A packet core is the software at the centre of a mobile network that checks your SIM, keeps track of your phone as it moves between towers, and passes your data to and from the internet. It is also called the mobile core or the core network. In 4G it is called the evolved packet core (EPC), and in 5G it is called the 5G core. Packet here means a small block of data. Everything you send, from a message to a video, travels as packets.
The core runs on servers in the operator's data centres, not on the towers. A tower's RAN software receives your phone's radio signal and sends the packets onward. They travel over microwave or fibre links to the core. The core then does three jobs.
First, it checks who you are. Your SIM holds a secret key, and the operator holds a copy of the same key. The core sends your phone a random number. The SIM uses its key to calculate an answer, and the core checks the answer against its own calculation. If the answers match, the SIM is genuine. The secret key itself is never sent over the air.
Second, it tracks where you are. Take, for example, a passenger on a bus from Pune to Nashik who watches a cricket match on her phone for the whole four-hour ride. Her phone moves from one tower to the next dozens of times. Each time, the core tells the network to send her packets to the new tower. Her phone keeps the same internet address all the way, so the video does not stop.
Third, it connects you to the internet. One function of the core, called the user plane, passes every packet between the operator's network and the internet. As it does, it counts the megabytes used. It reports this count to telecom billing and real-time charging, which deducts them from her prepaid data balance. If the balance reaches zero, the charging system tells the core to stop or slow her data. Rules on how to treat each kind of traffic can come from deep packet inspection (DPI) systems. When a court or an authorised agency orders it, the core also sends a copy of one person's calls and data to lawful interception systems.
The core used to be a set of sealed boxes from one equipment maker. Now it is mostly software that runs on ordinary servers, often in containers like any cloud application. This is one case of hardware-software disaggregation. It means an operator can add capacity by adding servers. It also means a company can run a small core of its own, for example for private 5G inside a factory.
How is the core different from the radio access network? The radio network handles the radio link between the phone and the tower. The core never sees a radio signal. It decides who may use the network, where to send each packet and what to charge.