Congestion Avoidance Control And RED Algorithm Modification Research

With the unceasing development of Internet scale > people have an ever-growing demand for the quality of service of computer networks (QoS). And nowadays, in the high-speed network multimedia's application not only has the very high bandwidth requirement to the network, but also requires intelligence transmission low delay and the low vibration and so on, needs to provide the end_to_end control and guarantee of QoS. The network congestion is a great factor to affect the network quality of service , so the implementation congestion control is prerequisite for other QoS mechanism normal work .At present, congestion control mechanism of Internet and quality of service are the central issues of the current research.The active queue management mechanism( AQM) is, which the IETF recommends, the essential technology based on the router congestion control, which combines with the TCP end-to-end congestion control , being a main method to solve the congestion control question of the present Internet .By evaluation the state of network and foretelling the appearance of the congestion, AQM can drop the packet purposefully so that the sending end can be informed of the state of network and then adjust it's sending rate. But the current algorithms aren't still perfect in terms of response's time, stability and sensitivity to the environment and so forth. In this paper, the advantages and disadvantages of the existent algorithms are concluded based on analysing the current prevalent congestion control algorithms RED in detail, an improved algorithm CARED of active queue management (AQM) is proposed.As a result, CARED algorithm has made the modifications of original RED algorithm in the computational method of probability of packet drop -P_b and the adapting of the parameter -P_max. First, based on the fuzzy math, instead of the probability function of drop, we use the membership function of ascend demi-cauchy distribution. Original RED algorithm based on the average queue size of the buffer to adjust probability of drop, and when the average queue size reaches the Q_max(denoted by Q_max), the dropping rate increases linearly from P_max to 1. This jump will aggravate the jitter of buffer queue. CARED improve probability calculation used a ascend half-cauchy distribution function, to extend the scope of control from the minimum threshold to the buffersize. It can also achieve the smoothness from part to complete marking or discarding. Second : CARED calculates the average queue length in the router unit- time-gap, and by comparing it with the maximum value and the minimum value, the interpolation can be obtained. Based on the interpolation's size, CARED can adjust dynamically the size of P_max, and therefore adjust the sending rate of congestion notification to the source end in time and maintain the stability of the queue length, in order to avoid the unnecessary delay of transmission and vibration .The algorithm is verified in NS2 network simulation machine as well. By the indication of a series of simulation experiments, CARED can validly adapt the change of network flow effectively, hold stability of queue length , reduce phenomenon occurrence of the queue overflow or the idle greatly. And It is superior to the RED algorithm in maintaining the stability of queue length and enhancing the utilization ratio of the links.