04-07-2013, 02:45 PM
A Backbone-Aware Topology Formation (BATF) Scheme for ZigBeeWireless Sensor Networks
A Backbone-Aware.pdf (Size: 1.54 MB / Downloads: 19)
Abstract
In a tree-structured ZigBee wireless sensor network, nodes close to the root of
the tree (i.e., hot-spot nodes) may exhaust their power earlier than those distant from the root
due to heavy loads on packet forwarding. This hot-spot problem is inherent in tree-structured
networks and may demand extra energy to recover from failures of hot-spot nodes. In this
paper, the backbone-aware topology formation (BATF) scheme is proposed to alleviate the
hot-spot problem. BATF utilizes power-rich nodes to form a backbone tree that does not
suffer from the hot-spot problem. Each power-rich node independently initiates a ZigBee
tree network that attracts associations from ZigBee-compliant devices in order to distribute
packet-forwarding loads over a larger set of nodes. Issues of BATF such as the partition
of address space and ZigBee-compliant routing are discussed in detail. Simulation results
confirm that BATF does alleviate the hot-spot problem as it improves network lifetime as
well as data collection capability. Comparisons with native ZigBee protocols show that the
improvement comes from our protocol design rather than simply introducing power-rich
nodes.
Introduction
ZigBee [1] is a wireless sensor network (WSN) standard developed by ZigBee Alliance [2].
It has been widely applied to information collection applications, especially ones such as climate
monitoring, soil status monitoring, river status monitoring, etc, that require periodical
monitoring of status in target areas. ZigBee supports three networking topologies: star, tree,
and mesh. Compared to the other two, tree topology has advantages in high connectivity and
low routing overhead, which is more suitable for periodical monitoring applications. However,
a tree-structured ZigBee network may suffer from the hot-spot problem, which refers
to the phenomenon that nodes close to the root of the tree (i.e., hot-spot nodes) may exhaust
their power earlier than those distant from the root due to a heavy load on packet forwarding.
This is because, in most applications, data packets flow from every node into the root, and
hot-spot nodes forward packets for all its descendants in the tree. As a result, a hot-spot node
is bound to run out its energy earlier, resulting in a link breakdown between its parent and the
subtree rooted at it. Extra efforts should be done to recover from such failures, which wastes
precious energy, thus degrading network lifetime to some extent.
An Overview of ZigBee’s Operations
In this section, tree-structured ZigBee network and its main mechanisms are introduced. As
shown in Fig. 1, a tree-structured ZigBee network [1] includes a coordinator, the root of
the tree, and several ZNs as tree nodes. Each ZN has a unique network address and a depth
value which is the distance (in terms of hop count) between the ZN and the root of the tree.
For example, ZN F has network address 37 and depth 3. Furthermore, ZNs can be further
classified into two types: router nodes and leaf nodes. Router nodes can have children, while
leaf nodes cannot. ZN A in Fig. 1 is a leaf node, and ZN F is a router node.
Packets issued by ZNs are routed along the tree. When a ZN receives an incoming packet,
it determines the next-hop node of the packet without consulting a routing table. The destination
address of the packet alone suffices for making the routing decision. That is, network
address is not only used for node identification but also for routing. In order to perform the
networking operation as described above, two main mechanisms are required: tree-structured
ZigBee topology formation mechanism and the corresponding routing protocol, which are
illustrated in the following two subsections.
Backbone-Aware Topology Formation (BATF)
In a conventional ZigBee tree, only the root of the tree acts as a ZigBee coordinator. In BATF,
however, each PN is utilized as a virtual coordinator, fromwhich a ZigBee-compliant tree can
independently develop. A ZigBee device joins one of these trees and sees the virtual coordinator
as a conventional ZigBee coordinator. All PNs are connected by another tree network
(referred to as backbone tree). The hot-spot problem is alleviated because BATF introduces
more ZNs with a depth value 1 in order to share traffic loads, and the load for each of the
ZNs is significantly reduced. Figure 2 shows the typical topology yielded by BATF where
the backbone tree connects the coordinator and all PNs. In general, the resulting topology is
effectively a connected forest consisting of several PN-rooted ZigBee trees. Consequently,
the depth value of each ZN is generally reduced.
Routing in BATF
Although network address is no longer flat, standard ZigBee routing rules still work in PNrooted
trees. That is, a ZN routes a packet to one of its children only if the destination address
of the packet falls within the range of the address space allocated to that child; otherwise,
the ZN sends the packet to its parent. It follows that a packet with destination address falling
into the range of other PN-rooted ZigBee tree will be routed to the backbone tree first. We
give a formal mathematical verification as follows.
Simluations
We used NS-2 [16] as the simulation tool to evaluate BATF in periodical data collection
wireless sensor network applications. We adopted NS-2.32 as our simulation platform, in
which the 802.15.4 modules developed by Zheng [17] have been included. On the basis of
the 802.15.4 architecture, we developed the ZigBee tree routing module and BATF module.
As shown in Fig. 5, the module blocks with shadows are themodules we developed, while the
others are the 802.15.4 modules. In the BATF module, two main functions are included: (1)
Backbone Routing and (2) ZN Attraction Function. Backbone routing in BATF uses the firstlevel
address and routing tables to forward a packet to a correct PN. ZN Attraction Function
is responsible for depth information modification, neighbor ZN selection, and re-association
triggering.
Conclusion
We have presented BATF, a network infrastructure for tree-structured ZigBee networks that
utilizes PNs as virtual coordinators for regular ZigBee devices. BATF distributes the traffic
load of hot-spot nodes by organizing all PNs into a backbone tree.Yet it is ZigBee-compatible
from the perspective of ZNs. In other words, BATF demands no modification from regular
ZigBee devices. The simulation result has shown that BATF mitigates the hot-spot problem
as it successfully improves network lifetime as well as data collection capability.