njg@cbnewsk.att.com (nick.j.gnesda) (04/25/91)
I have some questions which I believe ( hope ) others may have already developed/devised an answer to. Any help they can offer would be greatly appreciated. The questions are specific to the V.32 and V.42 ( and their "bis" variants" ) modems. It seems most people use these modems for asynchronous communications does anybody use these modems to connect to X.25 packet switching networks using synchronous ports? I am not a modem expert by any means but I have read the claims of various modem manufactures. In general many V.32/V.42 modems claim synchronous and asynchronous communication capabilities. Most provide an AT command set for dialing, others in addition to the AT command set support a standard called V.25bis, from what I can tell by reading some CCITT specs V.25bis defines a synchronous dialing command set, which replaces the AT commands. V.25bis can be used by HDLC ( X.25 ) and byte synchronous ( BISYNC ) protocols I wish to establish communications with other computers over synchronous ports and I would like to use V.42bis modems which offer some very nice price/performance statistics but am uncertain as to how to provide the needed dialing flexibility. Here's some more questions for any synchronous V.32 and V.42 modem users. If you dial multiple locations and utilize synchronous communications in an automated fashion; how are you establishing the modem connection? Is anyone using V.25bis? Thank You Nick Gnesda ===================================================================== = Nick Gnesda AT&T-BL Place Your Disclaimer = att mail: hrmso!njg Here = internet: njg@hrmso.att.com =====================================================================
lstowell@pyrnova.pyramid.com (Lon Stowell) (04/26/91)
In article <1991Apr25.154758.8842@cbnewsk.att.com> njg@cbnewsk.att.com (nick.j.gnesda) writes: > >I have some questions which I believe ( hope ) others may have >already developed/devised an answer to. Any help they can offer >would be greatly appreciated. The questions are specific >to the V.32 and V.42 ( and their "bis" variants" ) modems. > > > It seems most people use these modems for asynchronous > communications does anybody use these modems to connect > to X.25 packet switching networks using synchronous > ports? > Yes. When you run a sync interface, V.42 and V.42bis are not applicable....nor is speed mapping or any of the other async features. The modems typically provide clocking to the DTE's so if the modems alter speeds (on V.32 error rate) the DTE's will follow. A V.32 modem typically offers 9600 sync and some fall back to 4800 on noisy links. V.32bis offers 14.400 sync, some fall back on noisy lines. V.32 modems work extremely well for SDLC dial-up or even X.25 IF (and only IF) the modem configuration is matched to the DTE configuration. X.25 is not normally a dial protocol, but you can usually spoof the DTE sufficiently to make it work over a dial connection. SDLC may be implemented as either dial or leased line over V.32. >V.25bis can be used by HDLC ( X.25 ) and byte synchronous ( BISYNC ) >protocols > V.25bis offers a bit oriented dial mode that uses the broadcast link address and Unnumbered Information frames. SOME DTE's can be altered to provide this.....X.25 ones have more difficulty than SDLC ones....as X.25 typically uses only address 01 and 03, where SDLC primary stations typically support FF. >I wish to establish communications with other computers over >synchronous ports and I would like to use V.42bis modems V.42bis is NOT a sync protocol. It is for async data compression. It must be disabled (as must V.42, MNP, etc. > If you dial multiple locations and utilize synchronous > communications in an automated fashion; how are you > establishing the modem connection? It depends on your DTE and how much you can hack it. Some can use async AT commands, then go sync. (this is rare and has a gotcha...) Bisync stations (IBM type) have problems with the V.25bis Byte Oriented dial unless you have direct access to the link level inputs. Bit sync devices need to support Addx 0xFF and UI frames with Poll/Final bit parsing in order to use V.25bis HDLC. > > Is anyone using V.25bis? I have, you normally have to write your own link level code to issue the dial strings. If X.25 you will also have to be VERY aware of the peculiarities of V.32 modems in raising/lowering their EIA leads. Some spoofing of timers is required as well......how will depend on your specific machine and whether or not you have source code acess for the sync ports. Most sync users today (w/o V.25bis) use DTR Dial or just use an external RS-366 or V.25 (parallel) dialler....which requires a separate port....