There can be three different types of data transmission possible. They are
Ø Transmission from a device to the coordinator
Ø Transmission from the coordinator to the device
Ø Transmission between any two devices.
In a star topology only the first two transmission techniques are possible. Transmission between any two devices is not supported, where as in a peer to peer network all the three types of transmissions are possible. The transmissions can be carried out again in either of two ways, depending on if the beacon transmissions are allowed or not. The current study is focused on a beacon enabled network. Hence the following paragraphs will find no reference of a non-beacon enabled transmission techniques.
The following subsections will go through the steps followed by each mode of transmission.
Transmission from a Coordinator to a Device (Fig: 2.8)
Ø The coordinator has data to be transmitted to the device.
Ø It indicates this in the pending address fields of its beacon.
Ø Devices tracking the beacons, decode the pending address fields.
Ø If a device finds its address listed among the pending address fields, it realizes it has data to be received from the coordinator.
Ø It issues a Data-Request Command to the coordinator.
Ø The coordinator replies with an acknowledgement.
Ø If there is data to be sent to the device, it would transmit the data.
Ø If acknowledgements are not optional, the device would respond with an acknowledgement.
Figure 2.8: Coordinator to Device Transmission
Transmission from a device to a coordinator (Fig: 2.9)
Ø The device first listens to the beacon.
Ø On finding the beacon, it synchronizes first to the superframe structure. This process lets it know the start and end time of the Contention access period.
Ø The device will now compete with its peers for a share of the channel.
Ø On its turn, it will transmit the data to the coordinator.
Ø The coordinator may reply back with an acknowledgement, if it is not optional.
Figure 2.9: Device to Coordinator Transmission
Transmission between two devices
There is no predefined manner in which there can be a direct communication between two devices in the network. However, the suitable methods of transmission can be by mutual synchronization techniques, or direct transmission using unslotted CSMA-CA. But either technique has their downsides. The synchronization technique, even though look simpler, is harder to implement. Similarly, direct transmission might degrade the throughput performance of the PAN.
