 The dialed digits are transmitted from subscriber A to the local exchange, as explained in Section 2.3. When a set of digits is received by exchange A, it analyzes the dialed digits to determine to which Subscriber A Subscriber B Exchange Exchange IAM (Initial Address Message) SAM (Subsequent Address Message) B-number received, ACM (Address Complete Message) ringing signal to B Ringing tone B-subscriber lifts ANC (ANswer signal, Charge) the handset Conversation CBK (Clear BacK) B-subscriber hangs up A-subscriber hangs up CLF (CLear Forward) RLG (ReLease Guard) Figure 2.11 CCS between exchanges. The Telecommunications Network: An Overview direction it should route the call. From this information it looks up an address of the exchange to which it should send the signaling message for call connection. Then the exchange builds a data packet that contains the address of exchange B. This signaling message, called the initial address message (IAM), is then sent to exchange B. The remaining digits that did not fit into the IAM are transmitted in one or more subsequent address messages (SAMs). When all the digits that identify subscriber B are received by exchange B, it acknowledges this with an address complete message (ACM), to confirm that all digits have been successfully received. This message also contains information about whether the call is to be charged or not and if the sub- scriber is free or not. Exchange B transmits the ringing tone to subscriber A and the ringing signal to subscriber B, and telephone B rings. When subscriber B lifts the handset, an answer signal charge (ANC) is sent in order to activate charging. Exchange B switches off the ringing signal and ringing tone. Then both exchanges connect the speech channel through SO the conversation can start. When subscriber B hangs up, exchange B detects an on-hook condition and sends a CBK to exchange B. Exchange A responses with CLF signal. All exchanges on the line transmit the CLF mes- sage to the next one, and each receiving exchange acknowledges it wi