There are mainly four data components, which are working together with Alert SC.

1)      Messages-Waiting (MW)

2)      Messages-Waiting-Indication (MWI)

3)      Messages-Waiting-Data (MWD)

4)      Mobile-Station-Not-Reachable-Flag (MNRF)

Messages-Waiting (MW): Service element that makes a PLMN store information

(Messages-Waiting-Indication), listing those SCs that have made unsuccessful short message delivery

attempts to MSs in that PLMN.

Messages-Waiting-Indication (MWI): Data to be stored in the HLR and VLR with which an MS is associated, indicating that there is one or more messages waiting in a set of SCs to be delivered to the MS (due to unsuccessful delivery attempt(s)).

Messages-Waiting-Data (MWD): A part of the MWI to be stored in the HLR. MWD consists of an address list of the SCs which have messages waiting to be delivered to the MS.

Mobile-Station-Memory-Capacity-Exceeded-Flag (MCEF): A part of the MWI to be stored in the HLR. MCEF is a boolean parameter indicating if the address list of MWD contains one or more entries because an attempt to deliver a short message to an MS has failed with a cause of MS Memory Capacity Exceeded.

Mobile-Station-Not-Reachable-Flag (MNRF): The part of the MWI to be stored in the VLR and the HLR. MNRF is a boolean parameter indicating if the address list of MWD contains one or more entries because an attempt to deliver a short message to an MS has failed.

The MWD, MCEF and MNRF are updated in the following way:

1a) When a mobile terminated short message delivery fails due to the MS being temporarily absent (i.e. either IMSI DETACH flag is set or there is no response from the MS to a paging request), the SC address is inserted into the MWD list (if it is not already present) and the MNRF is set (if it is not already set).

1b) When a mobile terminated short message delivery fails due to the MS memory capacity being exceeded, the SC address is inserted into the MWD list (if it is not already present) and the MCEF is set (if it is not already set)

2a) When either the HLR or VLR detects that the MS (with a non-empty MWD and the MCEF clear in the HLR and the MNRF set in the VLR) has recovered operation (e.g. has responded to a paging request), the HLR directly or on request of the VLR will invoke operations to alert the SCs within the MWD . Once the Alert SC operations have been invoked, the MNRF is cleared. After each SC is alerted by the HLR, the address for that SC is deleted from the MWD. If the MCEF is set in the HLR, the HLR clears the MNRF, but does not invoke operations to alert the SCs within the MWD and data are not cleared from the MWD.

2b) When the HLR receives (via the MSC and the VLR) a notification that the MS (with a non-empty MWD and the MCEF set in the HLR) has memory capacity available to receive one or more short messages, the HLR will invoke operations to alert the SCs within the MWD . Once the Alert SC operations have been invoked, the MNRF is cleared in the VLR and the MCEF and MNRF are cleared in the HLR. After each SC is alerted by the HLR, the address for that SC is deleted from the MWD.

2c) When the HLR receives from the SMS-GMSC a notification that a short message has been successfully delivered from an SC to an MS for which the MCEF is set and the MWD are not empty, the HLR will invoke operations to alert other SCs within the MWD . Once the Alert SC operations have been invoked, the MCEF and MNRF are cleared in the HLR.

After each SC is alerted by the HLR, the address for that SC is deleted from the MWD. The SC which successfully delivered the message is also deleted from the MWD, if present.

2d) When the HLR receives (via the MSC and the VLR) a notification that the MS has memory capacity available to receive one or more short messages but the MCEF is not set and the MWD are empty, the HLR acknowledges the notification but does not alert any service centre.

Ref : GSM 03.40 standard Documents

Today we are going to discuss performace of operating system.

Trashing .
Thrashing happens when a hard drive has to move its heads over the swap area many times due to the high number of page faults. This happens when memory accesses are causing page faults as the memory is not located in main memory. The thrashing happens as memory pages are swapped out to disk only to be paged in again soon afterwards. Instead of memory access happening mainly in main memory, access is mainly to disk causing the processes to become slow as disk access is required for many memory pages and thus thrashing.
The OS can reduce the effects of thrashing and improve performance by choosing a more suitable replacement strategy for pages. Having a replacement strategy that does not cause memory areas to be written to disk that have not modified since been retrieved reduces thrashing. Using replacement strategies that allow little used rarely accessed pages to remain in memory while the most required pages are swapped in and out.

Thrashing

The theoretically optimal page replacement algorithm
Not recently used
First-in, first-out
Least recently used
We will discuss all the algorithms on coming weeks . Bye.