Registered Jack (RJ) connectors were
previously the standard for telephone connectors. More recently, RJs
have been used to connect not only the telephones, but also 10BASE-T,
100BASE-TX, and Token Rings.
Telephone lines use the RJ-11
connector, which connects four-wire cables, the two inner connectors
for one phone line and the two outer for another line. Modems are
restricted to using RJ-11 because it is an analog connector. RJ-12,
which is rarely used, is a six-wire version of the RJ-11 used for more
complex telephone systems.
RJ-45 connectors have eight wires and
are used for network technologies, which require four pairs of wires,
such as Ethernet and Token Ring networks. RJ-45 connectors are
specifically designed for digital signals. If an analog modem uses a
RJ-45 connector, either the connection will not function or the analog
modem will stop functioning and burn out. RJ-45 connectors are mainly
used to connect 10BASE-T cabling (or the like) to a hub or bridge in
order to connect to another media type, such as using a BNC connector
for 10BASE2.

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