[VoiceOps] PRI loss/level settings - DTMF issues

Jay Hennigan jay at west.net
Fri Mar 18 19:59:41 EDT 2011

On 3/18/11 3:35 PM, Tim Donahue wrote:
> Jay,
> We have seen this with the Adtran TA900 series boxes before.  Have you
> checked that the lbo parameter is set appropriately in the t1 interface
> settings?  This fixed has fixed echo problems for us in the past.

I think we're talking apples and oranges here.  LBO controls the level
of the digital T1 signal itself.  Problems here will show up as CRC or
other T-1 errors.  The characteristics of the TDM audio encoded in the
T-1 in terms of echo, DTMF, etc. are not going to be affected by LBO at
all.  As long as the T-1 receiver sees the 1.544 Mbits/s stream of ones
and zeros, the dB level of the T1 bitstream won't have any effect on
level of the u-law DTMF or voice encoded therein.

Within the Adtran for FXS lines, and within the Shoretel for the PRI
B-channels, there is a configurable level setting for transmit and
receive.  This is an adjustment in the form of a digital pad or volume
control that adjusts the DSP going from the analog to the digital realm
and will affect the volume of the signal heard at the earpiece or sent
from the mouthpiece.

For example, a milliwatt test tone will be measured at the same level at
the other end regardless of LBO on the T-1 but will be affected by the
configurable gain settings on the FXS or PRI.

>From my old Bell-head days, there were standards in the form of a "Loss
and level plan" for PBXs connecting to the network to ensure that voice
levels are audible and don't result in blasting or "singing".

I seem to recall that customer PBXs are recommended for -6 to the
network and -3 from the network but can't locate that spec.

(This is way different from the -16 and +7 from 0 TLP carrier levels for
those of us who have been around for a while...)

The Shoretel is currently at +/-0 dB in both directions, and the other
party reports that the voice coming from the Shoretel seems noticeably
loud.  I'm of the opinion to just set it to -6 and -3 and see what
happens but was hoping for some verification from the community that
this is a reasonable approach.

> Other than that, the only reliable solution we have found when these
> sort of problems crop up is installing a more configurable box, such as
> the Audiocodes Mediant 1000.  More expensive, definitely, but we have
> replaced 3 or 4 Adtrans that were having problems ranging from echo, to
> DTMF not registering properly or duplicating digits, that were deployed
> at client sites and the Audiocodes has always been able to fix the
> problems they were having.

If the main difference is the ability to tweak the levels, we have that
in the Shoretel itself.

> If  you do happen find a solution for controlling the levels on the
> Adtran T1 interfaces, I'd love to know what it is.  I have contacted
> Adtran's support a few times about it and I haven't gotten any
> confirmation about if it is possible out of them yet.

T-1 signal levels, use LBO.  Voice gain from handsets to the T-1 is
adjustable on the FXS/FXO interface config.  Voice gain to a CAS/PRI, to
the best of my knowledge, isn't settable on a TA900.

Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
Your local telephone and internet company - 805 884-6323 - WB6RDV

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