3
Telephone Line
These modems may be connected to the following circuits:
•
Direct exchange lines of the public switched telephone network, but not shared service or 1 plus 1 carrier
systems. The line should provide either loop disconnect or multifrequency signalling.
•
Private speechband circuits provided by British Telecommunications, Hull City Council or Mercury
Communications Limited.
•
They may also be connected as an extension to a compatible PABX, although there is no guarantee of
correct working in all circumstances.
This modem is suitable for connection to BT circuits with signalling at a nominal frequency of 2280 Hz and
may be connected to multipoint or point-to-point circuits. The apparatus does not require signalling or
otherwise use the frequency range 0–200 Hz.
No d.c. interaction is intended between the modem and the B.T. network.
This apparatus may be directly connected to a speechband circuit or connected to relevant branch system for
speechband circuits.
Receive-to-Transmit Loss (RTL)
The RTL measurement provides an indication of the signal attenuation between the receive pair and transmit
pair of a four-wire circuit. The RTL number for the 3900 Series modems is 75.
The sum of the RTL numbers of terminal apparatus at opposite ends of a particular point-to-point circuit shall
be equal to or greater than 28; and in the case of apparatus permitted connection before 5 August 1984 it shall
be assumed for the purpose of this calculation to have an RTL number of 14 unless it has been tested and
specially allocated a number other than this.
Apparatus having an RTL number greater than 47 must be used to terminate the distant end of this circuit.
Failure to do so is a breach of the conditions of approval of this apparatus.
Connection Arrangements
If any other apparatus, including cable or wiring, is connected between the apparatus and the point of
connection to any speechband circuit, then all that other apparatus shall comply with the following:
1. The overall transmission characteristics of all that other apparatus shall be such as to introduce no
material effect upon the electrical conditions presented to one another by the apparatus and the
speechband circuit; and
2. All that other apparatus shall comprise only:
(i) apparatus approved for the purpose of connection between the apparatus and a speechband circuit;
and
(ii) cable or wiring complying with a code of practice for the installation of equipment covered by this
part of BS6328 or such other requirements as may be applicable.
