# Caching Transient Contents in Vehicular Named Data Networking: A Performance Analysis
## 6 Performance Evaluation
### Urban Scenario  
The first set of results in Figure 4 focuses on the urban scenario, when varying the Zipf skewness parameter, \(\alpha\), in the range 1, 1.5, 2, 2.5 to model different content popularity distributions.  
Figure 3(a) reports the cache hit ratio (the metric equally applies for the different lifetimes of contents). Not surprisingly, the CEE strategy exhibits the poorest performance. Due to the broadcast nature of the wireless medium, in fact, many neighbouring vehicles are likely to receive the same Data packets at the same time instant and they all cache the same information, with a neat penalty in terms of content diversity. Better performance is obtained with the RC strategy which, by introducing the probabilistic caching decision, increases the content diversity in the network and facilitates cache hits.  
In FDC, a probabilistic decision is also foreseen, but it prioritizes caching of contents with a longer residual lifetime, hence it increases the cache hit ratio, and, consequently, reduces the content retrieval delay, as shown in Figure 3(b). The cache hit ratio metric increases for all the compared schemes when the Zipf skewness parameter increases. Indeed, as \(\alpha\) increases, requests from multiple consumers concentrate on a few contents from the catalog and the more popular contents are kept in the cache. This increases the chance for a request to be satisfied and a lower number of hops for the content retrieval is experienced, see Figure 3(c).  
Figure 3(d) shows that, as a further benefit, the proposal allows reducing the number of Data packets which are exchanged into the network compared to the benchmark schemes. The largest load is experienced by the CEE scheme and it can be observed that the latter one is the less sensitive strategy to the Zipf skewness parameter. This happens because, by caching all incoming contents indiscriminately, CEE unavoidably results in a high redundancy. When multiple nearby vehicles  
Figure 4: Performance metrics in the urban scenario, when varying the Zipf skewness parameter \(\alpha\) (number of consumers equal to 20).  
receive a broadcast content request and have a match in the CS, they all try to answer with the Data packet. Although V-NDN implements collision avoidance techniques, based on defer times and overhearing, hidden terminal phenomena cannot be completely avoided and multiple redundant packet are transmitted.  
Figure 5 reports the cache hit ratio and retrieval delay metrics when varying the number of consumers from 20 to 50, for the Zipf skewness parameter \(\alpha\) set equal to 1.5. It can be observed that performance improves as the number of consumers increases from 20 to 40. In this case, the value of \(\alpha\) ensures that many requests by multiple consumers concentrate on the more popular contents, thus, ensuring a high cache hit ratio and a low retrieval delay. Moreover, content sharing is facilitated due to the broadcast nature of the wireless medium. The supremacy of the proposed solution compared to the benchmark schemes is confirmed also under such settings.  
Notwithstanding, no more improvements are experienced when the number of consumers reaches the value of 40. Indeed, with a higher number of consumers also the number of distinct contents increases: more contents are requested that are less popular, which implies a higher traffic congestion in the network and a lower cache hit ratio.