Gigabit and 10 Gigabit Cabling Technology - LAN Wiring - page 295

This problem appears to lie only with the backward compatibility of Category 6
connecting hardware. So, any cable that meets Cat 6 standards should work fine in
a Cat 5/5e application. There is total compatibility between Cat 5e and Cat 5 com-
ponents, although performance may not stretch to the enhanced level.
Category 7 presents an entirely different problem, as both cable and connect-
ing hardware are quite different from the lower categories. Cat 7 cable consists of
up to four pairs of individually shielded 23-gauge wire, with an overall shield.
Category 7 connectors have been generally defined with two options: one a unique
dual-purpose RJ-style modular connector, and the other a non-RJ connector that
keeps each pair totally separate and separately shielded. Note that modified RJ-style
connector designs may present another backward-compatibility issue, as we had
with Cat 6.
If the non-RJ connector is used, any reverse compatibility will be provided
through the use of adapter cables that can translate the connector types, much like
those the fiber systems have been using to adapt the SC to older ST and SMA con-
nectors. For more information on advanced performance categories, refer to
Chapters 3, 5, and 16.
Testing Legacy Category 5 Installations. The specifications for Category 5 were
developed before all the engineering for gigabit speeds was done. At that time, trans-
mission was primarily unidirectional on each of the two pairs of the cable, and life
was good. The main parameters that caused link failure were attenuation and
crosstalk (NEXT), and the other cable parameters were usually all right, if those
two requirements were met.
The unexpected use of all four pairs for gigabit, in a split-signal, 3:1 reversible
manner, brought out the problem of the differing velocities of propagation between
the pairs. A common cable manufacturing practice is to vary the pitch of the twist-
ing to minimize crosstalk between pairs. This results in each pair having a slightly
different twist-per-inch, therefore decreasing the unwanted coupling between pairs
(the source of crosstalk). However, this practice also causes the pairs to vary in their
respective velocities of propagation—how long it takes for a signal to traverse a
length of cable.
If we split the gigabit signal among four pairs, as Gigabit Ethernet does, the
four signals will arrive at the remote end at slightly different times. This phenome-
non is called delay skew . There may be no way for the remote receiver to reassem-
ble the signal, as the bit times may virtually overlap. The only way that this
parameter can be controlled is to specify it and use cable and components that meet
this tighter delay skew spec. See Table 12.7.