Configuring 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.
In R2 signaling, the calling side acts as the originating PBX, and the called side acts as the terminating PBX. Signaling sent by the originating PBX is called forward signaling, and signaling sent by the terminating PBX is called backward signaling, as shown in Figure 15.
Figure 15: R2 signaling elements
R2 signaling includes digital line signaling and interregister signaling. Digital line signaling conveys status information about E1 trunks to describe whether the trunks are seized, released, or blocked. Interregister signaling transmits and requests calling and called numbers.
ITU-T digital line signaling
Digital line signaling monitors the state of a trunk and controls calls. It can identify the following state changes:
The calling party goes off-hook and seizes the line.
The called party goes off-hook and answers the call.
The calling party releases the call.
The called party releases the call.
Digital line signaling sets the line to be idle or seized according to the state of the trunk. This signaling is transmitted through timeslot 16. The two transmission directions of each line have four bits (A, B, C and D) as flag bits, with C and D bits fixed to 01. The forward line signaling adopts af and bf bits, and the backward line signaling adopts ab and bb bits, as shown in Table 2.
Table 2: Line signaling bit description
Bit | Description | Value = 0 | Value = 1 |
|---|---|---|---|
af | Identifies the state of the originating device and indicates the state of the calling line. | Off-hook, seized | On-hook (idle) |
bf | Indicates a fault state from the originating side to the terminating side. | Normal | Faulty |
ab | Indicates the state of the called line (on-hook or off-hook). | Off-hook by called party | On-hook by called party |
bb | Indicates the state of the terminating device (idle or seized). | Idle | Seized or blocked |
Table 3: State code of line signaling
State of the circuit | Signaling code | |||
|---|---|---|---|---|
Forward | Backward | |||
af | bf | ab | bb | |
Idle or release | 1 | 0 | 1 | 0 |
Seized | 0 | 0 | 1 | 0 |
Seizure-ack | 0 | 0 | 1 | 1 |
Answer | 0 | 0 | 0 | 1 |
Clear-back | 0 | 0 | 1 | 1 |
Clear-forward | 1 | 0 | 0/1 | 1 |
Blocked | 1 | 0 | 1 | 1 |
Unblocked | 1 | 0 | 1 | 0 |
R2 digital line signaling exchange includes the following processes:
Call establishment.
When the circuit is idle, the originating side sends a forward seizure signal (00) to the terminating side, and the terminating side sends back a seizure acknowledgment signal (11). Then the circuit is seized, and interregister signaling exchange begins. When the called party picks up the phone, the terminating side sends a backward answer signal (01). After the originating side recognizes the received signal, it establishes the call.
Figure 16: Call establishment
Call release at the originating side.
The originating side sends a clear-forward signal 10. When the terminating side recognizes the clear-forward signal, it sends a backward signal 10. When the originating side recognizes the backward signal 10, it releases the call.
Figure 17: Call release at originating side
Call release at the terminating side.
The terminating side sends a clear-back signal 11 to the originating side, and the originating side sends back a clear-forward signal 10. When the terminating side recognizes the forward signal 10, it sends a release-guard signal 10. When the originating side recognizes the release-guard signal 10, it releases the call.
Figure 18: Call release at terminating side
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, the circuit is blocked. In this case, the originating side still sends the forward signal 10 to indicate that the line is idle. When the terminating side unblocks the circuit, it sends a backward signal 10 to indicate that the line is idle. The originating side maintains the forward signal 10 and unblocks the local-end circuit for the next call.
ITU-T interregister signaling
Interregister signaling transmits and requests calling and called numbers. It adopts the multifrequency compelled (MFC) mode and includes forward signaling and backward signaling. Forward signaling exchange includes Group I and Group II, and backward signaling exchange includes Group A and Group B. When the originating side recognizes the seizure acknowledgment signal, interregister signaling 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 4: Forward Group I signals
Designation | Definition |
|---|---|
I-1 through I-10 | Digits 1, 2, 3, 4, 5, 6, 7, 8, 9, and 0, responsible for sending number information to the terminating side. |
I-11 | Spare for national use. |
I-12 | Request refused. |
I-13 | Connected to tested device. |
I-14 | Spare for national use. |
I-15 | Address identification terminator and pulse terminator (used in international calls). |
Group A backward signals—Control signals used for controlling and acknowledging Group I forward signals.
Table 5: Group A backward signals
Designation | Definition |
|---|---|
A-1 | Send next digit. |
A-2 | Send last but one digit. |
A-3 | Address-complete; changeover to reception of Group B signals. |
A-4 | Congestion in the national network. |
A-5 | Send calling party's category. |
A-6 | Address-complete, charge, and set up speech conditions. |
A-7 | Send last but two digits. |
A-8 | Send last but three digits. |
A-9 | Spare for national use. |
A-10 | Spare for national use. |
A-11 | Send country code indicator. |
A-12 | Send language or discrimination digit. |
A-13 | Send nature of circuit. |
A-14 | Request for information on use of an echo canceller (is an incoming half-echo suppressor required?). |
A-15 | Congestion in an international exchange. Terminate interregister signaling interaction. |
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 6: Group II forward signals
Designation | Definition |
|---|---|
II-1 | Subscriber without priority. |
II-2 | Subscriber with priority. |
II-3 | Maintenance equipment. |
II-4 | Spare for national use. |
II-5 | Operator. |
II-6 | Data transmission. |
II-7 | Subscriber (or operator without forward transfer facility), for international use |
II-8 | Data transmission (for international use). |
II-9 | Subscriber with priority (for international use). |
II-10 | Operator with forward transfer facility (for international aid use). |
II-11 through II-15 | Spare for national use. |
Group B backward signals—Acknowledge Group II signals and indicate the status of the called party.
Table 7: Group B backward signals
Designation | Definition |
|---|---|
B-1 | Spare for national use. |
B-2 | Send special information tone. |
B-3 | Subscriber line busy. |
B-4 | Congestion. |
B-5 | Unallocated number. |
B-6 | Subscriber line free, charge. |
B-7 | Subscriber line free, no charge. |
B-8 | Subscriber line out of order. |
B-9 through B-15 | Spare for national use. |
Figure 19 shows the exchange process requesting calling party information, which is typical of R2 interregister signaling.
Figure 19: ITU-T R2 interregister signaling exchange process
