## 4 MAC Protocol Design
### Hybrid TDMA/CSMA  
As mentioned earlier, the ISWN is expected to complete both on-board sensing and subsystems' data transmission, which leads to expanded scale and diversified data requirements. Especially, for the on-board AOC system, the delay performance of these nodes should be strictly maintained. Time-division multiple access (TDMA) and carrier sense multiple access with collision avoidance (CSMA/CA) are regarded as two possible solutions for multi access method. Usually, the CSMA/CA provides a simple, distributed, burden-less scheme for adaptive traffic, while the TDMA method offers a centralized, scheduled scheme, in which the target QoS can be guaranteed by resource assignment. In a CSMA/CA network, it is difficult to ensure the delay performance especially when the network is large in scale. In addition, the packet delay will increase rapidly with the increasing traffic load, which may lead to difficulties in real-time attitude and orbit control. Thus, CSMA/CA is not a suitable choice for an intra-satellite network, which contains dozens of nodes. As for TDMA, a fixed slot assignment-based TDMA network presents stable packet delay and requires very accurate duty cycling. Compared to the CSMA/CA, it increases the communication cycle when the network traffic is low, since every node must wait until its turn to communicate. To meet a variety of requirements, the MAC protocol is expected to have the characteristics of combing TDMA with CSMA/CA. Hybrid protocols have been applied in some scenarios, such as vehicular ad-hoc networks ([HTC-mac [36]] and Her-mac [37] and wireless sensor networks (Z-MAC [38], Tree-MAC [39] and MCLMAC [40], etc. ), and they have been proved to achieve improved throughput and reduced control overhead. They mainly aim to accomplish efficient access for multi-hop communication or/and mobile nodes. In an on-board wireless network, the location and the number of network nodes are fixed, and only one-hop communication is involved, so its topology and data requirements are different from either of them. Thus, a new hybrid TDMA/CSMA scheme for the intra-satellite network is proposed here. As an on-board network for intra-satellite, a star topology is adopted, in which the data is periodically transmitted. The on-board computer (OBC) is set as the master node, while other subsystems are set as the slave nodes. In traditional wired satellites, the synchronization was achieved with a plus-per-second (PPS) mechanism, in which the time information flows were transmitted by a wired data bus. Similarity, in this work, the synchronization between the OBC and other nodes \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type** & **Data Rate** & **Delay** & **Examples** \\ \hline A & High & Sensitive \& insensitive & Payloads, on-board computer, telemetering and tele-control system, power management system, etc. \\ \hline B & Low & Sensitive & Thrusters, reaction wheels, GPS, gyroscope, magnetometer, star camera, Sun sensors, etc. \\ \hline C & Low & Insensitive & Thermal subsystem, mechanical subsystem temperature sensors, solar panel and other sensors \\ \hline \hline \end{tabular}
\end{table} 
Table 2: On-board data classification. is achieved by radio beacons. The network works in a periodic mode, where both center controlled scheduling and contention access are adopted. The network communication cycle is divided into three periods:  
1. TDMA-based Main Communication period (MCP);
2. Backup communication period (BCP);
3. CSMA/CA-based Extended Communication period (ECP);  
The main communication phase provides center scheduled slots which are assigned by the nodes' ID. Nodes in type A and type B will communicate in this period. By contrast, the extended communication phase provides contention-based slots, in which an improved slotted CSMA/CA strategy is performed and nodes in type C will transmit their packet in this period. Due to the orbital characteristics, the on-board applications are periodic, thus we assume the number of the active nodes changes periodically too.