EVALUATING ADJACENT CHANNEL INTERFERENCE IN IEEE 802.11 NETWORKS Wee Lum Tan 1, Konstanty Bialkowski 1 1 Queensland Research Laboratory National ICT Australia Brisbane, QLD, Australia Email: {weelum.tan, konstanty.bialkowski}@nicta.com.au Marius Portmann 1,2 2 School of ITEE The University of Queensland Brisbane, QLD, Australia Email: marius@itee.uq.edu.au Abstract The performance of 802.11-based multi-channel wireless mesh networks is affected by the interference due to neighboring nodes operating on same or adjacent channels. In this paper, we have performed extensive measurements on our conducted testbed to evaluate the effects of adjacent channel interference (ACI) in 802.11 networks, under the exposed terminal and hidden terminal scenarios. By varying the path loss and the channel separation distance between two nodes, we investigate the effective attenuation needed in order to completely eliminate the ACI between the two nodes. Using node throughput as a metric, our results confirm that for low path loss between two 802.11 nodes, there still exists interference between the two nodes even though they are operating on non-overlapping channels. Our results also show that we require 37dB 45dB less attenuation to completely eliminate the ACI between two nodes operating on non-overlapping channels, compared to when both nodes are operating on the same channel. Our results are useful to network planners in terms of the placement of mesh nodes and the assignment of channels on the nodes in an 802.11-based multi-channel wireless mesh network. I. INTRODUCTION IEEE 802.11 networks operate in the unlicensed frequency band of 2.4GHz and 5GHz. The 2.4GHz frequency band is divided into 11 channels (in the USA) and 13 channels (in Europe). The center frequency of neighboring channels is spaced 5MHz apart, and each channel occupies a nominal bandwidth of 20MHz, as shown in Fig. 1. As can be seen in Fig. 1, there are at most three non-overlapping channels in the IEEE 802.11b/g 2.4GHz frequency band, i.e. channels 1, 6, and 11. It is assumed that two 802.11 nodes within transmission range of each other will not interfere with each other as long as they are operating on non-overlapping channels, i.e. the channel separation distance should be at least five channels. Conversely, two nodes that operate on adjacent channels that partially overlap (e.g. channels 1 and 2; or channels 1 and 5) will cause some degree of interference on each other. This interference is termed adjacent channel interference (ACI). In 802.11 networks, the effects of ACI can result in the following two consequences: The triggering of unintended transmission back-offs due to spurious carrier sensing between two transmitters operating on adjacent channels that overlap Channels 20MHz 1 2 3 4 5 6 7 8 9 10 11 5MHz Figure 1. IEEE 802.11 channels in the 2.4GHz ISM frequency band The corruption of frames at the receiver due to interfering signals from a nearby transmitter operating on an adjacent channel Due to these detrimental effects of ACI, works on 802.11- based multi-radio, multi-channel wireless mesh networks have only considered non-overlapping channels in their study [1], [2]. On the other hand, there have also been works [3], [4], [5] that exploited partially overlapped channels to allow more links to transmit simultaneously, in order to increase the capacity in a multi-channel mesh network. Recently, there have been studies [6], [7], [8] that showed through experiments that 802.11 nodes operating on non-overlapping channels can still interfere with each other due to the near-far effect. However, these studies and their results were obtained from wireless testbeds deployed in indoor office environments, which are readily affected by other interference sources such as external 802.11 networks, Bluetooth devices, or even microwave ovens. In addition, the amount of path loss in their experiments is crudely controlled through the limited and discrete variations of the physical distance between two nodes or links. In this paper, we evaluate the effects of ACI using a conducted testbed in which all external interferences can be controlled (to be described in Section II). In addition, we are able to control the quality of all links in the network through programmable RF signal attenuators. As such, we are able to have better control over the amount of path loss between the nodes in our experiments, compared to previous works [6], [7], [8]. We evaluate the effects of ACI under exposed terminal and hidden terminal scenarios, as shown in Fig. 2(a) and 2(b). By doing so, we can separately investigate the effects of ACI on spurious carrier sensing (using the exposed terminal scenario) and the effects of ACI on interference noise at the receiver (using the hidden terminal scenario). Our results show that for
low path loss between two 802.11 nodes, there still exists interference between the two nodes even though they are operating on non-overlapping channels. As expected, when we increase the amount of path loss and the channel separation distance between two 802.11 nodes, the effect of ACI decreases correspondingly. Our results also show that we require 37dB 45dB less attenuation to completely eliminate the ACI between two nodes operating on non-overlapping channels, compared to when both nodes are operating on the same channel. The remainder of this paper is organized as follows. We describe our measurement setup and methodology in Section II; present and discuss our measurement results in Section III; and summarize our key findings and conclude this paper in Section IV. II. EXPERIMENTAL SETUP Our conducted testbed consists of four IEEE 802.11b/g nodes which use the Atheros AR5001X chipset. The 802.11 nodes are connected via co-axial cables and a programmable RF signal attenuator, as shown in Fig. 2. In addition, each 802.11 node is placed in an RF shielded box, which is able to provide 85dB of isolation. This ensures that RF signal propagation between the nodes occurs only along the co-axial cables, and that our nodes are not affected by any external interference. The transmission power of each node is set to its default value of 19dBm. Links are established between each 802.11 node, by using a signal splitter/combiner. In our testbed, all devices use standard SMA connections, and are matched to 50-Ohms to ensure reduced reflections. The RF signal attenuator (JFW 50P-1708-SMA) allows for attenuation level stepping between 0dB and 63dB (with 1dB step size), for RF signals with a frequency range between 0.1-6GHz. In our experiments, we also initially added a 10dB fixed attenuator, and then replace it with a 50dB fixed attenuator, into our testbed. Coupled with the additional losses of 13dB due to the co-axial cable, splitter and connectors, we are then able to control the amount of path loss in our testbed to be within the range of 23dB 126dB. These two extremes of path loss correspond to a perfect link and a completely disconnected link. In this paper, we evaluate the effects of ACI under the exposed terminal [9] and hidden terminal [10] scenarios, as shown in Fig. 2. By increasing the value on the RF signal attenuator from 0dB to 63dB (as well as replacing the 10dB fixed attenuator with a 50dB fixed attenuator), we are effectively decreasing the amount of carrier-sensing between sender nodes S1 and S2, in the exposed terminal scenario decreasing the amount of interference from interferer sender S2 on receiver node R1, in the hidden terminal scenario In our testbed, the effects of the increased attenuation on the link between the two nodes are intended to emulate the fading RF signal as the physical distance between two nodes is increased. R1 S1 S2 R2 S1 R1 S2 R2 Figure 2. Exposed terminal and hidden terminal measurement setup TABLE I. MEASUREMENT PARAMETERS Data Flow S1 R1 Link rate 54 Mbps Channel 2 Interfering Flow S2 R2 Link Rate 54 Mbps Channel 2 11 Signal enuator 0 63 db Table I shows the parameters used in our measurements. All our nodes are running Linux kernel version 2.6.28, and MadWifi 1 version 0.9.4. We use Iperf 2 to send UDP traffic (packet size 1448 bytes) from node S1 to node R1, and from node S2 to node R2. In our measurements, both links S1-R1 and S2-R2 are continuously transmitting data packets, i.e. the links are saturated. The Request-to-Send (RTS) and Clear-to- Send (CTS) mechanism is turned off. The link between S1-R1 is fixed at channel 2, while the link between S2-R2 is varied from channels 2-11. The link rate of both the S1-R1 link and the S2-R2 link is fixed at 54Mbps. In all cases, the links autorate feature is turned off. All our results are averaged over more than 32,000 packets in a measurement period of 20 seconds. III. (a) = RF signal attenuator RESULTS AND DISCUSSION As a reference, we have conducted an experiment with just one active data link (using S1-R1), saturated with UDP traffic at the link rate 54Mbps. From this experiment, we determined that the maximum data throughput achievable is about 32-33Mbps. This throughput value would serve as an indicator of when the effective attenuation on the RF signal attenuator is enough to eliminate all ACI in the exposed terminal and hidden terminal scenarios. 1 http://madwifi-project.org 2 http://dast.nlanr.net/projects/iperf Exposed Terminal (b) Hidden Terminal
A. Exposed Terminal Results Fig. 3 shows the saturated throughput results on data link S1-R1 and interfering link S2-R2 in the exposed terminal scenario, with the interfering link s operating channel varied on channels 2-8. The results achieved when the interfering link is operating on channels 9-11 are similar to that of channel 8, and thus are not shown. The total throughput graph represents the sum of the individual throughputs achieved on the data and interfering links. In Fig. 3, we see that when the effective path loss between the sender nodes S1 and S2 is low (in the range between 23dB 58dB), both nodes will suffer from spurious carrier sensing (due to the ACI), even though the nodes are operating on nonoverlapping channels. This is evidenced by the low throughput achieved by both the data and interfering links. These results emphasize the importance of proper planning in the design of a multi-radio mesh router in terms of antenna selection, antenna separation distances, and the radio transmit powers. Fig. 3 also shows that there seems to be unfair sharing of the wireless medium bandwidth between both links when the effective path loss is low. However, we observe from the total throughput graph that the sum of the individual throughput values from the data and interfering links is approximately 32-33Mbps, which is the same as the maximum data throughput that can be achieved on a saturated data link. We also note that as the effective path loss increases, the effects of ACI and spurious carrier sensing decreases correspondingly. This can be seen from the throughput values on the total throughput graph, which steadily increases and finally reaches the total maximum data throughput of 64-65Mbps after the effective path loss exceeds a certain threshold value. This threshold value is the effective path loss value required to completely eliminate the ACI and spurious carrier sensing between the data and interfering links. Fig. 4 show the threshold path loss values as we vary the channel separation distance between the data and interfering links. As expected, the threshold path loss value decreases for increasing channel separation distance. Furthermore, the threshold path loss value required to completely eliminate the ACI between two nodes operating on non-overlapping channels (i.e. five channels apart) is 37dB less compared to when both nodes are operating on the same channel. We also see that when the channel separation distance is three channels or less, the threshold path loss value is within the small range between 100dB 105dB. Conversely, when the channel separation distance is five channels or more, the threshold path loss value ranges from 60dB 68dB. This suggests that when assigning channels on neighboring links in a multi-channel wireless mesh network, it is not practical to assign channels with a separation distance of three channels or less as the amount of ACI would be too prohibitive to overcome. On the other hand, if we were to assign non-overlapping channels, our results suggest that assigning channels with a separation distance of five channels would be approximately as good as assigning channels with a separation distance of six channels or more. In addition, the results in Fig. 4 show that we may be able to utilize channels with a separation distance of four channels (i.e. partially overlapped channels) as long as the actual physical distance between the two neighboring links are large enough to render the ACI negligible. This will be advantageous in terms of improving the spectrum utilization and increasing the throughput in a multi-channel wireless mesh network. Figure 3. Saturated throughput results on data and interfering links in exposed terminal scenario
Figure 4. The required threshold path loss to completely eliminate ACI in exposed terminal scenario B. Hidden Terminal Results Fig. 5 shows the throughput and packet loss results on the saturated data link S1-R1 in the hidden terminal scenario, with the interfering link s operating channel varied on channels 2-8. Similar to Section III.A, the results achieved when the interfering link is operating on channels 9-11 are similar to that of channel 8, and thus are not shown. We see that when the effective path loss between the interferer sender node S2 and the receiver node R1 is low (in the range between 23dB 45dB), the hidden terminal interference from the interferer sender node S2 caused the effective throughput on data link S1-R1 to be as low as 5Mbps with a 30% packet loss ratio, even though the data and interfering links are operating on nonoverlapping channels. However, we note that as the effective path loss and the channel separation distance between the data and interfering links increases, the effects of ACI and hidden terminal interference on the receiver node R1 decreases correspondingly. This is shown by the increasing throughput and lower packet loss ratio that are achieved on the data link. We also plot the threshold path loss value (i.e. the effective path loss value required to completely eliminate the hidden terminal interference of sender node S2 on receiver node R1) as we vary the channel separation distance between the data and interfering links. In Fig. 6, we see that the threshold path loss value required to completely eliminate the ACI between two nodes operating on non-overlapping channels (i.e. five channels apart) is 45dB less compared to when both nodes are operating on the same channel. Compared to Fig. 4, we note that the threshold path loss values in Fig. 4 are higher by about 10 15dB. This is because a node s carrier-sense sensitivity is higher compared to the SINR (signal-to-interference-andnoise-ratio) threshold required for correct reception of frames. Figure 6. The required threshold path loss to completely eliminate ACI in hidden terminal scenario Figure 5. Throughput and packet loss results on saturated data link in hidden terminal scenario IV. CONCLUSIONS We have evaluated the effects of adjacent channel interference (ACI) under exposed terminal and hidden terminal scenarios. Our results show that for low path loss between two
802.11 nodes, there still exists interference between the two nodes even though they are operating on non-overlapping channels. As expected, when we increase the amount of path loss and the channel separation distance between two 802.11 nodes, the effect of ACI decreases correspondingly. Our results also show that we require 37dB 45dB less attenuation to completely eliminate the ACI between two nodes operating on non-overlapping channels, compared to when both nodes are operating on the same channel. In addition, our results show that we may be able to assign partially overlapped channels (with a channel separation distance of four channels) to two neighboring links as long as the actual physical distance between the two links are large enough to render the ACI negligible. This will be advantageous in terms of improving the spectrum utilization and increasing the throughput in a multichannel wireless mesh network. Finally, our results are useful to network planners in terms of the placement of mesh nodes and the assignment of channels on the nodes in an 802.11- based multi-channel wireless mesh network. ACKNOWLEDGMENT National ICT Australia is funded by the Australian Government as represented by the Department of Broadband, Communications and the Digital Economy and the Australian Research Council through the ICT Centre of Excellence program; and the Queensland Government. REFERENCES [1] M. Alicherry, R. Bhatia, and Li Li, Joint Channel Assignment and Routing for Throughput Optimization in Multi-radio Wireless Mesh Networks, ACM MobiCom, 2005. [2] M. Kodialam and T. Nandagopal, Characterizing the Capacity Region in Multi-radio, Multi-channel Wireless Mesh Networks, ACM MobiCom, 2005. [3] A. Mishra, V. Shrivastava, S. Banerjee, and W. Arbaugh, Partially Overlapped Channels Not Considered Harmful, ACM SIGMetrics, 2006. [4] A. Mishra, E. Rozner, S. Banerjee, and W. Arbaugh, Exploiting Partially Overlapping Channels in Wireless Networks: Turning a Peril into an Advantage, ACM/USENIX Internet Measurement Conference, 2005. [5] H. Liu, H. Yu, X. Liu, C-N. Chuah, and P. Mohapatra, Scheduling Multiple Partially Overlapped Channels in Wireless Mesh Networks, IEEE ICC, 2007. [6] P. Fuxjager, D. Valerio, and F. Ricciato, The Myth of Non-Overlapping Channels: Interference Measurements in IEEE 802.11, Wireless on Demand Network Systems and Services (WONS), 2007. [7] J. Nachtigall, A. Zubow, and J-P. Redlich, The Impact of Adjacent Channel Interference in Multi-Radio Systems using IEEE 802.11, IEEE IWCMC, 2008. [8] P. Li, N. Scalabrino, Y. Fang, E. Gregori, and I. Chlamtac, Channel Interference in IEEE 802.11b Systems, IEEE GlobeCom, 2007. [9] P. Karn, MACA A New Channel Access Method for Packet Radio, ARRL/CRRL Amateur Radio 9th Computer Networking Conference, 1990. [10] F. Tobagi and L. Kleinrock, Packet Switching in Radio Channels: Part II The Hidden Terminal Problem in Carrier Sense Multiple-Access and The Busy-Tone Solution, IEEE Transactions on Communications, vol. 23, no. 12, pp. 1417 1433, 1975.