Virtual Channel Management for Densely Deployed IEEE 802.15.4 LR-WPANs∗Tae Hyun Kim1, Jae Yeol Ha2, Sunghyun Choi3, and Wook Hyun Kwon4School of Electrical Engineering and Computer ScienceSeoul National University, Seoul, KoreaEmails:[email protected],[email protected], and {2hjy8099,4whkwon}@cisl.snu.ac.krAbstractThe number of channels specified for IEEE 802.15.4Low-Rate Wireless Personal Area Networks (LR-WPANs) istoo few to operate many applications of WPANs in the samearea. To overcome this limit, we introduce Virtual Chan-nel, a novel concept to increase the number of availablechannels when various WPAN applications coexist. A vir-tual channel is basically created via superframe schedulingwithin the inactive periods in a logical channel preoccupiedby other WPANs. To maximize the coexistence capability ofWPANs using virtual channels, we propose (1) Least Col-lision superframe scheduler (LC-scheduler), (2) less com-plex heuristics, and (3) Virtual Channel Selector (VCS) toefficiently manage multiple available logical channels. Inaddition, a simple but practical synchronization method isdeveloped to compensate different time drifts among coex-isting WPANs. The simulation results demonstrate that a re-markable improvement on the coexistence capability of the802.15.4 can be achieved through the proposed schemes.1 IntroductionIEEE 802.15.4 for Low-Rate Wireless Personal AreaNetworks (LR-WPANs) [1] has been standardized for low-rate, low-cost, low-power, and short-range wireless net-working. We expect that this standard, which defines bothPhysical (PHY) and Medium Access Control (MAC) layers,would be widely adopted for various applications includ-ing industrial automation, home control, cable replacement,and wireless sensor networks. One can easily imagine thatmany applications of this standard, as enumerated in Ta-ble 1, would operate simultaneously in a given area so thatthe corresponding WPANs coexist in the same area.However, the available logical channels are very lim-∗This research was in part supported by University ITRC. Tae HyunKim and Sunghyun Choi are with the School of Electrical Engineering andINMC, Seoul National University, Seoul 151-744, Korea.ited.1That is, the current IEEE 802.15.4 specifies 27 logicalchannels across 868MHz, 915MHz, and 2.4GHz frequencybands. Especially, in the case of 2.4GHz ISM band, otherwireless technologies, such as IEEE 802.11 Wireless Lo-cal Area Networks (WLAN) and IEEE 802.15.1 Bluetoothshare the same frequency band. As discussed in [1, 2], con-sidering the coexistence between IEEE 802.15.4 and IEEE802.11, only four available channels (out of 16) remain forthe 802.15.4 networks. Despite this coexistence problem,the recent literature has been limited to the analysis of the802.15.4’s performance degradation resulted by other coex-isting wireless technologies. In [3, 4], the authors analyzethe packet error rate of the IEEE 802.15.4 devices, coex-isting with an 802.11 WLAN. Based on the analysis, theyconclude that a careful channel selection and assignment re-solve the coexistence issue.In fact, the adoption of a carrier sense medium accesswith collision avoidance (CSMA-CA) algorithm in IEEE802.15.4 can ameliorate this coexistence problem. Thanksto the inherent nature of CSMA-CA, coexisting WPANs areable to operate simultaneously even if devices from thoseWPANs in the same transmission range attempt to accessthe medium. However, as the number of such devices in-creases, the networks would suffer from severe performancedegradation. Therefore, the separation among coexistingWPANs either temporally or spectrally is definitely requiredto guarantee a reliable operation of the networks.In this paper, to enhance such a limited channel avail-ability, we introduce a novel concept, called Virtual Chan-nel, into IEEE 802.15.4 to increase available channels.Basically, a virtual channel is a newly-created conceptualchannel via superframe scheduling within the inactive pe-riods in a logical channel preoccupied by other WPANs,which parallels the adoption of “temporal agility” for IEEE802.15.4 networks. To maximize the coexistence capabilityof WPANs, we develop Least Collision superframe sched-uler (LC-scheduler) and two heuristic algorithms, Simpli-1In the 802.15.4 PHY specification, the term ‘logical channels’ repre-sents disjoint physical frequency channels. An 802.15.4 WPAN operatesin a logical channel.Table 1. IEEE 802.15.4 Target Applications [5]Category Application Delay BOVital Heart-rate monitor 1-5 sec 6-8Monitoring Body heat monitor 1 min 12Personal equipment control 50 ms 2Consumer Remote controls 100 ms 3Electronics PC-peripherals 50 ms 2Control of blinds/shades/ 1 sec 6rollers/windowsDimmer/switches 200 ms 4Automatic Electricity/gas/water AMR No 14Meter Reader boundAlarm/Security Smoke detector 1 sec 6System Burglary and social alarms 1 sec 6Access control 1 sec 6Water leakage alarms 1-5 sec 6-8Environmental Temperature/carbondioxide/ 1-5 min 12-14Monitoring humidity/vibration, HVACIndustrial Facility control 100 ms 3Automation Monitoring critical equipment 1 sec 6fied LC-scheduler (SimLC-scheduler) and NEarest VacancySearch (NEVS) under the assumption that only one logicalchannel is given to the scheduler.There have been remarkable research efforts for differ-ent scheduling algorithms in both wireline and wireless net-works so far. For example, in the field of IEEE 802.11WLAN research, scheduling algorithms have been devel-oped for the purpose of quality of service (QoS) provision-ing and energy saving [10, 11, 12]. Nevertheless, the con-sidered superframe scheduling problem is fundamentallydifferent from the existing scheduling problems due to suchdistinct constraints as (1) binary exponential difference be-tween beacon intervals with different beacon order valuesand (2) the lack of an interactive exchange of the requiredinformation caused by its inter-network scheduling charac-teristics.Shifted to the case that multiple logical channels areavailable, e.g., 4 logical channels as discussed above, theproblem to select one of those logical channels should bealso considered. By using the investigated characteristicsof the applications in Table 1, we design Virtual ChannelSelector (VCS), which offers more efficient virtual chan-nel management. Lastly, we consider a time drift prob-lem and provide Inter-WPAN Synchronization (IWS) sinceall our proposals are based on the perfect time synchroniza-tion among coexisting WPANs in the