The Cisco Unified Communications Manager allocates a global call identifier (GlobalCallID_callId) each time that a Cisco Unified IP Phone is taken off hook or a call is received from a gateway. The GlobalCallID_callId is allocated sequentially on a Cisco Unified Communications Manager server, independent of calls running on other call servers in the cluster. Cisco Unified Communications Manager writes the GlobalCallID_callId value to a disk file for every 1,000th call. When Cisco Unified Communications Manager restarts for any reason, it assigns the next 1000th number to the next GlobalCallID_callId.
For example, when a successful call gets made, the GlobalCallID_callId value in the CDR specifies 1001. For the next call, the GlobalCallID_callId value specifies 1002, and so on. When Cisco Unified Communications Manager restarts, the value for the next call in the CDR gets assigned 2001. The numbers continue sequentially from there until Cisco Unified Communications Manager restarts again. For the next restart, the GlobalCallID_callId value specifies 3001.
The maximum value that gets assigned to the GlobalCallID_callId is limited to 24 bits. When this limitation occurs, the GlobalCallID_callId value gets reset to 1.
The GlobalCallID_callIds in the CDR file may not be in sequential order in the CDR flat file. If a call with GlobalCallID_callId = 1 lasts longer than the call with GlobalCallID_callId = 2, then the CDR records for GlobalCallId_callId = 2 are written before GlobalCallId_callId = 1. GlobalCallID_callIds may be completely missing from the CDR flat file. If the first CDR record has GlobalCallID_callId = 1, and the second CDR has GlobalCallID_callId = 3, that does not mean that the CDR for GlobalCallID_callId = 2 is missing. GlobalCallID_callId = 2 did not meet the criteria to generate a CDR. The failure to generate a CDR can occur because while the first and third call were successful, the second call was never completed; or, GlobalCallID_callId = 2 could be part of a conference call. Each call leg in a conference call is assigned a GlobalCallID_callId that is overwritten in the conference GlobalCallID_callId. The original GlobalCallID_callId may not appear in the CDR flat file.
If the GlobalCallID_callId field is missing from the CDR record, CAR generates an error for that particular record. Additional information on CDR errors is available in the "Configuring CDR Error Reports" chapter of the CDR Analysis and Reporting Administration Guide.
For Cisco Unified Communications Manager Release 5.x and later releases, the value in the GlobalCallId CDR field survives over Cisco Unified Communications Manager restarts. In Release 4.x and earlier releases, even though the GlobalCallId field is time-based, the field gets reused under conditions of heavy traffic. Because of this behavior, problems can occur with customer billing applications and the ability of CAR to correlate CMRs with CDRs and to correlate conference call CDRs. For Release 5.x and later releases, GlobalCallId redesign ensures the field retains a unique value, at least for a certain number of days. Now, the last used globalCallId_callId value gets written to disk periodically (for every x number of calls). The value gets retrieved after a Cisco Unified Communications Manager restart, and the new globalCallId_callId value begins with this number plus x.