This example implements a BAP Unicast Client on top of the selected BLE host stack (Bluedroid by default; NimBLE via the sdkconfig.defaults.nimble overlay). It scans for a peer advertising the ASCS service UUID with a complete local name of BAP Unicast Server, opens an ACL connection, initiates pairing, exchanges MTU, and starts GATT service discovery.

Once the peer is reachable, the client uses the BAP unicast client API (esp_ble_audio_bap_unicast_client_discover, esp_ble_audio_bap_stream_config, esp_ble_audio_bap_unicast_group_create, esp_ble_audio_bap_stream_qos, esp_ble_audio_bap_stream_enable, esp_ble_audio_bap_stream_connect, esp_ble_audio_bap_stream_start) to discover sink and source ASEs, configure each with the LC3 16_2_1 unicast preset, build a unicast group, set QoS, enable, and ISO-connect the streams. Source (TX-from-server) streams are started by this client; sink (TX-to-server) streams are started by the server.

The TX path in stream_tx.c registers each newly started sink stream with example_audio_tx_scheduler and feeds it SDU-sized buffers driven by esp_ble_audio_bap_stream_send. The PACS callbacks (pac_record, endpoint, location, available_contexts) print the discovered peer capabilities.

Requirements

A board with BLE 5.2, ISO, and LE Audio support (e.g. ESP32-H4, ESP32-S31)
Peer device running the paired example

Configuration

Open menuconfig:

idf.py menuconfig

No build-time options — runtime defaults are baked into source.

Security & Pairing

This example inherits a Just-Works pairing model (LE Secure Connections, no MITM) with bonding enabled from the shared init at ../common_components/example_init/ble_audio_example_init.c.

Build & Flash

The base sdkconfig.defaults defaults to the Bluedroid host; idf.py automatically merges the per-target overlay (sdkconfig.defaults.$IDF_TARGET). To build with NimBLE host instead, layer sdkconfig.defaults.nimble on top via -DSDKCONFIG_DEFAULTS.

Bluedroid host (default)

idf.py set-target esp32h4
idf.py -p PORT flash monitor

NimBLE host

idf.py set-target esp32h4
idf.py -DSDKCONFIG_DEFAULTS="sdkconfig.defaults;sdkconfig.defaults.esp32h4;sdkconfig.defaults.nimble" -p PORT flash monitor

For esp32s31, replace the chip overlay accordingly.

(Exit serial monitor with Ctrl-].)

Example Flow

app_main initializes NVS, the BLE host, runs app_host_init for host-specific setup (GAP callback, plus a Bluedroid-only GATTC app registration so we have a gattc_if for outgoing aux-open), brings up the LE Audio common layer, registers stream ops on every sink/source slot, registers the BAP unicast client callbacks, calls set_device_name() (uses LOCAL_DEVICE_NAME from central.h), then calls ext_scan_start.
Passive extended scanning runs forever; scan_data_cb matches the complete local name BAP Unicast Server and parses the ASCS service-data AD for announcement type plus a 32-bit available-contexts field.
On a connectable match ext_scan_recv calls ext_scan_stop and then conn_create (NimBLE: ble_gap_connect; Bluedroid: esp_ble_gattc_aux_open); the resulting ESP_BLE_AUDIO_GAP_EVENT_ACL_CONNECT saves the handle and calls pairing_start (NimBLE: ble_gap_security_initiate; Bluedroid: esp_ble_set_encryption).
After the security-change event the client triggers exchange_mtu. On NimBLE this issues ble_gattc_exchange_mtu; on Bluedroid the GATTC adapter auto-configures MTU inside BTA_GATTC_Enh_Open, so exchange_mtu drives esp_ble_audio_gattc_disc_start directly. gatt_mtu_change then calls esp_ble_audio_gattc_disc_start (the retry is idempotent — -EALREADY is treated as success), and once both MTU and discovery are reported discover_sinks runs.
discover_cb chains sink discovery into discover_sources; endpoint_cb records each EP into the next free sinks[]/sources[] slot, and source completion kicks configure_stream, which configures every EP with unicast_preset.codec_cfg.
After the last config_cb, create_group builds pair_params (sinks first, then sources) with ESP_BLE_ISO_PACKING_SEQUENTIAL and calls esp_ble_audio_bap_unicast_group_create, then set_stream_qos.
When is_all_stream_qos_set returns true enable_stream walks every stream; on the final enable_cb, connect_stream ISO-connects each pair (sink first if available); when all are up start_stream starts only source streams. Sink streams are started by the server, and stream_started_cb calls stream_tx_register to begin TX.

Expected Log

TAG: BAP_UCL.

Startup and scanning:

I (xxx) BAP_UCL: Scanning for unicast server...
I (xxx) BAP_UCL: Unicast server found: type ... contexts 0x...

ACL, security, MTU, discovery:

I (xxx) BAP_UCL: Connected: handle ... role ... peer ...
I (xxx) BAP_UCL: Security: handle ... level ... bonded ...
I (xxx) BAP_UCL: MTU updated: handle ... mtu ...
I (xxx) BAP_UCL: Service discovery started: handle ...
I (xxx) BAP_UCL: Service discovery complete: handle ...

ASE discovery:

I (xxx) BAP_UCL: [SNK] Discovering...
I (xxx) BAP_UCL: [SNK #...] Endpoint discovered
I (xxx) BAP_UCL: [SNK] Discovery complete
I (xxx) BAP_UCL: [SRC] Discovering...
I (xxx) BAP_UCL: [SRC #...] Endpoint discovered
I (xxx) BAP_UCL: [SRC] Discovery complete

Config / group / QoS / enable / connect / start:

I (xxx) BAP_UCL: [SNK #...] Config ACK'd
I (xxx) BAP_UCL: [SNK #...] Stream configured, QoS preference:
I (xxx) BAP_UCL: [SNK #...] Configured
I (xxx) BAP_UCL: Pair[...]: SNK=... SRC=...
I (xxx) BAP_UCL: Unicast group created (... pair(s))
I (xxx) BAP_UCL: [SNK #...] QoS ACK'd
I (xxx) BAP_UCL: [SNK #...] QoS set
I (xxx) BAP_UCL: [SNK #...] Enable ACK'd
I (xxx) BAP_UCL: [SNK #...] Enabled
I (xxx) BAP_UCL: Pair[...] connecting (SNK first)
I (xxx) BAP_UCL: [SNK #...] ISO connected (pair[...])
I (xxx) BAP_UCL: [SNK #...] Connected
I (xxx) BAP_UCL: [SRC] Starting all source streams
I (xxx) BAP_UCL: [SRC #...] Stream started
I (xxx) BAP_UCL: [SNK #...] Stream started
I (xxx) BAP_UCL: [SNK #...] Started (SDU ..., interval ... us)

Disconnect:

I (xxx) BAP_UCL: [SNK #...] ISO disconnected, reason 0x...
I (xxx) BAP_UCL: Disconnected: handle ... reason 0x...

Peer Pairing

Run unicast_server on a second board.

Server boots and starts connectable extended advertising with the ASCS UUID and BAP Unicast Server name.
Client boots and scans; on matching the name and ASCS service data it cancels scanning and issues conn_create (NimBLE: ble_gap_connect; Bluedroid: esp_ble_gattc_aux_open).
After ACL is up the client initiates pairing/security; both sides log Connected: and Security:.
Client exchanges MTU and runs GATT discovery, then discovers sink ASEs followed by source ASEs on the server.
Client drives ASCS Config -> QoS -> Enable -> ISO Connect on every ASE; the server returns its QoS preference and accepts each operation.
Client starts source streams; server auto-starts each sink stream from stream_enabled_cb, after which the client's TX scheduler feeds SDUs and the server's stream_recv_cb accumulates RX metrics.