## 4 Analysis of Requirements and Feasibility of Assumptions for CB
In addition, it should be noticed that most of the motes are based on the TI CC2420 [19] radio chip [2]. A general scheme of a mote is shown in Figure 3. In particular, this design is used by \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Frequency** & \(\mathbf{\lambda}\) & **Period** & **Offset** & **(Delay (ns))** \\ \hline
**(MHz)** & **(cm)** & **(ns)** & \(\pi\)**/8** & \(\pi\)**/4** & \(\pi\)**/2** \\ \hline
2450 & 12.244 & 0.40 & 0.025 & 0.05 & 0.1 \\
868 & 34.56 & 1.15 & 0.071 & 0.143 & 0.287 \\
416 & 72.06 & 2.403 & 0.15 & 0.3 & 0.6 \\
195 & 153.73 & 5.127 & 0.32 & 0.64 & 1.281 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Detail of the \(\lambda\) and period for some IEEE 802.15.4 (n) frequencies and the specific delay required for offsets(\(\delta\)) of \(\pi\)/8, \(\pi\)/4 and \(\pi\)/2. a TelosB mote [18]. In Figure 3, we see the connection scheme between the radio chip TI CC2420 and the MicroController Units (MCU), and the decoupled clock of the MCU and the clock of the radio transceiver, which makes the synchronization more difficult. Both modules, MCU and transceiver, are connected by the data bus and the interrupt pin that detects the field of Start Frame Delimiter (SFD) in the outgoing and incoming frame, as defined in standard IEEE 802.15.4 [1]. The CC2420 transceiver is based on an internal crystal oscillator of 16 MHz, with an accuracy of \(\pm\)40 ppm, and it has an SFD output pin that triggers the internal counter of the MCU, allowing the exchange of time stamps between the motes for time synchronization. The SFD pin is activated both in the transmission process, at the instant the transceiver is sending the SFD byte of the outgoing frame, and in the reception process, at the instant the transceiver is receiving the SFD byte of the incoming frame. Both options allow improving time synchronization. This is because the sender can take the local time to time stamp the messages for the synchronization protocol as late as possible (reducing the time uncertainty), just when the SFD field of the packet is sent, and the receiver can take its local time as soon as possible, just when the SFD is detected (improving time uncertainty). This uncertainty is mainly due to the MAC protocol, but, with these time stamps, it is partially removed. This process is known as MAC layer time stamping, and it allows the time stamping after the network access. Nonetheless, we cannot meet all the requirements unless we relax these requirements. However, some of these requirements are available in commercial motes, in particular \(R\)-3, \(R\)-4 and \(R\)-5. Table 2 summarizes the main features of most popular commercial motes based on IEEE 802.15.4. We specify the microcontroller, the radio transceiver (and power transmission), the antenna and the frequency band as well as if they meet these requirements with yes/no (Y/N). The main constraint to do CB is determined by the type of antenna, given by \(R\)-3. Motes are required to have an omnidirectional antenna. From Table 2, the Iris mote is not valid due to its dipole, while the rest can adapt the antenna (external or with a connector) or have an omnidirectional one, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Platform (Manufacturer)** & **Microcontroller, Transceiver** & **Power** & **Antenna** & **Frequency** & \(R\)-3, 4, 5 \\ \hline TelosB (Memisic) & MSP430, CC2420 & 0 dBm & Inverted-F & 2.4 GHz & YYY \\ Meshbean Amp (Atmel) & ATmega128, AT86 RF230 & 5 dBm & external & 2.4 GHz & YYY \\ Meshbean 900 (Atmel) & ATmega128L, AT86 RF212 & 20 dBm & external & 900 MHz & YYY \\ MICAz (Memisic) & ATmega128L, CC2420 & 0 dBm & external & 2.4 GHz & YYY \\ Iris (Memsic) & ATmega128L, AT86 RF230 & \(-\)3 dBm & dipole & 2.4 GHz & NYN \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main features of the most popular commercial motes. Figure 3: Detail of the TI CC2420 transceiver connected to the microcontroller. such as TelosB. In addition, the power transmission for the Iris more is lower than the others.