home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Internet Standards
/
CD2.mdf
/
ccitt
/
1992
/
q
/
q32.asc
< prev
next >
Wrap
Text File
|
1991-12-30
|
4KB
|
81 lines
Recommendation Q.32
REDUCTION OF THE RISK OF INSTABILITY BY SWITCHING MEANS
For any connection between two-wire terminations, the transmission plan
admits a certain risk of instability. In an international connection,
Recommendation G.122 defines for each national network its responsibility in this respect.
It is recognized in section 2 of that Recommendation that during certain
phases of the call, the risk of instability could in certain circumstances become
excessive; this refers in particular to conditions other than that of an
established connection, viz. during set-up, clear-down and changes in a connection.
Appropriate precautions must then be taken by the switching services.
Techniques applicable to analogue exchanges which will afford a reduction
of the risk of instability for a national network have been shown in earlier
versions of Recommendation Q.32 (Red Book 1985 and earlier). For digital exchanges
these methods are as a rule not equally suitable, however, it should be noted that,
with today's digital networks giving 4-wire transmission down to the local
exchanges and with corresponding terminating losses, the transmission plan may often
not require extra loss during setting-up, etc., conditions.
Recommendation G.121, section 6.2 calls for a sum of losses round the a-t-b path of at least 6 dB; calculating according to Recommendation G.122,
section 2.2, this would be some four times the standard deviation, corresponding to a
risk of about 3 in 10 000. (The six calls per thousand risk called for in
Recommendation G.122 corresponds to about 3.25 times the standard deviation.) The switching
services thus only need to maintain this minimum loss in cases where it is
reduced in the conditions mentioned.
The use of a restricted value of loss (rather than total interruption of
the 4-wire loop) allows the passage of information tones or recorded
announcements or of communication with an operator, and of national use for non- chargeable
calls. Although as a rule digital pads are deprecated, the reasons for this are
all concerned with their presence in an established connection, and do not apply to
their use for the present purpose.
1.4 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 calls;
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.
1.5 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 the 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.