User's Manual

178 Voice API Programming Guide — June 2005
R2/MF Signaling
Figure 23. Multiple Meanings for R2/MF Signals
In general, Group I forward signals and Group A backward signals are used to control the call setup
and to transfer address information between the outgoing register (CO) and the incoming register
(CPE). The incoming register can then signal to the outgoing register to change over to the Group II
and Group B meanings.
Group II forward signals provide the calling party’s category, and Group B backward signals
provide the condition of the called subscriber’s line. For further information, see Table 23,
“Purpose of Signal Groups and Changeover in Meaning”, on page 178 describing the purpose of
the signal groups and the changeover in meanings.
Signaling must always begin with a Group I forward signal followed by a Group A backward signal
that serves to acknowledge the signal just received and also has its own meaning. Each signal then
requires a response from the other party. Each response becomes an acknowledgment of the event
and an event for the other party to respond to.
Backward signals serve to indicate certain conditions encountered during call setup or to announce
switch-over to changed meanings of subsequent backward signals. Changeover to Group II and
Group B meanings allows information about the state of the called subscriber’s line to be
transferred.
Forward
Signals
Backward
Signals
Group I
Meanings
Group II
Meanings
Group A
Meanings
Group B
Meanings
Table 23. Purpose of Signal Groups and Changeover in Meaning
Signal Purpose
Group I Group I signals control the call set-up and provide address information.
Group A Group A signals acknowledge Group I signals (see exception under signal A-5 below) for call
set-up, and can also request address and other information. Group A signals also control the
changeover to Group II and Group B meanings through the following signals:
A-3 Address Complete - Changeover to Reception of Group B Signals: Indicates the
address is complete and signals a changeover to Group II/B meanings; after signal A-3
is sent, signaling cannot change back to Group I/A meanings.
A-5 Send Calling Party’s Category: Requests transmission of a single Group II signal
providing the calling party’s category. Signal A-5 requests a Group II signal but does
not indicate changeover to Group B signals. When the Group II signal requested by
A-5 is received, it is acknowledged by a Group A signal; this is an exception to the rule
that Group A signals acknowledge Group I signals.
Group II Group II signals acknowledge signal A-3 or A-5 and provide the calling party category (national
or international call, operator or subscriber, data transmission, maintenance or test call).
Group B Group B signals acknowledge Group II signals and provide the condition of the called
subscriber’s line. Before Group B signals can be transmitted, the preceding backward signal
must have been A-3. Signals cannot change back to Group I/A.