Throughput with Bluetooth Low Energy Technology#

Introduction#

This document describes the maximum achievable bitrates on Blue Gecko devices (EFR32BGxx) with different Bluetooth SDK versions. However, the principles can also be applied to legacy devices (BLExxx), taking into account their parameters.

The application-level data, throughput with Bluetooth Low Energy is affected by many factors. In particular, two operations below have a different throughput calculation:

1. Acknowledged data transfer. In this use case, the reception of all data packets is acknowledged by the receiver. The receiver sends a response for every read/write request in the next connection interval. The connection is reliable but the throughput is low.

2. Unacknowledged data transfer. In this use case, packets can be sent sequentially without waiting for acknowledgment from the other side. This ensures much higher throughput, but a less reliable connection.

Acknowledged and unacknowledged operation can be mixed on the same connection by sending packets of different types. The following packet types result in acknowledged operation:

• Read (read request)

• Write (write request)

• Indication

• Prepare write

• Signed write

The following packet types result in unacknowledged operation:

• Write without response (write command)

• Notification

Throughput Calculation for Acknowledged Data Transfer#

For acknowledged data transfer, the throughput depends on the following parameters:

• Connection interval

• MTU size

• Attribute protocol operation

Connection Interval#

The connection interval specifies the frequency of sending data, which varies between 7.5 ms up to 4000 ms. After the sender sends data (or request), the sender has to wait for the receiver to send an acknowledgment. Therefore, one (GATT) operation takes two connection intervals.

The lower the connection interval, the higher the potential data rate. For example, using the smallest connection interval of 7.5 ms e.g., with 125-byte useful payload / connection interval, the radio can (assuming no lost packets and no re-transmissions) achieve a theoretical data rate, as follows:

1000 ms / (2 * 7.5 ms) * 125 bytes = 8,333 bytes/sec = 66,666 bps

On the other hand, when using the largest connection interval of 4000 ms e.g., with 22-byte useful payload, data rate will be reduced to:

1000 ms / (2 * 4000) ms * 22 bytes = 2.75 bytes/sec = 22 bps

MTU Size#

Maximum Transfer Unit (MTU) specifies the number of bytes that can be sent within one GATT operation. In other words, it's the number of bytes that can be sent within two connection intervals. MTU size can be set for each connection. However, it has an upper limit, which varies with Bluetooth stack versions. The maximum MTU size for each Bluetooth stack version is summarized in the following table

Note that the MTU size depends on both sides. For example, if the remote device supports a smaller MTU size, the smaller MTU size will be used. The higher the MTU size, the higher the throughput. Twice the MTU size doubles the throughput.

Attribute Protocol (ATT) Operation#

MTU size includes the GATT header, which has a variable length and means that the useful payload is a bit smaller than the MTU. The size of the GATT header depends on the operation type, hence the maximum useful payload is different for different operations. This is summarized in the following table: 

Maximum Achievable Throughput with ACK#

For acknowledged operations, the maximum throughput can be achieved with the following parameters:

• Connection interval: 7.5 ms

• MTU size: 250 bytes

• Attribute protocol operation used: Read

This results in a maximum throughput of

1000 ms / (2 * 7.5 ms) * (250 - 1) bytes = 16,600 bytes/sec = 132,800 bps

Throughput Calculation for Unacknowledged Data Transfer#

For unacknowledged data transfer, the throughput depends on the following parameters:

• Packet size

• Attribute protocol operation

• PHY (physical layer) bitrate

• Connection interval

Packet Size#

Data over Bluetooth is sent in packets. Multiple packets can be sent within one connection interval sequentially (with 150 us inter-frame spacing). Packets have variable length, but their length has an upper limit. The maximum PDU (protocol data unit) size of a packet depends on the Bluetooth stack version, as summarized in the following table:

Twice the packet size does not mean double throughput because the larger packets take more time to send. As a result, less packets can be sent during the same time interval. However, the larger the packet size, the smaller the overhead, which increases the application level data throughput.

For example, for a 27-byte PDU it takes the following time to send out a packet:

328 us + 150 us + 80 us + 150 us = 708 us

328 us is the effective transmitting time, 150 us is IFS (interframe space), 80 us is receiving, 150 us is IFS. This results in a theoretical data rate calculation, as follows:

27 byte / 708 us = 38,135 bytes/sec = 305,080 bps

For a 251-byte PDU, it takes the following time to send out a packet:

2120 us + 150 us + 80 us + 150 us = 2500 us

This results in a theoretical data rate calculation, as follows:

251 byte / 2500 us = 100,400 bytes/sec = 803,200 bps

Note that the packet size depends on both sides. If the remote device supports a smaller packet size, the smaller packet size will be used.

For more details about packet timing, see the following: https://blog.bluetooth.com/exploring-bluetooth-5-how-fast-can-it-be

Attribute Protocol Operation#

The PDU size includes the L2CAP header, which is 4 bytes long and the GATT header which has a variable length. This means that the useful payload of a packet is a bit smaller than the PDU size. The size of the GATT header depends on the operation type, hence the maximum useful payload is different for different operations. This is summarized in the following table: 

The theoretical effective data rate when sending out notifications with 251-byte PDU is as follows:

(251 – 4 – 3) byte / 2500 us = 97,600 bytes/sec = 780,800 bps

If the MTU is larger than the PDU, multiple packets can be sent within one GATT operation. In this use case, only the first packet contains the L2CAP header and the GATT header, and the following packets contain only data within the PDU.

Physical Layer (PHY) Bitrate#

Bluetooth 4.2 specifies the PHY bitrate to be exactly 1 Mbps.

Bluetooth 5 (introduced in Bluetooth SDK v2.3) specifies PHY bitrates as follows:

• 2 Mbps

• 1 Mbps.

• 500 kbps (1 Mbps with 1:2 convolution coding)

• 125 kbps (1 Mbps with 1:8 convolution coding)

These bitrates only define the symbol time and not the effective throughput. However, the higher the PHY bitrate, the higher the throughput. Double bitrate means nearly double throughput. It is not exactly double because of the constant IFS (IFS = 150 us for both 1 Mbps and 2 Mbps).

The following table summarizes the time it takes to send out a packet with different PDU sizes and PHY bitrates. The times include transmitting + IFS + receiving + IFS.

For example, if you use 2 Mbps PHY with 251-byte PDU, the theoretical throughput is:

(251 – 4 – 3) byte / (1060 + 150 + 40 + 150) us = 174,285 bytes/sec = 1,394,280 bps

For more details see: https://blog.bluetooth.com/exploring-bluetooth-5-how-fast-can-it-be

Connection Interval#

If acknowledgment is not required, any number of packets can be sent within one connection interval. As a result, the connection should not directly influence the throughput. Data packets, however, must be aligned so that the first packet starts at the start of the connection interval. For example, for a 7.5 ms connection interval, 251-byte PDU, 2 Mbps PHY, you can send 4 packets in one connection interval. The 5th packet does not fit and has to wait for the next connection interval to start.

This means that the effective throughput with these parameters is as follows:

4 * (251 – 4 – 3) byte / 7500 us = 130,133 bytes/sec = 1,041,066 bps

To avoid the overhead introduced by skipped packages, adjust the connection interval so that the remaining time at the end of the connection interval is minimal. Longer connection intervals usually mean smaller overhead because of less frequent re-adjustment.

However, if the connection interval is too long the following applies:

1. Too many packets may be queued while waiting for next connection interval and you can run out of memory.

2. You may have to wait much more to resynchronize if the connection breaks because of consecutive CRC errors. Long connection interval is hence not recommended in noisy environments where CRC errors are expected.

Maximum Achievable Throughput without ACK#

For unacknowledged operation, the maximum throughput can be achieved with the following parameters:

• Connection interval: 12.5 ms

• PDU size: 251 bytes

• Attribute protocol operation used: Write without response / Notification

• PHY bitrate: 2 Mbps

In this use case, 8 packets fit into one connection interval (8*1400 us = 11200 us) and the theoretical throughput is:

8 * (251 – 4 – 3) byte / 12,500 us = 156,160 bytes/sec = 1,249,280 bps

Considerations for Smart Phones#

The connection parameters and the MTU size depend on both devices participating in the connection. When connecting to a smart phone, the devices start to negotiate the connection interval and the MTU size. Depending on the smart phone and OS version, the minimum connection interval can be much greater than 7.5 ms and the maximum MTU size can be less than the MTU size supported by the Blue Gecko. This has a critical effect on throughput. The negotiated parameters are signaled by the stack events sl_bt_evt_connection_parameters and sl_bt_evt_gatt_mtu_exchanged, so you can check what parameters are supported by your phone.

Reference: https://blog.bluetooth.com/exploring-bluetooth-5-how-fast-can-it-be