A Distributed Lo cation System for the Active Oce y Andy Harter, Andy Hopp er y Olivetti Research Limited University of Cambridge Old Addenbro oke's Site Computer Lab oratory 24a Trumpington Street Pembroke Street Cambridge Cambridge CB2 1QA CB2 3QG United Kingdom United Kingdom Novemb er, 1993 Computer and communications systems continue to proliferate in the oce and home. Sys- tems are varied and complex, involving wireless networks and mobile computers. Mobility itself intro duces many new issues [Weiser93]. However, systems are underused b ecause the range of control mechanisms and application interfaces is to o diverse. It is therefore p ertinent to consider what mechanisms might allow the user to manipulate systems in simple and ubiquitous ways, and how computers can b e made more aware of the facilities in their surroundings. Knowledge of the lo cation of p eople and equipment within an organisation is such a mech- anism. Annotating a resource database with lo cation information allows lo cation based heuristics for control and interaction to b e constructed. This approach is particularly attractive since lo cation techniques can b e devised which are physically unobtrusive and do not rely on explicit user action. This article describ es the technology of a system for lo cating p eople and equipment, and the design of a distributed system service supp orting access to that information. The application interfaces which are made p ossible by,or b ene t from this facility are presented. 1 Lo cation Technology The top ology of a xed network of wireless receivers is the basis for a lo cation system. The p osition of a mobile wireless transmitter is determined by the identity of the receivers within the network. Increasing the density of receivers allows the transmitter p ower to b e reduced and the granularity of lo cation to b e re ned. Lo cation b oundaries b ecome 1 well-de ned if the wavelength is such that transmissions do not pass through walls. This prop erty is exploited in the ORL infra-red network. 1.1 Ro om scale lo cation Infra-red base sensors o ccupy xed p ositions within a building. Sensors are connected bya wire network which provides a communication path to the network driver, and distributes low-voltage p ower Figure 1. Sensors are eight-bit addressable, and contain bu ering to simplify proto cols on the wire network. A single sensor may also b e connected to the serial p ort of a convenientworkstation. A baseband mo dulated incoherent infra-red carrier of wavelength 900nm is the basic medium. Pulse-p osition mo dulation is used and a physical layer proto col de nes how discrete pulse sequences form packets. A data link layer proto col de nes messages and supp orts di erent devices using the infra-red network [Harter93]. Alow data rate of 9600 baud allows transceivers to b e small and low-p owered, which is imp ortant for mobile devices. Room 1 Room 2 Room 3 Base Base Base Workstation Ethernet Base or Corridor 4 ATM Base Mobile Base Network Driver Room 4 Figure 1: Infra-red sensor arrangement 1.2 Lo cating p eople People are lo cated to the ro om scale bywearing a p ersonal infra-red transp onding com- 1 puter, the Active Badge [Want92a,b]. Each badge p erio dically transmits an infra-red message containing a globally unique co de. The range of the system is around thirty me- tres, and line-of-sight is not necessary. Pressing one of two pushbuttons causes the badge to transmit immediately and act as a primitive but ubiquitous signaling device. The badge also has a receive capability and can interpret a range of messages. A small sp eaker and 1 Active Badge is a trademark of Ing. C. Olivetti & C., S.p.A. 2 two visible LEDs provide a basic paging facility. A small amountofinternal state can b e set and subsequently read. The p ower budget is managed by only transmitting every ten seconds, whichisinkeeping with the rate at which a p erson mightmovebetween zones. A light dep endent comp onent increases the transmission interval in darkness, for example when a badge is in a drawer or pocket. In addition, the receiver is only enabled for a p erio d after each badge transmission. Base sensors are able to store-and-forward messages at this rendezvous Figure 2. Badges run continuously on conventional batteries lasting around one year. Sensor 1 2 3 Badge 6 5 4 System 8 7 9 10 11 1 Badge transmits unique code 6 Badge transmits, enables receiver 2 Message buffered in sensor < 2 milli-seconds 7 Sensor transmits command < 10 seconds 3 Message delivered to system 8 Badge executes command 4 System downloads command 9 Badge issues reply 5 Command stored in sensor 10 Reply buffered in sensor 11 Reply delivered to system Figure 2: Active Badge communication sequence 1.3 Lo cating equipment Equipment is lo cated by attaching an infra-red transmitting computer derived from the Active Badge. The equipment badge is required to run continuously for several years, whichisachieved by increasing the transmission interval to ve minutes, and removing the receive capability. Much equipment is quite static and it is reasonable to wait a while for the system to realise equipment has moved. The equipment badge has a so cket allowing two digital inputs to b e monitored. The input values are rep orted in the p erio dic transmission, providing wireless telemetry of equipment state. Awell known wireless communication problem is the reception of the same transmission at multiple bases. The problem is apparent with the infra-red network, where a zone may haveanumb er of bases. Transmissions for b oth typ es of badge contain a sequence numb er, allowing duplicates to b e ltered at any level in the system. 1.4 Desk scale lo cation Finer grain lo cation is supp orted with a hybrid radio/infra-red scheme. Low-p owered radio transmitters generate a low-frequency carrier which is pulse-width mo dulated with one of a discrete set of widths. A passive tuned circuit in the badge detects the eld, whichis 3 sampled and co ded into each infra-red message. Overlapping elds can b e detected but not resolved. Typically transmitters and aerials are placed so as to form zones surrounding desks Figure 3. The range of the eld is controlled by transmitter p ower and aerial size, and is adjusted to corresp ond roughly to an arm's length. Badges are stimulated to transmit immediately on rst entering a eld. Infra-red room zone Sensor 4 Badge Radio desk zone 1 3 2 1 Badge ends power down mode 2 Radio field sampled and decoded 3 Badge transmits radio field identity 4 Equipment badges also decode fields Figure 3: Desk scale lo cation 1.5 Prop erties The lo cation technology has a numb er of prop erties which in uence the design of a dis- tributed software architecture. First, the spatial granularity correlates with natural b ound- aries, which de ne convenient units of interaction. Second, the temp oral granularityisas ne as the p ower budget of a continuous active technology allows. An imp ortant p ointis that lo cation information has a real-time element. If information is not timely,itmaybe of no use. Third, information is actively generated and regularly broadcast. The system can then cache information and the last known lo cation of p eople or ob jects is available on demand. 1.6 Deployment Over 1500 badges and 2000 sensors are deployed in the research community at ORL, a numb er of Europ ean universities, DEC research lab oratories, Bellcore and MIT Media Lab. The largest single system is at Cambridge University Computer Lab oratory, where over 200 badges and 300 sensors are in daily use. Equipment badges and desk scale lo cation technology are deployed at ORL, where around 200 items of equipment are badged, and each of some 50 or so desks is identi ed. To a reasonable level of b oth space and time, the lo cation of p eople and equipment is therefore known. The technology has proved to b e an extremely e ective to ol in the construction of large- scale exp eriments to discover some of the prop erties and uses of lo cation information. It has b een b oth a fo cus for designing and building software systems supp orting lo cation, 4 and an inspiration for creating lo cation-aware applications. These activities at ORL are describ ed b elow. 2 Lo cation System Architecture The main thrust of the design is towards a scalable architecture. Lo cation, naming, distribution mo dels and transaction management, are all in uenced by this goal. 2.1 Lo cation Lo cation can b e viewed as an attribute of an ob ject. In abstract form, the value of `lo ca- tion' is a key of no particular imp ortance other than to relate co-lo cated ob jects. In a more direct form, the value of `lo cation' has meaning by itself, b eing chosen from some top olog- ical or geographic naming scheme. Ecient searching of the ob ject space by lo cation key is required, since the co-lo cation function will b e evaluated often by contextually aware applications. The lo cation attribute is dynamic, and the mechanism to store it must sup- p ort a high frequency of up dates. It is therefore infeasible to construct global databases of lo cation information.