II. RELATED WORK AND BACKGROUND

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1 Comarson of Bluetooth Interconnecton Methods usng BlueProbe Sewook Jung, Alexander Chang, and Maro Gerla Deartment of Comuter Scence Unversty of Calforna, Los Angeles {sewookj, acmchang, Abstract Many Bluetooth researches are targeted to form effcent scatternets. However, scatternets do not have much oularty n general because many Bluetooth devces do not suort scatternet caabltes and hgh moblty makes t dffcult to kee the connectvty. Instead we choose temorary nterconnecton of conets to reduce scatternet formaton overhead. To comare several nterconnecton methods, we roose BlueProbe. BlueProbe fnds out the allocated tme slots sze, whch makes t at for Bluetooth style TDMA caacty measurement. BlueProbe s alcable to both conet and scatternet whether a cross traffc exsts or not. Results based on our real testbed measurements show that the nterconnecton method s more mortant than the ho length and the number of smultaneous flows, whereas revous researches based on smulatons concentrate on these two factors. Based on these results, we show effcent nterconnecton methods among Bluetooth Pconets. I. INTRODUCTION Bluetooth s a short-range wreless ersonal area network technology that suorts ad-hoc network. U to eght nodes are formed n a star-tye cluster, called conet. The node n the center of the star s called master and surroundng nodes are called slaves. Two slaves cannot communcate drectly. So, master should forward ackets between two slaves when one slave transmts ackets to the other. Pconets can be nterconnected va brdge nodes and these nterconnected conets form a scatternet. Brdges are the nodes that take art n more than one conet wth a tme-sharng method. However, scatternet s not suorted n all Bluetooth devces. Asynchronous Connectonless Lnk (ACL) s used to transmt data and t has 625μs sze tme slots. Data acket tyes vary based on how many tme slots (1, 3, or 5) are used and whether Forward Error Correcton (FEC) s used or not. FEC ackets are DM1, DM3, and DM5 (dgts ndcatng the number of slots used). Non-error correcton ones are DH1, DH3, and DH5. The latest Bluetooth Secfcaton 2.0 ntroduces the Enhanced Data Rate (EDR) ackets and they are 2-DH1, 2-DH3, 2-DH5, 3-DH1, 3-DH3, and 3-DH5 [1]. Ths aer has two man contrbutons. Frst, we roose a new caacty measurement tool that can be used for TDMA style rotocols. Prevously made tools do not work well for TDMA style rotocols. We roose BlueProbe that combnes acket length adataton, acket bundlng, and ng methods to measure ath caacty. Second, we comare several conet nterconnecton methods and show the roer ways to nterconnect Bluetooth devces II. RELATED WORK AND BACKGROUND A. Caacty Measurement AdHoc Probe [2] s a ath caacty robng method that s at for ad hoc network. It s based on CaProbe, whch s a well known bottleneck lnk caacty estmaton tool for wred and last-ho wreless networks [3]. AdHoc Probe uses one-way estmaton whereas CaProbe uses round-tr estmaton. It measures the maxmum rate achevable on an unloaded ath when ntermttent envronmental roblems are factored out. Fxed sze robng acket ars are sent back-to-back from the sender to the recever. The sender adds sendng tmestam n every acket. The recever calculates the one-way delay (OWD) and cks u roer acket ar by choosng two ackets that have the mnmum OWD sum, and calculates the caacty. Dovrors et al used dserson of acket ar and acket tran (more than 2 acket) to calculate caacty [4]. However, they used total dserson of acket tran to calculate caacty. B. Throughut Estmaton Kss Kallo et al analytcally estmated and showed the effect of ath length on throughut and ower consumton [5]. The throughut decreases wth the ncreasng number of hos and the ower consumton ntally decreases as the throughut decrease, and t later ncreases because of acket losses that ncur retransmssons. C. Scatternet Formaton Several algorthms [6, 7] are roosed to form and mantan a scatternet. In [6], BlueStar collects nformaton of other conets and t nterconnects conets based on ths nformaton. In [7], toology calculaton s used to fnd out number of slaves n a conet, number of conets servced by each brdge node, and number of loos n the network, n order to form a scatternet. These scatternet algorthms concentrate more on toologes of scatternet than nterconnecton methods. III. BLUEPROBE BlueProbe s based on AdHoc Probe, the one-way estmaton technque. AdHoc Probe measures the maxmum rate achevable on an unloaded ath. However, BlueProbe s dfferent from AdHoc Probe n several ways. Frst, BlueProbe calculates bottleneck node s allocated tme slot caacty for a certan ath nstead of the maxmum osle

2 Fgure 1. BlueProbe Tme Slot Fllng u caacty. AdHoc Probe calculates the best rate as the caacty and therefore t cannot roerly calculate a node s allocated tme slot caacty. When one Bluetooth node s shared wth several lnks, that node wll dvde ts tme slot for each lnk wth tme-sharng method. Second, BlueProbe uses tme slot fllng method to fully utlze ts tme slots whereas AdHoc Probe does not coner tme slot at all. When the frst and second robe ackets are located n the same tme slot as n Fgure 1. (a), the caacty wll be much hgher than the theoretcal one. When they are located n dfferent tme slots but each robe acket does not fully utlze ts tme slot as n Fgure 1. (b), the caacty wll be much lower than the theoretcal one. When each robe acket s located n each successve tme slots and each tme slot s fully flled u (each acket s fragmented nto two DH5 ackets) as n Fgure 1. (c), the caacty s very close to the theoretcal one. Thus, BlueProbe tres to fnd the roer acket length to fll each tme slot. Thrd, BlueProbe uses several methods each alcable to dfferent uroses. To calculate the exact caacty, t uses acket length adataton method whch tres to fnd the correct acket length to fll a tme slot. Ths method takes tme to fnd the correct acket length. In order to estmate the caacty n short amount of tme, BlueProbe uses acket bundle method n whch t sends multle ackets back to back to reduce the effect of cross traffc and thus smulates the real data transfer. In moble stuatons, connectons and dsconnectons are very frequent whch requre dfferent aroach to measure and udate caactes. For such cases, BlueProbe uses acket bundle and ng method to detect connectons and dsconnectons usng ng. When a connecton s detected t uses acket bundle and quckly calculates the caacty. If the connecton lasts long enough, the caacty s udated usng acket bundle. Detals are dscussed n the followng subsectons. A. Packet length adataton (PLA) Bluetooth devces usually select acket tye deendng on the ayload length and lnk qualty. In a good lnk qualty stuaton, DH acket s used nstead of DM acket to ncrease Fgure 2. BlueProbe Packet Length Adataton Method throughut. If IP ackets are longer than DH5 acket ayload sze (339 bytes), t wll be fragmented nto several DH5 ackets. So, we choose IP acket sze as the multle of DH5 ackets to fully fll u each tme slot. The IP acket sze starts from one DH5 acket ayload length, and ncreases u to ten DH5 acket sze, n the ncrement of one DH5 acket sze (1 DH5, 2 DH5 s,, 10 DH5 s), to fnd out the roer IP acket sze whch fully flls u a tme slot. In every fxed nterval, a acket ar (two IP ackets) s transmtted. AdHoc Probe comutes one-way delay sum (OWD_Sum) for each acket ar. It then cks the acket ar wth the mnmum OWD_Sum (Mn_OWD_Sum) to be used n calculatng the caacty. However, n TDMA style rotocol, MIN_OWD_Sum may not be the roer caacty measurement as shown n Fgure 2. (a). We ck the mnmum OWD among frst ackets (denoted as Mn_OWD_1) to fnd out the smallest queueng delay case. We also ck the acket ars n whch the frst acket s OWD s n the lowest 10% (denoted as Low_OWD_1), and the caacty s calculated by the average of each acket ar s caacty. In Fgure 2 (a), Mn_OWD_Sum used n AdHoc Probe calculates caacty whch s the total caacty of a certan node. Even f frst acket has small OWD, second acket s OWD (denoted as OWD_2) may be small or large as n Fgure 2 (b) and (c). Based on ths assumton, Mn_OWD_1 chooses the smallest OWD of frst ackets, and ths wll overestmate or underestmate the caacty when OWD_2 value s small or large, resectvely. Low_OWD_1 method calculates the average of these two cases, and therefore t wll be more accurate than Mn_OWD_1 method, when tme slots are not fully flled u. When tme slots are fully flled u, the standard devaton of caactes, calculated from each acket ar, s the lowest. Therefore we can detect the roer IP acket sze. When roer IP acket sze s used, the above Mn_OWD_1 and Low_OWD_1 methods show almost same caacty. B. Packet bundle (PB) Packet bundle method reduces caacty measurement tme. To fll u a certan lnk s tme slot, sender transmts 100 DH5

3 Fgure 3. BlueProbe Packet Bundle Method ackets at the same tme wthout delay and recever calculates dsersons of adjacent ackets. We check dsersons of every two adjacent ackets and regard the longer one as the start of tme slot. If dsersons are relatvely small (dfference to average s smaller than standard devaton), that means those ackets are transmtted n the same tme slot. If dsersons are relatvely long (dfference to average s bgger than standard devaton), that means those ackets are transmtted n dfferent tme slots. We choose these ackets as the start onts of tme slots. Caacty can be calculated as the total acket sze transmtted between the two consecutve start onts dvded by the sum of dsersons of the same erod. Fgure 3 shows detals of ths method. Dsersons D2 and D4 are longer than D1 and D2. So, we assume tme slots are started at the begnnng of acket 3 and 5 that have dsersons of D2 and D4. Caacty s calculated as (2*DH5)/(D3+D4). C. Packet bundle and ng (PB+PING) Ths method s at for the temorary scatternet connecton. Because Bluetooth scatternets are not always mantaned, they are connected at certan tmes but are dsconnected at other tmes. So, we use ng to check the exstence of a ath between source and destnaton. If there s a ath, acket bundle method s used to calculate the current caacty n a few seconds, and then ng s used agan to detect future dsconnecton. If the connecton stays longer than certan erod (usually several mnutes), the caacty s re-calculated usng the acket bundle method agan. Wth ths aroach, we can calculate the eak caacty when connecton s made and the average caacty as ( C D ) / D. The erod caacty s C and the duraton of erod s D. In dsconnected erod, C s 0. Thus PB+PING can be aled to a moble stuaton n whch nodes are frequently connected and dsconnected. IV. CAPACITY ANALYSIS Caacty of a sngle lnk n Bluetooth Scatternet s estmated lke the followngs. A. Bluetooth Scatternet Lnk Caacty In [4], throughut s calculated lke the followngs. θ = C mn ( f q ) (1) (, j) ( s, d ) mn ( f (, j) ( s, d ) j q ) j j j denotes the smallest usable bandwdth orton on the lnks of a connecton (s,d) (.e the bottleneck), whle qj s the acket success rate (PSR) of the lnk (, j). C s the maxmum caacty of a Bluetooth rado lnk. If a node has several lnks and lnk (, j) s used as a tmesharng method, node s total caacty s dstrbuted to each lnk. fj s calculated as the caacty of lnk (,j) dvded by the number of flows that ass ths lnk. Lnk caacty deends on the number of lnks connected to a certan node and swtchng tme (only on brdge node). Lnk caacty l s calculated lke the followngs. l = 1 / + α (2) l l N slave N master = 1 / σ + α (3) /( N master + 1) l Fgure 4. BlueZ Protocol Stack = 1 σ + α (4) mb mb _ m 1/( N master + 1) mb ) / = ( σ N + α (5) slave mb _ m l s the lnk caacty of a ure conet lnk. Lnks connected to slaves are sharng the caacty of master. α s are extra caactes f other lnks connected to a same node do not use ther maxmum caactes. For examle, three slaves are connected to one master. One lnk uses DH5 acket n one drecton and DH1 acket n the other drecton. Other lnks use DH1 ackets for both drectons. In ths case, addtonal 5 DH1 ackets (5 slots total) are transmtted for one DH5 acket transmsson (5 slots). So, l = 5 /(5 + 5) = In (1), l = 1/ 3 +α. So, α = l s the lnk caacty of a slave brdge. Slave brdge s actng as several masters slaves. So, a slave brdge node s caacty s dvded by the number of ts masters. Certan length tme slots are allocated from each master and the swtchng tme σ s allocated between tme slots. l s the lnk caacty of a master brdge when t s actng as a slave for that lnk. In ths case, the node s caacty s dvded by the number of ts masters lus one because the master brdge node has ts own conet. σ s the swtchng tme between tme slots. mb l s the lnk mb _ m caacty of a master brdge when t s actng as a master for that lnk. In ths case, the node s caacty s dvded by the number

4 of ts masters lus one as n l case. After that, the dvded caacty s allocated agan by the number of slaves n ts own conet. V. BLUETOOTH TESTBED In ths secton, we resent the Bluetooth Testbed envronment that we used for evaluatng our aroach. We used BlueZ Bluetooth rotocol stack for Lnux [8] and Belkn Bluetooth USB Adators (F8T003v). BlueZ conssts of HCI Core, HCI USB devce drver, L2CAP rotocol module, BNEP confguraton, and testng utltes. BlueZ rotocol stack s descrbed n Fgure 4. Belkn Bluetooth USB Adators use CSR Bluecore2 ch and suort lmted scatternet that can have u to two masters and seven slaves. We used and rogram n bluez-utls-2.13 [8] to make PAN (Personal Area Network) connecton (make lnks between nodes) and used brdge-utls [9] to brdge several connectons at a certan node. VI. RESULTS A. BlueProbe We evaluate BlueProbe methods wth AdHoc Probe and theoretcal results to verfy ts correctness. 1) Pconet 1-to-1 connecton To verfy BlueProbe, we frst used t for 1-to-1 connecton. There s only one connecton between Master and Slave to smlfy the test. BlueProbe PLA (Mn_OWD_1, Low_OWD_1), BlueProbe PB, and AdHoc Probe are used as caacty calculaton methods. L2test and Ierf (UDP, TCP) are used to calculate the real data throughut. Theoretcal calculaton s also used to comare aganst the results. Results are shown n Fgure 5. AdHoc Probe shows overestmaton for Slave-to-Master transfer. However, BlueProbe PLA shows almost same result as the real measurements (L2test and Ierf_UDP) n both Master-to-Slave and Slave-to-Master cases. BlueProbe PB shows slghtly lower caacty than the real measurements. Packet bundle method makes traffc fully use u caacty n a short erod. Usually, α n secton IV s 0, but t ncreases when there s no other traffc. Because of ths short erod, α s not fully ncreased. Theoretcal caacty s based on asymmetrc lnk usage of DH5 n one drecton and DH1 n the other. It assumes α s fully ncreased. However, t does not coner the rocessng and the queung delays n each node and thus t s always hgher than real measurements. 2) Pconet and Scatternet 2 Hos Connecton Smlar tests are erformed for 2 hos connectons. 3 knds of connecton tyes (S-M-S, M-MB-S, and M-SB-M) are used as n Fgure 7 (a), and both transfer drectons are used for M-MB-S case. L2test only suorts one-ho connecton, and theoretcal calculaton requres α and σ values n secton IV whch are deendent on Bluetooth chs and nterconnecton methods. Thus they are not used for ths test. In all cases, AdHoc Probe shows overestmatons because t calculates the maxmum osle caacty. BlueProbe PLA and PB show the caactes very close to the real measurement (Ierf). Results are shown n Fgure 6. B. Mult-ho Connecton Several connectons are selected for mult-ho tests as n Fgure 7 and results are shown n Fgure 8. 1) 2 Hos Connecton In 2 hos connectons, S-M-S (2H1) case (one conet) has hgher ath caacty than others. M-MB-S (2H2) and M-SB-M (2H3) cases are nterconnectons of conets va Master Brdge and Slave Brdge, resectvely. Caacty of a sngle conet (2H1) s much hgher than that of nterconnecton cases (2H2 and 2H3) because there s no conet swtchng tme σ n 2H1. Interconnecton va Master Brdge case (2H2) shows more than twce the caacty of nterconnecton va Slave Brdge (2H3). Slave brdge should wat for each master s oll ackets n snff slot before transmttng ackets, whereas Master brdge should wat only one master s oll acket [10]. Thus ther caactes are dfferent. 2) 3 Hos Connecton S-M-MB-S (3H1) case uses one Master brdge, M-MB-MB-S (3H2) case uses two Master brdges, and S-M-SB-M (3H3) uses one Slave Brdge. The 3H1 shows hgher caacty than 3H2 because t uses one less brdge. Even f 3H3 uses one brdge, t shows lower caacty than 3H2 case that uses two brdges.ths result shows that the caacty deends more on the tye of brdge than the number of brdges. Fgure 5. 1-to-1 Connecton Fgure 6. 2 hos Connecton

5 Fgure 7. Mult-hos Connecton Selectons 3) 4 Hos Connecton M-MB-MB-MB-S (4H1) case uses three Master Brdges, S-M-MB-MB-S (4H2) case uses two Master Brdges, and S-M-SB-M-S uses one Slave Brdge. Even f 4H1 uses one more Master Brdge than 4H2, the caactes are almost same. Ths shows that the n long-ho connectons, master brdges are as effcent as a sngle conet. Moreover, 4H1 and 4H2 cases show hgher caactes than 4H3 case. Ths result also shows that the caacty deends more on the tye of brdge. 4) 5 Hos Connecton M-MB-MB-MB-MB-S (5H1) case uses 4 Master Brdges but shows hgher caacty than that of S-M-MB-SB-M-S (5H2) case that uses one Master Brdge and one Slave Brdge. 5H1 uses 5 hos connectons but shows hgher caacty than that of 4H3 case. Ths result shows that the nterconnecton method s more mortant than the ho length. C. Effcent Bluetooth Pconet Interconnecton Method Three nterconnecton methods are used as n Fgure 9. In the fgure, left es are orgnal conets and rght es are nterconnected conets (Scatternet or multle new conets). There are two stages, such as Pconet Stage and Interconnecton Stage. Same duratons are used for both stages. After the duraton exres, one stage wll change to the other one. Addtonal connecton and setu tme s requred to change stages. We assume two alcaton flows (1 to 4, 3 to 2) exst. The flows cannot transfer acket durng Pconet Stage. BlueProbe PB+PING method s used to measure caacty. Due to frequent connectons and dsconnectons, other caacty measurement methods are not alcable. PB+PING can measure caacty n a few seconds n Interconnecton Stage and t also fnds out the duraton of Interconnecton Stage to calculate the eak and average caactes. Results are shown n Fgure 10. Multle 1-to-1 connecton Fgure 8. Mult-hos Connecton cases show hgher caactes than those of scatternet cases (va Master Brdge or Slave Brdge) n both Peak and Average. Even f 1-to-1 case has longer connecton setu tme, t suorts multle transfers durng Interconnecton Stage and therefore t has hgher eak and average caactes. D. Cross Traffc Cross Traffc can share a certan node or a certan lnk. UDP Cross traffc s generated by Ierf rogram for ths test. 1) Node Sharng Cross Traffc 5 knds of (Traffc, Cross traffc) cases n Fgure 11 (a) are used for ths test. Results are shown n Fgure 12. When cross traffc throughut s 0, all cases show ther eak values because maxmum α values are used. When cross traffc ncreases, caacty decreases but t s not changed a lot after cross traffc reaches a certan rate. We choose maxmum cross traffc throughut as 355kbs based on the theoretcal maxmum caacty of a sngle lnk of 1-to-4 (1 master, 4 slaves) conet connecton that has one 2-ho flow. Even f cross traffc reaches 355kbs, cross traffc UDP ackets are droed and thus the throughut calculated by the recever s same as the caacty calculated by BlueProbe. Ths result shows that the caacty can be changed by nterconnecton tye and BlueProbe can be aled to node sharng cross traffc cases. 2) Lnk Sharng Cross Traffc Traffc can share a lnk as n Fgure 11 (b). BlueProbe PLA and PB methods are used to comare results. Also the receved cross traffc throughut usng Ierf s used to comare BlueProbe methods. Results are shown n Fgure 13. Fgure 9. Pconet Interconnecton Selectons Fgure 10. Pconet Interconnecton

6 Fgure 11. Cross Traffc Selectons When cross traffc throughut s 0, caactes are at the hghest because α s used at ts maxmum. When cross traffc s at 10kbs, caactes decrease as α s reduced. After that, caactes become stable untl cross traffc reaches 355 kbs. Ths result shows that BlueProbe can also be aled to lnk sharng cross traffc cases. VII. CONCLUSION In ths aer, we resent a caacty measurement tool, BlueProbe, tuned for TDMA style rotocol wth or wthout cross traffc. It combnes three methods such as PLA, PB, and PB+PING. When nterconnectng conets usng brdges, caactes calculated usng PLA and PB methods are almost same as the real measurements. In one-ho connecton case, PLA shows better result than PB whch shows underestmaton. However, PLA method takes long tme (several mnutes) whereas PB takes very short tme (several seconds). PB+PING s the best and only choce when connecton s not ermanently mantaned. Caacty measurements on mult-ho connectons show that nterconnecton va Master brdge s better than va Slave brdge. Because of that, longer ho length connecton shows hgher caacty than shorter one. Thus roer choce of brdge tye s more mortant than ho length and Slave Brdge should be avoded f osle. Fgure 12. Cross Traffc (Node) Interconnectons of conets show that multle 1-to-1 connectons have hgher caacty than connectons va Master Brdge or Slave Brdge. Therefore, temorary 1-to-1 connectons are also effcent ways for nter-conet transfers. In the future, we lan to measure caacty on moble envronments. PB+PING method shows ts caablty to measure caacty for temorary connectons, and thus t can be aled to moble stuatons. We wll also try to use BlueProbe to other TMDA rotocols, such as IEEE and IEEE REFERENCES [1] Bluetooth SIG,. Bluetooth Secfcaton v2.0, 2004 [2] Lng-Jyh Chen, Tony Sun, Guang Yang, Medy Young Sanadd, and Maro Gerla, Ad Hoc Probe:Path Caatcy Probng n Wreless Ad Hoc Networks, IEEE Internatonal Conference on Wreless Internet (WICON), 2005 [3] Roht. Kaoor, Lng-Jyh Chen, L Lao, Maro Gerla, and Medy Young Sanadd, CaProbe: A smle and accurate caacty estmaton technque, Proceedngs of ACM SIGCOMM, [4] Constantnos Dovrols, Parameswaran Ramanathan, and Davd Moore What do acket dserson technques measure?, IEEE Conference of Comuter Communcaton (Infocom), [5] Casaba Kss Kallo, Sewook Jung, Lng-Jyh Chen, Mauro Brunato, and Maro Gerla, Throughut, energy and ath length tradeoffs n Bluetooth scatternets, IEEE Internatonal Conference on Communcatons (ICC), [6] Stefano Basagn and Chara Petrol, A scatternet Formaton rotocol for ad hoc networks of Bluetooth devces, IEEE Vehcular Technology Conference (VTC), 2002 [7] Leng Huang, Hongyuan Chen, T.V.L.N. Svakumar, Tsuyosh Kashma, and Kaoru Sezak, Imact of Toology on Bluetooth Scatternet, Internatonal Journal of Pervasve Comutng and Communcatons, 2005 [8] BlueZ Bluetooth rotocol stack for Lnux, htt:// [9] Lnux Ethernet Brdgng software, htt://brdge.sourceforge.net/ [10] Smon Baatz, Matthas Frank, Carmen Kuhl, Peter Martn, Chrstoh Scholz, Adatve Scatternet Suort for Bluetooth usng Snff Mode, IEEE Conference on Local Comuter Networks (LCN), 2001 Fgure 13. Cross Traffc (Lnk)