barcode project in vb.net The Data Link Layer in Software

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The Data Link Layer
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The Data Link Layer is responsible for ensuring bit-level integrity of the data being transmitted. In short, its job is to make the layers above believe that the world is an error-free and perfect place. When a packet is handed down to the Data Link Layer from the Network Layer, it wraps the packet in a frame. In fact, the Data Link Layer is sometimes called the frame layer. The frame built by the Data Link Layer is made up of several fields, shown graphically in Figure 2-35, that give the network devices the ability to ensure bit-level integrity and proper delivery of the packet, which is now encased in a frame, from switch to switch. Please note that this is different from the Network Layer, which concerns itself with routing packets to the final destination. Even the addressing is unique: Packets contain the address of the ultimate destination, used by the network to route the packet properly; frames contain the address of the next link in the network chain (the next switch), used by the network to move the packet along, switch by switch.
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Figure 2-35 Data Link Layer frame
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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
2
As the diagram illustrates, the beginning and end fields of the frame are called flags. These fields, made up of a unique series of bits (0111110), can only occur at the beginning and end of the frame they are never allowed to occur within the bitstream inside the frame through a process that we will describe momentarily. These flags are used to signal to a receiving device that a new frame is arriving or that it has reached the end of the current frame,3 which is why their unique bit pattern can never be allowed to occur naturally within the data itself it could indicate to the receiver (falsely) that this is the end of the current frame. If the flag pattern does occur within the bitstream, it is disrupted by the transmitting device through a process called bit stuffing or zerobit insertion, in which an extra zero is inserted in the middle of the flag pattern, based on the following rule set. When a frame of data is created at an originating device, the very last device to touch the frame indeed, the device that actually adds the flags is called a Universal Synchronous/Asynchronous Receiver-Transmitter (USART). The USART, sometimes called an Integrated Data Link Controller (IDLC), is a chipset that has a degree of embedded intelligence. This intelligence is used to detect (among other things) the presence of a false flag pattern in the bitstream around which it builds the frame. Since a flag is made up of a zero followed by six ones and a final zero, the IDLC knows that it can never allow that particular pattern to exist between any two real flags. So, as it processes the incoming bitstream, it looks for that pattern and makes the following decision: If I see a zero followed by five ones, I will automatically and without question insert a zero into the bitstream at that point. This is illustrated in Figure 2-36. This, of course, destroys the integrity of the message, but it doesn t matter. At the receive device, the IDLC monitors the incoming bits. As the frame arrives it sees a real flag at the beginning of the frame, an indication that a frame is beginning. As it monitors the bits flowing by, it will find the zero followed by five bits, at which point it knows, beyond a shadow of a doubt, that the very next bit is a zero which it will promptly remove, thus restoring the integrity of the original message. The receiving device has the ability to detect the extra zero and remove it before the data moves up the protocol stack for interpretation. This bit stuffing process guarantees that a false flag will never be interpreted as a final flag and acted upon in error.
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