Quick Convergecast in ZigBee/IEEE 802.15.4 Tree-Based Wireless Sensor Networks

1

Quick Convergecast in ZigBee/IEEE 802.15.4 Tree-Based Wireless Sensor Networks

Yu-Chee Tseng and Meng-Shiung Pan

Department of Computer Science

National Chiao Tung University, Taiwan

(in ACM MobiWac, 2006, candidate of best paper award)

2

Outline

Introduction Minimum delay beacon scheduling (MDBS) problem Algorithms for the MDBS problem

Optimal solutions for special cases Centralized tree-based assignment Distributed slot assignment

Simulation results Conclusions

3

Outline




4

Introduction

In many surveillance applications, convergecast is an important operation sensors periodically report sen

sed environmental events to a sink

ZigBee is a developing standard which is considered to satisfy the needs of WSN

PHY Layer

MAC Layer

Network & Security

Application Framework

Applications

802.15.4

ZigBeeSpecification

sink

sensor

5

Goal

To design protocols to achieve low-latency convergecast in ZigBee tree-based wireless sensor networks Why low-latency?

The late-arrived sensory readings are meaningless Why ZigBee tree-based network?

Devices in ZigBee tree-based network can operate in low-power mode

6

Contributions

Define a minimum delay beacon scheduling (MDBS) problem for ZigBee tree-based WSNs

Prove MDBS problem is NP-complete Find special cases in MDBS Propose centralized and distributed algorithms, whic

h are compliant to the ZigBee standard

7

Network scenario

In a tree network, routers can send regular beacons to support low duty cycle operations

Active Active

data from end devices

data from end devices

AB C

A

B

C

Sink

ZigBee router ZigBee end device

A’s beacon sche:

A wakes up to hear C’s beacon and report data

To C To C

Zzz .. Zzz …. Zzz ..

Active Active

C’s beacon sche:

ZigBee coordinator

8

Superframe structure in a ZigBee tree network According to ZigBee standard, beacons are scheduled in the fro

nt of non-overlapped active portions Superframe structure of IEEE 802.15.4

A superframe can contain 2BO-SO non-overlapped active portions (slots)

Beacon interval = u × 2BO

1

Active portion = u × 2SO

2 3 2BO-SO

★ In WSN, beacon interval >> active portion

u=aBaseSuperframeDuration

9

Schedule beacons in a ZigBee tree network When choosing a slot, routers should consider

interferences from other routers

Indirect interferenceTwo routers have indirect interference if they have at least one common neighbor

Direct interferenceTwo routers have direct interference if they can hear each other’s beacons

A B A B

C

10

Schedule of C

report report

Schedule of B

Schedule of A

A beacon schedule example

Latency from B to C is almost one beacon interval !!! Can up to 4 min. in ZigBee

AB C

Sink

B collects data here!!!

B reports to C here!!!

11

Schedule of B

Schedule of A

Schedule of C

report report

A better beacon schedule example

Latency from B to C is at most one active portion !!!

AB C

Sink

B collects data here!!!

B reports to C here!!!

12

Outline




13

Minimum delay beacon scheduling problem

0 1 0

1

0

7

3

2

4

3

5

Given G = (V, E), GI = (V, EI), and k slots A router i can be assigned to slot a s(i), where

s(i) [∈ 0, k-1] (choosing a proper active portion) s(i) ≠ s(j) if (i, j)∈EI (avoiding direct and indirect nterference)

6

s(i)=?

k=8

routers comm. linkInterference relationship

14

Minimum delay beacon scheduling problem(hop latency) The latency from i to j, where (i, j)∈E, is defined as

dij = (s(j)-s(i)) mod k (difference of slot number between i and j)

0 0

1

0

7 2

4

3

5

6

Hop Latency: 2

k=8

i

j 3

1

i

j

Hop Latency: (4-7)%8 = 5

15

Minimum delay beacon scheduling problem(report latency of a node) The report latency of router i is the sum of per hop delay from

i to the sink

0 0

1

0

73

2

3

5

i

4

6

1

Report Latency: 3k=8

16

Minimum delay beacon scheduling problem(convergecast latency) The convergecast latency is the maximum report latency betw

een all routers L(G)

0

1

0

2

3

54

6

1

37

0

Convergecast Latency: 7+5+2 = 14

k=8

critical path

17

Minimum delay beacon scheduling problem Definition of Minimum Delay Beacon Scheduling (MDBS) prob

lem Given G=(V, E), G’s interference graph GI=(V, EI), and k availabl

e slots, the MDBS problem is to find an interference-free slot assignment s(i) for each i∈V such that the convergecast latency L(G) is minimized

Definition of Bounded Delay Beacon Scheduling (BDBS) problem Given G = (V,E), G’s interference graph GI = (V, EI), k available sl

ots, and a delay constraint d, the BDBS problem is to decide if there exists an interference-free slot assignment s(i) for each i V ∈such that the convergecast latency L(G) ≤ d

18

Minimum delay beacon scheduling problem Theorem 1: The BDBS problem is NP-complete

Proof: 1. Given a solution, we can check if L(G) ≤ d in polynomial time.

2. We then prove that the BDBS problem is NP-hard by reducing the 3

conjunctive normal form satisfiability (3-CNF-SAT) problem to a

special case of the BDBS problem in polynomial time.

19

Outline




20

Optimal solutions for special cases Regular linear network

Theorem 2. For a regular linear network, if k ≥ h + 1, a bottom-up slot assignment can achieve a report latency of |V | − 1, which is optimal. Each node has an interference relation with any node within h

hops from it.

21

Optimal solutions for special cases Regular ring network

Theorem 3. For a regular ring network, assuming that k ≥ 2h and [(|V |−1) / 2] ≥ 2h, a heuristic slot assignment can achieve a report latency L(G) = [(|V |−1) / 2] + h, which is optimal within a factor of 1.5 [ ] means floor function

22

Centralized tree-based assignment Given G = (V,E), GI = (V, EI), and k, our centralized slot assignment

heuristic algorithm is composed of three phases: Phase 1: From G, construct a BFS tree T rooted at sink t Phase 2: Traverse T in a bottom-up manner. For each vertex v visited, w

e first compute a temporary slot number t(v) for v as follows. If v is a leaf node, we set t(v) to the minimal nonnegative integer l such that for

each vertex u that has been visited and (u, v) E∈ I, (t(u) mod k) ≠ l. If v is an in-tree node, let m be the maximum of the numbers that have been a

ssigned to v’s children. We then set t(v) to the minimal nonnegative integer l >m such that for each vertex u that has been visited and (u, v) E∈ I, (t(u) mod k) ≠ (l mod k).

After every vertex v is visited, we make the assignment s(v) = t(v) mod k. Phase 3: Traverse vertices from t in a top-down manner. When each vert

ex v is visited, we try to greedily find a new slot l such that (s(par(v)) − l) mod k < (s(par(v)) − s(v)) mod k, such that l≠s(u) for each (u, v) E∈ I , if possible. Then we reassign s(v) = l.

Each in-tree router tries to find a slot that induces the least report latency to its children

To further reduce the report latency of routers

23

Centralized tree-based assignment:Example (k=8)

E

A

DC

B0 1 0

1

02

2 3 2

4

3

5

6

Interference neighbors’

slots 0 and 1

3

4

Convergecast Latency: 6

Report Latency from 6 4

s(C) must be larger than s(A)

24

Distributed slot assignment

Based on the observation that each router can consider the neighbors within 2r as interference neighbors r is the default transmission range

Each router uses larger transmission power to exchange HELLOs with its interference neighbors The HELLO packet contains the sender’s slot information

25


This algorithm is triggered by the sink t setting s(t) and then broadcasting its beacon. A router v≠t that receives a beacon will find itself a slot as follows. Node v sends an association request to the beacon sender.

If v fails to associate with the beacon sender, it stops the procedure and waits for other beacons.

If v successfully associates with a parent node par(v), it computes the smallest positive integer l such that (s(par(v))− l) mod k≠s(u) for all (u, v) E∈ I and s(u) = NULL. Then v chooses s(v) = (s(par(v)) − l) mod k as its slot.

Then, v broadcasts HELLOs for a time period twait. If it finds that s(v) = s(u) for any (u, v) E∈ I such that u’s ID is larger than v’s ID, then v has to choose another slot assignment and going back to the above step.

After twait, v can finalize its slot selection and broadcast its beacons.

Each router tries to find a slot that induces the least report latency to its parent

26


t

AB

2

4

0 1

3

5

5

2

4

3

beacon

7

beaconAsso. req.6

6

I choose 6!!

ID 1 ID 10

Need to find another slot

Start to send its beacon

Convergecast Latency: 7

27

Outline




28

Simulation results We compare our algorithms to a random slot assignment sche

me (RAN) In RAN, each router randomly chooses a slot which does not inte

rfere with its interference neighbors CTB =centralized tree-based; DSA=distributed slot assignment

Fixed tx range Fixed network size

Centralized algo. outperforms others The larger tx range implies the

more interference neighbors

5 to 7x better6 to 9x better

29

Outline




30

Summary

We have define a new minimum delay beacon scheduling problem

This is the first work that models the quick convergecast in ZigBee/IEEE 802.15.4 based WSNs

Our solution is compliant to the standard and can be implemented easily

Documents

Quick Convergecast in ZigBee/IEEE 802.15.4 Tree-Based Wireless Sensor Networks