IEEE 802.15.4 MAC

Design Drivers

• Extremely low cost.

• Ease of implementation.

• Reliable data transfer.

• Short range operation.

• Very low power consumption.

Overview

Full function device (FFD).

• Any topology.

• Network coordinator capable.

• Talks to any other device.

• The FFD can operate in three modes serving.

  • Device.

  • Coordinator.

  • PAN coordinator.

Reduced function device (RFD).

• Limited to star topology.

• Talks only to a network coordinator.

  • Cannot become a network coordinator.

• Very simple implementation.

Star Topology

Mesh (Peer-Peer) Topology

Cluster Tree Topology

Combined Topology

General Frame Structure

4 Types of MAC Frames:

• Data Frame.

• Beacon Frame.

• Acknowledgment Frame.

• MAC Command Frame.

MAC layer

Managing PANs.

• Channel scanning (ED, active, passive, orphan).

• PAN ID conflict detection and resolution.

• Starting a PAN.

• Sending beacons.

• Device discovery, association/disassociation.

• Synchronization (beacon/nonbeacon).

• Orphaned device realignment.

Transfer handling.

• Transaction based (indirect transmission).

  • Beacon indication.

  • Polling.

• Transmission, Reception, Rejection, Retransmission.

  • Acknowledged / Not acknowledged.

• GTS management.

  • Allocation / deallocation / Reallocation.

  • Usage.

Superframe

A superframe is divided into two parts.

• Inactive: all devices sleep

• Active:

  • Active period will be divided into 16 slots.

  • 16 slots can further be divided into two parts.

    • Contention access period

    • Contention free period

Beacons are used for

• starting superframes.

• synchronizing with associated devices.

• announcing the existence of a PAN.

• informing pending data in coordinators.

In a beacon enabled network,

• Devices use the slotted CSMA/CA mechanism to contend for the usage of channels.

• FFDs which require fixed rates of transmissions can ask for guarantee time slots (GTS) from the coordinator.

The structure of superframes is controlled by two parameters: beacon order (BO) and superframe order (SO).

• BO decides the length of a superframe.

• SO decides the length of the active potion in a superframe.

For channels 11 to 26, the length of a superframe can range from 15.36 msec to 215.7 sec.

• which means very low duty cycle.

Each device will be active for 2^(-(BO-SO)) portion of the time, and sleep for 1-2^(-(BO-SO)) portion of the time.

In IEEE 802.15.4, devices’ duty cycle follow the specification.

GTS Concepts

A guaranteed time slot (GTS) allows a device to operate on the channel within a portion of the superframe.

A GTS shall only be allocated by the PAN coordinator.

The PAN coordinator can allocated up to seven GTSs at the same time.

The PAN coordinator decides whether to allocate GTS based on:

• Requirements of the GTS request.

• The current available capacity in the superframe.

A GTS can be deallocated.

• At any time at the discretion of the PAN coordinator or

• By the device that originally requested the GTS.

A data frame transmitted in an allocated GTS shall use only short addressing.

The PAN coordinator shall be able to store the info of devices that necessary for GTS, including starting slot, length, direction and associated device address.

Each device may request one transmit GTS and/or one receive GTS.

A device shall only attempt to allocate and use a GTS if it is currently tracking the beacon.

If a device loses synchronization with the PAN coordinator, all its GTS allocations shall be lost.

The use of GTSs be an RFD is optional.