BLE peripheral throughput limit

问题

We are developing a BLE sensor Peripheral to work with an iPad, that requires the following throughput of data on the BLE notification characteristic (no acknowledge) using a TI CC2541 BLE module and a custom profile:

One 20 bytes (GATT maximum standard packet) every 10ms, or since we appear to have a limit of 4 packets per connection interval, this equates to one connection interval every 40ms. Throughput required is 2,000 bytes per second, the TI website recommends the CC2541 BLE solution be used for several sensor devices requiring this level of data throughput.

The profile for the BLE module is set with min and max connection intervals of 20ms and 40ms respectively, which should suffice. The "Bluetooth Accessory Design Guidelines for Apple Products" document suggests that the minimum and maximum connection intervals we set, as above are correct. We are using the latest iPad and Apple tools for iOS 6 on a new Mac Mini / Mac Book.

With a simple test program on the iPad, we can get the link to work well sending 20 byte packets to the BLE Peripheral at intervals of 20ms, however once we lower this to 10ms as required we start loosing packets or getting corrupt packets, we have the FIFO empty interrupt turned off so we can handle the sending to the BLE module FIFO quicker, and we are using the maximum Baud rate of 230400 to send the 20 byte packets to the BLE TX FIFO from the micro.

We realise we are at the top end of the BLE transfer limit, and of what is possible. Can anyone advise if there is a solution to achieving 2000 bytes per second throughput using the TI CC2541 BLE chip / module with an up to date iPad?

回答1:

We use TI 2540 (BLE stack version 1.3.2) succesfully with iPad/iPod/iPhone (iOS 6.x and 7.x). We currently send 75 notifications of 20 bytes per second => 1500 bytes/second. But I have tried to send 125 notifications and that worked as well.

Of course the more you send the greater likelihood of loosing data, e.g., less time to resend a NACK'ed message.

I have experienced that iOS' BLE stack may enter a mode where it begins to NACK messages continuously. If this happens you will loose a lot of messages. I have reported an error to Apple about this. (This problem seems to have been fixed in iOS 7.1.beta3/4.)

I currently have:

// Minimum connection interval (units of 1.25ms, 80=100ms) if automatic parameter update request is enabled
#define DEFAULT_DESIRED_MIN_CONN_INTERVAL     10

// Maximum connection interval (units of 1.25ms, 800=1000ms) if automatic parameter update request is enabled
#define DEFAULT_DESIRED_MAX_CONN_INTERVAL     20

Yes, it doesn't conform with Apple's guidelines. But I believe they can be relaxed in our case.

UPDATE: I have also tried to use an iDevice as peripheral, i.e., BTLE between two iDevices. Here I have sent 150 messages per second without any problems. Check the code here.

回答2:

Here are few observation on throughput that we found during our RnD on iPhone with BLE. The below data is based on write with response.

1. iPhone 8 (BLE 5.0) as Central and Linux desktop (Ubuntu 16.04 with BLE dongle 4.0): MTU = 2048 : Throughput - 2.5 KiloBytes per sec.
2. iPhone 8 (BLE 5.0) as Central and Android OS with BLE version 4.2 as Peripheral(Xiomi Mi A1): MTU = 180 : Throughput - 2.5 KiloBytes per sec.
3. iPhone 8 (BLE 5.0) as Central and iPhone 7 plus (BLE 4.2) as Peripheral : MTU = 512 : Throughput - 7.1 KiloBytes per sec.
4. iPhone 8 (BLE 5.0) as Central and Samsung S8 (BLE 5.0) as Peripheral : Samsung S8 failed to work as peripheral
5. iPhone 8 (BLE 5.0) as Central and iPhone 8 plus (BLE 5.0) as Peripheral : MTU = 512 : Throughput - 15.5 KiloBytes per sec.

As you can see, as the MTU value increases, we get maximum throughput. But we cannot increase to any limit. The above MTU values are the default maximum allowed MTU value as per the given configuration. [MTU - Maximum Transmission Unit. i.e maximum bytes that can be sent in one write request]

Comments are welcome to the above data.

回答3:

Are you sending "write without response" commands? You can send 4 packets per connection event this way. Using you previous 20ms connection interval, you would be sending 4 packets with 20 bytes every 0.02 seconds. Putting that together: 4*20/0.02 = 4000 bytes per second easy.

I highly doubt you are getting corrupt data. The link layer adds a CRC and a 2 bits of "next expected" to BLE packets to ensure A) all the bits are received correctly and b) packets were not sent out of order. The TI stack and iOS control the link layer so I doubt you've botched that.

回答4:

iOS 7 seems to have made some optimizations regarding the throughput levels for BLE transfers. Try it again on an iOS 7 device.

回答5:

You don't really pose a question, but I can verify that your desired limit of 2000 bytes/sec is possible.

check out the selected answer on this forum post (http://e2e.ti.com/support/wireless_connectivity/f/538/p/353327/1244676.aspx#1244676) to see how we made it work.

来源：https://stackoverflow.com/questions/18240978/ble-peripheral-throughput-limit