on exploiting transient contact patterns for data forwarding in delay tolerant networks

On Exploiting Transient Contact Patterns for Data Forwarding in Delay Tolerant Networks

Wei Gao and Guohong Cao

Dept. of Computer Science and EngineeringPennsylvania State University

Outline

IntroductionTrace-based Pattern FormulationData Forwarding ApproachPerformance EvaluationSummary & Future Work

Data Forwarding in DTNs Carry-and-Forward methods

Mobile nodes physically carry data as relaysMajor problem: appropriate data forwarding metric for

relay selectionData forwarding metric measures the node’s capability of

contacting others in the future

B

A C

0.7

0.5

Our FocusEffective data forwarding metric for a short time

constraintTransient node contact pattern

Consider different time periods in a dayNode contact pattern may vary temporallyRelays selected based on cumulative contact patterns

may not be the best choice during a short time period

Transient Contact Pattern Transient contact distribution

Highly skewed during different time periods Example: different people contact during different time periods

in a day Cannot be differentiated from the cumulative contact rate

Contact frequency

Transient Contact Pattern Transient connectivity

Some nodes remain connected during specific time periods to form Transient Connected Subnets (TCS)

Example: a student remains connected with his classmates during the class

Multi-hop communication within the TCS

TCS

10:00AM

11:00AM

Transient Contact Pattern Node A sends data to a destination at 12pm with a 6-hour

time constraint Valid time period: 12pm-18pm A node forwards data to another node with higher data

forwarding metric

Cumulative contact rate Transient

contact rate

Direct contact

Transient connected subnet

Major ContributionsFormulate transient node contact patterns based

on realistic DTN tracesDevelop data forwarding metrics to analytically

predict node contact capability with better accuracy

Outline

IntroductionTrace-based Pattern FormulationData Forwarding ApproachPerformance EvaluationSummary & Future Work

TracesRecord user contacts at university campusVarious wireless interfaces

Bluetooth: periodically detect nearby peersWiFi: associate to the best Access Point (AP)

Transient Contact DistributionSkewed distribution of node contacts

Over 50% between 12pm and 16pm

Less than 7% between 22pm and 7am

Transient Contact Distribution Alternative appearances of on-period and off-period

Contact process during on-periods is stable and predictableContacts during off-periods are random and unpredictable

On-periodAn on-period is determined by a set of contacts

happened at time . For ,

EndStart

Distribution of On/Off-Period LengthAccurately approximated by normal distribution

hours for both traces

On-periods are short

Transient ConnectivityDistribution of contact duration

Many contacts with non-negligible duration

Over 20% last longer than 1 hour

Transient Connectivity The temporal change of TCS size of a mobile node

Formulated as Gaussian function

Outline

IntroductionTrace-based Pattern FormulationData Forwarding ApproachPerformance EvaluationSummary & Future Work

Data Forwarding Metric The capability Ci of node i to contact other nodes

during time tc: the current time te: the data expiration timeThe expected number of nodes that node i can contact

N: the total number of nodes in the networkcij: pairwise contact probability between node i and j

Pairwise Contact Probability Basic idea: only based on the contact process during

on-periods

Case 1: tc is within anon-going on-period

Case 2: the next on-period starts before te

Case 1: tc is within an on-going on-period

, where

Pairwise Contact Probability

Contact process within an on-period is modeled as Poisson

PDF of the on-period length

T = te – tc

Case 2: the next on-period starts before te

, where

Pairwise Contact Probability

The last on-period ends before tc

PDF of the off-period length

Exploiting Transient ConnectivityA node indirectly contacts all the nodes in a TCS

if it contacts any one node in the TCS

Temporal change of TCS size:

Outline

IntroductionTrace-based Pattern FormulationData Forwarding ApproachPerformance EvaluationSummary & Future Work

ComparisonsData forwarding metrics

Contact Counts (CC)BetweennessCumulative Contact Probability (CCP)

Data forwarding strategiesCompare-and-ForwardDelegation ForwardingSpray-and-Wait

Performance ComparisonCompare-and-Forward in the UCSD trace

100% 20%Better delivery ratio

Similar forwarding cost

Two Cases of Contacts

70%

30%

SummaryEffective data forwarding metrics for data

forwarding in DTNs with a short time constraintImproves delivery ratio with similar forwarding cost

Transient node contact patternsTransient contact distributionTransient connectivity

Future workThe overhead of identifying transient contact patternThe temporal evolution of social network structure

Thank you!

http://mcn.cse.psu.edu

The paper and slides are also available at:http://www.cse.psu.edu/~wxg139

Length Distribution of On/Off-Period

On-periods are generally shortMost contacts are covered by on-periodsWe have hours for both traces

Characterizing Transient Contact Pattern

Transient contact patterns are characterized at real-time at each nodeFor nodes i and j, the parameters of their on/off-period

are updated every time they contactEach node detects its TCS whenever it contacts

another node A detecting beacon message is broadcasted within the TCS Each node in the TCS replies to the original sender upon

receiving the beacon.

Prediction Error

The start of a new on-period?Random contact in an off-period?

Node contact randomnessUnclassifiable contact

Off-periods longer than 24 hours Most contacts happen during on-periods

The two cases are only found at nodes with low contact frequency

Different values of Ton

8 8

Node Buffer ConstraintForwarding only one data item

A node simply drops the data in cases of limited bufferThe consideration of node buffer constraint is trivial

Forwarding multiple data itemsThe consideration of node buffer constraint is

formulated as a knapsack problemVarious data forwarding metrics can be applied

Related Work Node contact capability is estimated from various ways

Prediction of node mobility and co-location events Semi-Markov chain, Kalman filter

Node contact pattern as abstraction of node mobilityExploitation of social network concepts

Centrality, social community, homophily Various metrics apply to the same forwarding strategy

Single-copyMultiple-copy

Spray-and-Wait, Delegation Forwarding