Page 308 - From GMS to LTE

294 From GSM to LTE-Advanced Pro and 5G

networks of another network operator in some or all parts of the country. When such
an agreement is made it is often necessary to update SIM cards over the air as their list
of ‘forbidden networks’ usually contains the identities of the other national network
operators to prevent registration attempts to other networks, which would in any case
reject such attempts. For the majority of network operators, such an over‐the‐air update
of SIM cards is not very difficult as the mechanism is already in place, e.g. to regularly
update the list of preferred international roaming networks.
In some cases network operators share only some radio network technologies but not
others. For example, two network operators might share an LTE access network in some
parts of the country but still have their own 2G and 3G networks. In such a scenario a
mobile device would stay on the 2G and 3G network of the home network operator even
if the LTE network of a competitor is available with whom a national roaming agree-
ment exists. This is because mobile devices will always prefer their home network over
any other networks. As such a behavior is not desirable in many national roaming situ-
ations, the home network can send a list of ‘equivalent PLMNs’ (equivalent Public Land
Mobile Networks) in Attach, Location Update, Routing Area Update and Tracking Area
Update messages. As the term suggests, all networks that are identified in this list by
their Mobile Country Code (MCC) and Mobile Network Code (MNC) shall be treated
as equal to the home network. In the example above in which only the LTE network is
shared, the home network operator has to broadcast GSM SIB2‐quarter and UMTS SIB
19 System Information messages that contain LTE reselection parameters. If such mes-
sages are not sent the mobile device will not search for LTE cells of the other network
operator while it is camped on a 2G or 3G cell of the home network operator.

4.15.2 MOCN (Multi‐Operator Core Network)
While national roaming is usually used only temporarily when new network operators
appear on the market or after a merger of two network operators, Multi‐Operator Core
Network (MOCN) extensions specified in 3GPP aim at providing the means for long‐
term sharing of radio infrastructure. In a traditional radio access network setup, a base
station only transmits the MCC and MNC of one network operator in the System
Information messages that are periodically broadcast to all mobile devices. In the MOCN
network sharing approach, all parts of a base station are shared and the base station
broadcasts several operator identities in the System Information messages. This means
that the same radio channels are used by several network operators. In practice it can be
observed that the network operators sharing a single base station often pool their spec-
trum holdings. Methods to govern which operator is then allowed to use how much of the
spectrum from this pool are not standardized and are specific to infrastructure vendors.
In LTE, the list of core networks that this base station is connected to is included in
the System Information Block (SIB) 1 and has already been part of the initial LTE speci-
fication. Therefore, MOCN is supported by all LTE‐capable devices today. In UMTS,
the feature was introduced in 3GPP Release 6 and the list of networks is sent in the
Master Information Block (MIB). In practice, virtually all UMTS‐capable devices in use
today support the feature. Unfortunately, support for MOCN in GSM was only intro-
duced in 3GPP Release 11 and as a consequence there are a large base of legacy devices
still used in networks today that are unable to detect and use the network list if sent by
a GSM base station.