Each packet normally consists of a preamble, start-of-frame delimiter and data. The preamble has nibbles each having a particular format. A header substituted for preamble nibbles by an individual one of the originating devices in a plurality, and an individual one of the ports in such originating device, indicates such originating device and such port. Such port in such originating device sends such modified packet to others of the originating devices and to an observing station. The header format is such that the last nibble in the header and the remaining preamble portion will not be confused with any two (2) nibbles in the header. A particular one of the originating devices indicated in the data converts the header back to the preamble format and transmits the converted packet to a receiving station. The observing station records the individual originating device, and the individual port in such device, indicated in the header. Each packet includes at its end a trailer formed from a plurality of nibbles and indicating whether or not a collision has occurred between such packet and a packet from another one of the originating devices. The trailer in each packet may also indicate additional information—e.g. whether the packet (a) is a normal packet originally formed, (b) is a runt packet, (c) is from an unacceptable source and (d) the delay between each packet end and the next packet start. The information in the trailer for each packet passes to the observing station.