BLUETOOTH is a low-power, open standard for implementing

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1 Proceedings of the 39th Hawaii International Conference on System Sciences - 26 Bluetooth Discovery with Multiple Inquirers Brian S. Peterson, Rusty O. Baldwin, and Richard A. Raines Department of Electrical and Computer Engineering, Air Force Institute of Technology, USA Brian.Peterson@wpafb.af.mil, Rusty.Baldwin@afit.edu, Richard Raines@afit.edu Abstract Although the discovery time between a pair of Bluetooth devices has been well characterized, the impact of multiple inquiring devices on the discovery time has not. Discovery time must be considered when forming or maintaining Bluetooth networks with multiple inquiring devices. The presence of a second inquiring device can significantly delay, and even preclude, the discovery of a scanning device by an inquiring node. When a scan window opens, each inquiring device does not have an equal likelihood of discovering the scanning node since one inquirer may consistently transmit on the scan frequency before the other. The discovery time probability density function with multiple inquiring devices is presented. I. INTRODUCTION BLUETOOTH is a low-power, open standard for implementing PANs [][2]. It is a popular protocol with 4 million Bluetooth-enabled phones shipped worldwide and over, new Bluetooth products being developed by more than 2, companies [3]. It uses a slow hop frequency hopping spread spectrum scheme with 79 -MHz frequency slots (23 in some countries) in the 2.4 GHz band. Members of a Bluetooth piconet hop together among the 79 frequencies (numbered -78) with a sequence that is a function of the master s free-running counter (CLK) and the first 28 bits of the master s 48 bit address. A Bluetooth device must be discovered and incorporated into a Bluetooth network, or piconet, to follow the piconet s hop sequence. The master of a Bluetooth piconet coordinates time-division duplex transmissions of up to seven active slaves by alternating between master and slave transmissions in 625 μs time slots. The discovery process requires the piconet master to be in the inquiry substate when a potential slave device is in the inquiry scan substate. Although a piconet is limited to eight active devices, piconets can be connected into scatternets by nodes that are members of multiple piconets [4]. The discovery time between an inquiring and scanning device has been fully characterized [5]. Furthermore, the expected inquiry time for multiple devices alternating between inquiring and scanning has been characterized with some simplifying assumptions [6]. However, neither characterization considers the impact of the dependencies of multiple inquiring devices hop patterns on the inquiry time. Due to the limited hop patterns, a second inquiring device can prevent an inquiring device from discovering a scanning node. In Section II, the Bluetooth discovery process is presented. The interaction of inquiry hop sequence is discussed in Section III and the discovery time probability density functions (pdfs) with multiple inquiring devices are presented in Section IV. II. BACKGROUND A Bluetooth node enters the inquiry substate for a period of time to discover other nodes to form a piconet; the inquiring node acts as the master. Bluetooth devices typically use the specification recommended.24 s as the inquiry period [2][6][7]. A node in the inquiry scan substate, in contrast, searches for nodes in the inquiry substate to form a piconet; the scanning node acts as a slave. A. Inquiry Substate A device in the inquiry substate transmits inquiry packets on two pseudo-random frequencies during a normal packet time slot. Inquiry packets consist of a 68 bit General Inquiry Access Code (GIAC) or Dedicated Inquiry Access Code (DIAC). The GIAC is used when searching for any Bluetooth device. A node may use one of several DIACs to search for devices with specific characteristics [][2]. The inquiring device waits for a response 625 μs later on the same frequencies from a prospective slave device in the inquiry scan substate. The inquiring device continues this process while collecting responses until the inquiry period is complete or an acceptable number of devices have been discovered. The device may leave the inquiry substate to service synchronous (SCO) links or to immediately page a discovered device. The page process uses the response from a discovered device to contact prospective slaves and incorporate them into the piconet. An inquiring device may also wait until the inquiry period is complete to page devices that are accepted into the piconet. Although some have recommended that a device enter the inquiry substate on a regular basis [2][6][8], others only inquire when explicitly commanded [9]. The inquiry substate uses a 32-frequency partition of the 79 frequencies similar to that used by a piconet in the connection state [2]. Unlike in the connection state, however, this partition remains constant. A node uses an address associated with the GIAC (i.e., 9E8B33 6 ) or one of the DIACs to generate the frequency hopping sequence. This address determines the 32- frequency subset used by the inquiry/inquiry scan substates. Although generated using the resident CLK and the GIAC (or DIAC) address, the hop sequence within the partition appears random to an outside observer. The 32 frequencies used by the GIAC hop sequence are spread across the Bluetooth spectrum by a spreading process in the final stage of the /6/$2. (C) 26 IEEE

2 Proceedings of the 39th Hawaii International Conference on System Sciences - 26 Frequency Hop Selection Kernel [2]. Placement within the spectrum is irrelevant for analysis and this spreading stage is ignored. Thus, the frequencies used by the inquiry substate for the GIAC address are designated as -5 and 53-78, rather than the Bluetooth frequency spectrum designations used after the spreading process which doubles the frequency number modulo 79 (i.e.,,2,4,6,8,,27,29,3,...77) [2]. This set of 32 frequencies is further segmented by the inquiry procedure into two 6-frequency trains, A and B. A device in the inquiry substate chooses the A or B train for initial transmission and switches between the trains every 2.56 s. The train used initially is not significant [2] and the initial train selection process is implementation specific. The frequencies within these trains change over time as shown in Figure where the A train are the frequencies in the white boxes and the B train are the frequencies in the shaded boxes. The 6 frequencies in a train at a given instant are called the train s membership []. A frequency entering the A train is shown as a thick bordered box and a frequency switching to the B train is shaded with a left-hash. The trains exchange one member every.28 s based on bit changes in the free-running counter, completely swapping membership every 2.48 s. As membership changes, the swapped frequencies retain their position in the transmitted train. Thus each train consists of only 32 possible sequences. t s t +.28 s t s t s t +5.2 s t s t s t s t +.24 s t +.52 s t +2.8 s t +4.8 s t s t s t s t +9.2 s t s t s First Frequency Lower Range Re-ordered Frequencies A-train B-train Fig.. Shifting of the inquiry trains [] Second Frequency Upper Range A-train addition for that cycle B-train addition for that cycle B. Inquiry scan/inquiry response substates A device enters the inquiry scan substate to make itself available to discovery by an inquiring device. To account for the hop sequence randomness, the scanning device only changes frequency every.28 s. Since the scan frequency changes every.28 s, and the train changes every 2.56 s, most implementations only scan for.25 ms [2][] and then move to the connection (i.e., normal operation) or a standby state for the remainder of the.28 s. Using a scan of length.25 ms rather than ms compensates for any timing misalignment and allows the scanning device to receive at least one full inquiry train sequence. When an inquiry packet is received during a standard inquiry scan, the scanning device waits 625 μs and enters the inquiry response substate to return a FHS packet to the master [2]. The FHS packet contains the slave device address and CLK values. The inquiring device either continues transmitting packets for the duration of the inquiry substate dwell time to find other neighboring devices or jumps to the page substate to immediately page the scanning device before continuing to inquire. After transmitting the FHS packet, the scanning device advances the scan frequency by adding an additional.28 s offset to the CLK, waits a back-off time uniformly distributed on [, ms], and re-enters the inquiry scan substate. Before doing so, it is allowed to enter the page scan substate in case the inquiring device immediately pages it. In v. of the specification, a FHS packet is not sent until after the device waits for the back-off period to elapse and receives a second inquiry packet. This was shown to cause unnecessary delay with little reduction in FHS packet collisions [5][2]. Additionally, an interlaced scan was added in v.2 of the specification in which the scan window is immediately followed by a scan using a frequency from the other train. In the standard inquiry scan substate, a new scan frequency is used every.28 s based on the scanning device s CLK. Inquiry scan frequencies change over time resulting in the scan frequency staying within the same inquiry train as shown in Figure 2 []. For example, at t =, the scan frequency is 6 and in the A train. At t =.52 s, frequency 6 is in the B train. However, the scan frequency has shifted to 7, which is in the A train, even though it was in the B train at t =. Since the scan frequency changes every.28 s, a device typically opens a window every.28 s when in the inquiry scan substate. The scan frequency used at the beginning of a scan window is assumed to be the frequency used for the entire scan window. This prevents loss of scan capability due to oscillator re-tuning during the window. s.28 s 2.56 s 3.84 s 5.2 s 6.4 s 7.68 s 8.96 s.24 s.52 s 2.8 s 4.8 s 5.36 s 6.64 s 7.92 s 9.2 s 2.48 s Re-ordered Frequencies A-train Random Inquiry Scan frequency B-train Fig. 2. Inquiry scan frequency remaining in a train [] C. Discovery Using the standard inquiry scan, the inquiry time distribution is shown in Figure 3 for an isolated inquiring/scanning pair. When a device begins inquiring, the time until the scanning device opens a scan window is uniformly distributed 2

3 Proceedings of the 39th Hawaii International Conference on System Sciences - 26 on [,.28 s]. Since a scan window may already be open when the inquiry begins, the inquiry packet is received uniformly distributed on [,.28 s] if the scan frequency is in the train used (probability.5). If not, the train changes at 2.56 s and the inquiry packet is received uniformly on [2.56 s, 3.84 s]. Exceptions occur with.275 probability, when the membership of the train changes during the scan window [5]..4 Inq2 inquiry hop sequence Inq inquiry hop sequence Scan Window Frequency = 64 Inq inquiry hop sequence b) Fig. 4. Inq2 receives inquiry packet.3 f TI (t).2. 6 (s) in the following analytical model of one of many conditional pdfs which comprise the inquiry time pdf. Consider the event, M, that Inq begins inquiring with a train containing the scan frequency of an arbitrary scanning device. The scanning frequency is equally likely to be in either train giving Fig. 3. f TI (t) Unconditional probability density for the standard inquiry scan, P (M) =.5. () III. HOP SEQUENCE INTERACTION When multiple devices inquire, discovery time may increase. When two nodes, Inq and Inq2, enter the inquiry substate simultaneously and use the train containing the scan frequency, discovery by one of the nodes will be delayed since a scanning node can only respond to one inquirer and then enters the back-off period (and possibly the page scan substate) before scanning again. If Inq uses the train not containing the scan frequency, the scanning node will receive its inquiry packet uniformly on [2.56 s, 3.84 s] unless the scanning frequency changes trains when the scan frequency offset advances by an additional.28 s after each FHS packet is transmitted. In this case, Inq s packet is received immediately after the back-off period is complete after the transmission of an FHS packet to Inq2. The limited number of inquiry train sequences further complicates determining the discovery time. If Inq and Inq2 both use train that contain the scan frequency, the scanning device may consistently receive packets from and respond to Inq2 each time a scan window opens until Inq2 uses a train not containing the scan frequency or the scan window begins just before Inq transmits its inquiry packet on the scan frequency. For example, when Inq2 transmits on the scan frequency slightly before Inq, there is a small interval where the scan window can open and receive Inq s packet as shown in Figure 4a. On the other hand, when the nodes use the scan frequency 9 time-slots apart as shown in Figure 4b, each node has an equal probability of being the first to use the scan frequency within a scan window. Although the relationship between the hop sequences is well defined, an analytical model depicting the inquiry time with n inquirers is not feasible due to the numerous relationships between membership changes and train changes and the multiple back-off periods that must be included when discovery is delayed due to other inquiring devices. This is illustrated The scanning device opens a scan window at a time uniformly distributed on [,.28 s]. The pdf of the time the scan window initially open is designated f T (t). IfInq2 inquires with a train not containing the scan frequency when the scan window opens (which occurs with probability.5), the scanning device is discovered at a time uniformly distributed on [,.28 s] as shown in Figure 5a. If Inq2 is inquiring with a train containing the scan frequency when the scan window opens, Inq2 may discover the device first. Unconditionally, (i.e., with no knowledge of when Inq and Inq2 transmit the scan frequency) the probability that Inq discovers the scanning node first when both nodes trains contain the scan frequency is.5. If Inq2 discovers the node first, the next opportunity for Inq to discover the scanning node occurs when it opens a scan window after the back-off period elapses. Since the back-off period, with pdf f TBO (t), is uniform on [,.64 s], the time the scan window will re-open is the convolution f TBO (t) f T (t) as shown in Figure 5b. During a back-off period, several events can occur that affect the probability Inq discovers the node when the scan window re-opens: Inq due to the scan frequency offset (with probability.625), Inq2 due to the scan frequency offset (with probability.625), Inq due to the CLK change (with probability.6), Inq2 due to the CLK change (with probability.6), the membership of Inq can change so the scan frequency is no longer in Inq s train (with probability.6), the membership of Inq2 can change so the scan frequency is no longer in Inq2 s train (with probability.6), and Inq2 can switch the train used (with probability.25). 3

4 Proceedings of the 39th Hawaii International Conference on System Sciences - 26 b) c) f T (t M) f T (t M)* f TBO (t) f TD (t M) Fig. 5. Discovery time distribution when inquiry packet is received during the first scan window b) when inquiry packet is received during the second scan window after being discovered first by another inquirer c) in the presence of another inquirer when M occurs Since these events are not mutually exclusive, the conditional distribution including each combination is complex. If the scan frequency is no longer in the train used by Inq, Inq cannot discover the device until after it changes trains at t =2.56 s (unless Inq2 continues to discover the device, advancing the device s scan frequency offset until it is back in the train used by Inq). If Inq2 discovers the device when the scan window reopens, the scanning device again waits for a back-off period before opening a scan window once again. Assuming none of the events listed above occur during any of the back-off periods and that each inquiring node has an equal likelihood discovering the scanning node when a scan window opens, the discovery time pdf for Inq before the train change at t =2.56 sis f TD (t M {T D < 2.56 s}) = 3f T (t) + f T (t) f TBO (t) + (2) 4 8 f T (t) f TBO (t) f TBO (t) +... (3) 6 and is shown in Figure 5c. Under these circumstances, the probability that Inq discovers the node before the train change at t = 2.56 s is.99. However, due to the relationships of the scan frequencies within each train, it is unlikely that each inquiring node has an equal likelihood of discovering the scanning node when a scan window opens. When Inq does not initially contain the scan frequency (i.e., P (M)), the conditional distributions are more complex since Inq2 may initially contain the scan frequency and cause the scanning node to go through back-off periods and advance the scan frequency offset and possibly into the train initially used by Inq. Considering this and the possibility of the events listed above which may occur during each back-off period, analytically deriving the discovery time distribution is deemed impractical. IV. RESULTS Since an analytical model is not feasible, a simulation model must be used to model the inquiry time with multiple inquirers, rather than be used solely to verify a derived model. A Matlab simulation was developed which simulated a single scanning node over a perfect channel in the presence of multiple inquiring devices. Although not an exact model, this provides an estimate of the discover time pdf which provides developers an estimate of the delay that multiple inquirers can cause. It also provides an estimate of the devices that may not be discovered by a specific inquirer due to the presence of other inquirers. Discovery time is defined as the time needed for an inquiring device (Inq) to receive an FHS packet from the scanning node. Since Inq may never discover the scanning node if another inquiring device consistently transmits on the scan frequency immediately before Inq and changes trains simultaneously with Inq, the simulation ends at 5 s. It is unlikely that Bluetooth devices will transmit longer than.24 s due the the interference generated and power considerations [5]. The simulation was run using 25 randomly generated scanning devices with random CLK values and up to five inquiring devices. A scanning device and inquiring device were randomly generated for each set of simulation runs and a baseline inquiry time was determined. The CLK/address settings for each preceding simulation were repeated in each subsequent run as additional inquirers were added in subsequent simulation runs. Thus, the inquiry times in the 5 simulation runs were comparable for each scanning device. When Inq was the first device to discover the scanning node, the discovery time was the same as when no other inquirers were present. When another inquirer discovered the scanning node first, the Inq was generally delayed as shown in Figure 6, although it was occasionally accelerated when M occurred but the scan frequency offset change caused the scan frequency to shift into the train before the change at t =2.56 s. The percentage of devices that are not discovered within 5 s rises quickly as additional devices inquire as shown in Table I. V. CONCLUSIONS Although the single inquiring device inquiry time has been well characterized, the effect of multiple inquirers is difficult 4

5 Proceedings of the 39th Hawaii International Conference on System Sciences - 26 b) Simulation pmf Simulation pmf Inquiry time (s) No other inquirers One other inquirer No other inquirers Rises to.6 Four other inquirers Inquiry time (s) Fig. 6. Inquiry time pmf with one other inquiring node b) four other inquiring nodes [9] B. Peterson, R. Baldwin, and R. Raines, Packet Error Rate Distribution Between Random Bluetooth Piconet Pairs, accepted to Wireless Personal Communications. [] B. Peterson, R. Baldwin, and R. Raines, Inquiry Packet Interference in Bluetooth Scatternets, in ACM Mobile Computing and Communications Review, pp Volume 8, Issue 2, April 24. [] O. Kasten and M. Langheinrich, First Experiences with Bluetooth in the Smart-It s Distributed Sensor Network, in Proceedings of the International Conference on Parallel Architectures and Compilation Techniques, 2, Sept. 2. [2] G. Zaruba, Accerlated Neighbor Discovery in Bluetooth Based Personal Area Networks, in Proceedings of the 22 International Conference on Parallel and Distributed Processing TEchniques and Applications (PDPTA 2), Las Vegas, NV, USA, Jun, 22. TABLE I INQUIRY TIME WITH MULTIPLE INQUIRERS Inquiring Devices Mean Inquiry % not found after 5 s to model and has not been considered. Due to the relationship between inquiry hop sequences, the impact extends beyond the independent probability that one device delays discovery by another simply because it discovers a scanning node first. The presence of a second inquirer may in fact prevent a inquiring device from discovering a scanning node until the second node leaves the inquiry substate. As the number of inquiring devices increases, the probability that a specific inquirer will not be able to discover a scanning device rises significantly. This relationship must be considered for scatternet formation as well as scatternet maintenance analysis. REFERENCES [] Specif ication of the Bluetooth System, Core Version., Bluetooth SIG, 999. [Online]. Available: [2] Specif ication of the Bluetooth System, Core Version.2, Bluetooth SIG, Nov 23. [Online]. Available: [3] L. Godell, M. Nordan, T. Lapolla, M. Mendez WLAN And Bluetooth Update: Beyond The Hype, [Online]. Available: [4] D. Jayanna, and G.V. Zaruba, A Dynamic and Distributed Scatternet Formation Protocol for Real-life Bluetooth Scatternets, in Proceedings of the 38th Hawaii International Conference on System Sciences, Hawaii, USA, Jan, 25. [5] B. Peterson, R. Baldwin, and J. Kharoufeh, Bluetooth Inquiry Characterization and Selection, accepted to IEEE Transactions on Mobile Computing. [6] G. Zaruba and V. Gupta, Simplified Bluetooth Device Discovery Analysis and Simulation, in Proceedings of the 37th Hawaii International Conference on System Sciences, Hawaii, USA, Jan, 24. [7] R. Woodings, D.Joos, T. Clifton, and C. Knutson, Rapid Heterogeneous Ad Hoc Connection Establishment: Accelerating Bluetooth Inquiry Using IrDA, Proceedings of the Third Annual IEEE Wireless Communications and Networking Conference (WCNC 2), Orlando, Florida, Mar 22. [8] T. Salonidis, P. Bhagwat, L. Tassiulas, and R. LaMaire, Distributed Topology Construction of Bluetooth Personal Area Networks, in Proceedings of IEEE INFOCOM 2, Anchorage, Alaska, Apr 2. 5