The IEEE 802.15.4 standard specifies the physical and MAC layers for low-rate wireless personal area networks (PANs). Its protocol stack is simple and flexible, and does not require any infrastructure, which is suitable for short range communications. For these reasons, it features ease of installation, low cost, and a reasonable battery life of the devices. The physical layer of the IEEE 802.15.4 standard has been designed to co-exist with other IEEE standards for wireless networks, for example, IEEE 802.11 and IEEE 802.15.1 (Bluetooth). It features activation and deactivation of the radio transceiver and transmission of packets on the physical medium.
The MAC layer provides data and management services to the upper layers. The data service enables transmission and reception of MAC packets across the physical layer. The management services include synchronization of communications, management of guaranteed time slots, and association and disassociation of devices to the network. In addition, the MAC layer implements basic security mechanisms.
Overview of the MAC layer
The MAC layer defines two types of nodes: reduced function devices (RFDs) and full function devices (FFDs). The RFDs are meant to implement end devices with reduced processing, memory, and communication capabilities, which implement a subset of the MAC layer functions. In particular, the RFDs can only be associated with an existing network and they depend on FFDs for communication. One RFD can be associated to only one FFD at a time. The FFDs implement the full MAC layer and they can act either as a PAN coordinator or as a generic coordinator of a set of RFDs. The PAN coordinator sets up and manages the network. In particular, it selects the PAN identifier and manages association or disassociation of devices. In the association phase, the PAN coordinator assigns to the new device a 16-bit address. This address can be used alternatively to the standard 64-bit extended IEEE address, which is statically assigned to each device. The FFDs cooperate to implement the network topology. The actual network formation is performed at the network layer, but the MAC layer provides support to two types of network topologies: star and peer-to-peer. In the star topology, one FFD is the PAN coordinator and is located in the star center. All the other FFDs and RFDs behave as generic devices and can only communicate with the coordinator, which synchronizes all the communications in the network. Different stars operating in the same area have different PAN identifiers and operate independently of each other. An example of the star topology is shown in Fig. 1 (a). In the peer-to-peer topology, each FFD is capable of communicating with any other device within its radio range. One FFD acts as the PAN coordinator, and the other FFDs act as routers or end devices to form a multihop network, as shown in Fig. 1 (b). The RFDs act as end devices and each RFD is connected only with one FFD.
Fig. 1. Network topologies supported by the IEEE 802.15.4 MAC layer [S. Chessa].
Channel access
The MAC protocol has two types of channel access: with a superframe structure and without a superframe structure. The channel access with a superframe structure is used in star topologies (it can also be used in peer-to-peer topologies organized in trees) and provides synchronization between nodes to enable energy savings of the devices. The channel access without a superframe structure is more general and can be used to support communications in arbitrary peer-to–peer topologies.
Data-transfer models
The standard supports three models of data transfers: end device to the coordinator, coordinator to an end device, and peer to peer. The star topology uses only the first two models because the data transfers can happen only between the PAN coordinator and the other devices. In the peer - to-peer topology, all three models are possible because data can be exchanged between any pair of devices. The actual implementation of the three data-transfer models depends on whether the
network supports the transmission of beacons.
MAC layer services
The MAC layer provides data and management services to the upper layer. Each service is specified by a set of primitives, which can be classified into four generic types, use all or part of the four primitives depending on its needs.
Request: It is invoked by the upper layer to request for a specific service.
Indication: It is generated by the MAC layer and is directed to the upper layer to notify the occurrence of an event related to a specific service.
Response: It is invoked by the upper layer to complete a procedure previously initiated by an indication primitive.
Confirm: It is generated by the MAC layer and is directed to the upper layer to convey the results of one or more service requests previously issued.
Data service
The data service comprises one main service that exploits only the request, confirm, and indication primitives. The DATA.request primitive is invoked by the upper layer to send a message to another device. The result of a transmission requested with a previous DATA.request primitive is reported by the MAC layer to the upper layer by the DATA.confirm primitive, which returns the status of the transmission. The DATA.indication primitive corresponds to a “receive” primitive: it is generated by the MAC layer on the receipt of a message. To pass the received message from the physical layer to the upper layer.
Management services
The management services of the MAC layer include functionalities for PAN initialization, device association/disassociation, detection of existing PANs, and other services exploiting some of the features of the MAC layer. As an example, we describe the protocol and functionalities of the ASSOCIATE service here. This service is invoked by a device wishing to be associated with a PAN that it has already identified by preliminarily invoking the SCAN service. The ASSOCIATE.request primitive takes as parameters the PAN identifier, the coordinator address, and the 64-bit extended IEEE address of the device.
The primitive sends an association request message to a coordinator. Since the association procedure is meant for beacon enabled networks, the association request message is sent during the contention access period (CAP) using the slotted carrier sense multiple access with collision avoidance (CSMA-CA) protocol. The coordinator acknowledges the reception of the association messages immediately. However, this acknowledgment does not mean that the request has been accepted. On the coordinator side, the association request message is passed to the upper layers of the coordinator protocol stack (using the ASSOCIATE.indication primitive), where the decision on the association request is actually made. If the request is accepted, the coordinator selects a short 16-bit address that the device may use later in place of the 64-bit extended IEEE address.
Further reading
- J. Zheng and A. Jamalipour (Editors), “Wireless Sensor Networks: A Networking Perspective,” Chapter 13, pp. 407-431, Wiley, Hoboken, NJ, USA, 2009.
- IEEE 802.15.4 Task Group, http://www.ieee802.org/15/pub/TG4.html
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