## 5 Discussion of the Results
### Federations on Ground  
When the balloons are still on the ground, multiple federations were established to verify the correct operation of the payloads. Among all of them, Figure 12 presents the packet sequence of a federation. This sequence represents the different packets exchanged between the balloons and the ground station scheduled on time. Moreover, this representation identifies the types of packets using different colors and shape markers. For instance, all the blue markers correspond to packets exchanged in a downlink connection, while the red markers identify those exchanged in a federation. The rhombus (\(\diamond\)) and the times (\(\times\)) markers represent respectively the packets to establish and close a connection (i.e., a downlink or a federation). In the case of a federation, these are the request, the accept, and the close packets. The dot (\(\diamond\)) and the triangle (\(\triangle\)) markers correspond respectively to data and acknowledgment packets. Moreover, the packet that corresponds to a retransmission is represented with an \"R\" below the marker. Finally, the green markers correspond to publications of the service. This kind of representation helps to understand the periods when a downlink or a federation is established with a dashed line of the corresponding color. Returning to Figure 12, a federation between Balloon C and Balloon B is established from 12:01:00 to 12:02:30. At the very beginning, the ground station opens a downlink connection with Balloon C, which starts to download its data for 50 s. At this moment, the  
Figure 11: SNR of packets that establish a federation, such as publish (blue dots) and accept (orange dots) packets. The three regions are remarked with dashed boxes. Figure 12: Packet sequence of federations established on ground. balloon has downloaded all its data, and starts to publish the availability of the download service. Balloon B and Balloon A receives this publication and decide to request the service to download its data. Balloon B reacts faster than Balloon A, being able to establish the federation with Balloon C. Although Balloon A does not receive any accept packet, it retransmits the request packet. After three retransmissions, it stops. Meanwhile, Balloon B transmits data to Balloon C which downloads it to the ground station. As part of the test, Balloon C is turned off during the federation (at 12:02:16). After that, Balloon B performs different retransmissions of the last data packet because it has not received its acknowledgment. At 12:02:23, it decides that the connection is lost, and closes the federation internally. Up to four federation packets are downloaded from Balloon B during the federation. Figure 13 presents the memory usage of Balloon B and Balloon C. Please note that the buffer usage is measured every 20 s in each balloon. When Balloon C establishes the downlink connection, it reduces the usage to zero by directly downloading them to the ground station. When the federation is established, Balloon B reduces its usage by using the federation. Once the federation is closed, Balloon B keeps producing new data packets and thus increasing the usage of the buffer. The throughput achieved with the federation is 184.7 bps, while the direct downlink has 756.9 bps of throughput. Both results can be compared with the maximum throughput according to the definition of Equation (6). In this scenario, the downlink communication has 4.3% of PER, and the federation one has 20.0%. The throughput achieved corresponds to 756.9 bps, and 184.7 bps respectively, which is equivalent to 71.0% and 98.2% of the maximum throughput with this PER. This performance indicates that the software implementation of the FSSExp payload is impacting the achieved throughput, being not possible to retrieve the maximum value in the federation case. Specifically, the processing time related to each packet increases the RTT value, which reduces the corresponding throughput. As in the federation case there is an intermediate node, this decrease is more explicit.