Beacon Signal

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108 Computer

In ancient times, beacons were hill-top fires or lights that served aswarning signals to spread word of invasion, natural disaster, or otheremergencies. Today, beacons are pri-

marily radio, ultrasonic, optical, laser,or other types of signals that indicate theproximity or location of a device or itsreadiness to perform a task. Beacon sig-nals also carry several critical, constantlychanging parameters such as power sup-ply information, relative address, loca-

tion, timestamp, signal strength, avail-able bandwidth resources, temperature,and pressure.

Although transparent to the user com-munity, beacon signals have made wire-less systems more intelligent and human-like. As Table 1 shows, they are an inte-gral part of numerous scientific and com-mercial applications ranging from mobilenetworks to search and rescue operationsand location tracking systems.

CELLULAR NETWORKS

Base stations periodically broadcast

one beacon signal per second to identifywireless subscribers in a given area.Mobile devices listen for new beacons,add those they detect to the device’s activebeacon kernel table, locate the nearestbase station, and establish rapport so theycan initiate dialogue with the outsideworld using the base station as a gateway.

Beacon signals carry information suchas a cellular network identifier, time-stamp, gateway address, paging area ID,and other base station parameters. In the

United States, these signals are transmit-ted via the analog Advanced Mobile

Phone System, a first-generation tech-nology, and its second-generation suc-cessor, the Cellular Digital Packet Datasystem. The second-generation GlobalSystem for Mobile communications isthe cellular standard used throughoutEurope and Asia.

Figure 1 illustrates how a cellular net-work uses beacon signals when a mobilephone user is in a location outside hissubscription area—for example, justafter getting off an airplane. When theuser switches on the handheld device, thebeacon signal activates a roaming ser-

vice, and the user registers and commu-nicates through the closest base station.Implementation of the system typicallyoccurs at three levels: user-level process-ing at the base stations, user-level pro-cessing at the mobile user, and kernelmodulation at the mobile user.

WIRELESS LANS

A wireless local-area network links agroup of wireless devices controlled byan access point that periodically broad-

casts a beacon signal that includes thetraffic map, indicating availability of buffered packets for specific LAN nodes.The signal awakens nodes that are insleep mode because of long idle time andresynchronizes them to receive data fromthe beacon. Such buffering occurs for allbroadcast and multicast messagesintended for a group of nodes. WirelessLANs primarily use two different unli-censed industrial, scientific, and medical(ISM) bands, one for analog and mixedsignals and another for digital signals.

Because wireless ad hoc networks areformed instantaneously without anyfixed infrastructure, they require peer-to-

peer mode routing. “Hello” signals func-

tion as beacons to inform nearby mobilenodes about existing neighbors. Most ad

hoc networking routing protocols—including table-based routing, on-demand distance vector routing, andassociativity-based routing—use beaconsignals. Ad hoc networks utilize the sametwo ISM bands as wireless LANs.

GLOBAL POSITIONING SYSTEM

GPS technology originally was de-signed to accurately navigate weaponsfrom mobile platforms to enemy targets.Operated by the US Department of Defense, 24 satellites orbiting approxi-mately 11,000 miles above the Earth

constantly transmit beacon signals con-taining orbital position information, thecurrent time, and error correction data.An Earth-based GPS receiver triangulatessignals from the nearest four satellites tointerpret a general position by checkingtheir clock reading, location, and orbit.

Satellite signals can reflect off atmos-pheric particles as well as buildings,mountains, and other objects beforereaching the GPS receiver, thereby lead-ing to calculation errors. Nevertheless,

Beacon Signals:What, Why, How,and Where?Sergei Gerasenko, Abhijit Joshi, Srinivas Rayaprolu,Kovendhan Ponnavaikko, and Dharma P. AgrawalUniversity of Cincinnati

C O M M U N I C A T I O N S

Beacon signals are an integral

part of various new

applications, including all

wireless systems.



researchers have extended GPS technol-ogy for use in a number of scientific andgeneral-purpose applications such asopen-water navigation, search-and-res-cue operations, precision agriculture, andgeographic surveys.

SEARCH-AND-RESCUE SYSTEMS

Satellite-assisted search-and-rescue sys-tems monitor and respond to emergencybeacons generated by individuals locatedaboard aircraft or ships. For example,Cospas—the abbreviation for Russia’sSpace System for the Search of Vessels inDistress—provides search-and-rescuesatellite-aided tracking by activating 5-

Watt RF burst signals of approximately0.5-second duration every 50 seconds at406 or 121.5 MHz. The vessel or aircraftin distress transmits information aboutthe country of registration and identifi-cation to Cospas-Sarsat low-earth-orbitor geostationary-earth-orbit satellite sub-systems, and they forward the data to adesignated mission control or nationalrescue coordination center.

MOBILE ROBOTICS

Researchers have proposed usingautonomous mobile robot systems to

facilitate distributed coordination andnavigation in applications ranging fromspace exploration to hazardous environ-ment and military operations. In theseapplications, robots can pick up beaconsignals, define the pallet locations, andself-organize to lift the pallets and carrythem to their destination. After theydeliver a payload to its destination, therobots disperse to continue the operation.When robots use beacon signals, local-ized information is adequate to providean autonomous system, and the algo-rithm does not require them to know the

distance from their farthest teammate.

LOCATION TRACKING

The Fraunhofer Institute for Com-puter Graphics in Rostock, Germany, hasdeveloped Mobis, a mobile informationsystem (http://www.rostock.igd.fhg.de/fhg_igd/abteilungen/a3/projects/ xyberscout/mobis) that combines elec-tronic beacons and a Palm Pilot to pro-vide self-guided museum tours. As avisitor approaches various locations

where electronic beacons are installedthroughout the facility such as exhibits,meeting points, or way crossings, thePalm Pilot receives a beacon signal thatuses the beacon range and object-sensing

device to automatically call up informa-tion or directions.

Beacon signals can be either weak,modulated radio signals such as a 2.4-GHz ISM band Bluetooth standard or

October 2001 109

Table 1. Applications and characteristics of beacon signals.

Application Frequency band Information carried

Cellular networks 824-849 MHz (Advanced Mobile Phone Cellular IP network identifier

System/Cellular Digital Packet Data system) Gateway IP address

1,850-1,910 GHz (Global System for Paging area ID

Mobile communications) Timestamp

Wireless LANs 902-928 MHz (industrial, scientific, and Traffic indication map

medical band for analog and mixed signals)

2.4-2.5 GHz (ISM band for digital signals)

Ad hoc networks 902-928 MHz (ISM band for analog and Network node identity

mixed signals)

2.4 to 2.5 GHz (ISM band for digital signals)

Global Positioning 1575.42 MHz Timestamped orbital map and

System astronomical information

Search and rescue 406 and 121.5 MHz Registration country and ID of vessel

or aircraft in distress

Mobile robotics 100 KHz-1 MHz Position of pallet or payload

Location tracking 300 GHz-810 THz (infrared) Digitally encoded signal to identify

user’s location

Aid to the impaired 176 MHz Digitally coded signal uniquely

identifying physical locations

Base station

at visited

location

Mobile

user 1Beacon signal exchange

2

Request for registration

5

Authentication/rejection

3

A u t h e

n t i c a t

i o n r e q u

e s t

4

A u t h e

n t i c a t

i o n r e s p o

n s e

Home site

Figure 1. Using a mobile phone outside the subscription area. (1) A mobile phone listens for new

beacons and, if it detects one, adds it to the active beacon kernel table. If the device determines

that it needs to communicate via a new base station, kernel modulation initiates the handoff

process. (2) The mobile phone locates the nearest base station via user-level processing. (3) The

visiting base station performs user-level processing and determines who the user is, the user’s

registered home site for billing purposes, and what kind of access permission the user has.

(4) The home site sends an appropriate authentication response to the base station currently

serving the user. (5) The base station at the visited location approves or disapproves user access.



110 Computer

tures. Constructing simple beacons thatuse either conventional or solar-poweredbatteries is relatively inexpensive. Thespecification for RF transmission of bea-cons remains the same in different areas,and the ALF device should choose thecorrect area every time a user activates it.

Beacon signals help synchronize,coordinate, and manage electronicresources using miniscule band-

width. Researchers continue to improvetheir functionality by increasing signal

coverage while optimizing energy con-sumption. Beacon signals’ imperceptibil-ity and usefulness in minimizing com-munication delays and interference arespurring exploratory efforts in manydomains ranging from the home to outerspace. ✸

pulsed infrared beams. Infrared beaconsrequire a line of sight between the mobiledevice and beacon to identify the user’sorientation, while radio beacons areinsensitive to location.

AUDITORY LOCATION FINDER

An auditory location finder is a geo-graphically programmable device thatreceives digitally coded signals from a setof beacons in a custom-built environmentto indicate a user’s whereabouts. ALFdevices are particularly useful for visuallyimpaired and elderly persons. An ALFdevice can identify the area, junctions,streets, and pedestrian crossings and pro-

vide specific information about stores,post offices, clinics, public telephones,ATM machines, and other services.

To increase accuracy and prevent sig-nal overlaps, ALF systems use strategi-cally placed, short-range beacon signalsto identify important locations and fea-

Sergei Gerasenko, Abhijit Joshi, Srinivas Rayaprolu, and Kovendhan Ponnava-ikko are graduate students in the Depart-ment of Electrical and Computer Engi-neering and Computer Science at the Uni-versity of Cincinnati. Contact them at {sgerasen,joshiabh,srayapro,[email protected]}.

Dharma P. Agrawal is the Ohio Board of Regents Distinguished Professor of Computer Science and Computer Engi-neering in the Department of Electrical and Computer Engineering and Com-

puter Science at the University of Cincin-nati. Contact him at [email protected].

C o m m u n i c a t i o n s

Editor: Upkar Varshney, Department of CIS,

Georgia State University, Atlanta, GA

30002-4015; voice +1 404 463 9139; fax +1

404 651 3842; [email protected]

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