Link Range and (In)Stability on Sensor Network Architecture Bhaskaran Raman, Kameswari Chebrolu, Naveen Madabhushi, Dattatraya Y. Gokhale, Phani K. Valiveti, Dheeraj Jain Indian Institute of Technology, Kanpur The First ACM International Workshop on Wireless Network Testbeds, Experimental evaluation and CHaracterization (WiNTECH 2006), A MOBICOM 2006 Workshop Presenter : Ahey Date : 2007/05/07

Outline Motivation External antenna communication range study Link stability measurements Conclusions Discussions

Motivation (1/2) The authors have two claims here: (1) Use of external antenna  improve communication range  more single hop links  simplify protocol design  energy efficient (2) Dynamic metric based routing questionable due to the high variability of RSSI

Motivation(2/2) What should be the network architecture? – What is the radio communication range? Study communication range, with the use of external antennae – Expected number of hops from/to base node Temporal stability of links’ error rate, RSSI, LQI – Does dynamic distributed routing make sense? – If so, at what time scale? If not, what else?

Outline Motivation External antenna communication range study Link stability measurements Conclusions Discussions

External Antenna: Preliminaries Cost: $50-$120 Form factor: 0.5-1m, kg dBi ( decibels relative to isotropic) G is said to be the gain of the antenna When we want to compare antenna gain to an absolute level, we use dBi = 10·log10(G) The antenna gain is referenced to an isotropic antenna Link symmetry is not affected by antenna type because transmit gain = receive gain

Experimental Setup (1/2) Tmote sky with CC2420 ( compliant) Internal antenna: 3.1dBi gain External connector: SMA – Grid (24dBi, 8 o ), sector (17dBi, 90 o ), omni (8dBi) |  beam width Several antenna combinations (1) internal-internal, (2) omni-internal, (3) sector-internal, (4) grid-internal, (5) omni-omni, (6) sector-omni, (7) grid-omni.

Experimental Setup (2/2) Transmitter: – 6000 packets, about 2 minutes, with one 24 byte packet sent every 20ms – Transmit power: 0dBm (max. possible) Receiver: – TOSBase, connected to laptop – Collect RSSI, LQI values Environment: – Dense foliage (with heavy path loss) – Narrow road (with mostly line-of-sight path)

Range in Dense Foliage Receiver: – on tripod (1.5m) – Internal ant. only Transmitter: – 1.5m – 6000 packets (60 bins x 100 pkts) Computer avg. pkt. error rate and avg. RSSI over 100 packet bins

Range in Narrow Road (1/2) Transmitter: – at 3.8m for sector/grid antennas Rows with bold font Indicate the approximate range of the particular transmitter antenna Not always the farther, the higher

Range in Narrow Road (2/2)

Implications of Link Range More one-hop nodes, lesser # hops ==> better network lifetime Foliage: range of about 90m – Useful for applications such as the redwood study – Can have just a single-hop network! Volcano monitoring: m range reported Range of about 800m with grid antenna – Useful in situations like Volcano monitor, BriMon

Outline Motivation External antenna communication range study Link stability measurements Conclusions Discussions

Controlled Calibration: Setup Step attenuator: varied from 0dB to 93dB 5000 packet = 50 bins x 100 packets For each bin: error rate, and avg. RSSI The receive sensitivity can be found

Receive Sensitivity around 90dB

RSSI Variability in Other Environment

Results similar to Emnet 2006 “RSSI is underappreciated” Good correlation between RSSI and error rate above some threshold But larger spread region (large variability) for multi-path prone environment (foliage) LQI variability similar to RSSI variability – 1/LQI, 1/PSR metrics would be unstable

Temporal variability in RSSI Omni-50m, foliage, avg. pkt. error rate is 7.2% (neither close to 0% or 100%), without binning Large variation (about 15dB)

good unstable

Outline Motivation External antenna communication range study Link stability measurements Conclusions Discussions

Conclusions Range: m – Number of one-hop nodes can be increased – Better life-time Variability in time-scales of 2s, 20s, hours – Dynamic metric-based routing may not be useful – Plan for “ good” links, use centralized routing Problems arise when RSSI window overlaps with the steep region – Provide sufficient link margin to plan the deployment to have “ good” links at the beginning – Dynamic routing metrics unnecessary

Conclusions Base node is a powerful node anyway – Can do centralized routing –Design, implementation, network might easier Think real hard before falling for: randomly deployed sensor nodes, self-organizing, distributed dynamic routing – Good for solving nice problems on paper – Practical value questionable

Outline Motivation External antenna communication range study Link stability measurements Conclusions Discussions

Contributions & Strength:  A good survey about using external antenna to improve radio range  Give some insight about the possibility of one- hop sensor network and dynamic routing (for outdoor application)  Provide temporal variation of RSSI within one node (which is not discussed in the previous Emnet paper)

Discussions Weakness:  Though RSSI variation is large, it can be a not bad link quality predictor as the experiment results show. The conclusion here is too strong but not totally convincing.  The duration of the experiment is not long enough ( about 2 hours).

Relevance to our research Han got similar conclusion for our testbed. Static routing metrics (to decide whether it is a stable link by RSSI from the very beginning) may be enough. However, for the medium quality links, more experiments can be done to quantify the accuracy using RSSI, LQI as the predictor of PRR dynamically. Is one-hop or multi-hop better? Need more network overhead v.s reachability analysis.