  While there are continuous efforts to introduce new communication systems and
standards, it is legitimate to ask the question: how can one send additional
bits by minimally changing the systems that are already operating? This is of a
significant practical interest, since it has a potential to generate additional
value of the systems through, for example, introduction of new devices and only
a software update of the access points or base stations, without incurring
additional cost for infrastructure hardware installation. The place to look for
such an opportunity is the communication protocol and we use the term *protocol
coding* to refer to strategies for sending information by using the degrees of
freedom available when one needs to decide the actions taken by a particular
communication protocol. In this paper we consider protocol coding that gives a
rise to *secondary communication channels*, defined by combinatorial ordering
of the user resources (packets, channels) in a primary (legacy) communication
system. We introduce communication models that enable us to compute the
capacity of such secondary channels under suitable restrictions imposed by the
primary systems. We first show the relation to the capacity of channels with
causal channel state information at the transmitter (CSIT), originally
considered by Shannon. By using the specific communication setup, we develop an
alternative framework for achieving the capacity and we discuss coding
strategies that need to be used over the secondary channels. We also discuss
some practical features of the secondary channels and their applications that
add value to the existing wireless systems.
