Bandwith use of ros topic

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Hi, I'm going to run my ros system on multiple computers, connected only by a wifi link of (possibly) bad quality. However I want to use data from multiple usb web cams.

I'm wondering how ros distributes the various message on the network and if it makes a difference where roscore is located.

Example Scenario:

Machine 1:
  Node 1:
    outgoing topics:
      camera1/image_raw
      camera1/image_compressed

Machine 2:
  Node 2:
    outgoing topics:
      camera2/image_raw
      camera2/image_compressed

Machine 3:
  Node 3.1:
    incoming topics:
      camera1/image_raw
      camera2/image_raw
    outgoing topics:
      control/steering
      control/debug

  Node 3.1:
    incoming topics:
      camera1/image_raw
      control/steering

1) Which messages are actually send?

2) Do the camera*/image_compressed topics take up bandwith, even though they are never subscribed to?

3) is camera1/image_raw send twice over the network?

4) is camera2/image_raw also send to machine 1?

5) Does it matter whether roscore is started on machine 1 or machine 3?