After looking at the "pktcNcsEndPntConfigTxGain"
and "pktcNcsEndPntConfigRxGain" MIB objects more closely, here are
some concerns with these
objects resurrected in draft-08 since being obsolete in
draft-02.
The
concerns are two-fold:
1. The real value in having
this solution for future proofing:
As Rich pointed out, we are trying
to future proof the unknown requirements. If this becomes important in the
future then we can figure it out then, it is impossible to solve everything up
front without understanding all the problems and consequences. The draft-02 has obsolete these two objects after the
extensive discussions. Are the problems these two MIB objects are presumably
solving important enough to consider the draft change given the current late
stage.
2. Implementation problems associated with the
functionality introduced by these two objects
We understand the benefits of having configurability to
meet different country specifications. However, the gain/loss plan is only one
component in trying to adapt to different country deployments. The MIB doesn't
address other items such as complex impedance, ring voltage, etc. By trying to
address the gain/loss in isolation we are actually creating implementation
difficulties. This is already solved by MTA manufactures through more
intelligent and coordinated methods using both H/W and
S/W
Providing the ability to dynamically adjust gains
means that you are inherently limiting the dynamic range of the DSP to try and
address all corner case scenarios. This is a huge implementation problem. Often
the per country matching of complex impedances and gains is done in H/W or
within the analog interface components like the SLIC. These interfaces are
configured for a given country specifications independent of signaling MIBs.
This creates a problem when you want to have the gain setting larger than the
natural country default. The only solution to meet the desired MIB value is to
generate a loud signal (likely clipping within the DSP) in order to adjust for
the analog hardware
gain.
Colleagues,
This is a proposal to establish the Transmit and Receive gain (loss)
plan parameters in the SigMIB to provide a solution to the setting of
supervisory tone levels at the a-b termination points and it also provides an
intelligent approach to future migration of a dynamic gain (loss) plan based
on the connection requirements.
The first point to consider in this proposal is when the MTA passes in
band signaling tones. Per
CableLabs and ETSI these tones are passed to the POTS a-b (T/R) termination
points through signal paths that apply a known loss. This is usually implemented in the
POTS SLIC circuitry either with fixed loss devices or provisioned by the
vendor/operators.
However, when directed by the Call Agent the MTA may generate the
supervisory tones locally. The
same loss plan specifications per CableLabs and ETSI for in band signaling are
assumed to still be active when the tones are locally generated. Therefore it is necessary for the
internal supervisory tone generator to compensate for the analog gain (loss)
in the MTA (Typically in the POTS SLIC circuitry) to achieve the desired level
at the a-b (T/R) termination point.
While this gain (loss) may be known to a vendor during manufacture, the
second consideration in this proposal suggests that this gain (loss) may be
required to be changed based on far end (analog loop) connection
characteristics that change depending on the actual far end connection
point.
While CableLabs
PKT-SP-EMTS-PRIMARY-I01-001128 Clause 8.4.4 and Telcodia GR-909-CORE R5-17
guildance suggest a 2 dB loss plan default setting, there is also guildance in
GR-909-CORE R4-134 referencing ANSI T1.401-1988 Figure 24 that DTMF levels may
be as high as +4 dBm under low loop current conditions. Per Telcordia GR-506-CORE Clause
15.1.5, the DTMF receiver acceptance levels must be greater than -25 dBm and
less than 0 dBm. Considering a +
4dBm DTMF level applied to a 2 dB loss plan would resulting a DTMF PCM coding
level greater than the GR-506-CORE guidance of 0 dBm maximum for the DTMF
detector. The setting of the
default value to -4 dB gain, ensures proper DTMF signal quality for proper
detection and is also consistent with Telcordia GR-909-CORE which references
TR-NWT-000057 for parameter settings and where 6.1.8 DLC System Loss O-11
suggests a DLC transmission path loss in each direction o the RT line unit
should be 4 dB +/-2 dB (- 4 dB gain).
Guidance in TIA/EIA TSB122-A and ETSI ES 202 020 further offers the
need to adjust the gain (loss) plan based on if a given MTA is attempting a
connection to one of several analog POTS termination points. In reviewing these documents if one
considers the MTA as the referenced ONU unit, various connection cases can be
made (ONU to OPS, DGS, WAN, DAL, FXO, FXD, or ATT or these terminations back
to the ONU) where the recommended gain (loss) plan per connection vary between
0 dB and 9 dB in loss or in some cases 3 dB of gain.
While it is recognized that current standard connection establishment
protocols (NCS, SIP) do not support dynamic gain (loss) plan negotiation or
assignment at this time, it should be recognized that by establishing the MIB
now will enable future protocol updates to improve the end user
experience.
Finally, a text enhancement is proposed to clarify the
pktcSigDevToneDbLevel tone level setting measurement with respect to complex
tones (tones sometimes consisting of more than one frequency).
Respectfully,
Phillip Freyman
Distinguished Member of the Technical Staff
Motorola Connected Home
Solutions (formally BCS)
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pktcNcsEndPntConfigTxGain
Integer32,
pktcNcsEndPntConfigRxGain
Integer32
pktcNcsEndPntConfigTxGain OBJECT-TYPE
SYNTAX
Integer32
UNITS
"dB"
MAX-ACCESS
read-write
STATUS
current
DESCRIPTION
"The per line transmitter (A/D) gain. A positive number reflects a signal
gain, a negative number reflects a signal loss. This Object may provision the gain or
it may be used to document a non-provisionable gain between the telco (POTS)
a-b (T/R) terminals and the analog codec maximum PCM coding limit (PCM maximum
coding limit).
Based on the default G.711 Vocoder maximum of 3.14 or
3.17 dBm the -4 dB gain default provides Vocoder coding protection against TE
maximum signals while also providing an initial loss to minimize analog signal
echo.
DEFVAL {
-4 }
::= {
pktcNcsEndPntConfigEntry 42 }
pktcNcsEndPntConfigRxGain OBJECT-TYPE
SYNTAX Integer32
UNITS
"dB"
MAX-ACCESS
read-write
STATUS
current
DESCRIPTION
"The per line receiver (D/A) gain. A positive number reflects a signal
gain, a negative number reflects a signal loss. This object may provision the gain or
it may be used to document a non-provisionable gain for use with the
pktcSigDevToneDbLevel Object to set the desired level at the a-b (T/R)
terminals.
Based on the default G.711 Vocoder maximum of 3.14 or
3.17 dBm the -4 dB gain default provides a maximum analog signal level at the
a-b (T/R) termination point. It should be noted that ETSI TS 101 909-4
provides guidance of 11 dB loss (-11 dB gain) and based on this reference a
DEFVAL { -11 } in international markets may be considered.
DEFVAL {
-4 }
::= {
pktcNcsEndPntConfigEntry 43 }
pktcSigDevToneDbLevel OBJECT-TYPE
SYNTAX TenthdBm
(-250..-30)
UNITS
"dBm"
MAX-ACCESS
read-write
STATUS
current
DESCRIPTION
"This is the decibel level for each analog signal (tone) that is
locally generated (verses in band supervisory tones) and sourced to the a-b
terminals (TE connection point).
Per the PktcSigDevToneEntry this DbLevel represents the level required
for each tone where each tone may consist of multiple frequencies which may be
summed or amplitude modulated. This Object must reflect the desired level at
the Telco (POTS) a-b (T/R) terminals including the affect of the
pktcNcsEndPntConfigRxGain setting on the delivered tone. The wide range of levels for this
Object is required to provide signal generator levels across the wide range of
gains but does not imply the entire range is to be achievable given the range
of negative values of gain (positive loss). This Object must be set for each
tone so as to generate the combined frequency level at the a-b (T/R)
terminals."
DEFVAL {
-40 }
::={pktcSigDevToneEntry 2 }