Triple-Play Service Deployment

Chapter 8: Troubleshooting VoIP
Common VoIP service degradation factors
A number of effects can occur on the service delivery network that
will negatively impact the perceived quality of service. Echo and
packet loss, slip, and/or delay are a few of the most common that
providers must identify and eliminate prior to releasing VoIP
service to the customer.
Echo effect on voice quality
The most common cause of echo is an impedance mismatch
between local and long distance circuitry.Telephone handsets use
4 wires to carry a call while the long distance network uses a single
2 wire pair. To interface between these wiring schemes a device
called a hybrid is used. However, a hybrid causes signal reflection
due to an impedance mismatch. Unfortunately there is no way to
balance the long distance to local connection because looking
from the local side, the long distance network’s loop impedance
varies depending on the length of the local loop, number of
phones, type of phones, presence of load coils, and other factors. A
secondary, but potentially significant, source of echo is acoustic
feedback from certain types of phones such as speaker phones.
The echo effect of can be addressed through suppression or
cancellation. Suppression is the simpler method and simply turns off
‘receive’ while ‘transmit’ is active. With the cancellation method, the
transmitted sound is saved.Then, if a weakened version of that sound
appears on the receive side, it is deleted from the signal. Echo
cancellation is usually implemented in a digital signal processor (DSP).
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