Study of ZigBee Wireless Mesh Networks

Jing Sun software deptment

Shenyang Normal University Shengyang, China

e-mail: sunjing431@tom.com

Xiaofen Zhang software deptment

Shenyang Normal University Shengyang, China

e-mail: xiaofen_zh2003@163.com

Abstract—ZigBee is the new standard that has been developed for low cost, low data rate and low power consumption wireless netwrok. And mesh is a type of netork architecture. This paper provides a brief overview of the available IEEE 802.15.4 topologies, and then delvesing the mesh topology, decribing its advanges, disadvanges and application, and presents a detailed study on open research issus in ZigBee mesh networks. Theoretical network Capacity and real capacity formulas influced by coverage and response time are discussed, followed by analysing the route discovery and route cost in detail. Finally, we introduce the security feature including trust center and security modes.

Keywordt:ZigBee;mesh networks;routing analysis; capacity analysis)

I. INTRODUCTION

IEEE 802.15.4 [1]defines the “physical layer” (PHY) and the “medium access layer” (MAC). PHY defines a low-power spread spectrum radio operating a t 2.4 GHz with a basic bit rate of 250 kilobits per second. There are also PHY specifications for 915 MHz and 868 MHz that operate at lower data rates. MAC layers defines frame formats for forming a network based on short(16-bit) and long (64-bit) addresses. A long address is unique to each physical node and does not change, A short address is assinged to a node when it joins the network.

The ZigBee Specification[2], released in December 2004 amd recently turned publicly available, specifies the protocol layers above IEEE 802.15.4. the network(including security services) and application (including device objects and profiles) layers.

II. MESH NETWORK TOPOLOGY

Two basic types of network topologies are defined[1][2][3] in the IEEE 802.15.4 standard, according to the networking requirements of the applications: the star topology and the mesh topology. Three types of devices are defined: coordinator, router and end device. The ZigBee coordinator is responsible for staring the network and for choosing certain key network parameters but the network may be extended through the use ZigBee routers. Router forms the network backbone.

The mesh topology, also called peer-to-peer, includes a PAN coordinator, which is nominated, for instance , by virtue of being the first device to communicate on the

channel. However, the communication paradigm in the mesh topology is decentralized, where each device can directly communicate with any other device in its radio range.

This mesh topology enables enhanced networking flexibility, but it induces an additional complexity for providing an end-to-end connectivity between all devices in the network. Basically, the mesh topology operates in ad hoc fashion and allows multiple hops to route data from and device to any other device.

ZigBee aims to create general-purpose, inexpensive, and self-organizing mesh networks. Applications such as industrial control and monitoring, wireles sensor networks, asset and inventory tracking would benefit from such a topology[4]. It also allows multiple hops to route messages from and device to any other device in the netwoik. It can provide reliabilety by multipath routing. Figure 1 shows ZigBee mesh topology.

Figure 1. mesh topology model

Advantages include: robust multihop communication; network is more flexible; lower latency. Disadvantages include: cannot form superframes; route discovery is costly; needs storage for routing table.

III. MESH NETWORK ANALYSIS

A. Capacity analysis The size of network address is 16 bits, so ZigBee is

capable to accept about 65535 devices in a network. Transportion time is 15ms between nodes. In fact, the capacity of ZigBee networks is effected by many factors such as Network coverage and response time. The Following

2009 Ninth International Conference on Hybrid Intelligent Systems

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DOI 10.1109/HIS.2009.164

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analysized capacity in ideal state for ZigBee mesh network[5].

1) Flat mesh network capacity: Mesh network's structure is quite complex, as a result of the network multi-path, the network scan time analyzes is also quite complex, the following replaces the circular by the square region to make the brief analysis. As shown in Figure 2.

Figure 2. ZigBee mesh network diagram

Assuming a straight line in any two cross-points to place a node, and the central node at the center of the entire network topology location, then the energy and focal point for direct communication (one hop) nodes have 8 (around the center of the surrounding eight node), while the central node jump required the consumption of two to reach the node has 16, the three have jumped 24…,The entire network scan time calculated as follows:

T=15ms*8(1+4+9+ +n*n) N=8(1+2+3+ +n) Where T is scan time of entire network, n is the network

layer, N is the network node numbers. When T = 20s, the calculation of available n = 7, N = 224,

namely the mesh network to meet the longest in the 20s at the time of the scan cycle of the maximum capacity of 224 network nodes. To each node of the communication distance of 100 meters calculation, then the entire network coverage for the 1.4 * 1.4 = 1.96 square kilometers area.

If the central node loacated at the edge of the entire network topology, then, it is clear that the network will increase the number of layers ,thus lengthened the system's scan time, that reduces the capacity of the entire network.

Based on the above analysis, the mesh network as much as possible of the central node in the network topology layout of the center, the more close to the edge of the system will scan time longer, in the limited scan time, the entire network capacity will become smaller.

2) Space meshed networ: The space meshed network is more complex compared with the flat meshed network. It is conceivable that in the distribution of 10-storey 20 rooms on each floor of the building, each room provided with a communications node comprising the network structure. In this topology, the entire network scan time calculated as follows:

T = N * t + n*15ms (1 + 2 + 3 + … + N – 1) Where T is the entire network scanning cycle, t is the

single-scan cycle, n is the number of nodes on each plane, N is the space-rise.

Based on the above formula, if the plane of each layer can be seen as a flat mesh structure network, the entire

network is composed of certain vertical distribution's plane meshed network. 25 nodes in each calculation, the space can not exceed a maximum of 9 layers.

If the plane of each layer can be seen as a second branch of the linear structure of the network, the entire network is composed of the certain vertical distribution's two branch linear network. 24 nodes in each calculation, the space can not exceed a maximum of 6 layers.

Based on the above analysis, the spatial mesh network system performance and each node the number of planar layers have a direct relationship, in order to increase network capacity, should be minimized for each plane layer nodes.

B. Routing protocol analysis 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 mesh network usually use Ad hoc On Demand Distance Vector (AODV) algorithm in order to meet the objectives such as cost-effictiveness and path robustness[6][7][8][9].

1) Route Discovery (AODV): The route discovery in a ZigBee network is similar to the AODV routing protocol[10]. The source node broadcasts a route request (RREQ) packet and then intermediate nodes rebroadcast RREQ if they have routing discovery table capacities and the cost is lower. Otherwise, nodes will relay the request along the tree. Once the destination node receives the RREQ, it responds by unicasting a route reply (RREP) packet to its neighbor from which it received the RREQ. The destination will choose the routing path with the lowest cost and then send a route reply.

In this more in-depth look at Route Discovery, we use a specific example to demonstrate route discovery. Consider again an attempt to discover a route from device S to device D. Using Figure.4, we observe that between these two devices are ZigBee routers A,B,C,E,F and G. Looking at box(a) on the left of Figure.4, we observe that device S initially broadcasts the Route Request with a path cost of 0 ( the default setting for the initial transmission ), represented by the number at the head of the arrow. Devices A,B and D detected this initial broadcast, and then broadcast the Route Request after adding an additional cost, based on the received signal strength from deviceS. For instance, device C adds to its own broadcast, a value of 9 to the cost for a total of 9, based on the received signal strength.

We observe in box (b) in the middle of Figure 3, that three Route Requests were observed by device F. Device F then adds the cost based on received signal strength, selects the lowest cost of these and broadcasts the Route Request. Device F also stores the device that provided the lowest cost route to it (after adding the cost from the received signal strength). If the Route Reply returns through F, F will then send its Route Reply to this device.

In box (c ), deviece D then adds the cost based on received signal strength of all incoming Route Requests, observes the lowest path cost of each of these, and then selects the route with the lowest cost. For this example, let us assume that the lowest cost route came from E. This will be the selcted route, and device D will then send a Route

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Response back through device E, which then propagates the Route Response back along its identified lowest cost route, ultimately yieding the Route Reply path shown in Figure 5. Thus, the forward route will be S-B-E-D.

Figure 3. Route Request

Figure 4. Route Response

2) Routing cost: The ZigBee routing algorithm uses a path cost metric for route comparison during route discovery and maintennance. It is supposed that the length of path P is L as an ordered set of devices [D1,D2,…,DL] and a link, [Di,Di+1], as a sub-path of length 2, then the path cost

C{P}=]},{[ 1

1

1i

L

ii DD

Where each o fthe value C{[Di,Di+1]} is referred to as a link cost. The link cost C{l} for a link l is a function with values in the interval[0…7] defined as:

Where pl is defined as the probability of packet delivery on the link l.

The question that remains, howerer, is how pl is to be estimated or measured. The most straighforward method ,available to all, is to form estimates based on an average over the per-frame LQI value provided by the IEEE 802.15.4-2003 MAC and PHY. Even if some other method is used, the initial cost map average LQI values onto C{l} values. It is strongly recommended that implementers check their tables against data derived from tests on production hardware, as inaccurate costs will hamper the operating ability of the ZigBee routing algorithm.

C. Security ZigBee security, which is based on a 128-bit AES

algorithm, adds to the security model provided by IEEE 802.15.4. IEEE 802.15.4 and ZigBee provides four basic security services: authentication, message integrity, message confidentiality, and replay protection. ZigBee’s security services include methods for key establishment and transport device management, and frame protection. ZigBee specification defines secuity for the MAC, NWK and APS layers. Security for applications is typically provided through Application Profiles [11][12][13][14][15].

ZigBee was created with security as a primary objective, using three main security-design principles.

Only a device that has joined the network and successfully received the network key will be able to have its messages communicated more than one hop across the network; End-to-end security is enabled such that only source and destination devices have access to their link key; If an application needs more secruity than is provided by a given network, it shall form its own separate network with a higher security level.

1) Trust Center: To coordinate above security services, ZigBee networks utilize the concept of a trust center. The Trust Center decides whether to allow or disallow new devices into its network. The Trust Center may periodically update and switch to a new Network Key. It first broadcasts the new key encrypted with the old Network Key. Later, it tells all devices to switch to the new key. The Trust Center is usually the network coordinator, but is also able to be a dedicated device. It is responsible for the following secuity roles

Trust Manager, to authenticate devices that requires to join the network Network Manager, to miantain and distribute network keys. Configuration Manager, to enable end-to-end sevurity between devices.

2) Security modes: ZigBee uses three types of keys to manage security: Master, Network and Link.

Master keys are optional keys which not used to encrypt frames. Instead, they are used as an initial shared secret between two devices when they perform the Key Establishment Procedure (SKKE) to generate link Keys. Keys that origiante from the Trust Center are called Trust Center Master Keys, which all other keys are called Application Layer Master Keys.

Network keys perform security Network Layer security on a ZigBee network. All devices on a ZigBee network share the same key. High Security Network Keys must always be sent encrypted over the air, while Standard Secuity Network Keys can be sent either encrypted or unencrypted. Note that High Security is supported only for ZigBee PRO.

Link keys are optional keys which secure unicast messages between two devices at the Application Layer. Keys that originate from the Trust Center are called Trust

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Center Link Keys, while all other keys are called Application Layer Link Keys.

ZigBee PRO offer two different security modes:Standard and High in table 1.

TABLE I. SECURITY MODES:STANDARD AND HIGH

Feature Standard HighNetwork Layer security

provided using a Network Key APS Layer secruity using

Ling Keys Centralized control and

update of keys Ability to switch from active

to secondary keys Ability to derive Link Keys

between devices Entity authentication and

permissions table supported In Standard Security mode, the list of devices, master

keys, link keys and network keys can be maintained by either the Trust Center or by the devices themselves. The Trust Center is still responsible for maintaining a standard network key and it controls policies of network admittance. In this mode, the memory requirements for the Trust Center are far less than they are for High Security mode.

In High Secuity mode, the Trust Center maintains a list of devices, master keys, link keys and network keys that it needs to control and enforce the policies of network key updates and network admittance. As the number of devices in the network grows, so too does the memory required fo the Truse Center.

The ZigBee standard defines a comprihensive securiy architecture and trust management model, including frame encryption, authentication, and integrity at each layer of the ZigBee protocol stack. ZigBee also defines a Trust Center that provides the essential security functions of key management, network management, and device configuration. These sevurity capabilities will enable application designers to create policy-based security features into their product offering and allow customers to depoly secure LR-WPAN networks.

IV. CONCLUTION

Although the ZigBee Alliance was formed in 2002, only recently have certified platforms become available for designers to create real-world applications and embedded product offerings. LR-WPAN technology has been evolving rapidly, with the ZigBee specification having undergone its most recent revision in December of 2006. However, as more certified chip sets, OEM modules, and ZigBee-enbled devices become available, users will have the opportunity evaluate the technology and consider deployment within their enterprise environment.

This paper has presented a broad overview of ZigBee mesh network. The particulary, we are interested to analysis network capacity, routing and secruity. ZigBee mesh network size can accept about 65535 devices according to 16

bit network address, while in the real ZigBee network, the capacity is greatly influenced by practical applications. The paper describes the formulas in the ideal state which the network effected by coverage and response time, and then analysis the mesh routing algorithm (AODV) in detail and routing cost. Finally, the focus has been on decribing the security features and making recommendations regarding the secrue implementation of wireless communication technology within industrial environments.

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