- Open Access
TCP-based window-size delegation method for TXOP Exchange in wireless local area networks
© Nishio et al; licensee Springer. 2011
- Received: 2 February 2011
- Accepted: 11 August 2011
- Published: 11 August 2011
We propose a TCP window-size delegation method for downlink TXOP (transmission opportunity) Exchange in wireless local area networks (WLANs). In our method, the 'compliant' stations (STAs) cooperatively use their available bandwidth in accordance with their throughput demand. We realize our method only with minimal modifications of the TCP functions of a proxy server, which lets one station (the TXOP provider) delegate TCP window size to another station (the TXOP client) so that the provider delegates its TXOPs in WLANs to the client. Our method enables an STA to flexibly delegate TXOPs to another STA without adversely affecting the legacy STAs, which is confirmed by computer simulations. We also confirmed that our method requires no modification to legacy access points and STAs.
- Bandwidth delegation
- TCP window size
- TXOP Exchange
- Wireless local area networks
In offices, homes, and public spaces, IEEE 802.11 wireless local area networks (WLANs) has been extensively used to provide wireless Internet connection services. In WLANs, stations (STAs) connected to an access point (AP) compete for transmission opportunities (TXOPs) using the carrier sense multiple access with collision avoidance (CSMA/CA). TXOPs are almost equally assigned to the STAs and the AP without considering each STA's throughput demand, especially when the traffic load is heavy. Although many quality of service (QoS) control mechanisms applicable for WLANs have been proposed including IEEE 802.11e, IntServ and DiffServ [1–3], they are not widely used basically because they require 'physical replacement' of existing APs or edge routers.
We previously focused on the use of TXOP Exchange for the uplink [4, 5]. In this paper, we focus on extending TXOP Exchange so that it can be used for the downlink as well. Unlike the uplink, we cannot realize TXOP Exchange for downlink only by MAC-layer modification at STAs since the packets are sent to the STAs from the connecting AP by using the AP's TXOP; in other words, the STAs share TXOPs of the AP for their downlink communications. On the other hand, AP sends a packet in the top of sending queue of the AP. Therefore, to enable an STA to delegate its TXOPs to another STA, we need to control the number of packets in the AP sending queue. Considering the implementation constraint and cost, we propose a transport-level control method that uses proxy servers to control the number of packets arriving at the AP. A proxy server plays roles of a coordinator between STAs and a manager for TCP connections of compliant STAs. In this method, a TXOP provider delegates a chance to increase its TCP window size to the TXOP client, which is controlled by the proxy server. The proxy server also decreases the window size for the TXOP provider where as that for the TXOP client would be decreased with the legacy TCP. This method does not require any modifications of the APs or the legacy STAs and is applicable to access networks other than WLANs since it is based on end-to-end TCP-level controls.
The rest of this paper is organized as follows. Section 2 briefly introduces our TXOP delegation method for the uplink and show how it works. In Section 3, we describe out TXOP delegation method for the downlink in detail. In Section 4, we observe how our method works through computer simulations and verify that it enables an STA to delegate throughput to another STA, while the other STAs see the same throughput as before. We also discuss the performance with varying the number of other STAs and round trip time (RTT). Finally, we mention the related work and conclude our paper in Sections 5 and 6.
4.1 Simulation setup
We evaluate our delegation method using QualNet simulator . We assume a network in which a provider and a client download data using TCP from a corresponding server, while the other STA download data directly from another corresponding server. Each of the STAs used only one flow. We assume that a bandwidth between the proxy server and the corresponding server for the provider and the client is large enough not to limit throughputs for them. We also idealize wireless channels to make our discussion simple. For instance, when an STA with lower channel quality delegates its TXOPs to another STA with higher channel quality, the overall transmission efficiency might improve. This issue should be included in future work.
4.2 Dependence of throughputs on α and β
We first demonstrate how a conventional method works. As mentioned in Section 3, a simple way to increase throughput of an STA is to assign multiple flows for the STA. Figure 4 shows the throughputs of each STA as a function of the number of flows assigned to STA A. With increase the number of flows for STA A, the throughput for STA A increases, while throughputs for STAs B and C decrease. As shown in this example, the conventional method may differentiate throughputs but it affects the non-compliant STAs significantly.
4.3 Performance of window-size delegation method in WLANs
Next, we evaluate the scalability of our method through the following two scenarios, in which the same simulation parameters are used as in Figure 11 except the number of STAs and RTTs between corresponding servers and the AP.
Our window-size delegation method in Section 3 enables an STA to delegate its throughputs to another STA in accordance with the required throughput without any effect on other STAs. Our method requires only a proxy server which is compliant with our method. To the best of our knowledge, any conventional methods cannot do this. In this section, we introduce several conventional methods as below.
Many flow level QoS control methods have been proposed including IntServ and DiffServ architectures [2, 3, 9, 10]. In [2, 3, 9], their approaches are basically to control bandwidths and/or delays of flows between edge routers. They can differentiate throughput and/or delays among flows, while they require replacements or modifications on edge routers, which are limited to their application range. On the other hand, it has been discussed how to prioritize throughputs for specific STAs with only modifications on a server . In , when a server receives duplicate ACKs from prioritized STAs, the server decreases congestion window size of flows for other altruistic STAs instead of the prioritized STA's congestion window size. However, this method cannot ensure to increase a throughput of prioritized STA when the number of altruistic STAs is not satisfactorily large.
On the other hand, in WLANs, MAC level QoS control methods also have been proposed including a QoS standard of WLANs called IEEE 802.11e . The AP equipped with IEEE 802.11e can prioritize packets classified as specific traffic like video and voice and differentiate throughputs for them from the other traffic. However, it does make it without giving some effect on other STAs in the network especially when the network includes non-compliant STAs. Cooperation methods have been also discussed [11, 12] in WLANs. They discussed cooperation in packet forwarding, which can be also effective but is a different model from ours. Furthermore, most of the conventional cooperation methods in wireless networks including  and  were discussed only in the link or/and physical layer.
We proposed a TCP window-size delegation method for downlink TXOP Exchange in WLANs. In our method, a proxy server lets one station (the TXOP provider) delegate TCP window size to another station (the TXOP client) so that the provider delegates its TXOPs in CSMA/CA to the client. Our method enables an STA to flexibly delegate TXOPs to another STA without adversely affecting the legacy STAs, which is confirmed by computer simulations. We also confirmed that our method requires no modification to legacy APs and STAs and needs only minimal modifications of the TCP functions of the proxy server.
We would like to mention that our method enables N-providers to delegate their window size to M-clients simultaneously, although, in this paper, we consider only a case where a provider delegates its window size to a client. We will observe the case where N-providers delegate their window size to M-clients in the future work. Future work also includes a design of a coordination algorithm that ensures fair incentives for cooperation between STAs.
This work is supported in part by the National Institute of Information and Communications Technology (NICT), Japan, under Early-concept Grants for Exploratory Research on New-generation Network.
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