# Review and Evaluation of MAC Protocols for Satellite IoT Systems Using Nanoatellites
## 5 MAC Protocols for Satellite IoT and M2M
### Reservation and Adaptive Protocols
#### 5.2.2 Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) with RTS/CTS  
A classical protocol in wireless network, the carrier sense multiple access with collision avoidance (CSMA/CA) defines the monitoring of the channel before sending a message. If a node senses the channel is busy, it refrains from transmitting and enters an exponential backoff stage; otherwise, it issues a reservation request by means of a small broadcast message (i.e., the request-to-send (RTS) packet). When there are no collisions and the receiver decodes the RTS correctly, another broadcast message granting the reservation (i.e., the clear-to-send [CTS] packet) is sent by the receiver. Upon reception of the CTS, the sender proceeds to send the data packet [59]. Figure 9 illustrates the exchange of packets when a successful reservation is placed in CSMA/CA with RTS/CTS. The protocol has been evaluated in a LEO satellite environment considering different back-off distribution functions [60].  
The maximum performance regarding throughput of this protocol varies between \(S\simeq 0.5\) and \(S\simeq 0.8\), since its performance depends on factors such as the packet length, number of nodes, and the number of hidden terminals, among others. As reported in [59], for an example network with 10 stations and no hidden terminals, the protocol achieves a normalized throughput of \(S=0.75\) when the load is \(C=0.8\). In the presence of hidden terminals--a common scenario for a satellite system with ground terminals distributed over a large area--if there is a 5% probability of hidden terminals for a total of 10 stations on ground, the normalized throughput is \(S=0.65\). By increasing the number of stations to 50 in the same case, the normalized throughput falls to \(S=0.57\). \(PLR\) values for this protocol are not reported since packet losses are avoided when using the reservation mechanism.  
Figure 8: R-Aloha operation: five users contending for channel access. User three transmits successfully in slot three of frame \(k\), and reserves the same slot in future frames \(k+1\) and \(k+2\). Users one and five achieve successful transmissions in frame \(k+1\), and place their reservations in frame \(k+2\) for slots two and one, respectively. Red packets correspond to active reservations.  
In terms of complexity, CSMA/CA is still a simple protocol: it does not impose high processing demands on the nodes beyond the ability to transmit and receive. Conversely, the power consumption of CSMA/CA is high, since each node must permanently listen and monitor the channel.  
CSMA/CA is widely used in wireless IoT technologies, such as ZigBee, D7 AP, and other short-range wireless sensor networks [49; 61]. However, for the protocol to be implemented on a satellite IoT network, it may be impractical for two reasons: first, the probability of hidden nodes in a satellite scenario is high due to a wide geographic distribution area; and second, the long delays of the different devices on ground make the channel sensing ineffective even when there are no hidden terminals.