Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!seismo!mcvax!unido!tub!cabo From: cabo@tub.UUCP Newsgroups: comp.dcom.modems Subject: Re: Network protocols questions - (nf) Message-ID: <51700003@tub.UUCP> Date: Wed, 1-Apr-87 07:52:00 EST Article-I.D.: tub.51700003 Posted: Wed Apr 1 07:52:00 1987 Date-Received: Sat, 4-Apr-87 19:06:40 EST References: <5893036@CU20B.UUCP> Lines: 71 Nf-ID: #R:CU20B:-589303644:tub:51700003:000:3432 Nf-From: tub!cabo Apr 1 13:52:00 1987 () [...] putting together a course on data communication protocols [...] () Can anybody out there help? Although I'm not the one who teaches the data communications courses here, I'll try to throw in my USD 0.02. () Is X.21 really used anywhere? Yes, it is used as the base for the (pre-ISDN) line switched digital networks in Europe like Datex-L (DATa EXchange/Line-switched) in Germany. Since Datex-L in turn is the base network for the German TELETEX network, a friend and I did have the pleasure to implement X.21 last year. () I suspect not, because it requires special interfaces (as () described in X.24), which are probably not (yet?) widespread. It does require special interfaces (called RS-422 in the US), but in Europe you just have to satisfy the PTT requirements when you want to connect to public networks. () How about X.21bis, which works with V-series interfaces (e.g. V.24, i.e. () RS-232)? Actually, there are two versions of X.21 (based on V.11 == RS422): A dialling version and a simple connect/disconnect version (without any way to give a number for the remote end) usually called the leased-line version. X.21bis is just a way to express the leased-line X.21 protocol on RS232 wires. If you have a working RS232 (with all the modem control signals correctly handled) you essentially have a working X.21bis, thus I could say X.21bis is used all over the world. The X.21 dialling protocol is only defined on RS422 wires and is weird enough that you essentially need hardware assistance to do it right (we managed to do it in software on a Z80 in assembly language, shudder). I'd recommend to stay away from the X.21 dialling protocol at all costs. () I suppose Hayes modem language could be considered a "de-facto" standard () physical layer protocol...(?) I think so, too; but line-switching (be it X.21 or Hayes modems) doesn't fit well into the OSI model; it moves layer 3 aspects down to layer 1. Now, how do we get the European PTTs to accept this de-facto standard? () It seems hard to believe that Bisync can actually work under noisy () conditions, since many of the commands and responses are not () error-checked. Do sites that depend on Bisync abound in horror stories? If Bisync does work, it works by accident. Every other description would be euphemic. RSCS (the protocol BITNET is based on) only really works because it has a second data link layer (with sequence numbers) above BSC. () Also, I don't understand how transparent data mode is supposed to work, in () light of the fact that SYNs that are inserted into the data by the () INTERFACE (e.g. idles) are supposed to have DLEs stuffed in front of () them, presumably by the SOFTWARE that already gave the data to the () interface to send. Or are there special Bisync interfaces that actually () understand all of this...? There are nice chips out there that do all the crubby BSC stuff (the R68561 comes to mind, I run my UREP on this GREAT chip), but you can do everything necessary on a dumb chip like the Z8530 SCC or the 8274. You are right, it is difficult to design a clean interface to a BSC line driver, but it is difficult to do anything cleanly in less than 10 meters of distance from an IBM defined protocol. Regards, Carsten -- Carsten Bormann, Communications and Operating Systems Research Group Technical University of Berlin (West, of course...)