BS EN 61158-4-2:2014

1   Scope

   1.1   General 
   The data- link  layer provides basic time-critical messaging communications between  devices  in an automation environment. 
   This protocol provides communication opportunities to all participating data- link  entities, sequentially and in a  cyclic  synchronous manner. Foreground  scheduled  access is available for time-critical activities together with background  unscheduled  access for less critical activities. 
   Deterministic and synchronized transfers can be provided at  cyclic  intervals up to 1 ms and  device  separations of 25 km. This performance is adjustable dynamically and on-line by re-configuring
     the parameters of the  local link  whilst normal operation continues. By similar means, DL  connections  and new  devices  may be added or removed during normal operation. 
   This protocol provides means to maintain clock synchronization across an  extended link  with a precision better than 10 μs. 
   This protocol optimizes each access opportunity by concatenating multiple DLSDUs and
     associated DLPCI into a single  DLPDU , thereby improving data transfer efficiency for data- link  entities that actively source multiple streams of data. 
   The maximum system size is an unlimited number of  links  o...