Today’s high- speed gigabit based equipment works
much differently than its 10 Mbps predecessors and as such requires
better performance from the cabling infrastructure to support
these applications. In particular, full duplex, parallel transmission,
echo transmission and advanced signal processing present new constraints
that demand higher performance cabling.
FULL DUPLEX, PARALLEL TRANSMISSION
To reach speeds of 1 gigabit or more, the transmission
scheme becomes more complex. Instead of two pairs, four pairs
are required for transmission (often referred to as parallel transmission).
And instead of simplex transmission, each pair must support full
duplex transmission, which means it must transmit and receive
in both directions simultaneously. The cross-talk noise induced
on a pair now comes from three transmitted signals on the receiver
end (power sum NEXT) and three transmitted signals on the far
end (power sum FEXT).
The initial ANSI/EIA/TIA 568A did not account
for added cross-talk noise from the 5 additional transmitted signals.
It defines the minimum requirements for a pair-to-pair simplex
transmission. A cable may pass the Category 5 requirements, TSB-67
field test, and work at 10 Mbps speeds, yet fail under the loads
of full duplex, parallel transmission.
In addition to power sum cross-talk, a new noise
source arises when full duplex transmission is used. When the
transmitter injects signal on to a pair, any reflection of that
signal can end up as noise in that pair’s own receiver (transmission
echo). Signal reflections are caused by impedance mismatches such
as connecting a 98 ohm cable to a 101 ohm connector, or by impedance
variations in the cable.
The amount of signal reflection is quantified
by a new parameter called return loss. This measurement is similar
to structural return loss, but is more stringent to better represent
signal reflection for the entire channel. Again the initial ANSI/EIA/TIA
568A did not address return loss and Category 5 compliant cables
with poor impedance characteristics may fail because of excessive
noise from transmission echo.
Encoding is the signaling method used to send
information down a wire pair. While there are many different encoding
schemes, they all determine how many Mbps can be sent for a given
frequency. In order to reach gigabit speeds, higher level encoding
is a requirement. But the complexity of the sophisticated processors
that provide such high data rates makes them very sensitive to
the cabling’s ACR headroom. Minimally compliant Category 5 cable
may support lower speed applications, but they can falter when
migrating to more complex encoding.
For transmission, the signal to noise ratio (SNR)
at the receiver will determine the bit error rate (BER) of the
circuit. Bit errors ultimately determine the performance level
of the network. Since SNR can not be determined until the network
is up and running, the closest approximation of a passive channel
is its attenuation to cross-talk ration (ACR). ACR is the best
indicator of how well a network will perform. While ACR is the
most important gauge of network performance, it does not account
for ambient noise – noise from electromagnetic sources that permeate
the environment in which the cable is placed. Fluorescent lights,
radio transmitters, motors, etc. can create noise that is coupled
onto a cable. The ability of the cable to be immune to these noise
sources is related to the cable’s balance. The better the cable
balance (the degree to which each conductor in a pair is identical)
the more the cable performs like fiber optics in being immune