Table of Contents
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.
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.
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 bit | Meaning | Values |
---|---|---|
A | Line status | 1=on hook, 0=off hook |
B | Condition | 1=failure, 0=normal |
C | Fixed | Always 0 |
D | Fixed | Always 1 |
Table 2.2. Backward line signals for digital MFC/R2
CAS bit | Meaning | Values |
---|---|---|
A | Line status | 1=on hook, 0=off hook |
B | Condition | 1=seized, 0=idle |
C | Fixed | Always 0 |
D | Fixed | Always 1 |
Table 2.3. ITU-T Q.421/Table 1
Circuit state | Forward AB | Backward AB |
---|---|---|
Idle/Released | 10 | 10 |
Seized | 00 | 10 |
Seizure acknowledged | 00 | 11 |
Answered | 00 | 01 |
Clear-back | 00 | 11 |
Clear-forward (before clear-back) | 10 | 01 |
Clear-forward (after clear-back) | 10 | 11 |
Blocked | 10 | 11 |
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.
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.
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
MF | Designation | Meaning |
---|---|---|
1 | I-1 | Digit 1 (Language: French, if first signal sent in intl. link) |
2 | I-2 | Digit 2 (Language: English, if first signal sent intl. link) |
3 | I-3 | Digit 3 (Language: German, if first signal sent in intl. link) |
4 | I-4 | Digit 4 (Language: Russian, if first signal sent in intl. link) |
5 | I-5 | Digit 5 (Language: Spanish, if first signal sent in intl. link) |
6 | I-6 | Digit 6 (Language: Spare, if first signal sent in intl. link) |
7 | I-7 | Digit 7 (Language: Spare, if first signal sent in intl. link) |
8 | I-8 | Digit 8 (Language: Spare, if first signal sent in intl. link) |
9 | I-9 | Digit 9 (Discriminating digit, if first signal sent in intl. link) |
10 | I-10 | Digit 0 (Discriminating digit, if first signal sent in intl. link) |
11 | I-11 | Country code indicator, outgoing half-echo suppressor required |
12 | I-12 | Country code indicator, no echo suppressor required |
13 | I-13 | Test call indicator (call by automatic test equipment) |
14 | I-14 | Country code indicator, outgoing half-echo suppressor inserted |
15 | I-15 | Signal is not used |
Table 2.6. ITU-T Q.441/Table 7 - group II forward signals
MF | Designation | Meaning |
---|---|---|
1 | II-1 | Subscriber without priority |
2 | II-2 | Subscriber with priority |
3 | II-3 | Maintenance equipment |
4 | II-4 | Spare |
5 | II-5 | Operator |
6 | II-6 | Data trannsmission |
7 | II-7 | Subscriber (or operator without forward transfer facility) |
8 | II-8 | Data transmission |
9 | II-9 | Subscriber with priority |
10 | II-10 | Operator with forward transfer facility |
11 | II-11 | |
12 | II-12 | |
13 | II-13 | Spare, for national use |
14 | II-14 | |
15 | II-15 |
Table 2.7. ITU-T Q.441/Table 8 - group A backward signals
MF | Designation | Meaning |
---|---|---|
1 | A-1 | Send next digit (n+1) |
2 | A-2 | Send last but one digit (n-1) |
3 | A-3 | Address-complete, changeover to reception of Group B signals |
4 | A-4 | Congestion in the national network |
5 | A-5 | Send calling party's category |
6 | A-6 | Address-complete, charge, set-up speech conditions |
7 | A-7 | Send last but two digit (n-2) |
8 | A-8 | Send last but three digit (n-3) |
9 | A-9 | Spare, for National use |
10 | A-10 | |
11 | A-11 | Send country code indicator |
12 | A-12 | Send language or discrimination digit |
13 | A-13 | Send nature of circuit |
14 | A-14 | Request for information on use of an echo suppressor |
15 | A-15 | Congestion in an international exchange or at its output |
Table 2.8. ITU-T Q.441/Table 9 - group B backward signals
MF | Designation | Meaning |
---|---|---|
1 | B-1 | Spare, for national use |
2 | B-2 | Send special information tone |
3 | B-3 | Subscriber's line busy |
4 | B-4 | Congestion (after changeover from Group A to B) |
5 | B-5 | Unallocated number |
6 | B-6 | Subscriber's line free, charge |
7 | B-7 | Subscriber's line free, no charge |
8 | B-8 | Subscriber's line out of order |
9 | B-9 | |
10 | B-10 | |
11 | B-11 | |
12 | B-12 | Spare, for National use |
13 | B-13 | |
14 | B-14 | |
15 | B-15 |