## 7 Performance Evaluation  
### Performance Evaluation of the CryptoSuite  
In this section we report the performance analysis of our cryptographic suite through experimental tests performed within the UAN experimental campaign conducted on May 2011 in Norway [5]. Figure 8 shows the topology of the underwater acoustic network, which constituted the testbed during the experiments. It is composed of a gateway GW, two fixed nodes (FN1 and FN2) and two UAVs (FLG1 and FLG2). Figure 9 shows the real devices we used in the experiments. Notice that the gateway (Figure 9(a)) is equal to a fixed node (Figure 9(c)) except for the optical fiber that connect the device to the asset (blue cable). UAVs are of Polaga type, manufactured by GraalTech Inc. [36] (Figure 9(b)). From a computing platform standpoint, all these computers are equipped with a 1 GHz VIA EDEN Ultra Low Voltage Processor, 1 Gbytes of RAM (DRAM DDR2 533/400 on SO-DIMM socket Chipset VIA CX700M), and run the Linux Ubuntu 10.4 Operating System. All underwater nodes are equipped with an acoustic modem developed by Kongsberg Maritime, a UAN partner, and providing a maximum transmission rate \(r\textsubscript{max}\) 500 bits/s. The GW is placed at a depth of 96 m. FN1 and FN2 are at a depth of 96 m and 39 m, at a distance from the GW of 163.5 m and 860 m, respectively. The nodes FLG1 and FLG2 are placed at a depth of 20 m and 15 m, with an initial distance from the GW of 677.8 m and 687.1 m, respectively. These values are summarized in Table 1.  
Figure 8: The test-bed. Figure 9: The nodes used in the experiments. In the experiments, we used AES in the CBC-CTS mode as cipher for confidentiality and SHA-2 as hash function for integrity. SHA-2 produces a 256-bit output that we truncate at 32 bits. It follows that guaranteeing secrecy and integrity gives rise to a message expansion of 4 bytes. In the experiments, the payload varied in between 32 and 78 bytes. This means that the 4 bytes message size increment due to security varies in between 5.13% and 12%. This increment, of course, reflects on the increased transmission time. Underwater acoustic communication is particularly affected by noise and subject to the highly variable sea environmental conditions. For this reason, an increased message length due to security might reflect in an increased likelihood of message loss or corruption. In order to evaluate this impact of security on performance we have considered _Average Delivery Ratio_ (ADR), which is the ratio between the average number of received messages and the number of sent messages. In practice, the average delivery ratio measures the fraction of messages the network is able to deliver. Ideally, the ADR should be equal to 1. We have compared the ADR when security is disabled to the ADR when security is enabled. The results are reported in Figure 10. In particular, the figure shows the ADR for nodes FN2 and FLG2. We note that the ADR when security is enabled is smaller than that when security is disabled. In particular, when security is enabled, the ADR seems to suffer a decrement of \(4.9\%\) in the case of a fixed node, and \(7.8\%\) in the case of a mobile node. These variations in the ADR could be caused by an increase in the message length due to security. On the other hand, such a small percentage variations may be well depend on the change of sea conditions during the day as experiments, with security enabled and disabled respectively, were conducted at a distance of quite a few hours from each other for logistic and organizational reasons. More details about environmental conditions--e.g., water, winds, salinity, or temperatures--can be found in [5]. For these reasons, we can safely conclude that the cryptographic suite add a negligible overhead to the underwater acoustic communication. In an aside, the highly variable sea conditions and their impact on the communication performance become particularly evident when you consider delay.