# EAM signaling

The protocol known as pulsed E and M (EAM) is defined by national specifications in a few countries. For compatibility with older analog equipment, this protocol exploits MFC-R2 compelled register signaling (as defined by the CCITT in Recommendations Q.441, Q.442) while using only one bit for line signaling.

The pulsed E and M protocol uses one bit in each direction for line signaling. The bit is pulsed. A signal is defined by the bit being flipped from its idle value, and being flipped back after a certain time. Two types of pulses are defined, long and short. They carry different meanings in the context of the current state of a call.

Different countries use different bits to implement line signaling, with different idle states.

This topic describes:

- Signaling states

- Register signaling

## Signaling states

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

State | Outbound | Direction | Inbound
--- | --- | --- | ---
Idle | Idle code |  | Idle code
Seizure | Short pulse |  | Idle code
Seizure acknowledged | Idle code |  | Short pulse (usually not necessary)
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.
Ringing | Idle code |  | Idle code
Answer - conversation state | Idle code |  | Short pulse
If the inbound side rejects the call, the outbound side clears forward by sending a long pulse. The inbound side acknowledges with another long pulse, and the line is back in the Idle state.
Clear forward | Long pulse |  | Idle code
Idle | Idle code |  | Long pulse
During conversation, the outbound protocol can receive billing pulses to signal that a unit of cost has been billed to the call. A billing pulse is always a short pulse.
Answer - conversation state | Idle code |  | Idle code
Billing pulses | Idle code |  | Short pulse
Answer - conversation state | Idle code |  | Idle code
Depending on which side hangs up the call first, a clear back signal or a clear forward signal is generated. If outbound sends a clear forward signal, inbound always acknowledges with a release guard signal. If inbound sends a clear back, outbound sends a clear forward. Depending on national specifications, inbound can still be required to acknowledge with a release guard. Idle follows.
Inbound hangs up first: Clear back | Idle code |  | Long pulse
Clear forward | Long pulse |  | Idle code
Release guard (in some countries) | Idle code |  | Long pulse
Idle | Idle code |  | Idle code
Outbound hangs up first: Clear forward | Long pulse |  | Idle code
Release guard | Idle code |  | Long pulse
Idle | Idle code |  | Idle code

## Register signaling

The pulsed E and M protocol uses the same multiple-frequency compelled scheme as the MFC-R2 protocol to perform register signaling.

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 is repeated.

Different kinds of information are transferred from the outbound to the inbound exchange in this way. The pulsed E and M 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 indicating if the call is to be billed or free

The pulsed E and M protocol implementation gives developers control over all of these features.