What is backhaul?

A network that reaches people or devices can be divided into three parts:

• Access, the link between the end device and the nearest piece of network equipment: a phone to a cell site, a sensor to a gateway, a laptop to a Wi-Fi access point.
• Backhaul, the link from that equipment back toward the centre of the network.
• Core, the central network and services, and from there the internet.

Every request that leaves the local area depends on the backhaul. A device can have a strong link to its access point and still reach nothing beyond it if the backhaul is down.

In 5G, the radio access network can be split into several units, and the links between them have names of their own. ITU-T’s technical report on transport networks for 5G (GSTR-TN5G) describes three:

• Fronthaul, between the remote radio unit and the distributed unit (DU).
• Midhaul, between the distributed unit and the central unit (CU).
• Backhaul, between the central unit and the 5G core network, and between central units.

Not every deployment has all three. When the CU and DU are in the same place, there is no midhaul. The report also describes small cell and hot-spot deployments where the radio, DU and CU are integrated, and there is only backhaul.

Backhaul does not have to be a cable. The 3GPP specification for 5G radio access includes integrated access and backhaul (IAB), which enables wireless relaying: an IAB node serves devices and carries its own backhaul over the same 5G radio, to a donor node connected to the core. 3GPP notes that backhauling can occur over a single hop or over multiple hops, which makes an IAB network a multi-hop relay chain whose job is to reach the backhaul.

Low-power IoT networks use the same word for the same role. In LoRaWAN, end devices send radio messages to gateways, and gateways forward them to a network server. The Things Network’s documentation says gateways connect to the network server over a backhaul such as cellular, Wi-Fi, Ethernet, fibre-optic or 2.4 GHz radio links. The radio side and the backhaul are separate choices: a site can have good LoRa coverage and a poor backhaul, or the reverse. For how this differs from a mesh, see mesh networking vs LoRaWAN.

The same shape appears in field data collection. A data logger or a site gateway gathers readings locally, and its modem or radio link to the office is its backhaul.

Broadband policy uses a related term. The US National Telecommunications and Information Administration (NTIA) defines middle mile infrastructure as “any broadband infrastructure that does not connect directly to an end-user location”. Under that definition, the backhaul links that carry a site’s traffic toward the internet backbone are middle mile infrastructure.

A mesh of phones has no fixed gateway, but the same role still exists. Phones exchange data with each other over short-range radio, and the backhaul is whatever path one of them has to the internet at the time: mobile data, Wi-Fi, or a connection to a relay server. The exit point can move from one phone to another as people move, and at times no phone in the group has one. The design questions are the same as for a fixed gateway: which device is allowed to carry data upstream, and what waits while none can.

A backhaul can be absent by design, as on a remote site, or temporarily, after a cut cable, a storm or an outage at the provider. What still works depends on where the logic lives.

• Local services keep working if they run on the access side. Devices on the same Wi-Fi, mesh or gateway can still talk to each other if nothing they need sits beyond the backhaul. This is one reason to put work at the edge, covered in what edge computing is.
• Data waits if the gateway or devices store it. A gateway that buffers readings until its backhaul returns turns an outage into a delay. The device-side version of this is covered in how to collect device data without internet.
• Data takes another route if devices can relay to a neighbour that does have a backhaul, or if a visiting phone or vehicle carries it out, which is data muling.

When designing for an unreliable backhaul, decide which functions must work with no backhaul at all, how long data must wait, and which devices are trusted to forward data upstream when they do have a connection.