The switching system receives, originating from an external network facility, data units according to a point-to-point transmission interface format, transporting packets of a higher layer protocol. The data units originating from the external network facility are processed so as to recover first packets. Test traffic carried by second packets of the higher layer protocol is generated, and the first and second packets are multiplexed. The stream of multiplexed packets is converted into data units of the point-to-point transmission interface format, which are re-transmitted to the switching system.