it is a protocol of ITU-T (國際電信聯盟 電信標準化部門). it helps to keep operation effectively even bitsrate-errors occuring.
They use X.25 protocol to make Packets-SW in Network. This protocol enables carrier to detect error and offer covery.
Tele Carrier -------------------------------------- PSN
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---- DTE, data terminat equipment (workstations & hosts)
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---- PSE, packet sw exchange
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---- PAD, packet assembler/diassembler (end device to DCE)
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---- DCE, data circuit-terminate equipment
see Frame Relay. (bneficial to save bandwidth)
SVC: sw virtual circuit
PVC: permanent virtual circuit