VDSL Bonding Vectoring white paper

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VDSL Bonding Vectoring white paper
www.flukenetworks.com 2006-2017 Fluke
Corporation
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VDSL Bonding Vectoring white paper
The Promise of 100 Mbps and Vectored Services
Raises Bar on Network Quality
The next generation of VDSL2 technology call
Bonding Vectoring promises performances upwards
of 100Mbs on your existing copper network (FTTN).
This allows you to economically deliver
significant performance boost to your customers
without costly fiber investment. But can your
copper plant handle the demands of bonding
vectoring ?

• Table of contents
• VDSL Bonding Technology
• VDSL Vectoring Technology
• FEXT Noise and NEXT Noise
• Speed and Bandwidth
• Preparing Copper Cable for 100MBs

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VDSL Bonding Vectoring white paper
You Cant Fake Fast
The Promise of 100 Mbps and Vectored Services
Raises Bar on Network Quality
As a provider of VDSL2 based residential
services (IPTV, High Speed Internet, etc.) you
have already made a significant investment in
your copper network. But will it be fast enough
to support customer demands over the next several
years? Given that cable companies are already
offering network speeds upwards of 100 Mbps, what
are the challenges facing your network to provide
a similar performance and beyond? How will you
meet those challenges? The good news is that you
dont have to make that huge investment in a
fiber-to-the home network. Once again, next
generation copper based technologies are coming
to the rescue. By using a combination of new
VDSL2 based copper bonding and vectoring
technologies, you will be able to deliver
fiber-like speeds (upwards of 100 Mbps) that
rival those of the competition. These are
serious speed increases that will require a
network free of impairments to reliably support
these ultra-fast speeds. Now, more than ever is
the time to truly condition your network to
deliver this performance. You cant ignore
impairments or marginal network conditions.
Simply put, you cant fake fast. What is the
best way to prepare the copper network to deliver
on the promise of 100Mbs services to customers?
Read on to get answers to these questions and
more as we take a deep dive into what it takes to
get the copper network ready for 100 Mbps speeds.
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VDSL Bonding Vectoring white paper
VDSL Bonding Technology
VDSL bonding technologies address the need for
higher speeds by dealing with the DSL signal
itself two or more DSL signals are better than
one. Pair bonding is the process of combining the
capacity of one or more pairs in such a way that
the bonded pairs act like one BIG capacity pair
(see Figure 1 VDSL2 2 Pair Bonding). The data
carrying ability of each pair remains the same,
but bonding two equally capable pairs together
effectively doubles the total data carrying
capacity. It is kind of like a super highway
more lanes mean a BIGGER capacity to carry
traffic.
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VDSL Bonding Vectoring white paper
VDSL Vectoring Technology
DSL vectoring technologies address the need for
higher speeds by dealing with speed killing
noise. DSL receivers deliver high speeds when
noise levels are low. Therefore, it makes sense
to mitigate noise at the receiver to keep speeds
high. The total noise present at a DSL receiver
is best thought of as a group of individual noise
components that add together to create a total
noise problem. Among those noise components
that contribute significantly to total noise
would be Near End Crosstalk noise (NEXT) and Far
End Crosstalk (FEXT) noise. DSL Vectoring is a
complex microprocessor intensive method of
mitigating a specific kind of noise - FEXT noise
(see Figure 2 FEXT Noise). Today, several DSLAM
vendors are providing VDSL2 vectoring solutions.
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VDSL Bonding Vectoring white paper
NEXT Noise
In the early days of DSL planning, it became
clear that the best way to mitigate NEXT noise
was to keep the Downstream and Upstream
transmission bands separate from one another.
Such an approach would not add significantly to
the cost of silicon.
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VDSL Bonding Vectoring white paper
FEXT Noise
Dealing with FEXT noise is more challenging. In
those early days of DSL planning the computing
power required to cancel FEXT noise at the
silicon level would not have been cost effective.
However, today vendors are shipping DSLAMs with
cost effective VDSL2 vectoring FNEXT noise
mitigation capabilities.
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VDSL Bonding Vectoring white paper
Speed Possibilities
DSL speeds are measured in bits-per-second
(bps). The 100 Mbps speed objective bandied about
today came from the ITU-T G.993.2 VDSL2 standard
which defines a VDSL2 17a profile with a minimum
bi-directional net data rate capability of 100
Mbps (the Shannon-Hartley theoretical limit of a
17 MHz bandwidth is closer to 246 Mbps). The
standard also defines a VDSL2 30a profile with a
minimum capability of 217 Mbps (the
Shannon-Hartley theoretical limit of a 30 MHz
bandwidth is closer to 417 Mbps). When pair
bonding is used in conjunction with DSL
technology, there is a significant jump in total
speed. Two bonded pairs as used in VDSL2
deployments also introduce the possibility of a
third phantom channel to further increase speed
- 300 Mbps or higher may be possible.
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VDSL Bonding Vectoring white paper
Bandwidth
Anytime there is an increase in bandwidth, there
will be a need to re-visit existing copper
network qualification methods. Bandwidth is the
available spectrum for transmission. POTS service
only uses 4 KHz of bandwidth. Qualifying copper
pairs for POTS is relatively easy. But then along
came analog modems and fax machines that used
frequencies within that 4 KHz bandwidth that had
not been qualified. Do you remember all those new
trouble tickets? Now imagine what happens when
you upgrade from an 8 MHz bandwidth network
(VDSL2 Profile 8d) to a 17 MHz network (VDSL2
Profile 17a). Sure, you have qualified the
network for 8 MHz, but what about that part of
the bandwidth from 8 to 17 MHz that wasnt
qualified? How do you know it will be free of
frequency dependent trouble? Over the years
there has been a steady increase in the bandwidth
requirements of DSL technologies
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VDSL Bonding Vectoring white paper
Bandwidth
The 17Mhz and 30Mhz are the new VDSl2 profiles
being adopted to support bonding vectoring to
achieve 100Mbs speeds. The 17Mhz profile is
expected to be the most widely adopted profile
for bonding vectoring due to an attractive
combination of rate (supports up to 100Mbs with
bonding vectoring) and reach (loop length up to
2kft or 600m).
Bandwidth Spectrum Requirement xDSL Technology Typical Speeds
1.1 MHz ADSL Up to 8 Mbps
2.2 MHz ADSL2 Up to 20 Mbps
8 MHz VDSL2 Profiles 8a, 8b, 8c, 8d Up to 30 Mbps
12 MHz VDSL2 Profiles 12a 12b Up to 40 Mbps (when bonding is used)
17 MHz VDSL2 Profile 17a Up to 100 Mbps (when bonding vectoring are used) at up to 2Kft (600m) This bandwidth spectrum profile is expected to be the most widely adopted due to excellent balance of rate (speed) and reach (loop length)
30 MHz VDSL2 Profile 30a Up to 217 Mbps (or even higher when bonding vectoring are used). Note this requires very short loops, typically less than 500ft (61m).
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VDSL Bonding Vectoring white paper
Preparing the Copper Cable for 100 Mbps or Higher
Speed Services
Now that you have an understanding of the
technologies, standards and speed possibilities,
lets focus on preparing the copper network for
100 Mbps or even higher speed services. The
addition of bonding and vectoring technologies to
existing VDSL2 deployments will require accurate
records and additional qualification tests.
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VDSL Bonding Vectoring white paper
Records Accuracy

• Records verification is a process unto itself.
  Telephone companies maintain database records
  (cable, pair, binding post, status, etc.) about
  copper pairs. These records are changing to keep
  pace with advancing DSL technologies. They need
  to be periodically refreshed to reflect the
  current state of pairs in the field. When a
  customer calls in to request high speed DSL
  service it is important that customer agents
  quickly respond with accurate information as to
  whether or not that service can be provided. Here
  are some of the new challenges in a VDSL2 bonded
  and vectored network related to records accuracy
  
• There will be an increase of bonded pairs in the
  new network that will require a better
  understanding of total pair capacity. How many
  pairs do you have to work with?
• Pair selection for bonding will require a better
  understanding of the utilization of available
  pairs. How many pairs are going to each customer?
  How long are the pairs? Are there spare pairs in
  the serving terminal?
• VDSL2 vectoring can be done at the board or
  system level. Since vectored noise mitigation
  only works for common VDSL2 based services, how
  will you manage these vector groups?
• The DSLAM vectoring engine can only mitigate so
  much FNEXT noise. How will you manage its
  resources?

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VDSL Bonding Vectoring white paper
Pair Qualification Testing

• A pair that is free of trouble will deliver the
  highest speeds. Here are some of the new
  qualification testing challenges for a VDSL2
  bonded and vectored network that makes use of
  bandwidths out to 17 MHz or 30 MHz
• The total speed of bonded pairs will be limited
  by pair quality. Pairs that are free of trouble
  and with the best longitudinal balance will
  deliver the highest speeds.
• Noise is a speed killer. Is there noise in the
  new bandwidth?
• Noise on one pair can crosstalk onto adjacent
  pairs. Is the total crosstalk noise at a DSLAM
  limiting the speeds for all? Will the high level
  of FNEXT noise overwhelm the DSLAM vectoring
  engine that is attempting to mitigate this noise?
• Signal attenuation is a speed killer. Is there
  frequency dependent trouble in the new bandwidth
  that will attenuate the signal?
• Bridged-Tap can reduce speeds. The new network
  will have to deal with the effects of even
  shorter bridge-tap lengths.
• Clearly it is still important to perform the
  same core physical layer testing required for
  VDSL2 qualification - tests that identify foreign
  voltage, insulation resistance breakdown, opens
  (capacitance), noise and bridged-tap. But the new
  network will need some additional tests that
  identify any frequency dependent trouble in the
  new untested bandwidth.

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VDSL Bonding Vectoring white paper
Pair Qualification Testing

• The following describes the comprehensive set of
  qualification tests we recommend for VDSL2
  bonding vectoring to support 100Mbs service.
• First, the following core tests help find some
  common line faults
• Foreign AC and DC Voltage
• The only DC voltages allowed on a copper pair
  are those that have been designed to be there.
  Unfortunately, DC voltages from other pairs may
  appear if the cable has been compromised water,
  broken insulation, etc.
• The only AC voltages allowed on a copper pair
  are those that have been designed to be there.
  Unfortunately, AC voltages couple onto copper
  pairs from all those power lines that run
  parallel with telephone cables.
• Insulation Resistance
• There should be high insulation resistance
  between copper pairs (Tip-to-Ring, Tip-to-Ground,
  Ring-to-Ground). Unfortunately, over time cables
  begin to deteriorate resulting in a lower
  insulation resistance.
• Capacitance
• Copper cable is designed to have the same mutual
  capacitance between the wires that make up a pair
  (Tip and Ring). However, it is possible for this
  capacitance to change over time due to bad
  splices, water, bridged-tap, etc. Also, the
  Tip-to-Ground capacitance should be very close to
  the Ring-to-Ground capacitance.

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VDSL Bonding Vectoring white paper
Pair Qualification Testing

• Bridge Tap Detection
• The International Engineering Consortium
  describes a bridge tap as any unterminated
  portion of a loop not in the direct talking path
  between the CO or DLC and the customers premise
  equipment (telephone or modem). When a signal is
  reflected from the unterminated portion, the
  reflection can interfere with the original
  signal. Bridge taps, particularly short taps, are
  much more of a problem in VDSL than in ADSL due
  to the higher frequency of VDSL. Bridge taps
  cause what is referred to as a notch effect,
  which can adversely affect the carriers at these
  frequencies. In general, the length of bridge tap
  will determine the severity of the notch.
• Finding and removing bridge taps is more
  critical than ever before when it comes to
  providing reliable 100Mbs performance at 17Mhz
  and 30Mhz profiles.
• But as we now know, bonding and vectoring
  performance requires additional tests that you
  may not have been performing. These additional
  tests are important especially out to 17Mhz and
  30Mhz to find faults that could impact
  performance at these higher frequencies.

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VDSL Bonding Vectoring white paper
Pair Qualification Testing

• Bandwidth Noise (out to 30 MHz)
• Noise is a speed killer. Noise should be
  analyzed across the entire bandwidth being used.
  The noise on a tested pair must be broken down
  into its component parts such that proper
  mitigation techniques can be applied. VDSL2
  Vectoring technology deals specifically with the
  cancelation of FEXT component noise it will not
  be able to deal with RF or impulse noise. Along
  with loss, this test provides key information
  related to bit rate estimations. The noise level
  at a receiver will limit transmission.
• Wideband Longitudinal Balance (out to 30 MHz)
• When you hear the term longitudinal, think
  Tip-to-Ground or Ring-to-Ground. When you hear
  the term balance, think equality between
  Tip-to-Ground and Ring-to-Ground. When you hear
  the term impedance, think frequency dependent
  resistance.
• A pair is longitudinally balanced IF the
  Tip-to-Ground impedance and Ring-to-Ground
  impedance is equal for each frequency in the
  transmission bandwidth.
• Why is it important for pairs to have good
  longitudinal balance?
• Todays telephony transmission methods use
  differential mode transmitters to send signals
  across twisted pairs to differential mode
  receivers capable of rejecting common mode noise.
  Since most noise is common mode, the receiver can
  effectively reject it. However, when a pair is
  unbalanced at a particular frequency, any noise
  at that frequency will appear as differential
  mode noise to the receiver, and the receiver will
  not be able to reject it

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VDSL Bonding Vectoring white paper
Pair Qualification Testing
Since reducing noise is a critical aspect for
vectored services, it is important that the line
is longitudinally balance. In addition, since
VDSL2 utilizes distinct sub-carriers are
4.3125Khz spacing (or bins) it is important
that the longitudinal balance test be performed
at each of this frequency across the entire 17Mhz
or 30Mhz operating spectrum. Eliminating a
longitudinal balance problem has a double benefit
a properly balanced pair will generate less
disturber noise and will have greater immunity to
disturber noise generated by adjacent pairs. Of
course the ability to perform this Wideband
Longitudinal test single-end (without requiring a
far end device) would be best to reduce test and
troubleshooting time.
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VDSL Bonding Vectoring white paper
Pair Qualification Testing
7. Bandwidth Loss Measurement (out to 30 MHz)
Having the ability to measure the loss of each
frequency within a useable bandwidth (the
attenuation curve) reveals important information
about the overall transmission ability across a
pair. Utilizing a single-ended technique to make
this measurement eliminates the requirement to
have a test set on the opposite end of the pair.
Along with noise, this test provides key
information related to bit rate estimations. The
attenuation curve accurately represents the
signal level at the receiver along with any loss
due to physical faults (bridged-tap, bad splices,
etc.). 8. Bandwidth Bit Rate Estimations
Accurate DownStream and UpStream bit rate
estimations provide key insight into the ability
of a pair to deliver on the 100 Mbps performance
target. A properly designed bit rate estimation
algorithm knows how to utilize all sources of
information (physical tests AND database
inquiries) pertaining to the tested pair to
provide an accurate estimate of expected
DownStream / UpStream rates. Physical tests
reveal important information related to
attenuation and noise on the pair. Database
inquiries reveal important information about the
effects of other pairs in the binder on the
tested pair. Bonding and vectoring introduce new
complexities that must be accounted for by a good
rate estimate algorithm
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VDSL Bonding Vectoring white paper
Pair Qualification Testing

• Bonding and vectoring introduce new complexities
  that must be accounted for by a good rate
  estimate algorithm
• Impact of noise and attenuation across two pairs
• Changeable bandwidths (e.g. 17a versus 30a)
• Noise contributions of adjacent pairs
• Expected improvements when vectoring is turned on
  
• Expected improvements when bonded pairs are used
  instead of a single pair
• Lost performance due to bridged-tap
• Lost performance due to foreign noise
• Performance impact on neighboring pairs
• When done correctly, this estimation can serve
  as the sole pass or fail indicator of a tested
  pair.

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VDSL Bonding Vectoring white paper
Conclusion
In conclusion, you can see that your networks
ability to support the latest VDSL2 bonding
vectoring technology to deliver up to 100Mbs
requires that it be free of impairments. The need
to identify problems such as 17Mhz noise and
longitudinal balance are now more important than
ever before. You are pushing your network more
than ever, delivering more bandwidth than ever
before. This requires the highest quality
network, you cannot mask these impairments if you
want to achieve upwards of 100Mbs. You cannot
fake FAST.
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