Effects of Network Elements on Echo 35
Figure 2-5 Tail circuit in a VoIP network.
Gateways have two types of PSTN interfaces: digital (ISDN BRI, T1/E1) or analog (E&M,
FXO, FXS). Recalling that bits don’t leak, further refine your search for echo sources to the
analog elements of the tail circuit. You can extend the echo-free digital zone out from the
gateway to the point of digital-to-analog (D/L) conversion in the PSTN, as shown in
Figure 2-6.
Figure 2-6 Tail circuit with both analog and digital links.
Effects of Network Elements on Echo
The following network elements in a VoIP network can have an effect on echo:
Hybrid transformers
Telephones
Routers
Quality of service (QoS)
Effect of Hybrid Transformers on Echo
Echo sources are points of signal leakage between analog transmit and receive paths.
Hybrid transformers are often prime culprits for this signal leakage. Figure 2-7 shows an
analog tail circuit with a hybrid transformer.
PSTN
PSTN WAN
London tail circuit
WAN
Alice
PBX
GW
T1
PBX
E&M
FXO:FXS
T1
PBX
Tail Circuit
Analog tail circuit (echo sources)
36 Chapter 2: Understanding Echo Analysis
Figure 2-7 Detail of analog tail circuit with a hybrid transformer.
The analog telephone terminal is a 2-wire device, with a single pair of conductors used to
carry both the Tx and Rx signals. For analog trunk connections, known as 4-wire
transmission, two pairs of conductors carry separate Tx and Rx signals. Digital trunks
(T1/E1) can be considered virtual 4-wire links because they also carry separate Tx and Rx
signals.
A hybrid is a transformer that is used to interface 4-wire links to 2-wire links. It is a non-
ideal physical device, and a certain fraction of the 4-wire incoming (Rx) signal will be
reflected back into the 4-wire outgoing (Tx) signal. A typical fraction for a properly
terminated hybrid is about –25 dB (ERL = +25 dB). This means that the reflected signal
(the echo) will be a version of the Rx signal attenuated by about 25 dB. Remember, an echo
must have both sufficient amplitude and sufficient delay to be perceived. Echo strength of
–25 dB relative to the talker’s speech level is generally quiet enough to not be annoying,
even for relatively long delays of 100 ms.
Echo strength is expressed in decibels (dB) as a measurement called echo return loss
(ERL). The relation between the original source and the ERL is as follows:
Original source amplitude = Echo amplitude + ERL
Therefore, an ERL of 0 dB indicates that the echo is the same amplitude as the original
source. A large ERL indicates a negligible echo.
The ERL is not a property of the hybrid alone, however. It depends on the load presented
by the terminating device, which might be a telephone or another PBX. The hybrid has a
certain output impedance that must be balanced by the input impedance of the terminating
device. If the impedances are not matched, the returning echo fraction will be larger (the
ERL will be smaller) and the echo will be louder.
You can expect a certain amount of impedance mismatch (a few tens of ohms) because a
normal hybrid connection will yield ERLs in the range of 20 to 30 dB. However, it is
possible that one device could be provisioned for an output impedance of 900 ohms, and
the terminating device provisioned with an input impedance of 600 ohms, which would
yield a large echo, and would be expressed by a small ERL.
The main point to remember about hybrids is this: Ensure that output and input impedances
are matched between the hybrid and the terminating device.
Alice
PBX PBX
E&M
FXO:FXS
Hybrid
Analog 4-wire link Analog 2-wire link

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