The duration of each backoff period is 20 symbols.
Specifically, once a device has a frame to transmit, it randomly chooses a backoff delay (number of backoff periods) from the range, where BE is the backoff exponent. All other frames are transmitted using CSMA.
In this MAC scheme, acknowledgment frames are transmitted immediately, without using the CSMA method. The asynchronous CSMA IEEE 802.15.4 MAC is similar to the generic CSMA operation and the IEEE 802.11 MAC. The data frames may be acknowledged, depending on their 'Acknowledgment Request' indication. At that time, data frames can be exchanged between the coordinator and the end device in any direction. Once the Association response is received and acknowledged, the end device is associated with the PAN. Such devices may periodically activate their radios to check whether any frames are pending for them, instead of continuously using their radios to receive a frame immediately.
In general, this mechanism is very useful for battery-powered devices of low-traffic networks (e.g., sensor networks). The IEEE 802.15.4 standard uses the term indirect transmission to refer to this mechanism for transmitting frames. Instead, the IEEE 802.15.4 coordinator first stores the Association Response locally it is only transmitted when the end device sends a Data Request and the coordinator acknowledges it. In contrast to IEEE 802.11, the coordinator does not follow the acknowledgment of an Association Request with an immediate transmission of an Association Response. Then it transmits an Association Request to the coordinator of this PAN and the coordinator acknowledges it. Upon the collection of beacons during passive scanning, the end device chooses the PAN with which it would like to associate. Passive scanning means that the device sniffs to collect beacon frames from PAN coordinators (who may have received their Beacon Request in the case of active scanning). Active scanning means that a device first transmits a Beacon Request and later on it performs passive scanning. First, devices that would like to join a network perform either active or passive scanning.
Network SetupĪn IEEE 802.15.4 PAN (personal area network) is set up by a standard process between end devices and PAN coordinators. This example provides an extensive simulation of the non-beacon-enabled, asynchronous, CSMA-based IEEE 802.15.4 MAC. The beacon-enabled MAC allows two different MAC periods: (i) a synchronized-CSMA MAC period, and (ii) a time-slotted, contention-free MAC period. The non-beacon enabled MAC is an asynchronous CSMA (Carrier-sense Multiple Access) MAC, which is very similar to the IEEE 802.11 MAC. The IEEE 802.15.4 MAC specifies two-basic MAC modes: (i) non-beacon-enabled, and (ii) beacon-enabled MAC. Such standards find application in home automation and sensor networking and are highly relevant to the Internet of Things (IoT) trend. The IEEE 802.15.4 MAC and PHY layers provide the basis of other higher-layer standards, such as ZigBee, WirelessHart®, 6LoWPAN and MiWi. The IEEE 802.15.4 standard specifies the MAC and PHY layers of Low-Rate Wireless Personal Area Networks ( LR-WPANs).