Services and protocols of the physical layer

Among the more important new works on the Physical Layer are perhaps the CCITT Draft Recommendation X.211, Physical Layer Service Definition, and the emerging Physical Layer protocols of the ISDN's (Integrated Services Digital Networks). Discussed along with these new developments, which came in time after the definition of the OSI model, are some of the widely used classic older standards and specifications, such as X.21, V.24, and V.54, which do not always fit nicely within the model layers but which deal substantively with the Physical Layer. This paper starts with an overview of Physical Layer documents, continues with-a detailed discussion of architectural service specifications, and concludes with a discussion of related standards.