"Vendor-Affected" WLAN Experimental results:a Pandora's Box?

Giuseppe Bianchi*, Domenico Giustiniano*, Luca Scalia** and Ilenia Tinnirello*** Universita degli Studi di Roma - Tor Vergata, Dipartimento di Ingegneria Elettronica, Italy

** Universita di Palermo, Dipartimento di Ingegneria Elettrica, Elettronica e delle Telecomunicazioni, Italy

Abstract-Experimental results are typically envisioned asthe ultimate validation reference for any theoretical and/orsimulation modelling assumptions. However, in the case ofWireless LANs, the situation is not nearly as straightfor­ward as it might seem. In this paper, we discuss to what(large) extent measurement results may depend on proprietaryundocumented algorithms implemented in the vendor-specificcard/driver employed. Specifically, we focus on the experimentalstudy of IEEE 802.llb/g outdoor links based on the widely usedAtheroslMADWiFi card/driver pair. We show that unexpectedperformance divergences do emerge in two classes of comparativeexperiments run on a same outdoor link: broadcast versusunicast, and 802.llg versus 802.1 lb. We provide experimentalevidence that the severe performance degradation emerging inthe broadcast case and in the 802.11g case, are not caused byphysical reasons, such as interference, multipath, or differentPHYIMAC settings, such as preamble type/size, etc. Rather,they are "simply" caused by two different AtheroslMADWiFiproprietary algorithms. The conclusions are quite worrying. Onone side, the scientific community seems still largely unaware ofthe existence of such vendor-proprietary solutions. This is crucialas, in most cases, lack of awareness may mislead to erroneousinterpretation of the results. On the other side, the inclusion ofproprietary solutions in the equipments is a growing trend, andthe two findings documented in this paper might be just the veryfirst ones emerging out from a Pandora's box...

I. INTRODUCTION

The IEEE 802.11 [1] technology for Wireless LANs isenvisioning an unabated market success. What started out as aniche standard for cable replacement is today a certified (Wi-Fialliance) technology, used in a variety of scenarios includingoutdoor hot-spots, long distance links, mesh networks, etc.

The thorough understanding of what this technology canor cannot do, in the highly heterogeneous application sce­narios envisioned, cannot be clearly confined to analyticalor simulation studies only. Indeed, the impact of complexenvironmental conditions, such as sources of interference andradio propagation phenomena, is only roughly captured insidethe modelling assumptions laying at the basis of analytical orsimulation studies [2]. As such, a large amount ofexperimentalassessment work in a variety of environments has been carriedout by the research community [3]-[8].

However, it is not easy to setup reproducible tests andreliable trace collectors. For this reason, the 802.11 WorkingGroup is heading towards the specification of a recommendedpractice standard (802.11 T, at the date of writing in the statusof draft standard, version 1.01) for the evaluation of the802.11 wireless performance. But measurement methodology

978-1-4244-2036-0/08/$25.00 ©2008 IEEE.

is not the only issue. Experimental trials indeed rely ondrivers and cards produced by real-world vendor. The factthat the devices employed in a trial are "Wi-Fi certified"does not guarantee by itself that these driver/card pairs arefully compliant to the 802.11 standard. Indeed, as extensivelydiscussed in [9], several driver/card pairs may behave in adifferent manner from what mandated by the 802.11 standard,and may even employ different fundamental MAC parameters(such as minimum Contention Window, Extended Inter FrameSpace, etc).

In their experimental work, most of the research communityrelies on a small set of driver/card pairs, typically either IntelPrism cards with the HostAP driver [10], or Atheros cards withthe MADWiFi driver [11] (the latter having emerged as the de­facto standard in more recent experimental work). The mainreason for these choices is that the above mentioned driversare open-source. As such, they provide the ability to carefullycontrol (and eventually change) most of the MAC-layer set­tings. Moreover, they provide the programming flexibility tocustomize experimental tests and trace collections. Finally, thefact that the majority of the research community has convergedtowards a small set of driver/card pairs undoubtedly helps inthe reproducibility of the results.

Goal of this paper is to raise awareness that, in additionto the possible lack of compliancy to the standard in terms ofMAC-layer operation [9], some driver/card pairs do implementproprietary optimization solutions, which, being not clearlyadvertised and documented, may not be widely known bythe research community. This would not be a problem aslong as these mechanisms were always beneficial in termsof measured performance. However, this paper shows that,in some specific environmental conditions (frequently foundin outdoor scenarios) these mechanisms may dramaticallyaffect the performance achieved. As such, lack of awarenessof even their existence may severely mislead the researchcommunity in the attempt to explain or justify the actualexperimental findings through physical (i.e., wireless channelrelated) reasons.

In more details, this paper builds on an extensive set ofexperimental results devised to assess the quality of short­medium range outdoor 802.11 wireless links, typical of anoutdoor mesh network university campus scenario. The equip­ments used in the experiments are based on Atheros cards withMADWiFi driver. Results show that there are cases in whichthe link performance are totally unexpected and unpredictable.

First, for a same link and comparable Signal to Noise Ratio(SNR) conditions, performance may unnaturally diverge in thecase of unicast and broadcast transmissions, the latter beingseverely penalized over the former. Second, in most outdoorlinks, the 802.11g physical mode at the lowest 6 Mbps ratedramatically underperforms the 802.11b mode at the highest11 Mbps rate.

Our search for a neat and clear physical justification of these"weird" results has lasted for more than two years. Only thanksto the execution of specifically targeted measurement tests, andto a closer look at the driver/card implementation details, weultimately found out that these phenomena were not causedby physical reasons, such as wireless channel propagationconditions. Rather, performance impairments were "simply"caused by two proprietary mechanisms: an undocumentedtransmit diversity algorithm1, and an interference mitigationscheme whose description we ultimately found only in apatent [14]. The dramatic impact of such mechanisms over theexperimental performance is illustrated in the remainder of thispaper. After a brief summary, in Section II of the measurementscenario and methodology, Section III describes the perfor­mance impairments emerging for broadcast transmissions, andcaused by the transmit diversity algorithm. Section IV showshow the interference mitigation algorithm may have severedrawbacks in outdoor 802.11 g transmissions. Conclusions aredrawn in section V.

II. MEASUREMENT SCENARIO

The experimental tests discussed in this paper have beenindependently carried out at the Rome Tor Vergata campusand at the Palermo University campus. We tested a totalnumber of 14 links (9 in Rome and 5 in Palermo), differingin terms of distance (ranging between 50 and 205 meters)and obstruction (from partially obstructed by surroundingobstacles to almost free-space). Owing to the well knownlink asymmetry (see e.g., [15], and indeed confirmed also byour results), measurements have been independently carriedout for both directions of each deployed link, thus gathering28 independent measurement collections. Each link has beenanalyzed in a stand-alone operation, with all the other linksinactive. The measurement results reported and discussed inwhat follows refer to a subset of these links, where theanomalies induced by the driver/card transmit diversity andinterference mitigation algorithms are most evident. As as­sessed through spectral analyzer, the two campuses show avery different level of interference generated by adjacent/co­channel interference. In the Rome Tor Vergata campus, wecould be able to find an operative channel away fronl anysource of interference, thus achieving an interference-freetrial. Conversely, the Palermo campus was strongly interferedin all the available operative channels, and thus we usedmeasurements taken in this environment for analyzing theimpact of the interference mitigation scheme.

1More precisely, two different transmit diversity mechanisms implementedin different parts of the MADWiFi driver stack; extensive discussions andtechnical details can be found in [12], [13].

The wireless nodes deployed over the campus roofs werenet4826 Soekris boards [16], with a Pyramid Linux dis­tribution [17] running a 2.6.18 kernel. Such boards havebeen equipped with AR5212 Atheros 802.11 a/b/g compliantmini-pci cards presenting two antenna ports, to which weconnected two rubber duck external omni-directional (on thehorizontal plane) antennas, devised respectively for 802.11bigand 802.11 a transmissions. The first antenna had a gain of 5dBi at 2.4 GHz, and the second one had a gain of 3 dBi at 5GHz.

The card driver was a customized version of the MAD­WiFi one, extended [8] to allow statistic collection at bothtransmitter and receiver sides, and their subsequent off-linecross-correlation.

The performance metrics considered in this paper for as­sessing the quality of a link are the Delivery ProbabilityRatio (DPR) and the per-frame RSSI (Receiver Signal StrengthIndicator). The DPR is the probability that a generic trans­mitted frame is successfully received. In the case of unicastframes, the DPR is defined as the probability that a singleasynchronous DATAlACK handshake, regardless of whetherthis is a new transmission or a retransmission, is successfullyconcluded. In the case of broadcast frames, since no ACK istransmitted, the DPR is measured at the receiver as the prob­ability that a DATA frame is correctly decoded. If ambiguityoccurs, to distinguish the DPR measured for unicast framesfrom that measured for broadcast frames, we will use for thislatter the notation DPR-RX (DPR at the receiver). RegardingRSSI, we recall that it is an estimate of the signal power at thereceiver and is provided by each manufacturer on a proprietaryscale. Atheros NICs measure RSSI in terms of SNR referred tothe noise floor power. Thus, in what follows, we will simplyrefer to SNR. To obtain per-frame SNR measurements, wedisabled the smoothing filter natively provided by the driver.For convenience of plotting (and for further elaborations asshown when discussing the broadcast measurement cases),we provided a custom smoothing on the collected measures.Unless otherwise stated, each plotted sample is obtained byaveraging the consecutive RSSI samples collected in a timewindow of 200 msec.

For testing the links, we generated 100 ICMP echo requestsframes per second, whose datagram size has been set to theunusual value of 1601 bytes (i.e. approximately up to 1.3Mbps goodput) in order to facilitate, during post-processing,detection and filtering of interfering frames. The correspondingICMP echo reply was disabled to avoid data traffic travelling inthe opposite direction. Each measurement was performed overa 90 seconds period of time. In all experiments, the automaticrate selection and the RTS/CTS mechanism were disabled.The link rate was set to a fixed static value while the MACretry limit was set to 7. The transmitted EIRP (EquivalentIsotropically Radiated Power) was set to 20 dBm.

III. BROADCAST YS. UNICAST TRANSMISSIONS

The results reported in this section are gathered on theinterference-free testbed of the Rome Tor Vergata campus,

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broadcast transmission is accounted when the DATA frame issuccessfully decoded at the receiver, unlike the unicast casewhich also requires a successful reception of the return ACKat the transmitter.

A justification for the almost perfect 50% DPR-RX mea­sured for the broadcast case emerges from the careful analysisof figure 2. This figure reports the SNR values obtained in thefirst 4 seconds of the same broadcast measurement of figure 1,but in this case the SNR samples are obtained by using a timewindow set to 40,96 ms (40 times the IEEE 802.11 1.024 msTime Unit - TU) instead of the previous 800 ms. This muchshorter time window reveals a periodic bi-modal fluctuationof the SNR. In particular, it shows that the measured SNRswitches every ~ 400 ms (more precisely, 400 TU, i.e., 4beacon intervals) from a high value to a much lower value(about 10-15 dB less). Conversely, for the unicast traffic, wefound a stable SNR, even with a small 40.96 ms time window.Figure 2 shows a periodic SNR pattern which envisions "good"periods, where the SNR is in the order of 20 dBs and hence wemay expect a successful transmission of the DATA frame inalmost 100% of the cases, followed by "bad" periods, wherean SNR in the order of 6-8 dBs basically leads to a closeto 0% Delivery Probability Ratio. This justifies the ~ 50%average DPR-RX measured throughout the whole experiment.Moreover, this finding also explains why, in our measurements,most ofthe other links did experience a broadcast DPR-RX inan intermediate range.

Although the periodic SNR pattern perfectly justifies thebroadcast link performance, it was very hard to understandwhy this clearly artificial pattern does arise (and in fact ourwork [18] erroneously conjectures the existence of a powercontrol mechanism in the Atheros card). A clean and neatexplanation emerged as long as we found the existence ofa transmit diversity algorithm implemented in the MADWiFidriver. This algorithm is devised to adaptively select the besttransmitting antenna, among the two available ones. In the caseof broadcast frames, since the transmitter cannot rely on anyfeedback on the success ratio experienced by transmissions oc­curring through either of the two antennas, the policy adoptedby the vendor is to simply switch antennas on a periodic

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for the 802.11 b physical mode operating at 11 Mbps. Similarresults have been found for the other rates [18].

Figure 1 shows the Delivery Probability Ratio versus themeasurement time, for two links with comparable averageSNR (for sake of readability, only the SNR samples mea­sured in the broadcast case are plotted). In the first case,we transmitted broadcast frames and measured the DPR-RXas the percentage of DATA frames correctly received at thereceiver. In the second case, we transmitted unicast frames andmeasured the DPR as the percentage of successful DATA/ACKhandshakes. Each DPR sample as well as each SNR sampleare averaged over a temporal window of 800 ms.

A first surprise is that the broadcast and unicast DPRs arewidely different, even if the SNR is the same. But what is reallyunexpected is that the unicast case outperforms the broadcastone. Indeed, the average DPR value computed over the wholemeasurement time, for unicast traffic, is 1]=0.84. Conversely,the average DPR-RX value for broadcast traffic is 1]=0.50.Since in both cases the SNR is the same (stably fluctuatingaround 12-13 dB) and the frames were transmitted at the samerate, it would have been intuitive to find a superior (or at mostequal) performance for the broadcast case. In fact, a successful

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IV. 802.11 G VS. 802.11 B PHYSICAL MODE

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engineering in [13]) and may degrade the DPR performanceof intermediate quality links also for unicast transmissions.

The discussion carried out in this section takes the leadfrom the results presented in figures 5 and 6. These figureshow the DPR performance for a given outdoor link whenframes are transmitted with 802.11 b set to 11 Mbps (figure 5)and with 802.11g set to 6 Mbps (figure 6). Both figures showthat the SNR (dashed line) is comparable. However, we notethat the performance achieved by 802.11g at 6 Mbps (time­varying DPR indicated with solid line, with average DPR TJ ==0.02) significantly underperforms that of 802.11b at 11 Mbps(TJ == 0.66). The superiority of 802.11b when employing a ratehigher than that used by 802.1g is unexpected and counter­intuitive. However, it was confirmed by several other outdoorexperiments, which show that, in most outdoor links, 802.11gat the minimum rate (namely 6 Mbps) deterministically worksworse than 802.11b at the maximum rate (11 Mbps). Thesefindings were first raised in a thorough investigation carriedout in our earlier work [8]: this paper had in fact shown that,in the case of 802.11g, synchronization errors where the maincause of frame loss. However, no explanation could be found

2The relevant MADWiFi setting is sysctl dev. wifiO.txantenna=i: if i = 1 or2 the card is forced to use the considered antenna port; if i = 0 the transmitdiversity mechanism is enabled. By default, transmit diversity is enabled.

basis. However, this policy (activated by default!) has severeconsequences in terms of performance when the two antennasexperience different propagation conditions or, as in our case,when the two antennas have different radiated power. Werecall that our setting, characterized by one antenna devisedfor 5GHz and a second one devised for 2.4 GHz, is the mostobvious one in trials employing 802.11a/b/g cards. Moreover,we remark that problems are deemed to emerge in all caseswhere the two available antennas are not homogeneous (interms of polarization and/or gain).

The confirmation that the poor quality of a broadcast link isdue to this transmit diversity mechanism is shown in figure 3which compares the performance achieved by broadcast trafficwith transmit diversity enabled and disabled2. As shown in thefigure, for the specific link under consideration, the resultingDPR performance in the case of transmit diversity disabled isabout 85% (comparable with the unicast case), and about thedouble of that experienced with diversity enabled (42%).

Finally, we conclude this section by pointing out that theunicast case is not immune from transmit diversity side-effects.Indeed, there is a second Atheros-specific transmit diversityalgorithm used for unicast traffic. We found out that, in linkswith intermediate quality, also for the unicast case, a fluctua­tion in the SNR measured at the receiver may occur (althoughwith a completely different pattern than the broadcast case- see details in [13]). This is experimentally illustrated infigure 4, where a hi-modal distribution of the measured SNRclearly emerges. However, the transmit diversity algorithm forunicast frames appears to be different from that one used forbroadcast frames, and specifically appears to take advantageof the feedback provided by the received ACK frames, forestimating on which antenna the transmission should occur.This explains why this mechanism is not noticeable in goodquality links (once the 2.4 GHz antenna is selected, it isused for all the subsequent transmissions). Nevertheless, suchmechanism is overly simplicistic (see details about its reverse

TABLE IDPR (%) FOR 802.IIB/G WITH/WITHOUT ANIoN FIVE SELECTED LINKS

at that time to justify the emergence of such an high rate ofsynchronization errors.

We finally found an answer through a set of experimentscarried out at the Palermo campus, using the latest, as oftoday, stable version of the MADWiFi driver (0.9.3.3). For thisversion, a patch was first devised for disabling an Atheros­proprietary interference mitigation algorithm via the driverinterface. Such an interference mitigation scheme proposedby Atheros is called adaptive Ambient Noise Immunity (ANI).The patent [14] identifies different sources of interference andproposes different actions for reducing their adverse effects onthe receiver. Basically, these actions are devised to modify thereceiver sensitivity according to the surrounding noise level, bychanging the minimum power level required for activating thereception process and revealing the channel as busy. Note thatmost cards are equipped with both the OFDM/CCK receiver,for working in 802.1Ib/802.1Ig modes. The two receiverswork in parallel at each channel activity detection, until themost likely modulation scheme is identified by a dedicatedvoting block. Thus, the ANI algorithm works on two differentcarrier sense thresholds, whose tuning scale is independent andheterogeneous.

Figure 7 surprisingly shows that the ANI algorithm, devisedto mitigate the impact of interference and thus aimed at provid­ing a performance benefit, in practice may dramatically impairthe performance themselves. Figure 7 repeats the 802.1Igmeasurement on the same previous link, but with the ANIalgorithm disabled at both the transmitter and the receiver side(for comparison, the case of ANI enabled - indeed the defaultcase in Atheros cards - is also plotted, taken from figure 6).Note that we disabled the ANI algorithm at the transmitter

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Delivery probability ratio without/with ANI at 6 Mbps (802.11 g).

Link ANIOn ANI Off6 Mbps 11 Mbps 6 Mbps 11 Mbps

A 2.2 66.0 67.3 61.9B 6.0 37.8 67.9 72.5C 1.5 19.3 35.8 53.6D 2.4 82.5 73.6 80.9E 2.2 71.5 82.2 67.4

side too, because the tuning of the carrier sense thresholds alsoaffects the transmission operations. In fact, the transmitter ismore/less greedy for revealing the channel as idle according tothe increment/decrement of the carrier sense sensitivity. Fromthe figure, we notice that, switching the ANI scheme off, thedelivery ratio suddenly improves from almost 0 (1} == 0.02) toalmost 70% (11 == 0.67).

We suspect that this phenomenon is properly due to theANI actions on the OFDM receiver. In fact, in presence ofinterfered links, the patent states that the OFDM thresholdsare increased for first, because the OFDM receiver is morevulnerable to false detects due to spurious signals3• However,this action also reduces the probability of detecting actual datapackets, which in outdoor links are strongly attenuated, andavoiding collisions with actual interfering transmissions. Inother words, the sensitivity reduction of the OFDM receiver,on which the ANI algorithm acts for first, has the dramaticeffect to lose the reception of most of the outdoor data packets,erroneously considered as interference.

In order to confirm these considerations, table I comparesthe ANI effects for five tested outdoor links (link A beingthe one whose results were presented in the previous figures5, 6, and 7). From the table, we see that the ANI schemeremarkably degrades the DPR performance at 6 Mbps, in allthe considered links. By disabling the ANI scheme, in linkA and link E, which are almost line-of-sight, the DPR at 6Mbps is higher that the one at 11 Mbps. For the 11 Mbps802.11b transmissions, the performance comparison betweenthe ANIon and the ANI off cases is not straightforward.In fact, for links A, D, and E the ANI scheme slightlyimproves the link performance, because it effectively reducesthe false packet detections due to the parallel OFDM receiver.Conversely, for links Band C, the ANI scheme degrades theDPR performance, because also the CCK sensitivity thresholdsare increased, as a result of very high false detect rates. Weconfirmed these conclusions by observing that for links Band C, several interfering 802.11b networks were heard inproximity of our receiver.

V. CONCLUSIONS

In this paper we have shown that two proprietary schemes,implemented in the Atheros/MADWiFi card/driver pair, mayhave severe side-consequences on the transmission quality ex­perienced in 802.11 WLAN outdoor links. First, in trials whereheterogeneous antennas are employed (e.g. meant to support802.11a/b/g cards), the lack of disabling a transmit diversityalgorithm literally halves, in virtually all cases, the broadcasttransmissions performance. Unicast traffic is affected as well,although in this case the transmit diversity algorithm uses thefeedbacks provided by the reception of ACKs, and hence itsnegative impact emerges only in intermediate quality links(which however are frequent in outdoor deployment). Second,the interference mitigation scheme (ANI) implemented in the

3A false detect is an interfering signal which is erroneously considered as avalid packet and whose power would not have prevented the correct receptionof a concurrent genuine packet.

Atheros card is shown to cause severe performance degrada­tion in several 802.11g outdoor links. Quite impressively, linksthat, with ANI disabled, work fine, with ANI activated mayexperience a frame loss ratio as high as almost 100%.

We are especially worried by the lack of awareness, amongmost of the research community, of even the existence ofsuch mechanisms. Since they (at least the two specificallyaddressed in this paper) are enabled by default, lack of properawareness of their existence and/or of their technical operationmay lead researchers to erroneously interpret experimentalfindings. Moreover, we suspect that existing deploymentsmight underperform only because the network administratorsare not aware of these mechanisms and of the benefits broughtabout, in some cases, simply by their disabling.

Apart from the examples discussed in the paper and specifi­cally focusing on the MADWiFi/Atheros case, we clearly can­not exclude the existence of many other proprietary schemesemployed in commercial 802.11 equipments, provided bydifferent vendors. On the contrary, we suspect that these fewcases (which we could properly interpret and assess only aftera significant research effort) are just the first ones emergingfrom a much wider Pandora's Box. We fully understandthat proprietary solutions are the bread-and-butter of marketcompetition among vendors. However, the fact that thesesolutions may not be directly and/or extensively documented,but only generically advertised in terms of claimed benefits, isthreatening. Indeed, as shown in this paper, only the detailedknowledge of such mechanisms allows to use them properly,i.e., only when they actually do provide benefits.

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