What is PLMN for a Network Operator?
The full form of PLMN is Public Land Mobile Network. It is the geographical area a mobile network operator covers for voice and data services to a mobile subscriber using a SIM card. For example, an operator may have one PLMN in the whole city. Based on the location, it can be a home PLMN or a Visitor PLMN. We will explain each in detail.
A PLMN connects to other PLMNs to provide inter-operators roaming, messaging, and data services. Other PLMN may belong to the same operator or another.
How is PLMN addressed in the network?
It is identified by an identifier known as PLMN ID. The identifier has a combination of MCC and MNC. This combination makes it unique globally.
From the IMSI of a SIM card, home PLMN can be identified. The home PLMN of a mobile subscriber is called HPLMN, which has subscription profiles of subscribers in the HLR. A mobile operator may have multiple PLMNs in the same country with a different mobile network code (MNC).
HPLMN :
Every SIM card has an HPLMN, where the subscriber profile is configured. While roaming, a subscriber changes its location. This may change in roaming PLMN. The new one may be the same as HPLMN or its other network’s PLMN if an Airtel sim card is purchased from New Delhi, India. The subscriber’s HPLMM is in New Delhi, India.
HLR is the node in HPLMN, where a new subscriber has all subscription information (SMS, Data, Voice), and the sim card keeps updating its location to the HLR during roaming. The home network has the message and billing nodes.
The SMSC is for messaging, and IN-SCP is for real-time charging.
VPLMN:
In roaming, a subscriber attaches to the core network via the access network. The roaming core network belongs to a PLMN. This is called Visiting Public Land Mobile Network or VPLMN. The VPLMN and HPLMN can be the same or different, depending upon the location of the mobile phone and the serving mobile operator. A mobile operator has VPLMN-based changing and roaming policies.
PLMN network id:
For any network, there is an identifier. PLMN network id for a mobile network identifies the roaming location uniquely. An integer value contains MCC and MNC. To transfer value on the wire, fixed decoding is done. Decoding is defined in the 3GPP spec.
| MCC Digit1 | MCC Digit1 |
| MNC Digit 3 | MCC Digit1 |
| MNC Digit 2 | MNC Digit 1 |
The 3GPP has specified a diameter AVP called Visited PLMN id, which contains the MCC-MNC in encoded format. Whenever a UE attaches to the LET network over the 3GPP s6a interface. Messages from MME to the HSS over diameter protocol have Visited PLMN Id AVP.
In the case of 3G, the PLMN id is present with roaming VLR information.
The HPLMN network id is derived from the IMSI, and the VPLMN network id is derived from the roaming information present in information while the subscriber attaches to the network.
Is the PLMN id is identical for the 3G and LTE networks?
Yes, in LTE, we have IMSI as the primary identity. The network is addressed based on the MCC and MNC values.
Now how to decode the PLMN id?
For a typical user, getting MCC and MNC from an encoded value is impossible. Here we show an example program in C++ for getting MCC and MNC from an encoded buffer.
#include <iostream>
#include <string>
using namespace std;
int main ()
{
unsigned char plmn_id[] = {0x31, 0xf4, 0x01}; /* Encoded PLMN Id */
string mcc, mnc;
unsigned char ch = 0;
int i;
/* First Digit of MNC */
ch = plmn_id[0] & 0xf;
mcc.push_back(ch + '0');
ch = (plmn_id[0] & 0xf0) >> 4;
mcc.push_back(ch + '0');
ch = plmn_id[1] & 0xf;
mcc.push_back(ch + '0');
/* DEcoding MNC 8 */
ch = plmn_id[2] & 0xf;
mnc.push_back(ch + '0');
ch = (plmn_id[2] & 0xf0) >> 4;
mnc.push_back(ch + '0');
ch = (plmn_id[1] & 0xf0) >> 4;
if(ch != 0x0f)
{
/* If MNC is 3 digits log */
mnc.push_back(ch + '0');
}
cout << "MCC = "<< mcc <<":" << "MNC = " << mnc << endl;
return 0;
}
After compilation and execution the display will be :
#g++ -o plmndecoded decodePlmnId.cpp
# ./plmndecoded
MCC = 134:MNC = 10
Elements of PLMN:
There are multiple components of a public land mobile network. These can be divided into two groups. One is the access network, and the other is the core network.
The access network connects the phone to the core network. The phone connects to the access network over a wireless or radio interface. The core network connects to the access network over a wired interface. The common access network is used for packet-switched and circuit-switched services. Each generation of mobile telephony has a different access network. The following mentions the generation and access network types.
GERAN, 2G access network. BTS and BSC are nodes that cover the access network.
UTRAN: This is the access network for 3G. RAN and NodeB are two nodes in the UTRAN access network.
E-UTRAN: It’s the access network for LTE. eNodeB is the node that is in the access network.
The Core network does the signaling for the setup of voice and mobile internet communication. A signaling network is the control plane, and the actual data or voice network is the user plane. In 2G/3G, there are separate nodes for circuit and packet services.
MSC is for circuit switching or voice; SGSN is for packet switching or the internet in 2G and 3G. While attaching to a VPLMN, the SIM card sends authentication and update location requests to the HPLNM. The home PLMN has the configuration of the subscriber for authentication, billing, roaming, and messaging.
Home PLMN may have inbound roamers or outbound roamers. If a mobile subscriber is roaming in another PLMN, it’s called an inbound roamer for the roaming network. It is an inbound roamer from the mobile SIM provider’s home network. Both network operators should have a roaming agreement.
Call Barring Outside Home PLMN:
Mobile operators can block incoming or outgoing calls if a subscriber roams outside the home network. There are two supplementary services described in GSM. HLR has provisioning information for the services. The following is a detailed description.
Barring Of all incoming calls when roaming outside HPLMN.
This disables a mobile subscriber to call when VPLMN differs from HPLMN. This is useful where national roaming needs to control. MCC remains the same in national roaming, but the MNC code changes.
Barring Of outgoing calls not directed to Home PLMN country: Supplementary service blocks all outgoing calls not dialed to the home country. This service comes into the picture when subscribers roam to another country.