Infrastructure Management &
Monitoring for Business-Critical
ContinuityTM
KVM Cable Length Best Practices Guide
What Customers Need to Know About Cable Length and Video Quality
Cable Length and Video Quality
Table of Contents
2
3
4
5
Executive Summary
Overview of Cable Length and Video Quality
Video Quality Test Results
Conclusion
Executive Summary
Avocent has an ongoing goal to provide and ensure best-in-class solutions for IT management. As part of
this goal, Avocent has historically chosen a maximum KVM cable length of approximately 33 feet (10 meters). This number was selected because KVM will work in that range, regardless of the conditions.
In reality, there is actually no specific cable length at which video will just stop working. There are, instead,
several factors that combine to affect the video quality. Cables longer than 33 feet (10 meters) may be
used if the customer’s environment is carefully made to fit as closely as possible to the “ideal” conditions
set forth in this document.
2 Cable Length and Video Quality
Overview of Cable Length and Video Quality
When cable longer than Avocent’s recommended maximum length
is used, video does not just suddenly stop working. Instead, it will
begin slowly degrading. Video quality gradually degrades at the
local port, and digitizing the signal just causes even more problems.
The point at which the video actually becomes unusable is
subjective. Some users may be more sensitive to sluggish or jumpy
mouse movements than others, for example. Avocent’s definition of
acceptable video quality is video that is considered visually identical
to the source using the minimum network bandwith to transmit
changes and producing zero packets generated on static screens.
The key factor in the amount of network traffic is the amount
of changes in the video. These changes are recognized by the
Avocent Dambrackas Video Compression® (DVC) algorithm, which
compares every single pixel from one frame to the next. Every
recognized pixel change is packaged and sent from the appliance
to the remote user. As network activity increases, mouse latency
increases, which means that mouse movement may become
sluggish or jumpy.
Video noise is another key contributor to the level of network
traffic. Theoretically, if a screen does not change, no packets should
be generated. In reality, video noise causes some pixels to appear
to change, even though there was no actual change. The appliance
recognizes video noise as a screen change and sends those
packaged pixel changes, thus generating more network traffic.
Factors Affecting Video Quality
There are seven variables that combine to affect video quality and
network traffic: quality of video source, quality of cable, twist ratio
of the cable, resolution used, length of the cable, number of patch
panels and interference. To achieve the best possible video quality,
careful attention must be shown to these factors.
•Quality of video source. Lower quality video cards will usually
result in a noisier video source, and thus more network traffic,
even on a static screen. Although the overall quality of video
cards has improved over time, there are still some poor quality
video cards being used. In addition to using better quality video
cards, video noise can be reduced by enabling the video noise
compensation feature, an appliance setting in the DSR® switch.
This feature tells the digital subsystem to raise the threshold
for what counts as a change in video, basically eliminating any
very slight video changes. Keep in mind, however, that if these
minor changes in the target video are not detected, remote
users will not see those minor changes either.
•Quality of the cable. Poor quality cable can increase
video noise in the signal path as well. Therefore, Avocent
recommends using CAT-5e and CAT-6 UTP cabling. With
all shielded cable, the maximum distance is halved. For this
reason, UTP is the preferred cable for AMX. In addition, you
must consider not only the quality of the cable itself, but also
the quality of the end connections. If the end connectors are
not properly terminated, you may experience lower video
quality and higher network traffic.
•Twist ratio of the cable. Twist ratio is how many twists per foot
are on each pair of wires in the cable. Removing the individual
wires, straightening them out and laying them side by side will
show that none of the wires are the same length. The longer
the length of the cable, the greater the difference in the length
of separate wires. This difference creates a different delay for
each of the three colors in the video. If the difference in delay
becomes great enough, the three colors can be out of phase
and seen in the video as “ghosting” or “smearing.”
•Resolution used. The frequency of the carrier for the video
signal is higher on a 1280 x 1024 source than it would be on a
1024 x 768 source. Higher frequencies are more susceptible
to noise and degradation over cable length. By using lower
resolution, video signals can travel further down the cable
before becoming too noisy to use.
•Length of the cable. The longer the cable, the more the
capacitance of the transmission line and the bigger the
difference in length of the pairs. This contributes to the
degrading of the signal and the greater variation in delay in the
three colors, a direct result of the length of the cable and twist
ratio. However, as was seen in Avocent’s testing, even the best
cable will degrade the video signals at very great lengths.
•Number of patch panels. Each patch panel will require the
cable to be terminated with an RJ connector. This involves
taking the signal off of the twisted pair and putting it onto the
blade of the male RJ, through the female RJ panel, and back
onto a twisted pair either on another 4-pair wire or possibly
larger pair bundles in structured cabling. Each time the signal
is moved from the cable through a patch panel back into a
cable, additional noise and crosstalk are generated. With
Virtual Media products, the data channel on the last two
wires is a much higher frequency than the non-Virtual Media
product. This additional high frequency can contribute noise
and cross talk on the video lines - especially through a lot of
patch panels.
•Interference. Electrical interference signals can also affect
signal quality. Customers need to be aware of the potential
negative effect of placing KVM cabling near sources such as AC
units, power generators and fluorescent lights.
3 Cable Length and Video Quality
Video Quality Test Results
Recently, tests performed by Avocent have prompted a change in
the maximum cable length number from 33 feet (10 meters) to
150 feet (45 meters). Avocent has determined that cables up to this
length, in an environment with carefully maintained variables, will
work without significant reductions in video quality or resolution or
increases in network traffic.
Each of those capabilities was tested at six different cable lengths
and was given a rating from one to four. As the graphs show, most
scores remained the same or very close to the same, even for cable
lengths as long as 500 feet. The exceptions are video quality and
Virtual Media quality. They decrease in performance at a cable
length of 150 feet (45 meters). This is why Avocent has determined
that a new number of 150 feet (45 meters) is an acceptable cable
length. It is extremely important to remember, however, that
these tests were performed in a controlled testing environment, as
discussed in Factors Affecting Video Quality, beginning on page 2.
•Window drag, minimize time and maximize time at local
and remote ports. As shown in Figure 1.2, out of window
drag, minimize time and maximize time, the window drag
quality at remote port was the only quality adversely affected
by cable lengths over 150 feet (45 meters). Window drag is
a large video change, creating a lot of extra network traffic,
which, combined with mouse movements, creates a network
bottleneck. As a result, the user may see a perceivable
delay in screen updates when using cable over 150 feet (45
meters).
Figure 1:2 : Window Quality Test Results
•Keyboard, video and mouse capabilities at local port or
during a DSView® management software session. As
shown in Figure 1.1, there was little to no change in mouse
or keyboard quality at any cable length. However, at cable
lengths of over 150 feet (45 meters), video quality decreased.
The video was usable, but there was some smearing and
tearing of text, and smaller details became more difficult to
view.
Figure 1:1 : KVM Quality Test Results
4 Cable Length and Video Quality
•Virtual Media quality at local port or during a DSView 3
software session. As shown in Figure 1.3, Avocent’s tests
found that at cable lengths greater than 150 feet (45 meters),
the signal latency in the high-speed data channel used for
Virtual Media was too high. As a result, the Virtual Media data
channel cannot be guaranteed to work when cable lengths
are greater than 150 feet (45 meters). At that point, the
DSR® switch reverts to normal KVM data channel speed, so
that the keyboard, video and mouse will still work properly.
Figure 1:3 : Virtual Media Quality Test Results
Conclusion
Determining the guaranteed operation of transporting a video
signal across a particular length of cable is a very complex issue.
Historically, a cable length of 33 feet (10 meters) resulted in what
Avocent considered to be acceptable video with acceptable network
traffic regardless of the environment. More recent test results show
that acceptable video can be achieved by using up to 150 feet (45
meters) with carefully controlled variables. As the cable length
continues to increase, the perceived quality of the remote video will
gradually begin to degrade at some point or create more network
traffic, resulting in increased mouse latency.
Because of the multiple factors that can affect performance at
various cable lengths and the subjectivity of video performance,
KVM configuration at cable lengths longer than the recommended
150 feet (45 meters) is not guaranteed by Avocent and should
be supported only after a customer conducts thorough tests and
achieves positive results from their own particular environment.
5 Cable Length and Video Quality
About Emerson Network Power
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by maximizing computing capacity and lowering costs while enabling the data center to operate at peak performance. For more information,
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