## 2 Wireless Channel Congestion Detection  
How to detect wireless channel congestion is important for congestion control design in wireless networks. Recent works have proposed several congestion metrics at the MAC layer.  
1. Retransmission counter: There exists a correlation between the number of RTS (Request to Send) retransmissions and channel congestion extent at the MAC layer [16; 17]. In [4], when the number of RTS retransmissions is greater than two, the wireless channel will be deemed as congested, and the TCP sender will decrease its sending rate. Though this method is very easy to deploy, the number of RTS retransmissions is not able to describe the congestion state accurately.
2. Frame Service Delay (FSD): The frame service delay is the interval from the time that the MAC layer begins to sense the channel for data transmission to the time that the acknowledgment is received successfully, which includes collision time and transmission time [18]. The greater the service delay is, the higher the probability of network congestion is. However, the hops that the flows are traveling through and the frame size greatly affect the service delay. Subsequently, the threshold value of the service delay is difficult to determine, resulting in the inaccuracy of congestion estimation.
3. Channel business ratio: The channel business ratio is defined as the ratio of link layer busy time (including collision time and data successful transmission time) to the total time [19; 20; 21; 22]. It can be shown in Equation (1). \[R_{b}=1-\frac{p_{i}\sigma}{p_{i}\sigma+p_{s}T_{suc}+p_{c}T_{col}}\] (1) where \(\sigma\) is the length of the backoff time slot, \(p_{i}\) is the probability that the observed backoff time slot is idle, \(p_{s}\) is the probability that there is one successful transmission, \(p_{c}\) is the collision probability that there are at least two concurrent transmission at the same time slot, \(T_{suc}\) is the average time period associated with one successful transmission and \(T_{col}\) is the average time period associated with collisions.  
As shown in Equation (1), the channel business ratio accurately reflects the contention and collision at the MAC layer. When the channel business ratio \(R_{b}\) increases, the channel utilization and network throughput will become higher. However, the channel business ratio is too complex to be deployed on the TCP protocol. Specifically, the key problem lies in the time interval involved in calculating \(R_{b}\) being difficult to set. The ideal situation appears only when the time interval is exactly equal to the transmission cycle of a frame. However, the transmission cycle of a frame will change with many factors, such as frame length, flow concurrency and channel status.  
Compared with the above solutions, this paper proposes a congestion notification scheme RECN, which uses the frame transmission efficiency as the congestion metric. With only a very small modification at the MAC layer, frame transmission efficiency characterizes the channel congestion status more accurately and can be deployed on the TCP protocol using the standard ECN signal method, improving the efficiency and fairness of the resource distribution in wireless ad hoc networks.