Bluetooth scatternet formation: A survey

Abstract

This paper describes the issue of piconet interconnection for Bluetooth technology. These larger networks, known as scatternets, have the potential to increase networking flexibility and facilitate new applications. While the Bluetooth specification permits piconet interconnection, the creation, operation and maintenance of scatternets remains open. In this paper, the research contributions in this arena are brought together, to give an overview of the state-of-the-art. First, operation of the Bluetooth system is explained, followed by the mechanism for link formation. Then, the issue of piconet interconnection is considered in detail. Processes for network formation, routing and intra- and inter-piconet scheduling, are explained and classified. Finally, the research issues arising are outlined.

Introduction

The Bluetooth (BT) technology, as specified in the Bluetooth System Version 1.1 [29], is the first flexible, mass market protocol for wireless ad hoc operation. This means that global control of the network is relinquished, permitting spontaneous deployment without dependence on fixed infrastructure. However, decentralised network organisation is required to enable self-planning and management, which is a key challenge in developing fourth generation technology.

The BT specification has been converted into IEEE standard 802.15 [16], [88]. From the outset, the specification has been mass-market oriented, driven by a special interest group (SIG) of major manufacturers, keen to develop a robust, flexible, and economically viable technology that can be easily incorporated into a multitude of devices to enhance their functionality via short connectivity, using low power radio links (10–100 m) in the unlicensed 2.4 GHz industrial, scientific and medical (ISM) band. Most current Bluetooth devices offer the modest range of 10 m using the (low) power class 2. This is a deliberate action, as Bluetooth is currently primarily a technology for personal area networks (PAN), where device battery power is limited. However, in the long term, higher power and extended range could lead to a range of further applications, beyond personal area networks. Additionally the technology may well prove a useful starting point for the development of other applications, and systems, such as those in the military arena.

In its current form, Bluetooth is well on the way to reaching impressive levels of penetration, helped by its royalty free status. It is estimated that 35 million chip-sets were produced in 2002, with an estimated rise to 510 million by 2006 [35]. The characteristics of the Bluetooth chip [96] have facilitated this, based on an occupancy of 90 mm², power consumption of 25 μA when idle with a peak of 25 mA, and a cost of less than 4 USA dollars per terminal for high volume production.

Since the conception of the Special Interest Group (SIG) in 1998 and the release of specification (v1.1 in February 2001), Bluetooth has received a considerable amount of attention, being vigorously marketed by the SIG who promise a technology to “seamlessly connect all your mobile devices”. Commercial interest frequently centres around its application (potential future application is given in [20] for example). An important limiting factor in developing future applications are the networking possibilities that the technology can facilitate. As currently specified, Bluetooth self-organises networks of up to eight active devices in piconets, but the specification permits much more. Piconet interconnection is possible, permitting multi-hop links between out-of-range devices. However, mechanisms for establishing and maintaining such scatternets remain open, challenged by the need to provide totally decentralised, high performance solutions for potentially dynamic environments.

Consequently, the extent to which the Bluetooth networking capabilities can be advanced are receiving particular attention, and this is the focus of our survey on the state-of-the-art. The rapid pace of research in this area has led to a large, dispersed and fragmented literature, which broadly seeks to further investigate and extend the operation of Bluetooth at the equivalent of the network layer in the traditional OSI model. This constitutes development for network formation, maintenance, scheduling and routing in the context of Bluetooth.

We begin by introducing the Bluetooth specification and the operation of the system for a single piconet. To support scatternets, we then consider the extension of functionality in the BT specification. The broad issues involved in defining a protocol to support scatternet operation are described in Section 2. In Section 3, we describe the general issues which influence the design of protocols in the context of Bluetooth. In Section 4, we consider the different possible topologies that have been proposed for scatternet applications. In Section 5 we consider the problem of forming and maintaining a scatternet, specifically the process of establishing device role. In Section 6, we describe the different techniques that have been applied to assess network performance. In Section 7, we consider the issue of routing. Finally, in Section 8 we address the problem of scheduling inter- and intra-piconet communication.

The original and first use of Bluetooth technology has been for small clusters of devices, which operate using the concept of a piconet. This is a collection of devices sharing the same communication channel. Devices such as laptops, mobile phones, PDAs are currently the main proponents of the technology, and it is likely to remain this way in the short term. However, there are a number of arguments which support the development of the technology for inter-connecting piconets to form scatternets. Like many technological developments, scatternets may precede concrete applications. However, high levels of market penetration will increase the density of Bluetooth enabled devices, thereby increasing the potential for scatternet based applications.

Scenarios requiring a greater amount of connectivity have received limited attention. In [38], the authors argue that scatternet functionality is important to allow flexible formation of Bluetooth personal area networks. Additionally, it is argued that scatternet functionality may also be used to improve the performance of a group of nodes that are either already part of a scatternet, or part of separate piconets. The roles of devices in such nodes may be rearranged to adapt to a new traffic distribution.

Bluetooth also can be used in a cellular fashion via access points for wireless LAN applications, which creates further opportunities for scatternet applications. Bluetooth WLANs are already in service for large scale commercial IP applications, such as conference scenarios like “Cebit 2003”, where 150 Bluetooth access points were provided for piconet formation [47]. Critics may be quick to dismiss the use of Bluetooth in this context, due to superior performance of the dominant wireless LAN technology, WiFi (IEEE 802.11b). This offers a higher data rate over a much greater distance (50 m versus 10 m for Class 2 Bluetooth devices). In [20], it is pointed out that while these comparisons are technically correct, Bluetooth has a much more powerful business model associated with it, based on two key points. Firstly, Bluetooth is designed for a myriad of wireless PAN applications. Secondly, the cost and size of the technology means that it is being included in equipment by default. These points mean that opportunities will exist to further network devices, including access points, for Bluetooth applications.

Finally, the low cost, power efficient specification of the technology means that there are potential Bluetooth applications in other ad hoc scenarios such as sensor networks [1]. These networks involve a large number of densely deployed wireless sensors, which need to be low power and low cost devices. The current literature acknowledges Bluetooth as being too expensive and too power consuming for sensor network application. However, there are a number of additional points which need to also be considered. The cost of a sensor network node should be less than 1 dollar to make the network feasible [74]. Clearly, the current Bluetooth production cost of 4 dollars [96] exceeds this, but it is feasible that this cost will fall further. In [27], it is argued that Bluetooth is not currently efficient for sensor networks because turning them on and off adds to the energy consumption. However, this has to be contrasted against the additional features that a Bluetooth sensor network could provide, including higher data rates than many sensor network solutions [1], off-the-shelf specification and compatibility with any other Bluetooth enabled device. This would enable direct access to other wired and wireless services. There may be scenarios where these advantages out weigh the increase in cost and additional power consumption. If Bluetooth were adopted in this context, scatternet formation for large networks would be essential.