Software-defined is a software claim, but it rests on a hardware change. Without
zonal architecture central compute
Zones own the wiring near them; the centre owns the software Central compute cockpit · ADAS · vehicle functions run here updatable over the air Front-left zone lights · door · mirror Front-right zone lights · door · sensors Rear-left zone door · seat · boot Rear-right zone door · seat · lamps Automotive Ethernet short local wiring to sensors and actuators · one high-speed link back to the centre Zonal wiring with central compute — Zones own short local wiring to whatever is physically near them. One fast link carries everything to the centre, where the actual function logic lives as software.The insight: group by place, not by purpose#
The older way grouped by function. A lighting controller connected to every lamp
in the vehicle — front, rear, interior, indicators — because they were all
"lighting".
The problem is geographic. Those lamps are metres apart in different corners of
the car, so a functional grouping guarantees long wire runs.
Zonal inverts it. A rear-left zone controller connects to the rear-left lamp, the
rear-left door motor, the rear-left seat and the boot latch — things that have
nothing functionally in common but are within half a metre of each other.
You would not send one van to deliver all the shoes in a city and a second van
for all the books. You send one van per district, carrying whatever is going
there.
Zonal is delivery by district. Functional grouping is delivery by product type,
and it is exactly as inefficient as it sounds.
What this actually saves#
Functional grouping Zonal Wiring Long runs across the vehicle Short local runs Harness weight 50–70 kg on a premium car Substantially less Assembly Largely manual, irregular routing Simpler, more repeatable Adding a sensor New wire to the relevant controller Connect to the nearest zone Connectors Many, and a common warranty fault Fewer
Weight matters more than it might seem. On an electric vehicle, mass directly
costs range, and range costs battery, and battery is the most expensive component
in the car. A harness saving compounds through the whole vehicle economics.
The zone controller does almost nothing#
This is the part people find surprising, and it is the key to the whole
architecture.
A zone controller is deliberately simple. It reads sensors, drives actuators,
converts between local buses and Ethernet, and forwards. It contains very little
decision-making.
The function logic moves to the centre. "Fold the mirrors when the car locks"
is not implemented in the door zone controller. It is software on central
compute that calls a mirror service and listens to a lock service.
That is what makes it changeable. Logic in a zone controller is embedded software
in a small box. Logic in central compute is software on a general-purpose
computer that can be updated over the air.
The feature: when the driver approaches at night, the puddle lamps illuminate
and the mirrors unfold.
Functionally grouped. The lighting ECU must learn about the key module. The
mirror ECU must learn about the lighting state. Both need software changes from
their suppliers, network signals must be added, the network design is revised,
and everything is revalidated. Months.
Zonal with central compute. A new piece of software on the central computer
subscribes to the key service and calls the lighting and mirror services, both of
which already exist. No new signals, no supplier involvement, no wiring. It ships
in an over-the-air update.
Same feature. The difference is entirely architectural.
Why Ethernet is required#
Zonal only works if the link between zone and centre is fast enough to carry
everything that used to be separate wires.
CAN
Raw speed is not the whole story. The essential addition is TSN
Without timing guarantees, Ethernet is fast but unpredictable, and unpredictable
is unusable for vehicle control. TSN is what makes the substitution acceptable.
Power distribution moves too#
An under-discussed part: zone controllers usually take over power switching
as well as signalling.
Instead of a central fuse box with a wire to every load, each zone has solid-state
switches for the loads near it. That means:
Fuses become software-configurable current limits
A fault can be isolated and reported rather than blowing a physical fuse
Loads can be shed intelligently when the battery is low
This is quietly one of the larger changes. "The fuse is a piece of software" has
consequences for diagnostics, for how faults are reported, and for what a
workshop actually does when something stops working.
What it costs#
Central compute is expensive. A capable automotive processor costs far more
than the ECUs it replaces, and it needs cooling.
Isolation must be rebuilt. A hundred separate boxes could not interfere with
each other. Software on one processor absolutely can — which is why hypervisors
and freedom from interference
A single point of failure appears. If central compute fails, a great deal
fails together. Real designs keep genuinely critical functions — braking,
steering, airbags — on their own hardware for exactly this reason.
That last point is worth stating plainly, because it is a common misconception:
central compute does not mean everything runs centrally. Safety-critical
control stays distributed and independent. What consolidates is the large,
complex, updatable software — cockpit, ADAS perception, connectivity, comfort.
What to check on a real programme#
How many zones, and what does each one own? Is it Ethernet with TSN, or Ethernet without timing guarantees? Does the zone controller contain logic, or only I/O? Logic in the zone
means the updatability benefit was not achieved.What stayed on dedicated hardware, and why? There should be a clear,
safety-driven answer.What still speaks CAN? Usually a lot, through a gateway.
Zonal groups wiring by location , not function — the geography is what makes
it cheaper.
Zone controllers are deliberately simple. The function logic moves to
central compute, which is what makes it updatable.Requires Automotive Ethernet with TSN — speed alone is not enough, timing
guarantees are.
Power switching moves to the zones , turning fuses into software.Central compute does not mean everything is central. Safety-critical control
stays on its own hardware.The cost is expense, rebuilt isolation, and a concentrated failure point.
Next#
How several operating systems share one processor without disturbing each other.
