MFS signaling

The MFS TCP implements the Spanish National CAS protocol, as specified by the Royal Decree 1562/1992, and amended by the Order 5309 of the Ministry of Industry, February 23, 1998.

Although E1 channel associated signaling (CAS) framing supports four signaling bits per direction, only one of them is used (in general) for MFS line signaling.

The signaling channels supporting the MFS line signaling protocol are referred to as Af in the forward direction and Ab 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 Bb bit might also be used, but only to convey billing pulses to the outbound equipment. The Bf bit is never used, and is always set to 1. The C and D bits are set to 0 and 1 respectively for both directions.

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

State

Outbound AfBf

Direction

Inbound AbBb

Idle

11

11

Seizure

01

11

Seizure acknowledged

01

01

The inbound side requests the address information, and the outbound side sends the data. This is accomplished by an in-band compelled sequence. 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

01

01

Answer - conversation state

01

11

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 Ab bit to 1.

Clear forward

11

01

Idle

11

11

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 is the Bb bit.

Answer - conversation state

01

11

Billing pulses

01

10

Answer - conversation state

01

11

Depending on which side hangs up the call first, a clear back signal or a clear forward signal is generated. There is then 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

01

01

Clear forward / release guard

11

01

Idle

11

11

Outbound hangs up first: Clear forward

11

11

Release guard

11

 

01

Idle

11

11


Register signaling is accomplished by a MF Socotel compelled scheme that is different from the R2 (CCITT Recommendation 441) compelled sequence. The basic concepts are summarized as follows:

The compelled sequence can only work if a very strict numbering scheme is in use in the network. The inbound side must know exactly the number of DID and ANI digits to expect in order to allow the compelled sequence to take place correctly. In Spain all telephone numbers are nine digits long. All extensions reachable through the national CAS protocol are five digits long.