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

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 configuring X.25 and the Mail External program, please reference the Microsoft Mail for PC Networks "Administrator's Guide," Appendix F "X.25 Settings for Mail."

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."