6.2. R2MF Overview

• 6.2.1. Direct Dialing-In Service
• 6.2.2. R2MF Multifrequency Combinations
• 6.2.3. R2MF Signal Meanings
• The Incoming Register Backward Signals Can Request:
• The Incoming Register Backward Signals Can Indicate:
• 6.2.4. R2MF Compelled Signaling
• 6.2.5. Related Publications

R2MF signaling is an international signaling system that is used in Europe and Asia to permit the transmission of numerical and other information relating to the called and calling subscribers' lines.

R2MF signals that control the call set-up are referred to as interregister signals. In the case of the signals sent between the central office (CO) and the customer premises equipment (CPE), 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. See Figure 15 .

Figure 15. Forward and Backward Interregister Signals

The focus of this section is on the forward and backward interregister signals, and more specifically, the address signals, that provide the telephone number of the called subscriber's line. For national traffic, the address signals can also provide the telephone number of calling subscriber's line for automatic number identification (ANI) applications.

R2MF signals that are used for supervisory signaling on the network are called line signals. Line signals are beyond the scope of this document.

6.2.1. Direct Dialing-In Service

Since R2MF address signals can provide the telephone number of the called subscriber's line, the signals may be used by applications providing direct dialing-in (DDI) service, also known as direct inward dialing (DID), and dialed number identification service (DNIS).

DDI service allows an outside caller to dial an extension within a company without requiring an operator to transfer the call. The central office (CO) passes the last 2, 3, or 4 digits of the dialed number to the customer premises equipment (CPE) and the CPE completes the call.

6.2.2. R2MF Multifrequency Combinations

R2MF signaling uses a multifrequency code system based on six fundamental frequencies in the forward direction (1380, 1500, 1620, 1740, 1860, and 1980 Hz) and six frequencies in the backward direction (1140, 1020, 900, 780, 660, and 540 Hz).

Each signal is composed of two out of the six fundamental frequencies, which results in 15 different tone combinations in each direction. Although R2MF 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 following tables for a list of the signal tone pairs:

• Table 5. Forward Signals CCITT Signaling System R2MF Tones
• Table 6. Backward Signals CCITT Signaling System R2MF Tones

Table 5. Forward Signals CCITT Signaling System R2MF Tones

R2MF TONES			DIALOGIC INFORMATION
Tone Number	Tone Pair Frequencies (Hz)		Group I Define	Group II Define	Tone Detect. ID
1	1380	1500	SIGI_1	SIGII_1	101
2	1380	1620	SIGI_2	SIGII_2	102
3	1500	1620	SIGI_3	SIGII_3	103
4	1380	1740	SIGI_4	SIGII_4	104
5	1500	1740	SIGI_5	SIGII_5	105
6	1620	1740	SIGI_6	SIGII_6	106
7	1380	1860	SIGI_7	SIGII_7	107
8	1500	1860	SIGI_8	SIGII_8	108
9	1620	1860	SIGI_9	SIGII_9	109
10	1740	1860	SIGI_0	SIGII_0	110
11	1380	1980	SIGI_11	SIGII_11	111
12	1500	1980	SIGI_12	SIGII_12	112
13	1620	1980	SIGI_13	SIGII_13	113
14	1740	1980	SIGI_14	SIGII_14	114
15	1860	1980	SIGI_15	SIGII_15	115

Table 6. Backward Signals CCITT Signaling System R2MF Tones

R2MF TONES			DIALOGIC INFORMATION
Tone Number	Tone Pair Frequencies (Hz)		Group A Define	Group B Define
1	1140	1020	SIGA_1	SIGB_1
2	1140	900	SIGA_2	SIGB_2
3	1020	900	SIGA_3	SIGB_3
4	1140	780	SIGA_4	SIGB_4
5	1020	780	SIGA_5	SIGB_5
6	900	780	SIGA_6	SIGB_6
7	1140	660	SIGA_7	SIGB_7
8	1020	660	SIGA_8	SIGB_8
9	900	660	SIGA_9	SIGB_9
10	780	660	SIGA_0	SIGB_0
11	1140	540	SIGA_11	SIGB_11
12	1020	540	SIGA_12	SIGB_12
13	900	540	SIGA_13	SIGB_13
14	780	540	SIGA_14	SIGB_14
15	660	540	SIGA_15	SIGB_15

6.2.3. R2MF Signal Meanings

There are 2 groups of meanings associated with each set of signals. Group I meanings and Group II meanings are associated with the 15 forward signals. Group A meanings and Group B meanings are associated with the 15 backward signals. See Figure 16 .

Figure 16. Multiple Meanings for R2MF Signals

In general, Group I forward signals and Group A backward signals are used to control the call set-up 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 7 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 acknowledgement of the event and an event for the other party to respond to.

Backward signals serve to indicate certain conditions encountered during call set-up 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.

Table 7. 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.

The Incoming Register Backward Signals Can Request:

• Transmission of address

· Send next digit

· Send digit previous to last digit sent

· Send second digit previous to last digit sent

· Send third digit previous to last digit sent

• Category of the call (the nature and origin)

· National or international call

· Operator or subscriber

· Data transmission

· Maintenance or test call

• Whether or not the circuit includes a satellite link
• Country code and language for international calls
• Information on use of an echo suppressor

The Incoming Register Backward Signals Can Indicate:

• Address complete - send category of call
• Address complete - put call through
• International, national, or local congestion
• Condition of subscriber's line

· Send SIT to indicate long term unavailability

· Line busy

· Unallocated number

· Line free - charge on answer

· Line free - no charge on answer (only for special destinations)

· Line out of order

NOTE:

The meaning of certain forward multifrequency combinations may also vary depending upon their position in the signaling sequence. For example, with terminal calls the first forward signal transmitted in international working is a language or discriminating digit (signals I-1 through I-10). When the same signal is sent as other than the first signal, it usually means a numerical digit.

See the following tables for the signal meanings:

• Table 8. Meanings for R2MF Group I Forward Signals
• Table 9. Meanings for R2MF Group II Forward Signals
• Table 10. Meanings for R2MF Group A Backward Signals
• Table 11. Meanings for R2MF Group B Backward Signals

Table 8. Meanings for R2MF Group I Forward Signals

Tone Number	Dialogic Define	(A) Primary Meaning (B) Secondary Meaning
1	SIGI_1	(A) Digit 1  (B) Language digit-French
2	SIGI_2	(A) Digit 2  (B) Language digit-English
3	SIGI_3	(A) Digit 3  (B) Language digit-German
4	SIGI_4	(A) Digit 4  (B) Language digit-Russian
5	SIGI_5	(A) Digit 5  (B) Language digit-Spanish
6	SIGI_6	(A) Digit 6  (B) Spare (language digit)
7	SIGI_7	(A) Digit 7  (B) Spare (language digit)
8	SIGI_8	(A) Digit 8  (B) Spare (language digit)
9	SIGI_9	(A) Digit 9  (B) Spare (discriminating digit)
10	SIGI_0	(A) Digit 0  (B) Discriminating digit
11	SIGI_11	(A) Access to incoming operator (Code 11)  (B) Country code indicator: outgoing half-echo suppressor required
12	SIGI_12	(A) Access to delay operator (code 12); request not accepted  (B) Country code indicator: no echo suppressor required
13	SIGI_13	(A) Access to test equipment (code 13); satellite link not included  (B) Test call indicator
14	SIGI_14	(A) Incoming half-echo suppressor required; satellite link included  (B) Country code indicator: outgoing half-echo suppressor inserted
15	SIGI_15	(A) End of pulsing (code 15); end of identification  (B) Signal not used

Table 9. Meanings for R2MF Group II Forward Signals

Tone Number	Dialogic Define	Meaning
1	SIGII_1	National: Subscriber without priority
2	SIGII_2	National: Subscriber with priority
3	SIGII_3	National: Maintenance equipment
4	SIGII_4	National: Spare
5	SIGII_5	National: Operator
6	SIGII_6	National: Data transmission
7	SIGII_7	International: Subscriber, operator, or maintenance equipment (without forward transfer)
8	SIGII_8	International: Data transmission
9	SIGII_9	International: Subscriber with priority
10	SIGII_0	International: Operator with forward transfer facility
11	SIGII_11	Spare for national use
12	SIGII_12	Spare for national use
13	SIGII_13	Spare for national use
14	SIGII_14	Spare for national use
15	SIGII_15	Spare for national use

Table 10. Meanings for R2MF Group A Backward Signals

Tone Number	Dialogic Define	Meaning
1	SIGA_1	Send next digit (n+1)
2	SIGA_2	Send last but one digit (n-1)
3	SIGA_3	Address complete; change to Group B signals; no change back
4	SIGA_4	Congestion in the national network
5	SIGA_5	Send calling party's category; change to Group II; can change back
6	SIGA_6	Address complete; charge; set-up speech conditions
7	SIGA_7	Send last but two digit (n-2)
8	SIGA_8	Send last but three digit (n-3)
9	SIGA_9	Spare for national use
10	SIGA_0	Spare for national use
11	SIGA_11	Send country code indicator
12	SIGA_12	Send language or discriminating digit
13	SIGA_13	Send nature of circuit (satellite link only)
14	SIGA_14	Request for information on use of an echo suppressor
15	SIGA_15	Congestion in an international exchange or at its output

Table 11. Meanings for R2MF Group B Backward Signals

Tone Number	Dialogic Define	Meaning
1	SIGB_1	Spare for national use
2	SIGB_2	Send special information tone to indicate long-term unavailability
3	SIGB_3	Subscriber line busy
4	SIGB_4	Congestion encountered after change to Group B
5	SIGB_5	Unallocated number
6	SIGB_6	Subscriber line free; charge on answer
7	SIGB_7	Subscriber line free; no charge (only for calls to special destinations)
8	SIGB_8	Subscriber line out of order
9	SIGB_9	Spare for national use
10	SIGB_0	Spare for national use
11	SIGB_11	Spare for national use
12	SIGB_12	Spare for national use
13	SIGB_13	Spare for national use
14	SIGB_14	Spare for national use
15	SIGB_15	Spare for national use

6.2.4. R2MF Compelled Signaling

R2MF interregister signaling uses forward and backward compelled signaling. Simply put, with compelled signaling each signal is sent until it is responded to by a return signal, which in turn 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.

Reliability and speed requirements for signaling systems are often in conflict, the faster the signaling, the more unreliable it is likely to be. 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.

The R2MF signal is composed of two significant events, tone-on and tone-off. Each tone event requires a response from the other party. Each response becomes an acknowledgement of the event and an event for the other party to respond to.

Compelled signaling must always begin with a Group I forward signal.

• The CO starts to send the first forward signal.
• As soon as the CPE recognizes the signal, it starts to send a backward signal that serves as an acknowledgement and at the same time has its own meaning.
• As soon as the CO recognizes the CPE acknowledging signal, it stops sending the forward signal.
• As soon as the CPE recognizes the end of the forward signal, it stops sending the backward signal.
• As soon as the CO recognizes the CPE end of the backward signal, it may start to send the next forward signal.

The CPE responds to a tone-on with a tone-on and to a tone-off with a tone-off. The CO responds to a tone-on with a tone-off and to a tone-off with a tone-on.

Refer to the following figures for more information:

• Figure 17. R2MF Compelled Signaling Cycle
• Figure 18. Example of R2MF Signals for 4-digit DDI Application

Figure 17. R2MF Compelled Signaling Cycle

Figure 18. Example of R2MF Signals for 4-digit DDI Application

6.2.5. Related Publications

For more information on R2MF signaling, you can refer to the following publications:

Specifications of Signaling Systems R1 and R2, International Telegraph and Telephone Consultative Committee (CCITT), Blue Book Volume VI, Fascicle VI.4, ISBN 92-61-03481-0

General Recommendations on Telephone Switching and Signaling, International Telegraph and Telephone Consultative Committee (CCITT), Blue Book Volume VI, Fascicle VI.1, ISBN 92-61-03451-9

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