HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New Millenniumanalog to digital cellular. CDPD works by transmitting 128-byte blocksof data over the cellular phone system, looking for unused time in thedigital cellular channels network, which is typically idle about 30% ofthe time. During the idle period, CDPD modems send short bursts ofdata packets to the cellular network. A cellular adapter connects a standardmodem to a cellular phone (via the RJ-11 jack) and enables it to beused for cellular communication.Unlike voice communications, CDPD automatically includes errordetectionand retransmission so that no data are lost. It also has dataencryption to secure the transmissions. Another advantage over RFmodems and circuit-switched cellular, is that CDPD modems can exchangedata with a designated host or service provider or be configuredto work with any land-line network and modem.CDPD modems use the same circuitry as digital cellular phonesand some can double as digital cellular phones. Some modems are designedto work with portable computers like the IBM ThinkPad. Othersare more flexible and work with any PC system.Satellite TechnologySatellite technology can connect almost any two points in theworld. Satellite equipment for a satellite-based phone includes the dish,stand, and transceiver. Some models only transmit Teletype and voice.Satellite units are called VSATs (very small aperture terminals).Wireless LANsFor local area wireless access between two machines, the hardwarecan be infrared or RF transceivers. Wireless local area networks(WLANs) may show significant growth. There are extensions for the802.11 standard for Ethernet-speed WLANs. Data rates are 20 Mb/s andhigher in the 5-GHz frequency range and 10 Mb/s in the 2.4-GHz band.The Wireless LAN Alliance (WLANA) is a not-for-profit consortium ofWLAN vendors which has pushed for the standard. It removes mostobstacles to wireless LAN interoperability.The modulation scheme used for the 2.4-GHz band was developedby Lucent Technologies and Harris Corporation. It is known as complementarycode keying (CCK). This technology works in a wide range of©2001 by The Fairmont Press, Inc. All rights reserved.