IEEE 802.15.4 Standard

This standard was developed for low-rate WPAN. It was designed to address the following needs in residential and industrial environment.

• Low data rate,
• Low power consumption,
• Low cost wireless networking among inexpensive, fixed, portable and moving devices.

It supports one-hop star and multi-hop peer-to-peer topologies. Among applications that benefit from this standard are automation and control in homes, factories and warehouses, monitoring for public safety, health and environment, and entertainment.

802.15.4 Protocol Stack

802.15.4 Physical Layer (PHY)

There are 14 PHY primitives with 2 PHY options based on direct sequence spread spectrum (DSSS) methods:

• 2.4 GHz PHY operates in the ISM band with data rate of 250 kbps,
• 868/915 MHz PHY operates in 868 MHz band in Europe; 915 MHz ISM band in the USA with data rates of 20 kbps and 40 kbps, respectively.

It supports one-hop star and multi-hop peer-to-peer topologies. There are 2 types of devices: full function device (FFD) and reduced function device (RFD). FFD is required to support all 45 MAC primitives and can function in any topology. It can talk to RFDs and FFDs.

RFD is required to support 38 MAC primitives under minimal configuration. It is limited to star topology and can only talk to FFD. It is meant for very simple apps. It cannot act as network coordinator.

A network requires at least one FFD as a network coordinator. FFD operates in one of 3 modes: PAN coordinator, router or device. The transmission range between 10 – 20 m.

802.15.4 Data Link Layer (DLL)

There are 2 sublayers: MAC and LLC. LLC is common among the 802 standards. MAC provides support for:

• Association and disassociation – allows for an automatic set up of a star network and the creation of a self-configuring peer-to-peer network
• Acknowledge frame delivery
• Channel access mechanism
• Frame validation
• Guaranteed time slot management
• Beacon management

The capabilities of 802.15.4 are limited compared to other WPANs and WLANs — they have small frame sizes, low bandwidth and low transmit power. The micro-controllers coupled with LoWPAN radios have limited memory and compute power. 6LoWPAN introduces an adaptation layer between the IP stack's link and network layers to enable efficient transmission of IPv6 datagrams over 802.15.4.

IEEE 802.15.4 Superframe Structure

There are two channel access methods:

1. Beacon enabled (BE) mode provides a power management mechanism based on a duty cycle. It uses a superframe structure that is bounded by beacons, i.e., special synchronization frames generated periodically by the coordinator node(s). The time between two consecutive beacons is called Beacon Interval (BI). It is defined using the Beacon Order (BO) parameter. Each superframe consists of an active period and an inactive period.

   During an active period, nodes communicate with their coordinator; during an inactive period they enter a low power state to save energy. The active period is denoted as Superframe Duration (SD) and its size is defined by the Superframe Order (SO) parameter.

   It can be further divided into a Contention Access Period (CAP) and a Contention Free Period (CFP). During CAP, a slotted CSMA-CA algorithm is used for channel access. During CFP, communication occurs in a TDMA style by using a number of Guaranteed Time Slots (GTSs), pre-assigned to individual nodes.

2. Non-beacon enabled (NBE) mode. There is no superframe. Nodes are always active (energy conservation is delegated to the layers above the MAC protocol) and use an unslotted CSMA-CA algorithm for channel access.

Shortcomings of 802.15.4

The following shortcomings of this standard makes it unsuitable for critical applications that have stringent requirements in terms of timeliness and/or reliability:

• Unbounded Delay. Because both the BE and NBE modes rely on CSMA-CA, it cannot provide any bound on the maximum delay experienced by data to reach the final destination.
• Limited communication reliability. The BE mode provides a very low delivery ratio even when the number of nodes is not very high due to the inefficiency of the slotted CSMA-CA algorithm used for channel access. A similar behavior can also occur in the NBE mode when a large number of nodes start transmitting simultaneously (e.g., in event-driven applications).
• No protection against interference/fading, which is very common in wireless networks. Unlike other wireless network technologies, such as Bluetooth and WirelessHART, the 802.15.4 MAC uses one channel and has no built-in frequency hopping mechanism to mitigate the negative effect of interferences and multi-path fading. Consequently, the network is subject to frequent instabilities and may collapse.
• Powered relay nodes. This standard supports both single-hop (star) and multi-hop (peer-to-peer) topologies. The BE mode could be used to form multi-hop PAN with a tree topology, where intermediate nodes do not need to always stay active. However, setting multi-hop topologies in BE mode requires complex mechanisms of synchronization and beacon scheduling that are not specified by the standard. To overcome these limitations, the intermediate relay nodes in multi-hop networks keep their radio on all the time, causing energy drain.

These shortcomings led to the introduction of the IEEE 802.15.4e standard.