Optimizing the Topology of Bluetooth Wireless Personal Area Networks Marco Ajmone Marsan, Carla F. Chiasserini, Antonio Nucci, Giuliana Carello, Luigi De Giovanni Abstract— In this paper, we address the problem of determin- ing an optimal topology for Bluetooth Wireless Personal Area Networks (BT-WPANs). In BT-WPANs, multiple communication channels are available, thanks to the use of a frequency hopping technique. The way network nodes are grouped to share the same channel, and which nodes are selected to bridge traffic from a channel to another, has a significant impact on the capacity and the throughput of the system, as well as the nodes’ battery life- time. The determination of an optimal topology is thus extremely important; nevertheless, to the best of our knowledge, this prob- lem is tackled here for the first time. Our optimization approach is based on a model derived from constraints that are specific to the BT-WPAN technology, but the level of abstraction of the model is such that it can be related to the slave slave & bridge more general field of ad hoc networking. By using a min-max for- mulation, we find the optimal topology that provides full network master master & bridge connectivity, fulfills the traffic requirements and the constraints posed by the system specification, and minimizes the traffic load of Fig. 1. BT-WPAN topology. the most congested node in the network, or equivalently its energy consumption. Results show that a topology optimized for some traffic requirements is also remarkably robust to changes in the traffic pattern. Due to the problem complexity, the optimal so- Master and slaves send and receive traffic alternatively, so as lution is attained in a centralized manner. Although this implies to provide full-duplex connections, and slaves are entitled to severe limitations, a centralized solution can be applied whenever transmit only when polled by the master. It is intuitive that the a network coordinator is elected, and provides a useful term of master is subject to higher traffic load, and thus higher energy comparison for any distributed heuristics. consumption, relative to slaves. Figure 1 shows an example of BT-WPAN topology, where overlapping piconets are deployed. Being a master or a slave I. INTRODUCTION is only a logical state for nodes. A unit can participate in two IRELESS Personal Area Networks (WPANs) is a new or more overlying piconets, although it can be master in one W wireless technology, which provides short-range con- piconet only. A master or a slave involved in the activity of nectivity between battery-operated portable radio devices, such more than one piconets can act as a bridge, allowing piconets as mobile phones, headsets and personal digital assistants. to form a larger network, a so-called scatternet. Because of WPANs are intended to operate at the 2.4 GHz ISM (Industrial, the use of different hopping sequences, a bridge cannot be ac- Scientific, Medical) band, where no license is required, using a tive in more than one piconet at a time; thus, bridges have to FHSS (Frequency Hopping Spread Spectrum) technique. This switch between piconets on a time division basis, and, while technology is based on the Bluetooth specification [1] and will switching, they must re-synchronize with the current piconet. become an IEEE standard under the denomination of 802.15 This implies a significant overhead that may severely effect the WPANs [2]. system performance. Bluetooth WPANs (BT-WPANs) are typically used to turn One of the most challenging problems in deploying a BT- battery-operated stand-alone devices located in the range of WPAN consists in forming a scatternet that meets the con- about 10 m into networked equipment. The network nodes are straints posed by the system specifications and the traffic re- organized into piconets, each of them composed of one master quirements. The way nodes are grouped into piconets, and device and up to seven active slaves, which are allowed to com- which nodes are selected as masters or bridges, has a signifi- municate with the master only. Each piconet uses a different cant impact on the capacity and the throughput of the system, frequency hopping sequence derived from the master address. as well as on the nodes’ battery lifetime – a crucial factor in net- work connectivity [3], [4], [5]. Knowing which topology opti- This work was supported by the Italian Ministry for University and Research through project RAMON. mizes the BT-WPAN performance is therefore of fundamental M. Ajmone Marsan, C. F. Chiasserini and A. Nucci are with Dipar- importance. timento di Elettronica, Politecnico di Torino, Torino, Italy. E-mail: So far, little research activity was devoted to algorithms for ajmone,chiasserini,nucci ¡ @polito.it. G. Carello and L. De Giovanni are with Dipartimento di Automatica e Informatica, Politecnico di Torino, Torino, Italy. the generation of efficient topologies for BT-WPANs, and for- E-mail: [email protected], [email protected]. mer work dealing with more general wireless ad hoc systems 0-7803-7476-2/02/$17.00 (c) 2002 IEEE. is not applicable. For instance, approaches based on the nodes’ another can be posed in the same way, under the constraints of position,¢ as the one proposed in [6], are suitable for networks the specific physical layer and traffic requirements [4]. that use a single communication channel, such as 802.11 wire- The remainder of this paper is organized as follows. Sec- less LANs, but cannot be applied to BT-WPANs where multi- tion II describes the network scenario and states the problem ple channels are made available by the FH scheme [4]. Algo- under study; Section III presents an Integer Linear Program- rithms proposed in the context of multihop wireless networks, ming (ILP) formulation of the problem. Numerical results are that control the topology by varying the nodes’ transmit power presented and discussed in Section IV. Section V concludes the [3], [7], [8], are not applicable as well. Indeed, BT-WPANs de- paper and points to some aspects that will be subject of future vices use a very low output power (namely, equal to 1 mW) and research. typically do not perform power control1. The issue of determining an optimal topology specifically for II. PROBLEM STATEMENT BT-WPANs is discussed in [4] but is not actually addressed there. The first attempt at finding a solution to the problem In BT-WPANs, connection establishment is a two-step proce- is represented by the work in [5]. Due to the problem complex- dure. By relaying on a universal frequency hopping sequence, ity, the authors adopt a statistical approach, i.e., they generate first an inquiry protocol is used to let a node discover the units random topologies and study the effect of the topology parame- located in its proximity, then a paging protocol is used to estab- ters on the system performance, deriving some useful insights: lish the communication link between two units. The unit that the bridging overhead and the number of links established be- initiates the procedure acts as the master of the connection, and tween nodes are found to have a major impact on throughput, the other unit as a slave, although roles can be exchanged later while slightly less important is the number of piconets that are on. A master or slave can become a slave in another piconet by created. being paged by the master of the other piconet; also, a unit par- In this paper, we solve the problem of forming an optimal ticipating in one piconet can page the master or slave of another BT-WPAN topology that minimizes the traffic load of the most piconet. In this case, the master or slave unit acts as a bridge congested node in the network, or equivalently its energy con- between the two overlapping piconets [1]. sumption, and is such that: In this paper, we refer to a master as a node that is assigned ¥ § 1) full network connectivity is guaranteed; to ¤ piconets and is the master in one of them, to a slave as 2) system specifications are met; a slave node that is assigned to one piconet only, and to a bridge ¥ 3) the traffic requirements are fulfilled; as a node that is assigned to ¤ overlapping piconets and is 4) specific restrictions are met, that may exist on the role a slave in all of them. that some nodes can play. Being BT-WPAN technology intended for local connectivity To the best of our knowledge, this problem was not solved so of battery-driven equipment, it supports low-power implemen- far. tations. The output power of BT-WPAN devices is limited to We provide a min-max formulation of the optimization prob- 1 mW and the entire radio tranceiver is designed for low power lem, and we solve it in a centralized manner, since the problem consumption. The major factor influencing the nodes’ energy complexity and the large number of parameters involved seem consumption becomes the amount of transmitted, received, and to prohibit a distributed approach. This is certainly a limitation processed traffic rather than the distance between transmitter of the proposed formulation. However, besides providing an and receiver. Notice that masters and bridges are therefore the upper bound to any topology derived through distributed heuris- nodes that experience the highest energy consumption in the tics, the centralized optimal solution can be applied whenever a network. network coordinator is elected. The work in [4] is an example Designing the BT-WPAN topology means determining the of asynchronous distributed protocol that can be used to select set of communication links between node pairs that are used a coordinator, which is in charge of determining the role of all to route traffic from each source to the corresponding destina- nodes in the scatternet.