4RF Aprisa XE, Руководство пользователя

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Configuring the traffic interfaces | 91
DFXO / DFXS loop interface circuits
Function
The function of DFXO / DFXS 2 wire loop interface circuits is to transparently extend the 2 wire
interface from the exchange line card to the telephone / PBX, ideally without loss or distortion.
The DFXO interface simulates the function of a telephone and a DFXS interface simulates the function
of an exchange line card. These circuits are known as ‘ring out, dial in’ 2 wire loop interface circuits.
Network Performance
The overall Network Performance is dependant on the number of D-A and A-D conversions and 2 wire
to 4 wire / 4 wire to 2 wire conversions in the end to end circuit (telephone to telephone).
To achieve the best overall Network Performance, the number of D-A and A-D conversions and 2 wire
to 4 wire / 4 wire to 2 wire conversions should be minimized.
Circuit Performance
The circuit quality achieved with a 2 wire voice circuit is very dependant on the external interface
parameters and the interconnecting copper line.
Short interconnecting copper lines (< 100 meters), have little effect on the circuit performance so the
interface parameters have the dominant affect on circuit performance.
As the length of the interconnecting copper line is increased, the attenuation of the analogue signal
degrades circuit performance but also the impedance of the copper line also has a greater effect on
the circuit performance. For this reason, complex line impedance networks (e.g. TBR21, TN12) were
created which model the average impedance of the copper network.
The factors that affect the quality of the circuit achieved are;
DFXO interface
•
The degree of match between the DFXO line termination impedance, the impedance of the
interconnecting copper line and the exchange line card line termination impedance.
This affects the return loss.
•
The degree of match between the DFXO line termination impedance, the impedance of the
interconnecting copper line and the exchange line card hybrid balance impedance.
This affects the exchange line card transhybrid balance.
•
The degree of match between the DFXO hybrid balance impedance, the impedance of the
interconnecting copper line and the exchange line card line termination impedance.
This affects the DFXO transhybrid balance.
•
The circuit levels of both the DFXO and the exchange line card.
DFXS interface
•
The degree of match between the DFXS line termination impedance, the impedance of the
interconnecting copper line and the telephone line termination impedance.
This affects the return loss.
•
The degree of match between the DFXS line termination impedance, the impedance of the
interconnecting copper line and the telephone hybrid balance impedance.
This affects the telephone transhybrid balance.
•
The degree of match between the DFXS hybrid balance impedance, the impedance of the
interconnecting copper line and the telephone line termination impedance.
This affects the DFXS transhybrid balance.
•
The circuit levels of both the DFXS and the telephone.