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Pervasive Computing Environments Based on Short Range Wireless Technologies: Issues and a Proposed System SAI-WING LEI2,
VINCENT K. N. LAU1, YU-KWONG KWOK1, STEVEN CHU2, LEO CHUNG2, CHUN-YI LEE2, AND CHUI-WA NG2 1
Department of Electrical and Electronic Engineering The University of Hong Kong, Pokfulam Road, Hong Kong 2 Cougar Wireless Limited Suite 702A, 625 King’s Road, Hong Kong
Email: {knlau, ykwok}@eee.hku.hk, {swlei, leochung, stevenchu, cylee, cwng}@cougarwireless.com
Abstract Pervasive computing environments are useful for efficiently disseminating and generating new information. However, many challenging research problems need to be solved before such a system can be realized. Several experimental and commercial systems exist but they only tackle a subset of the problems. We propose a system based on Bluetooth and identify some important research issues to be handled. Keywords: Bluetooth, CABER, pervasive computing, ad hoc networks, routing.
1 Introduction Computing is ubiquitous and is a daily activity. The next step might be to make computing a truly mundane task. Indeed, some “simple” computing tasks are really mundane. For example, the computations inside the microcontroller in a car, washing machine, TV, VCR, to name but a few. However, these belong to the embedded system world. In the desktop arena, although the age of booking a terminal for use to perform some computations has long gone, the “anywhere” and “anytime” computing environment is still under active research. Such an environment, necessarily wireless, which allows a user to perform usual computing tasks with a simple device, is what we call in this paper as a pervasive computing system [13]. Thus, we use a more relaxed notion of pervasive computing in that we do not just focus on the systems based on massive number of simple data sensors but we also consider those systems that allow users to perform usual computing tasks in a truly seamless manner. However, to build such a pervasive computing system, there are many crucial research and development issues to consider [15]. Firstly, the system has to provide an application environment that the user is very familiar with. It is of not much practical use if the user has to do a lot of configurations and adaptations while switching from a wired desktop environment to a wireless pervasive computing environment. Specifically, the user has to be able to do plug-and-play. That is, once a user enters such an environment, he/she can just make use of the hand-held device to do useful work right away. For example, the system should be able to automatically authenticate the user, properly setup communication connections, provide familiar user interface, etc. The user will just be prompted for which service to use. Mobility management is also of utmost importance. Because the user is always on the move. The system, consisting of numerous sensors or access points, should be able to track the user’s movement in such a way that the service is not interrupted. Furthermore, the system should also make use of the user’s location to provide location specific services for the user. For example, if a user walks near a bus-stop, the user
should be prompted with a list of bus routes and their expected arrivals in the next few minutes, if the user so wishes to see. To provide truly seamless communication, the connection/ session maintenance overheads must be minimized so that the user will not notice the setup and tear-down of connections, redirection of data transmissions, etc. To assist a smooth handoff, the users’ data should also be properly buffered, cached, or prefetched. Power management is a hot topic in this area also. This is because hand-held devices are inevitably running on battery power. The system should be intelligent enough so as not to require the user’s devices to perform excessive processing. The power management issue is actually related to the other issues such as the data management issue, in which judicious protocols should be used to efficiently sending/receiving data to/from the client’s devices. For example, intelligent broadcast protocols can be used to selectively sending general information to only those client devices that have not received the information before. In this paper, we first briefly describe several interesting pervasive computing systems under research or in commercial use. These systems nonetheless tackle only a subset of the important issues described above, albeit most of them are very successful systems by today’s standard. We then in Section 3 describe our proposed system which is envisioned as a practicable system, based on the Bluetooth technology [17], that attempts to tackle most of the important issues. Our proposed is still in an experimental stage and several research issues still need to be tackled. These issues are outlined in Section 4. We provide some concluding remarks in the final section.
2 Pervasive Computing Systems There are many pervasive computing systems being built in various academic and commercial institutions. In this section, we briefly describe several interesting ones with features solving different aspects of the technical challenges pertinent to a pervasive computing system. SoftNet Zone [19] provides wireless broadband access for travelers at major airports in the North America, by leveraging the existing airport Cyber Concierge (private computing rooms) that are operated by LapTop Lane (www.laptoplane.com). The technology is based on the IEEE 802.11b wireless LAN standard [18] which allows a maximum 10 Mbps wireless channel for each user. The major services provided by SoftNet Zone include Internet access and airport/ flight information retrieval. The system is pervasive in the sense that the user can enjoy a work environment with all convenient services albeit the physical environment is an airport. Ticketing and reservation will be available in the
future. Delta Airlines is the major strategic partner of SoftNet Zone and as such, the wireless services at airports are provided for Deltas customers on an exclusive basis. Users have to bring along an IEEE 802.11b wireless LAN card with their laptop computers in order to make use of the services. It is expected that the users will pay for the service via a monthly access fee. Delta also plans to work together with SoftNet Zone to develop broadband wireless Internet access onboard its aircraft, along with related content services. In summary, the SoftNet system has tackled the application environment aspect in a successful manner. BlueSky [3] is a wireless computing system consisting of access points, hand-held devices (e.g., PDAs), and a wired network. The system is targeted for devices such as Palm, Visor, and pocket PCs. The system design is generic in that the underlying communication and computing mechanisms are suitable for use in Bluetooth, HomeRF, or IrDA [3], [8], [12]. Specifically, each hand-held device is attached with a BlueSky wireless attachment module, which can communicate with an access point that acts as a layer 2/3 bridge between the wireless and wired part of the network. The hand-held device treats the attachment as a modem so that the device uses the dialup and PPP programs to connect to the wired network through an access point. The novelty in the BlueSky solution is that the PPP connection is kept alive while the mobile device moves from one access point to another. The communication between the access point and a server in the wired network is by tunneling such that the PPP connection terminate at the server rather than at the access point. Thus, when the device connects to a new access point, the system only needs to set up a new tunnel and the traffic can be properly redirected. In summary, the BlueSky system has tackled the mobility management, as well as connection/session maintenance problem very well. Infostations [7] is an architecture consisting of small and separated islands of coverage providing low-cost, low-power access to information services in a mobile environment. The ultimate design goal is to reduce the cost per bit of information transmission, motivated by the observation that the power consumption is a very critical factor in a mobile computing environment. The Infostations environment can be used for providing personal information services such as e-mail, fax, and voice messaging. Location-dependent information services (e.g., advertising, maps, or local hazard warnings) can also be supported. In a military scenario, Infostations system can also serve as a data relay between remote information sources and the low-cost, light-weight terminals carried by the soldiers. A major high performance enabling feature in the Infostations architecture is that the physical layer continually adjusts to the changing channel conditions so that the throughput can be maximized when conditions are good. Effectively, power consumption is also optimized. Caubweb [11] is a system that provides adaptive, ongoing read and update interaction with Web-based information. The system is designed in a way that the users can keep on “interacting” with the Web information even though connection is lost. The core technology in Caubweb is an application-specific transducer which is a platform-portable, nonshared, prefetching, caching HTTP proxy module. Such a transducer supports the specification and retrieval of “weblets” which are connected subsets of Web content. The disconnected mode of operation is based on intelligent caching strategies. A user can issue a PUT command into the cache at any time without regard to whether a network connection exists or not. In summary, the Caubweb system has tackled the data
management issue quite successfully, albeit it is currently not built with a fully functioning wireless communication network. The “Living Laboratory” [6] is an experimental environment built by a Georgia Tech research group for investigating the issues involved in a “truly” pervasive computing environment. In such an environment, the sensors (e.g., temperature, voice recognition, location tracking, pose tracking, etc.) are ubiquitous in the environment and are networked (currently using wired approach but will migrate to a wireless approach) so that the environment is “aware” of the users it is interacting with and is capable of unencumbered and intelligent interaction. The core technologies are mainly concerned with the signal-understanding methods to process the sensory data captured from these sensors. In summary, the so-called “Aware Home” environment illustrates that a convenient pervasive environment should be capable of handling much sensory data without user’s intervention.
3 The Proposed System for Information Hot-Spots 3.1 Bluetooth versus other Wirelesss Technologies Wireless LAN technologies are available now. Users can access the technologies, mostly based on the IEEE 802.11b standard, in products such as Lucent’s WaveLAN wireless card, Apple’s Airport wireless card, to name a few. However, these products are suitable only for PC platforms and, as such, cannot provide convenient wireless information services for users who just want to carry a truly pocket size light weight device (e.g., a cell phone, a PDA, or a hybrid of the two). On the other hand, the current cellular network technologies are far from satisfactory in providing meaningful wireless information services because of the low speed—9.6 Kbps in GSM, for example. While 3G can theoretically provide an always-on high speed wireless channel to a user, the costs of such access will inevitably be high due to the expensive 3G licenses. Thus, there is a need for a low power (hence light weight) and low cost technology for personal wireless information access. Bluetooth is, by and large, the most promising technology for such purposes. Bluetooth [17] is designed as a low-power and short-range wireless communication standard. Thus, Bluetooth is widely envisioned to be used in low bandwidth applications in the so-called personal area network (PAN) environment. Indeed, suggested applications are of cable-replacement type such as tetherless linking of hands-free earphone with a cell phone, wireless connection between a cell phone and a PDA, wireless connection among the low bandwidth peripherals of a PC, etc. There is no doubt that, due to its low cost, Bluetooth enabled devices will proliferate. In particular, hand-held information appliances such as PDAs, pocket PCs, and cell phones will be Bluetooth-enabled. Currently, there is roughly 500 million cell-phone users world-wide and the number will grow to 1.7 billion by 2010. Indeed, PDA vendors (such as Palm, HandSpring, TRG, etc.), pocket PC vendors (such as Compaq, Casio, etc.), and PC vendors (such as Toshiba, HP, etc.) are offering Bluetooth access hardware products. Given this trend, we can expect that people would like to implement more interesting and demanding applications based on Bluetooth. However, due to Bluetooth’s low bandwidth design, it may be difficult to support high bandwidth applications such as wireless video streaming using a Bluetooth channel in a large scale (e.g., stream the video to tens of users in a localized region).
3.2 The CABER Network In Cougar Wireless, an experimental pervasive computing system, targeted for palm-top devices (e.g., PDA), is being built. A major component in the system is the CABER†— Cougar Adaptive Bluetooth Enhanced Router. Specifically, the CABER can support multiple pico-nets simultaneously with much higher point-to-point bandwidth with client devices (1 Mbps for each client device; whereas, in standard Bluetooth, the maximum aggregate bandwidth in a pico-net with 7 devices is 1 Mbps only). The experimental system is a self-organizing high performance wireless data network in an information hot-spot, which is a localized physical environment from which a significant amount of timely and dynamic information is generated, transmitted, and to be disseminated to and used by the users located (in a physical sense or cyber sense) inside the environment. There are many examples of information hotspots, to name but a few: shopping malls, exhibition centers, museums, university libraries, wireless games centers, etc. In the following, we describe in detail the application of CABERs in an intelligent shopping mall. The CABER network architecture is depicted in Figure 1 below. The high-tech visitors‡, carry their Bluetooth devices (phones, PDAs, or notebooks), are connected to the mall’s CABERs or Baby CABERs (not fully wireless but with a 10/ 100BT port) via the wireless radio channels. The visitors will then be able to invoke a mall menu detailing the mall directory information, navigation map, traffic/weather information, and shop descriptions. Most importantly, the visitors can then view specific shops’ advertisements on sales and promotion items, thereby effectively enhancing the exposure of the shops in the mall. Furthermore, visitors can also view some advertisements and products information in the form high quality video clips. Indeed, the visitors can even preview some new CD or VCD titles being sold in some AV shops. Finally, the visitors can also access the Internet (WWW and email) freely using their Bluetooth devices. The critical components of the CABER network are the CABERs, Baby CABERs (fixed access points), and the high performance data server. The CABERs are used for communicating with the visitors’ devices (in the same piconets) and routing information packets to and from the data server. These CABERs are designed to be self-organizing and fault-tolerant in that a router fails will not bring down the network but instead the failed router will be by-passed by other routers in the packets forwarding process. The Baby CABER access points are high capacity hubs for handling the traffic to and from the CABERs. These access points are linked directly to the server using a high speed Ethernet LAN or switches. The data server is responsible for providing public access, Web/ WAP information serving, mall information serving, advertisements, CABER network mobility and location management, and coordination of merchant terminals. Of course, in environments such as a temporary exhibition hall where fixed cabling will be too expensive and cumbersome, we † CABER: a long heavy wooden pole thrown in the air as a trial of strength in the Scottish sport of tossing the caber [Oxford Advanced Learner’s Dictionary]. Nowadays information flow is in the air with strength. ‡ Nowadays, a shopping mall can have three types of visitors: ordinary visitors, cyber-visitors (visiting the mall’s portal), and high-tech visitors, who are ordinary visitors equipped with intelligent hand-held devices.
can employ a one-tier architecture composing only of CABERs, making the network fully wireless and selforganizing, and hence, more robust to changes in the physical environment.
3.3 Comparison with Cellular Data Services (GPRS and 3G) We would like to highlight a point that Bluetooth, unlike those CT-2 technologies, is not a low cost alternative of 3G cellular technologies. Instead, they are targeted for different application environments. For instance, 3G cellular technologies are designed for macroscopic coverage while Bluetooth is designed for short-range indoor access. They have the following differences in four aspects: (1) speed, (2) capacity, (3) service quality, and (4) service variety. • Speed: Currently, the circuit switched data rate of GSM is only 9.6kbps, whereas the practical data rate (packet switched) of up-coming technologies GPRS and 3G are 115kbps and 384kbps, respectively. However, the advertised speed are the peak rate. That is, the wireless data user data rate will be supported in burst of peak rates. It should be note that the sustainable burst duration is at most 2-10 sec. It should be noted that the packet burst duration is limited (e.g., 500msec each burst) and thus, the average data rate is even lower. In contrast, Bluetooth devices can communicate with each other at a sustainable data rate of 1Mbps. With such a considerably higher data rate, the application space of Bluetooth network is much wider. • Capacity: A 3G cell, for example, can only support only about 8–10 simultaneous high data rate users (at 384kbps). The size of a typical 3G cell is of a radius of 200m–10km. In contrast, for the same Bluetooth coverage area, which is covered by about 100 CABER access points, can support about 1200 (12 user per router, with 100 routers) high data rate users (at 1Mbps). Thus, this is a significant 120 fold increase in capacity (in terms of user population), largely contributed by the short range coverage and the much wider spectrum bandwidth of the ISM band (with 79MHz bandwidth at the 2.4GHz band) compared to the 3G bandwidth (about 10–15MHz). • Service Quality: As mentioned earlier, a shopping mall is an information hotspot, which means that tremendous amount of information is available for and needed by the customers. For example, a mall visitor would generally like to know the availability of services at favorite shops, or the most updated sales promotion information at favorite boutiques. The envisioned application scenario of the CABER network is as follows. Once a mall visitor enters the mall, his/her hand-held Bluetooth device (presumably with a high-quality LCD display, like the one used in latest models of Palm IIIc or Pocket PC) will be connected to the CABER network and will receive the frontpage of the mall menu, in the form of a small icon, just like the icon of the Yahoo! Messenger. From the menu, the customer can view the real-time availability or sales information about a shop in the form real-time videoclips (e.g., fashion show). Here, notice that the transmission resources requirement of realizing such services is high—a sustained data rate of at least 384kbps for more than 30sec. Such a requirement is only barely achievable by using 3G network, even if there is just a single user. Most importantly, if a 3G cellular network is used to access such information (in the form of a portal
mall visitor
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Baby CABER CABER
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Baby CABER Pico-net PSTN mall visitor
Figure 1: The two-tier CABER network architecture. frontpage on the Internet in HTML/WML format), the transmission has to go through the extremely congested public Internet and the heavily loaded cellular network gateway (e.g., the WAP gateway sitting between the Internet and the cellular network nowadays) which will for sure induce unpredictably long delay (on the order of tens of seconds). In contrast, in a CABER network, because the customer accesses the information from a “local” network, in which the information sources are in close connection with the customer’s device, the transmission of information is highly efficient and timely. This is illustrated in Figure 2 below. • Service Variety: As mentioned above, the customer’s hand-held device is effectively a “mobile-cyber-navigator” of the mall because the device can show information about the mall directory, traffic information, location information, and news. In addition, enabled by the CABER network, such a device can also be used for “previewing” products such as new CD and VCD titles, or books and magazines. Adventure type games can also be launched over the CABER network by making use of the customer’s devices. Indeed, the application space is only bounded by our imagination. This is also in sharp contrast with the cellular network which is merely a mobile low quality Internet surfing device. We would like to emphasize that the CABER networking technologies are designed for convenient wireless access data at information hotspot. Shopping mall is only an example. The CABER network can be deployed in other types of information hotspots such as theme parks (e.g., Ocean parks or DisneyLand), hotels, airports, MTR stations, wireless network games, and cruise liners.
3.4 Comparison with Wireless LAN Based
Infrastructure Wireless LAN technologies are also being used for providing information access for users on the move. The most notable example is the system provided by SoftNet Zone (www.softnetzone.com), which supports wireless broadband access for travellers at major airports in the North America, by leveraging the existing airport “Cyber Concierge” (private computing rooms) that are operated by LapTop Lane (www.laptoplane.com). The technology is based on the IEEE 802.11b wireless LAN standard. The major services provided by SoftNet Zone include Internet access and airport/flight information retrieval. Ticketing and reservation will be available in the future. Delta Airlines is the major strategic partner of SoftNet Zone and as such, the wireless services at airports are provided for Delta’s customers on an exclusive basis. Users have to bring along an IEEE 802.11b wireless LAN card with their laptop computers in order to make use of the services. It is expected that the users will pay for the services via a monthly access fee. Delta also plans to work together with SoftNet Zone to develop broadband wireless Internet access onboard its aircrafts. As mentioned earlier, the IEEE 802.11b standard is of a much more heavy weight compared with Bluetooth. Based on Bluetooth, users can access wireless information easily without the hassle of carrying a laptop computer. In addition, Bluetooth has in-built ad hoc networking facilities which allow seamless and transparent connection of devices within range, and thereby allows the development of innovative pervasive computing applications (e.g., automatic push-based synchronization of important local information such as mall shows schedules, etc.). IEEE 802.11b does not provide these facilities. Thus, the CABER networks can provide pervasive computing services and group communication services (e.g.,
mall information server
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(a) Internet access through cellular network Bluetooth phone/device
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Figure 2: (a) Accessing the Internet information source through the cellular network (a remote indirect approach), (b) accessing the Internet information source through the CABER network (a local direct approach). wireless network games), while SoftNet Zone just provides point-to-point per-user based wireless information services. Similarly, the CABER network is also believed to be superior to the RoofTop wireless access network (http:// www.nwr.nokia.com/rooftop/index.html) recently introduced by Nokia (also based on the 2.4 GHz ISM band).
3.5 Comparison with Wireline Broadband Access Points Obviously, wireline access points, such as those installed at a so-called cyber-cafe, suffer from a serious lack of mobility, poor capacity (fixed), poor scalability, and poor ubiquity. Indeed, the lack of mobility is a severe problem because a fixed access point naturally induces the occurrence of “traffic jam”— customers have no other choice to connect to the net. On the other hand, in the CABER network, if the capacity of some routers/access points is filled up, we fully expect customers can get network access by moving to other physical locations within the mall. Furthermore, even if we use the so-called “broadband” communication, the transmission speed is still limited by the outgoing leased line speed, which is in most cases on the order of 1.5Mbps. Thus, wireline broadband access points simply cannot compete with the high speed and ubiquitous CABERs approach. An analogy is to compare fixed telephone booths with cellular phones.
4 Research Issues in the Proposed System To make the above proposed system practicable in a real life scenario, we have to tackle a number of research and development issues. Capacity enhancement: The standard Bluetooth standard only supports 7 slaves in one piconet. Such a restriction is really a severe constraint for many useful applications such as
multicasting, files sharing, etc. More importantly, to build a geographically large network using Bluetooth access points, the capacity of each access point must be enhanced significantly (beyond the nominal 720Kbps) in order to support a reasonably large number of users at each location. In Cougar Wireless [16], novel techniques have been devised to increase the capacity of a Bluetooth access point by one order of magnitude. However, an even higher capacity is needed. Routing: For the CABER network (fully wireless) to be indeed self-organizing, a judicious and robust routing mechanism is a must. Routing serves the purposes of correctly delivering data to/from the clients, which are on the move, and carefully balancing the network’s traffic load to avoid hot-spots which may degrade service quality significantly. In a fully wireless self-organizing network like the CABER network, the routing problem is very challenging. For one thing, the logical topology (hence, the routing/forwarding information) of the network has to be highly adaptive to user mobility because the underlying physical environment changes rapidly. In an ad hoc network based on the Bluetooth technology, the situation is even more difficult to handle [1], [4]. This is because in Bluetooth, devices communicate based on the master/slave paradigm, and thus, inter-slaves communications have to go through the master. One master is responsible for a distinct piconet. But piconets can be connected to form scatternet (network of piconets) by having some devices acting as master and slave in a time-division manner, or acting as slave in two distinct piconets in a time-division manner. Interoperability: In a public area (e.g., mall), one can expect there are many devices using heterogeneous wireless standards operating in near-by regions. For example, there is no reason why there will not be a notebook computer based on IEEE 802.11b collocated with a Bluetooth PDA. Theoretically,
there will be interference because these different wireless technologies are all based on the 2.4GHz region in the ISM band, although practically the interference may be tolerable because of the different spread spectrum techniques used (direct sequence in IEEE 802.11b; fast frequency hopping in Bluetooth). There are also other wireless standards such as HomeRF, HIPERLAN, etc., which may also cause unpredictable interferences. Ideally, if the CABER (at least) can be interoperable with different wireless standards, the problem can be somewhat alleviated because the CABERs, being masters (with respect to the users’ devices), can judiciously control the scheduling of data transmissions to the slaves so as to avoid interference (for instance, a special hopping sequence can be used) [9], [10]. Much research still needs to be done in this area. Security: To deploy interesting (perhaps commercial) applications in such a wireless network, security is of utmost importance. The current practices in wireless standards such as the IEEE 802.11 are that the wireless channel just provide simple wire equivalent privacy (WEP) which is far from satisfactory from a business application’s point of view. Security has to be provided in higher protocol layers. Again, much research is needed. Power management: Because the hand-held devices usually rely on battery power, the software infrastructure in the network should be designed in such a way that the client side processing is minimized [14]. Caching and prefetching types of information retrieval techniques are widely envisioned as useful strategies to minimize client device power consumption. Indeed, broadcast protocols [2], [5] have been designed to efficiently deliver information to clients while minimizing the power requirements.
5 Concluding Remarks In this paper we have identified several critical issues in building a successful pervasive computing environment. We use a more relaxed notion of pervasive computing in that we do not just focus on the systems based on massive number of simple data sensors but we also consider those systems that allow users to perform usual computing tasks in a truly seamless manner. We briefly survey several interesting examples. We have also discussed in detail a system based on Bluetooth and the research issues involved.
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