to the receiver. The receiver then adds up the bits the same way the terminal did, and if they equal an odd number, the receiver assumes that everything has arrived correctly. If they don't equal an odd number, the receiver knows there is a problem but cannot cor- rect it. This is the trouble with parity checking; to determine that an error had occurred, you would have to look at the output report, and therefore errors can easily go unnoticed. Thus, parity checking is not the best technique when it comes to ensuring the correctness of information. Information Character Position Parity Bit Figure 5.6 Parity checking The OSI Reference Model and the TCP/IP Reference Model Receiver calculates own CRC, then checks that it matches sender's CRC Sender appends CRC Calculated Message Message Figure 5.7 Cyclic redundancy checking Synchronous terminals and transmission use a type of error control called cyclic redundancy checking (see Figure 5.7). This is the method mentioned earlier in the chapter, whereby the entire message block is run through a mathematical algorithm. A cyclic redundancy check (CRC) code is appended to the message, and the message is sent to the receiver. The receiver recalculates the message block and compares the two CRCs. If they match, the communication continues, and if they don't match, the receiver either requests retransmissions until the problem is fixed or it disengages the session if it is not capable of being fixed within some predeter- mined time frame. The OSI Reference Model and the TCP/IP Reference Model Before two computers or network devices can exchange information, they must establish communication, and this is where protocols come in. A network protocol enables two devices to communicate by using one set of rules. The OSI model and protocol standards help to ensure that networking devices are capable of working together over a network. Protocols are the hardware or software components that carry out the OSI model guidelines for transferring information on a network. A protocol may b