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Fractional E1/T1 Back-to-Back Configuration
Figure C-7. Fractional E1/T1 Back-to-Back Configuration
Echo Cancellation in Fractional E1/T1 Applications
Regular telephony systems use 4 wires circuits: one pair for the transmitted signal
(microphone) and one pair for the received signal (earphone). The transmission system is
comprised of a single pair (2 wires) and a hybrid component used for the required conversion
from 4- to 2-wires and vice versa.
The operation of the hybrid circuit is highly dependent on the impedance of the transmission
lines, and since this impedance may vary, optimal performance of the hybrid cannot be
assured. This imperfection results in the generation of an echo signal (the transmitted signal
at a level that is determined by the quality of the hybrid and the actual impedance of the line).
In short distance links, the echo generated in the far end has a negligible impact on the user,
since the delay time is insignificant. However, in some long distance links and in many
wireless links this delay may be significant and can reduce the quality of the
communications. People are used to hearing
themselves while speaking. However, they will be very uncomfortable when hearing
themselves after a delay of more than several milliseconds.
Figure C-8. The Source of Echo in Telephony
