# Software-Defined Heterogeneous Vehicular Networking: The Architectural Design and Open Challenges
## 4 Towards Software-Defined Heterogeneous Vehicular Networks
### Architectural Challenges
#### 4.2.5 Mobility-Aware Edge Caching  
Another significant challenge in SDHVNets is to formulate an optimal _edge caching policy_, i.e., for a given anticipated vehicle (or vehicular user) demand, it is imperative to determine which vehicular applications, services and/or content should be placed in each edge cache so as to minimize the average delay for such requests. If such or similar requests are not being catered to by the edge-based caches, fetching them from the remote back-end servers would induce a significantly larger delay, which becomes extremely critical especially in the context of vehicular safety applications. Since the size of the edge caches is considerably smaller in contrast to the global caches (i.e., local caches at most store \(10^{-3}\) or \(10^{-4}\) of the global cache content), it is quite essential to cache the right content at the right time and then to flush it out so as to create storage space for the next popular content. One possible way is to devise an _age threshold scheme_ that caches all content requested for more than a particular threshold \(\tau\), where \(\tau\) is the content age. Content popularity could also be ascertained by the _Zipf distribution_ which implies that the collection frequency decreases quite rapidly with rank and could be mathematically formulated as \(cf_{i}\propto 1/i\) or \(cf_{i}=ci^{k}\).  
Several researchers in academia and industry have already proposed a number of _cache management policies_, including, but not limited to, leave copy down, leave copy elsewhere, randomly copy one,  
Figure 6: Depicting broadcast storm and network slicing in a vehicular networking context.  
probabilistic caching, latency-aware caching, congestion-aware caching and search and PopCache. Furthermore, _cache evictions strategies_ are indispensable as they determine which entries need to be evicted from the cache in order to create sufficient space for the new entries and typically include first-in-first-out, least recently used, least frequently used and RANDOM schemes. Nevertheless, since vehicles traverse at very high speeds and content popularity subsequently changes at a dynamic pace as well, it is quite difficult to estimate the content popularity at any given time and location on an instantaneous basis, and therefore, intelligent dynamic edge-based caching algorithms need to be devised in this regard. In SDHVNets, the SDN controller possesses sufficient knowledge of a vehicle's serving edge node (EN) and the time it takes to traverse through the coverage of a respective EN along with the anticipated trajectories and the sequence of ENs in the anticipated trajectories of the vehicles. This thus facilitates the SDN controller to deploy the requisite content in advance on ENs in the anticipated trajectories of the vehicles. Furthermore, it is recommended that the content should be stored on the ENs where the vehicles have a real chance to acquire them, i.e., on slow-moving traffic regions or congested road intersections. Thus, intelligent mobility-aware edge caching architectures could ensure ultra-reliable and low-latency communication, in turn not only meeting the stringent QoS of diverse vehicular safety-critical and non-safety applications, but also guaranteeing the QoE of the vehicular users.