Hitless protection switching

A protection scheme for a frame-based primary signal, in which the optical signal is regenerated, the protection scheme comprising:

• receiving a plurality of secondary signals, each secondary signal including a plurality of sequentially numbered signal elements derived from the frame-based primary signal;
• aligning the plurality of secondary signals with each other by determining a differential delay between corresponding sequentially numbered signal elements of each secondary signal;
• compensating for the determined differential delay using at least one delay element to synchronise the plurality of secondary signals with each other;
• presenting a plurality of synchronised secondary signals to a switching element;
• and, switching on a frame-block by frame-block basis between said plurality of synchronised secondary signals to regenerate said primary signal.

Apparatus arranged to implement signal generation comprising the following:

• a plurality of receivers arranged to receive a plurality of secondary signals, each secondary signal including a plurality of sequentially numbered signal elements derived from the same primary frame-based signal;
• alignment means arranged to align the plurality of secondary signals with each other by determining a differential delay between corresponding sequentially numbered signal elements of each secondary signal;
• at least one delay element arranged to compensate for the determined differential delay to synchronise the plurality of secondary signals with each other;
• a switching element arranged to receive a plurality of synchronised secondary signals;
• and,
• a switch controller arranged to switch on a frame-block by frame-block basis between said plurality of synchronised secondary signals to generate at least one frame-based signal.

Apparatus arranged to implement a protection scheme for a frame-based primary signal, the apparatus comprising:

• a plurality of receivers receiving a plurality of secondary signals, each secondary signal including a plurality of sequentially numbered signal elements derived from the frame-based primary signal;
• alignment means arranged to align the plurality of secondary signals with each other by determining a differential delay between corresponding sequentially numbered signal elements of each secondary signal;
• at least one delay element arranged to compensate for the determined differential delay to synchronise the plurality of secondary signals with each other;
• a switching element arranged to receive a plurality of synchronised secondary signals;
• and,
• a switch controller arranged to switch on a frame-block by frame-block basis between said plurality of synchronised secondary signals to regenerate said primary signal.

In a synchronous transmission system comprising a first synchronous transmission apparatus arranged to transmit data signals across a network on at least two transmission paths;

• and a second synchronous transmission apparatus arranged to receive said data signals on said at least two paths, a method of switching between transmission paths, the method comprising:
• associating, at said first synchronous transmission apparatus, a tag with each element of said data signals, the tag indicating the position of the signal element within the data signal;
• aligning, at said second synchronous transmission apparatus, the respective data signals received on said at least two transmission paths;
• selecting, at said second synchronous transmission apparatus, between signal elements having corresponding tags;
• and outputting said selected signal elements in sequential order according to the positional information indicated by the respective -tags.