Information Technology Reference
In-Depth Information
sizes and pin configurations, ranging from four to eight positions, with from two to
eight contact pins. The popular styles are often referred to by a Universal Service
Order Code (USOC) “RJ” number, even though the connector may not actually be
used in the designated application called for by the USOC code. For example, the
common 6-pin modular telephone plug is often called an RJ-11, and the 8-pin mod-
ular plug is often called an RJ-45. The 8-pin modular plug is used in the TIA-568-
C wiring standard for both telephone and data. The 8-pin modular plug is also the
connector that is used for 10BaseT, 100BaseT, 1000BaseT, 100VG-AnyLAN, token-
ring/UTP, and many other LAN applications.
This 8-pin modular plug is probably the most subject to name abuse, because
it resembles the specialized RJ-45 connector. However, the RJ-45 wiring pattern
(which includes an interface programming resistor) is so radically different from
that of T568A and B that it really should not be called by that name at all. In this
book, we have tried to always use the “8-pin modular,” or simply “modular,”
description, as it leads to less confusion.
In truth, all modular plugs are identical until terminated. Then, they should
probably be called by a name that reflects their wiring pattern. For example, you
could certainly connect only four pins of an 8-pin modular plug in a 10/100BaseT
pattern. It would not truly be either T568A or B, because both require eight con-
nected pins. It would not be an RJ-45, because the wiring pattern would be wrong
and the programming resistor would be missing. Why not call it a 10/100BaseT con-
nector, as that would be its purpose and its unique wiring pattern? If all eight wires
are connected in an approved pattern, you could call it a “TIA” connector, or even
“T568A or B.” Well, until the habit is changed, we will probably still find people
RJ-45: What's in a Name?
The 8-pin modular connector is often improperly referred to as an “RJ-45.”
The terminology is in such wide use that even manufacturers who know
better frequently use the RJ-45 name in their marketing materials. Some will
call it RJ-45 “style” or “type.” In truth, the RJ-45 designation is for a particu-
lar interface most often used for programmable analog modem connections
to leased telephone lines. This true RJ-45 just happens to use the 8-pin (8-
position, 8-contact), 8P8C, modular jack and plug that are found in many
other LAN and telephone connections that have nothing whatever to do
with the original intent of the RJ-45 connection, Many other common inter-
faces use the same type of modular connector, including RJ-48,
10/100/1000BaseT, Token-Ring/UTP, UNI-PMD, T1, ISDN, and on and on.
