Figure 915 Star and PeertoPeer Topologies
- Figure 9.16. Cluster Tree Topology
- CID = 1 CID = 3
In a star topology data may be exchanged only between the network coordinator and the DEVs or network nodes (no direct communication between DEVs). There are two types of communication: uplink, from the DEVs to the network coordinator; and downlink, from the network coordinator to the DEVs.
In a peer-to-peer topology, each DEV can communicate with others within its radio range. There is no network coordinator. However, a DEV can become such a coordinator if it is the first device to communicate on the channel. A basic peer-to-peer network can be formed when a new DEV searches for another DEV with which it can communicate. If no such DEVs are found, the new DEV can become the network coordinator and wait for other DEVs to join.
A more complex topology that can be constructed out of the peer-to-peer topology is a cluster tree topology (Figure 9.16). In the figure, the circle area represents a cluster. Multiple clusters interconnect with each other, forming a multi-cluster network. Each cluster contains network nodes and a network node designated to be a cluster head (CH). A cluster head controls its cluster. All network nodes in a cluster are only FFDs (there are no RFDs in a cluster tree network). The cluster head of the first established cluster is called a designated device (DD). Therefore, the entire multi-cluster network will have only one DD. At the cluster formation phase, a DD forms the first cluster by assigning the cluster identifier zero and transmitting the beacon. A new network node that is not associated with any cluster receives the beacon and starts making the connection request to the cluster head (or DD in the case of the first cluster). The cluster head or DD will accept the new network node to become a member of the cluster. Several network nodes join the cluster and the cluster becomes bigger and bigger. The DD may promote a network node to be the cluster head and partition a big cluster into small clusters resulting in a multi-cluster network. A unique cluster ID (CID) is assigned to each cluster.
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