A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

Departamento de Tecnología Electrónica. University of MálagaETSI de Telecomunicación, Campus de Teatinos, 29071 – Málaga- SpainE-mail: ecasilari@uma.es

9th WSEAS Int.Conf. on APPLIED COMPUTER SCIENCE (ACS'09)

E. Casilari, J. Hurtado-López, J.M. Cano-García

UNIVERSIDAD DE MÁLAGA, SPAINGenova (Italy), 17th October 2009

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Index

1. Introduction: WPANs and 802.15.4/Zigbee

2. Overview of IEEE 802.15.4

3. Strategies to avoid beacon collision

4. Results

5. Conclusions

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Introduction: 802.15.4/Zigbee

Standards IEEE 802.15.4 (PHY and MAC) and Zigbee jointly describe a protocol stack for the definition of Wireless Personal Area Networks (WPAN).

Aimed at providing solutions for low-cost wireless embedded devices (transceivers under 1$) with consumption and bandwidth limitations

Low rate (up to 250 Kbps), short range (up to 10 m) communications

In immature state but appealing candidate to support a wide set of services, particularly for low consume domotic sensor networks (although real time services are also contemplated for services such as voice or biosignals)

Main challenge of 802.15.4/Zigbee: potentiality to set up self-organizing (ad hoc) networks capable of adapting to diverse topologies, node connectivity and traffic conditions.

Advantages of 802.15.4 mainly depend on the configuration of MAC sublayer

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Operation modes of 802.15.4

The MAC layer of IEEE 802.15.4 enables two alternative operational modes:

1. Non beacon-enabled (point-to-point) mode: Access control is governed by non-slotted CSMA/CA

Higher scalability but nodes must be active all time (elevated power consumption)

Real time constraints cannot be guaranteed

2. Beacon-enabled mode, A coordinator node periodically sends beacons to define and synchronize a WPAN formed by

several nodes

Nodes can wake up just in time to receive the beacon from their coordinator and to keep synchronized (power efficiency)

Synchronization permits to guarantee time slots (resources) to delay sensitive services

Main problem: scalability → Time must be divided between clusters

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Configuration of beacon enabled networks Two classes of nodes: the so-called Full-Function Devices (FFD) and the

Reduced-Function Devices (RFD).

Star topology: A FFD performs as the network ‘coordinator’, in charge of the communications of a set (or

‘cluster’) of RFD nodes (the ‘children’ nodes). The coordinator periodically emits a beacon to announce the network and to keep children synchronized

Beacon Interval (BI), divided in an active part and an inactive part. Active part consists of a ‘Superframe’ of 16 equally-spaced time slots.

Contention Free Period (CFP): guaranteed slots for certain nodes

Contention Access Period (CAP): nodes compete for the medium access

All the transmissions take place during the Superframe Duration (SD)

In the inactive period all nodes (including the coordinator) may enter a power saving mode to extend the lifetime of their batteries

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Structure of a 802.15.4 superframe

2 ; 2BO SOBI a SD a

Where a= 15.36, 24 or 48 ms when a rate of 250, 40 or 20 kbps is employed

Configuration of BO and SO: trade-off BO >> SO: almost all BI corresponds to the inactivity period, high power saving, low rate

can be achieved

Other case: lower power saving but higher rate

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Zigbee Cluster-trees Apart from the tree networks with a single coordinator, the Zigbee standard permits the

association of cluster coordinators to form cluster-trees.

One of the coordinator nodes assumes the central role: PAN or Zigbee Coordinator (ZC). The rest of the coordinators are Zigbee Routers (ZRs)

ZRs responsible for retransmitting the data from any ‘child’ node (leaf) within their clusters

Zigbee specification does not impose any protocol nor algorithm to create this type of networks

Existing commercial 802.15.4-compliants modules do not support the formation of cluster-tree topologies

Coexistence of more than one coordinator → possibility that beacons (simultaneously emitted by two adjacent coordinators) get lost due to collisions.

Beacon collision provokes children to desynchronize from the router

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Strategy to avoid beacon collision IEEE 802.15.4 Task Group 15.4.b has proposed two generic strategies to cope with beacon collision

Sequencing of the beacons and Superframes: in non-overlapped periods during the Beacon Interval

Advantages: Standard is respected, GTS can be implemented

Problems: scheduling of beacons within the different Beacon Interval and especially the duration of the superframes must be carefully designed. Otherwise: serious problem of scalability

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Objective

Assumptions:

Pessimistic case: Any node can interfere the rest, no radio planning (all nodes transmit in the same channel)→ Superframes cannot overlap

Hierarchical cluster-tree, all traffic flowing to the ZC (typical case of a sensor network)

Problem to solve: to define the superframe durations (SOi) of the clusters

Objective: to maximize the utilization of the BI

Condition to be accomplished in any case (for a network of NC coordinators: routers+ZC):

1 1

2 2C C

i

N NSOBO

ii i

BI a SD a

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Policies to distribute the Beacon Interval (I)

1. Equidistribution: All Superframe orders are set to the same value

1,i CSO SO i N 2 22log logBO

CC

SO BO NN

Problem: In most ZigBee/802.15 sensor networks, data are forwarded from the end nodes

(sensors) to a gateway or central node which most probably will reside in the ZC. This centralization may cause the ZC to become a traffic bottleneck if its SO is not higher than the rest.

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Policies to distribute the Beacon Interval (II)

2. Fixed Priorization of the superframe order of the coordinator: 2.A. Superframe order of the coordinator is set to twice the value of the rest

1

2,2

i CSO SO i NSO SO

1

2

2

1

2 2 2 ( 1) 2C

i

NC

i

NSOBO SO SO

Ci SD

SDi

a a a a N

22log 1 ( 1) 4 2 1BO

C CSO N N

1

2,

1i CSO SO i N

SO SO

2.B. Superframe duration of the coordinator is set to twice the value of the other superframes

1 2 22log log 11

BO

CC

SO BO NN

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Policies to distribute the Beacon Interval (III)

3. Topology based distribution: The order is particularized for each router

depending on the number of the leaf nodes

Proposal of an iterative algorithm:

li be the number of leaf nodes ‘depending’ of the i-th coordinator (or supported traffic)

The SOj of the coordinator with the highest lj is increased in one unit

If the BI is not exceeded by the sum of the SDs, the increase of SOj is admitted & lj is divided by two

The process is repeated while no SO can be increased without exceeding the BI

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Simulation parameters

Simulations in OMNeT++

Original model of 802.15.4 was extended to support cluster-trees

Three different network topologies: three-layer hierarchy in which leaf nodes (those generating traffic) do not have any children.

Simulations for different traffic loads of ‘upstream’ traffic

Network performance evaluated by means of the goodput (mean bit rate at which the ZC receives the data from the leaf nodes) and battery consumption

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Evaluated scenarios (II)

Scenario 3: routers support different trafficScenario 2: Coordinator supports several routers

Scenario 1: the coordinator support the same traffic than router

In all cases the number of leaf nodes is the same

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Results (I)

Scenario 1 Scenario 2

0 1 2 3 4 5 60

100

200

300

400

500

600

Packets per second (per leaf node)

Goo

dput

(Byt

es/s

)

Goodput of the network as a function of the traffic load

Same SO for all nodesPrioritization of ZC (SO1=2SOi)Prioritization of ZC (SO1=SOi+1)Topology based distribution

0 1 2 3 4 5 6 7 8 90

100

200

300

400

500

600

700

800

900

Packets per second (per leaf node)

Goo

dput

(Byt

es/s

)

Goodput of the network as a function of the traffic load

Same SO for all nodesPrioritization of ZC (SO1=2SOi)Prioritization of ZC (SO1=SOi+1)Topology based distribution

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Results (II)

Scenario 3

Results of the topologies in which the Zigbee coordinator concentrates the traffic (e.g.: the scenario 3) evidence that resources cannot be equally distributed among the clusters.

Scenario 1; limit case in which a router has to transport the same traffic of the Zigbee Coordinator. SO order of both clusters must be equal

More activity: more power consumption but a bad design of SO can also lead to a high battery consumption without increasing the network goodput

0 2 4 6 8 10 12 140

100

200

300

400

500

600

700

800

Packets per second (per leaf node)

Goo

dput

(Byt

es/s

)

Goodput of the network as a function of the traffic load

Same SO for all nodesPrioritization of ZC (SO1=2SOi)Prioritization of ZC (SO1=SOi+1)Topology based distribution

0 2 4 6 8 10 12 140

5

10

15

20

25

30

Packets per second (per leaf node)

mAh

Total Battery consumption in the network

Same SO for all nodesPrioritization of ZC (SO1=2*SOi)Prioritization of ZC (SO1=SOi+1)Topology based distribution

Scenario 3 (Power consumption)

A Study of Policies for Beacon Scheduling in 802.15.4 Cluster-Tree Networks

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Conclusions & Future Work

Problem of configuring SD is a key aspect for hierarchical 802-15.4/Zigbee cluster-trees

Even in small networks with less than twenty nodes a proper design of the duration of the 802.15.4 superframes is crucial to achieve a reasonable network performance.

An iterative strategy to design the SD of the nodes of a Zigbee network has been proposed.

SD is defined as a function of the topology (traffic)

Simple policies to distribute the beacon interval without taking into account the topology and traffic condition in the PAN leads to an inefficient network design

Future work should investigate the adaptation of this type of algorithms to more complex situations: node mobility, not all the routers interfere, etc.