R2 MF signaling is an international signaling system on E-1 that transmits numeric and other information relating to the called and the calling subscribers' lines. R2 MF signals that control the call setup are referred to as interregister signals.
For each call, whether an inbound or an outbound call, the entity making the call is the "calling party" and the entity receiving the call is the "called party." For an inbound call, the calling party is eventually connected to a central office (CO) that connects to the customer premises equipment (CPE) of the called party. For this inbound call, the CO is referred to as the outgoing register and the CPE as the incoming register. Signals sent from the CO are forward signals; signals sent from the CPE are backward signals. The outgoing register (CO) sends forward interregister signals and receives backward interregister signals. The incoming register (CPE) receives forward interregister signals and sends backward interregister signals.
For an outbound call, the calling party's CPE connects to the CO that switches the outbound call to the called party. For an outbound call, the signaling described above is reversed. That is, signals sent from the CPE are forward signals and signals sent from the CO are backward signals.
In addition, address signals can provide the telephone number of the called party's line. For national traffic, the address signals can also provide the telephone number of a calling party's line for automatic number identification (ANI) applications.
R2 MF signals used for supervisory signaling on the network are called line signals.
For example, a calling party sends the first dialed digits to the local CO. The local CO uses these digits to determine the next CO in the connection chain. The next CO uses these first dialed digits to determine if they are the destination CO or if the call is to be switched to another CO. Eventually, the call reaches the destination CO. At the destination CO, the call is received and acknowledged. The destination CO eventually gets the last dialed digits, which exactly identify the called party.
The destination CO checks the called party's line to determine if it is clear, idle, busy, etc. The destination CO then generates and sends a B-tone backwards to the calling party to indicate the condition of the line. If the called party's line is free, the destination CO applies ringing to the line and sends ringback tones backwards to the calling party. When the called party answers the call, the calling party is switched through to the called party. If the called party's line is busy, or in some other condition, the destination CO sends this information backwards to the calling party via R2 tones. The local CO sends all information received from the destination CO to the calling party. When calls are made in countries that adhere to the full R2 protocol standard (for example, Belgium), the condition of the called party's line is always returned to the calling party.
When traversing networks, protocols, or countries, R2 tonal information can be lost. For example:
2.4.1. R2 MF Multifrequency CombinationsR2 MF signaling uses a multifrequency code system based on six fundamental frequencies in the forward direction (1380, 1500, 1620, 1740, 1860, and 1980 Hz) and a different set of six frequencies in the backward direction (1140, 1020, 900, 780, 660, and 540 Hz).
Each signal is composed of two of the six frequencies, providing 15 different tone combinations in each direction. Although R2 MF signaling is designed for operation on international networks with 15 multifrequency combinations in each direction, in national networks it can be used with a reduced number of signaling frequencies (for example, 10 multifrequency combinations). See the Voice API Library Reference for your operating system for lists of these signal tone pairs.
2.4.2. R2 MF Signal MeaningsThe 15 forward signals are classified into Group I forward signals and Group II forward signals. The 15 backward signals are classified into Group A backward signals and Group B backward signals.
In general, Group I forward signals and Group A backward signals are used to control call setup and to transfer address information between the outgoing register (CO) and the incoming register (CPE). The incoming register can signal the outgoing register to change over to Group II and Group B signaling.
Group II forward signals provide the calling party's category and Group B backward signals provide the condition of the called subscriber's line. Group B signals, also called B-tones, are typically the last tone in the protocol. For example, typically a B-3 tone indicates that the called party's line is busy.
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; this Group A backward signal may request additional information. Each signal requires a response from the other party. Each response becomes an acknowledgment of the event and an event to which the other party must respond.
Backward signals serve to indicate certain conditions encountered during call setup or to announce switchover to changed signaling, for example, forward signaling switching over to backward signaling. Changeover to Group II and Group B signaling allows information about the state of the called subscriber's line to be transferred.
The incoming register backward signals can request:
The incoming register backward signals can indicate:
The meaning of certain forward multifrequency combinations may also vary depending upon their position in the signaling sequence.
See the Voice API Library Reference for your operating system for more details and definitions of R2 MF signals.
2.4.3. R2 MF Compelled SignalingCompelled signaling protocols vary from country to country and are grouped into two main categories, both of which are supported by the Global Call software:
The Global Call software provides network device independence by shielding the application from protocol-specific details while giving access to each protocol's full range of features. The compelled signaling feature uses tone generation and detection IDs that are defined at system initialization.
R2 MF interregister signaling uses forward and backward compelled signaling. With compelled signaling, each signal is sent until a response (a return) signal is generated. This return signal is sent until responded to by the other party. Each signal stays on until the other party responds, thus compelling a response from the other party. Compelled signaling provides a balance between speed and reliability because it adapts its signaling speed to the working conditions with a minimum loss of reliability.
Compelled signaling must always begin with a Group I forward signal. For an inbound call:
The above scenario describes the CPE handling of an inbound call. The roles of the CO and the CPE are reversed when the CPE makes an outbound call.
Click here to contact Telecom Support Resources
Copyright 2002, Intel Corporation