he next frame. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols Figure 2-36 Zero inserted here to prevent Bit stuffing, or possibility of a false flag zero-bit insertion 00101111101001 The next field in the frame is the address field. This field identifies the address of the next switch in the chain to which the frame is directed, and it changes at every node. The only address that remains constant is the destination address, safely embedded in the packet itself. The third field found in many frames is called the control field. It con- tains supervisory information that the network uses to control the integrity of the data link. For example, if a remote device is not respond- ing to a query from a transmitter, the control field can send a "mandatory response required" message that will allow it to determine the nature of the problem at the far end. It is also used in hierarchical multipoint net- works to manage communications functions. For example, a multiplexer may have multiple terminal devices attached to it, all of which routinely transmit and receive data. In some systems, only a single device is allowed to "talk" at a time. The control field can be used to force these devices to take turns. This field is optional; some protocols do not use it. The final field we will cover is the Cyclic Redundancy Check, or CRC, field. The CRC is a mathematical procedure used to test the integrity of the bits within each frame. It does this by treating the zeroes and ones of data as a binary number (which, of course, it is) instead of as a series of characters. It divides the "number," shown in Figure 2-37, by a carefully crafted polynomial value that is designed to always yield a remainder following the division process. The value of this remainder is then placed in the CRC field and transmitted as part of the frame to the next swi