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.