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1. INTRODUCTION
ZigBee is an established set of specifications for wireless personal area
networking (WPAN). WPAN Low Rate or ZigBee provides specifications for devices
that have low data rates, consume very low power and are thus characterized by long
battery life. ZigBee makes possible completely networked homes where all devices are
able to communicate and be controlled by a single unit.
When you hold the TV remote and wish to use it you have to necessarily point
your control at the device. This one-way, line-of-sight, short-range communication uses
infrared (IR) sensors to enable communication and control and it is possible to operate
the TV remotely only with its control unit. Add other home theatre modules, an airconditioner
and remotely enabled fans and lights to your room, and you become a
juggler who has to handle not only these remotes, but also more numbers that will
accompany other home appliances you are likely to use.
Some remotes do serve to control more than one device after memorizing' access
codes, but this interoperability is restricted to LOS, that too only for a set of related
equipment, like the different units of a home entertainment system.
Now picture a home with entertainment units, security systems including fire
alarm, smoke detector and burglar alarm, air-conditioners and kitchen appliances all
within whispering distance from each other and imagine a single unit that talks with all
the devices, no longer depending on line-of-sight, and traffic no longer being one-way.
This means that the devices and the control unit would all need a common
standard to enable intelligible communication. ZigBee is such a standard for embedded
application software and has been ratified in late 2004 under IEEE 802.15.4 Wireless
Networking Standards.
This kind of network eliminates use of physical data buses like USB and Ethernet
cables. The devices could include telephones, hand-held digital assistants, sensors and
controls located within a few meters of each other.
2. ARCHITECTURE
ZigBee stack architecture is made up of a set of blocks called layers. Each layer
performs a specific set of services for the layer above: a data entity provides a data
transmission service and a management entity provides all other services. Each service
entity exposes an interface to the upper layer through a service access point (SAP), and
each SAP supports a number of service primitives to achieve the required functionality.
The ZigBee stack architecture, which is depicted in figurel below, is based on the
standard Open Systems Interconnection (OSI) seven-layer model but defines only those
layers relevant to achieving functionality in the intended market space. The IEEE
802.15.4-2003 standard defines the lower two layers:
• The physical (PHY) layer
• Medium access control (MAC) sub-layer.
The ZigBee Alliance builds on this foundation by providing the network (NWK)
layer and the framework for the application layer, which includes the application
support sub-layer (APS), the ZigBee device objects (ZDO) and the manufacturer
defined application objects.
IEEE 802.15.4-2003 has two PHY layers that operate in two separate frequency
ranges: 868/915 MHz and 2.4 GHz. The lower frequency PHY layer covers both the
868 MHz European band and the 915 MHz band that is used in countries such as the
United States and Australia. The higher frequency PHY layer is used virtually
worldwide.
The IEEE 802.15.4-2003 MAC sub-layer controls access to the radio channel
using a CSMA-CA mechanism. Its responsibilities may also include transmitting
beacon frames, synchronization and providing a reliable transmission mechanism.
The responsibilities of the ZigBee NWK layer shall include mechanisms used to
join and leave a network, to apply security to frames and to route frames to their
intended destinations. In addition, the discovery and maintenance of routes between
devices devolve to the NWK layer. The NWK layer of a ZigBee coordinator is
responsible for starting a new network, when appropriate, and assigning addresses to
newly associated devices.
The ZigBee application layer consists of the APS, the Application Framework
(AF), the ZDO and the manufacturer-defined application objects. The responsibilities of
the APS sub-layer include maintaining tables for binding, which is the ability to match
two devices together based on their services and their needs, and forwarding messages
between bound devices. The responsibilities of the ZDO include defining the role of the
device within the network (e.g., ZigBee coordinator or end device), initiating and/or
responding to binding requests and establishing a secure relationship between network
devices. The ZDO is also responsible for discovering devices on the network and
determining which application services they provide.
3. DEVICE TYPES
There are three different types of ZigBee device:
• ZigBee coordinator (ZC): The most capable device, the coordinator forms the root
of the network tree and might bridge to other networks. There is exactly one ZigBee
coordinator in each network. It is able to store information about the network, including
acting as the repository for security keys.
• ZigBee Router (ZR): Routers can act as an intermediate router, passing data from
other devices.
• ZigBee End Device (ZED): Contains just enough functionality to talk to its parent
node (either the coordinator or a router); it cannot relay data from other devices. It
requires the least amount of memory, and therefore can be less expensive to
manufacture than a ZR or ZC.
4. MESSAGING
The three messaging modes are:
1.Direct addressing.
2.Indirect addressing.
3. Broadcast addressing.
4.1 DIRECT ADRESSING
Direct addressing assumes device discovery and service discovery have identified
a particular device and endpoint, which supply a complementary service to the
requestor. Specifically, direct addressing defines a means of directing messages to the
device by including its full address and endpoint information. Once devices have been
associated, commands can be sent from one device to another. A command is sent to an
application object at the destination address.
4.2 INDIRECT ADDRESSING
Use of direct addressing requires the controlling device to have knowledge of the
address, endpoint, cluster identifier and attribute identifier of the target device that it
wishes to communicate with and to have this information committed to a binding table
on the ZigBee coordinator prior to the creation of an indirectly addressed message
between the device pair.
A full IEEE 802.15.4 address amounts to 10 octets (PAN identifier plus 64-bit
IEEE address) and a further octet is required for the endpoint. Extremely simple
devices, such as battery-powered switches, may not want the overhead of storing this
information, nor the software for acquiring this information. For these devices, indirect
addressing will be more appropriate.
In Indirect addressing mode .when a source device wishes to send a command to
a destination, instead of including the address of the destination device (which it does
not know and has not stored), it omits the address and specifies indirect addressing via
the APSDE-SAP. The included source address, source endpoint and cluster identifier in
the indirect addressed message are translated via the binding table to those of the
destination device(s) and the messages are relayed to each indicated destination.
Where a cluster contains several attributes, the cluster identifier is used for
addressing and the attribute identifier is used in the command itself to identify a
particular attribute within the cluster. Attributes are not used in the indirect addressing
mechanism and are treated as a part of the data payload. The applications, however, can
parse and utilize the attributes as defined within their profile.
4.3 BROADCAST ADDRESSING
An application may broadcast messages to all endpoints on a given destination
device. This form of broadcast addressing is called application broadcast. The
destination address shall be the 16-bit network broadcast address and the broadcast flag
shall be set in the APS frame control field. The source shall include the cluster
identifier, profile identifier and source endpoint fields in the APS frame.
5. FRAME FORMAT
This sub-clause specifies the format of the NWK frame (NPDU). Each NWK frame
consists of the following basic components:
 A NWK header, which comprises frame control, addressing and sequencing
information.
 A NWK payload, of variable length, which contains information specific to the
frame type.
5.1 GENERAL NPDU FRAME FORMAT
The NWK frame format is composed of a NWK header and a NWK payload. The
fields of the NWK header appear in a fixed order, however, the addressing and
sequencing fields may not be included in all frames.