Transports
Messages travel from publishers to subscribers over transports. The publishers and subscribers on a transport all use the transport in common as a shared communication medium.
Each transport is a separate communication medium, insulated against crosstalk from other transports. The following diagram depicts this insulation using separate colors. Subscribers on transport T1 receive messages from Pub1 on T1, but not from Pub2 on transport T2, nor from publishers on any transport other than T1.
In the radio broadcast analogy, frequencies divide the airwaves into separate data streams. Each frequency is, in effect, a distinct communication medium that carries data to all devices tuned to that frequency. Radios tuned to frequency F1 receive the audio stream from the station that broadcasts on F1, but not from stations that broadcast on frequency F2, nor any other frequency.