Information Technology Reference
In-Depth Information
The TIA-568-C standard describes two different wiring patterns for wiring the
8-pin modular jack. The two patterns are called T568A and T568B, and are shown
in Fig. 6.13. The two patterns differ only in placement of Pairs 2 and 3 of the tra-
ditional 4-pair order. This order is defined as Pair 1 = Blue, Pair 2 = Orange, Pair 3
= Green, and Pair 4 = Brown. The T568A pattern places Pair 2 on Pins 3 and 6 of
the jack, and Pair 3 on Pins 1 and 2. The T568B pattern places Pair 2 on Pins 1 and
2 of the jack, and Pair 3 on Pins 3 and 6. The A-pattern is actually unique to the
TIA standard, but it is now the recommended pattern. The B-pattern is the tradi-
tional Avaya/Lucent (AT&T) 258A pattern specified in their premises distribution
system. Ironically, the A-pattern better follows the progression of wiring patterns
from the 2-pair pattern for 6-pin modular jacks that is used with many telephone
sets (the RJ-11/12 style jack). By the way, the “-” is used before the C in TIA-568-
C specifically so there is less confusion with the wiring pattern called T568B.
There has been considerable controversy as to which pattern is the proper one
to use. Depending upon whom you ask and where you ask, you can get strong opin-
ions on either side of the question. The TIA standard, however, is quite clear. The
“pin/pair assignments shall be… [T568A].” Optionally, the T568B may be used, but
only “if necessary to accommodate certain 8-pin cabling systems.” These certain
systems are ones that already use the optional pattern, for the most part. If you do
not need to connect to an existing system, cable, patch, etc., that uses T568B, you
should use the recommended T568A pattern.
What difference does it make which pattern you use—T568A or T568B?
Electrically, it makes no difference, as long as exactly the same pattern is used at
both
ends. Let's say that again: The two patterns are electrically equivalent, as long as both
ends of a cable run are wired with the same pattern. As has been said, “the electrons
don't know the wire color.” This means that as long as Pin 1 and Pin 2 on one end
of a link connect to Pin 1 and Pin 2 on the other end, and the same is true of Pins 3
and 6, you can use either pattern. Both the outlet and the telecommunications room
terminations (punchdown block or patch) of a horizontal cable run should be wired
with the same pattern, as should both ends of any patch or user cord.
Technically, you can use either pattern; however, some patches, fan-out cables,
equipment connectors, and adapters are prewired with the T568B pattern and you
must match that pattern at the other end of the cable for the link to work. You can,
however, mix patterns from one cable run to the next within the same link. For
example, a patch cord could be wired with either pattern without regard to how the
horizontal cable was terminated.
A common wiring pattern problem can occur in the horizontal cross-connect.
If you terminate the horizontal cable into a punch block first, then cross-connect to
a patch, you should be sure that the patch wiring matches that of the corresponding
