Nortel DMS-100 Series Maintenance Manual page 112

2-36 Functional overview
Log reports PM128 and PM181 indicate the overload condition in the OPM.
When call processing resumes, the system generates PM128 log with the
phrase LCM out of Overload.
Current extended memory LCM (XLCM) overload controls
The XLCM has 256 kbytes of memory. The XLCM has a specified number of
small, medium and large memory blocks, all of a fixed size. Domestic LCMs
and XLCMs use small and large memory blocks to receive external messages
and to send messages. Small memory blocks (SMB) are for utility purpose
uses like timer control blocks. Medium memory blocks (MMB) are for call
data blocks (CDB) use, which hold data associated with active lines.
In the current XLCM installation, overload is reported when the XLCM cannot
receive an external message, DMSX or Inter-Unit Communication (IUC).
This event occurs because the number of small or large memory blocks is not
enough. Some service degradation can appear before the OPM enters
overload.
The XLCM has four levels of throttling to prevent overload. Three levels work
according to the number of available SMBs, and conserve SMBs. The SMBs
are weighted to give terminating calls priority over originations.
The following describes four levels of throttling:
1. The XPM throttles messages to the XLCM a maximum of two in 50 ms
2. The XLCM appends the number of SMBs available for external messages.
3. The total number of SMBs available for external messages can be less
4. The total number of SMBs available for external messages can be less
For overload protection, the XLCM holds a reserve of small memory blocks.
These small memory blocks do not receive external messages. This limit
appears in the figure XLCM Overload Protection System. This condition
297-8361-550 Standard 04.02 December 2004
periods. This procedure helps control small peaks of very heavy traffic.
Sustained messaging at this rate can drive the XLCM into severe overload.
The XLCM appends these messages to each POTS origination message
and all messages that originate from P-phones. This number equals the
number of SMB in reserve subtracted from the total available SMBs.
When this number is less than 20, the XPM delays processing the
origination. This delay continues until this number returns to a minimum
of 20.
than 15. When this event occurs, the XLCM stops sending call processing
updates to the XLCM mate. This number equals the total available SMBs
minus the number of SMB reserve less than 15.
than 10. When this event occurs, the XLCM stops the scan of the BIC for
line scan changes. This number equals the total available SMBs minus the
number of SMB reserve less than 10.