User guide

20 RealCT Direct API Developer Guide
Chapter 2: Digit Handling
Defining R2 Signals
COs outside the United States, Canada, and Japan generally
use R2 digits to establish calls. Digital telephony systems in
these countries also use R2 digits to communicate with the CO.
R2 signals, which are also called R2-MF, involve a compelled
handshake between the transmitting and receiving ends to
confirm that each end receives the signal. Once the call is
established, callers use DTMF digits to navigate telephony
applications.
Although R2 signals are often used over E1 lines, there is no
relationship between the R2-CCITT line protocols used by E1
lines and the R2 signaling.
Timing the Signals
In R2 inter-register signaling, each signal from the transmitting
end requires an acknowledgment from the receiving end. The
handshaking process ensures that both ends receive the signal
properly.
The end that originates the call has outgoing registers and
sends forward inter-register signals. The end that receives the
call has incoming registers and sends backward inter-register
signals. A compelled signal between outgoing and incoming
registers uses the following steps:
1. The outgoing R2 register starts transmitting a forward
signal.
2. The incoming R2 register sends a backward
acknowledgment signal as soon as it receives the forward
signal.
3. The outgoing R2 register stops sending the forward signal
when it receives the acknowledgment.
4. The incoming R2 register stops sending the
acknowledgment when it stops receiving the forward signal.
5. The outgoing register transmits the next R2 signal when it
stops receiving the previous backward acknowledgment
signal.
Figure 5 shows the sequence of events in R2 compelled
signaling.