Realistic and Efficient Multi-Channel Communications in Wireless Sensor Networks

1. Realistic and Efficient Multi-ChannelCommunications in Wireless Sensor Networks Author: Yafeng Wu, John A. Stankovic, Tian He, and Shan Lin

2. Outline • References • Related Work • WSN Reality • The TMCP • PMIT Algorithm • Results

3. Introduction • A number of MAC protocols exist that utilize a single channel, but the throughput in these are insufficient to the needs of a high data rate network, such as the multi-media variety. • Current multi-channel protocols are not suitable for general WSN’s due to the small number of channels that are generally available. • Tree-based multi-channel protocol (TMCP) segregates the nodes into trees, delegating a channel to each tree. • The TMCP attempts to realistically solve the problem of multi-channel nodes in a WSN. • The TMCP uses the 802.15.4, or Zigbee, protocol.

4. Related Work • MMSN -- Multi-Frequency Media Access Control for Wireless Sensor Networks • TMMAC – TDMA based MAC protocol • MCMAC – Multi-Channel MAC protocol • All three of these are node-based and attempt to synchronize the channels between upstream and downstream nodes. All 3 use time slots to coordinate their transmissions. They face practical problems in real WSN’s: • A large number of orthogonal channels are needed in dense networks • Require precise time synchronization at the nodes. • Must account for channel switching delay and scheduling overhead • Complex protocols that call for more resources at the motes.

5. Multi-Channel Reality • First, find the number of available orthogonal channels. With 3 motes in a line, and 8 channels available to each mote, test the interference caused by the motes between channels. • Next, determine the interference of 802.11 networks. • By evaluating the packet reception ratio on all channels, it is determined that only three of the eight channels have a reasonably high packet reception ratio. • Furthermore, when the RSSI, or Received Signal Strength Indication dropped below -77dB the packet reception rate dropped dramatically. • Time synchronization errors play a crucial role in time synchronous protocols. For example, the Micaz mote, is considerably bad at time keeping and keeping it synchronized across nodes takes a considerable amount of overhead, without sufficient time synchronization the packet delivery ratio can drop by an incredible 80%.

6. The TMCP

7. TMCP Revealed • Firstly, partition the network into multiple vertex-disjoint subtrees all rooted at the base. • Since each tree has a single channel there is not a need for time synchronization. • Use coarse-grained channel assignment, which uses much fewer channels. • Simple channel coordination scheme. • reduce packet losses by eliminating inter-tree interferences and exploiting spatial reuses of parallel transmissions among subtrees.

8. TMCP Revealed (continued). • TMCP has 3 components • Channel Assignment • Given k orthogonal channels this module segregates the nodes into k subtrees and assigns one unique channel to each subtree. By using orthogonal channels the intranode interference is greatly reduced. • Channel Detection • Data Communication • The DC component manages the data through each subtree.

9. Interference view

10. PMIT Algorithm • Definition 1: INT(u) = {v|v ∈ D(v, Iv)}, where D(v, Iv) is the interference disk with node v in its center and radius Iv, and the interference value of a node u is defined as int(u) = |INT(u)| • Iv= (1+α)×Rv, and α > 0 implies that all of u’s neighbors belongs to INT(u). • Definition 2: The intra-tree interference value of a tree T is defined as int(T) = max{int(u) : u is a non-leaf of T} • Given k orthogonal channels, partition the network into k vertex-disjoint trees with minimizing the maximum intra-tree interference value of all trees, or PMIT. • This problem is NP-complete.

11. Results • Between 1.6 and 2.7 times the throughput compared to the spanning tree protocol with a single channel. • Decreases collisions. • Greater scalability. • 2.8 times greater aggregate throughput with 42% less latency. • When the workload increases the performance of the TMCP becomes unstable.