Couplers have a premium side and a secondary side. Which means that theoretically 16 coupler types exist: 4 media types (TP1, PL, IP, RF) for the premium side and same for the secondary side, i.e. 4 x 4 = 16.
These are combinations that can be excluded because they technically make no sense:
- couplers with IP as secondary medium
- couplers with RF as primary medium
- couplers with PL as primary & secondary medium
Couplers with IP as secondary medium are not useful because IP in intended to be used as backbone medium, i.e. in order to connect (network) segments together. Couplers with RF as primary medium are not useful because RF is not suitable as backbone medium, i.e. it is not supposed be used for connecting (network) segments together. A PL/PL coupler does not make sense because PL is an open medium, i.e. it is based on the public power grid. After excluding these combinations 7 useful combinations are left (16 - 4 - 4 - 1 = 7).
Some couplers could be useful but are nevertheless not available:
An IP/PL coupler could be used in order to connect PL installations together via an IP backbone. An IP/RF coupler could be used to in order to connect RF installations (RF domains) together via an IP backbone. An PL/RF coupler could be used to in order connect RF installations (RF domains) together via a PL backbone. After excluding these combinations the 4 combinations are left.
These are the 4 available combinations:
TP1/TP1 couplers are known as 'Backbone Coupler', 'Line Coupler' or 'Repeater'. TP1/PL and TP1/RF couplers are called 'Media Coupler'. IP/TP1 couplers are either known as 'IP Interface' or as 'IP Coupler'.