What it takes to build a 5G network
A plain-language explainer of how private and public 5G networks are built: cells, MIMO, handover, and how many radios, basebands and cores a network needs.
You do not need a telecom background. This page explains the parts of a 5G network and how a phone reaches the internet through a radio on a pole. It also shows what changes when the network serves one company instead of a whole island.
Six boxes in a row: phone, radio, baseband, core, transit and internet. A green packet travels along a rail under the boxes from the phone on the left to the internet on the right, resting under each box in turn, and a small white reply comes back. The links between the boxes are named air, fronthaul, backhaul, exit and ip.
- the packet, left to right
- the reply, back to the phone
A 5G network is three boxes and the links between them
Everything else on this page is detail. If you understand these three boxes, you understand the structure of the whole network.
The radio
The radio is a weatherproof box with antennas. It sits on a pole, a tower, or a ceiling. It changes digital data into radio waves, and radio waves back into digital data. Each radio covers an area, and that area is a cell. More radios give more coverage.
Many suppliers build open-interface radios ("O-RU"). These radios communicate with the baseband in a standard language. That means you do not depend on one supplier.
The baseband
The baseband is the computer that runs the cellular protocol, the rules that phones and the network use to communicate. It decides when each phone can send and receive data, and it encodes that data. When a phone moves between cells, the baseband manages the handover from one radio to the next. It has two parts. The DU is near the radio and works in real time. The CU is central, and its timing is less strict.
OCUDU is open-source baseband software that runs on ordinary servers. A network has one DU at each site and a few CUs that serve all sites.
The core
The core is the control centre of the network. It knows the identity of every SIM and decides if that SIM can connect and where its data goes. When the phone moves between cells, the core keeps its connection active.
Open5GS is an open-source core. It runs as a set of small services on one or two servers. One core serves the whole network.
And the links
Fronthaul connects the radio to the DU. This link is a short, fast fibre cable with strict timing. Midhaul connects the DU to the CU. Backhaul connects the site to the core and the internet. This link can be fibre, microwave, or satellite. The further a link is from the radio, the more relaxed the link can be.
Four boxes in a row: radio, DU, CU and core. The link between radio and DU is the fronthaul, with microsecond timing. DU to CU is the midhaul, with millisecond timing. CU to core is the backhaul, with tens of milliseconds. A scale under them runs from strict on the left to relaxed on the right.
- the link, as it lights in order
The words you will hear, each with a picture
There are twelve terms. Each term has a picture first and a sentence second.
Cell
The area that one radio can serve. Phones inside the circle connect to that radio. Where circles overlap, phones can switch from one radio to the other.
A radio on a pole in the middle. Three green rings spread out from it. Three small white dots, the phones, sit inside the rings and light in turn as each ring reaches them.
Cell sizes
The idea of a cell is the same at five scales. Large cells cover more ground with less capacity per person. Small cells cover less ground with more capacity per person.
Five green circles drawn to relative scale, from a large macro cell on the left, through micro, pico and femto, to a tiny indoor cell on the right. The scale runs from kilometres on the left to metres on the right.
Macro: a radio on a tower. It reaches for kilometres, has 3 sectors, and runs at tens of watts. Micro: a radio on a pole or a rooftop. It reaches a few hundred metres and runs at a few watts. This is the cell that does most of the work in island and campus builds. Pico / femto: a radio on a lamp post or a wall. It runs at hundreds of milliwatts. Indoor: a ceiling unit like a Wi-Fi access point.
Sector
One pole can carry three radios, and each radio points in a different direction. Each radio covers one sector. Together the three sectors make a full circle with three times the capacity.
Three green wedges around one white dot, the pole. Each wedge covers a third of the circle and is labelled sector A, B or C. The wedges light in turn, clockwise.
Spectrum and bands
Spectrum is radio waves at different frequencies. Low frequencies travel far and go through walls. High frequencies carry more data. A band is a named slice of the spectrum (n78 is 3.3 to 3.8 GHz). A licence from the regulator says who can use the band, where, and how much of it.
A horizontal bar with three green segments: low on the left, mid in the middle, high on the right. Under the bar: the low end reaches far with less data, the high end carries more data over a shorter reach. A line below says most new builds start in the mid band.
TDD: the two directions take turns
In the mid band, both directions use the same frequency. The radio and the phones take turns many times in a millisecond. Each turn is a slot. A few slots carry data down to the phones, and one slot carries data up from them. This is why uploads are slower than downloads.
Six slots in a row on one frequency. Five green slots carry data down to the phones; one white slot carries data up from them. A marker labelled now steps from slot to slot and the slot under it lights.
- down, to the phones
- up, from the phones
MIMO: more antennas, more streams
A radio with one antenna sends one stream of data. A radio with four antennas can send four streams to four phones at the same time, or four streams to one phone. "4T4R" means four transmit paths and four receive paths. Massive MIMO (32 or 64 antennas) adds beamforming: the radio aims its energy at each phone and does not spread it across the whole cell.
Antenna elements on the left and phones on the right. Four antennas send four streams, to one phone or to four phones. This is the common outdoor micro radio. Buttons switch between one, two, four and sixty-four antennas.
- a stream of data
- an antenna element, a phone
Handover
A session is a phone's live call or data connection. When a phone moves from one cell into the next, the network moves the session to the new radio and does not drop it. The baseband decides when the move happens. The core keeps the session alive. If the handover is good, the user does not notice it.
Two overlapping circles, cell A and cell B, each with a pole. A phone walks from the left cell to the right cell and back. The edge of the cell that serves the phone is white, and the word handover shows for a moment at the crossing. A green signal runs from pole A down to the overlap and up to pole B, so the session is never broken.
- the session, kept alive
- the edge of a cell; white while it serves the phone
Split 7.2: where the radio ends
The open standard sets the line between the radio and the baseband at a specific point. This point is called "split 7.2". The radio does the fast signal work. The baseband (the DU) does the rest. Because the line is standard, any radio that follows the standard can work with any DU that follows the standard. This is the main purpose of Open RAN.
Two boxes, radio and baseband, with a dashed green line between them labelled split 7.2. The radio does the fast signal work; the baseband does the rest.
- the standard cut
Timing (PTP and GNSS)
TDD only works if every radio agrees on the time to within a millionth of a second. Each site has a small clock. A satellite signal (GNSS) keeps this clock accurate. The site shares the time with the radio through a protocol called PTP. Without a clock, there is no cell.
A clock face on the left with a satellite marker above it labelled GNSS. The hand ticks once a second, and with each tick a green pulse runs along two links to the boxes on the right, radio and DU, so both share the same time.
- the tick, shared
SIM and PLMN
Every phone has a SIM. The SIM holds a secret key and a network ID called a PLMN (a country code plus a network code). The core checks the key before the phone can connect. A private network issues its own SIMs. A public network gets its PLMN from the regulator.
A green SIM card on the left sends PLMN plus key along a line to a box labelled core on the right. The core checks the key, then the word connected lights under it.
UPF: where traffic leaves
The core has one part that handles every data packet: the User Plane Function (UPF). If you put the UPF near the internet connection, traffic takes a short path. On a satellite-fed site, put a small UPF at the site itself. Then packets do not go up to the satellite, down to the central UPF, and back again.
Two rows. The top row, labelled central, runs from a site box up to a UPF box and down to an internet box. The bottom row, labelled local, runs from a site with its own UPF straight to the internet. Two green packets leave at the same moment; the local one arrives while the central one is still travelling.
- a packet; both leave at the same moment
- the links
Open M-Plane
A team must configure radios, upgrade them, and watch them for faults. The management plane ("M-Plane") is the standard way to do this over the network. Open M-Plane is open-source software for the M-Plane. With it, a small team can run hundreds of radios from one screen.
A box labelled Open M-Plane at the top, with the words one screen beside it. A green command leaves it, runs down to a bus, fans out left and right, and each of five radios drawn as white poles lights as the command reaches it.
- one command, to every radio
What it takes to build a private 5G network
One organisation owns the network, its own SIMs and its own land. Examples are a port, a resort, a utility, a factory or a government campus. It uses the same three boxes at a small scale.
What it looks like
The organisation puts a small number of radios on its buildings and poles. Beside each radio stands one small DU server. One CU and one core run on a couple of servers in a cupboard or in a nearby datacentre. A backhaul link connects the network to the internet. The organisation gives its own SIMs to its own devices: cameras, tablets, sensors, vehicles and staff phones.
What makes it easy
- There are few sites, and they are usually on land that the organisation controls.
- The organisation knows its devices, so it can plan the coverage and the capacity. It does not have to guess.
- A data-only network is normal. Voice calls can stay on the public network.
- The licence is local: a private-network class licence plus a frequency authorisation.
- There is one core, with no billing and no roaming. It also has no interconnect, which means no links to other operators' networks.
Four site boxes on the left, each with a radio, a DU and a clock. Their backhaul links join one trunk that carries a green signal into a dashed central room on the right. The room holds three boxes: CU, core and M-Plane. A short line leaves the room to the internet.
- backhaul, to the central room
- the room and its links
What it takes, in order
Design
Walk the site. Decide how many radios you need, where they go, which band they use, and whether they go indoors or outdoors. A radio-propagation model, a calculation of how far the signal reaches, models the coverage of the site plan before you buy anything.
Licence
Apply to the regulator for a private-network class licence and a frequency authorisation for the band. Every radio model must also have type approval, which is an approval for that model of radio, before you can import it.
Lab first
Pair the exact radio with the baseband (the DU and the CU) and the core in a lab. Fix the integration problems in the lab, where they are cheap to fix. Later, on a pole in the rain, they are not cheap to fix.
Build the sites
At each site, install a pole or a bracket, power, lightning protection, the radio, the DU server, the clock and the backhaul link. Outdoor hardware must survive heat, salt and wind.
Bring it up
Switch on the core, then the CU, then the DU servers, then the radios. Give the SIMs to the devices. Walk the site with test phones. Compare the coverage you measure to the coverage the design modelled.
Run it
Watch the alarms and the counters, which are the numbers the network reports about its own traffic. Apply the upgrades, and keep spares on the island. A small network is a part-time job for one trained person plus remote support.
What it takes to build a public 5G network
A public network serves anyone with a phone, across a whole island. The three boxes are the same. Three things change: the scale, the redundancy, and a list of obligations that the software does not cover.
What changes
- Spectrum. You need a nationwide licence for a block of a band. You win it or the regulator assigns it to you, and you pay for it per megahertz (MHz) of spectrum per year. It is often the largest recurring cost in the whole network.
- Sites. A public network has tens to hundreds of sites, on towers and rooftops that you lease, with a backhaul link to each one.
- Redundancy. You need two core sites and CU pools, groups of CUs that share the work. Then the failure of one server does not take the island offline.
- Voice. Phone calls need an IMS, a separate system that the core connects to. A data-only launch does not need it.
- Charging and billing. Prepaid plans, top-ups and invoices need a billing system that the core reports to.
- Lawful intercept and emergency calls. Lawful intercept and emergency calls are licence conditions almost everywhere.
- Interconnect and roaming. Interconnect carries calls to other networks. Roaming serves the phones of visitors and of residents abroad.
- Operations. You need a 24/7 watch, field technicians and a customer-care desk.
Five site boxes on the left, the last one standing for the many more sites. Their backhaul links join one trunk that splits and carries a green signal into two dashed rooms, core site A and core site B. Each room holds a CU pool and a core. A dashed link between the rooms says one backs up the other. Under the figure, a row of pills lists the systems not in the open stack: IMS for voice, billing, lawful intercept, interconnect, roaming and a NOC around the clock.
- backhaul, to both core sites
- the rooms and their links
What it takes, in order
Licence and spectrum first
An operator licence and a spectrum assignment from the regulator can take longer than the build. Do not start anything else until you know the band and the timetable.
Radio plan for the island
The radio-propagation model uses the terrain, the population, the roads and the buildings. It shows how many sites you need, where they go, and how many sectors each one has. The site count decides everything after it.
Sites and backhaul
Lease the towers and rooftops. Order a microwave or fibre link to each site. Size the power supply and the batteries to match the local grid. The construction work at each site is often the largest item in the build cost.
Two core sites, CU pools
Build the core twice, in two places, so that a power cut or a flood at one site does not stop the network. Spread the CUs across the two sites.
The obligations column
Add billing, lawful intercept, emergency-call routing and interconnect, and add IMS if there is voice. Each one is a product or a contract that you connect to the core.
Launch in phases
Start in the towns with a data-only, prepaid service. After the first sites are stable, extend the coverage to the roads, and add voice and roaming. A 24/7 watch starts from the first day.
How many of each thing?
Drag the slider to change the site count. The approximate rule is one DU server per site and one CU for every few dozen sites. The same rule gives one core for a private network and two cores for a public network.
This plan uses 4 sites with one radio each. One CU and one core share a single room. If downtime matters, add a cold spare server for the core, a second server that stays off until you need it.
A sizing tool. Two buttons choose a private or a public network; a slider sets the number of sites from 1 to 60. Six counts update: radios, DU servers, CUs, cores, timing clocks and backhaul links. A picture redraws with the chosen sites in a row above the core room or rooms. This plan uses 4 sites with one radio each. One CU and one core share a single room. If downtime matters, add a cold spare server for the core, a second server that stays off until you need it.
- backhaul, sites to the core
- a site, a room, a link
Private and public, side by side
| Topic | Private 5G | Public 5G |
|---|---|---|
| Who connects | Your own SIMs, your own devices. | Anyone with a phone. |
| Where | Your buildings and land. | A whole island, on leased towers and rooftops. |
| Licence | Private-network class licence plus a frequency authorisation. | Operator licence plus a nationwide spectrum assignment, with a fee per MHz per year. |
| Sites | 1 to 10. | Tens to hundreds. |
| Radios per site | Usually 1, a micro radio or an indoor radio. | Usually 3 sector radios. |
| DUs | One per site. | One per site. |
| CUs | One. | Pools of CUs, spread across two core sites. |
| Cores | One. | Two, geo-redundant: in two separate core sites, so that the network continues if one site fails. |
| Voice | Not necessary. Phones use the public network for calls. | An IMS is necessary for voice calls. Without it, the launch is data-only. |
| Billing | None. | Billing and charging system. |
| Obligations | Type approval, licence conditions. | Lawful intercept, emergency calls, interconnect, coverage conditions. |
| Operations | One trained person plus remote support. | 24/7 watch, field team, customer care. |
| Time to first cell | Months. | The spectrum timetable sets the schedule. The build comes after it. |
The software layer (OCUDU, Open M-Plane, Open5GS) is open source in both columns. Sites, spectrum, power, backhaul and the obligations column set the cost of a network. The software licences do not set it.
Open stack, engineered end to end
telecogeco designs, integrates and tests 5G networks built on open-source software: OCUDU for the baseband, Open M-Plane for the radios and Open5GS for the core. It also models coverage before a single radio is ordered. The steps on this page apply to both private campus networks and island-scale public networks.
Written by telecogeco, a ugeco brand. The diagrams are simplified. The counts in the sizing tool are rough estimates. A real design starts from a terrain model and a site survey.