[cisco-nas] X.75 and modem dial-in, autocommand to TCP

[Aaron Leonard]

Hi Gert,

> a customer of ours currently has a dedicated PABX which is feeding an
> ISDN PRI into a bank of about 20 "ISDN modems" (USR Courier I-Modem,
> which do X.75 and "V.34 stuff" modem calls).  Modems are connected
> to serial terminal servers, which in turn connect to an AIX unix system.
>
> Customer is unhappy, because these take up lots of space, the PABX is
> expensive, modems crash and need a reset, USR Courier I-Modems are not
> sold "new" anymore, and so on.
>
> So the idea was to build something with a router - PRI channel into
> router, "transparent TCP stream" out of the router to the AIX system,
> to be handled by a something like telnetd.  We can handle everything on
> the AIX side, the questions are on the router side :-)
>
>  - as far as I understand AS5xxx boxes, this should be doable with
>    an AS5350 or the like
>   

Yes.

>  - what do I need to terminate X.75 calls?  Do I need modem DSPs, or 
>    will the X.75 be handled by the "digital side"?
>   

The framing for incoming "X.75" calls (which we call "LAPB-TA") are
handled by the onboard framers in the T1/E1/T3 trunk card plus IOS
software, so no DSPs are used for this application.

>  - I assume that X.75 calls will end up on a "line async ...", 

actually on a vty line


> so I can
>    put an "autocommand" there, to immediately connect from the router
>    to the host at CONNECT time (*no* prompting or any sort of messages
>    from the router is permitted).
>   

Yep.

>    Will that work?  It MUST be a) completely 8-bit transparent (Zmodem
>    transfers over X.75) and b) completely silent (end user call-up scripts
>    get upset if 'yet-unseen' messages show up).
>   

Should work.

Here's the config for this, more or less.  Here I am assuming that you
intend to support both voiceband modem modulations (V.90, V.34, V.32,
etc.) and LAPB-TA, on calls into your E1, but nothing else (not V.110,
V.120, sync PPP ...)


ip telnet quiet
aaa authentication login DIALIN none
aaa authorization exec DIALIN none
no banner incoming
!
interface serial BLAH:15
  isdn incoming-voice modem
  autodetect encapsulation lapb-ta
!
line vty 0 4
  location first 5 VTYs are used for administrative telnet/ssh
  transport input ssh telnet
line vty 5 34
  location VTYs for LAPB-TA (X.75) callers
  login authentication DIALIN
  authorization exec DIALIN
  autocommand telnet 1.2.3.4
  no exec-banner
  escape-character NONE
  special-character-bits 8
line 1/0 1/59
  location TTY lines for modem callers
  login authentication DIALIN
  authorization exec DIALIN
  autocommand telnet 1.2.3.4
  no exec-banner
  escape-character NONE
  special-character-bits 8

>  - is there Cisco gear that can handle X.75 on BRI lines (so we could
>    develop and test this on an ISDN S0 port, without disconnecting the 
>    whole PABX)?
>   

Actually, you could configure your 5350 to do TDM switching for all
calls other than those to a special test number.  So then you could hook
things up like this:

{PSTN}--E1--[[port 0]  5350  [port 1]]--E1--[PABX]

(5350 config commands outside the scope of this message)

Then once you've got things working to your test #, just unconfigure the
TDM switching and turn off your PABX :)

But ... yes, any ISR (1800/2800/3800) that supports data BRI calls,
should also support LAPB-TA.  You will need an image that supports this
(check Feature Navigator http://www.cisco.com/go/fn - "ISDN LAPB-TA").

>  - some of the clients have 'issues' with X75 window and block sizes - can
>    this be tuned on the Cisco side, as in "we need to have the X75 block
>    size set to 4096 byte on reception, 8192 on transmission"?
>   

Ooh, not aware of any way to do that.  (Nor have I ever heard of anyone
*need* to.)

> If Cisco cannot do this - any other gear that's still sold and supported
> (important!) that one could use to terminate X.75 and modem calls?
>
> thanks,
>
> gert

I'll let someone else try that one.

Cheers,

Aaron