# Location-Based Deferred Broadcast for Ad-Hoc Named Data Networking
## 4 Experimental Evaluation  
The purpose of this experiment is to study the impact from the collision-avoidance timer on the performance of interest delivery in a deferred broadcast scheme. The impact is measured as the interest-receiving gain, which is the ratio of the total received number of interest packets over the total  
Figure 4: Interest and Data Forwarding Workflows. transmitted number of interest packets. In a dense network, collisions could happen if the timer value is not appropriate, leading to low interest-receiving gain. The experiments are performed using ORBIT (Open-Access Research Testbed for Next-Generation Wireless Networks) [31]. All the active nodes from the main grid according to the Testbed Portal were installed with _baseline-ubuntu-12-04-32 bit.ndz_ image and MadWiFi [32]. Each node is equipped with at least one wireless card, and the transmittable signal power can be adjusted. By changing the Wi-Fi signal power, we are able to emulate network density. Since our goal is to evaluate the broadcasting performance in dense network scenarios, and the signal propagation speed is much faster than vehicles' moving speed, it is reasonable to emulate a static environment. Figure 5 gives the grid availability and the topology of our experiment. The code we run was V-NDN, which was the basic ad-hoc NDN code and has been tested [33; 34]. It contains NDN daemon that works as a router and defines the network faces required by NDN. It is responsible for maintaining both CS and PIT tables as well as sending and receiving network packets. If a consumer application requests the road traffic information about a given region of interest (in our example, it is the top-right corner in Figure 5), it sends an interest packet and waits to receive the data. The nodes in ORBIT didn't have GPS devices. But ORBIT supports experiments with mobility by emulating locations using software and connectivity. We use this mechanism to provide continual GPS data during the experiment. The experiment mimics a broadcast of an interest packet from the center node _node10-10_ for traffic information about the region. All of the neighboring nodes inside its transmission range receive the interest packet and rebroadcast it after a deferred time. Specifically, we give one case in Figure 5 where node7-6 and node6-7 will receive the interest packet at the same time and schedule to rebroadcast at the same time because they have the same distance to node10-10 and the same distance to the region of interest. Their FP timers will be the same. Therefore, the CA timer will play an important role in avoiding collision when the two nodes rebroadcast the interest. During the experiment, we measure how interest-receiving gain changes when the range of the random numbers that the CA timer will draw from changes. A large range will yield smaller collision probabilities. The interest-receiving gain is the ratio of received interests over the transmitted interests. Note that the ratio may be larger than 1 (100%) because one transmitted interest could be received by many neighbors. Figure 6 presents the result from our experiment. The maximum value of each range  
Figure 5: Open-Access Research Testbed (ORBIT) Experiment Topology. increases as shown in x axis. It shows that small ranges up to 1000 lead to low interest-receiving gain, which suggests more packet collisions. When the range increases, the interest-receiving gain increases. The figure shows a critical increase of the interest-receiving gain at the range maximum of 1500 us, and a relatively flat level afterwards. This means that given the density used in the experiment, 1500 us is large enough to absorb the potential collisions. The result of this experiment can be used as a guideline in setting up the collision-avoidance timer. Considering that a larger timer value will result in a prolonged delivery delay, for the network density in this experiment, the best maximum number of the range of the random numbers for the collision-avoidance timer is around 1500 microseconds.