If students in an online class are distributed across the continent or around the world, then synchronous discussion sessions may be problematic for some of them due to the time differences. The 24/7 nature of distance learning provides flexibility for students to contribute asynchronously, but that flexibility is lost for synchronous sessions.
The Blackboard course management system that holds my City Colleges of Chicago online courses has a statistics tool that shows that some students are accessing the course site at all hours, but between 2 and 7 a.m. there is a big drop-off in log-ins. For another course, the results could be quite different, depending on where the non-local students live. There may need to be some flexibility in scheduling synchronous sessions. If students are just one or two time zones away, then there is little need to adjust the synchronous schedule, but if they are on the other side of the world, a 3 a.m. session, however difficult to accomplish, might be advisable.
But it may be impossible to make the synchronous sessions convenient for all the students enrolled in an online class. This may be the case even for some students in the same time zone as the instructor because everyone has different work and sleep schedules. To conduct synchronous sessions at different times each week can help to attract long-distance participants, but it may be difficult for a busy faculty member who has a regular schedule of classes to accomplish this degree of variability.
Because some students may not be able to participate, I think synchronous sessions should be considered optional, though suggested, components of an online class. Perhaps a small amount of extra credit could be awarded to those students who do participate. The synchronous sessions should be recorded and quickly made available to the entire class so that those who were unable to participate can at least view the session.