CommScope® Enterprise Data Center Design Guide - Public ...

36
www.commscope.com
With advancements in construction and materials, twisted pair cables are now produced with
exceptional bandwidth that can deliver high-speed transmission over horizontal (90 meter)
distances. To achieve high speeds on twisted pair, all four pairs are used to simultaneously
transmit and receive (full duplex parallel transmission). With all four pairs in use, TIA-568 has
standardized performance values that measure not only performance within the pair, but
among all four pairs. These are:
Near End CrossTalk (NEXT) is the ‘noise’ one pair induces into another and is measured in
decibels at the receiver. Higher numbers are better.
Attenuation to CrossTalk Ratio (ACR) is NEXT minus insertion loss/attenuation. Higher
numbers are better.
Attenuation to CrossTalk Ratio Far End (ACRF) is a Category 6A specification for the
‘noise’ one pair induces into another measured in decibels at the receiver minus insertion
loss/attenuation. Higher numbers are better. Also known as Equal Level Far End CrossTalk
(ELFEXT).
PowerSum Near End CrossTalk (PSNEXT) is a computation of the unwanted signal coming
from multiple transmitters at the near-end into a pair measured at the near-end. Higher
numbers are better.
PowerSum Attenuation to CrossTalk Ratio (PSACR) is PSNEXT minus insertion
loss/attenuation. Higher numbers are better.
Power Sum Attenuation to CrossTalk Ration Far End (PSACRF) is a computation of the
‘noise’ coming from multiple transmitters at the near-end into a pair measured at the far-end
and normalized to the received signal level. Higher numbers are better. Also known as
PowerSum Equal Level Far End CrossTalk (PSELFEXT).
Far End CrossTalk Loss (FEXT loss) is the unwanted signal coupling at the near-end
transmitter into another pair measured at the far end. Higher numbers are better.
Alien Near End CrossTalk (ANEXT) is the ‘noise’ introduced into a circuit by nearby
channels or con- nections. Higher numbers are better.
Alien Far End CrossTalk (AFEXT) is the ‘noise’ introduced into a circuit by nearby channels
or con- nections measured at the far end. Higher numbers are better.
Return Loss (RL) is the strength of signal reflected back by the cable terminated to 100
ohms. Like structural return loss (SRL), it is a negative number. A higher absolute value is
better (i.e. [ - ]20 dB is better than [ - ]10 dB).
Propagation Delay is the time required for a signal to travel from one end of the
transmission path to the other end.
Delay Skew is the difference in propagation delay of the two conductors with the
most/least delay.
Twisted Pair Cable Performance
Category 6A, Category 6 and Category 5e cables are capable of supporting full duplex
parallel transmission required by gigabit Ethernet and can deliver fast transmission protocols
such as broadband video.
A horizontal twisted pair link should deliver a minimum of 10 dB of PSACR at 100 MHz.
While some equipment can accept signal as low as 3 dB, 10 dB is a good rule of thumb.
However, an experienced designer knows that factors like transmission echo and impedance
mismatch can cause crippling power loss and the breakdown of the channel. Using a cable
with higher bandwidth, especially in links approaching the 90 meter limit, will keep high speed
networks performing as required. Many network problems are eliminated by installing cables
with the extra ‘headroom’ provided by higher bandwidth.