# MFC-R2 signaling

The MFC-R2 protocol is defined by the CCITT (now ITU) Recommendations Q.421-Q.442, (CCITT Blue Book Volume VI, Fascicle VI.4, Geneva 1989). Many country-specific variations exist. The MFC0 TCP is programmed by the parameters described in the following tables to implement the specifications of all supported countries and network operators.

Although E1 channel associated signaling (CAS) framing supports 4 signaling bits per direction, only 2 of them are used for R2 line signaling. Thus the signaling channels supporting the R2 line signaling protocol are referred to as Af and Bf in the forward direction, and Ab and Bb, in the backward direction. The forward channel indicates the condition of the outbound switch equipment and reflects the condition of the calling party's line. The backward channel indicates the condition of the called party's line (the inbound equipment).

The other bits in either direction (the C and D bits) usually have fixed values. However, their values may change from network to network.

This topic describes:

- Signaling states

- Register signaling

## Signaling states

The following table describes the signaling states of a typical call:

State | Outbound AfBf | Direction | Inbound AbBb
--- | --- | --- | ---
Idle | 10 |  | 10
Seizure | 00 |  | 10
Seizure acknowledged | 00 |  | 11
The outbound side starts to send the address information using in-band compelled MF tones. The inbound side completes the compelled sequence by accepting or rejecting the call, using the last backward compelled tone. If the call is accepted, the inbound side plays a ring tone on the line, and then signals that the call was answered by setting the Ab bit to 0.
Ringing | 00 |  | 11
Answer - conversation state | 00 |  | 01
If the inbound side rejects the call, the outbound side clears forward by setting the Af bit to 1. The inbound side goes back to idle by setting the Bb bit to 0.
Clear forward | 10 |  | 11
Idle | 10 |  | 10
During conversation, the outbound protocol can receive billing pulses to signal that a unit of cost has been billed to the call. The bit used to carry a billing pulse depends on national specifications.
Answer - conversation state | 00 |  | 01
Billing pulses | 00 |  | 11 or 00
Answer - conversation state | 00 |  | 01
Depending on which side hangs up the call first, a clear back signal or a clear forward signal, is generated. Depending on national specifications, there might be a period of time in which the inbound side holds a release guard state, which is the same as clear back but happens when the outbound side is already in the idle state. Idle follows.
Inbound hangs up first: Clear back | 00 |  | 11
Clear forward | 10 |  | 11
Idle | 10 |  | 10
Outbound hangs up first: Clear forward | 10 |  | 01
Release guard | 10 |  | 11
Idle | 10 |  | 10

## Register signaling

The MFC-R2 protocol uses a multi-frequency compelled scheme to perform register signaling (to exchange address information).

The outbound exchange starts by putting on the line a forward tone that represents the first digit of the called address. The inbound exchange detects the tone and answers with a backward tone, which acknowledges the previous forward tone and requests another digit. The inbound exchange can use different tones in the backward direction, each carrying a request for a different piece of information. The outbound exchange interprets the request and sends the appropriate digit.

When the outbound exchange detects the backward tone, it stops the current forward tone. When the inbound exchange detects the end of the forward tone, it stops its backward tone. When the outbound exchange detects the end of the backward tone, it starts the next tone, representing the next digit, and the cycle starts again.

Different kinds of information are transferred from the outbound to the inbound exchange in this way. The MFC-R2 protocol supports:

- DID digits (called party address)

- ANI digit (calling party address)

- Caller category (for instance, normal subscriber, pay phone, operator)

- Caller toll category (in some countries)

- The information if the call is to be billed or free

The NMS MFC-R2 protocol implementation gives developers control over all of these features.