The Superframe structure
The superframe structure (Fig: 2.4) is an optional part of a WPAN. It is the time duration between two consecutive beacons. The structure of the superframe is determined by the coordinator. The coordinator can also switch off the use of a superframe by not transmitting the beacons. The superframe duration is divided into 16 concurrent slots. The beacon is transmitted in the first slot. The remaining part of the superframe duration can be described by the terms, CAP, CFP and Inactive. The superframe is used to provide vital statistics like synchronization, identifying the PAN and the superframe structure, to the devices connected in a Wireless PAN. This information is critical for the operation of the PAN in a Beacon enabled network.
Figure 2.4: The SuperFrame Structure
The superframe structure (Fig: 2.4) is an optional part of a WPAN. It is the time duration between two consecutive beacons. The structure of the superframe is determined by the coordinator. The coordinator can also switch off the use of a superframe by not transmitting the beacons. The superframe duration is divided into 16 concurrent slots. The beacon is transmitted in the first slot. The remaining part of the superframe duration can be described by the terms, CAP, CFP and Inactive. The superframe is used to provide vital statistics like synchronization, identifying the PAN and the superframe structure, to the devices connected in a Wireless PAN. This information is critical for the operation of the PAN in a Beacon enabled network.
Figure 2.4: The SuperFrame Structure
Contention Access Period: It is the time duration in symbols during which the devices can compete with each other to access the channel using CSMA-CA and transmit the data.
Contention Free Period/Guaranteed Time Slots: It is the time duration for which certain low-latency application devices are given exclusive rights over the channel and the devices can directly start transmitting the data. There can as many as 7 slots assigned for GTS transmissions. These transmissions start immediately after the contention access period.
Inactive Period: It is the time period during which the coordinator goes to a power save mode and it would not interact with the PAN. Therefore, during this time, there will be no beacon transmissions. This implies that the devices also go to sleep mode for this duration.
Superframe Duration: The total time duration of the CAP, CFP (GTS) and a Beacon. The Superframe duration doesn’t include the inactive period.
Beacon Interval: It is the time duration between two successive beacons.
Synchronization is key for better throughput in the network. Every device in the network when ready to transmit data should compete for the channel. But to compete for the channel, they should know when the contention access periods start. And this is what the superframe structure or truly, the beacon transmission does. This information is embedded into the beacon, and the device receiving the beacon can extract this information and get ready to compete for the channel. Similarly is the case when a device wants to exclusively transmit in the GTS mode. It is the coordinator that would assign a device access to the GTS.
The structure of the superframe structure is determined by two parameters. The Superframe Order (SO) and the Beacon Order (BO). The superframe order is the variable which is used to determine the length of the superframe duration. Similarly the Beacon Interval is determined by the variable BO.
For BO=15 shall indicate that there are no beacon transmissions. Also for SO = BO (Fig: 2.5), the beacon interval is same as the superframe duration indicating there is no inactive portion. Similarly, when BO is greater than SO (Fig: 2.6), indicates there is an inactive portion present in the superframe.
BO = SO
BO > SO
The beacon interval and the active and inactive part of the superframe are calculated, in the following computation.
Beacon Interval
aBaseSuperFrameDuration = aBaseSlotDuration×aNumSuperframeSlots
aBaseSlotDuration = 60symbols
aNumSuperFrameSlots = 16
aBaseSuperFrameDuration = 60 × 16symbols = 960symbols
Lets calculate the beacon interval with BO=8 and SO=7.
Similarly the Superframe duration can be calculated using the superframe order as follows
And finally the inactive portion of the superframe can be calculated as,
The figure indicates all the time periods for a superframe with BO=8 and SO=7.
