A Routing Vector Method (RVM) for Routing in Bluetooth Scatternets Pravin Bhagwat IBM, Thomas J. Watson Research Center Yorktown Heights, NY pravinb@us.ibm.com Adrian Segall Dept. of Electrical Engineering Technion, Israel segall@ee.technion.ac.il Abstract—Bluetooth is a promising new technology for short range wire- less connectivity between mobile devices. Initially, Bluetooth will be used as a replacement for point-to-(multi)point cables. However, in due course of time, solutions for forming multihop ad hoc networks over Bluetooth (referred to as scatternets) will be needed. In this paper, we explore the de- sign space of routing protocols over Bluetooth scatternets. We argue that ad hoc routing protocols for Bluetooth must make a different set of design compromises compared to those being developed by the MANET working group. These differences result primarily from the specific characteristics of the Bluetooth physical and link layer. Our proposed routing method is based on the concept of route vector, which is an efficient method for encoding source route paths in Bluetooth scatternets. We describe the protocols for route discovery and packet for- warding. Our design illustrates three main design compromises, namely minimization of soft-state, protocol simplicity, and bandwidth conserva- tion, all of which are crucial for efficient operation over small size Bluetooth scatternets. I. I NTRODUCTION Bluetooth [1],[2] is a single chip, low-power, wireless com- munication module that will fit inside mobile phones, laptops, palm computers, digital cameras, and cordless headsets and al- low users to interconnect these devices easily and quickly with- out the need for cables. Bluetooth utilizes a short-range radio link to exchange information, enabling effortless wireless con- nectivity between devices and other peripherals. The radio op- erates in the globally available 2.45GHz ISM band, allowing in- ternational travelers to use Bluetooth-enabled equipment world- wide. Since the radio link is based on frequency-hop spread spectrum, multiple channels (frequency hopping sequences) can co-exist in the same wide band without interfering with each other. Two or more units sharing the same channel form a pi- conet, where one unit acts as a master, controlling the commu- nication in the piconet and the others act as slaves. The MAC layer protocols for communication between the master and the slaves in a piconet are well defined by now [2]. Bluetooth channels use a frequency-hop/time-division-duplex (FH/TDD) scheme. The channel is divided into 625-msec in- tervals, called slots, where a different hop frequency is used for each slot. The master transmission starts in even-numbered s- lots, while the slave transmission starts in odd-numbered slots. Packets can be 1,3 or 5 slot long and are transmitted in consec- utive slots. A slave is allowed to transmit in a given slot if the master has addressed it in the preceding slot. There is no direct This work was performed when the author was visiting the IBM T.J.Watson Research Center MAC layer communication between slaves. One of the goals of the present work is to propose higher layer protocols for al- lowing inter-slave unicast and broadcast communication within a piconet. In the Bluetooth architecture, multiple piconets with overlap- ping area of coverage can co-exist since their frequency hopping patterns are mutually orthogonal. A group of interconnected pi- conets is referred to as a scatternet. A node can participate in two or more piconets on a time sharing basis, i.e., at any instan- t the node can be active in only one piconet. Such a unit can receive packets from one piconet and relay them to the other pi- conet(s) it is connected to. Protocols for inter-piconet commu- nication among units in a scatternet have not been defined yet. A plausible solution is to model a scatternet as an instance of an ad hoc network and use already known methods of routing [3], [4], [5], [6]. While this approach is certainly feasible, it may not be the most efficient solution. Bluetooth layer presents a unique set of features and limitations which known ad hoc routing pro- tocols are not optimized for. For example, due to packet size limitation at the Bluetooth baseband layer, all MANET style solutions will require fragmentation and reassembly of packet- s at each relay node. This will lead to two problems; 1) in- creased buffering requirement at each node and 2) higher store and forward delay at each hop. Instead, if the forwarding is supported at the Bluetooth slot level, delay as well as buffering requirement will be reduced (at a marginal cost of link efficien- cy). In many ways, this tradeoff is similar to the hop-by-hop IP routing Vs. ATM switching tradeoff. There are conceptual similarities between the Bluetooth baseband layer and ATM cell transport layer and the overhead of running MANET style algo- rithms over Bluetooth is similar to the overhead of performing hop-by-hop IP routing over ATM networks. The other distinguishing feature of scatternets is their topol- ogy of interconnection. Scatternets and wireless LANs, for ex- ample, differ in terms of their communication capabilities. T- wo bluetooth nodes cannot hear each other unless they form a master-slave pair. This is in contrast to the wireless LAN- s where any two nodes within proximity can hear each other’s transmissions. It is also anticipated that scatternets will differ from classical ad hoc networks in terms of applications, traffic characteristics, mobility patterns, and scaling requirements. In most application instances, scatternets will be quasi-static, short lived, and small. Therefore, scalability and adaptivity features