add global channel service

Change-Id: I1fecdfd7f4f6dbea53008c453f1f93679e4bd351
add comments

Change-Id: I9a0edeb1b3107228fc1ae248ed7f3f1b2aa2d068
Merge changes I9a0edeb1,I1fecdfd7 into 0.5.x

* changes:
  add comments
  add global channel service