As far as the Server computer is concerned, ATS connections are logged in using the credentials of the process running ATSD.EXE. (If running as a service, the default user will be "SYSTEM", but you can change that on the Log On tab of the Services applet Properties dialog.) In any case, since all the ATS connections will all be running as the same user on the same server machine, there may be some difficulty in identifying connections. Aside from implementing your own application login procedure, one way you will be able to identify connections is with the /C switch to SYSTAT. This will show in place of the login names (which will all be the same for the Telnet connections) the IP addresses of the client machines.
Up to three ATS sessions from the same client workstation together count as only one "node" against the A-Shell license. This is similar to the way it works with A-Shell/Windows peer-to-peer sessions. In both cases, SYSTAT/C will show the machine identification with a suffix of ":01" for the first window, ":02" for the second, etc. The difference is that the machine identification for ATS sessions will be the IP address of the client workstation.
