When a peripheral and central device enter into a connection, there is a set of connection parameters that are exchanged. Some of them have a standard start value, for backwards compatibility, while some of them are dictated by the central device, and are included in the connection request packet.
The connection interval and the connection supervision timeout were briefly discussed in the previous topic, and are set by the central in the connection request packet, in addition to the peripheral latency. Peripheral latency allows the peripheral to skip waking up for connection events if it doesn’t have data to send.
The radio mode (1M, 2M or coded PHY) is set to 1M by default for backwards compatibility, but can be changed during the connection. The data length and MTU are also set for backwards compatibility, but we will take a look at how to change these in the exercise section of this topic.
A Bluetooth LE device spends most of its time “sleeping” (hence the “Low Energy” in the name). In a connection, this is accomplished by agreeing on a connection interval saying how often the devices will communicate with each other. When they are done communicating, they will turn off the radio, set a timer and go into idle mode, and when the timer times out, they will both wake up and communicate again. The implementation of this is handled by the Bluetooth LE stack, but it is up to your application to decide how often you want the devices to communicate by setting the connection interval.
When two devices are connected, they agree on a parameter that determines how long it should take since the last packet was successfully received until the devices consider the connection lost. This is called the supervision timeout. So if one of the devices is unexpectedly switched off, runs out of battery, or if the devices are out of radio range, then this is the amount of time it takes between successfully receiving the last packet before the connection is considered lost.
Peripheral latency allows the peripheral to skip waking up for a certain number of connection events if it doesn’t have any data to send. Usually, the connection interval is a strict tradeoff between power consumption and low latency or delay in communication. If you want to reduce the latency, but still keep a low power consumption, you can use peripheral latency. This is particularly useful in HID (Human Interface Devices) applications, such as computer mouse and keyboard applications, which usually don’t have any data to send, but when it has data to send, we want to have very low latency. Using the peripheral latency option, we can maintain low latency but reduce power consumption by remaining idle for several connection intervals.
PHY radio mode
Normal Bluetooth LE (1M PHY) transmits at 1Mbps. However, in Bluetooth 5.0, both high-speed (2M PHY) and long-range (Coded PHY) radio modes were introduced, (as covered in PHY: Radio modes). This gives is two more options.
First, we can increase the modulation scheme to use 2Mbps for higher transmit rates. This either means that you can transfer the data faster, and go back to sleep faster to conserve more power, or you can use that extra time to send even more data, practically doubling the throughput of a Bluetooth LE connection. This does however come with the cost of a slightly shorter range.
The other option is to use Coded PHY which results in a significant increase in range, but at the cost of lower throughput.
Data length and MTU
The data length and MTU (Maximum Transfer Unit) are two different parameters, but they often go hand in hand.
The Maximum Transfer Unit (MTU) is the number of bytes that can be sent in one GATT operation (for example, a send operation), while data length is the number of bytes that can be sent in one Bluetooth LE packet. MTU has a default value of 23 bytes, and data length has a default value of 27 bytes. When MTU is larger than data length, such as MTU being 140 bytes while data length is 27 bytes, the data will be segmented into chunks of the data length’s size. This means that, for your application, it appears like one message is being sent, but on the air, the data is actually split into smaller segments
Ideally, you want all of your data to be sent in one packet, to reduce the time it takes to send the data, so in Bluetooth 4.2, data length extension (DLE) was introduced to allow the data length to be increased from the default 27 bytes to up to 251 bytes. Packing everything together also reduces the number of bytes you need to transmit over the air, as every packet includes a 3-byte header. This saves both time and power, and in turn allows for higher throughput in your Bluetooth LE connection.
The relation between data length and MTU is not one-to-one. On air, the data length can be up to 251 bytes, while the actual payload that you can send is a maximum of 244 bytes. This is because the 251 byte Data PDU payload needs an L2CAP Header of 4 bytes, and an Attribute header of 3 bytes. This leaves you with 251 – 4 – 3 = 244 bytes that you can actually populate with payload data.
Below is a figure showing what it looks like to send a message with 40 bytes before and after changing the default data length. It is clear that sending all the data in one packet leads to less radio on time.
Updating the connection parameters
The connection interval, supervision timeout and peripheral latency are dictated by the central, but the peripheral can request changes. However, it is always the central that has the final say with these requests. So in the case where the central is your phone, it is the OS running on the phone that decides whether to accept or reject the new parameters in the connection parameter request.
As for the PHY radio mode, data length and MTU, these cannot be chosen only by the central. Since the ability to change these parameters was introduced in later releases of the Bluetooth Specification, they are always set to their default values when a connection is first established. When the connection is first established, either device can request to update these parameters with new values. The other device will then either send its supported values or state that it does not support updating one or more of those parameters.
Taking the data length as an example, this is always 27 bytes when the connection is first established . Then let’s say that the peripheral wants to update this to 200 bytes, and sends a request to do so. The central may then reply with a message saying it can do 180 bytes, and then they will agree on having the data length set to 180 bytes.
The default value for the PHY radio mode is 1M, and the default MTU is 23.