IEEE 802.15.4 Developer’s Guide

Introduction

The IEEE 802.15.4 working group focuses on standardization of the bottomtwo layers: Medium Access Control (MAC) and Physical access (PHY). And thereare mainly two options available for upper layers:

  • ZigBee - proprietary protocol from the ZigBee Alliance

  • 6LoWPAN - IPv6 networking over low rate personal area networks

The goal of the Linux-wpan is to provide a complete implementationof the IEEE 802.15.4 and 6LoWPAN protocols. IEEE 802.15.4 is a stackof protocols for organizing Low-Rate Wireless Personal Area Networks.

The stack is composed of three main parts:

  • IEEE 802.15.4 layer; We have chosen to use plain Berkeley socket API,the generic Linux networking stack to transfer IEEE 802.15.4 datamessages and a special protocol over netlink for configuration/management

  • MAC - provides access to shared channel and reliable data delivery

  • PHY - represents device drivers

Socket API

int sd = socket(PF_IEEE802154, SOCK_DGRAM, 0);

The address family, socket addresses etc. are defined in theinclude/net/af_ieee802154.h header or in the special headerin the userspace package (see eitherhttps://linux-wpan.org/wpan-tools.htmlor the git tree athttps://github.com/linux-wpan/wpan-tools).

6LoWPAN Linux implementation

The IEEE 802.15.4 standard specifies an MTU of 127 bytes, yielding about 80octets of actual MAC payload once security is turned on, on a wireless linkwith a link throughput of 250 kbps or less. The 6LoWPAN adaptation format[RFC4944] was specified to carry IPv6 datagrams over such constrained links,taking into account limited bandwidth, memory, or energy resources that areexpected in applications such as wireless Sensor Networks. [RFC4944] definesa Mesh Addressing header to support sub-IP forwarding, a Fragmentation headerto support the IPv6 minimum MTU requirement [RFC2460], and stateless headercompression for IPv6 datagrams (LOWPAN_HC1 and LOWPAN_HC2) to reduce therelatively large IPv6 and UDP headers down to (in the best case) several bytes.

In September 2011 the standard update was published - [RFC6282].It deprecates HC1 and HC2 compression and defines IPHC encoding format which isused in this Linux implementation.

All the code related to 6lowpan you may find in files: net/6lowpan/*and net/ieee802154/6lowpan/*

To setup a 6LoWPAN interface you need:1. Add IEEE802.15.4 interface and set channel and PAN ID;2. Add 6lowpan interface by command like:# ip link add link wpan0 name lowpan0 type lowpan3. Bring up ‘lowpan0’ interface

Drivers

Like with WiFi, there are several types of devices implementing IEEE 802.15.4.1) ‘HardMAC’. The MAC layer is implemented in the device itself, the deviceexports a management (e.g. MLME) and data API.2) ‘SoftMAC’ or just radio. These types of devices are just radio transceiverspossibly with some kinds of acceleration like automatic CRC computation andcomparison, automagic ACK handling, address matching, etc.

Each type of device requires a different approach to be hooked into the Linuxkernel.

HardMAC

See the header include/net/ieee802154_netdev.h. You have to implement Linuxnet_device, with .type = ARPHRD_IEEE802154. Data is exchanged with socket familycode via plain sk_buffs. On skb reception skb->cb must contain additionalinfo as described in thestructieee802154_mac_cb. During packet transmissionthe skb->cb is used to provide additional data to the device’sheader_ops->create function. Be aware that this data can be overridden later(when socket code submits skb to qdisc), so if you need something from that cblater, you should store info in the skb->data on your own.

To hook the MLME interface you have to populate the ml_priv field of yournet_device with a pointer tostructieee802154_mlme_ops instance. The fieldsassoc_req, assoc_resp, disassoc_req, start_req, and scan_req are optional.All other fields are required.

SoftMAC

The MAC is the middle layer in the IEEE 802.15.4 Linux stack. At the moment, itprovides an interface for driver registration and management of slaveinterfaces.

NOTE: Currently the only monitor device type is supported - it’s IEEE 802.15.4stack interface for network sniffers (e.g. WireShark).

This layer is going to be extended soon.

See header include/net/mac802154.h and several drivers indrivers/net/ieee802154/.

Fake drivers

In addition there is a driver available which simulates a real device withSoftMAC (fakelb - IEEE 802.15.4 loopback driver) interface. This optionprovides a possibility to test and debug the stack without usage of real hardware.

Device drivers API

The include/net/mac802154.h defines following functions:

structieee802154_dev*ieee802154_alloc_device(size_tpriv_size,structieee802154_ops*ops)

Allocation of IEEE 802.15.4 compatible device.

voidieee802154_free_device(structieee802154_dev*dev)

Freeing allocated device.

intieee802154_register_device(structieee802154_dev*dev)

Register PHY in the system.

voidieee802154_unregister_device(structieee802154_dev*dev)

Freeing registered PHY.

voidieee802154_rx_irqsafe(structieee802154_hw*hw,structsk_buff*skb,u8lqi)

Telling 802.15.4 module there is a new received frame in the skb withthe RF Link Quality Indicator (LQI) from the hardware device.

voidieee802154_xmit_complete(structieee802154_hw*hw,structsk_buff*skb,boolifs_handling)

Telling 802.15.4 module the frame in the skb is or going to betransmitted through the hardware device

The device driver must implement the following callbacks in the IEEE 802.15.4operations structure at least:

struct ieee802154_ops {     ...     int     (*start)(struct ieee802154_hw *hw);     void    (*stop)(struct ieee802154_hw *hw);     ...     int     (*xmit_async)(struct ieee802154_hw *hw, struct sk_buff *skb);     int     (*ed)(struct ieee802154_hw *hw, u8 *level);     int     (*set_channel)(struct ieee802154_hw *hw, u8 page, u8 channel);     ...};
intstart(structieee802154_hw*hw)

Handler that 802.15.4 module calls for the hardware device initialization.

voidstop(structieee802154_hw*hw)

Handler that 802.15.4 module calls for the hardware device cleanup.

intxmit_async(structieee802154_hw*hw,structsk_buff*skb)

Handler that 802.15.4 module calls for each frame in the skb going to betransmitted through the hardware device.

inted(structieee802154_hw*hw,u8*level)

Handler that 802.15.4 module calls for Energy Detection from the hardwaredevice.

intset_channel(structieee802154_hw*hw,u8page,u8channel)

Set radio for listening on specific channel of the hardware device.

Moreover IEEE 802.15.4 device operations structure should be filled.