PSTN

Name

PSTN -- PSTN Modem Support.

Description

The following text describes a facility which was inherited from DOS XRouter, but which probably has no relevance nowadays?...

XRPi may be connected to one or more Public Switched Telephone Network (PTSN) modems, for dial-in and dial-out operations.

Dial-in would, for example, allow users and/or sysops to connect to XRPi and operate it using a dumb terminal or a standard terminal package such as Telix or Hyperterm.

Alternatively, after login, the link may be switched into SLIP or PPP mode for TCP/IP operations, behaving in exactly the same way as a dial-up Internet Service Provider.

Dial-out allows Xrouters to be linked with each other or with an Internet service provider, for the purposes of on-demand wired routing, or Internet Connection Sharing.

A single modem may be used for both dial-in and dial-out operations on the same port, although not at the same time of course!

Suitable Modem Types

Almost any modem is suitable, providing it can be initialised by a single string of characters and can be configured to disconnect when DTR is dropped.

Hardware Configuration

External modems should be connected to a serial port using a cable with at least 8 connections, namely TXD, RXD, RTS, CTS, DTR, DSR, DCD and ground. The RI (ring indicator) connection is not needed.

Internal modems should be configured to use a spare COM number and IRQ.

Software Configuration

Each modem requires an ASYNC interface definition in XROUTER.CFG, with COM (or IOADDR & IRQ if non-standard) configured for the appropriate serial port or modem card. You should use PROTOCOL=MODEM, FLOW=1 and MTU=576.

To each "modem" interface should be attached a PORT with at least INTERFACENUM and ID specified. If the modem requires an initialisation string, add INITSTR=<initstring>, e.g. to set the modem into auto answer mode use "INITSTR=ATS0=1". If you don't include the INITSTR keyword, the modem configuration is not altered.

If your modem does not, by default, hang up when the RS232 DTR signal is dropped, you should configure it to do so by including "&D2" in the initialisation string, for example: "INITSTR=ATM0S0=1&D2".

Example MODEM Interface and Port Configuration

In XROUTER.CFG:

            INTERFACE=5
                TYPE=ASYNC
                COM=3
                SPEED=57600
                PROTOCOL=MODEM
                FLOW=1
                MTU=576
            ENDINTERFACE

            PORT=2
                ID=PSTN Modem port
                INTERFACENUM=5
                INITSTR=ATS0=1
            ENDPORT

If you will be allowing incoming calls, you must set up a callsign and password entry for each user in USERPASS.SYS.

Dial-in Operation

If you have configured the modem for auto-answer, PSTN callers must successfully complete a callsign and password challenge before they are allowed to use the XRPi command interface.

The callsign must be a proper amateur radio callsign, i.e. not a "username". If a valid callsign is not given, or if the callsign is not found in USERPASS.SYS, or if an incorrect password is supplied, the user is immediately disconnected. If this sounds unforgiving, it is meant to be! It will cost hackers the price and time delay of a separate phone call for each attack.

If the callsign and password challenge is successfully completed, the caller will be allowed full access to the command shell, exactly the same as a radio caller.

XRPi will disconnect the caller after 15 minutes of inactivity. You may initialise the modem to disconnect after a shorter interval if necessary.

The XLINK command

If the caller (e.g. using NOS) wishes to establish a TCP/IP link with XRPi, the XLINK command is used to switch the ASCII link into SLIP ("XLINK SLIP") or PPP ("XLINK PPP") mode. XRPi will respond with "Entering SLIP mode" or "Entering PPP mode", and will thereafter no longer respond to ASCII commands. SLIP or PPP mode may only be terminated by disconnection.

In order to use SLIP or PPP modes, XRPi must have at least a global IPADDRESS, and you must set up IP routing to the caller's IP address on the modem port. You could either allow each caller to use their own IP address, and have one routing entry for each caller, or you may choose to require all callers on a particular port to use the same IP address (since only one may connect at any time) and set up a single routing entry.

For example, you could tell each of your SLIP/PPP callers to set their IP address to 192.168.73.88, which is one of the "unregistered" addresses anyone can use. If your modem is on port 2, you would add the following entry to IPROUTE.SYS:

	route add 192.168.73.88/32 * 2 d

Which means "route datagrams for 192.168.73.88 directly on port 2 using datagram mode".

No ARP entry is necessary for the caller, because SLIP and PPP do not use "hardware addresses".

XLINK PPP mode

XLINK SLIP mode requires no extra configuration, but PPP mode optionally uses an extra file to configure the PPP system for receive operations on the modem port. For example, you may wish to use one IP address when making outgoing connects and a different one when receiving incoming connects.

The optional file is named "PPPHOST.n" where n is the number of the modem port, e.g. "PPPHOST.2". You may have a separate file with a different configuration for each modem port if required. The file should be located in the same directory as XRPi itself and may contain any PPP configuration command.

See the description of PPP commands for details of how to configure PPP.

The PPP link inactivity timeout defaults to 5 mins, but can be overridden by including the PPP IDLE command in the PPPHOST.n file.

See also

DIAL(2) -- Dial a PSTN Connection
DUN(2) -- Dial Up Networking Commands.
PPP(2) -- PPP Configuration Commands.
PPPHOST.n(8) -- PPP Configuration File(s)
XLINK(1) -- The XLINK Command.
XROUTER.CFG(8) -- Main Configuration File.