| 摘要 |
PROBLEM TO BE SOLVED: To obtain an ISDN communication system having high line efficiency and reliability. SOLUTION: When a user performs calling communication to a center 3 side from a meter 13 side by using user/user information communication service, there are cases where a user exchange 9 of a network sometimes decides it as congestion, so that the meter 13 side may receive response corresponding to the condition of being in the midst of a conversation. For instance, an NCU 11 sets telegraphic message to a UUI(user/user information) of a SETUP message and transmits the SETUP message to the user side exchange 9 (step 204). While the exchange 9 decides it as congestion, the exchange 9 returns a REL 21 (release) message, where busy 28 or usable line unavailable channel 29 is set to a reason flag 27 corresponding to the condition of line being busy (step 206). Congestion is decided from the REL 21 (release) message received by the NCU 11, and a retry is performed after prescribed time that is shorter than a normal retry passes, (e.g., four seconds) (step 208). When the response corresponding to the condition of line being busy is received, the NCU 11 repeats retries for a prescribed number of times.
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