Bluetooth LE Connection Parameters

The connection parameters for a BLE connection are a set of parameters that determine when and how the central and peripheral in a link transmit data. The parameters are exchanged when the central and peripheral initially connect. The Bluetooth Framework provides methods to read, change, and monitor the current connection parameters. On Windows 11 and above, there is no problem to read and change the connection parameters. However, on Windows 10 there is no documented way to do anything with the Bluetooth LE connection parameters. Fortunately, the Bluetooth Framework can at least read the connection parameters when your application uses the standard Microsoft Bluetooth driver. And of course, if you use the BLED112 Bluetooth dongle, you can do anything you need with the connection parameters on any Windows platform.

Custom Connection Parameters

There are two ways to set the connection parameters for the current connection: use the predefined or custom connection parameters. If you ever used the standard Microsoft Bluetooth API, you can find that there is no way to read the connection parameters on the Windows 10 platform. The Bluetooth Framework allows you to do that. Also, with the standard Microsoft Bluetooth API on Windows 11, you can set only the predefined connection parameters. But the Bluetooth Framework provides methods to set any customer connection parameters. This is one of many other reasons to use the Bluetooth Framework instead of the standard Bluetooth API.

The connection parameters are important for BLE communication because they affect the power consumption, the data transfer speed, and the stability of the connection. The connection parameters for a BLE connection are a set of parameters that determine when and how the central and peripheral in a link transmit data. The parameters are exchanged when the central and peripheral initially connect. It is always the central that actually sets the connection parameters used, but the peripheral can send a connection parameter update request that the central can then accept or reject. The three different parameters are:

Connection Interval

The connection interval determines how often the central will ask for data from the peripheral. When the peripheral requests an update, it supplies a maximum and a minimum wanted interval. The connection interval must be between 7.5 ms and 4 s. The connection interval time has an inverse effect on the data throughput of the BLE connection, as shown in the following equation:

DataThroughput = (8 * BytesPerPacket * PacketsPerInterval * 1000) / ConnectionInterval

To save power, we need to change the connection interval to a much longer period when we know the connection is going to be in an idle state for a period of time.

Slave Latency

By setting a non-zero slave latency, the peripheral can choose not to answer when the central asks for data up to the slave latency number of times. However, if the peripheral has data to send, it can choose to send data at any time. This enables a peripheral to stay sleeping for a longer time if it doesn't have data to send but still send data fast if needed. Furthermore, slave latency can be applied to help the peripheral (slave) device to further reduce power consumption. This parameter is helpful to avoid changing connection parameters frequently to achieve both high-speed data transfer and low power consumption during idle.

Connection Supervision Timeout

This timeout determines the timeout from the last data exchange until a link is considered lost. The central will not try to reconnect before the timeout has passed, so if you have a device that goes in and out of range often and you need to notice when that happens, it might make sense to have a short timeout.

The effective connection interval is equal to the amount of time between two connection events, assuming that the slave skips the maximum number of possible events if slave latency is allowed (the effective connection interval is equal to the actual connection interval if slave latency is set to 0).

The slave latency value represents the maximum number of events that can be skipped. This number can range from a minimum value of 0 (meaning that no connection events can be skipped) to a maximum of 499. The maximum value must not make the effective connection interval (see the following formula) greater than 16 s. The interval can be calculated using the following formula:

Effective Connection Interval = (Connection Interval) * (1 + [Slave Latency])

Consider the following example

Connection Interval - 80 (100ms)
Slave Latency - 4
Effective Connection Interval - 100ms * (1 + 4) = 500ms

When no data is being sent from the slave to the master, the slave transmits during a connection event once every 500 ms.

In many applications, the slave skips the maximum number of connection events. Consider the effective connection interval when selecting or requesting connection parameters. Selecting the correct group of connection parameters plays an important role in the power optimization of the Bluetooth Low Energy device. The following list gives a general summary of the trade-offs in connection parameter settings.

Reducing the connection interval

• Increases the power consumption for both devices.
• Increases the throughput in both directions.
• Reduces the time for sending data in either direction.

Increasing the connection interval

• Reduces the power consumption for both devices.
• Reduces the throughput in both directions.
• Increases the time for sending data in either direction.

Reducing the slave latency (or setting it to zero)

• Increases the power consumption for the peripheral device.
• Reduces the time for the peripheral device to receive the data sent from a central device.

Increasing the slave latency

• Reduces power consumption for the peripheral during periods when the peripheral has no data to send to the central device.
• Increases the time for the peripheral device to receive the data sent from the central device.

The Bluetooth Framework allows you to change the connection parameters individually as well as to use predefined settings. Below you can find a description of the predefined connection parameters.

Balanced

The balanced set of connection parameters, offering a balance between throughput and power usage.

Power Optimized

The more power-efficient set of connection parameters. Optimized for power usage at the expense of throughput. Also allows for more simultaneous connections to other Bluetooth devices.

Throughput Optimized

The more aggressive set of connection parameters, optimized for faster throughput at the expense of power usage. Also reduces the number of simultaneous connections that can be made to other Bluetooth devices.

Connection parameters are initially set when a connection is established between the devices. According to the BLE specification, upon connection establishment, the connection interval shall be set to the recommended value (30-50 ms), with 0 slave latency. And when no further action is pending and TGAP time elapses, the central device checks the advertising/scan response data or the peripheral preferred connection parameters characteristic (PPCP) to determine the connection parameter to be used for this connection, which triggers a connection parameter update procedure. On the peripheral side, it can also trigger a connection parameter update procedure after the TGAP time when no further action is pending. Connection parameters can be changed from the peripheral side during the connection, but the negotiation result with the central may not always be successful.