Hit-less protection switching method and apparatus for ATM transmission lines

A hit-less protection switching apparatus for ATM transmission lines comprising:

• normal receiving means for receiving ATM signals from a normal transmission line;
• emergency receiving means for receiving ATM signals from an emergency transmission line;
• first selecting means for selecting assigned cells from normal received signals;
• second selecting means for selecting assigned cells from emergency received signals;
• a normal cell buffer for storing the assigned cells selected from said normal received signals;
• an emergency cell buffer for storing the assigned cells selected from said emergency received signals;
• means for reading a first assigned cell from said normal cell buffer with a first delay of a predetermined time and reading a second assigned cell from said emergency cell buffer with a second delay of a predetermined time simultaneously;
• means for comparing contents of said first assigned cell with contents of said second assigned cell and detecting discrepancy between said contents;
• means for changing a difference between said first delay of a predetermined time and said second delay of a predetermined time, when a state of the discrepancy between said contents is continuously detected for a predetermined time, so as to dissolve said state of the discrepancy;
• and
• third selecting means for selecting one of said assigned cells read from said normal cell buffer and from said emergency cell buffer as communication information based on operation commands,
• wherein said changing means includes means for once stopping read-out of a previously outputted one of said assigned cells from said normal and emergency buffers and inputting a subsequently outputted one to said third selecting means in the meantime when an idle cell is received as a received signal by one of said normal and emergency transmission lines from which said assigned cells are previously outputted.