PC Ext Gen: Questions Asked About X.25 and External [Part 1]

• Microsoft Mail for PC Networks, versions 3.0a, 3.2, 3.2a, and 3.5
• Microsoft Mail Multitasking MTA, version 3.2
• Microsoft Mail Multitasking MTA for Windows NT, version 3.5

SUMMARY

When you set up the Mail External program (EXTERNAL.EXE) computers so that they can communicate via X.25, it may require a substantial amount of configuration. The following section may answer some of the related questions.

MORE INFORMATION

The following are some of the frequently asked questions related to X.25:

1. Q. What X.25 boards are supported by the Microsoft Mail Gateway to
      X.400?

   A. The Eicon X.25 board is supported.

2. Q. Do I have to run a separate instance of the Mail External program
      to monitor or use each of the four channels of the Eicon card?

   A. Yes, an instance of External can handle only one X.25 virtual
      circuit (VC). If there must be two concurrent X.25 sessions, then
      two X.25 External instances must be configured with the following
      parameter:

         COMMTYPE=X25<card-type>

3. Q. What is the difference between these terms: virtual channels,
      virtual circuit (VC), physical channels, logical channels, ports,
      sessions, logical channel number (LCN), and logical session number
      (LSN)?

   A. The term virtual channel is confused with virtual circuit. Virtual
      channels do not exist; instead, it is a physical channel or port.
      Each Eicon board has one or four ports.

      A physical channel is the physical link provided by the X.25 vendor
      (for example, Infonet) to communicate with each port.

      A virtual circuit is the logical connection or session. A virtual
      circuit is the X.25 network layer term. The virtual circuit exists
      at the time of the communication only.

      The logical channel number (LCN) is the same as the virtual circuit
      number (VCN) but at the X.25 datalink layer. Each Eicon port can
      handle up to 99 virtual circuits; each of them has its own VCN.

      The logical session number (LSN) is the term used by Eicon
      to lump the two terms LCN and VCN together; the sessions open over
      a physical port one at a time.

4. Q. Is the virtual channel number the same thing as X.25 port numbers?

   A. A port (physical channel) is not the same as a virtual circuit (VC).
      Each Eicon board can have up to four ports; each port can handle up
      to 99 VCs. Each Eicon card can handle up to 396 VCs through four
      ports.

5. Q. How can we specify the virtual circuits?

   A. Virtual circuits (VC) cannot be specified, which is the X.25 packet
      and logical channel number (X.25 datalink layer term). You can only
      specify the port number (physical channel) and a subaddress number.

      The Eicon card manages which VC and logical channel number (LCN) or
      logical session number (LSN) will be assigned to the X.25 session.
      The assigned LCN is transparent to the user and will be reassigned
      after the VC is dissolved.

6. Q. Do I have to specify the port number? If so, how is this done?

   A. If you have multiple X.25 ports supported on the Eicon card under
      OS/2, then specify each channel with the parameter:

         X25PortNumber=<n>

      where "n" is the port number, in one of the four External instances
      running under the OS/2 MMTA. If you do not specify a port number, the
      card may default to port 1.

      It is recommended that you use the parameter, X25PortNumber, for
      each instance. You will also need the parameter for the Eicon board:

         CommType=X25Eicon

      In addition, you will need the long X.121 address from the X.25
      vendor. The two digit subaddress specifies the originating address
      of the External program. The two digit subaddress is required for
      the Eicon boards. For example,

         X25subaddress=<x25originatingaddress>.

7. Q. How do you use each of the virtual circuits (conversations) provided
      by the X.25 vendor for every port of the Eicon card?

   A. Run a separate instance of External for every virtual circuit. When
      the first channel is busy, all calls (in or out) will be routed to
      the second channel, etc. However, to achieve 99 concurrent X.25
      connections (virtual circuits), you need 99 instances of the Mail
      External program. Each instance of the External handles one VC.

8. Q. How do you specify the X.25 destination address and X.25 listen
      address?

   A. Each physical connection (node or DCE) has an X.25 address
      associated with it. Specify the node listen address in the
      EXTERNAL.INI file by the parameter:

         X25subaddress=313023022

      (plus the optional two-digit subaddress), to the MTA computer ID
      on the local area network (LAN). Enter the listen address from
      the vendor for example:

         Port=<long X.25 Address>=<InfoNet DCE>

      Specify the destination MTA address in the External-Admin, Setup, and
      put the address (for example, 313023022) plus any optional two-digit
      subaddress in the local EXTERNAL.INI file.

For additional information regarding the configuration of an X.400 gateway using X.25, refer to the Microsoft Mail Gateway to X.400 "Administrator's Guide."