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- Linux IEEE 802.15.4 implementation
- Introduction
- ============
- The IEEE 802.15.4 working group focuses on standardization of bottom
- two layers: Medium Access Control (MAC) and Physical (PHY). And there
- are mainly two options available for upper layers:
- - ZigBee - proprietary protocol from the ZigBee Alliance
- - 6LoWPAN - IPv6 networking over low rate personal area networks
- The linux-wpan project goal is to provide a complete implementation
- of the IEEE 802.15.4 and 6LoWPAN protocols. IEEE 802.15.4 is a stack
- of 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 messages
- and a special protocol over genetlink 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 the
- include/net/af_ieee802154.h header or in the special header
- in the userspace package (see either http://wpan.cakelab.org/ or the
- git tree at https://github.com/linux-wpan/wpan-tools).
- One can use SOCK_RAW for passing raw data towards device xmit function. YMMV.
- Kernel side
- =============
- 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 device
- exports MLME and data API.
- 2) 'SoftMAC' or just radio. These types of devices are just radio transceivers
- possibly with some kinds of acceleration like automatic CRC computation and
- comparation, automagic ACK handling, address matching, etc.
- Those types of devices require different approach to be hooked into Linux kernel.
- HardMAC
- =======
- See the header include/net/ieee802154_netdev.h. You have to implement Linux
- net_device, with .type = ARPHRD_IEEE802154. Data is exchanged with socket family
- code via plain sk_buffs. On skb reception skb->cb must contain additional
- info as described in the struct ieee802154_mac_cb. During packet transmission
- the skb->cb is used to provide additional data to device's header_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 cb later, 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 your
- net_device with a pointer to struct ieee802154_mlme_ops instance. The fields
- assoc_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. This moment it
- provides interface for drivers registration and management of slave interfaces.
- NOTE: Currently the only monitor device type is supported - it's IEEE 802.15.4
- stack interface for network sniffers (e.g. WireShark).
- This layer is going to be extended soon.
- See header include/net/mac802154.h and several drivers in
- drivers/net/ieee802154/.
- Device drivers API
- ==================
- The include/net/mac802154.h defines following functions:
- - struct ieee802154_dev *ieee802154_alloc_device
- (size_t priv_size, struct ieee802154_ops *ops):
- allocation of IEEE 802.15.4 compatible device
- - void ieee802154_free_device(struct ieee802154_dev *dev):
- freeing allocated device
- - int ieee802154_register_device(struct ieee802154_dev *dev):
- register PHY in the system
- - void ieee802154_unregister_device(struct ieee802154_dev *dev):
- freeing registered PHY
- Moreover IEEE 802.15.4 device operations structure should be filled.
- Fake drivers
- ============
- In addition there is a driver available which simulates a real device with
- SoftMAC (fakelb - IEEE 802.15.4 loopback driver) interface. This option
- provides possibility to test and debug stack without usage of real hardware.
- See sources in drivers/net/ieee802154 folder for more details.
- 6LoWPAN Linux implementation
- ============================
- The IEEE 802.15.4 standard specifies an MTU of 127 bytes, yielding about 80
- octets of actual MAC payload once security is turned on, on a wireless link
- with 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 are
- expected in applications such as wireless Sensor Networks. [RFC4944] defines
- a Mesh Addressing header to support sub-IP forwarding, a Fragmentation header
- to support the IPv6 minimum MTU requirement [RFC2460], and stateless header
- compression for IPv6 datagrams (LOWPAN_HC1 and LOWPAN_HC2) to reduce the
- relatively large IPv6 and UDP headers down to (in the best case) several bytes.
- In Semptember 2011 the standard update was published - [RFC6282].
- It deprecates HC1 and HC2 compression and defines IPHC encoding format which is
- used in this Linux implementation.
- All the code related to 6lowpan you may find in files: net/6lowpan/*
- and net/ieee802154/6lowpan/*
- To setup 6lowpan interface you need (busybox release > 1.17.0):
- 1. Add IEEE802.15.4 interface and initialize PANid;
- 2. Add 6lowpan interface by command like:
- # ip link add link wpan0 name lowpan0 type lowpan
- 3. Set MAC (if needs):
- # ip link set lowpan0 address de:ad:be:ef:ca:fe:ba:be
- 4. Bring up 'lowpan0' interface
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