ZigBee is a high level protocol allowing the communication of small radios, with reduced consumption, based on the standard IEEE 802.15.4 for the networks with personal dimension (Wireless Personal Area Networks: WPANs).

Ratified on December 14th, 2004, the specifications of ZigBee 1.0 are available near the members of the industrial community ZigBee Alliance.

The purpose of this technology is the communication of short distance such as already technology Bluetooth proposes it, while being less expensive and simpler. As example, the traditional ZigBee nodes require approximately 10  % of the code necessary to the implementation of nodes Bluetooth or wireless networkings, and the most elementary ZigBee nodes can thus go down until 2  %!

In 2005, an estimate of the unit costs for a ZigBee node revealed a price of 1,10  $ per unit within the framework of a production in very great number. It is of course necessary to add the price of the microcontrolor who orders the circuit and increases the price slightly. For information, technology Bluetooth, launched in 1998, at that time announced an unit price of production ranging between 4 and 6  $, now gone down again with less than $3.

Comparison of the various protocols without current wire:

History

• 1998 : As of the arrival of wireless technologies WiFi and Bluetooth, the first outlines of networks of the ZigBee type made their appearance within the framework of applications where preceding technologies were not usable. In particular, of many research were carried out on networks organizing itself automatically and composed of small radios. Technology Bluetooth inspired the ZigBee protocol much.

• May 2003 : The standard IEEE 802.15.4 is announced (often associates wrongly with the ZigBee protocol).

• Be 2003 : Philips semiconductors decides to give up the regrouping around ZigBee within ZigBee Alliance. It is an hard blow for the project which does not profit any more of the support from this great group.
• October 2004 : ZigBee Alliance announces that the number of inscriptions doubled to arrive at more than one hundred of companies in 22 country.
• December 14th, 2004 : Ratification of the first specifications of ZigBee.
• June 13rd, 2005 : ZigBee Alliance publishes the first official specifications of the version ZigBee 1.0 which are from now on available in free remote loading.

Applications

The initial specification of ZigBee proposes a slow protocol whose operating range is relatively weak, but of which reliability is rather high, the weak cost price and considerably reduced consumption.

One thus finds this protocol in embarked environments where consumption is selection criteria. Thus, the house automation and the many sensors which it implements particularly appreciate this protocol in full rise and whose configuration of the ring main system is done automatically according to the addition or of the suppression of nodes. One finds also ZigBee in industrial controls, the medical applications, the smoke detectors and of intrusion.

The nodes are conceived to function several months (up to two years for less consuming) in complete autonomy thanks to a simple alkaline pile of 1,5V.

Presentation of the stack ZigBee

The gasoline even of a protocol is to give a framework of operation to a communication. Thus, the IEEE defines within the framework of its standard IEEE 802.15.4 the framework of this protocol for the low layers (physics and mac).

It is thus necessary to implement the layers moreover high level (network and applicatif in our case) so that this model is perfectly functional. The protocols ZigBee can thus function, in theory, on several mac supports but are in general present for media 802.15.4. It is ZigBee Alliance which deals with this part of the protocol by providing a stack (or crushes) reference. This one is reserved to the members of alliance who must implement it in their solutions.

If one wishes to set up a device which can connect itself to a ZigBee network, it will be necessary to follow one of the three following cases:

• Is to belong to ZigBee Alliance and thus to profit from its technological contributions, in particular concerning this stack of communication. It should be known that the inscription with this structure costs approximately $3500 for a company.

• Is to take again a product developed by one of the members of ZigBee Alliance and to have the stack, specific to this product, developed by the selected manufacturer.
• Is to develop its clean stack in agreement with the last specifications available. That of course represents a very important work which must adapt to the system (hardware and software) selected to set up the device. If this development is carried out with business ends, it will have to be validated by ZigBee Alliance.

Routing of the messages

Routing on the level network

On the level of the layer network , the routing is either direct , or indirect .
The routing is direct when a device wanting to transmit data knows the address network of the recipient. This address is thus transmitted in the screen to reach and act on the device envisaged.

In the contrary case, the indirect routing is made when a device does not know the address of the recipient. Equipment of the router type or coordinator makes the relation with the true recipient according to the table of routing and the table of discovered roads.
A device which does not have the capacities of routings (ZED) owes router the data according to the hierarchical routing (it reassembles the tree).

The table of routing contains the data on the recipients. It is about the address TO of the road and the next device to reach “to approach” the recipient.

The table of discovered of a road contains information on the sources of the message. It stores the original address of the device which made the request and addresses device which will transmit the data as an intermediary (between the source and the destination). It contains also the costs of transmission between the source until the current node and of the node to the recipient. It can thus adapt the road to be more powerful by updating the addresses to be used.

The choice of a road, when several routers in parallel relay information, is done compared to the router (or coordinator) on the level of the second node at least. At the time of the request for creation of road, the table will receive several requests starting from the same original address. It then will compare the “costs” of transmission to choose the way having the weakest cost.

The algorithm of routing suggested by ZigBee Alliance for the mesh networks is AODV ( Ad hoc One-Demand Vector Routing ). It is a protocol of routing known as " réactif" : a road is established only on request. The advantage is that it does not charge the traffic.

Routing at the applicatif level

The routing at the applicatif level is done thanks to the table of connection , contained in the coordinator or a router.

The connections make it possible to create logical bonds between complementary devices of application and elements of ends (sensors). The table of connection also makes it possible to associate with an attribute of a device in entry several attributes of devices at exit or the reverse. The table of connection is implemented in the ZigBee coordinator. The choice of this device comes owing to the fact that the ZigBee coordinator is necessary to the network. The second interest is seen that the coordinator is essential to the network, it must be (in general) supplied with the sector. These two reasons make that the table of connection will be always accessible.

The table of connection rests on three criteria standardized by ZigBee Alliance:

• the profile'

One profiles makes it possible to create a interopérable application and distributed. It is thus a question of defining formats of messages and the treatment of the actions to make it possible devices to ask, transmit data and to know to interpret them.

Profile are developed by the companies to allow to meet specific needs. For example, the first profiles existing is made to manage the lamps and of the switches ( home control lighting ). This profiles allows six types of exchanges of messages of control.

Profile allow to also create a standard around each application to allow the interworking of the systems.

• the cluster

The clusters are associated with floods of data entering or outgoing. The identifiers of clusters are single in one profiles. The clusters make it possible to bind two devices by the association of a cluster in entry and of a cluster at exit by supposing that they belong to same profiles. In fact two devices are dependant if they share the same need (receiving side) and the same resource (transmitting side). The table of connection ( binding table ) contains for each cluster an identifier to define it (on 8 bits) and addresses it of the two devices (source and destination).

• the attribute

An attribute defines a sensor or an actuator. It is the element which describes way the most precise use of device (for example a sensor of movement, a buzzer, a lamp, etc).

The table of connection is the applicative layer which makes it possible to manage the table of routing and the table of discovered roads. It is it which will make it possible to associate the statement of a sensor on a device a specific action on another device through all the layers of the ZigBee protocol. It is a way of simplifying the access when the network contains many connections and devices: the recognition between the devices which dialog makes compared to their “families” (profile them and clusters) and their qualities (attributes) common.

See too

• 802.15.4

External bonds

• Official site of ZigBee Alliance

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