Media Access and Link Control Layer ZigBee will use the IEEE 802. 15.4 MAC with 15.4a modifications, to support up to 64,000 nodes in a variety of simple connection topologies. In an extended network, device access to the physical channel is controlled using a combination of TDMA and CSMA/CA. A "superframe" structure (Figure 10-9) breaks the time period between beacon transmissions into 16 time slots. Beacons are transmitted by the PAN co-ordinator at predefined intervals of between 15 ms and 252 seconds, and are used to identify and synchronise devices in the network. These beacon messages are sufficiently infrequent not to suffer collisions, and are therefore not subject to CSMA/CA. The 16 time slots are divided into two access periods, a contention based access period when devices will use CSMA/CA to determine periods when they are able to transmit, and a contention free period when devices will use guaranteed time slots, assigned by the PAN co-ordinator (TDMA). The Wireless PAN Standards Super frame N-1 Super frame N Super frame N+1 Contention based access period 15 mS Inactive Beacon Optional guaranteed time slots Beacon Figure 10-9: ZigBee Superframe Structure combination of predefined beacon intervals and guaranteed transmission time slots allow sensing devices to conserve power during extended sleep periods, waking only to check in on beacons or to use a guaranteed transmission slot. CSMA/CA used during the contention access period is very similar to that used in IEEE 802.11 networks. Devices listen before transmitting and backoff a random number of time slots before attempting to transmit. The backoff period is doubled each time a collision is sensed. A transmitting device can set a header bit to request an acknowledgement (ACK) for a message and will retransmit a message a fixed number of times if the ACK is not received. A non-beacon mode is also defined for application in which the controller does not need to conserve power and where device transmission is SO infrequent that a collision avoidance stra