ok from the top to bottom, application layer hands DATA to presentation layer, presentation layer does nothing to the DATA and hands it to session layer, session layer does nothing to the DATA and hands it to transport layer, transport layer will seperate the DATA into SEGMENTS and put a header containing segment number, port number on each SEGMENT (encapsulating) and hands it down to network layer. network layer put another header containing IP address and other info on each encapsulated SEGMENT to form PACKETS, then hands it to datalink layer. datalink layer put a header containing MAC address and other info and a trailer containing the checksum on each PACKET to form FRAMES. and sent them to physical layer. physical layer does nothing to the content of the frames but only translate them into signals and then transmit them to the receiver. the physical layer of the receiver translate the signals back to FRAMES, hands it to datalink layer. datalink layer check the mac address, if it's the receiver's mac address, it'll pill off the frames headers and trailers and hands the naked PACKETS to network layer. the network layer will check the iP address, if it's the receiver's IP address it'll pill off the network headers and send the naked SEGMENTS to transport layer. transport layer will read the info in each segment, puts the segments into correct order according to segment numbers (and request retransmitting of missing segments), assembles them back into DATA and give them to his boss.
