Chapter 2. An introduction to MFC/R2

Table of Contents

Signal types in MFC/R2
Line signals for the digital version of MFC/R2 (defined in ITU-T Q.421)
Line signals for the analogue version of MFC/R2 (defined in ITU-T Q.411)
Inter-register signals (defined in ITU-T Q.441)
Where are the signals and supervisory tones generated, and how are they used?

This section provides and introduction to how the MFC/R2 protocol works. It is intended to help those not familiar with MFC/R2 who need enough understanding to debug a new installation which is not working properly. If you need a more detailed description of the protocol you should obtain copies of the ITU-T Q.400 series specifications, and any national specifications which might be applicable. Although the ITU defined MFC/R2 internationally, few countries use the protocol exactly as defined by the ITU. It is generally necessary to check national specifications for an exact definition of the local MFC/R2 variant.

Signal types in MFC/R2

MFC/R2 was originally used to provide register to register signaling over analogue copper wiring at a higher speed than had been possible with pulse dialing. To do this, MFC/R2 continued to use DC voltage conditions on the line to indicate things like seize and clear, as older systems had done. However, it replaced the decadic pulsed digits of older systems with dual tone signals. The tone generators and decoders are only needed during call setup, and may be reallocated to other circuits once a call is established. Monitoring for the end of the call is handled by the line signals alone. This was critically important when MFC/R2 was first developed, as the tone processing hardware was very expensive, and needed to service as many calls as possible.

As time passed there was a need to operate MFC/R2 over analogue circuits which had no DC continuity. A 3825Hz tone was introduced for this purpose. The line signal was encoded as the prescence or abscence of this tone on the wires. 3825Hz may be considered an out-of-band tone on these older circuits. When E1 circuits were introduced, MFC/R2 was adapted to work over them. Thus, today there are both analogue and digital versions of MFC/R2. The analogue versions are now rarely used. The digital version, over E1s, is still widely used.

MFC/R2 channels may be configured for only incoming calls or only outgoing calls. However, most national variants permit each channel to handle both incoming and outgoing calls. The protocol does not avoid collisions, but can gracefully detect a collision and reallocate a call to another channel.

The terms forwards and backwards are heavily used in descriptions of MFC/R2. Forwards is the direction from the calling party to the called party. Backwards is from the called party to the calling party.

Line signals for the digital version of MFC/R2 (defined in ITU-T Q.421)

The MFC/R2 digital line signals are the ABCD bits of the channel associated signaling (CAS) in timeslot 16 of an E1. They represent the states of the line, and are similar to the states of an analog line. In general, only bits A and B are used. In most systems bits C and D are set to fixed values, and never change. There are some national variants where bit C or D may be used for metering pulses.

Table 2.1. Forward line signals, digital version

CAS bitMeaningValues
ALine status1=on hook, 0=off hook
BCondition1=failure, 0=normal
CFixedAlways 0
DFixedAlways 1

Table 2.2. Backward line signals for digital MFC/R2

CAS bitMeaningValues
ALine status1=on hook, 0=off hook
BCondition1=seized, 0=idle
CFixedAlways 0
DFixedAlways 1

Table 2.3. ITU-T Q.421/Table 1

Circuit stateForward ABBackward AB
Seizure acknowledged0011
Clear-forward (before clear-back)1001
Clear-forward (after clear-back)1011

Charging signals (line signaling with metering, Q.400 Series Supplement No.6)

During the conversational phase of a call, some countries send metering pulses from the called switch to the calling switch. These are encoded as pulses of the line signals, typically pulsing backwards bit A. To avoid confusion with clear-back, a new table was defined in Supplement No.6 of the ITU specifications. This replaces the clear-back state with a forced-release state.

Table 2.4. ITU-T Q.400 series supplement No.6/Table 1

Circuit stateForward ABBackward AB
Seizure acknowledged/meter0011
Clear-forward (forced release)1000
Clear-forward (before clear-back)1001
Clear-forward (after clear-back)1011
Forced release0000

Line signals for the analogue version of MFC/R2 (defined in ITU-T Q.411)

The analogue line version of MFC/R2 is rarely used these days, and will not be covered in detail. It uses only a single line signaling bit. This was originally encoded by DC conditions on the line. Later it was encoded as the prescence or abscence of a 3825Hz tone on the line. This frequency carries well enough on even poor quality lines to function properly on a single link in the chain, and allowed paths without response down to DC to be used. If passed through an E1 link without translation, this bit is usually carried in the A bit of the CAS bits. The remaining bits (B, C and D) are fixed, so the line signals are represented as 1XXX and 0XXX.

Inter-register signals (defined in ITU-T Q.441)

The inter-register, or inter-switch, signals in MFC/R2 signaling are encoded as the prescence of 2, and only 2, out of 6 specific tones, spaced at 120Hz intervals. Two sets of tones are defined - one for forward signals, and one for backward signals. There are 15 combinations of 2 out of 6 tones, so there are 10 signals for the digits 0 to 9, and 5 additional signals available for supervisory purposes.

Some of the Bell system signaling schemes use similar 2 out of 6 tones signals. However, the actual frequencies used are different. Also, the Bell system uses only one set of 6 frequencies. MFC/R2 uses a separate set of frequencies for the forward and backwards directions.

The inter-register signals are send in-band. They may pass transparently through several nodes in the network between the two terminating switches. The signals are arranged in groups. When a call begins the calling end uses group I signals, and the called end uses group A. The called end may tell the calling end to switch to using group II and group B signals, or to switch back to group A. In some countries there are also groups III and C, used for caller number transfer. Groups III and C do not exist in the ITU specifications.

MFC/R2 uses a system called compelled signaling. To ensure the sending end never sends signals too fast, each signal from the sending end results in an acknowledgement from the receiving end. The sending end is instructed signal by signal what it should send next - a dialed digit, a digit of caller ID, etc.

Table 2.5. ITU-T Q.441/Table 6 - group I forward signals

1I-1Digit 1 (Language: French, if first signal sent in intl. link)
2I-2Digit 2 (Language: English, if first signal sent intl. link)
3I-3Digit 3 (Language: German, if first signal sent in intl. link)
4I-4Digit 4 (Language: Russian, if first signal sent in intl. link)
5I-5Digit 5 (Language: Spanish, if first signal sent in intl. link)
6I-6Digit 6 (Language: Spare, if first signal sent in intl. link)
7I-7Digit 7 (Language: Spare, if first signal sent in intl. link)
8I-8Digit 8 (Language: Spare, if first signal sent in intl. link)
9I-9Digit 9 (Discriminating digit, if first signal sent in intl. link)
10I-10Digit 0 (Discriminating digit, if first signal sent in intl. link)
11I-11Country code indicator, outgoing half-echo suppressor required
12I-12Country code indicator, no echo suppressor required
13I-13Test call indicator (call by automatic test equipment)
14I-14Country code indicator, outgoing half-echo suppressor inserted
15I-15Signal is not used

Table 2.6. ITU-T Q.441/Table 7 - group II forward signals

1II-1Subscriber without priority
2II-2Subscriber with priority
3II-3Maintenance equipment
6II-6Data trannsmission
7II-7Subscriber (or operator without forward transfer facility)
8II-8Data transmission
9II-9Subscriber with priority
10II-10Operator with forward transfer facility
13II-13Spare, for national use

Table 2.7. ITU-T Q.441/Table 8 - group A backward signals

1A-1Send next digit (n+1)
2A-2Send last but one digit (n-1)
3A-3Address-complete, changeover to reception of Group B signals
4A-4Congestion in the national network
5A-5Send calling party's category
6A-6Address-complete, charge, set-up speech conditions
7A-7Send last but two digit (n-2)
8A-8Send last but three digit (n-3)
9A-9Spare, for National use
11A-11Send country code indicator
12A-12Send language or discrimination digit
13A-13Send nature of circuit
14A-14Request for information on use of an echo suppressor
15A-15Congestion in an international exchange or at its output

Table 2.8. ITU-T Q.441/Table 9 - group B backward signals

1B-1Spare, for national use
2B-2Send special information tone
3B-3Subscriber's line busy
4B-4Congestion (after changeover from Group A to B)
5B-5Unallocated number
6B-6Subscriber's line free, charge
7B-7Subscriber's line free, no charge
8B-8Subscriber's line out of order
12B-12Spare, for National use