Developed to utilize X.25 (XCOM) and BiSync (BSC), there was no TCP/IP equivalent service within Tymnet. To continue use of this service after the shutdown of Tymnet, a solution was selected. A special version of Tymnet Engine node code which allows nodes and interfaces to communicate with one another and the rest of the network was created. Instead of relying on the "supervisor" to validate calls, a table of permitted connections was defined per customer to allow an incoming call to be made from the HSA interface to the BBXS interface to the XCOM interface and on to the Tandem computer. In effect, a "Tymnet Island" consisting of a single Tymnet node that accepted calls for a pre-determined list of clients was utilized by EDI*Net. No supervisor needed.