1. ASCENT: Adaptive Self-Configuring sEnsor Networks Topologies.
Alberto Cerpa and Deborah Estrin, In Proceedings of the Twenty First International Annual Joint Conference of the IEEE Computer and Communications Societies (INFOCOM 2002), New York, NY, USA, June
November 21, 2018
Haeyong Kim Seoul National Univ.

2. Distributed sensor network scenario
Ex) A habitat monitoring sensor network
Deployed in a remote forest by dropping sensor nodes from a plane or by placing them by hand
Ad-hoc deployment
Cannot expect the sensor field to be deployed in a regular fashion
Uniform deployment does not correspond to uniform connectivity
Energy constraints
Expend as little energy as possible to maximize network lifetime
Unattended operation under dynamics
Preclude manual configuration, pre-configuration
Haeyong Kim Seoul National Univ.

3. Assumptions Reacts when links experience high packet loss
High enough node density to connect the entire region
When node density is low, ASCENT mechanism does not applicable (generally, all nodes should be used to form an effective network)
Does not consider partition (partition detection and repair techniques are leaved to future work)
Haeyong Kim Seoul National Univ.

4. Primary contributions
Use of adaptive techniques
Applications configure the underlying topology based on their needs while trying to save energy to extend network lifetime
Use of self-configuring techniques
React to operating conditions measured locally
Use of adaptive techniques : Does not presume fairness, degree of connectivity or capacity required
Haeyong Kim Seoul National Univ.

5. ASCENT design ASCENT adaptively elects “active” nodes “passive” nodes
Stay awake all the time and perform multi-hop packet routing
“passive” nodes
Periodically check if they should become active
Two-hop network example
Only some nodes are active. Very high message loss (because range) sending “help messages” passive neighbor nodes join the network
Neighbor receive help message, it may decide to join the network (becomes an active neighbor) signals the existence of new active neighbor to other passive neighbors by sending a “neighbor announcement message” continues until the number of active nodes stabilizes on a certain value
Delivery of data from source to sink more reliable
Haeyong Kim Seoul National Univ.

6. ASCENT state transitions
Random timer turns on the nodes to avoid synchronization
When a node starts, test state
Exchange data and routing control messages and sets up a timer (Tt), sends neighbor announcement messages with node ID
Before Tt expires, neighbors > NT (neighbor threshold) or DL (data loss rate) > DL before entering in the test state passive state
Passive  test : active neighbor가 보낸 help 메시지. LT (loss threshold)
Passive mode : overhear all packet (even if not addressed to the passive node)  gather information
Haeyong Kim Seoul National Univ.

7. ASCENT parameter tuning
Choices left to the applications
NT (neighbor threshold)
the average degree of connectivity of the network
Trade-off between the energy consumed and/or the level of interference vs. the desired sensing coverage
LT (loss threshold)
The maximum amount of data loss that an application can tolerate
Application dependent
temperature measurements vs tracking of a moving target
Tt, Tp (timer of test state, passive state)
Trade-off of power consumption vs. decision quality
Ts (timer of sleep state)
Trade-off of energy saving vs. reaction time to dynamics
Level of interference = packet loss
Tt, Tp : Robust decision in presence of transient paket losses, neighbor determination에 영향
Haeyong Kim Seoul National Univ.

8. Neighbor and data loss determination
Measured locally by each node while in passive and test state
Each node increase the sequence number when each packet transmitted
When a sequence number is skipped, loss is detected
Assumes application data packets also have some mechanism to detect losses
N이 커질수록 loss가 커지게 되므로, loss threshold도 같이 증가 시켜야만
합리적인 N값을 얻을 수 있다.
그렇지 않으면 active neighbor가 증가함에도 불구하고 N값은 줄어들 수가 있다.
Haeyong Kim Seoul National Univ.

9. Neighbor and data loss determination (cont’d)
The number of active neighbors (N)
The number of neighbors with link packet loss smaller than the neighbor loss threshold (NLS)
NLS = 1- (1/N)
This formula worked best
N : the number of neighbors calculated in the previous cycle
NLS : neighbor loss threshold
Average data loss rate (DL)
Calculated based on the application data packets
Data losses are detected using data sequence numbers
If the message was not received from any neighbor, it is considered a data loss
Control messages are not considered
Help, neighbor announcement and routing control
Depending on the routing strategy, a node may receive multiple copies of the same application data packet.
Haeyong Kim Seoul National Univ.

10. ASCENT interactions with routing
runs above link and MAC layer below routing layer
is not a routing or data dissemination protocol
decides which nodes should join the routing infrastructure
Nodes become active or passive independent of routing protocol
Does not use state gathered by the routing protocol
Does not requires changing the routing state
Test state (actively routing packets)  passive state (listen-only)
Cause some packet loss
Improvement :
Traffic could be rerouted in advance by informing the routing protocol of ASCENT’s state changes
Haeyong Kim Seoul National Univ.

11. Conclusion ASCENT has the potential of ASCENT mechanisms were
significant reduction of packet loss
Increase in energy efficiency
ASCENT mechanisms were
responsive and stable under systematically varied conditions
Future work
Use of load balancing techniques to distribute the energy load
Use of wider area links to detect network partitions
Haeyong Kim Seoul National Univ.