# Priority-Based Content Delivery in the Internet of Vehicles through Named Data Networking +
## 2 Traffic Prioritization in IoV: What and Why?  
### Towards IoV Applications  
So far, the majority of research efforts worldwide considered a rough taxonomy of vehicular applications, by distinguishing safety data from non-safety data, where the first category accounts for high-priority messages crucial for the safety of drivers and passengers on wheels, whereas the second one may tolerate graceful performance degradation being mainly targeted to improve the traveling experience.  
Starting from this, designed solutions have been mainly devoted to ensuring the reliable and timely delivery of safety messages, prioritized over non-safety messages. Such a kind of prioritization has been tackled as follows.  
First, at the physical layer, dual-radio on-board transceivers are deployed to have one radio constantly tuned on a dedicated channel for the exchange of periodical and event-driven safety messages among vehicles without interference from other message types [13]. In the spectrum at 5.9 GHz allocated for VANETs, the dedicated safety channel is Channel 180 (named the Control Channel (CCH)) in Europe or Channel 172 in the US. Other (non-safety) data are, instead, expected to be transmitted on the second transceiver tuned into a different channel, i.e., a service channel (SCH). Multiple SCHs (up to five) are available for non-safety data, but there is not a clear indication about what type of traffic to transmit on which SCH [13].  
Then, the Medium Access Control (MAC) layer of IEEE 802.11 OCB [3] relies on differentiated channel access (i.e., the Enhanced Distributed Channel Access (EDCA)) in order to further prioritize safety transmissions. Through EDCA, different channel contention parameters, e.g., the contention window, the Arbitration Inter-Frame Space (AIFS), are assigned in order to ensure high-priority safety packets seize the channel before less crucial ones. Parameters are set for transmissions over the CCH to provide aggressively differentiated priorities.  
Moreover, at the upper layers, to further speed-up the exchange of safety messages, they are allowed to be exchanged by leveraging the lightweight Wireless Access for Vehicular Environment (WAVE) Short Message Protocol (WSMP) protocol, not to incur the overhead foreseen by heavy TCP/IP-based networking/transport protocols [14].  
The aforementioned taxonomy of applications is too ossified for the IoV, which is expected to encompass novel applications specifically emerging with vehicles becoming multi-faceted elements (equipped with cameras, sensors [15], radars, storage, processing [4] and positioning capabilities [16; 17]) of smart and connected cities able to interact also with nearby/remote people and objects [18; 19].  
The rich variety of IoV applications exhibits heterogeneous requirements in terms of delivery performance (i.e., latency, reliability, etc.) and space and time relevance. For instance, road traffic information (e.g., average speed in a given area) is locally-relevant data, the time validity of which lasts a few seconds or minutes; whereas the time validity of fee information about charging stations for electric vehicles and flyers of points-of-interest in a road area may span several hours.  
Moreover, traffic of the same type, e.g., video, may require different delivery priorities. For instance, the delivery of a video requested by an autonomous car to improve its perception of the surrounding area under foggy weather conditions should be prioritized over the retrieval of the trailer of a recent cartoon, for a baby in the backseat. Indeed, the former video should be promptly retrieved for safe driving, whereas the latter is less crucial, but still asking for Quality of Experience (QoE) demands to be satisfied.  
Despite such heterogeneous requirements, the MAC layer of 802.11 would treat video packets of both applications with the same Access Category (AC) parameters. Hence, we argue in favor of solutions able to complement the traffic differentiation capability at the MAC layer of the 802.11 technology to fulfil IoV requirements.