Bluetooth: LLPM
The Bluetooth® Low Latency Packet Mode (LLPM) sample uses the GATT Latency Service and the GATT Latency Client to showcase the LLPM proprietary Bluetooth extension from Nordic Semiconductor. You can use it to determine the transmission latency of LLPM-enabled connections, or to compare with different connection parameters and check their influence on the results.
Requirements
The sample supports the following development kits:
Hardware platforms |
PCA |
Board name |
Build target |
|---|---|---|---|
PCA10040 |
|
||
PCA10059 |
|
||
PCA10056 |
|
The sample also supports other development kits running SoftDevice Controller variants that support LLPM (see SoftDevice Controller proprietary feature support). You can use any two of the development kits mentioned above and mix different development kits.
Additionally, the sample requires a connection to a computer with a serial terminal for each of the development kits.
Overview
The LLPM is designed for applications in which the interface response time is critical for the user. For example, for virtual reality headsets or gaming mouse and keyboard.
The key LLPM elements are:
- LLPM connection interval (1 ms)
The connection interval defines how often the devices must listen on the radio. The LLPM introduces the possibility to reduce the connection interval below what is supported in Bluetooth LE. The lowest supported connection interval is 1 ms for one link.
- Physical layer (PHY)
Starting with Bluetooth® 5, the over-the-air data rate in Bluetooth Low Energy supports 2 Ms/s (mega symbol per second), which allows for faster transmission. The LLPM connection interval is only supported on LE 2M PHY. Otherwise, the SoftDevice Controller will deny the request command.
- QoS connection event reports
When reports are enabled, one report will be generated on every connection event. The report gives information about the quality of service of the connection event. The values in the report are used to describe the quality of links. For parameter descriptions, see
sdc_hci_vs_subevent_qos_conn_event_report_t(insdc_hci_vs.h).- Transmission latency
The definition of the latency used in this example counts the time interval from the sender’s application to the GATT service of the receiver. It demonstrates the performance of an LLPM-enabled connection that the receiver will receive the data approximately every 1 ms.
- GATT Latency Service
To measure the transmission latency from application layers, a GATT Latency service BT_UUID_LATENCY is included to compute the time spent. When the sender writes its timestamp to the BT_UUID_LATENC_CHAR characteristic of the receiver, the Latency service of the receiver will automatically reply back. Whenever the sender receives a response, it will use its current time and the corresponding timestamp written before to estimate the round-trip time (RTT) of a writing characteristic procedure (see Bluetooth Core Specification: Vol 3, Part F, 3.4.5 Writing attributes).
GATT Attributes Type
UUID
Property
Primary service
BT_UUID_LATENCY
Read only
Characteristic
BT_UUID_LATENCY_CHAR
BT_GATT_CHRC_WRITE, BT_GATT_PERM_WRITE
This sample transmits data between two development kits to measure the transmission latency in between. One of the devices is connected as a central and another is connected as a peripheral. The performance is evaluated with the transmission latency dividing the estimated round-trip time in half (RTT / 2).
By default, the following values are used to demonstrate the interaction of the connection parameters:
Parameter |
Value |
|---|---|
Connection interval |
80 units (100 ms) |
LLPM connection interval |
Lowest interval (1 ms) |
Physical layer (PHY) |
LE 2M PHY |
Building and running
This sample can be found under samples/bluetooth/llpm in the nRF Connect SDK folder structure.
To build the sample with Visual Studio Code, follow the steps listed on the How to build an application page in the nRF Connect for VS Code extension documentation. See Configuring and building an application for other building scenarios, Programming an application for programming steps, and Testing and optimization for general information about testing and debugging in the nRF Connect SDK.
Testing
After programming the sample to both development kits, test it by performing the following steps:
Connect to both kits with a terminal emulator (for example, nRF Connect Serial Terminal). See Testing and optimization for the required settings and steps.
Reset both kits.
In one of the terminal emulators, type “c” to start the application on the connected board in the central (tester) role.
In the other terminal emulator, type “p” to start the application in the peripheral (peer) role.
Observe that the kits establish a connection.
The central outputs the following information:
Press any key to start measuring transmission latency
The peripheral outputs the following information:
Press any key to start measuring transmission latency
Press a key in the terminal that is connected to the peripheral.
Observe the terminal connected to the peripheral. The latency measurements are printed in the terminal. The latency is expected to be around 1 ms.
Transmission Latency: 1083 (us), CRC mismatches: 0
Press a key in the terminal that is connected to the central.
Observe the terminal connected to the peripheral. The measured latency on the peripheral becomes approximate 1 ms:
Transmission Latency: 1098 (us), CRC mismatches: 0
Observe the central switches to standard 7.5 ms interval right after performing latency measurements.
Observe the terminal connected to the central. The latency is higher now.
Connection interval updated: 7.5 ms Transmission Latency: 4592 (us), CRC mismatches: 0
![msc {
hscale = "1.3";
Central,Peripheral;
Central<<=>>Peripheral [label="Connected"];
Central<<=>>Peripheral [label="Discovered GATT Latency Service"];
Peripheral note Peripheral [label="Press any key to start measuring transmission latency"];
Peripheral note Peripheral [label="Read current timestamp: s1"];
Peripheral=>Central [label="Write Request (timestamp: s1)"];
Central>>Peripheral [label="Write Response"];
Peripheral note Peripheral [label="Read current timestamp: s2"];
Peripheral note Peripheral [label="Latency = (s2 - s1) / 2"];
Central note Central [label="Press any key to set LLPM short connection connection interval (1 ms)"];
Central<<=>>Peripheral [label="Switched to LLPM connection interval"];
Central note Central [label="Press any key to start measuring transmission latency"];
Central note Central [label="Read current timestamp: m1"];
Central=>Peripheral [label="Write Request (timestamp: m1)"];
Peripheral>>Central [label="Write Response"];
Central note Central [label="Read current timestamp: m2"];
Central note Central [label="Latency = (m2 - m1) / 2"];
}](../../../_images/mscgen-a232fcffd0b0efd0c32b1ae476bbf6e5688552c2.png)
Sample output
The result should look similar to the following output.
For the central:
*** Booting Zephyr OS build v3.3.99-ncs1-2858-gc9d01d05ce83 *** Starting Bluetooth LLPM example I: SoftDevice Controller build revision: I: 01 2d a8 d4 0d e5 25 cf |.-....%. I: a3 48 8d 2f 56 e0 59 c8 |.H./V.Y. I: 24 df 3d 58 |$.=X I: HW Platform: Nordic Semiconductor (0x0002) I: HW Variant: nRF52x (0x0002) I: Firmware: Standard Bluetooth controller (0x00) Version 1.43053 Build 635768276 I: Identity: FF:FA:56:08:E1:DD (random) I: HCI: version 5.4 (0x0d) revision 0x10b7, manufacturer 0x0059 I: LMP: version 5.4 (0x0d) subver 0x10b7 Bluetooth initialized LLPM mode enabled Choose device role - type c (central) or p (peripheral): Central. Starting scanning Scanning successfully started Filter does not match. Address: 1F:25:BE:AC:D4:47 (random) connectable: 0 Filter does not match. Address: 5E:29:3A:8C:5A:8D (random) connectable: 0 Filter does not match. Address: 2C:E8:67:EA:C9:27 (random) connectable: 1 Filters matched. Address: C3:06:3D:25:7F:73 (random) connectable: 1 Filter does not match. Address: EA:0E:68:82:9D:0C (random) connectable: 1 Filter does not match. Address: 1B:CC:D4:7C:9E:31 (random) connectable: 0 Connected as central Conn. interval is 3329 units (1.25 ms/unit) Security changed: level 2, err: 0 Service discovery completed Press any key to start measuring transmission latency Transmission Latency: 1037 (us), CRC mismatches: 0 Transmission Latency: 1296 (us), CRC mismatches: 0 Transmission Latency: 1342 (us), CRC mismatches: 0 Transmission Latency: 1419 (us), CRC mismatches: 0 Transmission Latency: 961 (us), CRC mismatches: 0 Transmission Latency: 1052 (us), CRC mismatches: 0 Transmission Latency: 1296 (us), CRC mismatches: 0 Transmission Latency: 1342 (us), CRC mismatches: 0 Transmission Latency: 1419 (us), CRC mismatches: 0 Transmission Latency: 961 (us), CRC mismatches: 0 Transmission Latency: 1052 (us), CRC mismatches: 0 Transmission Latency: 1296 (us), CRC mismatches: 0 Transmission Latency: 1342 (us), CRC mismatches: 0 Transmission Latency: 1419 (us), CRC mismatches: 0 Transmission Latency: 961 (us), CRC mismatches: 0 Transmission Latency: 1052 (us), CRC mismatches: 0 Transmission Latency: 1296 (us), CRC mismatches: 0 Transmission Latency: 1342 (us), CRC mismatches: 0 Transmission Latency: 1419 (us), CRC mismatches: 0 Transmission Latency: 961 (us), CRC mismatches: 0 Transmission Latency: 1052 (us), CRC mismatches: 0 Transmission Latency: 1296 (us), CRC mismatches: 0 Transmission Latency: 1342 (us), CRC mismatches: 0 Transmission Latency: 1419 (us), CRC mismatches: 0 Transmission Latency: 961 (us), CRC mismatches: 0 Connection interval updated: 7.5 ms Transmission Latency: 1922 (us), CRC mismatches: 0 Transmission Latency: 7064 (us), CRC mismatches: 0 Transmission Latency: 13244 (us), CRC mismatches: 0 Transmission Latency: 4592 (us), CRC mismatches: 0 Transmission Latency: 7019 (us), CRC mismatches: 0 Transmission Latency: 5691 (us), CRC mismatches: 0 Transmission Latency: 4592 (us), CRC mismatches: 0 Transmission Latency: 7019 (us), CRC mismatches: 0 Transmission Latency: 5706 (us), CRC mismatches: 0 Transmission Latency: 4577 (us), CRC mismatches: 0 Transmission Latency: 7019 (us), CRC mismatches: 0 Transmission Latency: 5706 (us), CRC mismatches: 0 Transmission Latency: 4592 (us), CRC mismatches: 0 Transmission Latency: 7019 (us), CRC mismatches: 0 Transmission Latency: 5691 (us), CRC mismatches: 0 Transmission Latency: 4592 (us), CRC mismatches: 0 Transmission Latency: 7019 (us), CRC mismatches: 0 Transmission Latency: 5706 (us), CRC mismatches: 0
For the peripheral:
*** Booting Zephyr OS build v3.3.99-ncs1-2858-gc9d01d05ce83 *** Starting Bluetooth LLPM example I: SoftDevice Controller build revision: I: 01 2d a8 d4 0d e5 25 cf |.-....%. I: a3 48 8d 2f 56 e0 59 c8 |.H./V.Y. I: 24 df 3d 58 |$.=X I: HW Platform: Nordic Semiconductor (0x0002) I: HW Variant: nRF52x (0x0002) I: Firmware: Standard Bluetooth controller (0x00) Version 1.43053 Build 635768276 I: Identity: C3:06:3D:25:7F:73 (random) I: HCI: version 5.4 (0x0d) revision 0x10b7, manufacturer 0x0059 I: LMP: version 5.4 (0x0d) subver 0x10b7 Bluetooth initialized LLPM mode enabled Choose device role - type c (central) or p (peripheral): Peripheral. Starting advertising Advertising successfully started Connection event reports enabled W: opcode 0x200a status 0x0d Connected as peripheral Conn. interval is 3329 units (1.25 ms/unit) W: opcode 0x2032 status 0x3a E: Failed LE Set PHY (-5) Security changed: level 2, err: 0 Service discovery completed Press any key to start measuring transmission latency Transmission Latency: 1083 (us), CRC mismatches: 0 Transmission Latency: 1312 (us), CRC mismatches: 0 Transmission Latency: 900 (us), CRC mismatches: 0 Transmission Latency: 1007 (us), CRC mismatches: 0 Transmission Latency: 1052 (us), CRC mismatches: 0 Transmission Latency: 1083 (us), CRC mismatches: 0 Transmission Latency: 1358 (us), CRC mismatches: 0 Transmission Latency: 915 (us), CRC mismatches: 0 Transmission Latency: 1007 (us), CRC mismatches: 0 Transmission Latency: 1052 (us), CRC mismatches: 0 Transmission Latency: 1098 (us), CRC mismatches: 0 Transmission Latency: 1296 (us), CRC mismatches: 0 Transmission Latency: 900 (us), CRC mismatches: 0 Transmission Latency: 1007 (us), CRC mismatches: 0 Connection interval updated: 7.5 ms Transmission Latency: 1052 (us), CRC mismatches: 0 Transmission Latency: 7247 (us), CRC mismatches: 0 Transmission Latency: 5950 (us), CRC mismatches: 0 Transmission Latency: 4867 (us), CRC mismatches: 0 Transmission Latency: 7293 (us), CRC mismatches: 0 Transmission Latency: 5996 (us), CRC mismatches: 0 Transmission Latency: 4913 (us), CRC mismatches: 0 Transmission Latency: 7385 (us), CRC mismatches: 0 Transmission Latency: 6088 (us), CRC mismatches: 0 Transmission Latency: 5004 (us), CRC mismatches: 0
Dependencies
This sample uses the following nRF Connect SDK libraries:
This sample uses the following sdk-nrfxlib libraries:
In addition, it uses the following Zephyr libraries:
include/console.h-
include/kernel.h
include/sys/printk.hinclude/zephyr/types.h-
include/bluetooth/bluetooth.hinclude/bluetooth/conn.hinclude/bluetooth/gatt.hinclude/bluetooth/hci.hinclude/bluetooth/uuid.hinclude/bluetooth/scan.hinclude/bluetooth/gatt_dm.h
References
For more information about the connection parameters that are used in this sample, see the following chapters of the Bluetooth Core Specification:
Vol 3, Part F, 3.4.5 Writing attributes
Vol 3, Part G, 4.9.3 Write Characteristic Value
Vol 6, Part B, 5.1.1 Connection Update Procedure
Vol 6, Part B, 5.1.10 PHY Update Procedure