Initial address message format. [From CCITT Rec. Q.723, Figure 3/Q.723, page 23
H1 contains a signal code or identifies the format of more complex messages. For instance,
there are four types of address message identified by H0
= 0001, and H1 identifies the
type of message, such as the following:
Address Message Type
Initial address message
IAM with additional information
Subsequent address message
Subsequent address message with signal unit
Moving from right to left in Figure 14.11, after H1 we have the call party subfield
consisting of 6 bits. It identifies the language of the operator (Spanish, English, Russian,
etc.). For example, an English-speaking operator is coded 000010. It also differentiates
the calling subscriber from one with priority, a data call, or a test call. A data call is coded
001100 and a test call is coded 001101. Fifty of the 64 possible code groups are spare.
Continuing to the left in Figure 14.11, two bits are spare for international allocation.
Then there is the message indicator, where the first two bits, B and A, give the nature
of the address. This is information given in the forward direction indicating whether the
associated address or line identity is an international, national (significant), or subscriber
number. A subscriber number is coded 00, an international is coded 11, and a national
(significant) number is coded 10.
Bits D and C are the circuit indicator. The code 00 in this location indicates that there
is no satellite circuit in the connection. Remember that the number of space satellite relays
in a speech telephone connection is limited to one relay link through a satellite because
of propagation delay.
Bits F and E are significant for common-channel signaling systems such as CCIS, CCS
No. 6, and SS No. 7. The associated voice channel operates on a separate circuit. Does
this selected circuit for the call have continuity? The bit sequence FE is coded:
Bits F and E
Continuity check not required
Continuity check required on this circuit
Continuity check performed on previous circuit