My Project | Low Rate Wireless Personal Area Networks in NS2

The new IEEE standard, 802.15.4, defines the physical layer (PHY) and medium access control sublayer (MAC) specifications for low data rate wireless connectivity among relatively simple devices that consume minimal power and typically operate in the Personal Operating Space (POS) of 10 meters or less. An 802.15.4 network can simply be a one-hop star, or, when lines of communication exceed 10 meters, a self-configuring, multi-hop network. A device in an 802.15.4 network can use either a 64-bit IEEE address or a 16-bit short address assigned during the association procedure, and a single 802.15.4 network can accommodate up to 64k (216 ) devices. Wireless links under 802.15.4 can operate in three license free industrial scientific medical (ISM) frequency bands. These accommodate over air data rates of 250 kb/sec (or expressed in symbols, 62.5 ksym/sec) in the 2.4 GHz band, 40 kb/sec (40 ksym/sec) in the 915 MHz band, and 20 kb/sec (20 ksym/sec) in the 868 MHz. Total 27 channels are allocated in 802.15.4, with 16 channels in the 2.4 GHz band, 10 channels in the 915 MHz band, and 1 channel in the 868 MHz band.
Wireless communications are inherently susceptible to interception and interference. Some security research has been done for WLANs and wireless sensor networks, but pursuing security in wireless networks remains a challenging task. 802.15.4 employs a fully handshaked protocol for data transfer reliability and embeds the Advanced Encryption Standard (AES) for secure data transfer.
In the following subsections, we give a brief overview of the PHY layer, MAC sublayer and some general functions of 802.15.4. The services of a layer are the capabilities it offers to the user in the next higher layer or sublayer by building its functions on the services of the next lower layer. This concept is illustrated in 3.1, showing the service hierarchy and the relationship of the two correspondent N-users and their associated N-layer (Or sublayer) peer protocol entities.
The services are specified by describing the information flow between the N-user and the N-layer. This information flow is modeled by discrete, instantaneous events, which characterize the provision of a service. Each event consists of passing a service primitive from one layer to the other through a layer SAP associated with an N-user. Service primitives convey the required information by providing a particular service. These service primitives are an abstraction because they specify only the provided service rather than the means by which it is provided. This definition is independent of any other interface implementation.
Services are specified by describing the service primitives and parameters that characterize it. A service may have one or more related primitives that constitute the activity that is related to that particular service. Each service primitive may have zero or more parameters that convey the information required to provide the service.









Figure 2.10: Service Primitive


A primitive can be one of four generic types:
Ø Request: The request primitive is passed from the N-user to the N-layer to request that a service is initiated.
Ø Indication: The indication primitive is passed from the N-layer to the N-user to indicate an internal N-layer event that is significant to the N-user. This event may be logically related to a remote service request, or it may be caused by an N-layer internal event.
Ø Response: The response primitive is passed from the N-user to the N-layer to complete a procedure previously invoked by an indication primitive.
Ø Confirm: The confirm primitive is passed from the N-layer to the N-user to convey the results of one or more associated previous service requests.

The IEEE 802.15.4 standard describes 14 PHY and 35 MAC primitives. The IEEE 802.15.4, WPAN supports two types of devices, The FFD and the RFD. The FFD is a full function devices supporting all the defined primitives of the standard. While, the RFD, is a reduced function device, degraded in terms of functionality. It supports a subset of these primitives. A total of 38 primitives are supported by RFDs. A few of those primitives are described here which are quite frequently used and are implemented in the zigbee modules.