how to select the right VDSL band plan to fit your application requirements, and to
make the right design choices to optimize your products performance
by Sigurd Schelstraete, Principal Engineer
and Ben Runyan, Director of Product Marketing, Ikanos Communications


VDSL2 represents an opportunity for communications vendors to deliver the higher bi-
directional bandwidth that carriers need for new services such as IPTV, video
conferencing, VOIP, peer-to-peer file sharing, and interactive gaming. From a design
standpoint VDSL2 is a far more robust and flexible technology than its predecessors, and
designers have a greater range of options about how to deploy it. Having more options,
however, can present distinct challenges.

This TechNote examines the technical attributes of the new VDSL2 standard and how
they impact designs and applications, and provides recommendations on what criteria to
consider in the design process.


VDSL2 Profiles

At the beginning of the VDSL2 standardization process, it was expected that it would
incorporate the best of the existing VDSL and ADSL standards and take into account the
applications likely to be encountered in deployment. Even at the early stages, however, it
became clear that the potential deployments were extremely diverse. Interest was
expressed in the following distinct deployment scenarios:
Extremely high symmetric bit rates (100/100 Mbit/s) for MxU applications
Deployment from CO/RT (exchange/cabinet) at VDSL1 rates with the possibility
of extending the reach beyond typical VDSL1 ranges
Low-complexity, high-density solutions for confined enclosures (eg cabinets)
Deployments in CO environments with strong ADSL crosstalk

VDSL2 can deliver for each of these deployment scenarios. However, it was recognized
that using a single system for each of these deployments would lead to an overly complex
design -- the different scenarios sometimes lead to conflicting system requirements
making the system too expensive. For instance, a system that has to contend with strong
ADSL2(+) crosstalk may benefit from raising its downstream transmit power to +20 dBm
(the limit for ADSL2 and ADSL2+), but using the same transmit power for the wideband
100/100 systems would negatively impact performance.

As a result, VDSL2 introduced profiles a concept which did not exist in VDSL1
because the application space was more homogeneous. Essentially, profiles are subsets
of the full VDSL2 set of capabilities and settings. Each profile is intended to address
specific deployment scenarios.
The ITU-T standard consented to (May 2005) specifies eight different implementation
profiles that provide different levels of upstream and downstream bandwidth for different
applications. To be able to claim VDSL2 standard compliance, a system must be able to
support at least one profile. These profiles and applications are summarized in Table 1.


Table 1: VDSL Profiles With Frequencies Corresponding To Band Plan 998

NOTE: the band plan extensions above 12 MHz are currently under study for North-
America. Once the band plan is known, the max DS frequency values will be specified.

The Application information in Table 1 describes the most common uses for the various
profiles. The 8-MHz (8a, 8b, 8c, 8d) and 12-MHz (12a, 12b) profiles fit mostly in the
CO/cabinet application space. Since these environments may incorporate longer reaches
most of these profiles require mandatory support of U0 (Upstream Band 0). Also, to
achieve this longer reach in the presence of crosstalk from legacy systems, the
downstream transmit power can go up to 20 dBm.

The higher-frequency profiles (17a and 30a) are mostly used to deliver high speed at
shorter distances -- up to 100 Mbit/s of downstream bandwidth over short distances (eg
within multi-tenant and multi-dwelling units, or MxUs), with upstream speeds increasing
to 100 Mbit/s when frequencies up to 30 MHz are used. Profile 17a may also be deployed
from a cabinet over short distances. Because the MxU application involves short
distances no U0 support is required, and the transmit power is kept to a lower 14.5 dBm.

Any of the profiles can naturally be used in customer premises equipment (CPE) as long
as the CPE implementation uses the same profile as the equipment in the CO/RT
implementation.

Not required
Not required
Not required
Required
Required
Required
Required
Required
U0 for Long
Range
Not
specified
Not
specified
12.0 MHz
12.0 MHz
5.2 MHz
5.2 MHz
5.2 MHz
5.2 MHz
Max. US
Freq.
Cabinet (RT)
MxU

Cabinet (RT)
Exchange (CO)
Cabinet (RT)
Exchange (CO)
Exchange (CO)
Exchange (CO)
Cabinet (RT)
Exchange (CO)
Cabinet (RT)
Typical
Application
14.5 dbm
Not
specified
14.5 dbm
30a
14.5 dbm
Not
specified
14.5 dbm
17a
14.5 dbm
8.5 MHz

14.5 dbm
12b
14.5 dbm
8.5 MHz

14.5 dbm
12a
14.5 dbm
8.5 MHz

20.5 dbm
8b

14.5 dbm
8.5 MHz

17.5 dbm
8a

14.5 dbm
8.5 MHz

14.5 dbm
8d

14.5 dbm
8.5 MHz

11.5 dbm
8c
Max. US
Power
Max. DS
Freq.
Max. DS
Power
Profile
Not required
Not required
Required
Required
Required
Required
Required
U0 for Long
Range
Not
specified
Not
specified
12.0 MHz
12.0 MHz
5.2 MHz
5.2 MHz
5.2 MHz
5.2 MHz
Max. US
Freq.
MxU

Cabinet (RT)
Cabinet (RT)
Exchange (CO)
Cabinet (RT)
Exchange (CO)
Exchange (CO)
Exchange (CO)
Cabinet (RT)
Exchange (CO)
Cabinet (RT)
Typical
Application
14.5 dbm
Not
specified
14.5 dbm
30a
14.5 dbm
Not
specified
14.5 dbm
17a
14.5 dbm
8.5 MHz

14.5 dbm
12b
14.5 dbm
8.5 MHz

14.5 dbm
12a
14.5 dbm
8.5 MHz

20.5 dbm
8b

14.5 dbm
8.5 MHz

17.5 dbm
8a

14.5 dbm
8.5 MHz

14.5 dbm
8d

14.5 dbm
8.5 MHz

11.5 dbm
8c
Max. US
Power
Max. DS
Freq.
Max. DS
Power
Profile VDSL2 Design Challenges

The 8- and 12-MHz profiles are now fairly well understood by equipment designers as
these were designated in the initial VDSL specification (G.993.1) in 2004 and in regional
VDSL standards adopted since 2002. However, the 17- and 30-MHz profiles are
relatively new to the marketplace.

When designing a VDSL2 system for a given application it is important to realize that the
complexity will be determined to a large extent by transmit power, required bandwidth,
and the capability to support U0.


Transmit Power

Transmit power is one of the prime design parameters for a system and one of the guiding
principles in the definition of profiles. The transmit power of the line driver impacts at
least two other aspects of the system: power consumption and bandwidth.

Power consumption -- higher transmit power on the line will result in a system with
higher power consumption because the line driver requires higher bias current to deliver
the higher voltages to the line. Higher power consumption will limit the number of ports
(port density) of the VDSL2 line card.

Bandwidth -- higher transmit powers (like +20 dBm) cannot be delivered in combination
with very wide bandwidth. Again, the line driver is the main limitation. The line driver
will need a high bias current to support the high-voltage signals on the line. Typically,
working at higher bias current will somewhat reduce the linearity of the line driver as a
whole. In addition, the higher signals at the input side of the line driver are more likely to
steer the line driver out of its linear range. The non-linearities resulting from those two
effects will cause performance degradation at the higher frequencies.

Separately, the higher bias current will also increase the noise introduced by the line
driver. This may not be an issue at the lower frequencies since both the signal power and
the noise power are increased. At higher frequencies the signal power does not increase
when going from +14.5 dBm to +20 dBm (since the PSD limit has already been reached),
but the noise power does. As such the higher frequencies are negatively impacted by the
higher transmit power.

The relationship between bandwidth, transmit power, and power consumption can be
clearly seen in the profile definition (Table 1) and can be summarized as follows:
Profiles with large bandwidth (12a, 12b, 17a, 30a) have a transmit power that is
limited to +14.5 dBm (because high transmit power cannot be combined with
very wide bandwidths)
The only profile that supports +20 dBm has its maximum downstream frequency
limited to 8.5 MHz (because this high transmit power limits the maximum usable
bandwidth)

The profile intended for RT (cabinet) deployments (8c) has the lowest transmit
power (reflecting the desire for a system with minimal power consumption)

Designers must select line drivers and analog front ends (AFEs) which best meet the line
cards intended application. Since one line card may support several different profiles, the
tradeoffs between power output and bandwidth are not always easy ones to make.


Higher Bandwidth

In addition to transmit power higher bandwidth increases the complexity of the system as
well. As indicated previously there are limitations in providing high transmit powers over
large bandwidths, limitations mainly introduced by the line driver. To correctly transmit
and receive the higher frequencies, the AFE also needs to have good linearity in this
frequency range. For the high-bandwidth profiles like 17a and 30a careful design
considerations are especially needed for the DAC and ADC since sampling rates have to
be increased.

The support of higher bandwidths also has an impact on the complexity