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

HVAC Control in the New Millenniumencryption and decryption, while asymmetric encryption schemes usedifferent keys.Both schemes rely on the strength of the key being subject to publicscrutiny by the cryptographic community. The main advantage of symmetricencryption schemes is the speed with which clear text can beconverted to or from cipher text. Symmetric encryption operates almost1000 times as fast as asymmetric encryption.Asymmetric encryption schemes use public/private key pairs.They utilize a defined mathematical relationship between the public andprivate keys. The main drawback to using asymmetric schemes is thehigh computational load of the encryption/decryption algorithms.MSMQ uses a combination of asymmetric and symmetric encryptionschemes to encrypt the body of the message. Asymmetric encryptionis used to ensure the integrity of the transfer of the symmetric key.The MSMQ Queue Manager generates a random symmetric keyand uses it to encrypt the body of a message. It must also guarantee theauthenticity of any public key used. Digital certificates are commonlyuse to solve this problem. If messages contain sensitive information,they should be both authentic and encrypted to prevent eavesdroppingor tampering while they are in transit.Direct Digital ControlDirect digital control (DDC) technology is constantly changing. Inthe past several years, there has been a race between controls companiesto provide leading edge hardware and software. Many building siteshave been converted to or built with direct digital control systems.Most systems are very close to each other in performance. Thelatest movement has been towards the ability to easily transfer DDCsystem information into desktop software packages for data accumulationand processing.The program is an inherent part of the system. If the computer isnot programmed to stringent engineering practices, the DDC system canbe a liability rather than an asset. There are many DDC systems in usethat have no or little user interface, that function with no comfort problems,and are cost effective. This is because they are quite intelligent. Thesystem was designed specifically for a particular building and functionswell under all conditions. Users can make setpoint adjustments without©2001 by The Fairmont Press, Inc. All rights reserved.