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

Building Automation, Wireless Technology and the InternetAs long as the data are placed in an IP envelope, the network hasthe information it needs to get your packet from your computer to itsdestination. The information in a packet is usually between 1 and about1500 characters long. This prevents any one user of the network frommonopolizing the network. It also means that the network becomesslower as more people try to use it.Many information transfers are longer than 1500 characters andnetworks sometimes lose packets, or damage them in transit. Packetscan also arrive out of sequence. The next layer of the network providesa way to transfer longer messages and also takes care of some of thedegradation that can occur in the network.Transmission Control Protocol (TCP)TCP is the protocol that handles most of these problems. It makessure the packets arrive and pastes them together in the right order. TCPtakes the information you want to transmit and breaks it into sections.Each section is numbered so receipt can be verified and the data can beput back in the proper order. The sequence number is sent in a packetof its own. Some or all of your data are placed in a TCP packet.During transmission, packets can be lost, or affected by noise ontelephone lines. TCP handles this by calculating a checksum. This is anumber based on the parity of the transmitted bytes. Parity involvescounting the number of 1s and determining if this number is even (evenparity) or odd (odd parity).When the packet arrives at its destination, the receiving TCP softwarecalculates what the checksum should be and compares it to the onesent by transmitter. If they do not match, an error has occurred duringthe transmission. The TCP software throws out that packet and requestsa retransmission.The TCP packet is placed inside an IP packet and given to thenetwork for transmission. On the receiving end, the TCP software collectsthe packets, extracts the data, and puts it in the proper order. Ifsome of the information is missing, it asks the sender to retransmit them.When it has all the information in the proper order, it passes the data tothe application program that is using its services.TCP appears to the user as a dedicated wire between two applications.There is not a dedicated link between the sender and receiver©2001 by The Fairmont Press, Inc. All rights reserved.