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

HVAC Control in the New Millenniumnection, you can dial into an ISDN bridge or router from a remote siteand log in to a file server. While connected, you may get a phone call,or some other distraction, and stop work on your computer. Since theconnection is idle for a few minutes, the equipment could drop the linewhile maintaining your log-in to the file server. It does this by spoofingthe protocols.When you later require access to the file server, the ISDN equipmentre-establishes the connection, typically in less than one second.This is very fast compared to the time required to establish a connectionbetween two modems.ISDN equipment is termed customer premises equipment. There isa basic rate interface and a primary rate interface. The basic rate interface(BRI) uses the telephone company’s existing two-wire copper linesand multiplexes three channels over them. The primary rate interface(PRI) has 23 B channels and one D channel in the United States while 30B channels and one D channel are used in Europe. In the United States,PRI runs on a T-1 local-loop circuit. In Europe, an E-1 circuit is used.The B channel can be used to transmit voice, data, video, image, orfax. It can operate as circuit switched voice (CSV), circuit switched data(CSD), or packet switched data (PSD) which is also called packet modedata, or PMD. It runs at 56 Kbps or 64 Kbps, depending on the switchingmode and on whether the telephone system can handle clear-channeldata calls.The D channel provides call-setup times of about one second. TheD channel can also be used for packet-switched data. The ISDN equipmentcan use the traffic between sites to add additional B channels toincrease the throughput. The number of B channels used can fluctuateup and down throughout the day, providing increased bandwidth atpeak times and dropping back during low traffic periods to reducephone charges.ISDN equipment can also compress data with compression ratioscomparable to those of modems. Typically, ISDN achieves compressionratios of about 2:1, but 4:1 or even 8:1 are possible under ideal conditions.ISDN gives twice as many phone lines (B channels) per number ofcopper-wire pairs as analog, with packet-mode data on the D channelalso being available. With standard four-wire copper cable, analog telephonesand modems, you get a maximum of two telephone lines. ISDNusers can have four numbers on these same four wires along with twoadditional 9.6-Kbps packet-mode connections.©2001 by The Fairmont Press, Inc. All rights reserved.