Rendezvous Scheduling in Bluetooth Scatternets

P. Johansson*, R. Kapoor**, M. Kazantzidis**, M. Gerla** Ericsson Corporate Research* (Per.Johansson@ericsson.com)

Computer Science Department, UCLA** (rohitk, kazantz, gerla@cs.ucla.edu)

Abstract- Bluetooth scatter-nets are formed by interconnecting Bluetooth piconets in an ad-hoc fashion and consists typically of the handheld electronic devices from one or more user(s). Thus, scatternets may he used to form and interconnect ad-hoc Bluetooth personal area networks (PANS). The inter-piconet Bluetooth units, i.e. the gateways interconnecting the piconets in a scatternet, need to time division multiplex their presence in each of their piconets. This requires an inter-piconet scheduling (IPS) algorithm, operating in each inter-piconet unit in order to efficiently coordinate its presence with other Bluetooth units. In this paper an IPS algorithm based on periodic rendezvous points is proposed and analyzed with simulations. The algorithm is called Maximum Distance Rendezvous Point (MDRP) and utilizes the Bluetooth SNIFF functionality to establish the periodic rendezvous points between gateways and their peer nodes. Simulation results show that TCP works well with MDRP, hut the large round trip delays caused by the inter- piconet gateway nodes give a slowly growing flow control window for TCP. The latter will in particular have impact on the TCP performance in scatternets hosting “thin” (embedded) clients with limited dynamic memory capacity.

I. INTRODUCTION

The Bluetooth wireless technology [l] is a low power, low cost, short range RF technology that focuses on replacement of cables between electronic devices. It uses a fast frequency- hopping scheme and operates in the unlicensed Industrial Scientific-Medical (ISM) band at 2.4 GHz.

Even though pure cable replacement with point-to-point radio links is perhaps the most obvious use for Bluetooth, the capability to form Bluetooth networks opens up a whole new arena for applications. Networks of small handheld electronic devices will enable an environment where information may be accessed, exchanged and shared seamlessly among the devices in the network. Typically, such a network could consist of a cellular phone, a PDA, a notebook PC, an mp3 player, a DVD player etc., all of which are devices that a person carries around in his every day life both for work and pleasure [2]. Often, this kind of network is referred to as a personal area network (PAN). However, a PAN may, from time to time, also include devices that are not carried along with the user, e.g., an access point for Internet access or sensors located in a room. Moreover, devices from other users’ PANS may also be interconnected to enable sharing of information, which could be anything from business cards to multiplayer game interaction.

The Bluetooth network architecture is based on the piconet, a star topology, which is defined by two or more Bluetooth units that share the same frequency hopping channel. The central unit in a piconet (the master) may control up to seven units (slaves). Any Bluetooth unit is capable of becoming the master of a piconet.

The nature of PANS in general implies a minimum of precontiguration, i.e. it should be possible to establish the network in an ad-hoc fashion with minimum user intervention. In this context, Bluetooth has been tailored to provide such fimctionality through its inherent ad-hoc discovery and connectivity capabilities. Also, it is likely that most PAN devices will be battery-driven, which makes efficient use of power an important issue. The Bluetooth piconet architecture enables the controlling master unit to apply a strict medium access control through the use of a polling scheme. This gives a better control over power consumption and bandwidth usage compared to a random access scheme since slave units transmit only when scheduled by the master unit. Hence, the Bluetooth wireless technology provides functions that make it a natural choice for PAN connectivity.

In order to further enhance the networking capabilities of Bluetooth, piconets may be interconnected into scatternets, which requires some units to be present in two or more piconets. However, since a Bluetooth unit is expected to host only one radio transceiver, this presence needs to be handled in a time division manner, i.e. the inter-piconet unit will switch between the piconets. As the unit switches between the piconets, it can act as slave in one or more, but as master in at most one piconet. The inter-piconet units may also forward traffic between the piconets, i.e. operate as gateways between piconets. Since the inter-piconet unit cannot receive information from more than one piconet at a time, the need to co-ordinate the presence of masters and inter-piconet devices in each piconet is necessary to achieve a controlled performance. The work in this paper focuses on describing the inter-piconet scheduling issue in a scatternet. In addition, the performance of a proposed scheduling scheme is analyzed through simulation experiments. Results are presented for a mix of TCP and voice traffic traversing across different scatternet scenarios.

Most of the published work on Bluetooth so far has focused on the single piconet and issues related to scheduling of units within one piconet, e.g., [3] [6]. Some work has also

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throughputs achieved by the TCP connections. The lower throughput of TCP 2 is explained by the fact that dest5 (which is the gateway in TCP2’s path) is a slave in a piconet which contains 3 gateways; its share of the piconet bandwidth is, thus, reduced. Also, TCPl achieves a throughput slightly less than the 360 kbit/sec ideal throughput. This is due to the high round-trip-delay of TCP (Section A.l), which results in a slow initial increase of the TCP window.

The complementary cumulative delay distribution of the “prioritized” voice packets for both the voice connections is shown in Figure 9. It can be seen that about 90% of the voice packets have a delay of less than 330 msec, which is quite acceptable. In addition, very few (around 0.5%) voice packets are dropped due to queue overflow.

Complementary Cumulative Delay Distribution

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0.8 g 0.6 $ 0.4 E 8

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Delay (msec) -4

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F&i-e 9. Complementary Cumulative Delay Distribution for the two voice connections

VII. CONCLUSIONS

The introduction of Bluetooth scatternets enables a wider and more flexible use of Bluetooth network as means to create local and short range connectivity. However, the scatternet also presents a technical challenge with respect to network formation and scheduling of traffic within a Bluetooth network. In this paper the focus was set on the scheduling issues in general and on the inter-piconet scheduling (IPS) in particular. An overall description of the issues related to scheduling in scatternets was given. For instance, the intra-piconet scheduler (IRPS) and the IPS need to be able to interact in order to provide an efficient scatternet scheduler. A periodic rendezvous IPS algorithm was proposed, called the Maximum Distance Rendezvous Point algorithm (MDRP). MDRP uses a periodic superframe in which the rendezvous points are distributed according to a maximum spread approach. The MDRP algorithm may utilize the Bluetooth SNIFF mnctionality to set-up and maintain the periodic rendezvous points between the gateways and their peer nodes. MDRP was analyzed by means of simulations based on NS-2, enhanced with a Bluetooth scatternet model. The simulations covered experiments on an ad-hoc network

of Bluetooth nodes using AODV as the ad-hoc routing protocol.

The simulations presented herein showed that the TCP behavior is affected by the large round trip delay imposed by the inter-piconet gateways, causing TCP to grow its window slowly. Moreover, the simulations showed that the delays for voice packets, in presence of other TCP connections, are acceptable if head-of-line priority is applied to the voice packets in the gateway nodes. The TCP simulations also confirmed a well known relation between the bandiwdth- delay product and the available buffer capacity in the bottleneck nodes, which in the inter-piconet case is the gateway nodes. Hence, the choice of time granularity for the inter-piconet time division multiplexing may heavily depend on the buffering capacity in the Bluetooth devices. A fact that will affect the ability for “thin” clients to efficiently operate as gateway nodes in a scatternet.

REFERENCES

[l] Specification of the Bluetooth System - Core vol. 1 vl. 1, www.bluetooth.com

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[7] G. Miklos, A. Racz, Z. Turanyi, A. Valko, P. Johansson, “Performance Aspects of Bluetooth Scatternet Formation”, Poster session Mobihoc 2000, Boston, MA, Aubmst 11, 2000.

[8] N. Johansson, U. Kiirner, L. Tassiulas, “A Distributed Scheduling Algorithm for a Bluetooth Scatternet,” to appear at the Seventeenth International Teletraffc Congress, ITC’ 17, Salvador da Bahia, Brazil, September 24-28, 2001.

[9] “Network Simulator (NS-2)“, www.mash.cs.berkelev.edu/ns/ [lo] C. Perkins, E. Royer, “Ad-hoc on-demand distance vector

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[ 1 l] P.T. Brady, “A model for generating on-off speech patterns in two-way conversation”, Bell System Technical Journal, Sept. 1969, pp. 2445-2471.

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