Introduction to R2 signaling

ITU-T recommendations Q.400 through Q.490 define the R2 signaling standards. However, the R2 signaling standards implemented in different countries and regions are ITU variants.

R2 signaling includes two categories: digital line signaling and interregister signaling. Digital line signaling conveys status information about E1 trunks to describe whether the trunks are occupied, released, or blocked. Interregister signaling transmits such information as address in multi-frequency compelled approach. Generally, the calling side serves as the originating PBX and the called side serves as the terminating PBX. Signals sent by the originating PBX are called forward signals and those sent by the terminating PBX are called backward signals, as shown in Figure 18.

Digital line signaling is responsible for changing call statuses and conditions of a line. It functions to identify and detect these four states: calling party goes off-hook and seizes the line, called party goes off-hook and answers the call, calling party releases the call, and called party releases the call. Accordingly, it sets the line to be idle or seized. This signaling is transmitted in the 16th multiframe timeslot of PCM system. The two transmission directions of each line have 4 bits (A, B, C and D) as flag bits, with C and D bits fixed to 01. Therefore, the forward line signaling adopts a_f and b_f bits and the backward line signaling adopts a_b and b_b bits, as shown in the following table:

The following are typical R2 digital line signaling interaction procedures:

1. Call establishment.

   When the circuit is idle, the originating side sends a forward seizure signal to the terminating side. The terminating side then sends back a seizure acknowledgement signal after it recognizes the seizure signal. At this time, the circuits of the both sides are seized, and they start interregister signaling exchange. When the called party picks up the phone, the terminating side sends a backward answer signal. After the originating side recognizes the received signal, it establishes the call.

   Figure 19: R2 digital line signaling – call establishment

   

2. Originating side releases the call.

   The originating side sends a clear-forward signal 10. When the terminating side recognizes the clear-forward signal, it sends a backward signal 10 (release guard signal or clear-forward acknowledgement signal). After the originating side recognizes the backward signal 10, it releases the circuit.

   Figure 20: R2 digital line signaling – originating side releases the call

   

3. Terminating side releases the call.

   The terminating side sends a clear-back signal 11. After the originating side receives the clear-back signal, it sends a clear-forward signal 10. After the terminating side recognizes the forward signal 10 sent by the originating side, it sends a backward signal 10. After the originating side recognizes the backward signal 10, it releases the circuit.

   Figure 21: R2 digital line signaling – terminating side releases the call

   

4. Line released by forced release signal.

   When the terminating side supports metering signals, the system might send a forced release signal 00 instead of a clear-back signal 11 to release the line. This is to avoid collision between the clear-back signal sent by the called party and the metering signal.

5. Blocking in idle state or during conversation.

   After the originating side receives a blocking signal 11 from the terminating side when the circuit is idle or during conversation, it sends a forward signal 10. At this time, the circuit is blocked. When the terminating side unblocks the circuit, it sends a backward signal 10 in the corresponding line to indicate that the line is idle. The originating side should maintain the forward signal 10 and unblock the local-end circuit for next call.

6. Troubleshooting in idle state.

   If the terminating side receives a forward signal 11 from the originating side to indicate a device fault when the circuit is idle, the terminating side sends a backward signal 11. Then, the circuit is in faulty state. When the device recovers, the originating side sends the forward signal 10, and the terminating side responds with the signal 10. At this time the circuit regains normal state.

7. Troubleshooting during conversation.

   After the terminating side receives a forward signal 11 from the originating side to indicate device fault during conversation, the terminating side releases the line backward. At the same time, it sends a backward signal 11. Then, the circuit is in faulty state. When the device recovers, the originating side sends a forward signal 10, and the terminating side sends back a signal 10. At this time, the circuit recovers.

Interregister signaling controls automatic connection of circuits. It adopts MFC mode and includes forward signaling and backward signaling. Forward signaling exchange includes Group I and Group II, while backward signaling exchange includes Group A and Group B. When the originating side recognizes the seizure acknowledgement signal, the register begins to send the first digit of the called number, and waits for the response of Group A signaling from the terminating side.

• Group I forward signals—Include connection control signals and digit signals.

  Table 7: Forward Group I signals

• Group A backward signals—Control signals used for controlling and acknowledging Group I forward signals.

  Table 8: Group A backward signals

• Group II forward signals—Identify the calling party category. The system looks at the calling party category to decide whether the calling party can perform forced release or break-in.

  Table 9: Group II forward signals

• Group B backward signals—Identify the state of the called party, and acknowledge Group II signals and control connection.

  Table 10: Group B backward signals

The following figure shows the exchange process requesting calling party information, which is typical of R2 interregister signaling.