## 4 The Proposed Framework
### Sensing Data Aggregation in Groups  
Finally, we discuss how global fleet information can be cooperatively collected and distributed in two-tier MLTNs consisting of cyclist groups. To improve the sensing data collection of group cyclists, the GH is selected as the gateway in a group. During sensing data collection for a group of size \(n\), group cyclist \(i\) will transmit its sensing data \(SD_{i}\) to group cyclist \(i-1\) in sequence, according to the order \(\text{SN}_{i}=n,n-1,\cdots,1\). Thus, after group cyclist \(i\) receives the sensing data \(SD_{n},SD_{n-1},\cdots,\) and \(SD_{i+1}\) from group cyclist \(i+1\), it can immediately send the received sensing data and \(SD_{i}\) to group cyclist \(i-1\) without collisions with the sensing data of group cyclists \(n,n-1,\cdots,i+1\) because all group cyclists follow a common route and transmit their sensing data in order. In addition, group cyclist \(i-1\) will aggregate the sensing data of group cyclist \(i\) based on the relative position instead of the absolute position. Thus, the GH (i.e., group cyclist 1) receives the aggregated sensing data of all group cyclists from group cyclist 2 via the low-tier network and transmits it to the server via the high-tier network. On the other hand, the sensing data distribution from the GH to group cyclist \(n\) occurs as follows. After the server has collected global fleet information from all groups in \(G\), it transmits global fleet information to all GHs in \(G\). Then, each GH broadcasts global fleet information to the group cyclists behind it through multi-hop forwarding. When group cyclist \(n\) receives global fleet information, it will transmit its sensing data to group cyclist \(n-1\) again, thus repeating the process of cooperative sensing data collection and distribution. Note that the transmission order of sensing data collection is one by one from cyclist \(n\) to cyclist 1 (i.e., the GH) following their serial numbers so that there is no message contention during the sensing data collection process and the packet collision probability within a group can be minimized. Because each group cyclist may increase or decrease its speed, its SN in the group may change with time. To obtain the correct \(\text{SN}_{i}\) of all group cyclists \(i\), before the GH transmits the aggregated sensing data to the server, it will broadcast a _Reordering Beacon (ROB)_ to all group cyclists to update their serial numbers. The ROB contains a current serial number \(\text{SN}_{c}\) during the reordering process, which is equal to 1 when the GH broadcasts it. The receiver closest to the GH will rebroadcast first as it receives the ROB. When group cyclist \(i\) receives a ROB, it will calculate the distance \(d_{i}\) between the sender and itself. A smaller value of \(d_{i}\) is used to set a smaller backoff timer as follow:  
\[BT_{i}^{ROB}=\left\{ \begin{array}{ll}[0,2^{+1}-1]&0<d_{i}\leq\frac{1}{\rho}r \\ [2^{+1},2^{+2}-1]&\frac{1}{\rho}r<d_{i}\leq\frac{2}{\rho}r\\ \vdots\\ [2^{+\rho-1},2^{+\rho}-1]&\frac{\rho-1}{\rho}r<d_{i}\leq r\end{array} \right., \tag{1}\]  
where \(\rho\) is the number of backoff classes, \(r\) is the transmission range, and \(\tau\) is a small integer. When the group cyclist closest to the sender backs off to 0, it will set its SN to \(\text{SN}_{c}+1\) and rebroadcast the ROB with \(\text{SN}_{c}=\text{SN}_{c}+1\). The reordering process will be repeated until the last cyclist in the group rebroadcasts the ROB and then broadcasts a \(M_{update}\) message to all group cyclists ahead to update the current group size to its SN. Thus, all cyclists in the group will have the up-to-date group size and serial numbers. To reduce the number of rebroadcasts of IABs, \(M_{join}\), \(M_{update}\), \(M_{notify}\), \(M_{req}\), and global fleet information, the receivers that are further from the sender will rebroadcast earlier as they receive IABs, \(M_{join}\), \(M_{update}\), \(M_{notify}\), \(M_{req}\), and global fleet information. Below, we use IABs as an example.