Mobile networks: 4G and 5G
Your phone has no cable, yet it reaches the same internet as your laptop. Follow a phone in a moving car: by radio to a cell tower, over fibre to the operator's core network, out to the internet, and across to the next tower without dropping. Then see what 5G changes.
5 quick questions
Pick an answer to see why. Your score saves with your progress.
01What does a cell tower (base station) do with your phone's radio signal?
The base station (eNodeB in 4G, gNodeB in 5G) handles the radio side. A cable, usually fibre, carries the data on to the operator's core network.
02Why does the network check your SIM before letting you on?
The core looks up your identity, then phone and network prove to each other that they hold the same secret key. The key itself never travels over the air.
03Your car drives from one tower's cell into the next. What happens?
That is a handover. The phone reports that the next tower is stronger, the two base stations agree (over X2 in 4G, Xn in 5G), and the core sends your data to the new tower. Your IP address stays the same.
04Why do networks split an area into many small cells?
Spectrum is scarce. Each cell reuses the same frequencies, so more (smaller) cells means more phones served with the same slice of spectrum.
05What is the trade-off with 5G millimetre-wave (mmWave) bands?
mmWave (above 24 GHz) allows channels up to 400 MHz wide, so it is very fast, but the signal fades quickly and walls block it. Mid-band around 3.5 GHz balances speed (up to 100 MHz channels) and reach.
Glossary
- Cell
- The area one tower's antennas cover. A mobile network is a patchwork of cells.
- Base station
- The radio equipment at a cell tower: an eNodeB in 4G (LTE), a gNodeB in 5G (NR).
- Backhaul
- The link, usually fibre, that connects a base station to the operator's core network.
- Core network
- The operator's central systems: they check SIMs, track where each phone is, give out addresses and connect to the internet.
- SIM
- A small chip that holds your subscriber identity and a secret key, so the network knows who you are.
- Handover
- Moving a phone's connection from one cell to the next while it is in use.
- Spectrum
- The range of radio frequencies. Operators license slices of it and reuse them across cells.
- mmWave
- 5G radio bands above 24 GHz: very wide, fast channels with a short range.
Notes and sources
- 198.51.100.23 and 203.0.113.10 are RFC 5737 documentation addresses; the SIM shows the ITU-T E.212 test network code 001-01.
- The 20 km backhaul and the cell sizes in the drawing are illustrative.
- Many operators give phones an IPv6 address, or an IPv4 address shared between many phones through carrier-grade NAT.
- Backhaul is often fibre, sometimes a point-to-point microwave radio link.
- The core is really many separate functions (in 5G: AMF, SMF, UPF, AUSF, UDM and more); a special topic opens it up.
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