Path: utzoo!utgpu!watserv1!watmath!att!bu.edu!rpi!zaphod.mps.ohio-state.edu!samsung!uunet!cos!howard From: howard@cos.com (Howard C. Berkowitz) Newsgroups: comp.protocols.iso Subject: Re: question about transport layer protocol Message-ID: <42894@cos.com> Date: 28 Jan 91 13:48:23 GMT References: <12656004461.12.BILLW@mathom.cisco.com> Reply-To: howard@cos.UUCP (Howard C. Berkowitz) Organization: Corporation for Open Systems, McLean, VA Lines: 38 In article <12656004461.12.BILLW@mathom.cisco.com> BILLW@MATHOM.CISCO.COM (William "Chops" Westfield) writes: >Aren't X.21 and X.25 vastly different technologies? circuit switched >vs packet switched? It's really a worthwhile point, which I think is practical rather than pedantic, to remember that X.21 and X.25, respectively, are interfaces to networks rather than network specifications. X.21 specifies a circuit-switching interface to a network, which really could be packet-switching internally. X.25 specifies a packet-switching interface to a network which could be an X.25-like virtual call connection-oriented network, or DECnet, or SNA, or a LAN... To confuse things further, X.25 cites X.21 as is physical interface specification. Really, what is meant by this is the Level 1 (physical) part of X.21, which specifies a 15-pin connector and pin assignments. At the next level of confusion, very few people, certainly in North America, use the X.21 connector specification (the CONNECTOR, but not the logical pin definitions, IS used for T1 and other applications). Instead, people use X.21 _bis_, which is more or less RS-232C. [Actually, X.21 bis points to the physical connector specification in V.24]. RS-232C, V.24, and X.21 bis will interoperate in practice if one is careful, and sensitive to issues such as the screw thread on the locking connector (as opposed to the compatible pins). The EIA standard specifies an English 4-40 screw thread, while the international specs specify a metric screw thread. At COS, we finessed this in our testing and stack specifications by referring to X.21 bis, but asking the implementor to state whether he was using English or metric threads. -- howard@cos.com OR {uunet, decuac, sun!sundc, hadron, hqda-ai}!cos!howard (703) 883-2812 [W] (703) 998-5017 [H] DISCLAIMER: Opinions expressed are not necessarily those of the Corporation for Open Systems, its members, or any standards body.