Intra-cluster communications: In this case, both the communicating MNs reside in the same cluster.
On the other hand, the amount of intra-cluster communications grows significantly as k decreases.
In this scheme, the collision-free intra-cluster communications are organized by the cluster-heads using a TDMA scheme, and a CDMA scheme is overlaid on the TDMA to organize the interference-free inter-cluster communications.
For intra-cluster communications, different channels are assigned to adjacent clusters to reduce collisions, while for inter-cluster communications, cooperative MIMO links are scheduled to improve energy efficiency and concurrent transmissions are enabled by assigning different channels to them.
Furthermore, a multi-hop routing algorithm for inter-cluster communications is proposed which basically considers the tradeoff between distance and the energy of relaying nodes.
It is of great significance to determine the optimal number k of clusters, because the amount of inter-cluster communications increases with k.
Most cluster systems rely on security products like firewalls for their security, but they cannot guarantee security of intra-cluster communications, which can be a weak spot that hackers exploit for further security attacks.
For example, for a WSN with 150 sensors in a 250×250m2 field, with five available channels, the protocol can achieve three times saturated throughput while saving 14% energy per bit for inter-cluster communications.
A recent study by Lee and Kim (2007) [22] proposed a security framework to protect intra-cluster communications by encrypting and authenticating all packets with fine-grained security where any two communicating processes dynamically generate and share a cryptographic key, called a session key.
Communication among clusters of functional units in the stream processor impose a latency cost during both the message-passing phase and the parity-check phase of the TDMP algorithm with early termination; the inter-cluster communications latency is a significant factor in limiting the throughput of the decoder.
The HMAG provides a local mobility management for each cluster, reduces the signaling cost by integrating the functions of the AAA, provides a route optimization strategy for intra- and inter-cluster communications, and reduces handoff latency for the MNs intra-cluster movements.
