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The ACK could have been transmitted by another node (e.g. node C) in the range of node A, but could be misunderstood by node A as an ACK sent by node B.
Fig. 4.11. Format of the IEEE 802.15.4 acknowledgment frame 220.127.116.11. MAC command frame Fig. 4.12 shows the structure of the MAC command frame. The addressing fields contain the destination address fields and/or source address fields, depending on the values encoded in the frame control field. The MSDU is composed of a one-byte command frame identifier, which indicates the type of command and a variable length payload, which includes the MAC command itself.
4.6.9. Performance of an IEEE 802.15.4-2003 peer-to-peer link In many deployments, IEEE 802.15.4 is used in its peer-to-peer configuration. In this case, unslotted CSMA/CA is used. Some performance results for this mode are provided below in terms of throughput and delay.
The maximum user data throughput of a bulk data transmission between a single sender and a single receiver through an unslotted IEEE 802.15.4channel in ideal conditions is as shown in Table 4.2 . Note that these figures are the highest possible ones under each specific set of conditions (address length, use of ACKs, physical layer channel, etc.).
The maximum user data throughputs (at three different frequency bands) for 16 bit MAC address length and Reliable mode shown in Table 4.2 correspond to the bit rates per channel at PHY layer shown in Table 3.2 of Chapter 3. The range of latencies of a frame transmission between a single sender and a single receiver through an unslotted IEEE 802.15.4-2003 chan
nel in ideal conditions are as shown in Table 4.3 . In the case of reliable communications, the latency indicated in the table equals the round trip time, which includes the delay for reception of the ACK.
Range of latencies with unslotted IEEE 802.15.4-2003 Note that, as shown in Table 4.3, latency decreases as the available bandwidth increases.
4.6.10. IEEE 802.15.4e At the time of writing, the IEEE 802.15 Task Group 4e is working to develop an amendment to IEEE 802.15.4-2006. The purpose of this amendment is to provide additional functionality to the IEEE 802.15.4-2006 MAC layer, the better to satisfy requirements of industrial markets and for improved compatibility with the Chinese WPANs .
The application spaces currently being considered for IEEE 802.15.4e are:
Factory Automation, Process Automation, Asset Tracking, General Sensor Control (Industrial/Commercial, including Building Automation), Home Medical Health/Monitor, Telecom Application, Neighbourhood Area Networks and Audio.
4.7. Bluetooth Low Energy MAC
This section presents the main features of BT-LE MAC layer . The MAC fundamental mechanism used in BT-LE is a TDMA based polling scheme, whereby one device transmits a packet at a predetermined time and a corresponding device responds with a packet after a predetermined interval.
4.7.1. State diagram BT-LE operation at link layer is based on a state machine designed for low power operation. As shown in Fig. 4.13, the number of states and the amount of state transitions are both small.
The state diagram has five states, namely: standby, scanning, initiating, advertising and connecting. In the standby state, a device is inactive. The device may transition to the scanning state, in which a device listens for advertisers. From the standby state, a device may transition to the advertising state, in which a device sends advertisement messages for advertising itself, or to the initiating state, where a device wants to connect to another device. A device in the initiating state (hereafter referred to as the initiator) listens for advertisements. When such a message is received by the initiator, this device sends a Connect Request message to the advertiser for initiating a connection and enters the connecting state as a master. When the advertiser receives this message, it enters the connecting state as a slave.
This procedure is more simple than that of Bluetooth, where a connection always has to be started by a device as a master, and if the device wants to act as a slave during the connection, the roles of the devices have to be switched, which consumes a significant amount of energy.
The Connect Request message includes information about the connection, such as the frequency of communication, the frequency hopping sequence that will be used, etc.
4.7.2. Data transaction
BT-LE allows low latency transactions for data transmission. Fig. 4.14 illustrates the complete dialogue between two devices, where the advertiser wishes to transmit data to the initiator. Once the initiator has become a master, it requests the data from the advertiser. The advertiser then sends the data to the master, which replies with an acknowledgement if the data have been correctly received. If the slave has no more data to transmit, it indicates that it wants to close the connection by sending a link layer (LL) Terminate message.
Fig. 4.14. Connection and data transmission message chart
The total time between the transmission of the advertisement and the confirmation of the connection termination by the master is less than 3 ms. During this time, the radio is actually turned on for less than
4.7.3. Frame format This subsection presents the general frame format and some details about the structure of the advertisement frame and for the data frame.
18.104.22.168. General frame format Fig. 4.15 shows the general frame format of a BT-LE frame. The frame starts with a one-byte preamble, which contains one of the following two sequences: 01010101 or 10101010. These sequences allow receivers performing automatic gain control at the physical layer to increase the probability of correct reception of the frame. The next field is the Access address, which is a 32-bit identifier of the destination device. In contrast, Bluetooth addresses have 64 bits. In the advertising channel, the Access address is set to a broadcast address, which is a fixed value. In data channels, the Access address is a random number chosen by the master and transmitted in the Connect Request message. The random number is chosen for each connection, which is beneficial for security. The next field is the payload, which has a maximum size of 39 bytes. The last field is a 24-bit CRC, which is larger than the 16-bit CRC used in other radio technologies, such as IEEE 802.15.4. The use of a 24-bit CRC offers high robustness in noisy environments.
22.214.171.124. Advertising frame format An advertising frame includes a header field and a length field (see Fig.
4.16). The header indicates the type of advertising frame, which includes information about the intent of the frame. The length field is needed because the payload has a variable length.
126.96.36.199. Data frame format The data frame has the same general format as the advertising frame. The header contains two relevant bits used for flow control, namely: the sequence number and the next expected sequence number. Acknowledgements actually use the same frame format, but do not contain the payload field.
4.7.4. Acknowledgment and flow control
In Bluetooth, an acknowledgment is sent after every data transmission.
To minimize the amount of frame transmissions, BT-LE introduces a sliding window scheme. Data frames have a sequence number (which can be ‘0’ or ‘1’). Because data (and acknowledgment) frames contain the next expected sequence number, an acknowledgment is not needed after the transmission of every data frame. This approach (also used in other protocols such as TCP) contributes to energy-saving.
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5. Sensor node platforms The origins of WSNs can be found in the eighties in the area of military applications, but the origin of the concepts currently in use is the Smart Dust project , also funded by the US military. The objectives of this project were pretty ambitious but some designs based on Commercial Off-TheShelf define the basic architectural principles of WSNs. A wireless sensor node is composed of a set of hardware components that allow two carry out tasks such as measurement, processing and transmission. These elements are combined according to a general architecture, but nowadays there exists a large variability of these components that can result in different solutions optimised for certain applications. The content of this chapter presents the different elements and the possible options.
5.1. Sensor node architecture elements
A sensor or an actuator node has the ability to sense or activate an element, process the related information and transmit it. Also, such nodes are able to collaborate with other nodes in creating a network and moving the information either within the WSN or to some external server connected to another network like the Internet. To perform all these functions, a minimum set of elements is needed. This set comprises a processor (with the corresponding memory and input and output lines), a transceiver for the communication, one or several sensors and actuators and power supply. Fig. 1.1 from chapter 1 presents a view of these elements and their relationship.
The element that controls the sensor node is the processor. Due to limited processing capabilities, and as it includes memory and peripherals in addition to a Central Processing unit (CPU), the processing IC is recognised as a Micro Controller Unit (MCU). As the final applications of the sensor node are quite diverse, manufacturers offer a wide variety of MCUs. The range goes from 8-bit processors based on the 8051 microprocessor to a 32-bit architecture based on the Advanced RISC Machine (ARM) microprocessor, or newer versions such as ARM Cortex. These ARM processors are adopted by several manufacturers and can offer 32 MIPS (Million Instructions Per Second). In order to reduce the power consumption, these platforms use power sources as low as 1.8 V and support different clock rates.
Several manufacturers offer MCUs used in sensor nodes such as Atmel , Chipcon  or Freescale .