A Guide For the Clueless: IEEE 802.15.4 Standard for Low-Rate Wireless Personal Area Networks (LR-WPAN)

Author: Andrew D. Parker
Updated: July 14th, 2004

Introduction

The IEEE 802.15.4 LR-WPAN task group is charged with developing a standard for Low-Rate Wireless Personal Area Networks (LR-WPANs) [1]. This very new technology: the IEEE Standards board approved IEEE 802.15.4 draft 18 on May 12, 2003. Manufacturers are expected to make available 802.15.4 based products in the coming months.

Until recently, the primary activity in networking technology has been focused on productivity and entertainment applications running on PCs. For wireless technologies, this translates to high bandwidth. The attention of the wireless communications industry and researchers has now spread beyond the office and home to include new environments such as factories, hospitials, and agriculture. Furthermore, applications in these new areas are becoming increasingly dependent upon embedded systems. Traditional wireless technologies are often not suitable in this context due to reasons of practicality, hence the need for a new standard that focuses on these new special requirements.

What are the goals for IEEE 802.15.4?

Generally speaking, the applications that IEEE 802.15.4 addresses are characterized by their requirements for low power consumption and low cost of deployment [3].

Very low power consumption: On any wireless embedded system, the radio is often one of the largest consumers of energy--even more than the CPU. The embedded systems that are expected to utilize IEEE 802.15.4 for communication have extremely tight energy contraints, perhaps operating off of a small battery for a period of months or years.
Very low cost: The final cost of the component implementing the IEEE 802.15.4 standard must be small compared to the cost of the rest of the component. The reason for this is that the deployed applications are envisioned to be composed of possibly numerous, inexpensive, and disposable devices.

Basically, what are the properties of IEEE 802.15.4?

Table 1 summarizes the proprites of IEEE 802.15.4.

Table 1. Summary of high-level characteristics, taken from [3].
Property	Range
Raw data rate	868 MHz: 20 kb/s; 915 MHz: 40 kb/s; 2.4 GHz: 250 kb/s
Range	10 - 20 meters
Latency	Down to 15 ms
Channels	868MHz: 1 channel; 915 MHz: 10 channels; 2.4 Ghz: 16 channels
Frequency band	Two PHYs: 868 MHz/915 MHz and 2.4 GHz
Addressing	Short 16-bit or 64-bit IEEE
Channel access	CSMA-CA and slotted CSMA-CA
Temperature	Industrial temperature range -40 to +85 C

Furthermore, high reliability is acheived through message acknowledgements, error checking, and prioritized communication. Network layer protocols are expected to support star and mesh topologies.

How does IEEE 802.15.4 relate to other IEEE standards?

There are several other wireless IEEE standards in existence. Table 2 highlights IEEE 802.15.4 in context with these other standards.

Table 2. 802.15.4 in the context of other related IEEE standards, taken from [2].
Standards Committee	IEEE 802: LAN/MAN Standards Committee
Working Group	IEEE 802.11: WLAN Working Group	IEEE 802.15: WPAN Working Group
Task Group	802.11a/b/g	802.15.1: WPAN/Bluetooth	802.15.3a: WPAN High Rate/UWB	802.15.4: WPAN Low Rate/Zigbee
Promoter / Industry Alliance	Wi-Fi: Cisco, 3Com, Agere, Intersil, Compaq, Dell, Sony, Nokia, Symbol, etc.	Bluetooth SIG: Ericsson, 3Com, IBM, Intel, Motorola, Nokia, Agere, Toshiba, etc.	Wi-Media: Appairent, HP, Motorola, Philips, Samsung, Sharp, XtremeSpectrum	Zigbee Alliance: Honeywell, Invensys, Mitsubishi, Motorola, Philips, etc.

IEEE 802.11 is the Wireless Local Area Network (WLAN) Working group. WLAN technologies are not suitable for the scope of problems WPANs concern themselves with. For more information on WLANs, see [5].

Table 3 illustrates some general differences between WLANs, WPANs, and LR-WPANs.

Table 3. A comparison of LR-WPAN with other wireless technologies, taken from [4].
	WLAN (802.11)	Bluetooth-based WPAN (802.15.1)	Low-rate WPAN (802.15.4)
Range	~100 m	~10 - 100 m	~10 m
Data throughput	~2 - 11 Mbs	~1 Mbs	~0.25 Mbs
Power consumption	Medium	Low	Ultra low
Size	Larger	Smaller	Smallest
Cost/complexity	>6	1	0.2

The IEEE 802.15 working group defines three classes of WPANs characterized by data rate, power usage, and quality of service [4]:

802.15.1: AKA Bluetooth: Medium-rate: Voice applications, PDAs, etc.
802.15.3: High-data-rate, High quality of service. Good for multi-media applications.
802.15.4: Lower cost/power/data-rate/QoS than 802.15.1.

Note that IEEE 802.15.2 tackles coexistance issues between WLANs (802.11) and WPANs (802.15) [8].

What is the relationship between IEEE 802.15.4 and Zigbee?

As is the custom of IEEE 802 standards, 802.15.4 is charged only with creating the specifications at the physical layer and portions of the data link layer (DLL). The higher-layer protocols are left to industry and the individual applications.

The Zigbee Alliance is an association of companies involved with building higher-layer standards based on IEEE 802.15.4 [6]. This includes network, security, and application protocols. In many contexts, "ZigBee" is used interchangably with IEEE 802.15.4, just as "Bluetooth" is often used interchangably with IEEE 802.15.1. Sometimes this usage is not correct.

The following table illustrates where IEEE 802.15.4 lies in relation to ZigBee ("higher layers") in the context of the ISO-OSI layered network model.

Table 4. IEEE 802.15.4 in the ISO-OSI layered network model, taken from [3] and [4].
ISO-OSI Model	IEEE 802 Model
7. Application	Higher layers
6. Presentation
5. Session
4. Transport
3. Network
2. Data link	IEEE 802.2 LLC, type I	Other LLC
	SSCS
	IEEE 802.15.4 (MAC)
1. Physical	IEEE 802.15.4 868/915 MHz (PHY)	IEEE 802.15.4 2.4 GHz (PHY)

What is IEEE 802.2 LLC, type I?

IEEE 802.2 LLC, type I is Logical Link Control (LLC) sublayer common to the IEEE 802 standards [10]. For example, it is this sublayer that is used to encapsulate IP datagrams and ARP requests.

What approaches are used to make IEEE 802.15.4 low power and low cost?

In order to achieve the low power and low cost goals established by IEEE 802.15.4 and to inform design trade-off decisions, the following approaches are taken [4]:

Reduce the amount of data transmitted
Reduce the transceiver duty cycle and frequency of data transmissions
Reduce the frame overhead
Reduce complexity
Reduce range
Implement strict power management mechanisms (power-down and sleep modes)

All of these contribute to lower power and cost requirements. Here are some highlights of power and cost saving decisions made at the PHY and MAC layers [4]:

Physical Layer Decision Highlights:

2.4 GHz band: This frequency is unlicensed and widely available for use. Existing low-cost designs for this band makes device manufacturing more affordable.
868/915 MHz band: Lower power consumption than 2.4 GHz and are freely available for use in Europe and the United States respectively.
Data Rate: Power can be saved by maximizing the data rate for any given amount of data that needs to be transmitted. Thus IEEE 802.15.4 allows up to 250kbs. However, the real power savings comes from transceiver duty cycling, which is expected to be about 0.33% for many applications. The high data rate allows the standard to accommodate applications that need higher throughput, but that can tolerate increased latency in order to save power.

Media Access Control Decision Highlights:

Network Topology: The common configuration of an LR-WPAN will be a star or star-cluster topology. The master node at the center of the star (or cluster-head, in the case of star-clusters) simplifies control and synchronization of the nodes participating in the network.
Homogeneous vs. Heterogeneous Devices: At the MAC layer, components may be identified as a fully functional or reduce functional device. Therefore, if the application accommodates them, reduced functional devices may be attached to the network as leaf nodes, which simplifies their implementation and precludes these devices from forwarding messages, allowing them to save on power.
Message Structure: The structure used is very simple compared to other WLAN/WPAN standards, allowing for very short messages to conserve power.

Discussion

IEEE 802.15.4's most important contribution is that it is an enabling technology for the Proactive Computing agenda set forth by David Tennenhouse [7] in 2000. David Tennenhouse illustrated the disparity between where researchers are investing their resources versus where the "chips" are going: into embedded systems. Tennenhouse argued for a paradigm shift from interactive computing (human-centered) to proactive computing (embedded human-supervised systems). Low power and low cost networking technology is one of the major requirements in order to realize this vision.

It is too early to tell how successful IEEE 802.15.4 will be once deployed in the real-world. One concern is that IEEE 802.15.4 utilizes frequencies that are already crowded by other devices such as cell-phones, microwaves, and other IEEE 802 standards. IEEE 802.15.4 tries to be a good neighbor and is expected to have little impact on other networks such as IEEE 802.11b assuming some form of frequency management is used [9].

References

[1] IEEE 802.15.4 WPAN-LR Task Group Website: http://www.ieee802.org/15/pub/TG4.html
[2] Taekon Kim of Samsung, High Throughput Wireless Home Network Solutions: http://snrc.stanford.edu/events/industry-seminar/spring03/slides/taekon.pdf
[3] Callaway, E.; Gorday, P.; Hester, L.; Gutierrez, J.A.; Naeve, M.; Heile, B.; Bahl, V., Home networking with IEEE 802.15.4: a developing standard for low-rate wireless personal area networks, Communications Magazine, IEEE, Vol.40, Iss.8, Aug 2002 Pages: 70- 77
[4] Gutierrez, J.A.; Naeve, M.; Callaway, E.; Bourgeois, M.; Mitter, V.; Heile, B., IEEE 802.15.4: a developing standard for low-power low-cost wireless personal area networks, Network, IEEE, Vol.15, Iss.5, Sep/Oct 2001Pages:12-19
[5] 802.11 Working Group Website: http://www.ieee802.org/11/
[6] ZigBee Alliance Website: http://www.zigbee.org
[7] Tennenhouse, D. Proactive computing, Communications of the ACM, vol.43, (no.5), ACM, May 2000. p.43-50.
http://www.acm.org/pubs/articles/journals/cacm/2000-43-5/p43-tennenhouse/p43-tennenhouse.pdf
[8] 802.15.2 Task Group Website: http://grouper.ieee.org/groups/802/15/pub/TG2.html
[9] Howitt, I.; Gutierrez, J.A., IEEE 802.15.4 low rate-wireless personal area network coexistence issues, Wireless Communications and Networking, 2003. WCNC 2003. 2003 IEEE, Vol.3, Iss., 2003 Pages: 1481- 1486
[10] 802.2 Working Group Website: http://ieee802.org/2/