5.4 KiB
Bluetooth LE Protocol
Some data is made available via BLE so that a smartphone app can be used to display it in other use cases.
General
The BLE protocol contains some standardized services, but can also transport custom services. Each service is compromised of a set of characteristics which can be read or clients can even subscribe for changes.
Sadly we can't use any of the standardized service/ profiles to transmit V2X data. Even notifications about DENMs can't be transmitted since the Alert Notification Profile is only meant for the companion device to query notifications from the phone, but not the other way around.
So we will only use the Device Information Service to announce ourselves and use a custom service for everything else.
V2X BLE Service
V2X data is exposed via the service UUID c0b70000-d4f4-4000-ada8-f99a02ee315c (randomly generated UUID, with byte 3 and 4 set to zero) using the following characteristics:
| UUID | Type | Description |
|---|---|---|
| c0b70001-d4f4-4001-ada8-f99a02ee315c | PositionState |
Set position from external source |
| c0b70001-d4f4-4001-ada8-f99a02ee315c | RawMsgRx |
Raw CAM message (de-duplicated/ rate-limited) |
| c0b70001-d4f4-4002-ada8-f99a02ee315c | CamEvent |
Latest received CAM |
| c0b70001-d4f4-4003-ada8-f99a02ee315c | RawMsgRx |
Raw DENM message (de-duplicated/ rate-limited) |
| c0b70001-d4f4-4004-ada8-f99a02ee315c | DenmEvent |
Latest received DENM |
| c0b70001-d4f4-4005-ada8-f99a02ee315c | RawMsgRx |
Raw MAPEM message (de-duplicated/ rate-limited) |
| c0b70001-d4f4-4006-ada8-f99a02ee315c | RawMsgRx |
Raw SPATEM message (de-duplicated/ rate-limited) |
Message Format
Messages are encoded according to the Protocol Buffers specification since it provides decent binary size and has parser libraries for many programming languages.
All data types are defined in one .proto file: c_its-messages.proto.
Position State Input
When there's no GNSS receiver connected to the ESP, position, heading and speed information can be set using a PositionState message (see c_its-messages.proto).
Maximum message size (assuming realistic value ranges):
| field name | tag size | (max.) data size |
|---|---|---|
| timestamp_ms | 1 | 8 (I64) |
| position | 1 | 1 (LEN(10)) + sub-fields below |
| + latitude_deg | 1 | 4 (I32) |
| + longitude_deg | 1 | 4 (I32) |
| heading | 1 | 2 (VARINT(3600)) |
| speed | 1 | 2 (VARINT(~163 m/s)) |
| SUM | 6 + | 21 = 27 byte |
DENM Event
Each received DENM will be "published" as a DenmEvent message (see c_its-messages.proto).
Repetitions of the same message (e.g. no changes to the validity timeframe and location) may be dropped by the BLE server.
Maximum message size (assuming realistic value ranges):
| field name | tag size | (max.) data size |
|---|---|---|
| timestamp | 1 | 8 (I64) |
| validity_end_ts | 1 | 8 (I64) |
| station_id | 1 | 8 (I64) |
| seq_num | 1 | 3 (VARINT(65535)) |
| event_position | 1 | 1 (LEN(10)) + sub-fields below |
| + latitude_deg | 1 | 4 (I32) |
| + longitude_deg | 1 | 4 (I32) |
| event_heading_deg | 1 | 1 (VARINT(360)) |
| direction | 1 | 1 (VARINT(3)) |
| cause_code | 1 | 1 (VARINT(127)) |
| sub_cause_code | 1 | 1 (VARINT(127)) |
| SUM | 11 + | 40 = 51 byte |
Note: Cause code and sub-cause code are technically 0..255 in the ASN.1 spec, but that's mainly to ensure backwards-compatibility of the CHOICE with the old wire format. Realistically only cause codes up to 100 are defined and common sub-cause codes use values up to 22.
CAM Event
Each received (low-frequency) CAM will be "published" as a CamEvent message (see c_its-messages.proto).
Maximum message size (assuming realistic value ranges):
| field name | tag size | (max.) data size |
|---|---|---|
| station_id | 1 | 8 (I64) |
| station_type | 1 | 1 (VARINT(15)) |
| position | 1 | 1 (LEN(10)) + sub-fields below |
| + latitude_deg | 1 | 4 (I32) |
| + longitude_deg | 1 | 4 (I32) |
| vehicle_role | 1 | 1 (VARINT(15)) |
| vehicle_data | 1 | 1 (LEN(11)) + sub-fields below |
| + heading_deg | 1 | 2 (VARINT(360)) |
| + speed | 1 | 2 (VARINT(~163 m/s)) |
| + vehicle_length_dm | 1 | 2 (VARINT(1023)) |
| + vehicle_width_dm | 1 | 1 (VARINT(62)) |
| SUM | 11 + | 27 = 38 byte |
Raw Received Message
Received V2X messages will be "published" as a RawMsgRx message (see c_its-messages.proto).
Repetitions of the same message should be dropped by the BLE server.
Consecutive messages with no relevant changes may be dropped by the BLE server while keeping some minimal publishing rate, e.g.:
- only publish MAPEMs every 10 seconds (since content doesn't change, but client may be restarted after first MAPEM from an intersection was received)
- drop high-frequency CAMs (only publish "full" CAMs which include the low-frequency container)