Virtual build-out

Assemble the roadside and the vehicle, part by part

Pick a preset or add parts from the bin. The board draws how they connect. Click a part to read what it does, which standards it follows, what to ask the vendor and what goes wrong in the field. The system check runs on every change. When the build is right, export the bill of materials.

This is a training tool: no vendor endorsements, no prices, no design. It is meant for state and local DOT staff, integrators, transit and toll agencies and students who want to see the whole system before they buy a piece of it. ITS Roads designs and installs real systems.

Drag a part to move it (positions are only cosmetic). Click or press Enter on a part to read about it. Lines are drawn from the wiring rules, not by hand.

Bill of materials

Quantities, interfaces, power and standards come from the catalog. Power figures marked typical are typical; check the datasheet.

Drag a part to move it (positions are only cosmetic). Click or press Enter on a part to read about it. Lines are drawn from the wiring rules, not by hand.

Bill of materials

Quantities, interfaces, power and standards come from the catalog. Power figures marked typical are typical; check the datasheet.

The OBU and the vehicle CAN network

The controller area network (CAN) is the bus every module in a vehicle shares. The on-board unit listens to it for the BSM fields the vehicle knows better than GNSS: brakes, transmission, steering, lights, wipers. On a public road the OBU only listens. Writing to a production bus is done on a bench, on a hardware-in-the-loop rig, or through an OEM-approved gateway.

What the OBU takes from the vehicle

BSM field	Source	Car (J1979 PID or OEM frame)	Heavy vehicle (J1939)
speed	CAN (GNSS as fallback)	J1979 PID 0x0D vehicle speed; OEM wheel speed frames need the DBC	PGN 65265 CCVS wheel-based vehicle speed; PGN 65215 wheel speeds
heading	GNSS	GNSS course over ground	GNSS course over ground
brakes.wheelBrakes, brakePadel	CAN	OEM frame, needs the DBC	PGN 65265 CCVS brake switch
brakes.abs, traction, stabilityControl (scs)	CAN	OEM frame, needs the DBC	The J1939 PGN for the electronic brake controller (EBC1)
transmission	CAN	OEM frame, needs the DBC	The J1939 PGN for the electronic transmission controller (ETC2)
angle (steering wheel angle)	CAN	OEM frame, needs the DBC	The J1939 PGN for vehicle dynamic stability control (VDC2)
accelSet.yaw (angular velocity)	CAN or the OBU's inertial sensor	OEM frame, needs the DBC	The J1939 PGN for vehicle dynamic stability control (VDC2)
accelSet.long (accelerator and braking)	CAN or the OBU's inertial sensor	J1979 PID 0x11 throttle position for pedal, OEM frame for acceleration	PGN 61443 EEC2 accelerator pedal position
lights (exterior lights, turn signals)	CAN	OEM body control frame, needs the DBC	The J1939 PGN for operator external light controls (OEL)
wipers (OBU weather applications; the 2024 dictionary marks the old BSM weatherProbe field do-not-use)	CAN	OEM body control frame, needs the DBC	The J1939 PGN for operator wiper and washer controls (OWW)
pathHistory, pathPrediction	GNSS, computed by the OBU	Position fixes over the last 300 m	Position fixes over the last 300 m; PGN 65267 vehicle position is also on the bus
events.eventHardBraking (EEBL)	Computed from deceleration	Wheel speed or inertial sensor	PGN 65265 CCVS speed and the brake switch, or the inertial sensor
lightsUse, sirenUse (emergency vehicles)	Digital input or CAN flag	Light bar and siren relay into the OBU input	Light bar and siren relay, or an upfitter CAN flag

J1979 PIDs are requested over ISO 15765-4 and answered by the engine module. OEM frames are proprietary: every make uses its own IDs and scaling, written down in the DBC file. J1939 PGNs are standardized across heavy vehicles. Only the PGN numbers shown are quoted here; the others are named by their mnemonic.

OBD-II J1962 connector

Pin 4	Chassis ground
Pin 5	Signal ground
Pin 6	CAN high (ISO 15765-4)
Pin 14	CAN low (ISO 15765-4)
Pin 16	Battery +12 V (Type A) or +24 V (Type B)

CAN in one minute

ISO 11898: a differential pair (CAN high and CAN low) with 120 ohm termination at both ends of the bus.
Bits are dominant (0) or recessive (1); a dominant bit wins, so the lowest ID wins arbitration when two nodes start at once.
Cars: 500 kbit/s on the diagnostic connector, ISO 15765-4, 11-bit IDs, J1979 request and response.
Heavy vehicles: SAE J1939 at 250 kbit/s (J1939/11 and /15) or 500 kbit/s (J1939/14), 29-bit IDs that carry the PGN, and SPNs inside the data.
CAN FD raises the data phase up to 8 Mbit/s and the payload to 64 bytes; newer vehicles use it on internal buses.

CAN bench: a small virtual vehicle

Injection only on a bench. Frames are sent to a vehicle bus only on a bench, on a hardware-in-the-loop rig, or through an OEM-approved gateway with ISO 14229 UDS security access. An aftermarket OBU on a public road connects listen-only, through a CAN gateway or an OBD-II dongle with transmit disabled. Writing to a production bus is a safety and cybersecurity risk (ISO/SAE 21434, the NHTSA cybersecurity best practices of 2022, the vehicle warranty and the agency's liability). This bench uses illustrative IDs; every make uses its own, and you need the DBC or the J1979 request and response to decode a real one.

Frames on the bench bus (illustrative IDs, every make uses its own)

Decoded into BSM fields

Protocol stack

Where IEEE fits

Four bodies share the work. IEEE wrote the networking and security layers (1609.3 and 1609.2) that US C-V2X still uses on top of the 3GPP radio, plus the legacy DSRC radio (802.11p). SAE writes the messages (J2735) and the performance requirements (J2945). 3GPP writes the C-V2X radio (PC5). The FCC assigns the spectrum and licenses the RSUs.

Layer

C-V2X as deployed in the US

Legacy DSRC

Application

SAE J2735 message set (BSM, SPaT, MAP, TIM, PSM, SDSM, SRM, SSM), SAE J2945/x performance requirements, SAE J3161/1 the LTE-V2X profile used in the US

Same SAE messages and J2945 requirements

Networking

IEEE 1609.3 WSMP (WAVE Short Message Protocol) for the broadcast messages; IP for management traffic. IEEE 1609.12 assigns the PSIDs

IEEE 1609.3 WSMP, the same header

Security

IEEE 1609.2 signatures and certificates, issued by an SCMS

IEEE 1609.2, the same certificates

Access (radio)

3GPP LTE-V2X PC5 sidelink, Release 14 as deployed. NR-V2X (Release 16 and later) is not interoperable with it

IEEE 802.11p with IEEE 1609.4 channel switching. DSRC licenses sunset December 14, 2026 under FCC 24-123

Spectrum

FCC 24-123: 5.895 to 5.925 GHz, three 10 MHz channels; Part 90 RSU licenses in ULS, Part 95 OBUs licensed by rule

Transition out of the band by the sunset date

PC5 and Uu

PC5 is direct: vehicle to vehicle and vehicle to roadside, no network in between, which is why a BSM arrives in a few milliseconds. Uu is the cellular link (vehicle to network): it carries TIMs from a cloud service and the backhaul for management, logs and certificate top-ups over the mobile network. A real deployment uses both.

PSIDs (IEEE 1609.12)

0x20	BSM
0x8002	SPaT
0xE0000017	MAP
0x8003	TIM
0x27	PSM
0x8010	SDSM
0xE0000016	SRM
0xE0000015	SSM

The PSID sits in the WSMP header and tells the receiver which application a frame belongs to. It is not part of the J2735 payload.

Read the standards guide for the full list of documents and their current revisions.

This page is training material. ITS Roads can design, procure, install and integrate the real system: connected-vehicle services. Missing a part you want on the bench? Tell us.