Chapter 3
WIA-COMM-003 treats Vehicle-to-Everything (V2X) communication as a family of four modes. Each mode answers a different question about the road environment. V2V (Vehicle-to-Vehicle) lets a car learn what its neighbours are doing without relying on any fixed equipment. V2I (Vehicle-to-Infrastructure) lets it learn what the road itself is doing, such as signal phases and intersection geometry. V2P (Vehicle-to-Pedestrian) covers vulnerable road users, including pedestrians, cyclists and emergency-vehicle warnings. V2N (Vehicle-to-Network) connects the vehicle to cloud services for traffic management, remote monitoring and over-the-air updates.
The specification groups the message families under these modes as follows:
| Mode | Message families and functions named by the standard |
|---|---|
| V2V | Cooperative Awareness (CAM), Decentralized Environmental Notification (DENM), Collective Perception (CPM) |
| V2I | Signal Phase and Timing (SPaT), Map Data (MAP), Road Side Unit (RSU) communication |
| V2P | Pedestrian detection, cyclist awareness, emergency vehicle warning |
| V2N | Cloud traffic management, remote monitoring, OTA updates |
Two kinds of device sit at the edges of this model. The On-Board Unit (OBU) is the vehicle-side radio and processing element. The Road Side Unit (RSU) is its fixed counterpart at intersections, ramps and work zones. In the reference architecture, each OBU exchanges V2V messages with other OBUs and V2I messages with RSUs. It also exchanges V2P messages with pedestrian devices and reaches cloud services through V2N.
The standard also fixes a communication stack so that implementations divide responsibilities in the same way. From top to bottom, the layers are:
The security layer sits between message creation and the network. Signing therefore happens to a finished message rather than being left to each application. Because the access layer is a swappable slot, the same facilities-layer messages can ride on either radio family. That is how the standard supports both DSRC and C-V2X without defining two parallel message sets.
The stated scope is broad: protocols for all four modes, message formats, security and privacy, latency and reliability requirements, collision-avoidance algorithms and platooning coordination. The guiding principle is ๅผ็ไบบ้ (Benefit All Humanity). In practice this means the safety-critical path is optimised first. Confidentiality, for instance, is marked optional for safety messages, while authentication and integrity are not.
The standard accommodates two radio families. DSRC (Dedicated Short-Range Communications) is based on IEEE 802.11p. At the physical layer it uses 10 MHz channels with OFDM modulation (BPSK, QPSK, 16-QAM or 64-QAM). At the MAC layer it uses CSMA/CA, requires no association and supports both broadcast and unicast, with Quality of Service through EDCA. The absence of association matters for vehicles: two cars passing each other at highway speed have no time for a handshake.
C-V2X (Cellular V2X) covers LTE-V2X and 5G NR-V2X. It defines two operating modes. Mode 3 is network-based, with the cellular network scheduling resources. Mode 4 is direct communication over the PC5 interface, which works without network coverage. The specification summarises the radio families as follows:
| Parameter | DSRC (802.11p) | C-V2X (LTE) | 5G NR-V2X |
|---|---|---|---|
| Frequency | 5.9 GHz | 5.9 GHz | 5.9 GHz |
| Range | 300-1000 m | 500-1500 m | 1000-2000 m |
Ranges are quoted as bands rather than single values because propagation dominates the result. For DSRC the data sheet gives line-of-sight reach of up to 1000 meters and urban reach of 300-500 meters.
Spectrum is regionally fragmented, and the standard records the three allocations it addresses: 5.850-5.925 GHz in the US (75 MHz), 5.875-5.905 GHz in the EU (30 MHz) and 5.770-5.850 GHz in Japan (80 MHz). Channel numbering follows the 5.9 GHz plan. Channel 172 (5.860 GHz) is the safety-critical control channel, and channels 174 through 184 are service channels. A device that ships in several regions therefore needs region-aware channel configuration, not a single hard-coded frequency.
The message envelopes are where the numbers become binding. The standard states that performance requirements live alongside the formats because they constrain valid encodings. A BSM that cannot be produced and delivered within its latency budget is not useful, however well formed. The canonical set is:
| Message | Frequency | Purpose | Latency |
|---|---|---|---|
| BSM (Basic Safety Message) | 10 Hz | Position, speed, heading | <10 ms |
| CAM (Cooperative Awareness) | 1-10 Hz | Vehicle status | <100 ms |
| DENM (Event Notification) | Event-driven | Hazard warnings | <10 ms |
| SPaT (Signal Phase/Timing) | 10 Hz | Traffic signal info | <100 ms |
| MAP (Map Data) | 1 Hz | Road geometry | <500 ms |
The terminology reflects two lineages. BSM comes from SAE J2735, and CAM comes from the ETSI family. Both are carried because American and European deployments evolved separately, and vehicles sold across regions need to understand both. A BSM in the SDK looks like this:
const bsm: V2VMessage = {
type: MessageType.BSM,
position: { lat: 37.7749, lon: -122.4194, alt: 10 },
speed: 65, // km/h
heading: 270, // degrees
acceleration: { x: 0.5, y: 0, z: 0 },
timestamp: Date.now()
};
await v2x.sendV2V(bsm);The same SDK exposes detectCollisionRisk on received messages and subscribeToInfrastructure for signal data. The envelope carries only what a neighbour needs to reason about motion: where, how fast, which way, how it is accelerating and when the sample was taken.
V2V is a broadcast model. Vehicles transmit periodically, with no handshake or acknowledgment, and every vehicle in range receives. Transmission can also be event-triggered, for example for an emergency brake warning, a collision warning, a lane change notification or a hazardous location warning. The periodic layer gives everyone a shared picture, and the event layer interrupts that picture when something changes.
Although the BSM is nominally 10 Hz, the cooperative-awareness section defines a speed-dependent update schedule. The rate rises with the kinematic urgency of the vehicle:
| Condition | Update rate |
|---|---|
| Stationary | 1 Hz |
| Low speed (<50 km/h) | 2 Hz |
| Medium speed (50-100 km/h) | 5 Hz |
| High speed (>100 km/h) | 10 Hz |
| Emergency | 20 Hz |
The reasoning is that a stationary car does not need to flood a shared channel, while a car at highway speed covers a large distance between samples. Data elements shared include GPS position (latitude, longitude, elevation), speed and heading, longitudinal, lateral and vertical acceleration, vehicle dimensions, brake status and turn signals.
Collision detection is built on Time-to-Collision (TTC):
TTC = (d - L1 - L2) / (v1 - v2)
d = distance between vehicles
L1, L2 = vehicle lengths
v1, v2 = vehicle speedsTTC is meaningful only when the closing speed is positive. The reference algorithm returns no warning when the relative speed is zero or negative, meaning the target is moving away or travelling at the same speed. It also discards targets whose heading differs by more than 45 degrees, treating them as not in the collision path. Remaining cases fall into four threat levels:
The protocol-phase implementation of Forward Collision Warning (FCW) refines this. It keeps a warning threshold of 2.5 seconds and a critical threshold of 1.5 seconds, plus a minimum distance of 5.0 meters. For critical and warning outcomes it also computes the required deceleration from the kinematic relation v² = v₀² + 2ad, which gives a = v² / (2d) for the relative velocity. A warning therefore tells the vehicle how hard it would have to brake, in addition to saying that a conflict exists.
Two further applications use the same shared-state idea. Intersection Movement Assist (IMA) applies when the ego vehicle approaches an intersection (the reference uses a 50 meter radius). For each other approaching vehicle it predicts a path through the intersection and finds the conflict point where the paths cross. It then compares arrival times at that point. If the arrivals are within 3.0 seconds of each other, the vehicle is flagged as a threat, and threats are sorted with the most urgent first. The recommended action is SLOW_DOWN when the ego vehicle must yield, otherwise MONITOR.
Blind Spot Warning defines detection zones on each side. The API-phase text describes them as 0.5 m to 3.0 m from the vehicle side, from the B-pillar to 2 m behind the vehicle. The warning conditions are a vehicle detected in the zone together with an activated turn signal or detected lateral acceleration. The pairing is deliberate. A neighbour in the zone is normal in dense traffic and becomes relevant only when the driver shows intent to move laterally.
The documents give the TTC formula in two slightly different forms. The API-phase version subtracts vehicle lengths from the gap, while the protocol-phase FCW class divides raw distance by relative velocity. An implementer should therefore state which gap definition a given deployment uses, since it shifts the threshold crossings.
V2I moves the vehicle from reasoning about neighbours to reasoning about the road. The Road Side Unit (RSU) is the anchor. The standard lists its functions: broadcasting SPaT for traffic signals, transmitting MAP data for intersection geometry, collecting traffic data, relaying V2V messages to extend range, and emergency vehicle preemption. Deployment is prioritised by risk. Signalized intersections, highway on/off ramps, school zones and work zones rank highest. Parking areas, transit stations and toll plazas are medium, and rural highways and residential areas are lowest.
SPaT and MAP are complementary. MAP describes where the lanes are, and SPaT says what each signal group is doing right now. A vehicle joins them by looking up the signal group for its lane. The reference TypeScript reduces a SPaT message to a simple phase record:
interface SignalPhase {
state: 'red' | 'yellow' | 'green';
timeRemaining: number; // seconds
nextPhase: 'red' | 'yellow' | 'green';
nextPhaseTime: number;
}Once the vehicle knows the phase and its remaining time, it can offer Green Light Optimal Speed Advisory (GLOSA). The logic is two-sided. If the signal is red and the vehicle would reach the intersection before the red ends, it recommends a lower speed so the car arrives as the light turns green. If the signal is green but the vehicle would arrive after it ends, it advises speeding up or preparing to stop. This turns signal timing into an efficiency feature, since fewer unnecessary stops mean smoother flow.
Emergency vehicle preemption shows request-response messaging between a vehicle and the RSU. The vehicle sends a Signal Request Message (SRM) identifying itself, its type, its priority, its approach and departure directions, its ETA and the green time it needs:
{
"messageType": "SRM",
"requestID": 12345,
"vehicleID": "FIRE-001",
"vehicleType": "fire",
"priority": "emergency",
"route": { "approach": "north", "departure": "south" },
"eta": 15,
"duration": 120
}The infrastructure answers with a Signal Status Message (SSM) that echoes the requestID so the two can be correlated. In the example the status is granted, with a preemptionTime and a duration of 120. Because there are many simultaneous requesters, the requestID is the only thing that lets a vehicle know which response is its own.
Hazards that are not signals use the event channel. Work-zone warnings are expressed as DENM messages with an eventType such as roadWorks and a location. These are event-driven and carry the tightest latency budget in the message table (<10 ms). A static hazard can therefore reach an approaching vehicle in the same time budget as a dynamic collision warning.
Every message above is safety-relevant, so a forged BSM is a safety problem as well as a security one. The standard lists six security requirements: authentication, integrity, non-repudiation, confidentiality (optional for safety traffic), availability against denial of service, and privacy. Trust is anchored in a Public Key Infrastructure. A Root CA issues to an Enrollment CA, which provides long-term vehicle enrollment certificates, and to an Authorization CA, which provides short-term pseudonym certificates that are changed frequently. The certificate format follows IEEE 1609.2. Its fields include a validity period, a circular geographic region, appPermissions keyed by a PSID (0x20 denotes the Basic Safety Message), and an ecdsaNistp256 verification key.
A signed message wraps the BSM, CAM or DENM payload with header information, signer information (a certificate or digest), generation time, generation location and an ECDSA P-256 signature. The reference verification routine runs six checks in sequence:
The last two checks are specific to vehicles. A perfectly signed but stale message is a replay, and a perfectly signed message from outside the certificate's region is suspicious. A companion routine looks for position inconsistency against the vehicle's history, for example movement faster than the last reported speed would allow, which feeds misbehavior detection.
Authentication creates its own tension with privacy, because a car that broadcasts its position ten times a second can be tracked. The standard's answer is pseudonym certificates. These change every 5-10 minutes, are not linkable to each other, and are pre-loaded as a pool of 20 or more. It also defines privacy zones: no broadcast within 500 m of a registered home, and automatic protection of sensitive places such as hospitals, government buildings and religious sites. Location obfuscation is expressed as four levels. MAXIMUM suppresses position entirely, HIGH rounds coordinates to three decimals (about 100 m), MEDIUM to four decimals (about 10 m), and LOW or NONE broadcasts full precision (about 1 cm). Obfuscation trades accuracy for anonymity, so it is suited to non-safety uses rather than to the collision-warning path, which depends on precise positions.
Taken together, the chapter's points form one chain:
The overview has so far stayed at the level of messages and decisions made by individual vehicles. The next chapter builds on this vocabulary and moves to the coordination protocols above single-vehicle warnings. These include leader-follower platooning, in which a string of vehicles operates as one coordinated unit, and the wider technical foundation that makes such cooperation possible.