Why a Multichannel Protocol can boost IEEE 802.11 ... - CiteSeerX

Why a Multichannel Protocol can boost IEEE 802.11 Performance Andrea Baiocchi andrea.baiocchi@ uniroma1.it

Alfredo Todini alfredo@ net.infocom.uniroma1.it

Andrea Valletta valletta@ net.infocom.uniroma1.it

INFOCOM Department University of Roma “La Sapienza” Via Eudossiana, 38 00184 - Roma, Italy

ABSTRACT We analyse a CSMA MAC protocol for ad hoc wireless networks, that uses one control channel and a number of data channels. The data channel employed in each transmission is dynamically selected with an exchange of frames on the control channel. We present simulation results obtained with both the multichannel protocol and the IEEE 802.11 MAC protocol under different scenarios. We show that the multichannel protocol performs better than the single channel MAC under certain conditions, since the presence of hidden nodes has a smaller impact on its performance; we argue that this is mainly due to the separation operated between the control frames and the data frames.

Categories and Subject Descriptors C.2.1 [Computer-Communication Networks]: Network Architecture and Design—Wireless communication

General Terms Performance

Keywords

The most widespread WLAN standard, IEEE 802.11, uses channelized bandwidth, where MAC operation takes place within a given channel in a Basic Service Set (BSS) according to a CSMA-like protocol. In general, multiple access entails a time division component (possibly with random access). A capacity increase can be achieved by simultaneously using a number of different frequency channels, thus aiming at a distributed dynamic channel allocation. A number of works have addressed the multichannel paradigm, often as applied to the 802.11 WLANs [3][4][5][6][7]. Different multichannel protocols have been proposed and extensive simulations have been presented. It appears that a multichannel approach can yield very significant performance gains, but, to the best of our knowledge, no thorough analysis of the fundamental causes of such gains has been laid out. Many different simulation models have been used; a comparison among results of different works is therefore not feasible. This motivated us to perform a systematic analysis of the capacity attained by an 802.11-like multichannel MAC protocol. The whole system bandwidth is assumed to be divided into a control channel for RTS-CTS frames and equal sized data channels for data and ACK frames. We refer to an ad hoc network (i.e., without fixed infrastructure). The major results of this work are:

IEEE 802.11, MAC, collisions, hidden nodes, multichannel ad hoc networks

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1. the key to any performance improvement is the presence of hidden nodes: not all the stations can correctly receive and decode the information sent by other stations;

INTRODUCTION

Two major challenges in the evolution of wireless networks are the move towards packet oriented communications and an increase in the offered capacity well beyond that promised by UMTS. Wireless LANs (WLANs) give a partial response to these communication needs and this motivates a large number of works addressing improvements and enhancements of current WLAN standards.

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2. the performance gains strongly depend on the extent to which nearby stations interfere with one another even though they cannot communicate correctly1 . The distance among stations and the values of the defer and carrier detect thresholds [12] appear to have a strong influence. In general, a given station A can successfully transmit to nC other stations (out of the n stations making up the entire BSS); however, its transmissions do actually cause interference to nI ≥ nC stations, even though the most distant ones cannot correctly decode the transmitted information. In other words, nC stations can correcty receive A’s signals; other nI − nC stations cannot correctly receive 1

Some stations are out of the range of a given one as to a correct information decoding, yet they disturb the given station in a non-negligible way.

such signals, but their ongoing receptions can be disrupted by the interference coming from A. We found that multichannel based MAC protocols do not bring about any capacity gain if nC = n. In this case (named “full visibility” in the following) the best approach is to keep all communications within a single channel, exploiting the whole available bandwidth. On the other hand, performance gains become evident when nC  nI  n. The other very interesting finding is that the improvement is due more to the presence of a separate control channel (reserved to RTS and CTS packets) than to a large number of data channels. The optimization of the number of data channels is clearly a second order improvement on the overall system capacity; most of the gain is obtained by simply separating the signalling from the data traffic. The rest of the paper is so organized. In Section 2 the related work is discussed in detail to summarize the state of the art of the MAC level capacity evaluation of the multichannel paradigm. Section 3 gives a detailed description of the multichannel protocol analysed in this work. The simulation model and some performance results are presented in Section 4. Section 5 draws the conclusions.

in [5]. The authors adapt the IEEE 802.11 MAC layer to operate with multiple channels. One channel is used for control messages, the others for data. The sender includes in the RTS frames the list of free data channels, based on signal power measurements. The receiver then selects the channel on which it wants the communication to take place, and it sends this information on a CTS. The data packet, and its acknowledgment, are then sent on the selected channel. The authors show, through simulations, that this protocol performs better than the single channel 802.11 MAC, even in the case of fixed total bandwidth. Similar protocols are described in [6], [7]. The cited papers give many simulation results, which show a substantial improvement in the performance of a multichannel WLAN compared to a single channel one in the case of fixed aggregated bandwidth. In our view, however, they do not give a convincing explanation of this improvement; in fact, the results presented are obtained in a single, usually very complex, scenario, and the performance of the protocols in different situations is not analysed. We started from a protocol similar to the ones found in the literature, but we tried to study its behaviour more sistematically.

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3. DESCRIPTION OF THE PROTOCOL

MOTIVATIONS AND RELATED WORK

The idea of employing multiple channels with CSMAbased protocols is not new. In fact, in [1] multichannel CSMA protocols are shown to perform better than singlechannel ones, in the case of wired LANs. This is due to the reduction in the normalized propagation delay, which in turn reduces the collision probability. In wireless networks the normalized propagation delay is very small, this effect is therefore not relevant. There are, however, a number of reasons which make it worthwhile to investigate multichannel protocols for wireless LANs. Firstly, multiple channels could simply be a way to make use of additional bandwidth, thus increasing the global throughput of a network. For example, the IEEE 802.11b physical layer has 14 channels, 3 of which are orthogonal and thus available for concurrent use. Instead of statically assigning different frequencies to separate BSSs, one could use the additional channels to create a single BSS of higher capacity. Thus the allocation of the channels would be done dynamically and in a distributed way. A second motivation for the use of multiple channels is that they could provide some performance improvements with respect to a single-channel CSMA even in the case of fixed aggregated bandwidth. The idea is that more channels, by allowing concurrent transmissions, could reduce the number of collisions, and bring about a more efficient utilization of the bandwidth. Moreover, the hidden terminal problem, which seriously affects the performance of wireless LANs, could be relieved by an appropriate allocation procedure. A protocol for wireless LANs, in which each node listens to a unique channel, is proposed in [2]. A different kind of protocol, called multichannel CSMA, is proposed in [3]. The number of channels is fixed and much smaller than the number of nodes, and a terminal can transmit or receive on any channel. However, this protocol does not solve the hidden terminal problem. In [4] the protocol is extended to select the best channel based on the signal power received at the sender. An RTS/CTS-like reservation mechanism is introduced

The protocol studied in this paper derives from the IEEE 802.11 DCF with RTS/CTS. The main difference is that it adopts a multichannel scheme: the total bandwidth W is divided into one control channel (with bandwidth Wc ), used to carry RTS and CTS frames, and N data channels each with a bandwidth (W − Wc )/N used to carry data packets and acknowledgements. We assume that each node is able to do carrier sensing on the control channel and on all data channels simultaneously. We also assume that each node is equipped with two receivers, one for the control channel and one for the data channels, which could then “scan” over multiple channels in sequence. If this is done quickly enough, we can consider it to be simultaneous carrier sensing. We also assume that each node, using the couple of receivers, is able to receive packets on the control channel while it is receiving other packets on a data channel. However, a node cannot simultaneously receive multiple data packets; moreover, a trasmitting node is not able to receive other transmissions. As in 802.11, a node tracks the state of each channel with a timer called NAV (Network Allocation Vector); the NAV for a channel is started when the node receives control frames coming from other nodes, signalling that a transmission of a given duration will take place on the stated channel. Thus, physical carrier sensing is supplemented with a virtual carrier sensing. In the case of the single channel protocol, if in a given area the channel is free, each node in that area is able to receive packets. In the case of the multi-channel, instead, a channel could be free, but some nodes in the area may be trasmitting or receiving on a different channel; thus, they could be unable to receive. Therefore, when a node sends an RTS packet, it may find the destination busy; after failing to receive a CTS it would restart the backoff, doubling the contention window even if a collision did not occur. To avoid this we implemented some timers that provide information on the state of nodes, in the same way as the NAVs do for channel allocation. 1. A node A that wants to transmit checks that the in-

tended destination is free (it uses the list of timers just described, see also point 6), and it builds a list of idle data channels, on the basis of physical and virtual carrier sensing. The control channel is accessed in the same way as in 802.11; if there is at least one idle data channel, the node sends to the destination an RTS frame, which includes the list of free channels. 2. If, instead, the destination is busy or there are no free data channels, the node reenters the backoff procedure without doubling the contention window, since no collision has occurred. 3. When the destination node B receives the RTS packet, it selects a data channel among those present in its idle channel list and in the list included in the received RTS. If it finds a free channel, after a SIFS it sends on the control channel a CTS frame, including the index of the data channel to use in the subsequent transmission. If no idle data channel is found, node B does not send the CTS frame. 4. Other nodes receiving the RTS frame defer their transmissions on the control channel only for the duration of the ongoing RTS/CTS exchange; the data frame will be transmitted on a data channel, so the control channel will be idle after the CTS. 5. When node A receives the CTS it switches to the assigned channel and, after a SIFS, it starts to transmit the data packet. 6. Other nodes receiving the CTS set the NAV of the channel indicated in the CTS according to the duration field of the CTS, which is the duration of the subsequent data/ACK exchange. They also set the timers indicating that the nodes involved in the transmission exchange are busy2 . 7. After a SIFS, if node B has correctly received the packet, it transmits the ACK on the same data channel. It is important to note that in this protocol the virtual carrier sensing is less efficient than in the single-channel 802.11. In fact in 802.11 a node sets the NAV timer when it receives an RTS or a CTS packet. In our protocol only CTS packets can be used for this purpose, since RTS packets do not carry the index of a single channel, but a list of idle channels. This could be corrected by having the sending node reply to the CTS with a third control frame, in order to inform its neighbours of the data channel on which the subsequent transmission will take place. We chose not to implement this because we wanted to modify the 802.11 MAC layer as little as possible; moreover, a third frame would add overhead to the protocol. Notice also that, according to our assumptions, when a node is transmitting it cannot hear the packets exchanged on the control channel, so it will lose some information on the allocation of data channels. This is the price to pay for the simplicity of the protocol and of the terminals. 2 These timers are updated on the basis of the CTS frames, as is the case for the NAVs. The information carried by RTS frames is not used to update the timers because the RTS frame may collide, or the destination node may not be able to find a free channel; in this case both nodes would immediately become idle after the RTS.

4. PERFORMANCE EVALUATION 4.1 Simulation model The results shown in the following sections have been obtained with a multichannel MAC module developed by us for the ns-2.26 network simulator [8]; this module has been derived from the 802.11 MAC as implemented in ns2. A two-ray ground reflection radio propagation model has been adopted. We employ a simplified physical model, in which an interfering signal disrupts an ongoing reception with probability 1 if the interference power level is higher than a fixed threshold, and with probability 0 otherwise. With the ns standard values for the transmission power, carrier detect threshold and defer threshold we obtain that each transmission can be correctly received from a distance of up to 250 meters, while it can be heard and it can interfere with an ongoing reception from a distance of up to 550 meters. This model approximates a WaveLan radio interface operating at 2 Mbps. In all our simulations nodes are located on a square grid. To avoid edge effects, each side of the grid is seen as adjacent to its opposite side, thus obtaining a virtually infinite grid. In other words, in our model the terminals are laid over a torus-shaped surface. Traffic is generated according to independent Poisson processes, with mean interarrival time identical for all nodes. For each packet, one of the eight neighbouring nodes in the grid is randomly chosen as the destination. A fixed packet size of 1500 bytes is used for all simulations. We assume that channels are perfectly orthogonal to each other, so a transmission occuring on one channel does not disturb transmissions on other channels. We simulate a fixed aggregate bandwidth scenario, i.e. for the multichannel case we divide the same bandwidth used for the single channel among the multiple channels. We find that this scenario is more interesting as it allows us to find whether there are inherent performance gains due to the adoption of the new protocol. In order for the comparison between the single channel and the multi channel protocols to be fair, we must remove all the limitations introduced in the IEEE 802.11 standard in order to guarantee compatibility among terminals operating at different rates. This means that in the single channel case we use a rate of 2 Mbps for all transmissions, including the PLCP preamble and header which, according to the IEEE specifications, should be transmitted at a rate of 1 Mbps. In the multichannel case, the data rate used is always the maximum allowed in each channel (for both control and data channels).

4.2 Full visibility scenario We start from a scenario in which 100 nodes are placed on a 10x10 grid, with a grid spacing of 10 meters; in this way each node is within the transmission range of every other node; there are no hidden nodes. Figure 1 shows the mean throughput per node vs the offered load per node, for both the single channel and the multichannel MACs. In the multichannel case, the simulations have been performed in the case of four data channels; 20% of the bandwidth has been allocated to the control channel. Both the throughput and the load have been normalized to the total available data rate, i.e. 2Mbps. We can see that, in this scenario, no performance advantage comes from the adoption

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the throughput in saturation vs the grid distance, in a scenario with 256 nodes on a 16x16 grid. As usual, the area over which the terminals are arranged is modelled as torusshaped. We notice that, when the grid distance is such that the interference from a transmission does no longer reach all the other nodes (d ≥ 55 metres, with our assumptions), the throughput initially decreases, then it starts increasing thanks to channel reuse. At this point, the performance of the multichannel protocol becomes better than the performance of the single channel MAC.

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Figure 2: Throughput per node, with a grid distance of 175m To investigate what is happening, in Figure 3 we plot

Figure 3: Throughput per node vs grid distance The fact that channel reuse is not effective at first is due to the inefficiency of the RTS/CTS procedure under these conditions. In fact, it is possible that two nodes A and B are not able to successfully receive the RTS and the CTS frames transmitted by another couple of nodes, C and D (i.e., they are not within the transmission range of C and D); moreover, they may not be able to detect an ongoing transmission between C and D, if they cannot detect the interference. Consider, in the case of the standard 802.11 single channel protocol, the example in Figure 4, in which a data packet is being sent from D to C; nodes A and B are out of the transmission range of both C and D, thus they cannot have correctly received the RTS and CTS frames exchanged before the data packet was sent. Moreover, A and B are both out of the interference range of D, so they cannot sense the ongoing transmission. Node B, therefore, can send an RTS to A; however, C is in the interference range of B, so the RTS disrupts the reception of the data frame in C. In this case, the RTS/CTS protocol effectively fails to solve the problem of hidden nodes. While in the case of the single channel protocol data packets can collide both with other data packets and with RTS or CTS control frames, with our multichannel protocol data packets are protected from collisions with RTS/CTS frames, which are transmitted on the control channel. Data packets can collide only with other data packets or with ACKs and this can happen only if both source and destination of the transmission are unaware that another transmission was taking place (a data packet is transmitted only after the RTS/CTS exchange) and only if the same channel is selected. This is the case of Figure 4 where a collision between

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Figure 4: Illustration of how a data packet can be interfered two data packets could happen even with the multichannel protocol since both nodes A and B are unaware that D is trasmitting a data packet to C. In the case of the standard 802.11 MAC, instead, the collision can also take place if only the sender of the RTS is unaware of the ongoing transmission. See, for example, Figure 5; B, which is unaware of the ongoing transmission of a data packet from D to C starts sending an RTS to A, thus disrupting the reception at C. With the multichannel protocol, the RTS would not collide with the data packet, as they are sent on different channels; the data packet from B to A, if sent, would use a different data channel, as A would sense the transmission from D, and would not select the channel being used by D.

Figure 5: Illustration of how a data packet can be interfered by an RTS The root of the problem lies in the fact that the interference range of a station, i.e. the range within which other stations’ transmissions will be disrupted by the signal of the given station, is much larger than the transmission range. This is confirmed in Figure 6, in which we reduce the interference range to coincide with the transmission range; in this case the two protocols perform almost identically. As confirmation that the multichannel MAC performs better than the single channel because fewer collisions occur, in Figure 7 we graph the percentage of collided RTS and data packets vs the offered load for the same scenario of Figure 23 . We 3

Note that once saturation is reached, every node always

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Figure 6: Throughput per node, with a grid distance of 175m and an interference range coinciding with the transmission range see that the multichannel protocol is particularly effective at reducing the percentage of collided data packets. The percentages of collided data packets have also been used to label the points in Figure 3. First, we can see that for d < 55 metres there are no collisions involving data packets: the RTS/CTS protocol works correctly. For longer distances, when the physical carrier sensing is insufficient since the interference of a transmission does not reach all nodes, collisions involving data packets start to occur; in the case of a single channel their frequency is much higher than in the case of multiple channels. The implication of this is that the main reason behind the improvement in the performance of the multichannel MAC with respect to the single channel MAC does not lie in the number of data channels, but simply in the separation operated between signalling (the RTS/CTS frames sent on the control channel) and data. This is confirmed by Figure 2, which shows that even the multichannel protocol with a single data channel performs much better than the single channel MAC. The multichannel MAC operates best with a higher number of data channels; with a single channel there are inefficiences, e.g. during an RTS/CTS exchange the entire bandwidth devoted to data is unutilized. However the optimization of the number of channels brings about a marginal performance gain as compared to the improvement due to control and data channel separation.

5. CONCLUSION AND FUTURE WORK We have shown that a multichannel MAC protocol brings about a substantial performance improvement in terms of throughput with respect to the single channel MAC; however, this improvement is mainly due to the fact that the separation between RTS/CTS frames and data frames, operated by our protocol, alleviates the problems (high risk of collisions involving data packets) due to the large interference area. has a packet to send, and the total number of transmitted packets does no longer increase with a further increase of the offered traffic; thus, the percentage of collided packets also stays the same.

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Figure 7: Percentage of collisions vs load per node The issue of the interference area has been often neglected, but there are now a number of studies investigating it and offering solutions ([9], [10], [11]); to the best of our knowledge, however, this is the first time that an organization of the bandwidth in multiple channels has been explicitly proposed as a solution to this problem. In the future we intend to analyse the proposed protocol with a more realistic interference model. We will study the extension of the multichannel protocol to the case of a non-fixed number of non-orthogonal channels with variable capacity (this is the case of a CDMA-based physical layer). We will also investigate the possibility of finding an analytic expression for the saturation capacity achieved by the single channel and multichannel protocols in the presence of hidden nodes.

[6]

[7]

[8] [9]

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ACKNOWLEDGMENTS

This research has been funded by the Italian Ministry of Education, Research and University (MIUR) under the FIRB national project PRIMO.

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REFERENCES

[1] M. Ajmone Marsan and D. Roffinella, Multichannel Local Area Network Protocols, IEEE Journal on Selected Areas of Communication, 1983 [2] A. Muir and J. J. Garcia-Luna-Aceves, A channel access protocol for multihop wireless networks with multiple channels, IEEE International Conference on Communications, June 1998 [3] A. Nasipuri, J. Zhuang and S. R. Das, A Multichannel CSMA MAC Protocol for Multihop Wireless Networks, Proc. of IEEE Wireless Communications and Networking Conference (WCNC’99), September 1999. [4] A. Nasipuri and S. R. Das, Multichannel CSMA with Signal Power-based Channel Selection for Multihop Wireless Networks, IEEE Vehicular Tecnology Conference (VTC), September 2000 [5] S.-L. Wu, C.-Y. Lin, Y.-C. Tseng and J.-P. Sheu, A New Multi-Channel MAC Protocol with On-Demand Channel Assignment for Multi-Hop Mobile Ad Hoc

[10]

[11]

[12]

Networks, IEEE Wireless Communications and Networking Conference (WCNC), September 2000 N. Jain, S. R. Das and A. Nasipuri, A Multichannel CSMA MAC Protocol with Receiver-Based Channel Selection for Multihop Wireless Networks, Proc. of the 9th Int. Conf. on Computer Communications and Networks (IC3N), October 2001 Jiandong Li, Z. J. Haas, Min Sheng and Yanhui Chen, Performance evaluation of modified IEEE 802.11 MAC for multi-channel multi-hop ad hoc network, 17th International Conference on Advanced Information Networking and Applications (AINA), March 2003 VINT Group, UCB/LBNL/VINT network simulator ns (version 2.26) http://www.isi.edu/nsnam/ns. J. Li, C. Blake, D. Couto, H. Lee and L. Morris, Capacity of Ad-Hoc Wireless Networks, Proc. of ACM MobiCom 2001, Rome, July 2001 S. Xu and T. Saadawi, Does the IEEE 802.11 MAC Protocol work well in Multihop Wireless Ad Hoc Networks?, IEEE Communications Magazine, Volume 39 Issue 6, June 2001 K. Xu, M. Gerla, S. Bae, How effective is the IEEE 802.11 RTS/CTS Handshake in Ad Hoc Networks?, IEEE Global Telecommunications Conference, 2002 (GLOBECOM ’02), November 2002 ISO/IEC 8802-11; ANSI/IEEE Std 802.11, 1999 edn Information technology - telecommunications and information exchange between systems - local and metropolitan area networks - specific requirements. Part 11: wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications