This paper presents ZigZag, an 802.11 receiver design that combats hidden terminals. ZigZag's core contribution is a new form of interference cancellation that exploits asynchrony across successive collisions. Specifically, 802.11 retransmissions, in the case of hidden terminals, cause successive collisions. These collisions have different interference-free stretches at their start, which ZigZag exploits to bootstrap its decoding.ZigZag makes no changes to the 802.11 MAC and introduces no overhead when there are no collisions. But, when senders collide, ZigZag attains the same throughput as if the colliding packets were a priori scheduled in separate time slots. We build a prototype of ZigZag in GNU Radio. In a testbed of 14 USRP nodes, ZigZag reduces the average packet loss rate at hidden terminals from 72.6% to about 0.7%.

/pdf/zigzag-decoding-combating-hidden-terminals-in-wireless-282gpsraz4.pdf

Zigzag decoding: combating hidden terminals in wireless networks

The proliferation of hotspots based on IEEE 802.11 wireless LANs brings the promise of seamless Internet access from a large number of public locations. However, as the number of users soars, so does the risk of possible misbehavior; to protect themselves, wireless ISPs already make use of a number of security mechanisms, and require mobile stations to authenticate themselves at the Access Points (APs). However, IEEE 802.11 works properly only if the stations also respect the MAC protocol. We show in this paper that a greedy user can substantially increase his share of bandwidth, at the expense of the other users, by slightly modifying the driver of his network adapter. We explain how easily this can be performed, in particular with the new generation of adapters. We then present DOMINO (System for Detection Of greedy behavior in the MAC layer of IEEE 802.11 public NetwOrks), a piece of software to be installed in the Access Point. DOMINO can detect and identify greedy stations, without requiring any modification of the standard protocol at the AP and without revealing its own presence. We illustrate these concepts by simulation results and by the description of our prototype.

/pdf/domino-a-system-to-detect-greedy-behavior-in-ieee-802-11-2tt3y0u0i5.pdf

DOMINO: a system to detect greedy behavior in IEEE 802.11 hotspots

We propose a CDMA-based power controlled medium access protocol for mobile ad hoc networks (MANETs). Unlike previously proposed protocols, ours accounts for the multiple access interference (MAI), thereby addressing the notorious near-far problem that undermines the throughput performance in MANETs. Channel-gain information obtained from overheard RTS and CTS packets over an out-of-band control channel is used to dynamically bound the transmission power of mobile terminals in the vicinity of a receiver. By properly estimating the required transmission power for data packets, the proposed protocol allows for interference-limited simultaneous transmissions to take place in the neighborhood of a receiving terminal. Simulation results indicate that compared to the IEEE 802.11 approach, the proposed protocol achieves a significant increase in network throughput at no additional cost in energy consumption.

/pdf/cdma-based-mac-protocol-for-wireless-ad-hoc-networks-2eu1t3mjmj.pdf

CDMA-based MAC protocol for wireless ad hoc networks

In this paper, we propose a comprehensive solution for power control in mobile ad hoc networks (MANETs). Our solution emphasizes the interplay between the MAC and network layers, whereby the MAC layer indirectly influences the selection of the next-hop by properly adjusting the power of route request packets. This is done while maintaining network connectivity. Directional and channel-gain information obtained mainly from overheard RTS and CTS packets is used to dynamically construct the network topology. By properly estimating the required transmission power for data packets, our protocol allows for interference-limited simultaneous transmissions to take place in the neighborhood of a receiving node. Simulation results indicate that compared to the IEEE 802.11 approach, the proposed protocol achieves a significant increase in the channel utilization and end-to-end network throughput, and a significant decrease in the total energy consumption.

Power controlled dual channel (PCDC) medium access protocol for wireless ad hoc networks

In wireless networks, a frame collision does not necessarily result in all the simultaneously transmitted frames being lost. Depending on the relative signal power and the arrival timing of the involved frames, one frame can survive the collision and be successfully received by the receiver. Using our IEEE 802.11a wireless network testbed, we carry out a measurement study that shows the terms and conditions (timing, power difference, bit rate) under which this capture effect takes place. Recent measurement work on the capture effect in 802.11 networks [10] argues that the stronger frame can be successfully decoded only in two cases: (1) The stronger frame arrives earlier than the weaker frame, or (2) the stronger frame arrives later than the weaker frame but within the preamble time of the weaker frame. However, our measurement shows that the stronger frame can be decoded correctly regardless of the timing relation with the weaker frame. In addition, when the stronger frame arrives later than the weaker frame's arrival, the physical layer capture exhibits two very distinct patterns based on whether the receiver has been successfully synchronized to the previous weak frame or not. In explaining the distinct cases we observe that the successful capture of a frame involved in a collision is determined through two stages: preamble detection and the frame body FCS check.

http://morse.colorado.edu/~tlen5520/Papers/capture-effect-lee.pdf

An experimental study on the capture effect in 802.11a networks

We address issues with the performance of IEEE 802.11, when used in the adhoc mode, in the presence of hidden terminals. We present results illustrating the strong dependence of channel capture behavior on the SNR observed on contending hidden connections. Experimental work has illustrated that in a hidden terminal scenario, the connection having the strongest SNR is able to capture the channel, despite the use of the RTS-CTS-DATA-ACK 4-way handshake designed to alleviate this problem. Our results indicate that the near-far SNR problem may have a significant effect on the performance of an adhoc 802.11 network.

/pdf/unfairness-and-capture-behaviour-in-802-11-adhoc-networks-3rpf6wzbxu.pdf

Unfairness and capture behaviour in 802.11 adhoc networks

We investigate the performance of common capture models in terms of the fairness properties they reflect across contenting hidden connections. We propose a new capture model, message retraining, as a means of providing an accurate description of experimental data. Using two fairness indices we undertake a quantitative study of the accuracy with which each capture model is able to reflect experimental data. Standard capture models are shown to be unable to accurately reflect the fairness properties of empirical data. The message retraining capture model is shown to provide a good estimate of actual system performance in varying signal strength conditions.

/pdf/simulation-of-capture-behaviour-in-ieee-802-11-radio-modems-3hrgbenxsy.pdf

Simulation of capture behaviour in IEEE 802.11 radio modems

This paper addresses experimental measurements from an IEEE 80211 ad hoc network testbed, which indicate a strong signal strength dependence in the ability of a hidden terminal to gain access to the radio channel We present analytical results investigating the 'hidden terminal jamming' ability of the IEEE 80211 DSSS physical layer Results indicate that in a hidden terminal topology, the presence of an interfering transmission with a signal strength marginally greater than the transmission currently being received will result in an intolerable increase in BER, effectively jamming the ongoing transmission These results confirm previous experimental measurements which show that after a number of MAC layer timeout/retransmission periods, the original (weaker) connection is effectively prevented from gaining access to the channel

/pdf/hidden-terminal-jamming-problems-in-ieee-802-11-mobile-ad-51jeehxjyl.pdf

Hidden terminal jamming problems in IEEE 802.11 mobile ad hoc networks

In this paper we investigate the performance of common capture models in terms of the fairness properties they reflect across contenting hidden connections We propose a new capture model, Message Retraining, as a means of providing an accurate description of experimental data Using two fairness indices we undertake a quantitative study of the accuracy with which each capture model is able to reflect experimental data Standard capture models are shown to be unable to accurately reflect the fairness properties of the experimental data The Message Retraining capture model is shown to provide a good estimate of actual system performance in varying signal strength conditions

/pdf/modelling-capture-behaviour-in-ieee-802-11-radio-modems-mlqga79dtg.pdf

Modelling Capture Behaviour in IEEE 802.11 Radio Modems

Topology control in a wireless ad-hoc network allows better spatial reuse of the wireless channel and network resources. The existing topology control algorithms tend to optimise network power usage by keeping the topology connected but do not take the network application requirements into account. Mission critical applications which require explicit end-to-end bandwidth and delay guarantees may not find enough resources in the network with the existing network topology. We have devised a topology control algorithm for mission critical applications (TCMCA) in wireless ad-hoc networks, which adapts the network topology to improve the available resources for a set of mission critical applications (high priority services) in a network. TCMCA is a source based algorithm where topology control decisions are made on distributed network knowledge. The performance of TCMCA is evaluated for a static wireless network and compared against algorithms such as connect, LINT and full power transmissions. We demonstrate that TCMCA shows better support for mission critical services for varying number of mission critical applications in the network.

/pdf/tcmca-a-source-based-distributed-topology-control-algorithm-2jm5kq1t0s.pdf

C. Ware

Papers

Unfairness and capture behaviour in 802.11 adhoc networks

Simulation of capture behaviour in IEEE 802.11 radio modems

Hidden terminal jamming problems in IEEE 802.11 mobile ad hoc networks

Modelling Capture Behaviour in IEEE 802.11 Radio Modems

TCMCA: a source-based distributed topology control algorithm for mission critical applications in mobile ad-hoc networks