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A substantial variety of control algorithms to adjust carrier sensing, transmission power, and transmission rate have been proposed for IEEE 802.11 wireless networks in the recent literature. Their objectives range from maximizing throughput, spatial reuse, and fairness to minimizing interference and congestion within the network. However, only a f...
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... is used to broadcast UDP packets, hence no MAC-layer acknowledgement packets are generated. To understand the impact of turning carrier sensing on and off, we used the following scenario (where t is time) and follow the airtime utilization for Node A and Node B (see Figure 2 and Figure 3): ...
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... Finally, a similar situation appears in networks that perform transmit power control on a per-link or perpacket basis, e.g. as suggested by [1], [5], [6], [7], [8], [12], [15]. Fig. 6 illustrates a scenario where per-link power control leads to hidden node problem. ...
We present our current work-in-progress on the design and implementation of a hybrid TDMA/CSMA medium access architecture, hereafter referred to as hMAC, which can be used on top of commercial IEEE 802.11 off-the-shelf hardware. The software only solution is based on the popular Linux ATH9K softMAC driver and hence can be used with standard Linux systems using Atheros based wireless network devices. The proposed hMAC exploits the standard 802.11 power saving functionality present in the ATH9K device driver to enable control of the software packet queues. This allows the assignment of TDMA time slots on wireless link and traffic class basis. While the solution is placed only in the device driver, the CSMA/CA functionality on hardware level is still active. This enables inter-working with standard unmodified 802.11 devices. We tested our prototypical hMAC implementation in a small test-bed. Therefore, we implemented a centralized interference management scheme in which pairs of links suffering from a hidden node problem are assigned to TDMA time slots on a per-link basis. To show the benefits of the proposed hMAC approach we compared the results with standard 802.11 DCF and classical, i.e. per-node, TDMA. Finally, to enable collaboration with the research community, the hMAC source code is provided as open-source.
... For instance, several theoretical and practical works focus on independently performing power [1]- [6], rate [7]- [12] and carrier sense control [13]- [17], which aim at adjusting the power, rate and carrier sense parameters, respectively, according to the measured link quality. As the interactions among these mechanisms lead to interesting tradeoffs [18], in this paper, we focus on their joint operation. Theoretical solutions in this context typically either require changes to the MAC (e.g., introducing a new frame type or adding additional information to Beacon frames) [19], [20], or assume cross-layer information from the MAC layer such as information about wireless neighbors and medium access state per link [20]. ...
... There is a significant amount of both theoretical and practical work on independently performing power [1]- [6], rate [7]- [12] and carrier sense control [13]- [17]. However, the interactions among these mechanisms cannot be overlooked, as they lead to tradeoffs [18]. In theory, one can improve spatial reuse, and consequently network capacity, by reducing the power or increasing the carrier sense threshold. ...
While there has been extensive theoretical work on sophisticated joint resource allocation algorithms for wireless networks, these ideas are not typically followed up in WiFi (IEEE 802.11) based deployments. The reasons range from technical issues such as limitations of current driver implementations to administrative issues such as not having centralized control over neighboring deployments. To this end, in this work, we investigate the feasibility of and benefits from resource control in a real WiFi system. Specifically, we focus on rate and power control capabilities and realize an adaptive rate-power controller (Minstrel-Piano) by enabling a cross-layer communication interface between the IEEE 802.11 PHY and MAC layers. Our initial evaluation results show that Minstrel-Piano significantly decreases the transmit power per communication while maintaining the same link performance in different scenarios. This is expected to achieve better spatial reuse as using less transmission power decreases the overall interference in the network.
... We attribute this behavior to the receiver blocking effect mentioned at the outset. We obtained greater confidence in this conclusion when we found that MAC on the receiving (5GHz) interface never got into BUSY state [14] when the collocated 2.4GHz interface is transmitting. ...
... An 802.11 interface can be viewed as being in one of the following four states: transmit (TX), receive (RX), channel busy (BUSY) and IDLE [14]. The BUSY state occurs when energy detected on the channel is more than a specified threshold (e.g., -62dBm for Atheros chipset in 802.11a). ...
We study coexistence interference that arises between multiple collocated radio interfaces on 802.11 based multi-radio platforms used for mesh networks. We show that such interference can be so severe that it prevents concurrent successful operation of collocated interfaces even when they use channels from widely different frequency bands. We propose the use of antenna polarization to mitigate such interference and experimentally study its benefit in both multi-band and single band configurations. In particular, we show that using differently polarized antennas on a multi-radio platform can be a helpful counteracting mechanism for alleviating receiver blocking and adjacent channel interference phenomena that underlie multiradio coexistence interference. We also validate observations about adjacent channel interference from previous studies via direct and microscopic observation of MAC behavior.
... A unicast packet can be annotated with up to 4 rates and the number of retries to be used. The chip has, like the Atheros AR5212, 4 socalled performance registers which can be used to determine the dwell time in the states receiving, transmitting and busy[7]. It is also possible to get the information of the error vector magnitude per spatial stream. ...
... A unicast packet can be annotated with up to 4 rates and the number of retries to be used. The chip has, like the Atheros AR5212, 4 socalled performance registers which can be used to determine the dwell time in the states receiving, transmitting and busy [6]. It is also possible to get the information of the error vector magnitude per spatial stream. ...
In this paper, we present a configurable and inexpensive MIMO mesh network research platform based on IEEE 802.11n and open source software. There are two requirements for such a research testbed. First configurability: the researcher needs the ability to modify hard- and software on each ISO/OSI layer. Secondly, the testbed has to comprise hundreds of nodes to make sound conclusions on network performance. Therefore, a single mesh node has to be cheap. Both requirements have contradicting implications and a tradeoff is necessary. We propose a solution based on off-the-shelf 802.11n hardware with Atheros WiFi chips, the open source WiFi driver ath9k, and the Click Router API. This represents a good tradeoff. Procurement costs for a single network node are below 100$. The solution yet allows a variety of adjustments based on the open-source driver and a highly configurable WiFi hardware. We evaluated our platform and present measurement results.
... For instance, several theoretical and practical works focus on independently performing power [1]– [6], rate [7]–[12] and carrier sense control [13]–[17], which aim at adjusting the power, rate and carrier sense parameters, respectively, according to the measured link quality. As the interactions among these mechanisms lead to interesting trade- offs [18], in this paper, we focus on their joint operation. Theoretical solutions in this context typically either require changes to the MAC (e.g., introducing a new frame type or adding additional information to Beacon frames) [19], [20], or assume cross-layer information from the MAC layer such as information about wireless neighbors and medium access state per link [20]. ...
... In the literature, there is a significant amount of both theoretical and practical work on independently performing power [1]–[6], rate [7]–[12] and carrier sense control [13]– [17] . However, the interactions among these mechanisms cannot be overlooked, as they lead to trade-offs [18]. In theory one can improve spatial reuse, and consequently network capacity, by reducing the transmission power or increasing the carrier sense threshold. ...
IEEE 802.11 (WiFi) networks are used to provide Internet access anywhere anytime. However, their performance is far below the achievable limits when multiple participants share the same frequency spectrum in an uncoordinated manner. The major reason behind such inefficiency is the lack of practical resource allocation algorithms that adapt well to the current conditions in a wireless network dynam- ically and select the appropriate transmission parameters such as transmission rates and power levels. Most current practical schemes are rather simplistic and only change a single transmission parameter. For instance, Transmit Power Control (TPC) works at the WiFi PHY layer and commonly assigns a static and rather high power level to all packets. A per-link or packet scheme is expected to provide better performance, but typically increases complexity and requires higher-layer information, such as medium access state from the Medium Access Control (MAC) layer. Therefore, although performance improvements have been shown in theory, these ideas are largely uninvestigated in practice.
In this thesis, our main goal is to understand the feasibility and performance impact of a joint and per-link rate and power controller in a real WiFi system. To this end, we first enabled cross-layer communication of transmission power between the WiFi PHY and MAC layers in the Linux mac80211 subsystem. Based on our Atheros WiFi hardware, we also developed the in-kernel monitoring tool ‘RegMon’ that enables understanding and troubleshooting MAC and PHY-layer operations with a fine- grained time resolution across different Linux drivers.
We designed and implemented a distributed rate and power control algorithm, Minstrel-Blues, which does not rely on signal strength or channel state information, but uses local statistics from periodic sampling of different rate and power combinations. Essentially, Minstrel-Blues can run on any WiFi hardware that supports packet-level power and rate control capabilities. Minstrel Blues decides the data- rate, and consequently, the minimum power-level to support the chosen rate using a two-attribute utility function based on the throughput and power consumption of all rates. To expose the trade-off between throughput and network interference, we also introduced a weight parameter for the utility function, which tunes the importance of throughput in utility decisions.
Our results show that if the goal is on maximizing the per-link throughput, Minstrel-Blues can sig- nificantly reduce transmission power necessary to communicate per link, while maintaining the same throughput achieved with maximum transmit power. We call this mode of operation - Minstrel Pi-
i
ano. Based on experiments in BOWL, at 5Ghz, Minstrel-Piano shows significant overall throughput enhancements due to increasing spatial reuse. Our performance analysis concludes with experiments with different weight factors in a home network scenario, with 2-laptops and one access point. These experiments were run in the 2.4 GHz ISM band, and hence, they also show that our controller works well in an typical scenarios. As more and more WiFi Access Points are deployed and with upcoming IEEE 802.11 n and ac devices using wider channel widths, resource allocation is expected to become even more important to manage interference efficiently. To this end, our work significantly contributes to the understanding of rate, power and carrier-sense control in practice.
