The MOP module is located in the Application layer described by the
   Digital Network Architecture. MOP has a direct connection with the Data
   Link layer; thus, for certain functions, MOP can bypass the higher layers
   in the DNA protocol tower.  This is useful for nodes which do not yet have
   all the higher layers of DNA protocol towers installed.  Functions
   provided by the MOP module include down-line loading, up-line dumping,
   and communications testing.

  The hierarchy of MOP Module Entities is shown with indentation as follows:

                           MOP
                             Client
                             Circuit
                               Operation
                               Station

   The MOP entity is the top-level entity in the hierarchy of entities
   belonging to the MOP module.