针对无线上行链路的TCP跨层改进机制

在下一代蜂窝通信系统（例如,4G）里,传输控制协议（TCP）做为主要的端到端传输协议将随着互联网业务的开展而得到相当广泛的应用。本文根据上行无线链路对TCP的影响,提出了一种针对上行链路的TCP改进机制。该机制利用无线链路控制层（RLC）的反馈信息,实现了对TCP层定时器的自适应控制,从而大大降低了上行无线链路对TCP性能的影响。仿真结果表明,同传统的TCP机制相比,本文提出的TCP改进机制能够使得TCP在可靠性低,突发错误时间长以及切换时延高的无线移动环境更为有效地工作。     

一种改善TCP性能的链路层重传方案

有线网络中TCP拥塞控制机制是建立在网络丢包的基础之上的,所以该机制不能适应无线网络中高误码率造成的无线链路丢包的情况。无线链路层重传技术是改善网络性能因无线信道误码率较高而下降的一项重要措施。文中研究了WCDMA无线网络中链路层重传技术对无线TCP数据传输的影响,比较两种重传方案,通过OPNET仿真技术对其进行仿真比较,得出其中一种更有效的改善TCP传输性能的方案。     

基于TCP的卫星链路性能分析与改进策略

针对低误码率有线网络所设计的TCP协议,具有使用广泛、算法成熟、可移植性好等优点,然而卫星链路自身的高误码率、长传播时延、高时延带宽积等因素导致了TCP性能的大幅下降.这使得传统TCP协议应用到卫星网络时性能不理想.本文在梳理TCP协议特点的基础上,对卫星链路影响TCP性能的原因进行了分析,并提出了几种基于卫星链路传输TCP协议的性能改进方法.     

适用于卫星网络的TCP跨层改进机制

该文提出基于跨层信息交互,将链路层ARQ重传状态信息通知TCP的机制,避免了链路层重传引起的时延变化对TCP的不利影响。该机制使用完全可靠选择性重传ARQ为TCP提供可靠的链路,避免卫星链路上发生丢包,并且不必要求链路层保证包按序递交,消除了重排序的等待时延,适合带宽时延积较大的卫星网络。仿真结果表明,能显著提高TCP在卫星网中的性能,特别是在误帧率较高条件下。     

无线高速链路快速可靠传输方法

在无线信道上,链路层选择性重传与TCP重传相比,可只对发生错误的部分链路帧重传,不需重传整个数据包,可以大大节约链路带宽资源。因此高速无线链路的应用需要一种高效率的发送与接收窗口控制方法。该方法将分组序号作为访问窗口结构数组的下标,并且联合应用预设表进行查找的搜索方法,实现了对窗口的高效率快速操作和对高速无线信道的数据传输支持。通过实验,验证了这种设计的有效性。     

卫星链路中TCP传输性能改进与仿真

卫星信道与地面有线信道具有不同的特点,当TCP协议直接应用于卫星链路上时,其吞吐量等性能会受到影响。针对卫星链路的特点,分析了现有的卫星链路进行TCP传输的几种解决方案,使用NS 2仿真软件,建立了仿真模型,并在给定的仿真环境下得出了一些卫星链路下TCP传输性能和卫星spoofing技术的仿真结论。卫星链路具有时延大、误码率高等特点,仿真结果说明卫星spoofing技术和PS技术相结合的方案对卫星链路TCP传输性能改善是十分有益的。     

一种空间环境下的TCP传输技术研究

随着因特网技术的高速发展,如何利用基于可靠连接的传输控制协议(TCP)作为传输机制成为空间环境下测控通信的新问题。在研究航天科学数据业务的特点和空间传输信道特点及链路特性的基础上,分析了空间环境下TCP传输机制具有长时延和高误码率等缺陷。针对这些问题提出了用于空间高时延信道条件下的TCP高速传输的解决方法,并详细阐述2种方法的原理和实现途径。经过系统仿真,证明此方法可行,传输性能得到显著的提高。     

改善卫星融合网络随机丢包和间歇性中断的方法研究*1

当移动自组织网用户通过网关经卫星网络发送数据时,存在卫星链路的长传播时延、随机丢包以及移动终端的移动性引起链接的间歇性中断,导致TCP触发拥塞控制机制而降低传输性能降低。针对以上卫星网络的特点,提出了TCP M-Veno方法。在发送端对TCP Veno进行优化改进使得它能够缓解卫星网络中长的传播时延和随机丢包的影响。在网关中结合M-TCP算法来解决卫星融合网络移动终端间歇性中断。仿真表明,所提出的TCP M-Veno比NewReno、Veno以及M-TCP有更好的传输性能。     

基于HARQ的水声信道链路自适应技术

水声信道的多径扩展严重、环境噪声高、带宽受限、传输时延大等特点,极大地制约了水声通信系统的可靠性和有效性.链路自适应（LA）和混合自动重传（HARQ）常用来改善无线链路的可靠性和有效性,但是它们在水声环境中的性能与影响尚不明确.因此,文中主要研究水声信道中链路自适应和混合自动重传的联合优化.首先分析水声信道中不同的调制编码方式（MCS）的误码率性能.然后通过联合研究LA和HARQ技术在水声信道中的性能,文中提出一种适用于水声信道的联合LA和HARQ技术.     

一种基于数据重传率的无线自组织网络TCP公平性策略

IEEE 802.11 MAC协议被广泛应用于多跳无线网络的研究与示范网中,802.11协议采用RTS/CTS策略减少了隐终端 问题对无线TCP性能的影响。但在多条并行链路间,隐终端问题的存在使得中间链路节点不能有效的接入到信道,导致TCP性能和公平性严重下降。该文提出了一种新的基于传输层数据重传率方法,对于受到严重干扰的中间链路不断获取它的数据重传率,在重传率增加时动态减少802.11退避计时器竞争窗口大小,增加其接入信道能力,从而提高其TCP性能和公平性。仿真结果证明,使用基于传输层数据重传率方法,不仅能基本上保证并行链路TCP的稳定性,还可以极大地提高中间链路TCP公平性。     

Detecting spurious timeouts in wireless cellular networks using DS‐Agent

Large and sudden variations in packet transmission delays are often unavoidable in wireless networks. Such large delays, refer to as delay spikes (DSs), are likely to exceed several times the typical network round‐trip‐time figures, which can cause TCP spurious timeouts. The spurious timeouts lead to unnecessary retransmissions and reduction of the TCP sender's transmission rate, and degradation of TCP throughput. In this paper we propose a new scheme called DS‐Agent. The spurious timeout is detected by a DS‐Agent and thus TCP sender can response to this spurious timeout accordingly. The simulation results show the better performance of DS‐Agent scheme compared with F‐RTO and TCP Reno in the presence of DSs which is caused by mobility. Copyright © 2006 John Wiley & Sons, Ltd.     

Performance study of link layer and MAC layer protocols to support TCP in 3G CDMA systems

Providing support for TCP with good quality link connection is a key issue for future wireless networks in which Internet access is going to be one of the most important data services. A number of schemes have been proposed in literature to improve the TCP performance over wireless links. In this paper, we study the performance of a particular combination of link layer protocol (e.g., radio link protocol or RLP) and MAC retransmissions to support the TCP connections over third generation (3G) wireless CDMA networks. We specifically investigate two metrics - the packet error rate and the delay provided by RLP and MAC retransmissions - both of which are important for TCP performance. For independent and identically distributed (i.i.d) error channels, we propose an analytical model for RLP performance with MAC retransmission. The segmentation of TCP/IP packets into smaller RLP frames, as well as the RLP buffering process, is modeled using a Markov chain. For correlated fading channels, we introduce an analytical metric called RLP retransmission efficiency. We show that: 1) the RLP frame size has significant impact on the overall 3G system performance, 2) MAC layer retransmissions significantly improve the TCP performance, and 3) the RLP retransmission scheme performs better in highly correlated channels, while other scheme performs better in low correlated channels. Simulation results also confirm these conclusions.     

Considering spurious timeout in proxy for improving TCP performance in wireless networks

In this letter, we introduce a new proxy that effectively prevents unnecessary retransmissions from flowing over a wireless link on a path with sudden delay. The proposed Spurious Timeout (STD) algorithm detects spurious timeout based on the data and acknowledge sequence number. It responses to spurious timeout by filtering duplicate acknowledgements that can cause spurious fast retransmission. Simulation results show that the proposed STD algorithm performs better than, or as well as, other end-to-end mechanisms.     

Retransmission strategies for an acceptable quality of HDTV in the wireless and wired home scenario

We evaluate the performance of various retransmission schemes to achieve the goal of less than one visible distortion in twelve hours for high definition television. The focus is on an indoor wireless link and a DSL link, and we consider systems where the packets are protected by means of retransmissions with and without forward error correcting (FEC) codes. In order to achieve a satisfactory performance with restricted latency, we propose an unconventional retransmission procedure. The overall conclusion is that retransmissions without FEC achieve the best performance with the lowest latency, lowest overhead and lowest complexity, both in the wireless and the wired home scenario. Copyright © 2006 John Wiley & Sons, Ltd.     

TCP throughput and fairness performance in presence of delay spikes in wireless networks

Wireless networks are likely to experience delay spikes exceeding several times the typical round‐trip‐time figures, which can cause spurious timeouts that lead to unnecessary retransmissions and reduction of the TCP sender's transmission rate, and thus, the throughput of the TCP is degraded. This paper presents some research results on the effect of delay spikes caused by handover on TCP performance by using three different mobility models. It is shown that the throughput of TCP connection over a single bottleneck link is decreased in the presence of delay spikes significantly. Furthermore, it is shown that the fairness feature of TCP is also severely affected in the presence of delay spikes. Copyright © 2005 John Wiley & Sons, Ltd.     

基于明确丢失应答的异质网络传输控制

指出链路层策略在提高无线链路可靠性的同时,对传输层带来的影响,并采用明确丢失应答作为反馈信息,根据不同的数据丢失原因采取相应的措施,保证了对有线拥塞的及时检测和处理能力,使源端能在无线链路传输差错率高的情况下保持较高的传输速率,同时避免传输层与链路层之间不必要的重发竞争,从而提高了异质媒体网络上端到端的数据吞吐量。     

Alleviating contention-induced spurious timeouts in QoS-aware MANETs

The bandwidth is one of the most precious resources in Mobile Ad Hoc Networks (MANETs); their network protocols should therefore be able to achieve an increase in efficiency of bandwidth utilisation. TCP, which implements flow control to regulate the network traffic, plays a significant role in shaping the workload of the network. However, spurious TCP retransmissions can increase the link utilisation, consequently limiting the amount of spare bandwidth available as extensively reported in the literature. This problem is exacerbated by the presence of high priority traffic and traffic differentiation introduced in IEEE 802.11e. In this paper, we focus on the optimisation of TCP for resource efficiency, by limiting the number of contention-induced retransmissions. Using a cross-layer technique, TCP can be notified of the medium contention state and adjusts its retransmission timer accordingly. Our simulation results demonstrate the effectiveness of our proposal in various scenarios. A 15% gain in resource utilisation is obtained, fewer packets are retransmitted thus reducing the overall power consumption and TCP goodput is improved.     

Hierarchical cache design for enhancing TCP over heterogeneous networks with wired and wireless links

TCP is a reliable transport protocol tuned to perform well in traditional networks made up of links with low bit-error rates. Networks with higher bit-error rates, such as those with wireless links and mobile hosts, violate many of the assumptions made by the transmission control protocol (TCP), causing degraded end-to-end performance. We propose a two-layer hierarchical cache architecture for enhancing TCP performance over heterogeneous networks with both wired and wireless links. A new network-layer protocol, called new snoop (NS), is designed. The main idea is to cache the unacknowledged packets at both the mobile switch center (MSC) and base station (BS), to form a two-layer cache hierarchy. If a packet is lost due to transmission errors in the wireless link, the BS takes the responsibility to recover the loss. When a handoff occurs, the packets cached at the MSC can help to minimize the latency of retransmissions due to temporal disconnection. NS can preserve the end-to-end TCP semantics and is compatible with existing TCP applications. Its implementation only requires code modification at the BS and MSC. Simulation results show that NS is significantly more robust in dealing with unreliable wireless links and handoffs as compared with the original snoop scheme, as well as some other existing TCP enhancements.     

Improving reliable transport and handoff performance in cellular wireless networks

TCP is a reliable transport protocol tuned to perform well in traditional networks where congestion is the primary cause of packet loss. However, networks with wireless links and mobile hosts incur significant losses due to bit-errors and handoffs. This environment violates many of the assumptions made by TCP, causing degraded end-to-end performance. In this paper, we describe the additions and modifications to the standard Internet protocol stack (TCP/IP) to improve end-to-end reliable transport performance in mobile environments. The protocol changes are made to network-layer software at the base station and mobile host, and preserve the end-to-end semantics of TCP. One part of the modifications, called the snoop module, caches packets at the base station and performs local retransmissions across the wireless link to alleviate the problems caused by high bit-error rates. The second part is a routing protocol that enables low-latency handoff to occur with negligible data loss. We have implemented this new protocol stack on a wireless testbed. Our experiments show that this system is significantly more robust at dealing with unreliable wireless links than normal TCP; we have achieved throughput speedups of up to 20 times over regular TCP and handoff latencies over 10 times shorter than other mobile routing protocols.This work was supported by ARPA Contract J-FBI-93-153. This paper was in part presented at the ACM Mobile Computing and Networking Conference (Mobicom '95), Berkeley, California, 14–15 November 1995.     

Improving Wireless TCP Throughput by a Novel TCM-Based Hybrid ARQ

A novel hybrid ARQ (HARQ) scheme using a concatenated two-state trellis-coded modulation (CT-TCM) code is proposed for improving wireless TCP throughput. A distinguished feature of the proposed scheme is that the heavily punctured TCM codes are used for retransmissions of the corrupted data block, which are combined at the receiver with the previously received sequences of the same data block for decoding. By this method, significantly improved coding gain and efficient spectrum utilization can be achieved with very low complexity. A Markov model is developed to evaluate TCP throughput over the proposed HARQ in wireless link. By both analysis and simulation, we demonstrate that compared with other existing TCM-based ARQ schemes, significant improvement of TCP throughput over wireless links is achieved by the proposed CT-TCM HARQ while smaller buffer size is required at the access point.