I sometimes use the following table : +---+--------------+----------------------+ ! # ! Layer name ! Protocol Data Unit ! +---+--------------+----------------------+ ! 7 ! Application ! Message ! ! 6 ! Presentation ! Data ! ! 5 ! Session ! Dialog ! ! 4 ! Transport ! Segment ! ! 3 ! Network ! Packet or Datagram ! ! 2 ! Data Link ! Frame ! ! 1 ! Physical ! Bits ! +---+--------------+----------------------+ ! 0 ! Medium ! Signal ! +---+--------------+----------------------+ -----Original Message----- From: Michel Py [mailto:michel@arneill-py.sacramento.ca.us] > Bill Cunningham wrote > If datagram and packet is the same, then what are frames? > TCP is carried by PPP frames. I'm not sure about APEX (rfc 3340) This is not new, is it what we are referring to? +---+--------------+----------------------+ ! # ! Layer name ! PDU ! +---+--------------+----------------------+ ! 7 ! Application ! Messages ! ! 6 ! Presentation ! Messages ! ! 5 ! Session ! Messages ! ! 4 ! Transport ! Segments ! ! 3 ! Network ! Packets or datagrams ! ! 2 ! Data Link ! Frames ! ! 1 ! Physical ! Bits ! +---+--------------+----------------------+ Michel.