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SWRS095A

– FEBRUARY 2011 – REVISED MARCH 2011

Message Types

Although the data payload is fixed at 8 bytes per packet, ANT channels do support different types of messages

including broadcast messages, acknowledged messages and burst messages. The choice of message type to

use depends on the needs of a particular application.

Broadcast messages are messages that do not expect a response. This means that the sender of a broadcast

message (whether it be a slave or master) has no way of knowing if the message it sent was successfully

received by its intended recipient. Broadcast messages offer the lowest power consumption of the three different

message types. As these types of messages do not ellicit a response from their target, these messages are

ideally suited for applications where one-to-many type architectures are required. For example, a sport sensor

may broadcast data to a watch and smartphone simultaneously. Broadcast type messages are also useful for

sensor data that changes slowly relative to the channel period, and it is more important to have the latest data

rather than every single packet of data. For example, a temperature sensor is an ideal candidate for a broadcast

data type. Broadcast messages are the default message type used by a master channel and are the only data

type supported by transmit-only channels.

Acknowledged messages are messages that expect and ellicit a response from the receiving device. The

response to an acknowledged message is automatically handled by the ANT protocol (transparent to the

application MCU). On sending an acknowledged message type, the application MCU will be notified whether or

not the message successfully reached its target. Any retries must be handled by the application MCU, as ANT

does not re-transmit any messages that were not acknowledged. Acknowledged type messages are best suited

for control applications where the transmitter must know whether or not a message got through. For example, a

remote control or actuator is an ideal application for an acknowledged message type.

Burst messages are designed to allow the transfer of bulk data as fast as possible without compromising the

ability of a device to run using a coin-cell battery. A burst always starts on a channel-period timeslot and send

packets as fast as possible, potentially extending the channel period. The maximum data throughput for a burst

is 20 kbps. Each burst packet is re-tried by ANT up to five times if necessary. The success or failure of a burst

operation is communicated to the application MCU. The burst message type is most appropriate for sending

large amounts of episodic data. For example, a watch may upload data to a computer after a workout using

burst-type messages.

Pairing

In order to ensure that a slave is talking to the correct master, the slave must know the channel ID of the master.

If the channel ID is not known, the slave must attain the channel ID through a process called pairing. Pairing is

an issue that affects all wireless technologies. There are several methods available for pairing, each with its

merits and drawbacks. Ultimately, the best pairing solution is one that is most seamless to the user. ANT has

many pairing methods built in, ranging from simple to sophisticated, and allowing the designer to customize the

pairing experience for a particular application.

The simplest pairing method is for the slave to wildcard the channel ID (by explicitly setting it to 0). When the

channel ID is wildcarded, the slave connects to the first master it finds (provided it matches non-wildcarded

portions of the ID and the frequency and network number). This method is easy and relatively seamless to the

user. The user generally only must perform a UI operation on the slave device and then wait to connect to the

master. However, if the user is in a crowded environment, where several devices to pair with are available, this

method breaks down. For example, if a user is trying to pair to a heart-rate strap at a gym, the user may easily

pair to a device that is not the user

's own. Hence, a wildcard pairing method is generally suitable if pairing is

expected to happen only occasionally and in isolation.

To reduce the chances of pairing to an incorrect device in a crowded environment, ANT allows the user to put

the ANT channel into a pairing mode. This is done by setting a particular bit of the channel ID of the master

device and forcing the slave to only pair to devices that have this bit set. This method reduces the chances of

pairing to the wrong device; however, it forces the user to perform a UI operation on the slave device and on the

master device. This may not be practical both from a user point of view and from a production point of view, as

adding a button or switch may not be feasible for a particular device.

Another, more-powerful, method of pairing is to use the relative proximity of a slave device to a master device to

determine if pairing should occur. This is known as proximity pairing. ANT has a very simple interface that allows

the user to specify a proximity threshold, effectively blocking other devices that are outside of this threshold.

Once pairing has occurred, the effective transmission range is returned to normal, allowing the device to function

normally.

Copyright

© 2011, Texas Instruments Incorporated

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