Overview of IEEE 802.15.6 MAC

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.

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