| Parameter | Type | Length (octets) |
|---|---|---|
| Message type | F | 1 |
Blocking message: A message sent only for maintenance purposes to the exchange at the other end of a circuit, to cause an engaged condition of that circuit for subsequent calls outgoing from that exchange. When a circuit is used in the bothway m ode of operation, an exchange receiving the blocking message must be capable of accepting incoming calls on the concerned circuit unless it has also sent a blocking message. Under certain conditions, a blocking message is also a proper response to a reset circuit message.
Blocking Acknowledgement message: A message sent in response to a blocking message indicating that the circuit has been blocked.
Continuity Check Request message: A message sent by an exchange for a circuit on which a continuity check is to be performed, to the exchange at the other end of the circuit, requesting continuity checking equipment to be attached.
Loop back Acknowledgement message: A message sent in the backward direction in response to a continuity check request message indicating that a loop (or transceiver in the case of a 2-wire circuit) has been connected.
Overload Message: A message sent in the backward direction, on non-priority calls in response to an IAM, to invoke temporary trunk blocking of the circuit concerned when the exchange generating the message is subject to load control.
Reset Circuit message: A message sent to release a circuit when, due to memory mutilation or other causes, it is unknown whether for example, a release or a release complete message is appropriate. If, at the receiving end, the circuit is remotely blocked, reception of this message should cause that condition to be removed.
Unblocking message: A message sent to the exchange at the other end of a circuit to cancel, in that exchange, the engaged condition of the circuit caused by a previously sent blocking or circuit group blocking message.
Unblocking Acknowledgement Message: A message sent in response to an unblocking message indicating that the circuit has been unblocked.
Unequipped Circuit Identification Code message: A message sent from one exchange to another when it receives an unequipped circuit identification code.