d IP encapsulation, mapping to ATM, and then TDM, which likely included SONET/SDH as the underlying layer 1 transport. More recently, as traffic has grown and technology has matured into more capable and sophisticated network elements, IP has moved beyond ATM and directly on to SONET/SDH. Without consideration of such things as MPLS and other mechanisms to deal with GOS and QOS issues, it is instructive to consider Transport of the same pro- gram content over IP. A couple of important points to recognize and not forget: (1) IP networks rely on the underlying transport layers-Layers 1 and 2 for clocking and timing. Once a set of information is taken from the source and placed into packets it has no timing relationship until it reaches the other side of the network and the reversal of the encapsulation process takes place. (2) The packets may take different routes through the network and experience varying degrees of delay relative to one another. Obviously, this wouldn't work too well, SO there must be some kind of mechanism to ensure the information the packets carry is placed in the correct relationship to each other. How to do this is a very big story, and outside of the scope of this appendix. In short, the answer is protocols-processes that take the APPENDIX program content and encapsulate it into packets that the network carries and delivers, after which it is removed from the packets reassembled into program content. One example, and there are many, uses IP, UDP, and RTP where the sequence of events or results of running the protocols through their full cycle produces a sequence starting with the headers followed by the content. Picture 20-byte IP, 8-byte UDP, and 12-byte RTP headers and, finally, 4-188 byte MPEG packets with the content in them. Equivocating this to the TDM and ATM examples above is a bit of a convoluted exercise, but here goes: Each repetitive cycle includes 792 bytes (6,336 bits) total 752 bytes of content 40 bytes overhead (20+8+12) Note: The above is all static numbers and contains no 