Lord of the Rings. MSSP, MSDP ring protection

I was system architect in the telecommunication company several years ago.
And I am the author of the Network Managament System (NMS) for SDH, PDH, Ethernet networks and equipment.
The most interesting things in the SDH/PDH - theirs protection schemes:

• MSDP Ring (Dedicated ring). One unidirectional ring for main traffic and one for protection.
  
  
• MSSP Ring 2-Fiber (Shared ring). 1/2 of bidirectional ring for main traffic and 1/2 for protection.
  
  
• MSSP Ring 4-Fiber (Shared ring). One bidirectional ring for main traffic and one for protection.
  
  

It is possible to use protecting channels for not protected (NOP) traffic while ring works without breaks.