INFO: Modem Selection Algorithm for "Use Another Port If Busy"

ID: Q200467

The information in this article applies to:

Microsoft Routing & Remote Access Service Update for Windows NT Server version 4.0

SUMMARY

The information in this article explains the algorithm used by remote access service (RAS) when you select use another port if busy under Dialup Networking, and how this can cause connectivity problems with multiport serial adapters.

MORE INFORMATION

If you are using a multiport serial adapter to provide connectivity for numerous modems, the modems must be homogeneous if you want "use another port if busy" to work. The reason for this is that the list of ports is maintained in the order that TAPI enumerates them to RAS. The order is arbitrary, and Unimodem is not capable of assigning a particular order. TAPI is a neutral layer between the TSP (Unimodem) and the application (RAS), and enforces no rules about ordering on either side.

Applications should, therefore, not make assumptions that there is an order to the selection of COM ports. RAS merely assumes that the TAPI list is the list it should use when failing over. If the list is out of order, the COM ports will be tried out of order. Note that adding and removing devices will have the same affect on device order.

For example, you have a multiport serial adapter with 24 ports. COM 1 through 12 have analog modems installed. COM 13 through 24 have ISDN modems installed.

You will find that rollover to available ports will result in random instability and dropped connections when analog strings are sent to ISDN modems, and vice versa. This is also true for other types of modems that cannot communicate with each other for one reason or another. Currently, there is no way to configure which ports should be used for failover for particular devices. Because of this, applications must assume responsibility for choosing which ports they will dial when one is determined to be busy.

The current workaround to this problem involves disabling the RAS/RRAS servers attempt to use another port.

For RAS

To disable the automatic dialing of another port if the current port is busy, go to the Basic tab of the phonebook entry and click to clear Use another port if busy. This will prevent the RAS server from attempting to use another port if the device it is configured to use is busy.

For RRAS

To disable the automatic dialing of another port if the current port is busy on a RRAS router, you need to edit %Windir%\System32\Router.pbk:


  MEDIA=rastapi
  Port=nameofinterface
  OtherPortOk=1
  Device=rasinterface

Router.pbk is arranged as an .ini file with the name of demand dial interfaces as sections. Go to the section corresponding to the demand dial interface for which you want to change this behavior. Change the OtherPortOk field in this section to 0, instead of 1.

Additional query words:

Keywords : ntras
Version : winnt:4.0
Platform : winnt
Issue type : kbinfo