Digital Audio Interfacing and Networking 1501Often when installers accustomed to telephone
wiring install data cabling they will incorrectly use the
telephone USOC (Universal Service Ordering Code)
wiring scheme. This will result in a network that either
does not work, or has very high error rates, Fig. 39-46.
Normal Ethernet cable wiring such as shown in Fig.
39-47 is used for interconnecting unlike devices. In
other words it is used to connect the Network Interface
Card (NIC) in a station to a switch or repeater hub.
Connections between like devices such as a pair of
NICs, or between switches, repeater hubs, or switch to
repeater hub require a “crossover” cable wired per Fig.
38-48. This is because the data transmit pair must
connect to the receive input, and vice versa.
It is very easy to tell the difference between a cross-
over cable and a straight-through cable by looking at the
conductors in the RJ-45 connectors. If the wiring is
identical at both ends, you are holding a straight-
through cable, if it is different, you most likely have a
crossover cable.
Some hubs and switches have uplink ports that can
eliminate the need for crossover cables. Such a port is
wired with pairs 2 and 3 reversed internally. Make sure
that when connecting two switches or repeater hubs so
equipped, you only use the uplink port at one end.
Another caution is that often such an uplink port is not
an independent port but is wired internally to one of the
normal ports. In such a case make sure that only one of
the pair of ports is used.
Some switches employ an autoselect crossover
feature. This allows the use of either a straight-through
or a crossover cable on any port. The switch automati-
cally senses which cable type is in use and adjusts the
electronics to suit the cable.
Stranded patch cable sometimes has different colors.
Pair 1 Green and RedFigure 39-44. Standard EIA/TIA T568A (also called ISDN,
previously called EIA). One of the wiring schemes used for
Ethernet.Figure 39-45. Standard EIA/TIA T568B (also called AT&T
specification, previously called 258A). One of the wiring
schemes used for Ethernet.Figure 39-46. USOC (Universal Service Order Code), the
wiring scheme used for telephones. This must not be used
for data! The eight contact connector uses all pairs for four
lines. The six contact connector uses only center one–three
pairs for one, two, or three line phones.Pairs T/R Pin Wire ColorT3 1 White/Green
Pair 3
R3 2 Green
T2 3 White/Orange
R1 4 Blue
Pair 2 Pair 1
T1 5 White/Blue
R2 6 Orange
T4 7 White/Brown
Pair 4
R4 8 BrownPairs T/R Pin Wire ColorT3 1 White/Orange
Pair 2
R3 2 Orange
T2 3 White/Green
R1 4 Blue
Pair 3 Pair 1
T1 5 White/Blue
R2 6 Green
T4 7 White/Brown
Pair 4
R4 8 BrownFigure 39-47. Ethernet Standard (T568B colors) patch cord
wiring used for most interconnects. Ethernet usage shown
for 10Base-T and 100Base-T. Gigabit Ethernet uses all the
pairs.Figure 39-48. Ethernet standard (T568B colors) crossover
cord. Pairs 2 and 3 are reversed end to end. Used for
connections between like devices (NICs, switches or
repeater hubs).RJ-45 RJ-45
Pairs T/R Pin Wire Color Pin Ethernet
T3 1 White/Orange 1 TxData +
Pair 2
R3 2 Orange 2 TxData
T2 3 White/Green 3 RecvData +
R1 4 Blue 4
Pair 3 Pair 1
T1 5 White/Blue 5
R2 6 Green 6 RecvData
T4 7 White/Brown 7
Pair 4
R4 8 Brown 8RJ-45 RJ-45
Pairs T/R Pin Wire Color Pin
T3 1 White/Orange 3
Pair 2
R3 2 Orange 6
T2 3 White/Green 1
R1 4 Blue 4
Pair 3 Pair 1
T1 5 White/Blue 5
R2 6 Green 2
T4 7 White/Brown 7
Pair 4
R4 8 Brown 8