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

Building Automation, Wireless Technology and the InternetCircuit SwitchingThis is the common type of telephone data communications, wheretwo users or their sending and receiving modems have the exclusiveand full use of a telephone circuit or cellular channel until the connectionis released. Circuit switching modems can be analog or digital.Unlike circuit switching, with packet switching no circuit is used on adedicated basis. Data packets are sent along any channel or line availableat the time they are received.Digital cellular modems use digital rather than analog signals toconvey information. The advantage is that digital eliminates static, linenoise and signal loss during hand-offs. These handoffs occurs when cellular-basedcalls are transferred from one cell site to another as the userroams. Roaming occurs when you use a cellular phone in a city otherthan the one in which you originally set up your account.TDMA (Time Division Multiple Access) is a multiplexing methodfor digital cellular phones in which each user is assigned a time slot ona channel. Often, ten users are assigned to a channel.Code Division Multiple Access (CDMA) is a multiplexing methodfor digital cellular phones that uses a spread-spectrum technique. Allspeech bits (or phone calls) are assigned codes that are encrypted, transmitted,decrypted, then reassembled. By spreading the signals across thesame broad frequency spectrum and assigning each a unique code,CDMA allows more calls to occupy the same space as each cellularchannel is shared.RF ModemsRadio frequency modems operate in the radio band, which is theportion of the electromagnetic spectrum from 100 kHz to 20 GHz. Widearearadio-frequency services use an RF modem, which transmits andreceives data in packets 128 bytes in size.The advantage of packet data is that it optimizes throughput evenunder adverse line conditions. The sending modem sends a series ofpackets, without waiting to see if each is received successfully. If a packetis corrupted by noise or interference, the receiver asks the sending modemto resend the corrupt packet. The sending modem will continue tosending new packets until it receives this request. It then resends the errantpacket and resumes normal transmission. Since each packet is num-©2001 by The Fairmont Press, Inc. All rights reserved.