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

Building Automation, Wireless Technology and the InternetThe address field encoded into the frames determines if the messageis for the particular pager. If not, the decoder informs the controllerand the controller conserves power by shutting off parts of the receiver.The message data may be passed to the controller, which then deliversthe information to the user either by a liquid-crystal display (LCD) orspeaker.Wireless and wired functions are similar in some ways and in otherways they are not. Sending data over a wireless channel poses someproblems due to the channel’s corruptive nature, compared to datatransmission over phone lines, which offers a relatively clean channel.Compared to voice, data transmission via a wireless channel hastwo added complexities every bit is equally significant and must berecovered at the receiver, and the data rates are much higher.The multifunctional communicator’s DSP system must be capableof on-the-fly reconfiguration. The multiband and multimode capabilitiesof cellular-phones require the DSP to reconfigure the phone dependingon the user’s location. There are different carriers and standards thatmay be available locally. The software must reconfigure the unit tohandle a function like narrowband cellular phone, digital audio receiver,or wideband high-speed data transfer.Multiband capability means that the cell phone must be able toperform a search algorithm that determines the available air interface.Multimode capability determines the carrier, cellular standard, andquality level available.The DSP also performs filtering that was previously handled bythe RF transmitter/receiver. This allows the DSP to control the operatingfrequency band of the unit. This filtering takes large amounts of data athigh speed, which requires a high DSP throughput.One of the algorithms used in digital cellular phones is a speechcodec. This algorithm takes digitized speech and compresses it to a lowerbit rate to conserve transmission bandwidth. Another is error correction.The error protection and correction technique is called convolutional encodingand decoding, which adds redundancy and structure to the compressedvoice data to correct for bit errors. Convolutional decoding usesthe Viterbi algorithm to recover data bits from the received data. In asymmetrical-digital-subscriber-line(ADSL) systems data rates are so highthat a Reed-Solomon algorithm is combined with bit interleaving and acyclic redundancy check (CRC) to protect and recover data.Equalization is used in wireless receivers to clean up the received©2001 by The Fairmont Press, Inc. All rights reserved.