EBDP BUFFER SIZING STRATEGY 802.11 BASED WLANS

In this paper we present wired routers, for whom the sizing of buffers is an active research topic. The classical rule of thumb for sizing wired buffers is to set buffer sizes to be the product of the bandwidth and the average delay of the flows utilizing this link, namely the Bandwidth-Delay Product (BDP) rule. Surprisingly, however the sizing of buffers in wireless networks (especially those based on 802.11/802.11e) appears to have received very little attention within the networking community. Exceptions include the recent work in relating to buffer sizing for voice traffic in 802.11e WLANs, work in which considers the impact of buffer sizing on TCP upload/download fairness, and work in which is related to 802.11e parameter settings

Buffer Sizing for 802.11 Based Networks

We consider the sizing of network buffers in 802.11 based networks. Wireless networks face a number of fundamental issues that do not arise in wired networks. We demonstrate that the use of fixed size buffers in 802.11 networks inevitably leads to either undesirable channel under-utilization or unnecessary high delays. We present two novel dynamic buffer sizing algorithms that achieve high throughput while maintaining low delay across a wide range of network conditions. Experimental measurements demonstrate the utility of the proposed algorithms in a production WLAN and a lab testbed.Comment: 14 pages, to appear on IEEE/ACM Transactions on Networkin

Improving Performance for CSMA/CA Based Wireless Networks

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) based wireless networks are becoming increasingly ubiquitous. With the aim of supporting rich multimedia applications such as high-definition television (HDTV, 20Mbps) and DVD (9.8Mbps), one of the technology trends is towards increasingly higher bandwidth. Some recent IEEE 802.11n proposals seek to provide PHY rates of up to 600 Mbps. In addition to increasing bandwidth, there is also strong interest in extending the coverage of CSMA/CA based wireless networks. One solution is to relay traffic via multiple intermediate stations if the sender and the receiver are far apart. The so called “mesh” networks based on this relay-based approach, if properly designed, may feature both “high speed” and “large coverage” at the same time. This thesis focusses on MAC layer performance enhancements in CSMA/CA based networks in this context. Firstly, we observe that higher PHY rates do not necessarily translate into corresponding increases in MAC layer throughput due to the overhead of the CSMA/CA based MAC/PHY layers. To mitigate the overhead, we propose a novel MAC scheme whereby transported information is partially acknowledged and retransmitted. Theoretical analysis and extensive simulations show that the proposed MAC approach can achieve high efficiency (low MAC overhead) for a wide range of channel variations and realistic traffic types. Secondly, we investigate the close interaction between the MAC layer and the buffer above it to improve performance for real world traffic such as TCP. Surprisingly, the issue of buffer sizing in 802.11 wireless networks has received little attention in the literature yet it poses fundamentally new challenges compared to buffer sizing in wired networks. We propose a new adaptive buffer sizing approach for 802.11e WLANs that maintains a high level of link utilisation, while minimising queueing delay. Thirdly, we highlight that gross unfairness can exist between competing flows in multihop mesh networks even if we assume that orthogonal channels are used in neighbouring hops. That is, even without inter-channel interference and hidden terminals, multi-hop mesh networks which aim to offer a both “high speed” and “large coverage” are not achieved. We propose the use of 802.11e’s TXOP mechanism to restore/enfore fairness. The proposed approach is implementable using off-the-shelf devices and fully decentralised (requires no message passing)

Spurious TCP Timeouts in 802.11 Networks

In this paper, we investigate spurious TCP timeouts in 802.11 wireless networks. Though timeouts can be a problem for uploads from an 802.11 network, these timeouts are not spurious but are caused by a bottleneck at the access point. Once this bottleneck is removed, we find that spurious timeouts are rare, even in the face of large changes in numbers of active stations or PHY rate

An analytical model for performance evaluation of multimedia applications over EDCA in an IEEE 802.11e WLAN

We extend the modeling heuristic of (Harsha et al. 2006. In IEEE IWQoS '06, pp 178-187) to evaluate the performance of an IEEE 802.11e infrastructure network carrying packet telephone calls, streaming video sessions and TCP controlled file downloads, using Enhanced Distributed Channel Access (EDCA). We identify the time boundaries of activities on the channel (called channel slot boundaries) and derive a Markov Renewal Process of the contending nodes on these epochs. This is achieved by the use of attempt probabilities of the contending nodes as those obtained from the saturation fixed point analysis of (Ramaiyan et al. 2005. In Proceedings ACM Sigmetrics, '05. Journal version accepted for publication in IEEE TON). Regenerative analysis on this MRP yields the desired steady state performance measures. We then use the MRP model to develop an effective bandwidth approach for obtaining a bound on the size of the buffer required at the video queue of the AP, such that the streaming video packet loss probability is kept to less than 1%. The results obtained match well with simulations using the network simulator, ns-2. We find that, with the default IEEE 802.11e EDCA parameters for access categories AC 1, AC 2 and AC 3, the voice call capacity decreases if even one streaming video session and one TCP file download are initiated by some wireless station. Subsequently, reducing the voice calls increases the video downlink stream throughput by 0.38 Mbps and file download capacity by 0.14 Mbps, for every voice call (for the 11 Mbps PHY). We find that a buffer size of 75KB is sufficient to ensure that the video packet loss probability at the QAP is within 1%

Traffic modelling in WLANs and cellular networks.

Over the past several years there has been a considerable amount of research in the field of traffic modelling for WLANs and Cellular Networks as well as the integration of these networks. To date, the focus of published work has been largely on the operation of delay sensitive calls. Because the voice calls are no longer the only service in wireless and cellular systems, multi-service traffic networks now consist of integrated services with distinctive Quality of Service (QoS) requirements. Therefore, a number of different schemes have been proposed to deal with this problem. Most of these schemes only consider mobility and multi-service traffic characteristics. However, few studies have considered the impact of buffering of voice calls in integrated voice and data services. Therefore, we aim to make a critical investigation of existing traffic models and offer generic traffic schemes for WLAN and Cellular networks in order to analyse the impact of buffering of voice calls in hybrid networks. For this purpose, an analytical model for performance evaluation of a single server network with voice and data traffic is considered. In this system, voice is given priority but can be buffered in a limited way. The analysis shows that this approach can be used in fast mobile systems

ボイスIPネットワークのモデリングと高性能化

Tohoku University堀口 進課

Reducing Latency in Internet Access Links with Mechanisms in Endpoints and within the Network

Excessive and unpredictable end-to-end latency is a major problem for today’s Internet performance, affecting a range of applications from real-time multimedia to web traffic. This is mainly attributed to the interaction between the TCP congestion control mechanism and the unmanaged large buffers deployed across the Internet. This dissertation investigates transport and link layer solutions to solve the Internet’s latency problem on the access links. These solutions operate on the sender side, within the network or use signaling between the sender and the network based on Explicit Congestion Notification (ECN). By changing the sender’s reaction to ECN, a method proposed in this dissertation reduces latency without harming link utilization. Real-life experiments and simulations show that this goal is achieved while maintaining backward compatibility and being gradually deployable on the Internet. This mechanism’s fairness to legacy traffic is further improved by a novel use of ECN within the network