The network in one page

Three networks and a database

A mobile session crosses three networks and one database lookup. The radio access network carries the device's signal to a base station. The transport network carries that traffic to the core. The core authenticates the subscription, assigns an address and exits to the internet. The database is the subscriber store that tells the core whether the subscription may connect.

Each part answers a different question. The radio answers whether the device can reach a cell. Transport answers whether the cell can reach the core. The core answers who the subscriber is, which gateway to use and where the traffic leaves. The database answers whether the subscription is valid and where it is.

The SIM is not the phone. The SIM holds the subscription credential and the identity the core checks. The phone holds the radio and the computer. A phone with a working radio and no valid subscription cannot open a session; a valid subscription in a device with a broken radio cannot reach the cell.

The core separates two jobs. The control plane attaches the device, authenticates the subscription and sets the session up. The user plane carries the packets. A session can be attached and still carry nothing if the user plane is blocked, so a status screen that says "connected" is not proof that traffic flows.

Which layer owns the problem

Start with the layer that fails. No service usually points to the radio (coverage, band support, antenna), or to a core rejection when the subscription is barred. A device that attaches but sends nothing has a transport or core problem: backhaul, a congested cell, an APN mismatch or a gateway that rejects the session.

A page that loads while voice fails is usually a voice-path problem, not a radio problem. Voice over LTE runs through IMS, which has its own registration and its own APN. The device must support VoLTE, the subscription must allow it and the network must accept the IMS registration.

Transport is the layer buyers forget. The cell can be close and still fail if its backhaul is congested, cut or routed badly. A site with good radio and a saturated backhaul shows full bars and slow pages, which looks like a radio problem and is not.

Write the layer into the ticket. "No data in location X" sends the reader to the radio. "Attached, no IP address" sends them to the core. "Data works, calls fail" sends them to IMS. The label saves a day of guessing.

The identifiers behind the session

Four identifiers appear in a support ticket, and each answers a different question. The IMSI identifies the subscription: it is the MCC, the MNC and the MSIN. The ICCID identifies the card or profile container. The IMEI identifies the device. The MSISDN is the diallable number, which can move under portability.

The MCC and MNC form the PLMN. A neutral example is MCC 274 and MNC 01, a network in Iceland; the registry decides whether the MNC has two or three digits. A structurally valid fictional IMSI is 274011234567890: MCC 274, MNC 01, MSIN 1234567890. The number shows structure, not a live subscriber.

Use the identifier that matches the question. A blocked device is an IMEI question. A subscription that will not authenticate is an IMSI question. A card that was replaced is an ICCID question. A number that does not ring is an MSISDN question. Four numbers explains the split, and the MCC-MNC lookup names the network that owns a PLMN.

Where voice takes a different path

Voice on LTE is an IP service carried by IMS. The device registers with IMS in addition to attaching to the data core, and the IMS APN and the voice configuration must match the subscription. When they do not, data works and calls fail.

Check four things in order: the device supports VoLTE on the network's bands, the subscription is provisioned for voice, the IMS APN is present, and the network accepts the registration. A missing item in that list produces the same symptom, so test them one at a time.

What to ask a provider

Ask which layer the provider operates and which layer it buys. A mobile data provider may run the core and buy radio access, or resell a full mobile network. The answer tells you who can fix a radio problem and who can only raise a ticket.

Ask for the identifiers on the order: the IMSI range, the APN, the IP address pool and the network that serves each country. Ask what happens to a session when the home gateway fails. Ask which team owns the voice path. Put those answers in the design note, not in a slide.

Ask how the provider measures each layer. A radio claim needs a signal reading from the device. A transport claim needs a route and a round-trip time. A core claim needs a session log and an address. Without the layer, a measurement cannot be checked.

The network page shows a published network footprint. Use it as a map, then ask the provider to mark the same map for its service.