A Study of Policies for Beacon Scheduling in Cluster-Tree Networks 1 Departamento de Tecnología Electrónica. University of Málaga ETSI de Telecomunicación, Campus de Teatinos, – Málaga- Spain 9th WSEAS Int.Conf. on APPLIED COMPUTER SCIENCE (ACS'09) E. Casilari, J. Hurtado-López, J.M. Cano-García UNIVERSIDAD DE MÁLAGA, SPAIN Genova (Italy), 17 th October 2009

A Study of Policies for Beacon Scheduling in Cluster-Tree Networks 2 Index 1.Introduction: WPANs and /Zigbee 2.Overview of IEEE Strategies to avoid beacon collision 4.Results 5.Conclusions

A Study of Policies for Beacon Scheduling in Cluster-Tree Networks 3 Introduction: /Zigbee  Standards IEEE (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 /Zigbee: potentiality to set up self-organizing (ad hoc) networks capable of adapting to diverse topologies, node connectivity and traffic conditions.  Advantages of mainly depend on the configuration of MAC sublayer

A Study of Policies for Beacon Scheduling in Cluster-Tree Networks 4 Operation modes of  The MAC layer of IEEE 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 Cluster-Tree Networks 5 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 Cluster-Tree Networks 6 Structure of a superframe  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 Cluster-Tree Networks 7 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 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 Cluster-Tree Networks 8 Strategy to avoid beacon collision  IEEE 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 Cluster-Tree Networks 9 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 ( SO i ) of the clusters  Objective: to maximize the utilization of the BI  Condition to be accomplished in any case (for a network of N C coordinators: routers+ZC):

A Study of Policies for Beacon Scheduling in Cluster-Tree Networks 10 Policies to distribute the Beacon Interval (I)  1. Equidistribution:  All Superframe orders are set to the same value  Problem: In most ZigBee/ 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 Cluster-Tree Networks 11 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  2.B. Superframe duration of the coordinator is set to twice the value of the other superframes

A Study of Policies for Beacon Scheduling in Cluster-Tree Networks 12 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:  l i be the number of leaf nodes ‘depending’ of the i -th coordinator (or supported traffic)  The SO j of the coordinator with the highest l j is increased in one unit  If the BI is not exceeded by the sum of the SDs, the increase of SO j is admitted & l j 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 Cluster-Tree Networks 13 Simulation parameters  Simulations in OMNeT++  Original model of 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 Cluster-Tree Networks 14 Evaluated scenarios (II)  Scenario 3: routers support different traffic  Scenario 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 Cluster-Tree Networks 15 Results (I)  Scenario 1  Scenario 2

A Study of Policies for Beacon Scheduling in Cluster-Tree Networks 16 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  Scenario 3 (Power consumption)

A Study of Policies for Beacon Scheduling in Cluster-Tree Networks 17 Conclusions & Future Work  Problem of configuring SD is a key aspect for hierarchical /Zigbee cluster-trees  Even in small networks with less than twenty nodes a proper design of the duration of the 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.