Modular Jack Styles:
There are four basic modular jack styles. The 8-position and 8-position keyed modular
jacks are commonly and incorrectly referred to as RJ45 and keyed RJ45 (respectively). The
6-position modular jack is commonly referred to as RJ11. Using these terms can sometimes
lead to confusion since the RJ designations actually refer to very specific wiring
configurations called Universal Service Ordering Codes (USOC).
The designation 'RJ' means Registered Jack. Each of these 3 basic jack styles can be
wired for different RJ configurations. For example, the 6-position jack can be wired as an
RJ11C (1-pair), RJ14C (2-pair), or RJ25C (3-pair) configuration. An 8-position jack can be
wired for configurations such as RJ61C (4-pair) and RJ48C. The keyed 8-position jack can
be wired for RJ45S, RJ46S, and RJ47S.
The fourth modular jack style is a modified version of the 6-position jack (modified
modular jack or MMJ). It was designed by Digital Equipment Corporation® (DEC) along with
the modified modular plug (MMP) to eliminate the possibility of connecting DEC data
equipment to voice lines and vice versa.
Common Outlet Configurations:
Two wiring schemes have been adopted by the '568-A standard. They are nearly identical
except that pairs two and three are reversed. T568A is the preferred scheme because it is
compatible with 1 or 2-pair USOC systems. Either configuration can be used for Integrated
Services Digital Network (ISDN) and high speed data applications.
USOC wiring is available for 1-, 2-, 3-, or 4-pair systems. Pair 1 occupies the center
conductors, pair 2 occupies the next two contacts out, etc. One advantage to this scheme
is that a 6-position plug configured with 1 or 2 pairs can be inserted into an 8-position
jack and maintain pair continuity. A note of warning though, pins 1 and 8 on the jack may
become damaged from this practice. A disadvantage is the poor transmission performance
associated with this type of pair sequence.
10BASE-T wiring specifies an 8-position jack but uses only two pairs. These are pairs
two and three of the T568B and T568A schemes.
Token-Ring wiring uses either an 8-position or 6-position jack. The 8-position format
is compatible with T568A, T568B, and USOC wiring schemes. The 6-position is compatible
with 1- or 2-pair USOC wiring.
The MMJ is a unique wiring scheme for DEC® equipment.
ANSI X3T9.5 TP-PMD uses the two outer pairs of an 8-position jack. These positions are
designated as pair 3 and pair 4 of the T568A wiring scheme.
Modular Plug Pair Configurations:
It is important that the pairing of wires in the modular plug match the pairs in the
modular jack as well as the horizontal and backbone wiring. If they do not, the data being
transmitted may be paired with incompatible signals.
Modular cords wired to the T568A color scheme on both ends are compatible with T568B
systems and vice versa.
Straight-through or Reversed?
Modular cords are used for two basic applications. One application uses them for
patching between modular patch panels. When used in this manner modular cords should
always be wired "straight-through" (pin 1 to pin 1, pin 2 to pin 2, pin 3 to pin
3, etc.). The second major application uses modular cords to connect the workstation
equipment (PC, phone, FAX, etc.) to the modular outlet. These modular cords may either be
wired "straight-through" or "reversed" (pin 1 to pin 6, pin 2 to pin
5, pin 3 to pin 4, etc.) depending on the system manufacturer's specifications. This
"reversed" wiring is typically used for voice systems. The following is a guide
to determine what type of modular cord you have.
How to read a modular cord:
Align the plugs side-by-side with the contacts facing you and compare the wire colors
from left to right. If the colors appear in the same order on both plugs, the cord is
wired "straight-through". If the colors appear reversed on the second plug (from
right to left), the cord is wired "reversed".
Recommended Cabling Practices:
Do's:
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