Implication Of MAC Frame Aggregation On Empirical
Wireless Experimentation
Gautam Bhanage
∗, Rajesh Mahindra
†, Ivan Seskar
∗, Dipankar Raychaudhuri
∗∗
WINLAB, Rutgers University, RT 1 South, North Brunswick, NJ 08902, USA
†NEC Labs America, Princeton, NJ 08540, USA
{
gautamb, seskar, ray
}
@
winlab.rutgers.edu,
{
rajesh
}
@
nec-labs.com
Abstract— Wireless network emulator testbeds have become increasingly important for realistic, at-scale experimental evalua-tion of new network architectures and protocols. Typically, wire-less network performance measurements are made at multiple layers of the wireless protocol stack, i.e. link layer, MAC layer and network layer. This study highlights the impact of layer 2 frame aggregation that is enabled by default in the software drivers for commodity wireless 802.11 devices while it is still not a part of the core 802.11 standard. Using experimental measurements, it is shown that this feature has an impact across a diverse set of wireless experiments and should be considered while comparing results. Measurements on the ORBIT testbed show
that throughput measurements can vary up to a startling 25%
for certain packet sizes and the variance in receiver side inter-frame delays can almost double if MAC aggregation and preset transmission opportunities are not taken into consideration. Further results for VoIP traffic show a deterioration in jitter
of up to 8times when coupled with MAC layer aggregation in
802.11.
Index Terms— Frame aggregation, fast-framing, txop, madwifi implementation, 802.11e.
I. INTRODUCTION
Testbeds for wireless networking research such as OR-BIT [9], MiNT [10], EMULAB [19], and Roofnet [6] aim to provide the ability to control and measure important network parameters on actual wireless devices with a high degree of accuracy, reproducibility, and efficiency. Wireless emulation has been recently used for evaluating the performances of 802.11 protocols [13][21], ad hoc routing protocols [8], man-aged mode performance of networks [12], quality of service evaluations [25], power and rate control experiments [23], [20], [27], [7], and a host of other studies. The common factor in the success of the wireless testbeds mentioned above is the use of commodity wireless hardware devices (e.g., 802.11 Atheros/Intel NICs) along with open-source driver softwares (e.g., Madwifi driver [5]) that allows for flexible configuration and customization. This flexibility gives users a better control over protocols and softwares used over the radio nodes.
Despite having several advantages, certain driver implemen-tations might have unexpected features running by default to further optimize standard 802.11 behavior. In a recent work [11], the authors provide experimental evidence of unexpected outdoor link-layer performance of 802.11 Atheros chipset based cards [1] with Madwifi driver due to a driver
Research supported in part by National Science Foundation Grant # CNS-072505.
specific antenna diversity algorithm. Hence, executing exper-iments on such wireless testbeds requires careful monitoring throughout the experiment and an awareness of unexpected
non-802.11 conforming operations of the hardware-driver.
Such behavior of the driver should either be disabled or accounted for in the eventual analysis.
Through this study we show that though MAC-frame
ag-gregation orfast framing1 is not a part of the802.11standard,
open-source drivers can and do perform frame aggregation un-der certain circumstances (besides having aggregation enabled by default) which may not lead to conforming results with scientific experiments. Through our experiments we evaluate the frame aggregation functionality that is a part of the latest Madwifi drivers and report deviation from expected results. Contributions of this study can be summarized as follows:
1) We describe frame aggregation as implemented in the Madwifi driver and show that savings in overheads are determined only by the MAC back-off and the transmission rate,
2) Describe prerequisites for aggregation on the basis of reverse engineering of the Madwifi driver source code, 3) We show that throughput, inter-frame delay, frames
transmitted and jitter vary significantly from the ex-pected value (as per the standard).
4) Finally, we present three test cases that show a signifi-cant difference in performance for scientific experiments with default frame aggregation behavior.
Section II starts by describing the basics of frame aggre-gation, followed by its quantitative analysis and conditions for its working based on the Madwifi driver. Section III presents a detailed discussion on the results from experiments of first order measurements such as throughput and inter-frame delay. Section IV shows the practical implication of frame aggregation on scientific experiments through three case studies. Finally, Section V provides the concluding remarks.
II. FRAMEAGGREGATION
Observation of internet traffic studies [22] showed that more than half of the packets were smaller than 200 bytes leading to considerable control overhead and degraded efficiency. A few studies like [18] and [26] aim to alleviate this problem by performing frame aggregation above the MAC - SAP. Other
1This term in Madwifi world is based on the fast framing support from
the ATHEROS hardware. Frame aggregation was considered as a part of 802.11eD1.0, but is not a part of the final802.11e standard.
DIFS SIFS 802.11 MAC Frame Backoff ACK DIFS SIFS 802.11 MAC Frame Backoff ACK DIFS SIFS
Aggregated MAC Frame Backoff
ACK
16 8 28 MAC LLC MAC Sub-Frame I MAC Sub-Frame II PHY
24 8 4 0
UDP/IP LLC
Aggregation MAC DATA
Destination MAC
0 Source MAC LEN 2 6 6 2 A ) B ) C ) D ) E )
Fig. 1. Frame aggregation for the creation of a lumped MAC frame which has the two aggregated frames as the payload. This approach generates savings ondifs,backoff times and physical layer headers. Subparts (A) and (B) of the figure compare timelines for transmission with and without aggregation while (C), (D), and (E) describe the payload of an aggregated frame.
approaches like [24], [15] have been proposed to perform MAC frame aggregation to improve efficiency of real-time traffic transmissions over 802.11. Another study in [14] uses MAC frame aggregation in 802.11 mobile terminals for energy
savings and an increased battery life. Recently, the 802.11n
standard [3] has incorporated frame aggregation as a built in feature. Fast Framing is a technique employed by Atheros chip manufacturers to achieve higher throughput values as an
optimization for improving 802.11a/b/g system performance
by utilizing MAC frame aggregation (allowing frame sizes up
to 3000Bytes)2,3. This paper considers Madwifi’s
implemen-tation for frame aggregation on Atheros wireless cards. This technique may or may not be used by other manufacturers and their implementation details may be different. However, popularity of this driver/hardware combination prompted us to delve deeper into its working.
A. Basics
Frame aggregation in Madwifi combines two MAC frames as shown in Figure 1 and sends them as the payload of a single congregated frame. This single aggregated frame contains two sub-frames encapsulated in a 802.11 PHY and MAC header prior to transmission. Each sub-frame is a UDP/IP payload encapsulated by the Link-Layer control header. A custom Aggregation MAC header is added by the driver to each sub-frame containing details about the source and destination MAC addresses and the length of the subsequent sub-frame in bytes. This aggregation header helps the receiver to detect aggregation and push the congregated frame as two independent frames upward through the stack.
2We noticed this feature after considerable sniffing and investigation.
Though this feature may be well known in industry, the scientific community does measurements largely unaware of this mechanism.
3Packet trace analysis softwares likeWiresharkorEtherealare not able to
identify these frames. However, a study of the source code, comparison of frame sizes, and transmissions confirm frame aggregation.
B. Performance Estimation
To determine performance gains due to frame aggregation we estimate the channel time which includes the channel access time and transmission time. The total channel time (Ttotal) using standard802.11a protocol is given as the sum of
Tbackof f+Tdif s+Tphy+Tmac+Tllc+Tdata+Tack+Tsif s.
where Tdif s, Tsif s, and Tbackof f are taken in accordance
with the 802.11a standard [2]. Time taken to transmit PHY
header/preamble Tphy and a MAC ACK Tack are fixed
ir-respective of data transmission rates. Transmission time for
the MAC header Tmac, Link-Layer control Header Tllc and
UDP/IP payload Tdata depends on the physical layer
trans-mission rate used. The total channel time for an aggregated frame will change to:
Ttotal aggr=Tbackof f +Tdif s+Tphy+Tmac+Tllc
+ 2×(Taggr mac+Tllc+Tdata) +Tack+Tsif s (1)
where Taggr mac is the time spent to transmit the Madwifi
Aggregation header. The aggregation header is smaller than a standard MAC header since most details are provided by the header of the aggregated frame. Savings with aggregation can be calculated as:
Savingswith aggr=Tbackof f+Tdif s+Tphy
+ Tmac+Tack+Tsif s−(2×Taggr mac+Tllc) (2)
Since the Taggr mac and Tllc overheads added due to
ag-gregation (as shown in Figure 1) are always lesser than the times spent in sending multiple ACKs, PHY, MAC headers, and channel contention, aggregation always yields bounded savings in channel time. It is interesting to note that this value is not determined by the frame size, rather it varies only with theTbackof f and the transmission rate. Thebackoff is decided
on the number of slots after which a node can transmit its frame is largely correlated with the contention on the channel.
The transmission rate of a frame decides theTmacand theTllc
since these headers are transmitted at the same rate as data.
Figure 2 shows the efficiency of payload delivery in802.11a
seen for two different transmission rates with and without MAC frame aggregation. The effect of PHY header, ACK transmissions and inter-frame spacing is more pronounced for higher PHY rates since the PHY headers and MAC
acknowledgements are always transmitted at6M bps.
Substi-tuting standard values, we observe that the protocol overhead reduction with frame aggregation results in savings of 90 to 370μsecs of channel time. This value varies depending upon the amount of backoff determined by contention.
C. Preconditions For Aggregation
Enabling frame aggregation (which is done by default when loading a driver) in itself does not result in aggregation of every transmitted frame. Rather, it is a decision made on a per frame basis, which depends on a certain set of conditions. Studying these conditions is crucial since they enable us to understand when we may observe aggregation and thus, attribute certain performance characteristics to it. Frame aggregation mechanism is possible when the driver senses that
36M 36M−Ag 54M 54M−Ag 0 12 24 36 48 54
Effective Channel Utilization
Payload UDP/IP Headers MAC Headers Preamble + PHY Headers MAC ACK
IFS (SIFS+DIFS)
Fig. 2. Bandwidth utilization with aggregation for different transmission rates (36M and54M in this case) of1024byteframes. Since the transmission rate of Physical layer headers are at a constant speed of6Min802.11a, the benefits of aggregation are more at higher physical layer rates.
Given:Fragmentation threshold, channel rate, transmit queue
priority.
Relevant Checks Performed:
1: if (if depth txQueue < aggregation thresh), goto
RET0.
2: if (fragmentation threshold <2346), goto RET0.
3: if (mode is neither station nor AP), goto RET0. 4: if (in ap mode() and is ether multicast), goto RET0. 5: if (fast frames are enabled and current rate is less than the minimum rate for aggregation), goto RET0.
6: if (txoplimit is defined and the time for aggregated frame
transmission>txoplimit), goto RET0.
RET1: Aggregation possible. RET0: Aggregation not possible.
Result:Boolean - Aggregation possible or not.
Fig. 3. Factors affecting decision making in the driver for MAC frame aggregation as per the Madwifi 0.9.3 driver source code.
the channel is approaching saturation. Figure 3 shows some of the relevant checks performed in the driver before a decision is
made to aggregate a MAC frame4. One of the first checks is to
determine if the user has enabled fragmentation by setting the threshold to a value lesser than the MTU. The second check ensures that MAC aggregation works only in the access point and station modes and will not affect experiments performed in the ad hoc mode. Aggregation is also disabled with multicast. Another important requirement is that the transmission
queue must have at least aggregation thresh MAC frames
ready for transmission for frame aggregation to occur. A
default value of 3is used for theaggregation threshby the
Madwifi driver. The driver also makes sure that the aggregated
frame will be transmitted within the txoplimit set for the
interface to conform with the802.11e standard. Based on this
4Information provided in the figure is based on source code for the
Madwifi-ngdrivers and may vary for implementations.
20m 20m C1 C2 S AP 20dB Attenuation Ant - 1 Ant - 2 Ant - 3 Ant - 4
Fig. 4. Experiment setup on the ORBIT radio grid. Figure indicates scaled relative position of entities for measurements on the ORBIT grid. C1and C2represent the clients sending traffic to the AP. The four noise injection antennae (Ant - *) are located at the four corners of the grid with only Ant - 1 used to pump noise at the receiver running on the AP.Sis a sniffer used for inter-frame delay measurements.
P arameter V alue
Experiment Duration 120 secs
Averaging Duration Per sec
Operation Mode 802.11a Infrastructure
Frequency 5.18GHz
Chipset Atheros AR5212
Driver MadWiFi 0.9.3
Tx Diversity Disabled
Virtual CS Disabled
Fig. 5. Common experimental parameters used with ORBIT nodes. Other parameters such as channel rate and packet sizes which may vary with experiments are mentioned explicitly.
study some simple means of disabling frame aggregation are:
by a privateioctlto disable fast framing, source code change
or by making the fragmentation threshold smaller than2346.
III. FIRSTORDERRESULTS
We begin our empirical evaluation with results that illus-trate the impact of aggregation using metrics like: saturation throughput, MAC frames, inter-frame arrival times, and their variance.
A. TestBed And Topology
We use the ORBIT testbed facility [9] which consists of
400 802.11 wireless nodes arranged in a 20m×20m grid. As shown in the Figure 4, four noise injection antennae are incorporated in the testbed that allow controlled injection of AWGN noise at desired power and frequency.
The generic topology consists of a single wireless link
running between an AP and a client C1 which acts as the
sender as shown in Figure 4. To measure performance with
contention we add another client C2 sending traffic to the
AP. Care was taken in choosing the position of the wireless nodes such that the RSSI at the receiver was similar for the different nodes to avoid PHY layer capture that could affect our results. A node was chosen to operate in monitor mode to verify the aggregation of MAC frames over the wireless link. Figure 5 depicts the generic experimental parameters used for conducting the experiments. We generate traffic using Iperf [4] internet traffic generator tool.
5 10 15 20 25 30 35 40 5 10 15 20 25 Offered Load (Mbps) Observed Throughput (Mbps) Aggregation − ContentionNo Aggregation − Contention
Aggregation − No Contention No Aggregation − No Contention Channel Saturation Triggers Aggregation (a) 64 128 256 512 1024 1400 0 5 10 15 20 25 30
Packet Sizes (Bytes) Observed Throughput (Mbps) Aggregation − ContentionNo Aggregation − Contention
Aggregation − No Contention No Aggregation − No Contention Percentage improvement in
throughput due to aggregation decreases with packet size
(b) 9M 12M 18M 24M 36M 48M 54M 5 10 15 20 25 30
PHY Channel Rate (Mbps) Observed Throughput (Mbps) Aggregation − ContentionNo Aggregation − Contention
Aggregation − No Contention No Aggregation − No Contention Higher difference in throughput
at higher rates due to aggregation
(c)
Fig. 6. (a)Observed throughput with varying offered load with and without frame aggregation. Retries due to contention are costlier with aggregation. Hence the difference in throughput with and without contention scales with aggregation. (b) Throughput with varying packet sizes with and without frame aggregation. Results show that for smaller frame sizes throughput almost doubles, while the advantages diminish for larger frames. (c) Throughput measurements for varying transmission rates. We see that benefits of aggregation are more at higher transmission rate.
5 10 15 20 25 30 35 40 500 1000 1500 2000 2500 Offered Load (Mbps) MAC Frame Transmissions Agregation − Contention
No Aggregation − Contention Aggregation − No Contenion No Aggregation − No Contention Drop in frames transmitted
due to aggregation (a) 64 128 256 512 1024 1400 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500
Packet Sizes (Bytes) MAC Frame Transmissions Aggregation − Contention
No Aggregation − Contention Aggregation − No Contention No Aggregation − No Contention (b) 9M 12M 18M 24M 36M 48M 54M 0 500 1000 1500 2000 2500 3000
PHY Channel Rate (Mbps) MAC Frame Transmissions Agregation − Contention
No Aggregation − Contention Aggregation − No Contenion No Aggregation − No Contention More frames are transmitted
with aggregation at higher rates
(c)
Fig. 7. (a)Observed MAC frames transmitted with varying offered load with and without frame aggregation. Frames transmitted in saturation with aggregation drop to almost half those without aggregation. (b) MAC frames transmitted with varying packet sizes with and without frame aggregation. Number of MAC frames transmitted drop by half for larger frame sizes, while for smaller frame sizes the reduction in transmitted frames is lesser. (c) MAC frames transmitted as a function of the transmission rate. We observe that higher number of frames are transmitted for higher rates and the savings increase significantly.
B. Throughput Measurements
Our first experiment measures the throughput performance with aggregation as a function of offered load on the link. Figure 6(a) plots the throughput measured on a single link and a link with two senders (with contention) as a function of the offered load. Packet size throughout the experiment is
main-tained at 1024bytes, transmission rate at 36Mbps, and other
experiment parameters are maintained as shown in Figure 5. Aggregation is explicitly enabled or disabled by controlling the
fast framingoption in the driver. The results show that below saturation, performance of all 4 cases is similar. However, saturation throughput settles at different values for each of the test cases as offered load is increased. Maximum throughput is observed with the case of aggregation and no contention since it sees minimum overheads. Contention reduces throughput both with and without aggregation. However, the results are different due to higher costs associated with MAC retries in aggregation. With the same experiment setup we also measure the number of MAC frames transmitted.
Figure 7(a) shows the number of frames transmitted as a function of the offered load at the sender(s). As with the throughput measurements, results below saturation from all four cases are comparable. However, as the offered load is increased further, the number of MAC frames transmitted drop to almost half with the use of aggregation. Another interesting measurement revealed that as the channel is on
the verge of being saturated we see partial aggregation.
Packet traces captured at an offered load of 25Mbps with
a channel rate of 36Mbps revealed that some frames were
being aggregated while some were not. This conforms with our earlier investigation of the driver which reveals that a decision to aggregate is made on a per frame basis. This observation is particularly important since experiments making measurements with the channel barely saturated may see varying throughput depending on the amount of aggregation.
Our next experiment measures the pattern in throughput with varying application packet sizes. Figure 6(b) plots the observed throughput with varying packet sizes and a fixed
offered load of 50Mbps. These measurements show that
throughput nearly doubles with aggregation for small frame
sizes of64bytes, while it shows comparatively lesser gains for
larger frame sizes. As proven earlier savings in overheads are constant and only determined by the backoff and transmission rate. Since the relative overheads seen for small frames are higher, we observe better performance gains for small frame sizes and vice versa. It is to be noted that for large frame sizes, even though the throughput does not double with
ag-gregation, the change is significant (approximately 25%) to
affect scientific experiments. It can also be seen that the effect of contention is seen more with large frame sizes since the cost of retransmissions is higher.
Figure 7(b) shows the number of MAC frames transmitted for different frame sizes. We observe that the number of frames
0 5 10 15 20 25 0 500 1000 1500 2000 Frame Number
Observed Interframe space (microsecs)
No Aggregation With Aggregation − Receiver With Aggregation − Sniffer
(a) 64 128 256 512 768 1024 0 0.5 1 1.5 2
Frame Size (bytes)
Mean Inter−frame Delay (milliseconds)
No Aggregation With Aggregation − Receiver With Aggregation − Sniffer
(b)
Size NoAg Ag − R Ag − S
128 5e-4 5.5e-2 7e-4
256 6e-4 12.4e-2 1.2e-4
512 8e-4 34.2e-2 1.6e-4
1024 2.1e-4 108.9e-2 4.7e-4
(*all values in msecs)
(c)
Fig. 8. (a)Observed interframe space (µsecs) between MAC frames as seen with and without aggregation for the first25MAC frames. Frame size is maintained at 1024bytes in all scenarios. (b) Mean interframe arrival times as seen at the receiver and sniffer with and without frame aggregation. (c)Variance in inter-frame arrival times without aggregation at the receiver (NoAg), with aggregation at the receiver (Ag-R), and aggregation as seen by the sniffer (Ag-S).
with the use of aggregation. As the frame size reduces, the percentage reduction in the frames transmitted decreases. As discussed previously, the savings in channel time due to aggre-gation are fixed irrespective of the frame sizes used. Hence, it is possible to transmit more frames of smaller size because of the lesser transmission times when compared to large frames. To sum up, maximum advantage with throughput (doubling) is seen with smaller frame sizes while the maximum advantage with frame sizes (halving) is seen with larger frames. Thus, these measurements with frame sizes provides an insight into the inverse relationship between how aggregation affects the throughput and frames transmitted for different frame sizes.
Transmission rate is one of the major factors for determining the effect of physical layer overheads and hence the savings achieved due to aggregation. Figure 6(c) shows the measured throughput for various transmission rates at a fixed frame size
of 1024bytes. As expected and seen through the formal
anal-ysis, benefits of aggregation are more at higher transmission rates since we see a greater effect of the savings on higher overheads. Figure 7(c) shows the number of MAC frames transmitted for different frame sizes. We see an expected trend where the number of frames transmitted with and without aggregation increase with channel rate.
C. Inter-frame Arrival Measurements
Delay measures the difference in absolute time between when a frame is transmitted and the time when it is received. However, we measure interframe arrival times since delay
measurements require explicit synchronization5. Let us
as-sume that the delay for two consecutive frames i and j is
given as Di andDj. Each Dk is evaluated as the difference
in the time stamp at the receiver Rk from the senderSk. We
can evaluate the difference in delay of two consecutive frames
i andj, given byDj−Di as:
(Rj−Sj)−(Ri−Si) = (Rj−Ri)−(Sj−Si) (3)
Hence the difference in inter-frame arrival times given by
Rdif f =Rj−Ri is evaluated as:
Rdif f = (Dj−Di) + (Sj−Si) =Ddif f+δ (4)
5Synchronization across machines is possible to a granularity of a few
msecs using the network timing protocol daemon. However, achieving higher granularity is difficult using standard tools. Delay measurements can easily replace the inter-frame arrival measurements if this issue is fixed.
where Ddif f represents difference in delay of consecutive
frames andδdenotes difference in time stamps of consecutive
frames at the sender. As long as the frames are sent at a constant rate over a single link, with static channel conditions,
δ appears as a constant value. Thus we can use difference in
inter-frame arrival times as an approximation for difference in delays of consecutive frames.
To achieve accurate measurements with considerably slow CPUs at the sniffers, all temporal measurements are made
at 9M. While absolute values of delay might change, the
observed trends only scale with transmission rate. Figure 8(a)
shows the inter-frame delay seen for 25sequential frames at
the receiver. We observe that without aggregation the
inter-frame arrival times are constant at around1msecthroughout
the experiment. It is interesting that the measurements for aggregation vary depending on the point of measurement. The arrival times at the sniffer are homogeneous since it sees continuous flow of aggregated MAC frames. However, at the receiver, after the aggregated MAC frame is received they are split and pushed up the stack. Hence the arrival time for the first frame within the aggregated frame is equal to the arrival
time of the aggregated frame which is approximately2msecs,
and that of the second frame is the time required to simply
push it up the stack which is around 2μsecs.
Similar observations can be made from the Figure 8(b) which measures the mean of inter-arrival times between frames
over approximately 25 thousand sniffed MAC frames for
different frame sizes. Results show that the mean delays are comparable for the case with no aggregation and the measurement made at the receiver. This relationship holds because even with aggregation, though the first MAC frame is received after considerable delay the second MAC frame (which is a part of the aggregated frame) is available almost instantaneously. Mean inter-arrival time with the aggregation at the sniffer is approximately double than that without aggre-gation since the sniffer essentially measures the inter-arrival time for the aggregated frame.
Figure 8(c) displays the variance in inter-arrival frame times for different frame sizes. These results are derived from measurements shown in Figure 8(a) which shows that there is little or no variance in inter-frame arrival times for the cases with no aggregation and aggregation measured at the sniffer. However, the variance is high at the receiver since the
inter-−500 −48 −46 −44 −42 −40 −38 −36 20 40 60 80 100
Injected Noise At Receiver (db)
Percentage Of Saturation Throughput (%)
1024Byte, aggregation 1024Byte, no aggregation 64Byte, aggregation 64Byte, no aggregation
Fig. 9. Performance with MAC frame aggregation in noisy environments. Results presented with emulation on an indoor setup.
9 12 18 24 36 48 54 0 5 10 15 20
PHY Channel Rate (Mbps)
VoIP Jitter (milisecs)
No Aggregation Aggregation
Fig. 10. Measurement of VOIP jitter with different channel rates with and without aggregation. Jitter significantly increases with aggregation.
frame arrival time is high for the first frame and negligible for the second frame within the aggregated frame. The sniffer which measures performance under aggregation does not see the two separate frames and hence sees a very low variance in inter-frame times. These measurements are important since they show that point of measurement (at the sniffer or receiver) would produce a dramatic difference in delay and jitter with frame aggregation.
IV. CASESTUDIES
Our case studies show the effect of MAC frame aggregation on possible scientific experiments. We present a representative set of results which are by no means exhaustive but provide an insight into possibly inaccurate measurements or inferences that could be made.
A. Case 1: Topology Creation
Wireless emulation frequently requires the creation of multi-hop topologies [8], [28], mobility emulation and emulation of non-zero packet error rate (PER) links. Creation of multihop topologies in ORBIT is supported by noise injection in four corners of the grid. Studying impact of noise injection on topology creation is important with aggregation since noise affects varying frame sizes differently.
For our experiments we run a single link with the receiver closer to a noise antenna while the sender is as far as possible. Noise injection on all other antennae (except near the receiver) is turned down by introducing attenuation. Figure 9 shows the percentage of saturation throughput seen on the link with varying levels of noise injected at the receiver. We see that for the same frame sizes, throughput falls faster with aggregation
since the PER increases with frame size. This has a particular importance with topology creation since creating topologies with noise requires injection of just the adequate amount of noise (so that it does not impact other links). However, if the frame sizes vary due to aggregation this may introduce another dimension to the topology mapping problem [16].
B. Case 2: VOIP Performance
Voice over IP (VOIP) is a protocol for voice packet delivery over IP. An interesting feature of aggregation is that it can also aggregate frames across multiple IP flows. If appropriate tagging is not used at the IP layer, real time and best effort frames could be aggregated resulting in unexpected perfor-mance. At this time, most services like Gtalk, Skype, or VLC media player do not set the TOS (type of service) byte. Hence channel saturation by running other flows such as data, or streaming video could result in aggregation of small VOIP frames with possibly large data frames. This could cause a higher variation in the observed delays with the VOIP frames. To measure performance under these conditions we setup a link between an access point running a VoIP and a data receiver and a client running the corresponding senders. VoIP
traffic was emulated using theG.711.2codec with50pkts/sec
of real time traffic and a frame size of 92(64+overheads)
bytes. For different channel rates the measured jitter is as shown in the Figure 10. As seen, the jitter is always higher for the case with aggregation. At lower rates, the jitter with
aggregation is 8 times higher than that without aggregation
and at least6.5times higher at higher channel rates. Thus, real
time traffic is severely affected by aggregation in terms of the perceived jitter. Upon sniffing the wireless link, we observed
that about33%of the total VoIP packets were aggregated with
the1400byte packets of the competing data flow.
C. Case 3: Rate Adaptation Algorithms
Performance and stability of 802.11 rate adaptation algo-rithms is widely proven through implementation on madwifi drivers [23], [20], [27], [7]. Each of these algorithms rely on channel estimation metrics such as ETT (expected
trans-mission time) [23], packet loss [20], delivery ratio [27],
throughput [7], or SINR estimates [17]. Through this study we will study the impact of aggregation on rate selection.
We setup an experiment to measure the rate selected by the sample rate adaptation algorithm [7] over a single link with maximum transmit power. We select and plot the rate selected per frame for a consecutive sample of 250 sniffed frames in Figure 11(a). Since these measurements are taken under a controlled environment, the channel conditions have been shown to be consistent and repeatable. For the same conditions we observe that results without aggregation settle
at 54M most of the time while with aggregation we see
an oscillation between 54M and 48M. This performance is
attributed to higher frame loss with aggregation and becomes clearer with results in Figure 11(b). This figure plots the distribution of frames transmitted at different rates. We observe that for different transmit powers, the rate adaptation algorithm behaves consistently better without aggregation, sending more
0 50 100 150 200 250 40
48 54
Experiment Duration (secs)
Observed Packet Tx Rate (Mbps)
Aggregation No Aggregation (a) No AG AG No AG AG No AG AG 0 0.2 0.4 0.6 0.8 1 1.2 TRANSMIT POWER
Percentage of Transmitted MAC Frames
54Mbps PHY Rate 48Mbps PHY Rate 36Mbps PHY Rate MINIMUM MAXIMUM (b) No AGAG No AGAG No AGAG 0 0.2 0.4 0.6 0.8 1 1.2
Percentage of Transmitted MAC frames
54M 48M 36M 24M 18M 12M 9M
No Noise −39 dBm −37 dBm
(c)
Fig. 11. Measurements made for performance with auto rate adaptation using the sample rate adaptation scheme that is default with the Madwifi drivers. (a)The time plot shows a random snapshot of rates of 250 consecutive transmissions. We observe that the adaptation algorithm settles comfortable with no aggregation at a rate of54M but does not work so well when aggregation is enabled. (b)Shows the distribution of the rates at which the 50K sniffed packets were sent. We observe that this number varies widely with (AG) and without aggregation (No AG) for different transmit power levels. (c) A similar measurement is made here with varying levels of injected noise at the receiver to emulate deteriorating link conditions. We see a distinct difference in rate selection with (AG) and without (No AG) aggregation.
frames at a higher rate. A comparison with varying noise at the receiver in Figure 11(c) shows a similar trend. For the same injected noise at the receiver, we observe a lower rate selection for aggregation due to a possibly higher packet error rate. Thus rate adaptation algorithms should consider aggregation while comparing results obtained with their evaluations.
V. CONCLUSIONANDFUTUREDIRECTIONS
This study highlights conditions under which MAC aggrega-tion occurs and quantifies typical performance variaaggrega-tions seen with and without aggregation using both a theoretical model and experimentation. We show that the default aggregation used in commodity 802.11 devices can produce measurements which differ from those of a properly configured controlled experiment, and may thus lead to misleading conclusions. Specifically, we observe significant differences in throughput, number of frames transmitted, sniffed inter-frames times, and inter-frame times as seen at receiver. Topology creation
becomes harder with noise injection since link PERs are
aggregation dependent. Also, for real time services such as audio and video, despite increase in throughput, jitter increases with aggregation. Future work will involve detecting clients
with non-conforming MAC behavior in802.11hotspots.
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