Chris F.:Trinode Ethernet MAC Host interface and physical interface, with flow control between the two. Problems: Control logic managed by writing to status registers through host interface (serial/parallel conversion required). One host interface can control multiple MACs on the MDIO bus (bus arbitration is required). Must support multiple paths of control/types of operation in one set of hardware. How to do error control? On a link? In general? - Size of data, detection and correction, check and retry, forward extra information to reconstruct Data clock & clock crossings (large library of solutions) Physical constraints are part of the problem specification that makes up the LHS of a pattern. May be used to eliminate some of the potential solutions that might have been selected in other environments.