Insights into proper return path alignment Digital troubleshooting is a

Insights into proper return path alignment Digital troubleshooting is a
RETURN PATH ALIGNMENT
Insights into proper
return
path alignment
Digital troubleshooting is
a whole new ballgame
Figure 1: TV monitor connection diagram
Carrier generator
Editor’s note: As the cable industry rushes to
get into high-speed data, telephony and perhaps
other services requiring real-time transactions,
a critical network component is a reliable working return path. But that’s easier said than done,
many are finding. This article is part one of a
three-part series on cleaning up the return path
in preparation for advanced, two-way services.
This initial installment focuses on return path
alignment techniques; future articles will focus
on ongoing maintenance as well as return path
noise and ingress performance.
The signing of the Telecom bill, coupled with
Camera/modulator
consumer demand for interactive services in the
home, has created the potential for cable television companies to explore new sources of revenue. They now have the opportunity to provide
their subscribers with two-way services like
Internet communications, interactive TV and
telephony. But there’s a catch–cable operators
need to successfully activate their return paths
before they can deliver these potentially lucrative new services. As the industry begins to activate the return path, a host of problems are surfacing. Many of these problems can be avoided
if an accurate alignment procedure is followed.
This article provides insight into
Table 2: Return sweep alignment
some of the problems that have
been encountered while working
Pros
Cons
with operators on alignment techUses minimal
Slower ingress
niques.
forward bandwidth
response
H
L
Return
equipment
Spectrum analyzer
Table 1: TV monitor with 4-carrier generator
Pros
Cons
Utilizes readily
available hardware
Technician carries
multiple boxes
Ingress and gain
balance
on same screen
Requires 6 MHz
forward bandwidth for
each return monitor
Provides best
(variable) frequency
resolution
Real-time ingress
response simplifies
troubleshooting
Limited frequency
resolution
Technician carries
one box for
all measurements
Figure 2: Return sweep connection diagram
H
L
Return
equipment
Headend return monitor
42
Monitor
By Bill Morgan, R&D Project Manager,
Hewlett-Packard
Forward/return
sweep analyzer
Requires functional
return communications
(for sweep)
Customer impact
Over the years, cable operators
and technicians have become
familiar with the picture artifacts
related to poor alignment in the
forward path, but the artifacts
related to poor return alignment
are new. Techs are no longer dealing with picture artifacts, but communications
degradation. The ability to identify system
problems by the type of degradation is in its
infancy. These problems may include many of
the following:
1) Source levels from modems end up higher than practical.
2) Excessive return levels cause clipping of
the return laser, affecting all the signals on one
return.
3) Communications throughput is reduced
(poor response times).
4) Telephone calls are dropped or service is
delayed.
5) IPPV requests respond slowly or intermittently.
CED: COMMUNICATIONS ENGINEERING & DESIGN OCTOBER 1996
RETURN PATH ALIGNMENT
6) Customers experience service outages.
7) All of the above may
be intermittent.
Problems in the forward
path have typically been
diagnosed by visually
observing the degradation,
but the digital communications inherent in return services makes this method of
troubleshooting impractical. It also places even
more importance on the
quality of the initial return
path alignment. One unique
difference between maintaining the forward and
return paths is that each test
location in the forward path
is affected only by the
amplifiers closer to the
headend, but amplifiers in
either direction can affect
the current location in the
return path. For example,
when sweeping the forward
path, if the noise in the system increases, the technician knows the problem is
between the current testpoint and the headend.
However, in the return
path, the technician cannot
be sure. The source of the
problem could be on a different feeder or trunk altogether. In this case, not only
is the technician faced with
the challenge of finding the
noise/ingress problem, he or
she may also be faced with
the problem of not being
able to complete the testing
of the current amplifier.
If a well-documented
alignment procedure is followed, and the technicians
understand the inter-relationships of how the return
operates, these problems
can be minimized.
often, the narrow bandwidth
of the return path is used to
FREQ 0030.00
LEVEL -00.1
P-V 05.3
justify lower frequency reso+06
lution. In reality, a 5 MHz to
42 MHz return path still con+02
sists of more than three
octaves of bandwidth, only
one octave less than a state-02
of-the-art forward path. Many
DIR 005
035 REF 02
of the problems found and
resolved while aligning the
Figure 4: Low resolution (1.25 MHz/data point) return sweep
system will be repeated over
FREQ 0030.00
LEVEL -00.1
P-V 04.9
octaves and may be as signifi+06
cant in the lower frequency
octaves as in the upper.
+02
The sweep response traces
shown in Figures 3 through 5
provide an example of the
-02
benefits of improved freDIR 005
035 REF 02
quency resolution. Figure 3 is
the response of a return cable
Figure 5: 4-carrier generator (5, 13.5, 22 and 30 MHz)
system (11 amplifiers deep)
FREQ 0030.00
LEVEL -00.1
P-V 04.5
indicative of a significant
+06
reflection which could push
the error correction in a digital communications link to its
+02
limit. This sweep response
was taken using 135 kHz of
-02
frequency resolution.
DIR 005
030 REF 02
Figure 4 shows the same
system response using only
1.25 MHz of frequency resoFigure 6: Spectrum scan of T-10 calibrated return with excessive low end gain
lution. Note that the reflection
FREQ 005.00
LEVEL 17.9
which is obvious in Figure 3
+20
could be missed in Figure 4.
Figure 5 shows the same sys+00
tem response using a simulated four-carrier approach.
-02
It is important to be aware
DIR 005
Ingress
040
of the tradeoffs being made
when test equipment that
provides less frequency resoFigure 7: Return sweep of T-10 calibrated return with excessive low end gain
lution is selected. It is also
FREQ 0030.00
LEVEL -05.3
P-V 31.3
important to be familiar with
+30
the passives in the system
when using a carrier genera+10
tor for alignment.
Roll-off in the passives
may be compensated for by
-10
misadjusting the amplifier
DIR 005
035
slope. Some of the newer 1
GHz passives roll-off below
analyzer and video modulator to send the
10 MHz, so when using the carrier generator
Alignment methods
response downstream;
approach, the carriers should be placed at freThere are currently many methods being
2) Return sweep generator with headend
quencies that are flat through the passives. In
used to align the return, but only the two most
sweep receiver and ingress monitor. Tables 1
summary, a return sweep system with high freprevalent are discussed here because they do a
and 2 provide a short summary of the pros and quency resolution has several advantages:
good job of representing the range of capability: cons of these two methods (see page 42).
1) Flatness discontinuities and suckouts can
1) TV monitor and portable 2- or 4-carrier
One of the differences between the two align- be seen.
generator in the field with a headend spectrum ment methods is frequency resolution. Quite
2) Roll-offs at the band edges are visible,
Figure 3: High resolution (135 KHz/data point) return sweep
and diplexer problems may
be eliminated.
3) Reflections and return
loss problems show up as
ripples in the sweep
response and can be
repaired.
4) Modern sweep systems with short duration
sweep pulses can be used in
the presence of carriers
with minimal interference
and don’t take up the bandwidth required by CW carriers.
Ingress problems
If the return ingress is
extremely high, repairs may
be required prior to starting
the alignment process.
Experience shows that 70
percent of ingress problems
occur in the home, 25 percent in the drop, and only
five percent in the coaxial
trunk and feeder itself. It is
also becoming apparent that
a major contributor to the
ingress in the coaxial trunk
is actually common path distortion. Excessive ingress
can interfere with the sweep
systems, and may drive the
laser into compression, causing the output levels to be in
error.
In order to follow the
ful on new systems where
customers are not installed.
PEAK FREQ 017.85
LEVEL 05.0
If a new section is connect+15
ed into an existing system,
an ingress problem in the
+05
new section may degrade
or disable the existing system. There have been
-05
many articles written disDIR 005
045
cussing the source of
ingress and solutions, and
Figure 9: Return sweep of properly calibrated return
we have listed some of
FREQ 0040.00
LEVEL -02.1
P-V 02.5
them in the reference sec+02
tion (see page 53).
Figures 6 and 7 provide
an extreme example of
-02
how poor alignment techniques can aggravate
-06
ingress problems. This
DIR 005
045 REF 01
particular return path was
“aligned” using the level
process described in this article
of the T-10 carrier as a reference, but ignorfor return system alignment, one ing the slope of the return.
needs to be able to start from the
Because of the excessive gain at the low
fiber node and proceed through
end of the spectrum, the return amplifiers
the network (one visit per locawere pushed into compression by small
tion being the goal). Return path amounts of burst noise below 10 MHz. These
“blockers,” or some alternate
bursts of compressed noise affected the
methods of disconnecting the
entire return spectrum. Once the proper pads
return input to the amplifier cur- and equalizers were installed, the same small
rently being tested, need to be
amounts of noise caused no problems.
available to establish proper setThe traces shown in Figures 8 and 9 are of
up from the current location
a properly aligned return and the associated
back to the headend. This will be well-behaved noise response of the return.
an important step in reducing
Once again, it is important to have good frereturn path test time and meeting quency resolution in the headend spectrum
the one visit per location goal.
analyzer to effectively identify and trouBlockers are particularly usebleshoot sources of ingress.
Figure 8: Spectrum scan of properly calibrated return
Once the proper
pads and
equalizers were
installed,
small amounts of
noise caused no
problems
Pad
H
L
EQ
Figure 10: Typical fiber hub
Pad
H
Test point
-30 dB
L
Pad
EQ
Test point
-30 dB
Pad
Test point
-30 dB
H
L
EQ
Pad
H
Test point
-30 dB
L
Test point
-30 dB
RETURN PATH ALIGNMENT
Return path alignment using the HP CaLan Sweep/Ingress Analyzer.
Alignment process
The approach to alignment of the return path
is similar to the forward path in the sense that
it should be aligned for unity gain. In the return
path, unity gain is referenced to the input of the
amplifiers. In the forward path, unity gain is
referenced to the output of the amplifiers. In
each case, the gain of the amplifier is compensating for the loss of the section of cable
between the previous amplifier (closer to the
headend) and the current amplifier. Attention to
detail in the return path is critical to successful
alignment. A poorly aligned amplifier farther
out in the trunk may make alignment impossible because of excessive noise in the communications path.
Reference output
The first step in the alignment process is to
measure the output level at the headend for
each return path using a given reference level
input to the fiber node return laser. When
choosing this reference input level, consideration must be made for low-end optical and RF
noise floors, as well as high-end clipping
Table 3: Source level matrix
Sweep input level = carrier level - 10 dB = +20 - 10 = +10 dBmV
resulting from overdrive. A typical manufacturer’s specification for optimum input level to
the return laser is +20 dBmV. This level
assumes standard video carriers, and it is
becoming common practice to use this as the
reference level. Modern sweep systems are
designed to operate 10 dB or greater below
optimum carrier levels, so +10 dBmV is used
as the reference input level in this discussion.
An accurate and flat input to each laser is
necessary to establish the proper headend
reference, and this input level must be maintained for the return system to operate properly. This input may be provided by either of
the methods discussed earlier. The sweep
system with higher frequency resolution has
the advantage of allowing a tech to see problems in the return frequency response during
the alignment.
It is important to be familiar with the
amplifier and fiber optic block diagrams (see
Figure 10). Internal coupling and test point
variations determine the loss between the
sweep insertion point (IP) and the input to the
amplifier or laser. In our experience, not having this information has been a major contributor to alignment problems.
We recommend developing a level matrix
for your equipment which technicians can refer
to when setting the source level. A sample
matrix is provided in Table 3 (page 53). It may
be necessary to contact the manufacturers of
the specific hardware to verify the configurations. This table should provide a concise summary of the internal losses in the hardware and
the level required from the return sweep source
to provide a known level to the active device. It
is also necessary to have block diagrams of the
hardware with the available return test points.
The need for a readily available concise
summary of this information should not be
underestimated. It is often the case that
sweep technicians seldom know the configuration of the return test points, or what
the losses are to the active devices.
Normalize outputs
Type of hardware
Laser hub
Line extender
Trunk amp
Bridger
Sweep input level
Internal coupling loss
Test point loss
+10 dBmV
4 dB
30 dB
+10 dBmV
1 dB
20 dB
+10 dBmV
5 dB
20 dB
+10 dBmV
13 dB
20 dB
Total insertion point loss
34 dB
21 dB
25 dB
33 dB
Source level
+44 dBmV
+31 dBmV
+35 dBmV
+43 dBmV
Source level = sweep input level + IP = +10 +34 = +44 dBmV
With the correct input level to the fiber
node or first return amplifier, the output at
the headend is measured with a spectrum
analyzer or sweep receiver. Because the
output from each return will vary by different lengths of return fiber or coax, these
outputs should be normalized to the lowest
level return by attenuating the higher level
returns. This step creates a common output
at the headend for all returns, assuming
+10 dBmV input to the laser or first return
amplifier. This common output level is
referred to here as the “X” level.
Check and align sweep response
All subsequent amplifiers should be adjusted to re-establish the X level output at the
headend with the same +10 dBmV at each
amplifier input. The amplifier is adjusted using
the plug-in pad and equalizer for coarse
adjustment and the gain and slope controls for
the fine adjustment. The alignment should proceed from the fiber node or first return amplifier out, making sure each amplifier is calibrated properly before moving on.
Again, this may be done with a carrier generator approach, or a sweep system. Care must
be taken if using the carrier generator
approach because the frequency resolution is
limited, and flatness problems may be missed.
Once again, it is critical that the proper source
levels be used. The level matrix created earlier
minimizes the errors in this step.
The successful delivery of interactive services
to subscribers is dependent upon the proper
alignment of the return path. The key points are:
1) Attention to the alignment process is
absolutely critical;
2) Unity gain in the return path is as important as in the forward path; and
3) Good frequency resolution in both the
return sweep and spectrum monitoring test
gear can help identify many problems before
they become customer complaints.
In the November issue, ongoing maintenance of the return path will be addressed.
References
1. CableLabs, Digital Transmission
Characterization of CATV Systems, Nov. 1994.
2. Farmer, James O., Managing the Return
Spectrum to Optimize Interactive Revenue
Opportunities, 1995 NCTA Technical Papers.
3. Farmer, James O., Paul Gemme, Charles
Cerino, and Mark Millet, Two-Way Cable Plant
Telephony and Data Experience, 1996 SCTE
Emerging Technologies Proceedings Manual.
4. Johanson, Brian, Bob Chamberlain, and
Aravanan Gurusami, HFC Return System:
Management of Subscriber-Induced Noise,
1996 SCTE Emerging Technologies
Proceedings Manual.
5. Kim, Albert J., Two-way Plant
Characterization, 1995 NCTA Technical Papers.
6. Natarajan, Raja, and Paul Vilmur,
Providing High Reliability Telephony and Data
Services over HFC Networks, 1996 SCTE
Emerging Technologies Proceedings Manual.
7. Paff, Andy, Implications for the
Introduction of Telephony Services on the
HFC Architecture, 1996 SCTE Emerging
Technologies Proceedings Manual.
8. Prodan, Richard S., Majid Chelehmal,
Tom Williams, and Craig Chamberlain, The
Cable System Return Channel Transmission
Environment, 1995 NCTA Technical Papers.
9. Staniec, Thomas J., Making it Work:
Return Systems 101, CED, August 1995.
10. Stonebeck, Dean A., and William F.
Beck, Designing the Return System for Full
Digital Services, 1996 SCTE Emerging
Technologies Proceedings Manual.
RETURN PATH MAINTENANCE
Proactive return
path
maintenance
It’s time to sweat
the small stuff
By Bill Morgan, R&D Project Manager,
Hewlett-Packard
Editor’s note: This article is part two of a
three-part series on the return path. The
final installment will appear in the February
issue.
return, requires a schedule to make it happen.
You need a list of daily, weekly, monthly,
semi-annual and annual inspections and procedures to be performed. Establish a checklist
and keep it updated as the system architecture
changes. Information from the routine tests
should be recorded and kept in a database as a
history of the active network.
data, so a carrier-to-ingress measurement in
the return path must be made relative to an
intermittent data carrier or its expected level.
Cable operators are discovering that maintaining the return path is much more difficult
than the forward path. The cost of maintaining
a bi-directional system is currently two to four
times greater than the cost of maintaining the
forward path alone. Much of the increase in
labor is related to multiple trips to the same
location repairing self-inflicted problems. The
goal of return path maintenance is to minimize
trips to a given site by maintaining precise
gain alignment. This can be accomplished by
adding return path sweep testing and ingress
monitoring to the normal forward path test
program.
Sweep testing
Routine sweep testing of the forward and
return paths gives the technician many advann last month’s issue, we discussed return
tages over other alignment methods. Forward
path alignment procedures and solutions to
Return vs. forward
path sweep response data is valuable informasome of the problems that may be encountered
Preventive maintenance for the return path
tion for the technician as he is troubleshooting
while turning on the
the return path. The
return path. Activating the
improved frequency resoFigure 1: Reflections
return path is only the
lution inherent in today’s
FREQ 0030.00
LEVEL -00.1
P-V 05.3
+06
first step toward offering
return path sweep syssubscribers interactive
tems enables the techniservices. If the quality of
cian to find and fix prob+02
the service you provide is
lems before they become
not superior, your cuscustomer complaints.
-02
tomers will look to your
Some examples of return
DIR 005
035 REF 02
competitors. To keep the
path problems identified
competitive edge, you
by sweep testing are picwill need to be able to
tured in Figures 1 and 2.
find and fix problems
Maintenance costs
Figure 2: Diplexer grounding problems
before they are visible to
will be reduced if the
FREQ 0040.00
LEVEL +14.5
P-V 03.3
your customers. In this
technician sweeps the
+20
article, we will discuss
forward and return paths
the importance of a return
at the same time.
+16
path preventive mainteCombining the two tests
nance program and look
eliminates a second trip
at issues surrounding
to the same location and
+12
proactive maintenance of
minimizes housing openDIR 005
REF 045
the return path.
ings which can aggravate
As fiber is installed
ingress and leakage probdeeper into today’s cable
lems. In addition, many
systems, and the number of cascaded ampliis similar to many of the current maintenance
amplifier designs have return signal paths on
programs used in the forward path. These
fiers is reduced, the tendency is to pay less
the forward amplifier motherboard. Therefore,
methods include periodic system sweep, endattention to keeping the remaining amplifiers
if the forward amplifier is replaced, the return
operating at their optimum performance. But
of-line monitoring and periodic performance
amplifier may also be affected. Faulty
tests. Because the return path is configured
with the activation of return path services,
grounding of the modules in the amplifier can
differently and carries different services, some cause ingress problems in the return, but only
optimum performance is even more critical.
of the return path tests have changed. As an
Attention to detail is mandatory, regardless of
have a minor effect on the forward path.
example, a carrier-to-noise measurement in the Checking both forward and return paths at the
the number of cascaded amplifiers and the age
forward path can be made relative to a meaof the plant. Technicians need to be obsessed
same time can help eliminate future problems
sured visual carrier because the forward path
with details and investigate all discrepancies.
in both directions.
carriers are always active. Quite often, return
In other words, don’t ignore the small stuff!
The sweep tech needs to understand the
path carriers are only on when transmitting
Any maintenance program, forward or
importance of return path unity gain, the prop-
I
er levels to insert into the return, and what levels to expect at the headend.
In last month’s article (see October issue,
page 42), we discussed using a level matrix to
set the sweep source level. This matrix is just
as important for maintenance as it is for
alignment. If these levels aren’t well controlled, all the work done during initial alignment can be wasted. Routine sweeping of the
return should include sweeping the fiber link
and reconfirming the normalized levels at the
headend (the “X” level) to verify the performance of the optics.
Some manufacturers’ distribution equipment
will recommend different optimum input levels. This is sometimes found when comparing
the recommended input to the return amplifier
gain block and the fiber node return laser. This
is a deviation from the unity gain theory. The
gain block (return amplifier and cable loss)
can have either gain or loss in order to set the
signals to the correct level for the fiber node.
But the philosophy that each amplifier in the
return path is adjusted to compensate for the
loss of the section of cable
following (in
this case, the
cable and passives between
the fiber node
and the first
amplifier) still
holds true. If
your sweep
gear uses an
insertion point
loss variable
to adjust the
actual sweep
source level,
this loss may be adjusted to compensate for
the variation.
It is important to maintain a history of
amplifier serial numbers, pad and equalizer
values, and performance data for identifying
trends. Keeping a record of the previous
sweep results in the amplifier housing, or any
place easily accessible to the technician, will
also help to prevent or track down problems.
Technicians should compare the current and
previous results and question any discrepancies. Don’t just change pad or equalizer values if the sweep insertion level is correct, and
the levels at the headend are incorrect. Find
the problem! The goal is to continually
improve the performance of the system.
Small incremental improvements each time
the location is visited will keep the system at
peak performance.
The safest
approach is to
sweep and
align the new
return before
connecting it
Type of hardware
Table 1: Source level matrix
Laser hub
Line extender
Trunk amp
Bridger
Sweep input level
Internal coupling loss
Test point loss
Tap loss
Cable loss
+10 dBmV
9 dB
30 dB
+10 dBmV
1 dB
0 dB
24 dB
8 dB
+10 dBmV
5 dB
20 dB
+10 dBmV
13 dB
20 dB
Total insertion point loss
39 dB
33 dB
25 dB
33 dB
Source level
+49 dBmV
+43 dBmV
+35 dBmV
+43 dBmV
If it is necessary to sweep from a tap in the
return, you can use this same level matrix.
Determine the new source level by ignoring
the amplifier test point loss and adding the
loss from the tap to the amplifier output
(return input). The level matrix in Table 1
illustrates an example of this new calculation.
This same approach can be used for testing
the return path from any return insertion point
in the system.
Turning on new sections
cussed earlier. The return amplifier pad and
equalizer are then adjusted to achieve the
required level at the return input to the node.
This approach allows the technician to locate
and fix any ingress problems before connecting the new section to the existing plant. After
the new section is aligned, it can safely be
connected to the existing return and a final
check made for overall performance.
A more expedient approach is to maximize
the pads in the new return section or disable
the return amplifiers at the time of installation
and then connect the new section to the existing trunk. Now the new section can be aligned
in the same manner as the initial alignment
using a return sweep receiver at the headend.
The operator needs to be aware that using this
approach, an ingress problem in the new section may impact the existing return before the
ingress can be located and repaired.
Connecting new forward plant to an operational forward plant usually presents little risk
to the existing system, with the exception of
the power demands and tap insertion loss. In
contrast, connecting new return plant into an
existing system can create major problems. In
Figure 3, if the last three amplifiers were connected into the system with zero attenuation,
and the gain and slope controls set to the maximum output level, this could result in a signif- Ingress monitoring
icant increase in noise at the laser and disable
Some of the test equipment currently used
the return.
for sweeping the return path makes it easy to
It is extremely important to check the pad,
check the headend return ingress while sweepequalizer, gain and slope values before coning. Monitoring ingress as part of the sweep
necting the new equipment.
routine enables the technician to continually
The operator has a couple of choices when
lower the composite ingress distortion as part of
connecting new return plant to operational sec- the routine maintenance. In addition, having
tions. The safest approach is to sweep and
access to headend ingress while sweeping proalign the new return before connecting it. This
vides an excellent troubleshooting tool when
problems occur. By comparing the ingress at the
method requires a return sweep system with a
portable sweep
receiver placed
at the node and
H
connected to
L
the return
feeder under
test. When
using this
Figure 3: Typical system diagram
approach, the
New plant
sweep source
level is determined using
the same
source level
matrix dis-
RETURN PATH MAINTENANCE
Figure 4: Return path ingress performance
25.0
Another method is to record the number of
times ingress is detected above a given threshold. This method can be used to provide a
measure of percent availability. An example of
this type of data also collected over a two-hour
period is shown in Figure 5.
The NCTA engineering committee is in the
process of developing recommended measurement practices for the return path, and among
these efforts is radiated and conducted ingress.
Results from this effort should be available
early in 1997 and will provide guidelines for
these measurements.
20.0
15.0
Maximum
Average
Ingress level (dBmV)
10.0
5.0
0.0
-5.0
-10.0
-15.0
-20.0
-25.0
Summary
-30.0
The successful delivery of two-way services
to subscribers requires a higher reliability system. An effective proactive maintenance program can help achieve this level
of performance while reducing
resource demands overall. The
key points to remember are:
1) Return path maintenance
requires more diligence than the
forward path
2) Routine sweep testing of
the return will prevent problems
and minimize repeat trips
3) Ingress management
requires continuous monitoring
of the return path.
-35.0
5.00
10.00
15.00
20.00
25.00
Frequency (MHz)
30.00
35.00
40.00
Figure 5: Percentage of return path availability
100
90
Availability (percentage)
80
70
60
50
40
30
References
20
10
0
5.00
10.00
15.00
20.00
25.00
30.00
35.00
Frequency (MHz)
current field test location to the ingress at the
headend, the technician can isolate the source.
Continuously monitoring ingress will also
help the operator predict problems in the system by viewing slight changes in the system’s
performance over time. This process helps the
operator direct maintenance technicians to
potential trouble locations before system performance degrades.
Proper monitoring provides a measure of
the availability of the system and a look at the
performance of the return path. Problems in
the return path can cause an increase in the
ingress signals received at the headend. By
sampling, storing and time stamping these
measurements, the performance of the system
can be viewed over time. These measurements
can be compared to current measurements for
troubleshooting purposes. We may be able to
compare the
time of an
increase in
noise to an
installation of
a new section
of the system,
a particular
weather pattern, an accident, or even
the installation of a new
subscriber.
One method of tabulating ingress data is
to store peak and average values which give
an indication of long term performance. An
example of this type of data collected over a
two-hour period is shown in Figure 4.
Return path
maintenance
requires
more diligence
than the
forward path
1. CableLabs, “Digital
Transmission Characterization
of CATV Systems,” November
1994.
40.00
2. Farmer, James O., Paul
Gemme, Charles Cerino, and
Mark Millet, “Two Way Cable
Plant Telephony and Data
Experience,” 1996 SCTE Emerging
Technologies Proceedings Manual.
3. Kim, Albert J., “Two-way Plant
Characterization,” 1995 NCTA Technical
Papers.
4. Natarajan, Raja and Paul Vilmur,
“Providing High Reliability Telephony and Data
Services over HFC Networks,” 1996 SCTE
Emerging Technologies Proceedings Manual.
5. Paff, Andy, “Implications for the
Introduction of Telephony Services on the
HFC Architecture,” 1996 SCTE Emerging
Technologies Proceedings Manual.
6. Prodan, Richard S., Majid Chelehmal,
Tom Williams, and Craig Chamberlain, “The
Cable System Return Channel Transmission
Environment,” 1995 NCTA Technical Papers.
7. Staniec, Thomas J., “Making it Work:
Return Systems 101,” CED, August 1995.
INGRESS MANAGEMENT
Noise and ingress
performance
in the return path
Starting a
monitoring program
By Bill Morgan, R&D Project Manager,
Hewlett-Packard
The previous two parts of this series (in the
October and November 1996 issues of CED)
discussed the proper procedures for alignment
of the return path, and recommended proactive
maintenance practices to keep the return path
operating effectively. Cable operators are finding that ingress is the biggest roadblock to offer-
trum analyzer to the return path and check it
daily or whenever a problem is reported. This
can hardly be classified as a proactive maintenance approach, but it is better than waiting
for the customer’s complaint. As a first level
of monitoring, this same spectrum analyzer
can be connected to a computer and trace data
stored periodically to create a history of your
return path’s performance. The analyzer can
be configured to keep the sweep rate as fast as
Figure 1: BER vs. C/I ratio
1.00E+00
1.00E-01
1.00E-02
1.00E-03
BER
1.00E-04
1.00E-05
1.00E-06
1.00E-07
1.00E-08
1.00E-09
0
10
20
30
C/I ratio (dB)
ing the two-way services that customers are
demanding. This article will recommend several
ingress measurements to help quantify the
return’s performance, discuss the relationship
between ingress and BER, and provide some
suggestions for data handling. Prior to starting
an ingress monitoring program, it is critical that
your return path be properly aligned and your
technicians understand the importance of maintaining proper gain balance. For more detailed
information on this subject, we have provided
references at the conclusion of this article.
Spectrum monitoring
The most common method of return path
monitoring in use today is to connect a spec-
possible and still have the frequency resolution necessary to identify problems. The data
processing should include comparing the data
to thresholds and identifying alarm conditions,
as well as keeping a running average of the
data to characterize the performance of the
return path over long periods of time.
By comparing the data samples which fall
below a given threshold to the total test time,
the user is able to derive a rough approximation of percent availability vs. frequency.
There are many things that can be done with
the results from this type of test, and there is a
potential for generating massive amounts of
data. Because of this, it is important to establish good methods for compressing the data
into usable archives. Programs designed for
this type of monitoring are currently available.
BER vs. carrier/ingress
Although spectrum monitoring is a good
first line of defense, it is important to monitor
other key parameters in the return path and
repair problems before customers are affected
by slow response times or loss of service. Bit
Error Rate (BER) is often discussed as one of
the key parameters to use for monitoring
return path communication channels, because
this is a true measure of the performance
delivered to the customer. Each of the services
(cable modems, telephony, VOD, LAN, etc.)
require varying levels of performance, anywhere from one error per 10,000 bits (10-4) to
one error per 10 million bits
(10-7). BER is one of the measurements that
has traditionally been used in digital communications networks to monitor performance,
and in a completely digital network, BER is
an excellent metric to monitor. But in a mixed
RF/digital network such as the cable TV environment, BER has several disadvantages.
Figure 1 is a typical example of the variation
of BER vs. the carrier/ingress ratio (C/I) for a
given type of modulation and error correction.
The horizontal axis will shift depending on the
type of modulation used, and the sharpness of
the knee will vary depending on the type of
error correction. The important characteristic to
notice is the rapid increase in the BER as the
C/I degrades near the knee of the curve. With
only a 4 dB drop in C/I from 28 dB to 24 dB,
the BER increases from 1 x 10-7 to 4 x 10-4. In
an RF network, the advanced warning received
as the C/I degrades is limited if BER is the chosen parameter to monitor.
In addition, most of the digital services
being carried by the return path today do not
carry the reference bit stream required to make
a BER measurement. The time required to
make a BER measurement can be significant,
and is defined by the equation:
Time =
1
Data Rate X BER
Using this equation, we can calculate that
on a typical QPSK return carrier with a data
rate of 1.5 Mbps, the time to measure 1 x 10-8
BER will be almost 67 seconds. Because the
carriers are typically TDMA, the time required
to capture enough data is lengthened even further. Another drawback to measuring BER is
that it requires test equipment not currently
found in most cable systems.
Another method often used to measure data
transmission performance is monitoring packet
errors. This is a measure of data packets (as
few as 64 or as large as several thousand
INGRESS MANAGEMENT
How do we measure it?
The C/I ratio can be calculated directly by
first measuring the average power of the carrier, and then the average power of the noise in
the same frequency span. When measuring the
return path noise, you are actually measuring a
combination of many artifacts. They are, in
addition to noise from amplifiers, common
path distortion generated from the forward
path signals, second- and third-order distortion
generated from excessive signal levels, and
impulse noise and inducted interference coming from the drop and home. In general, all of
these artifacts degrade the performance of the
RF/digital communications link.
Figure 2 is an example of a 64 QAM carrier
constellation in the presence of noise. As the
noise approaches the knee of the BER vs. C/I
curve, the points on the constellation spread far
enough that they become indistinguishable from
each other, and data errors occur.
The biggest difficulty encountered when
monitoring noise in the return path is the presence of carriers. One option is to measure the
noise in a vacant portion of the return band.
Unfortunately, the magnitude of the noise in a
typical return path can vary quite a bit with
frequency. This approach also misses narrowband signals which fall on the carrier. But
measuring noise offset from the carrier will
help identify noise problems which cover
wider bands, typical of impulse noise.
Figure 2: 64 QAM constellation with ingress.
Gated spectrum monitoring
Figure 3: Wideband noise measurement.
bytes) which were re-sent because of errors
detected by the receiver. Monitoring packet
errors has the advantage of measuring the performance of an operating link without requiring embedded test code sequences. But packet
errors also have a similar sharp transition from
low errors to high errors as the C/I decreases.
A simpler and more effective approach to
monitoring data transmission performance in
an RF network is monitoring the C/I instead.
Once the type of digital modulation and the
error correction are defined, the relationship
between the measured C/I and potential BER
performance can be predicted. If necessary, a
specific digital link can be characterized by
measuring the BER as the C/I is varied. This
data will normally be provided by the test
equipment vendor, but should be verified anyway. The sensitivity and advanced warning
gained by monitoring C/I is significant.
What is needed is a method for measuring
noise in the presence of TDMA carriers. This
is an ideal application for a measurement
approach developed specifically for bursted
carriers. The video gate was developed for the
spectrum analyzer specifically for measuring
intermittent events. In this case, the video gate
can be triggered by the envelope of the detected data pulse. An in-depth discussion of gated
measurements is beyond the scope of this article, but gated measurements are discussed in
several articles referenced at the end of this
text. In addition, gated measurement application notes are available from the spectrum analyzer manufacturers.
By using a gated measurement approach,
and changing the video gating from the rising
to the falling edge of the burst detector, the
spectrum analyzer video can be gated both
when the carrier is on for the carrier measurement, and when the carrier is off for the noise
measurement.
Using currently available test equipment,
there are several other tests which can be performed to help quantify the magnitude of the
ingress in the return path.
Wide bandwidth average power
An average power measurement set up to
measure the entire return band will provide a
single quantity result (the ultimate in data
reduction) for the average noise performance
of the return path. This measurement algorithm is currently available in some spectrum
analyzers for measuring the average power of
digital carriers. To measure digital carrier
power, the analyzer samples the signal in predetermined increments across the bandwidth
of the carrier and integrates the samples to
arrive at the average power of the carrier.
This same measurement can be used to measure the average power of wide bands of noise
simply by increasing the measurement span to
the full bandwidth of the return path. Figure 3 is
becomes a frequency selective voltmeter with
the selectivity determined by the analyzer’s IF
resolution bandwidth, when it is tuned to a frequency of interest and set to 0 MHz span. The
analyzer’s video output can be sampled at a
high speed by a digital oscilloscope. The samples can then be compared to a user-defined
threshold to locate impulse or burst events in
the return path which exceed the threshold. If
you keep a log of the number, amplitude and
duration of the events, you will have a good
picture of the burst noise performance of your
return path. Figure 4 is an example histogram
from the results of a 24-hour test.
The CW TesterTM, developed by CableLabs,
is a variation on this test that measures the
interfering signal’s impact on the amplitude
burst noise in the return path is just beginning.
The NCTA Engineering Committee is in the
process of writing an addendum to the
Recommended Measurement Practices which
will cover the return path. The goal is to provide
measurement procedures which rely on currently
available test equipment, when possible.
What should I do with all of this data?
The goal of this effort is to provide different
methods of ingress measurement which can be
used as required, depending upon the nature of
the anticipated problems. Ideally, a combination
of all of the above should be used to give the
broadest picture of the return path’s performance.
If these measurements are made on the return
prior to service activation, you can select where
Figure 4: Burst event histogram results at 16.3 MHz.
600
From: 17:00 1/15/97
To: 17:00 1/16/97
Threshold = -15 dBmV
500
Events
400
300
200
100
0
0.1-1.0
uSec
1.0-10
uSec
10-100
uSec
an example of this measurement, with the start
and stop frequencies set to 5 and 30 MHz,
respectively. If carriers are active in the measured band, they will be included in the average
power result, but this measurement can be an
effective method for comparing the performance
of multiple returns before there are active carriers. This measurement provides a single number
for the result which is a good indication of the
average noise performance of the measured
path. This is not the best measurement for finding impulse problems or very narrowband interference. But it will help spot 2 dB or 3 dB variations in the average noise performance over
time, which are difficult to identify by looking at
a number of spectrum analyzer traces.
Time domain burst event counting
There are several variations on tests which
tabulate burst events. A spectrum analyzer
100-1,000
uSec
1.0-10
mSec
10-100
mSec
and phase of a reference CW carrier. The storage of data by the CW TesterTM is triggered
by sensing when the reference carrier is shifted far enough (in phase or amplitude) to cause
a data bit error. This additional phase information provides a unique signature of the burst,
which may make identification easier. There
are additional tests going on in which a highspeed oscilloscope is being used to monitor
the return without frequency selectivity. This
approach also has merit because the highspeed impulse noise in the return quite often
has a wide frequency spectrum. Tom Staniec
of Time Warner’s Excalibur Group has even
gone so far as to capture these impulses with a
high-speed digital scope and re-create them
with an arbitrary function generator in the lab
to simulate return path disturbances.
Stay tuned to these trials, because the evolution of the methods for capturing impulse and
100-1,000
mSec
1.0-10
Sec
10-100
Sec
carriers are assigned in the return band to provide the best performance for that carrier. Some
carriers, such as IPPV, can tolerate higher levels
of noise and can be placed in portions of the
band with higher noise/ingress. Telephony carriers, which have the lowest tolerance to impulse
noise, can be placed in the portion of the band
where the impulse events are at a minimum.
The amount of data captured in a good return
path monitoring program has the potential of filling even the largest computer disk drive in a
short period of time. It is important to implement
data compression which will prevent storing
massive amounts of data which will most likely
never be accessed. The spectrum analyzer scans
can be stored as the average, peak and minimum
of the data over specified periods of time.
Another approach to use when analyzing
spectrum scan data is to use a waterfall display,
which adds a third dimension to the graphs for
INGRESS MANAGEMENT
easier visualization of performance vs. time. A
good example of this was provided by John
Mattson and Joe Pendergrass of Arris Interactive
in a paper presented at the 1997 SCTE
Emerging Technologies Conference. But once
again, the amount of data stored is significant.
The wide-band average power data could be
stored as a graph of the average, maximum and
minimum over time, with time as the horizontal axis in this case. This approach makes it
easy to see trends in the data which may indicate gradual performance degradation.The best
method for displaying the burst event data is in
a histogram, with event length in the horizontal
axis and number of events in the vertical axis.
Archival data from any of these measurements can be used to compare the performance
of the plant as it grows, and subscribers are
added. With a historical perspective of the
return plant performance, gradual degradation
will not go unnoticed.
The successful delivery of interactive services
to your subscribers is contingent upon an efficient ingress management strategy. Know your
options for measuring ingress and be prepared to
be creative. Effective ingress management, along
with proper alignment techniques and a proactive
preventive maintenance program, will assure that
you are ready to provide the new services that
your customers are demanding.
Acknowledgements
The author would like to thank Beth
Armantrout, Syd Fluck and Jerry Green of
Hewlett-Packard for their help in creating
these three articles.
References
1) Staniec, Thomas J., “Return Systems 102:
What goes around...,” CED, December 1996.
2) Kim, Albert, Rogers Engineering and
CableLabs, “Two-way Cable Television
System Characterization,” April 1995.
3) Prodan, Richard, Majid Chelehmal, Tom
Williams and Craig Chamberlain, “The Cable
System Return Channel Transmission
Environment,” 1995 NCTA Technical Papers.
4) Prodan, Richard, Majid Chelehmal and
Tom Williams, “Analysis of Two-way Cable
System Transient Impairments,” 1996 NCTA
Technical Papers.
5) Raskin, D., D. Stoneback, J.
Chrostowski and R. Menna, “Don’t Get
Clipped on the Information Highway,” 1996
NCTA Technical Papers.
6) Chen, Helen and Don Gardina, “Testing
Digital Video on Cable TV Systems,” 1996
SCTE Cable-Tec Expo Proceedings.
7) Hranac, Ron and Thomas J. Staniec,
“Making Two-way Work,” 1996 SCTE CableTec Expo Proceedings.
8) Mayes, Rembrandt and Jeffrey Sauter,
“HFC Return Systems–A Tutorial,” 1996
SCTE Cable-Tec Expo Proceedings.
9) Huff, John L., “Time-Selective
Spectrum Analysis: Extending Spectrum
Analyzer’s Usefulness,” 1983 NCTA
Technical Papers.
10) Mattson, John and Joe Pendergrass, “A
Cost-Effective Approach to Return Path
Monitoring,” 1997 SCTE Emerging
Technologies Conference Proceedings.
11) Product Note #8590-2, available from
Hewlett-Packard, discusses gated measurements for the HP 8591C cable TV analyzer.
800-452-4844 x HPTV
www.hp.com/go/catv
5965-7860E
Reprinted with permission of CED magazine® October & November 1996 and March 1997. All rights reserved.
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement