This section briefly
describes the normal PLMN selection procedure performed by MS along
with modifications for network sharing.
Whenever MS is switched
on or has just returned to network coverage after being out of coverage, it
tries to select a network to register itself and receive network
services. Traditionally, each network broadcasts
its own PLMN-Id on common broadcast channels that are visible
to all MSs in that area.
The MS starts by scanning
for all the available radio networks in that area and creating an Available
PLMN list. It then refers to the Equivalent PLMN list and
Forbidden PLMN list (stored on its SIM) to prioritize
the Available PLMN list. Once this prioritized PLMN list
is available, the MS attempts registration with a PLMN
based on priority.
With network sharing
a single radio network is shared by more than one network operator. Information
about the availability of multiple operators must be propagated
to the MS so that it can correctly select a home or equivalent network
from all available networks.
To advertise availability
of multiple core network operators on a single radio network, broadcast
information has been modified to contain a list of PLMN-Ids
representing core network operators sharing the particular radio
network. The traditional PLMN-Id broadcast by
a radio network before network sharing support was available is
known as a "common PLMN Id".
An MS that does not
support network sharing (a non-supporting MS) sees
only the "common PLMN Id", while an MS
supporting network sharing (a supporting MS) is
able to see the list of PLMN-Ids along with "common
A supporting MS is
responsible for selecting an appropriate core network, while
the RNC and SGSN will help select an appropriate core network for
a non-supporting MS.