V2X Explained / C-ITS architecture
C-ITS architecture
How C-ITS is built.
Cooperative-ITS, the European framework for vehicles talking to each other and to the roadside, is defined as a stack of layers. Each one has a single job, and the layering is what lets the same road messages run over completely different radios.
The layers
Five layers, bottom to top.
Read from the road upward: the radio at the bottom, the meaning at the top, and trust applied through all of it.
Layer 5
Access
The radio
EN 302 663 · 3GPP
How the signal physically travels through the air. Two radios coexist in Europe: ITS-G5, a Wi-Fi variant for direct short-range links, and C-V2X, which reuses cellular technology. Everything above this layer stays the same whichever one carries it.
Layer 4
Networking & transport
Getting it to the right place
EN 302 636-4-1 · EN 302 636-5-1
Messages are addressed by geography, not by network address. GeoNetworking forwards a message to every station inside an area (the 300 metres approaching a junction, say), and a lightweight transport layer keeps each message type on its own channel.
Layer 3
Facilities
The meaning
EN 302 637-2 · TS 103 900 · TS 103 301
Where the actual messages live: awareness, hazards, signal timing, maps, priority requests. This is the layer a city or an operator cares about, because it is the one that carries road meaning rather than plumbing.
Layer 2
Security
Trust, across everything
TS 103 097 · TS 102 941 · IEEE 1609.2
Every message is signed, and every station carries certificates from a European public-key infrastructure. A receiver can tell a genuine roadside unit from an impostor before it acts on anything. Security is not a layer on top; it applies across the whole stack.
Layer 1
Management
Keeping it sane
EN 302 895 · TS 102 687
A Local Dynamic Map fuses everything a station hears into one live picture of its surroundings, and congestion control keeps the shared radio channel from saturating when traffic is dense.
How a message travels
From a hazard to a dashboard, in milliseconds.
A roadside unit detects ice on the carriageway. At the facilities layer it builds a hazard notification (a DENM) describing what and where. The transport layer tags it, and GeoNetworking marks it for delivery to every station in the affected stretch of road.
The security layer signs the packet with the unit's certificate. The access layer puts it on the air over whichever radio is fitted. A car approaching a few hundred metres back receives it, checks the signature against the European trust list, and only then shows the driver a warning. The whole exchange takes well under a tenth of a second, which is what makes it useful before the driver can see the hazard.
Why the layering matters
Swap the radio, keep the road meaning.
Europe has not settled on one radio, and it may never need to. Because the access layer is separated cleanly from everything above it, a deployment can change from ITS-G5 to C-V2X, or run both, without rewriting a single message, security rule or map.
That is the whole point of a layered architecture, and it is why a production stack built to it can outlast the radio debate. It is also how SkyV2X keeps one stack running across mixed, multi-vendor infrastructure.