A shell variable is a variable that is available only to the current shell. In contrast, an environment variable is available system wide and can be used by other applications on the system.A shell is the operating system's command interpreter. It processes the commands entered on the command line or read from a shell script file.
A token bus network is much like a token-ring network except that the ends of the network do not meet to form the ring. Instead, the network is still terminated at both ends.
A token is still required before a node can use the network. Like in a token-ring, it needs to include the address of the destination along with the data it needs to send. Although in the token bus, it implements a virtual ring on the coaxial cable.
Though both topologies use tokens, the similarities end there, as token bus uses a different topology and the token-passing protocol is different. In a token-ring network, the token and data is passed to the next physical node along the line, but in a token bus network, it does not matter where the nodes are physically located since token-passing is done via a numeric sequence of node addresses. The token or data is passed to the next sequential node address no matter if the physical location of that node is at the very end of the bus network. This is the virtual ring; the physical layout of the network will not change it.
Token bus networks are similar in concept to token ring networks in essence but different in practice. For one, it uses a different topology, a bus topology, and the method for passing tokens is different. Compared to a token ring which passes the token and data to the next physical node in the ring, token bus networks utilize a virtual ring where all the nodes have different sequence addresses which the token and data have to pass through. The physical location of the nodes will not matter in the token bus topology; it is the sequence address of the nodes that matter where the token would go next until it reaches its destination.
Token bus networks are mainly used for industrial applications such as manufacturing. The IEEE 802.4 working group has disbanded, meaning that the standard is no longer being updated and not seeing wide use.
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