progress bar code in vb.net Figure 2-36 Bit stuffing or zero bit insertion. in Software

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Figure 2-36 Bit stuffing or zero bit insertion.
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The third field found in many frames is called the control field. It contains supervisory information that the network uses to control the integrity of the data link. For example, if a remote device is not responding to a query from a transmitter, the control field can send a mandatory response required message that will enable it to determine the nature of the problem at the far end. It is also used in hierarchical multipoint networks 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 (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 switch. The receiving switch performs the same calculation and then compares the two remainders. As long as they are the same, the switch knows that the bits arrived unaltered. If they are different, the received frame is discarded and the transmitting switch is ordered to resend the frame, a process that is repeated until the frame is received correctly. This process can result in transmission delay, because the Data Link Layer will not enable a bad frame to be carried through the network. Thus, the Data Link layer converts errors into delays.
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Figure 2-37 CRC-based bitlevel error control.
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Protocols
Protocols
Error Recovery Options
A number of commonly used techniques enable receiving devices to recover from bit errors. The simplest of these is frame discard, the technique used by Frame Relay and ATM networks. In frame discard environments, an errored frame is simply discarded, period. No other form of recovery takes place within the network. Instead, the end devices (the originator and receiver) have the end-to-end responsibility to detect that a frame is missing and take whatever steps are necessary to generate a second copy. The reasons for this strategy will be discussed in the section on fast packet services. A second common technique is called forward error correction (FEC). FEC is used when (1) no backward channel is available over which to request the resend of an errored packet, or (2) the transit delay is so great that a resend would take longer than the application would allow, such as in a satellite hop over which an application is transmitting delaysensitive traffic. Instead, FEC systems transmit the application data with additional information that enables a receive device to not only determine that an error has occurred, but to fix it. No resend is required. The third and perhaps most common form of error detection and correction is called detect and retransmit. Detect and retransmit systems use the CRC field to detect errors when they occur. The errored frames are then discarded, and the previous switch is ordered to resend the errored frame. This implies a number of things: the frames must be numbered, some positive and negative acknowledgment system must be in place, the transmitter must keep a copy of the frame until its receipt has been acknowledged, and there must be some facility in place to allow the receiver to communicate upstream to the transmitter. Two recovery techniques are commonly utilized in synchronous systems. To understand them, we must first introduce a couple of transmission protocols used to meter the transmission of frames between switches. In early communications systems (back in the 1970s), the network was known to be relatively hostile to data transmissions. After all, if noise occurred during a voice conversation, no problem; it was a simple matter to ignore it, provided it wasn t too bad. In data, however, a small amount of noise could be catastrophic, easily capable of destroying a long series of frames in a few milliseconds. As a result, early data systems such as IBM s Binary Synchronous Communications (BISYNC) used a protocol called Stop-and-Wait that would only permit a single frame at a
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