Investigations On TCP Friendly Congestion Control In Wireless Network

In recent years, many real time multimedia stream applications (audio/video) emerge in Internet, such as video dibble seeding,IP telephone,long-distance education and so on. With the fast development of wireless network technology, real time multimedia stream operations will obtain the widespread application in the wireless network. Stream media data is produced based on certain code rate, to guarantee the playback quality of stream media, the sending rate of sender should satisfy a request of a minimum rate, if the sending rate is lower than this minimum rate, the sending data is invalid data, it could not to be played correctly even if data packets can be received correctly. At present the majority stream media use UDP as those transport layer protocol, but UDP protocol itself provides no congestion control mechanism, when many UDP stream of having no congestion control and TCP stream of having congestion control shared the networks bandwidth, the former will obtain the massive bandwidths unfairly, have suppressed the throughput of TCP stream greatly, possibly causes to congestion collapse finally. Therefore, in wireless environment researching TCP-friendly congestion control mechanism that both can guarantee the real time transport of multimedia stream operations and share the bandwidth with TCP fairly have important meaning.At present, the TCP-friendly congestion control mechanism suited for stream media transport is based on the wired network, which considered that packet loss is a networks congestion signal, so it has good performance in the wired networks. However, if these mechanisms are directly used in the wireless networks, their performances will drop, because the wireless link will be influenced by disturbance and burst characteristic of noise in the wireless network, the wireless link have very high error rate, this will cause coexisting of packet losses created by wireless channel and packet losses created by congestion, TCP friendly congestion control mechanism with a packet loss as congestion signal could not differentiate wireless packet losses and congestion packet losses, will decelerate excessively, the sender will be sending data packets with a rate that lower than the low sending rate, thus create the invalid transport, and cause the transport throughput to be low, influence streaming media playback quality seriously.TFRC (TCP-Friendly Rate Control) mechanism proposed by Floyd et al. is an excellent TCP-friendly congestion control mechanism based on the wired network at present, it guarantees TCP friendliness, and also has smooth sending rate, more suitable for transport of stream media. However, if TFRC is directly used in the wireless network, its performance will drop large scale, because TFRC is take the packet loss as congestion signal to trigger congestion control mechanism, but there are massive packet losses caused by wireless error code in the wireless network, TFRC is unable to differentiate congestion losses and wireless error losses, will decelerate excessively, the network throughput will reduce greatly. Moreover, TFRC has not considered the requirement of minimum sending rate of stream media applications, it would sending data packets with a rate that lower than the minimum rate , resulting the invalid transport, wasting networks bandwidth.In this paper, in view of the problem that the TFRC algorithm cannot differentiate congestion losses and wireless losses leading to the drop of performances, we proposed a new TCP-friendly congestion control mechanism that suit for transport of stream media in wireless network, named DTFRC. DTFRC uses one way delay to differentiate congestion losses and wireless losses at receivers. The experimental results indicated that DTFRC can differentiate congestion losses and wireless error code losses correctly, the sending rate maintains at the high level, the utility of wireless bandwidth maintains at the high level.In view of the problem that the TFRC algorithm cannot differentiate congestion losses and wireless losses result in performances drop, decelerating excessively, even sending invalid data packets with a rate that lower than the minimum rate, reducing network throughput greatly, affecting the playback quality of stream media, and consideration the service quality of streaming media transport and TCP friendliness, we propose a kind of TCP-friendly congestion control mechanism that guaranteed the minimum rate in wireless network, called WNMR-TFRC, WNMR-TFRC differentiates the loss situation at receivers according to the one way delay of the data packet, the"sufferable transient timer"is introduce at senders, the sender is sending data packets with the minimum rate in certain amount of time according to the current network congestion degree when the TCP-friendly sending rate calculated is lower than the minimum rate.The results of simulation show that WNMR-TFRC can differentiate congestion losses and wireless error code losses effectively, the sending rate maintains at the high level, is not lower than the requested minimum rate of stream media applications regardless of the network is free or congestion, not only had guaranteed the playback quality of streaming media, but also maintained the TCP friendliness simultaneously.