text: In practice, the most often overlooked cause is latency. Global Mirror has a round-trip-time 
  tolerance limit of 80 or 250 milliseconds, depending on the firmware version and the hardware 
  model. A message that is sent from the source IBM Spectrum Virtualize system to the target 
  system and the accompanying acknowledgment must have a total time of 80 or 
  250 milliseconds round trip. That is, it must have up to 40 or 125 milliseconds latency each 
  way. 
  
  The primary component of your round-trip time is the physical distance between sites. For 
  every 1000 kilometers (621.4 miles), you observe a 5-millisecond delay each way. This delay 
  does not include the time that is added by equipment in the path. Every device adds a varying 
  amount of time, depending on the device, but a good rule is 25 microseconds for pure 
  hardware devices. 
  
  For software-based functions (such as compression that is implemented in applications), the 
  added delay tends to be much higher (usually in the millisecond plus range.) The following is 
  an example of a physical delay. 
  
  Company A has a production site that is 1900 kilometers (1180.6 miles) away from its 
  recovery site. The network service provider uses a total of five devices to connect the two 
  sites. In addition to those devices, Company A employs a SAN FC router at each site to 
  provide FCIP to encapsulate the FC traffic between sites. 