PLMN,HPLMN,VPLMN,IPLMN,

Home PLMN (HPLMN) – the HPLMN is the GSM network that a GSM user is a subscriber of.
That implies that GSM user’s subscription data resides in the HLR in that PLMN. The HLR
may transfer the subscription data to a VLR (during registration in a PLMN) or a GMSC (during
mobile terminating call handling). The HPLMN may also contain various service nodes, such as
a short message service centre (SMSC), service control point (SCP), etc.


• Visited PLMN (VPLMN) – the VPLMN is the GSM network where a subscriber is currently
registered. The subscriber may be registered in her HPLMN or in another PLMN. In the latter
case, the subscriber is outbound roaming (from HPLMN’s perspective) and inbound roaming
(from VPLMN’s perspective). When the subscriber is currently registered in her HPLMN, then
the HPLMN is at the same time VPLMN.1


• Interrogating PLMN (IPLMN) – the IPLMN is the PLMN containing the GMSC that handles
mobile terminating (MT) calls. MT calls are always handled by a GMSC in the PLMN, regardless
of the origin of the call. For most operators, MT call handling is done by a GMSC in the HPLMN;
in that case, the HPLMN is at the same time IPLMN. This implies that calls destined for a GSM
subscriber are always routed to the HPLMN of that GSM subscriber. Once the call has arrived in
the HPLMN, the HPLMN acts as IPLMN. MT call handling will be described in more detail in
subsequent sections. When basic optimal routing (BOR) is applied, the IPLMN is not the same
PLMN as the HPLMN.