure 2.Routing performance in small stable networksTime (s)N e t w o r k L o a d (b i t /s )Figure 3. Routing performance in small unstable networksTime (s)N e t w o r k L o a d (k b i t /s )Figure 4.Routing performance in large stable networks4Time (s)N e t w o r k L o a d (k b i t /s )Figure 5. Routing performance in large unstable networksTo analyze the effect of mobility and dimensions to SRD and ERD in detail, we also simulated the routing performance for their changing separately. To see the effect of dimensions, we used a 300m*300m network, all the nodes were in the stable condition, and the communication parameters keep the same as above. We set the number of the nodes as 10,15,20,25 and 30. Fig.6 showed the result, from which, we may find the network load of ERD growing faster and the growing rate was more stable. The SRD performance was connected with the initial tree structure of the network. However, in the low memory cache situation, the routing request method is not always the most efficient one.Fig.7 demonstrates the effect of mobility factor. The network was 300m*300m and 30 nodes in total. With the same transmission parameters, we changed the expectation of the moving probability of every node. The expectation to stay was 150s, 50s, 20s, 5s and 1s. As illustrated, the SRD is much more sensitive to the node mobility, its performance declines fast with the rapid change of the network structure. On the other hand, the ERD may always maintain the routing efficiently with routing discovery progress. We also find the criticalboundary to the SRD better performance. The node expectation of the node moving probability to stay was around 20 up, which means the node may move about 3 to 4 times in 100 seconds duration in the network, the ERD and SRD have similar capability. In a network which the change of the structure is predictable, the ERD performs better when the nodes move more than 4 times on average in 100 seconds, andthe SRD could achieve better performance when nodes move less than 3 times.Node NumbersN e t w o r k L o a d (b i t /s )Figure 6. Routing performance for SRD and ERD for different node numberExpectaion of the probability of the node to stay (s)N e t W o r k L o a d (b i t /s )Figure 7. Routing performance for SRD and ERD for different nodemobilityV.C ONCLUSIONAs analyzed in the last paragraph, we found the FRD always had the worst performance in all the scenes we simulated. Although the forced routing made the network always find the shortest path. In the dynamic networks, ERD had the most efficiency. As mentioned in the third paragraph, the working mechanism of ERD was similar as the AODV, which could manage the routing well in the mobile networks. As to the stable network, ERD and SRD had similar efficiency in the small ones. However, the SRD based on tree routing, required no memory cache. SRD also had the lowest network load when the scale of the network expanded. Whereas the ERD might do reduplicate routing due to the restricted memory space. The SRD was the best routing option for the stable networks and the ERD performed most efficient in the unstable networks. We also find the tolerance of the network mobility for the efficiency of SRD. In a lager network, SRD has better performance when the nodes move less than 3 times on average in 100 seconds duration, and it performs poorly when the times of movement was more than 4 times.For our further work, we will try to make the node justify the network structure automatically in a simple way without too much memory and calculation requirements. In this way, we may find a self-adapting routing selection strategy for ZigBee networks.R EFERENCES[1]Taehong Kim, Daeyoung Kim, Noseong Park; Seong-eun Yoo andLopez T.S, “Shortcut Tree Routing in ZigBee Networks”, Wireless Pervasive Computing, 2007[2]Ran Peng, Sun Mao-heng and Zou You-min, “ZigBee Routing SelectionStrategy Based on Data Services and Energy-Balanced ZigBee Routing”.IEEE Asia-Pacific Conference on Dec. 2006 Page(s):400 – 404.[3]P. 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