The 73 standards behind a connected-vehicle deployment, and the role each one plays

What it does. The dictionary. Defines the MessageFrame wrapper, the message IDs and every data element: BSM (20), SPaT (19), MAP (18), TIM (31), PSM (32), SRM (29), SSM (30), RTCM (28), EVA (22), PVD (26) and the newer messages up to ID 50. Encoded with ASN.1 UPER, which is why a BSM fits in about 40 bytes.

Role in the system. Every box that sends or receives a V2X message encodes and decodes against it: the RSU, the OBU, the edge computer and the back office. A sender and receiver on different revisions can disagree on a field.

What it does. How a vehicle must build and send the BSM: 10 per second, the position accuracy, path history and path prediction, congestion control, and the security rules for the pseudonym certificates.

Role in the system. The OBU's rulebook. An OBU that passes J2945/1 produces BSMs that every other vehicle can act on.

What it does. The V2V awareness applications on top of the BSM: forward collision warning, emergency electronic brake lights, intersection movement assist, blind spot, do not pass, left turn assist. Defines what each one must detect and when it may alert.

Role in the system. The OBU's application layer for V2V warnings. Our simulator's V2V scenario follows these definitions.

What it does. How vehicles report weather observations (wipers, traction, temperature) through probe data and how the roadside turns them into weather alerts.

Role in the system. Agency weather programs and the probe data pipeline from OBU to TMC.

What it does. The RSM (ID 33) for work zones, reduced speed zones and lane closures, more precise than a TIM: lane-level geometry, worker presence, speed limits per segment.

Role in the system. The work zone trailer or RSU broadcasts it; the OBU displays it. Ships inside the J2735 package.

What it does. What goes in the SSP field of an IEEE 1609.2 certificate for each application: which messages and which content a certificate holder may sign (a signal controller's RSU may sign SPaT, a transit bus may request priority).

Role in the system. Ties the SCMS certificates to the applications. The SCMS provider provisions the SSPs the agency asks for.

What it does. How CACC and platooning vehicles exchange intent and control data and what gap, latency and fallback behavior they must keep.

Role in the system. The OBU in a platooning truck. Our corridor and toll scenarios follow its gap logic.

What it does. How a pedestrian, cyclist or worker device sends the PSM and how a vehicle must handle it, including the cluster and sensor-fusion rules.

Role in the system. VRU devices and the OBU that warns the driver; the RSU may relay or substitute SDSM.

What it does. What an RSU application must do: forwarding received messages, SPaT and MAP broadcast behavior, message logging and the interfaces to the back office.

Role in the system. The RSU and the edge computer behind it; it complements CTI 4001, which specifies the device.

What it does. How a vehicle asks for priority or preemption with the SRM, how the roadside answers with the SSM, and what the controller must do in between.

Role in the system. Transit, freight and emergency vehicles (the OBU builds the SRM), the RSU and the controller or edge computer that arbitrates the request.

What it does. How vehicles collect and hand over probe snapshots (position, speed, events) with the Probe Vehicle Data message, and the privacy rules around it.

Role in the system. The OBU and the back office that collects operations data.

What it does. The SDSM (ID 41): a roadside or vehicle sensor describing the objects it sees (people, bikes, cars, obstacles) with position, size, heading and confidence, so others can be warned about road users who carry no radio.

Role in the system. The edge computer or RSU with a camera, radar or lidar; the OBU that consumes it. Ships inside the J2735 package.

What it does. The US profile for running the SAE messages over 3GPP LTE-V2X PC5: channel, bandwidth, transmit power and antenna requirements, congestion control, and how the 1609 layers sit on the sidelink.

Role in the system. Every C-V2X OBU and RSU radio in the US. Vendors certify against it; it is the reason two makers' radios interoperate.

What it does. The code list a TIM uses to say things: 'stopped traffic', 'work zone', 'ice', 'reduced speed', each a number so the OBU can render it in any language.

Role in the system. Anyone who authors TIMs: the TMC, the edge computer, a work zone trailer. J2540/1 is the RDS phrase list and J2540/3 the national names list.

What it does. The levels 0 to 5 of driving automation. Not a V2X standard, but every automation application in the catalog is described against it.

Role in the system. Vocabulary for the agency and the vendor; nothing on the hardware bench implements it.

What it does. The map of the 1609 family: how the networking, security and channel layers fit together and where the SAE messages sit on top.

Role in the system. Reading, not implementing. Start here if the family is new to you.

What it does. The Secured Protocol Data Unit (SPDU) that wraps every broadcast message: the signature, the signer's certificate or its 8-byte digest, the PSID, the generation time and location, and the certificate format itself (permissions, validity, region, the encryption key). Also the encryptedData structure used for SCMS traffic and misbehavior reports.

Role in the system. The RSU and the OBU sign with it and verify with it; the hardware security module holds the keys; the SCMS issues the certificates it describes. A receiver drops anything it cannot verify.

What it does. How a device talks to the SCMS: enrollment, authorization certificate requests, certificate downloads, the Certificate Trust List and the Certificate Revocation List, misbehavior reporting, and the butterfly key expansion that lets one request produce many pseudonym certificates.

Role in the system. The RSU's and the OBU's SCMS client, and the SCMS provider's end-entity interface. This is what the bootstrap in the security lab walks through.

What it does. The WAVE Short Message Protocol (WSMP): the small header (version, transport PID, PSID, length, optional channel and power) in front of the SPDU, so a message needs no IP stack. Also how IPv6 runs over the same radio for management traffic, and the WAVE Service Advertisement.

Role in the system. The RSU and the OBU both use WSMP for every broadcast message; the PSID in its header tells the receiver which application the bytes belong to.

What it does. How a DSRC radio switches between the control channel and service channels on a 50 ms cadence and what that means for timing.

Role in the system. DSRC-era RSUs and OBUs only. C-V2X PC5 manages its channel inside 3GPP, so a C-V2X deployment does not use it.

What it does. The registry of Provider Service Identifiers (PSIDs): BSM 0x20, SPaT 0x8002, MAP 0xE0000017, TIM and RSM 0x8003, PSM 0x27, SDSM 0x8010, SRM 0xE0000016, SSM 0xE0000015. The same value appears in the WSMP header and in the certificate's permissions.

Role in the system. The RSU's store-and-repeat table, the OBU's receive filters and the SCMS certificates all use these numbers; a mismatch is the most common reason a message is ignored.

What it does. A payment protocol for tolling over the WAVE stack. Rarely used; US tolling by OBU is running on the J2735 toll messages and back-office accounts instead.

Role in the system. Toll operators evaluating payment over 5.9 GHz.

What it does. The radio US C-V2X runs on: a direct vehicle-to-vehicle and vehicle-to-roadside link (PC5 sidelink) that works without a cell tower, using sensing-based scheduling so hundreds of radios can share a 20 MHz channel. A BSM crosses it in a few milliseconds.

Role in the system. The modem inside every C-V2X RSU and OBU. SAE J3161/1 profiles it for the US; the FCC rules fix its channel and power.

What it does. The 5G sidelink: higher throughput, unicast and groupcast, lower latency. It is not interoperable with LTE-V2X on the same channel, so the US has stayed on Release 14 for safety messages.

Role in the system. Future radios. Buy for Release 14 compatibility today and ask vendors how they will coexist.

What it does. The ordinary cellular connection: vehicle to network, roadside to network. It carries TIMs from a cloud service, the backhaul for a cellular-connected RSU, certificate downloads and log uploads. Not for the time-critical safety broadcasts.

Role in the system. The backhaul router at a site without fiber, and the OBU's cellular modem for SCMS top-ups.

What it does. The DSRC radio: Wi-Fi style OFDM at 5.9 GHz with no association step. Folded into the base 802.11 standard since 2012.

Role in the system. Legacy RSUs and OBUs. Under FCC 24-123 they must stop transmitting in the band by December 14, 2026.

What it does. The successor to 802.11p: higher data rates and range, backward compatible with 802.11p. Not adopted for the US 5.9 GHz band, which the FCC assigned to C-V2X.

Role in the system. Reading only, unless you work outside the US.

What it does. The final US rules: the upper 30 MHz of 5.9 GHz for C-V2X as three 10 MHz channels that may be combined into 20 or 30 MHz, 33 dBm EIRP for RSUs with a height rule, OBU limits, and the DSRC sunset on December 14, 2026.

Role in the system. Every radio and every antenna height on the bench. The licensing planner and the build-out system check both enforce it.

What it does. The rules an RSU licensee lives under: who is eligible (90.388), one non-exclusive geographic license per entity and a ULS registration per RSU (90.389), channels and priority (90.390), power and height (90.391), NTIA coordination near federal sites (90.387(b)), and the 12 months to put a registration on the air (90.155).

Role in the system. The agency's license and every RSU site's registration. File on Form 601 with Schedule M in the Universal Licensing System.

What it does. Vehicles need no individual license: an OBU operates under a rule-based authorization, as long as it is a certified device and keeps to the EIRP limits in 95.3204.

Role in the system. Every OBU, aftermarket or built in. The vehicle owner files nothing.

What it does. A 5.9 GHz radio must be certified before it is marketed or used; the FCC ID on the label is the proof. Ask the vendor for the grant and the operating conditions it was tested under (bandwidth, power, antenna gain).

Role in the system. Procurement check for every RSU and OBU. The build-out system check asks for the FCC ID.

What it does. The procurement-ready RSU specification that replaced the older USDOT RSU specification: C-V2X PC5 radio, IEEE 1609.2 security with an SCMS client, NTCIP 1218 management, GNSS, message forwarding to the back office, environmental and power requirements, and the logging an agency needs.

Role in the system. What you write into the RSU bid. Vendors state conformance to it; OmniAir tests against it.

What it does. How SPaT and MAP must be produced so every vehicle reads an intersection the same way: SPaT ten times a second within 300 ms end to end, MAP once a second with 0.2 m node accuracy, time within 10 ms of UTC, and the verification tests. The phase two drafts split it into 4501/1 SPaT, 4501/2 MAP, 4501/3 security and 4501/4 testing.

Role in the system. The controller, the edge computer that builds SPaT, the RSU that sends it, and the agency that verifies the intersection before it goes live. The map builder follows its MAP rules.

What it does. A common data feed for work zone location, lanes, speed limits and workers present, so the same record can become a TIM or RSM on the road and a map layer in a navigation app.

Role in the system. The TMC's work zone system and the edge computer or trailer that turns records into messages.

What it does. The SNMP objects an agency uses to configure and watch an RSU: the store-and-repeat and immediate-forward message tables, radio settings, GNSS status, certificate status, interface and system health, and the forwarding of received messages to the back office.

Role in the system. The RSU's management interface and what the central system polls. This portal's RSU push writes into these tables.

What it does. Environmental, electrical and mechanical requirements for roadside V2X equipment: temperature, humidity, vibration, surge and power, in the style of the NEMA controller standards.

Role in the system. The RSU and cabinet hardware specification for harsh sites; often cited next to CTI 4001.

What it does. Which SNMP (v1 and v3) features, security and transport ITS devices must support, including the SNMPv3 authentication and privacy an RSU on an agency network should run with.

Role in the system. The switch, the RSU and the central software agree on this; it is why an RSU wants SNMPv3 credentials, not a community string.

What it does. The objects every NTCIP device shares: time, module identity, event logs, database transactions.

Role in the system. RSU and controller alike; the central system reads them first.

What it does. The controller's SNMP objects: phases, rings, timing plans, coordination, preempts and priority, and in v03 the SPaT data (phase states and the time-to-change values) the edge computer or RSU reads to build the SPaT message. Also the controller's interface for priority requests.

Role in the system. The signal controller, and the edge computer or RSU that turns its status into SPaT. Without a v03 controller you need a vendor adapter.

What it does. How a priority request reaches the controller: the Priority Request Server and Priority Request Generator roles, request objects and the status the controller reports back.

Role in the system. The controller and the edge computer that converts an SRM into a controller request; the SSM reports the result.

What it does. The controller hardware and Linux platform: engine board, power, the serial and Ethernet ports, and the ability to run more than one application on the same controller.

Role in the system. The controller in the cabinet, and one place the SPaT application can live.

What it does. The API that lets several applications share the ATC's front panel and field I/O without fighting each other.

Role in the system. Vendors writing the SPaT or priority application on the controller.

What it does. The modern cabinet: serial bus to the input and output assemblies, 48 VDC, the monitor unit, and the rack space and power an RSU, switch and edge computer borrow.

Role in the system. The cabinet on the roadside bench. NEMA TS 2 cabinets are the older alternative.

What it does. The controller and cabinet standard most installed intersections run on: the malfunction management unit, the SDLC bus, Type 1 and Type 2 configurations, and the NTCIP objects a TS 2 controller must support.

Role in the system. The existing cabinet most connected intersections are retrofitted into.

What it does. The rules for signs, signals and markings. A V2X message that contradicts the posted sign is wrong; the MUTCD also governs the work zone layouts a TIM or RSM describes.

Role in the system. The agency engineer who approves SPaT, MAP and work zone messages.

What it does. The governance over the US SCMS: which root CAs are trusted (the elector-signed CTL every device holds), how providers must operate, what a device must do to enroll, and how misbehavior and revocation work. IEEE 1609.2.1 is the protocol; these documents are the policy.

Role in the system. The SCMS provider you contract with, and the enrollment of every RSU and OBU. Production equipment ignores a certificate that does not chain to the CTL.

What it does. ECDSA on the P-256 curve is what 1609.2 signatures use in the US (Brainpool curves are used in Europe). A BSM signature is 64 bytes and verifies in well under a millisecond on an OBU.

Role in the system. Inside every signature on the air and inside the HSM. The security lab signs with exactly this.

What it does. The certification level of the hardware security module that holds a device's private keys. Agencies and the SCMS policy ask for a validated module so a stolen RSU cannot be turned into a forger.

Role in the system. The HSM inside the RSU and the OBU; a procurement line item.

What it does. ECIES wraps a one-time AES key to the recipient's public key; AES-128-CCM encrypts the payload with a 12-byte nonce and a 16-byte tag. This is how SCMS traffic and misbehavior reports are protected.

Role in the system. The SCMS client in the RSU and OBU, and the SCMS itself. Safety broadcasts are signed, never encrypted.

What it does. The vehicle's internal bus: a two-wire differential pair, frames of up to 8 bytes (64 with CAN FD), arbitration by identifier so the most urgent frame wins. Everything from wheel speed to brake status rides on it.

Role in the system. The OBU's connection to the vehicle. It reads speed, brake, steering and lights for the BSM; on a bench it can also write frames.

What it does. How requests and replies longer than one frame are split (ISO 15765-2) and the 500 kbit/s OBD-II CAN profile on the J1962 port (ISO 15765-4), the way a plug-in OBU asks the engine computer for speed or RPM.

Role in the system. The OBU plugged into the J1962 port of a passenger car or light truck.

What it does. The parameter IDs a vehicle answers on the diagnostic port: vehicle speed (PID 0x0D), RPM, throttle, and more. A polled value, not a broadcast, so it arrives slower than a native CAN frame.

Role in the system. The fallback a plug-in OBU uses when the vehicle's own CAN database is not available.

What it does. The CAN application layer for trucks and buses: 29-bit identifiers, parameter groups (PGNs) that broadcast speed, brakes, cruise state and more, with no polling needed. 250 kbit/s on J1939/11 and /15, 500 kbit/s on J1939/14.

Role in the system. The OBU in a transit bus, snow plow or truck. Transit and freight BSMs are built from J1939 data.

What it does. The 16-pin OBD-II connector under the dash: pins 6 and 14 are CAN high and low, 16 is battery, 4 and 5 are ground. Where an aftermarket OBU plugs in.

Role in the system. The physical interface for an aftermarket OBU; the build-out shows the pinout.

What it does. The diagnostic service set (read data by identifier, security access, routine control) a tool uses to talk to a controller over CAN. Writing to a vehicle through UDS is a bench and vendor activity, never a V2X application.

Role in the system. Bench work on the OBU's CAN interface; the safety rules on the build-out page apply.

What it does. The process for threat analysis and risk assessment over a vehicle's life. Any OBU that can write to CAN falls under it; NHTSA's cybersecurity best practices point to it.

Role in the system. The vehicle maker and the upfitter installing an OBU with CAN write access.

What it does. Temperature, vibration, load dump and power transients an in-vehicle electronic box must survive. The reason an OBU power feed wants a fused, ignition-switched supply and a proper ground.

Role in the system. The OBU and display installation in buses and trucks.

What it does. The safety integrity framework for vehicle electronics. An advisory OBU display is outside it; an application that acts on the vehicle (braking on a V2X warning) is not.

Role in the system. Automation and CACC applications; a reason most deployed V2X stays advisory.

What it does. How the controller, the edge computer and the RSU agree on the time. SPaT time-to-change values are in tenths of a second and the CI guide wants 10 ms of UTC, so every box needs a disciplined clock. The RSU's GNSS can serve as the time source for the cabinet.

Role in the system. Every box on the roadside bench; the system check flags a missing time source.

What it does. The corrections an RSU can rebroadcast inside the J2735 RTCMcorrections message so vehicles reach lane-level position accuracy.

Role in the system. The RSU with a GNSS reference and the OBU that applies the corrections.

What it does. The text sentences (GGA, RMC and others) a GNSS receiver streams: position, speed, heading, fix quality and time. The OBU's BSM and the RSU's time and position usually start here.

Role in the system. The GNSS receiver in the RSU and the OBU.

What it does. How the cabinet switch powers the RSU over the same Ethernet cable and how traffic is separated into VLANs (management, V2X forwarding, controller). An RSU with heaters can exceed Type 2.

Role in the system. The managed PoE switch and the RSU; the build-out adds up the PoE budget.

What it does. IEEE 1609.3 carries IPv6 over the air for management; the backhaul to the DOT network runs over IPsec or a TLS-protected VPN; SCMS traffic rides TLS.

Role in the system. The backhaul router, the RSU and the central system.

What it does. The protocol under NTCIP 1218 and 1202. Version 3 adds users, authentication and encryption, which an RSU reachable across an agency network needs.

Role in the system. The RSU, the controller and the central system; this portal's push uses it.

What it does. Surge protection and grounding of the pole-mounted RSU and its PoE run. Lightning on an unprotected run reaches the switch and the controller.

Role in the system. The surge protector and ground on the roadside bench.

What it does. Independent testing of radios, 1609 stacks, J2735 messages and security against the standards above, plus multi-vendor plugfests. A certified device appears on the OmniAir Qualified Products List.

Role in the system. A procurement requirement for RSUs and OBUs; the way an agency avoids testing interoperability itself.

What it does. Open source for the edge computer: V2X Hub turns controller data into SPaT, relays messages, hosts applications; the ODE and asn1_codec decode captures; the Connected Intersection tools verify SPaT and MAP against CTI 4501.

Role in the system. The edge compute device on the roadside bench and the agency's verification toolkit. This portal's codec is built from the same generated source.

What it does. The federal roadmap: targets for signalized intersections and highway miles with V2X by 2028 and 2036, the interoperability, security and standards expectations, and the funding paths.

Role in the system. The agency's planning document and the frame for a grant application.

What it does. A common set of assumptions about other road users' behavior that an automated vehicle may make. Referenced by automation applications that consume V2X data.

Role in the system. Automation programs; reading for everyone else.

What it does. Europe's BSM and its event message. Different encodings, different fields and a different rate rule (CAM adapts between 1 and 10 per second).

Role in the system. Recognize them in a European vendor's data sheet; a US deployment does not use them.

What it does. Europe's profile of IEEE 1609.2 certificates and signed messages, under the EU C-ITS Certificate Policy rather than the SCMS Manager. Close cousins, not interchangeable.

Role in the system. Context when a vendor says its HSM supports both.

What it does. The international profile of the J2735 intersection messages, so a European MAP and a US MAP share structure.

Role in the system. Multinational vendors and anyone comparing deployments.

What it does. The reference architecture (the ITS station) that ETSI and ISO work hangs on. The 1609 family plays the same role in the US.

Role in the system. Reading.