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.
Time division multiplexing (TDM) is a communications process that transmits two or more streaming digital signals over a common channel. In TDM, incoming signals are divided into equal fixed-length time slots. After multiplexing, these signals are transmitted over a shared medium and reassembled into their original format after de-multiplexing. Time slot selection is directly proportional to overall system efficiency.
Time division multiplexing (TDM) is also known as a digital circuit switched.
TDM was initially developed in 1870 for large system telegraphy implementation. Packet switching networks use TDM for telecommunication links, i.e., packets are divided into fixed lengths and assigned fixed time slots for transmission. Each divided signal and packet, which must be transmitted within assigned time slots, are reassembled into a complete signal at the destination.
TDM is comprised of two major categories: TDM and synchronous time division multiplexing (sync TDM). TDM is used for long-distance communication links and bears heavy data traffic loads from end users. Sync TDM is used for high-speed transmission.
During each time slot a TDM frame (or data packet) is created as a sample of the signal of a given sub-channel; the frame also consists of a synchronization channel and sometimes an error correction channel. After the first sample of the given sub-channel (along with its associated and newly created error correction and synchronization channels) are taken, the process is repeated for a second sample when a second frame is created, then repeated for a third frame, etc.; and the frames are interleaved one after the other. When the time slot has expired, the process is repeated for the next sub-channel.
Examples of utilizing TDM include digitally transmitting several telephone conversations over the same four-wire copper cable or fiber optical cable in a TDM telephone network; these systems may be pulse code modulation (PCM) or plesiochronous digital hierarchy (PDH) systems. Another example involves sampling left and right stereo signals using resource interchange file format (RIFF), also referred to as waveform audio file format (WAV), audio standard interleaves. Also synchronous Digital Hierarchy (SDH) and synchronous optical networking (SONET) network transmission standards have incorporated TDM; and these have surpassed PDH.
TDM can also be used within time division multiple access (TDMA) where stations sharing the same frequency channel can communicate with one another. GSM utilizes both TDM and TDMA.
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