The telephone exchanges are connected via E1 or T1 trunks which transport the speech from the calls. These trunks are divided into 64 kbit/s timeslots, and one timeslot can carry exactly one call. Each timeslot between two switches is uniquely identified by a Circuit Identification Code (CIC) that is included in the ISUP messages. The exchange uses this information along with the received signaling information (especially the Called Party Number) to determine which inbound CICs and outbound CICs should be connected together to provide an end to end speech path.
BASIC ISUP CALL FLOW:
1. When a call is placed to an out-of-switch number, the originating switch MSC1 transmits an ISUP initial address message (IAM) to reserve an idle trunk circuit from the originating switch to the destination switch . The IAM includes the originating point code, destination point code, circuit identification code,dialed digits and, optionally, the calling party number and name.
Note that the same signaling link(s) are used for the duration of the call unless a link failure condition forces a switch to use an alternate signaling link.
2.The destination switch examines the dialed number, determines that it serves the called party, and that the line is available for ringing. The destination switch rings the called party line and transmits an ISUP address complete message (ACM) to the originating switch to indicate that the remote end of the trunk circuit has been reserved.
3.When the called party picks up the phone, the destination switch terminates the ringing tone and transmits an ISUP answer message (ANM) to the originating switch.The ANM verifies that the calling party's line is connected to the reserved trunk and, if so, initiates billing.
4.If the calling party hangs-up first, the originating switch sends an ISUP release message (REL) to the destination switch . If the called party hangs up first, or if the line is busy, the destination switch sends an REL to the originating switch indicating the release cause (e.g., normal release or busy).
5.Upon receiving the REL, the destination switch transmits an ISUP release complete message (RLC) to the originating switch to acknowledge the release of the remote end. When the originating switch receives the RLC, it terminates the billing .
