| 摘要 |
Selective tributary switching is realized in a bidirectional transmission system by selectively switching, in accordance with the same rules governing the set-up an d take down procedures of full line-switching, only that portion of the bandwidth of the particular line which has been provisioned to be line-switched. The remainin g bandwidth can be left unprotected or, by for the first time combining line-switc hed ring functionality with path-switched ring functionality in the same ring transm ission system, some remaining bandwidth can be path-switched. Furthermore, another degree of switching freedom is achieved in a four optical fiber bidirectional ri ng transmission system by selectively span-switching, but not ring-switching, speci fic bandwidth on the line. To this end, communications circuit provisioning information is provided in the ring nodes as to whether a particular communicati ons circuit should be line-switched or not and, if not, whether it should be span- s witched, path-switched or left unprotected.
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