The IEEE 802.15.6 MAC operates either in one-hop start topologies, where a hub is responsible for coordinating access to the channel and a two-hop restricted tree topologies where a hub is the root of the tree and enlists the aid of device (relaying nodes) to route to a maximum of two hops.
The hubs are responsible for coordinating channel access by establishing one of the following three access modes:
- Beacon mode with superframe boundaries
- Non-beacon mode with superframe boundries
- Non-beacon mode without superframe boundaries
In all access modes hub and devices maintain time keeping using a superframe structure. In the first two cases the time base is common between hub and devices. In particular for beacon mode with superframe boundaries the time base is announced by the hub using beacons or Timed frames (T-Poll). For non-beacon mode with superframe boundaries the time base is announced by the hub using only Timed frames T-Poll. For the third and final access mode devices and hubs maintain time base independently.
The time base is organised in beacon periods (or superframes) and each superframe is further divided in slots. The number of slots can be from 1 to 256. The duration of the slot is variable and for the narrowband physical layer can be from 1 to 256 ms. This flexible superframe structure gives a great control over the length of slots and beacon periods unlike other protocols such as IEEE 802.15.4 which only allowed a small number of fixed (and depending on the application) oversized slots.
In a follow up post I will describe in more detail the beacon mode with superframe boundaries.