This example is for illustration purposes only. By convention, I frames are sent roughly every 400 ms. Also by convention, there are generally 10 to 12 frames between I frames. The mix of B frames and P frames is variable. Some users have elected not to use B frames at all but to use more P frames instead.
B frames tend to make pictures smoother on playback while consuming less bandwidth.
The problem is that they force the decoder to buffer P frames and compute B frames. This requirement increases decoder costs, which is a particular problem for cable TV set top manufacturers. General Instrument's Digicipher protocol has specifically excluded B frames in an effort to keep costs down. This architecture also has implications on networking. I frames anchor picture quality, because ultimately P and B frames are derived from them.