Understanding Smart Sensors - Nomads.usp

180 Understanding Smart SensorsTable 8.2RF Communication: Wireless LAN Protocols and Sponsoring AgenciesProtocol Sponsor(s) CommentsX.25 packet protocol CCITT Wireless extension of landline specification802.11 IEEE Proposed standard; uses ISM frequenciesCDPD McCaw Uses unused capacity on cellular voiceRAM Mobitex Ericcson Message packet data systemARDIS* Motorola and IBM Data packets at either 9.6 or 14.4 Kbps*ARDIS is a trademark of Motorola, Inc.protocols. Several RF-specific terms are defined in the glossary at the end of thisbook.8.2.1 Wireless Local Area NetworksWireless LANs (WLANs) operating at 18 GHz are able to propagate wellwithin a building yet diffuse rapidly outside. FCC licensing is required forWLANs in the digital termination service (DTS) band, 18–19 GHz, whichincludes the use of low-power radio communications within buildings. Each ofthe 10-MHz channels within that band can deliver 15 Mbps [7]. SomeWLANs also operate in the lower frequency range of 900 MHz to 6 GHz. Inaddition, there are a variety of infrared links for wireless networks. Thoseindoor systems have a range of 100m and data rates of 1 Mbps.8.2.2 FAX/ModemsReal-time sensing using a PCMCIA card fax/modem installed in any portablecomputer establishes a flexible, easily installed link to a base computer. Up to28,000 bps are already possible using available technology. PCMCIA cards arealso being designed specifically to handle sensor data [8]. One device providesthe modem function and a level of intelligence that can obtain informationfrom sensors located in remote or mobile devices.An error correcting protocol, derived from the asynchronous balancemode of the Consultive Committee of the International Telephone and Telegraph(CCITT)–defined X.25 packet protocol, retransmits only those framesthat are not recognized by the receiver (Figure 8.3). The modulation techniqueeasily synchronizes transmitted data. A basic HDLC concept and nonreturn-