Although certain signalling systems may have the capability to
provide an indication when an individual circuit is faulty, in
order to maintain the required availability of the public network,
it is considered necessary to provide alarm facilities to alert
maintenance staff when a group of circuits provided by a multiplex
transmission system is faulty.
An alarm indication can be initiated on failure of a FDM system
by means of pilot supervision. On failure of a PCM system, and
alarm indication initiated at both ends by the loss of frame
alignment (or multiframe alignment as appropriate) ,. These
failure indicators provide the means whereby the faulty circuits
can be removed from service automatically and, when the fault
condition no longer exists, be restored automatically by the
switching control of an international exchange (see § 1.4 below).
Additionally, the existence of such failure indications allow an
end-to-end indication of circuit availability which is a
prerequisite to the operation of Signalling System No. 7 without a
per call continuity check [see Recommendations Q.724 (TUP) and
Where transmission links comprise several transmission systems
in tandem, the protection against the effects of faulty
transmission on groups of circuits can only be maintained if the
primary multiplex structure is maintained from end-to-end together
with a transparency of alarm indications. In other cases the
provisions of §§ 2 and 3 below apply.
Following a transmission failure a number of specific signalling
actions are required to be carried out by the switching control of
an international exchange. These actions are designed to:
a) prevent failure of new call attempts;
b) provide appropriate failure indications on established
c) provide a means of releasing circuit connections beyond the
point of transmission failure. Paragraph 4 below details the
actions to be taken for circuits employing Signalling Systems Nos.
5, 6 and 7 (TUP and ISUP). For circuits employing Signalling System
R2, Recommendation Q.416 details the actions to be taken.
The recognition time used by the international exchange to
validate the alarm ON/alarm OFF states shall be 20 ± 10
milliseconds. The recognition time is defined as the duration that
signals representing the alarm ON/OFF states must be present at the
input of the exchange terminal equipment.
Following recognition of the alarm ON or alarm OFF states the
exchange shall carry out the actions detailed in § 4.