Q2. What are the contributions in this paper?
In this paper, the authors present solutions for the data gathering and routing problem with in-network aggregation in WSNs. The authors present Grid-based Routing and Aggregator Selection Scheme ( GRASS ), a scheme for WSNs that can achieve low energy dissipation and low latency without sacrificing quality.
Q3. What are the future works in this paper?
In particular, the authors developed GRASS ( Grid-based Routing and Aggregator Selection Scheme ), a scheme for WSNs that combines the ideas of fixed cluster-based routing together with application-specific data aggregation in order to enhance the wireless sensor network performance in terms of extending the network lifetime, while incurring acceptable levels of latency under data aggregation.
Q4. How many MAs can be used to solve the ILP problem?
in order to speed up the execution of the ILP solver, the authors limited the search space of the number of MAs to be about one half of the total number of LAs, which is the maximum number of second level aggregators under the two level aggregation assumption.
Q5. What is the function that is used to backtrack the route?
Whenever a node is inserted in the new-candidate-list, CBAH keeps a pointer to the predecessor node, which is required to backtrack the route(s) to the source node in case this node falls on the selected route.
Q6. Why does GRASS have more interference when the number of sources is large?
This is because in DD data forwarding paths from different sources may cross or overlap with each other anywhere in the network area, thus there is more interference when the number of sources is large, whereas in their scheme each LA source node sends data on the virtual grid, thus data flows on the grid faster and the routes are selected to balance power consumption in the network.
Q7. What is the power consumption at each node?
The power consumption at each node is also dependent on the determination of the actual routing of the data traffic, i.e., the authors must find the power consumed by each LA source node when participating in routing data over G.
Q8. Why did the authors not impose an upper bound on the number of MAs?
Since with multiple levels of aggregations the maximum number of MAs is n − 1, where n is the total number of LAs, the authors did not impose any upper bound on the number of MAs, as this will not provide any significant reduction in the search space.
Q9. What is the function that is invoked to determine the route of the source node?
After finding the set of routes for each source s in a group g (routes[g][s]) and in order to determine what route among these routes the source s will use, CBAH invokes a function called select-route(.), and described in Figure 5.
Q10. What is the importance of the candidate-list?
The importance of the candidate-list is that it allows one to quickly select the next LA node needed to expand the route search for a certain source node.
Q11. How much energy is consumed by a sensor node in receiving a 1000-bit data?
the energy consumed by a sensor node i in receiving a 1000-bit data packet is 1000*50 nJ/bit= 50µJ, while the energy consumed in transmitting a data packet from sensor i to sensor j is given by T = 50µJ + 100nJ/m2 × d2ij .
Q12. What is the main reason for GRASS to be able to improve the network lifetime?
Their results show that, when compared to other approaches in the literature, the proposed scheme is able to improve the network lifetime while incurring acceptable levels of latency and without sacrificing quality.
Q13. What constraint ensures that the amount of traffic sent to the other nodes is sufficient?
The following two constraints find the maximum power consumption over all LA nodes, while respecting the maximum power consumption limit of any LA node,ρ ≥ Pi/Ei , ∀ i (19)ρ ≤ 1/Tmin (20)The following constraint ensures that the power consumed at each LA node is sufficient to send the amount of required traffic to the other nodes.
Q14. What is the effect of number of MAs on the network lifetime?
Effect of number of MAs: the authors study the effect of varying the number of MAs, M , for a fixed value of n on the increase in the network lifetime in the 2L aggregation scheme.
Q15. What is the difference between the two-level and multi-level aggregation schemes?
If data gathering and reporting delay is not a concern, multi-level aggregation schemes would be a good choice in this case as these schemes consume less power and hence allow for longer network lifetime.
Q16. What is the heuristic for a network?
The heuristic CBAH will find routes on the graph G for each source node of each group such that power consumption in the network is balanced.