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APPENDIX D
Point-to-Point
Connection Using
Hybrids
PBX
OR
PHONE
HYBRID
A
2-WIRE
D-8
This network has 2-wire elements at each end. The elements may be
telephone instruments, 2-wire electronic Key Systems or older 2-wire
relay PBXs. Hybrids convert the 2-wire interfaces to 4-wire.
TIE
TMS-3000
TRUNK
CELP
4-WIRE
A
Hybrids generate a sidetone (echo) from the 4-wire input to the 4-wire out-
put in all but the most perfectly matched lines. The CELP algorithm adds
a considerable amount of propagation delay in the circuit. The CELP end-
to-end delay is long enough for talker echo to be perceived from the re-
mote end. Hence the CELP echo canceller will always be enabled.
CHECK LIST FOR HYBRID CIRCUITS
•
Compile the end-to-end Loss Plan.
•
Adjust levels with real loads in both directions. Use dBm meter in
"Bridge'' or "High Impedance'' mode. Add only enough gain to com-
pensate for the hybrid insertion losses.
•
Measure the Echo Return Loss at each CELP.
•
Calculate each CELP's CODEC Echo Return Loss from the measured
ERL and CELP transmit and receive gain values. Check for compli-
ance with the 6 dB rule.
•
Check effects of possible multiple echo paths via PBXs. This could
happen with dialed "tail'' circuits through the PBX.
TIE
TMS-3000
TRUNK
CELP
4-WIRE
B
PBX
OR
PHONE
HYBRID
B
2-WIRE
036R480DB
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