## 5 A Framework for SDN-SDR-Based Maritime Communications
### Performance Improvement of SDN-SDR over SATCOM
We can model \(d_{nodal}\) at the satellite as  
\[d_{nodal}=d_{proc}+d_{queue}+d_{trans}+d_{prop}, \tag{3}\]  
where \(d_{proc}\) is processing delay, \(d_{queue}\) is queuing delay, \(d_{trans}\) is transmission delay, and \(d_{prop}\) is propagation delay. When SATCOM is used, we expect \(d_{prop}\) to dominate in Equation (3) the other component terms. It should be noted that not every node's nodal delay components in every communication will be computationally significant, but it is important to treat them as non-trivial. Let us consider the communication needs of Destroyer-1 (see Figure 7), which is a smaller vessel in the CSG with limited bandwidth. With the proposed solution, Destroyer-1 can piggyback on the large bandwidth SATCOM data pipe of the Carrier. When Destroyer-1 needs access to information on the GIG or other on-shore sites, it will contact the Carrier with its request using the proposed SDN-SDR solution (see Figure 7). If the requested information has previously been accessed by the Carrier, then it will be stored locally so that future requests from other vessels in the CSG, such as Destroyer-1, can be satisfied at a fraction of the cost. If, however, Destroyer-1's requested information is not available locally on the Carrier, then the Carrier will request that information using a SATCOM data link, which can support more throughput than the other ships' links due to antenna size and gain advantages, as seen in Equations (1) and (2). This alleviates the burden on Destroyer-1's limited bandwidth SATCOM and potential long delays in accessing critical and time-sensitive information. In [59], the authors note that the typical SATCOM bandwidths found on small ships such as a Destroyer range from 256 to 512 kbps, while large ships such as the Carrier have the capacity for 4-8 Mbps. The cost of the extra-hop of communication from Destroyer-1 to the Carrier is insignificant compared to the cost that will be incurred--directly and indirectly--if Destroyer-1 were to make a direct SATCOM request. The round trip propagation time when communicating to a 35,000 km altitude geosynchronous satellite with a 25 degree slant angle is approximately 552 ms, i.e., \(d_{prop}^{SAT}=552\) ms. On the other hand, the line of sight communication for ships operating at a range of 25 km (i.e., Destroyer-1 to Carrier physical separation in this example) results in a round-trip propagation time of 0.17 ms, i.e., \(d_{prop}^{SDN}=0.17\) ms. Assuming Destroyer-1 has a SATCOM bandwidth of 512 kbps and the Carrier has a SATCOM bandwidth of 2 Mbps, assuming all the other nodal delay components are the same, the time to process the request for information made directly from Destroyer-1 will be \(4x\) slower compared to a request made via the Carrier. While this would add an additional hop with an added round-trip propagation of 0.17 ms, the overall request would still be processed almost \(4x\) faster. Therefore, with the proposed solution, the additional communication hop between Destroyer-1 and the Carrier is trivial compared to the overall round-trip propagation time. Finally, if the information requested by Destroyer-1 has been saved locally on the Carrier following a previous request, then the entire request can be satisfied without necessitating the use of SATCOM.