### 1. Introduction

### 2. Related Work

### 2.1 WAVE Standards

### 2.2 Related Works in Vehicular Networks

Message context m can be characterized by the message age (i.e., the time duration between the last broadcast and the last reception of a message).

Vehicle context v is described by a list of vehicle context information, such as the driving direction, distance to the last forwarder, current number of reachable neighbor nodes, and distance to adjacent nodes road type.

Information context i is specified by the time of day, purpose of traveling, distance to the source, information accuracy, information category, news value, and change rate of the information.

### 3. The Enhanced Message Priority Mechanism

### 3.1 Benefit-Value Algorithm

*β*(

*m*) of a message can be represented by:

#### 3.1.1 Sensor function

#### 3.1.2 Speed function

#### 3.1.3 Deceleration function

^{2}, the driving situation is interpreted as a normal situation. When it is between 2.7 m/s

^{2}and 5.5 m/s

^{2}, the proposed algorithm gives more weight to its messages. Finally, if it exceeds 5.5 m/s

^{2}, the highest weight values are assigned to the messages. Table 6 shows the resulting weight assignments of the deceleration function.

#### 3.1.4 Message life function

### 3.2 Back-off Counter Selection Algorithm

*λ*is a function of the benefit value

*β*(

*m*). Note that, to reduce the delay of the high priority messages, this function must be a non-increasing function of the benefit value

*β*(

*m*). To meet this requirement, we employed the following function for obtaining the value of parameter

*λ*:

*λ*. In Fig. 3, the highest probability is 0.3085 when the

*λ*is equal to 1. After that, the result is lower than 0.25. That means the two vehicles may have less chance to get into a collision because of the different parameters.

*β*(

*m*) can be received from the upper layer. If the contention window is larger than 20, as we proved before, the Gaussian distribution should be used. What’s more, we also add the back-off times in the variance part because we wanted to make the peak point lower and to flatten the probability curve. If the contention window is less than 20, the Poisson distribution will be applied to this part.

### 4. Performance and Results

^{2}. For every 0.1 second the system will be refreshed and will recalculate the environment factors. Forty nodes were calculated in our simulation. The data rate was set for 6 Mbps. The packet rate was 40 packets per second. The message could then be transmitted at most 3 hops. The size of the packet was 8 kbps and the length of the queue was 30. The parameter settings in the simulation are summarized in Table 8.