Mitel MiVoice Business Voice Quality And Troubleshooting Manual page 22

Voice Quality Troubleshooting Guide
In Scenario 3:
1. The PSTN is not aware of the equipment used on the MiVoice Business side of the PRI
interface.
2. If the MiVoice Business system introduces an echo causing delay in the IP path, you are
responsible for preventing (cancelling) the echo from entering the PSTN. Otherwise, echo
will be heard by the CPE VoIP phone user. Even though the PRI interface is 4-wire, there
could be a source of echo (for example, the LS trunk) on the other end of the connection.
3. There are two hybrids, one on each end of the 2-wire segment. This 2-wire segment
contributes reflections towards the MiVoice IP Phone. The MiVoice Business echo canceller
is responsible for cancelling reflections from sources that are bound for the MiVoice IP
Phone. The VoIP gateway is responsible for cancelling any echo delays that originate from
the CPE VoIP phone.
4. These two hybrids also each generate a reflection in the direction of the other CPE VoIP
phone. Since the delay through the VoIP gateway is long enough for these reflections to
be heard as echo by the CPE VoIP phone, the VoIP gateway must cancel this echo.
5. If the PSTN introduces a delay that is long enough for the reflections to result in echo, then
the PSTN is responsible for placing echo cancellers in the audio path. The PSTN will
normally insert a pair of echo cancellers, one at each end. These echo cancellers will
prevent 2-wire PSTN sets from hearing echo. MiVoice Business also has echo cancellers
behind the PSTN echo cancellers. If a PSTN echo canceller is operating properly, the
MiVoice Business echo canceller does not detect any echo and it disables itself. If the
PSTN echo canceller is faulty, the MiVoice Business echo canceller located downstream
could make the echo worse.
6. There are no sources of electrical echo in the Mitel CPE, because all the internal paths are
4-wire.
7. Both the MiVoice IP Phone and the CPE VoIP phone could generate acoustic, mechanical,
or crosstalk echo. The acoustic echo suppressors or acoustic echo cancellers on the hand-
sets are responsible for cancelling this echo.
8. The equipment that inserts the echo causing delays is responsible for cancelling any re-
flections that would be heard as echo because of this delay.
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Figure 5: Scenario 3