  We derive a new outer bound on the capacity region of broadcast traffic in
multiple input broadcast packet erasure channels with feedback, and extend this
outer bound to packet erasure relay networks with feedback. We show the
tightness of the outer bound for various classes of networks. An important
engineering implication of this work is that for network coding schemes for
parallel broadcast channels, the `xor' packets should be sent over correlated
broadcast subchannels.
