Compared with wired networks, wireless networks provide advantages in deployment, cost, size, and distributed intelligence. Wireless technology not only enables users to set up a network quickly, but also enables them to set up a network where it is in-convenient or impossible to wire cables. The “care free” feature and convenience of deployment make a wireless network more cost-efficient than a wired network in general... Continue Reading
A Low rate WPAN supports three different types of topologies.
- Star Topology
- Peer-to-Peer Topology
- Cluster Tree/Mesh Topology... Continue Reading
Network formation is part of the network layer functionalities. An example run of the various steps involved for a network formation is presented:
Star-Topology
- Assume a full function device is switched ON for the first time.
- It starts scanning its operating channels for possible beacon transmissions, from other PANs... Continue Reading
The LR-WPAN architecture is defined in terms of a number of blocks in order to simplify the standard. These blocks are called layers. Each layer is responsible for one part of the standard and offers services to the higher layers. The layout of the blocks is similar to the structure of the OSI layered architecture. But the IEEE 802.15.4 standard only defines the PHY and the MAC layers. The upper layers of networking and application have been left for the application developers. An LR-WPAN device comprises a PHY, which contains the radio frequency (RF) transceiver along with its low-level control mechanism, and a MAC sub layer that provides access to the physical channel for all types of transfer. The figure 2.3 depicts the layered architecture of IEEE 802.15.4
The Superframe structure
The superframe structure (Fig: 2.4) is an optional part of a WPAN. It is the time duration between two consecutive beacons. The structure of the superframe is determined by the coordinator. The coordinator can also switch off the use of a superframe by not transmitting the beacons. The superframe duration is divided into 16 concurrent slots. The beacon is transmitted in the first slot. The remaining part of the superframe duration can be described by the terms, CAP, CFP and Inactive. The superframe is used to provide vital statistics like synchronization, identifying the PAN and the superframe structure, to the devices connected in a Wireless PAN. This information is critical for the operation of the PAN in a Beacon enabled network... Continue Reading
There can be three different types of data transmission possible. They are
Ø Transmission from a device to the coordinator
Ø Transmission from the coordinator to the device
Ø Transmission between any two devices.
IEEE 802.15.4 Service Primitives
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... Continue Reading
The primitives indicate the functions organized by each layer. The PHY layer is responsible for the following tasks:
Ø Activation and deactivation of the radio transceiver
Ø Energy Detection
Ø Link Quality Indication Measurement
Ø Clear Channel Assessment
Ø Data Transmission and Reception
Data Transmission
When ever there is data to be transmitted, the MAC layer Management Entity calls upon the PHY layer with these primitives to transmit a data frame
Data Transmission
The following primitives are used for data transmission from the next higher layer. The result is indicated with confirm primitive. These responses are prepared with respect to its own requests for data transmission to the PHY layer.
MCPS-DATA.request : Requests the transmission of a data unit from the local SSCS entity. It prepares the corresponding MPDU from the incoming SPDU and this is passed on to the PHY layer for transmission
