Applying Priorities in Selecting a System

Having located a system within its GEO the system selection process must decide whether to stay on the acquired system or whether to continue to look for one that is more preferred. For the first acquired system, this is a decision of whether to stay or keep looking and then, for situations of reselection, the question becomes a comparison of whether the newly acquired system is better than the one you are on.

Exactly what these criteria are and how they are applied can vary widely across implementations; however, all will exhibit general behavior in terms of:

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Roaming preference & home SID/NID/list.

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SID/NID lockout list

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Priority

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Equal priority selection criteria

The first two can be considered to be filters that are applied to the system table to modify set from which selection can be made. The latter two can be considered as choice criteria from the resultant set of selectable systems.

5.3.9.1 Roaming Preferences

Earlier in this section we noted that the roaming preference can affect system selection. The roaming preference setting can be a system table filter that reduces the systems available for selection. If the roaming preference has a setting of ‘Home Only’ then any system in the system table with a roaming indicator other than ‘Roaming Indicator Off’ becomes unavailable for selection.

If the device has a populated ‘home SID/NID list,’ then, any systems listed become identified as selectable non-roaming systems. Use of this list with a PRL has the following side effects:

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Systems in the list that are not in the PRL enter the selectable set regardless of the PREF_ONLY setting in the PRL properties

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Systems in the list that are listed in the PRL as roaming systems become selectable as non-roaming systems

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Systems in the list that are listed in the PRL as negative systems shed their barred status and become selectable

The ‘home SID/NID list’ is a legacy from the days prior to PRLs and is typically not used in conjunction with a PRL.

5.3.9.2 SID/NID Lockout List

Section 5.1.1 described how some devices may have a SID/NID lockout list. This list too is effectively a system table filter. Any system listed becomes unavailable for selection. Use of this list with a PRL has the following side effects:

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Systems in the list that are listed in the PRL are barred from selection regardless of their description in the PRL

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Systems in the list that are not in the PRL are barred from selection as an available system (see section 5.3.10) regardless of the PREF_ONLY setting in the PRL properties

The ‘SID/NID lockout list’ is a legacy from the days prior to PRLs and is typically not used in conjunction with a PRL.

5.3.9.3 Priority

Priority is normally the primary distinguishing factor for system selection within a GEO, and it is the major indicator of whether a system is most preferred or not. It should be remembered that the scope of priority is limited to within the GEO. The prime objective of system selection is to get to the most preferred system.

Each system record does not require a unique priority; the same priority can be applied to multiple systems. When systems have the same priority they are all share level in the pecking order of systems. This means that situations arise where priority does not

distinguish between systems. In most cases this is perfectly fine and means that any one is as equally selectable as the other; however, there are situations where priority alone may not be enough, such as:

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Two systems have the same preference and different roaming indications

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Selection would require selection between two GEOs 5.3.9.3.1 Equal Priority, Different Roaming Indicator

Where two systems in the same GEO are compared and the only discernable difference is the roaming indicator, then, in general, a system will be more preferred if it is not roaming.

5.3.9.3.2 GEO Changes & System Selection

In the situation where the acquired system and the current serving system are listed in different GEOs then there are normally some rules to follow to ensure that the ‘best system’ is used. Some typical rules that would be applied would be:

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In the case where only one of the systems is listed in the PRL, then it is normally the PRL system that would be preferred.

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Where both systems are listed in different GEOs and the roaming indicators are different then typically ‘roaming indicator off’ would be preferred over ‘roaming indicator on’ or ‘roaming indicator flashing’; and, ‘roaming indicator on’ would be preferred over ‘roaming indicator flashing.’

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When one of the systems is of a higher preference order in its GEO then it would probably be preferred over the other.

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All other things being equal, the system operating on the channel that appears first in the acquisition table may be preferred.

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If one system is CDMA and the other is AMPS, typically the CDMA system would be preferred over AMPS.

In document prl maintenance (Page 68-70)