  We identify the common underlying form of the capacity expression that is
applicable to both cases where causal or non-causal side information is made
available to the transmitter. Using this common form we find that for the
single user channel, the multiple access channel, the degraded broadcast
channel, and the degraded relay channel, the sum capacity with causal and
non-causal side information are identical when all the transmitter side
information is also made available to all the receivers. A genie-aided
outerbound is developed that states that when a genie provides $n$ bits of side
information to a receiver the resulting capacity improvement can not be more
than $n$ bits. Combining these two results we are able to bound the relative
capacity advantage of non-causal side information over causal side information
for both single user as well as various multiple user communication scenarios.
Applications of these capacity bounds are demonstrated through examples of
random access channels. Interestingly, the capacity results indicate that the
excessive MAC layer overheads common in present wireless systems may be avoided
through coding across multiple access blocks. It is also shown that even one
bit of side information at the transmitter can result in unbounded capacity
improvement. As a side, we obtain the sum capacity for a multiple access
channel when the side information available to the transmitter is causal and
possibly correlated to the side information available to the receiver.
