Indoor Riding - ANT+ and Bluetooth

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ANT+ and Bluetooth are essential wireless technologies that enable indoor cycling. What do they do? How are they different? Why should you care?

Assumptions: In this post, I use the term indoor bike to mean smart bike or trainer. I own a Stages SB20 Smart Bike and use it in my examples. 

Protocols



ANT+ and Bluetooth are protocols that facilitate the transfer of data between apps and devices. In this context, they are like spoken languages. ANT+ is like English and Bluetooth is like French. Both communicate similar information using different verbs, nouns and ways of speaking.


ANT+ was introduced nearly 20 years ago to support monitoring of data from devices like heart rate monitors (HRMs) and bike sensors. Since then it has been extended to control and monitor fitness machines including indoor bikes.

Bluetooth was introduced around the same time to support things like handsfree phone headsets. It was updated from 2010 to 2017 to support heart rate monitors, bike sensors and indoor bikes. This set of capabilities became part of the Bluetooth Low Energy FiTness Machine Services standard or just BT FTMS.

Why Two Protocols  (Two Languages)?

ANT+ and BT FTMS evolved independently to support roughly the same functions. Device support varies. Older Garmin computers and HRMs support only ANT+.  Phones and tablets typically support only BT FTMS.  Newer devices, including indoor bikes, often support both ANT+ and BT FTMS. 

Pros and Cons


PRO

CON

ANT+

ANT+ is more efficient allows for connectionless communication.  This means you can easily interact with multiple devices. Zwift and a Garmin computer, can both connect to an indoor bike simultaneously. Several apps can get updates from an HRM. There is no limit to the number of devices that can send or receive data in the same riding session.
ANT+ is  less robust in noisy environments.  Home users probably won’t notice. Crowded wireless environments, like commercial gyms, can have issues. 

ANT+ also has little security. Hacking ANT+ is relatively easy. Fortunately, few bad actors  have been interested in messing with ANT+ (yet). 

BT FTMS

BT FTMS is more ubiquitous.  Most new indoor cycling devices and apps support it.  FTMS is also slightly more robust in noisy environments. 

BT FTMS supports only connection-based communication. This means you must always create a 1:1 bi-directional communication link between each device or app, even if you are just reading data. Most devices have limits on the number of connections. Here are some common examples: SB20 Smart Bike: 2, Wahoo HRM: 2, Stages Power Meters: 2, Apple TV: 3.  Riders are often frustrated when they run up against these limits, even in simple configurations. 


Profiles

Within the ANT+ and BT FTMS protocols are profiles. These are essentially topic areas that focus on different areas of interest. In spoken languages, profiles are disciplines like history or mathematics. Profiles exist for bike control, pedalling cadence, heart rate, cyclist power and more.

Here are related technical references:

Clients and Service Providers

Clients use a Service Provider to get leverage a particular type of functionality. Examples of Clients are Zwift, TrainerDay TrainerRoad or a Garmin computer. Examples of Service Providers are indoor bikes, HRMs or bike cadence sensors.  

Here's a summary of functionality by profile:

Type

Profile

Summary

Direction

Advertised by:

Indoor Bike

Power Meter

HRM

ANT+

Fitness Equipment Control (FE-C)

Control of smart bike/trainer to set resistance (0 - 100%), target power (watts) and simulation parameters (weight, terrain, wind and incline)

Client→Bike

✓



Broadcast workout data including power, cadence along with calculated speed and distance

Client←Bike

✓



Bicycle Power

Broadcast measured power

Client←Bike/PM

✓

✓


Bicycle Cadence

Broadcast measured cadence

Client←Bike/PM

✓

✓


Heart Rate Monitor 

Broadcast measured heat rate

Client←HRM



✓

BT FTMS

Bike Control

Control of smart bike/trainer to set resistance (0 - 100%), target power (watts) and simulation parameters (weight, terrain, wind and incline)

Client→Bike

✓



Power

Send measured power to client

Client←Bike/PM

✓

✓


Cadence

Send measured cadence to client

Client←Bike/PM

✓

✓


Heart Rate

Send measured heat rate to client

Client←HRM



✓


Advertising Services

Service Providers advertise the capabilities they provide. Indoor bikes typically advertise: Bike control, Bicycle Power and Cadence. Power meter cranks or pedals advertise Bicycle Power and Bike Cadence. HRMs advertise Heart Rate.

Example: Leveraging BT FTMS in Zwift

Zwift picks advertised capabilities from the devices you have powered up. The selected capabilities enable the Zwift simulation that most of us a familiar with. 

Here's what a typical Zwift ride looks like:


1. Zwift shows the Paired Devices page to select available ANT+ and/or BT FTMS capabilities.

1a. Advertised Indoor Bike capabilities are selected for BT FTMS Cadence, Power Source and Resistance (formerly Controllable).
.
1b. Advertised HRM capabilities are selected for the BT FTMS Heart Rate.

2. The Zwift ride starts, showing its simulated (avatar) world.

2a. Terrain and environmental elements, like drafting, causes Zwift to send Bike Control commands to the Indoor Bike to increase or decrease riding resistance.  

3. The Indoor Bike reflects riding physics. The rider responds by pedalling faster or slower while pushing harder or easier. 

3a. In this Stages SB20 smart bike example:  Right Crank measures power and send updates to the Left Crank. The Left Crank measures power and forwards consolidated L:R data to the Stages Bike using ANT+.

3b. The Indoor Bike sends BT FTMS Cadence and Bike Power data to Zwift once a second.

4. The HRM measures rider heart rate. 

4a. The HRM sends BT FTMS Heart Rate data to Zwift once a second.

Summary

ANT+ is well suited for the low data rates and allows virtually any number of listeners. It's perfect for HRMs and bike sensors. ANT+ is not available on most phones and tablets. 

BT FTMS is more widely supported. It works in noisier environments with higher data rates. Connection limits are a concern. Devices like an Apple TV or a smart bike can quickly use up all their connection slots causing confusion and frustration. 



Want to learn more?  See my post on Wireless is Good - and Complicated. 

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