Multichannel CSMA with Signal Power-Based Channel Selection for Multihop Wireless Networks Asis Nasipuri Division of Computer Science The University of Texas at San Antonio San Antonio, TX 78249-0667 E-mail:
[email protected]
Samir R. Das Department of Electrical & Computer Engineering and Computer Science University of Cincinnati Cincinnati, OH 45221-0030 E-mail:
[email protected]
Abstract We describe a new carrier–sense multiple access (CSMA) protocol for multihop wireless networks, using multiple channels and a distributed channel selection scheme. The proposed protocol divides the available bandwidth into N channels where the transmitting station selects an appropriate channel for packet transmission. The selection criterion is based on the interference power measurements on the channels. We show via simulations that this multichannel CSMA protocol provides a higher throughput compared to its single channel counterpart by reducing the packet loss due to collisions.
1 Introduction We consider multihop wireless networks without fixed base stations or any wireline backbone infrastructure. A mobile terminal (node) can exchange data packets directly with another node if it is located within its radio range. If the intended destination is located outside its radio range, packets are relayed via intermediate nodes located between the two nodes. The nodes can be mobile and thus the network topology can change dynamically. In literature, terms such as packet radio networks or ad hoc networks have also been used to describe such networks. Such networks are very useful in military, law enforcement, emergency rescue or exploration missions, and other applications where cellular infrastructure is unavailable or not cost-effective. There is considerable interest in using ad hoc networks in commercial applications as well where there is a need for ubiquitous communication services without the presence of a fixed infrastructure. The medium access control (MAC) protocol, which allows the nodes to share the radio medium is a key issue that
determines the performance of a packet radio network. A major concern in designing the MAC protocol for such networks is that traditional contention resolution methods that are used in wired networks, such as carrier sense multiple access (CSMA) [9], are not very effective in the wireless channel. This is due to the fact that propagation path losses in the wireless medium cause the signal power to vary with distance and hence the same signal is not heard equally by all nodes, which is required in CSMA. Hence the carrier sensed at the source node cannot correctly assess the interference level at the receiver site. This gives rise to effects such as the hidden and exposed terminal problems [18] which are the major causes of throughput degradation in wireless CSMA. The basic channel access method recommended by the IEEE 802.11 standard [4] known as carriersense multiple access with collision avoidance (CSMA/CA) also suffers from these problems. Several solutions to these problems have been suggested in recent years. The busy tone multiple access (BTMA) for systems with a base-station [18] and its counterpart for multihop networks, the dual busy tone multiple access (DBTMA) [3], propose to solve the problem by the transmission of tones on separate channels to indicate the state of the channel. This method requires the additional complexity of narrowband tone detection and the use of separate channels. A receiver initiated busy tone protocol that requires the transmission of a “request-to-send” control packet from the source followed by a busy tone from the destination, was suggested in [19]. Many other solutions have been proposed to perform a “virtual” carrier sensing at the receiver from the source node. The multiple access with collision avoidance (MACA) protocol [8] proposes that the source and destination nodes complete an exchange of Request-to-Send (RTS) and Clear-to-Send (CTS) packets before the data packet is sent. If the source receives the CTS reply, it is confirmed that the destination node is
free to receive the data packet under the prevailing channel conditions. The RTS/CTS exchange also informs the neighbors of the two nodes of a scheduled data transmission who can cooperate by not interfering during the ensuing data packet transmission. This mechanism has been found to be quite effective in covering the drawbacks of inadequate carrier sensing in wireless channels and is included in the IEEE 802.11 standard as an option over the basic channel access mechanism. Some other MAC protocols such as MACAW [2], FAMA [5], CARMA [6], also use the RTS/CTS exchange to reduce contention in the channel. However, the RTS/CTS method does not completely avoid carrier sensing of the wireless medium, and hence is also subject to its inherent disadvantages. Though a successful exchange of these short control packets reduce the probability of collisions of the larger data packets, the RTS and CTS packets themselves must rely on CSMA for their transmission. Hence these packets suffer losses due to collisions leading to wastage of bandwidth. The overhead for the transmission of these control packets increases with increasing probability of collisions. Hence, at heavy traffic loads it can lead to situations where the channel is mostly consumed by the transmission of RTS and CTS packets, only a few of which are successful, leading to a small data throughput [5]. Moreover, the efficiency of the RTS/CTS scheme has been found to degrade with increasing node mobility, as the mobile nodes are more likely to miss the RTS or CTS transmissions before sending data packets [7]. This provides a motivation for improving the efficiency of the basic medium access mechanism in wireless, which could be used for transmitting control or data packets. The key issue is to develop a framework that allows better adaptation to radio interference. To this end, we propose a multichannel CSMA protocol with distributed channel selection as a basic channel access mechanism. We divide the bandwidth into N orthogonal channels where N is smaller than the number of nodes. We assume that the nodes can sense the level of interference on all the channels and select any channel to transmit a packet. The channel selection criterion is aimed to reduce the amount of interference at the receiver. We show that this basic channel access scheme can provide a higher throughput than single channel CSMA/CA for the same available channel capacity, due a reduced number of packet collisions.
2 Related Work Multichannel random access protocols were first explored for wired networks in [1], where it was shown that the use of multiple channels with the same aggregate capacity can improve the throughput of CSMA protocols. In a wired medium, carrier sensing is perfect as the signal level is considered to be the same at all points in the network.
However, collisions may still occur due to non-negligible propagation delays. Packet collisions in these networks depend on the ratio of the propagation delay in the medium and the packet transmission time. Multichannel CSMA protocols in wired networks benefit from a reduction of this ratio with increasing number of channels. Research has indicated that multichannel MAC protocols have advantages in wireless networks as well, although for different reasons [11, 13, 15, 14, 13, 16, 6]. Leung [11] presented a class of reservation protocols using multiple channels that are applicable to star-configured satellite networks. Here, multiple channels were formed by timedivision multiplexing (TDM), and slot reservation algorithms were proposed. A similar multichannel framework based on time slots was considered in [13], where a multichannel reservation protocol was presented that reduced the delay in multicast packet transmissions in WLANs with access points. More recently, the performance of multichannel CSMA has been analyzed for a multihop wireless network [12], in which N nodes competed for any one of M available channels. Each node transmitted on a channel that is chosen randomly from the set of free channels sensed at that node. Though this method was shown to have a throughput advantage over single channel CSMA, the analysis was performed under simplistic assumptions such as a fully connected network without variations of the signal power amongst the different nodes. In [14], simulation studies on the performance of a multichannel MAC protocol for mobile ad hoc networks were presented. Assuming multiple CDMA channels, the authors presented a protocol in which the nodes identified and exchanged channel usage tables with each of its neighbors. An appropriate free channel was selected according to this information for packet transmission. Haas presented a multichannel MAC protocol for adapting to the dynamic conditions of a multihop wireless network [7]. It also uses channel usage tables at every node and a polling mechanism to schedule packet transmissions. A multichannel MAC that uses RTS/CTS transmissions to perform channel reservation was presented in [17]. In our earlier work [15], a multichannel MAC protocol was presented where nodes used their own channel usage history for channel selection. The idea was to allow “soft reservation” of channels, allowing nodes to access the same channels as much as mossible, thereby reducing contention for the same channel amongst neighboring nodes. It was demonstrated that this channel selection algorithm improved the throughput in multihop wireless networks by reducing the probability of collisions. All these protocols benefit from using multiple nonoverlapping channels for random access, and have varying levels of complexity. In this paper, we focus on the distribution of the level of interference on multiple channels. We propose a basic channel access mechanism that tries to dis-
tribute the interference on multiple channels evenly, thereby minimizing the average probability of packet loss from interference. One of our goals is to study the tradeoff for increasing the number of channels.
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3 Characteristics of Wireless CSMA We first discuss some key features of random channel access in packet radio networks that motivated this work. In wireless channels, the capability of a receiver to correctly detect a received signal depends on the ratio of the received signal strength to the total power of the noise and interfering signal at the receiver. Since the power of the desired signal and the interference both vary from point to point, it is difficult to say from the measured carrier power alone whether the channel is usable or not. A channel is generally considered “busy” if the total interference level measured on it lies above a pre-defined threshold. However, the same channel can be sensed to be “busy” at one point and “idle” at another. Moreover, even if the interfering signal lies below the specified threshold, it still contributes to the total interference to a receiver located at that point. This leads to various interference-related problems in wireless CSMA, affecting the effective throughput. Two of the well-known problems, the hidden-terminal problem and the exposed-terminal problem, are illustrated in Fig. 1. In Fig. 1(a), node A senses the channel to be idle while transmitting a packet to B. But the channel is busy at B due to the transmission from C. Hence the packet from A is not received by B (suffers a collision). Fig. 1 (b) demonstrates a situation where node B, wanting to transmit to A, backs off, as it senses the channel to be busy due to a transmission from C. However, the backoff may be unnecessary as the interference from C (B) may be too weak to affect reception at A (D). The hidden terminal problem may be resolved to some extent by using the RTS/CTS handshake. But it does not solve the problem with exposed terminals. Another interference related problem is illustrated in Fig. 2. Here, two neighboring transmissions of the receiving node B are individually below the channel sensing threshold at B. The interference from either one of C or E may not be harmful to B, but the total power from C and E affects B’s reception from A. The RTS/CTS handshake will be unsuccessful here as all the source and destination nodes are outside the carrier-sensing range of the three sources. Some advantages are gained from the propagation path loss characteristics of wireless channels as well. For instance, it facilitates “frequency reuse”, by which the same channel is simultaneously used at two locations in the network which are sufficiently far apart so that the transmissions do not interfere with each other. It also allows power capture, by which a receiver can recover one of two or more simultaneously received packets, all of whose re-
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ceived powers are above the prescribed threshold, if the signal power of one of the packets sufficiently exceeds the others [10]. Due to these reasons, a pre-defined fixed threshold for evaluating the channel busy/idle status is highly inadequate to predict the probabiltiy of success of a transmitted packet in wireless. It would be beneficial to utilize the level of interference for a better prediction.
4 Multichannel CSMA with Signal PowerBased Channel Selection In this section we describe our multichannel protocol. Assume that the wireless medium is divided into N orthogonal channels. The nodes can transmit and receive on all
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channels. However, a node may transmit or receive on any one channel at a given time. Each channel has a bandwidth of W=N , where W is the total available bandwidth. The channels can be formed by frequency division or code division. Due to the difficulty in time syncronization in ad hoc networks, creating channels by time division may be difficult. However, out scheme is independent of the mechanism used for seggregating the medium into multiple channels. We assume that a node can sense the carrier in all the channels and take a local decision in selecting a channel during packet transmission. An obvious advantage of using multiple channels is that it allows concurrent transmissions from a group of neighboring nodes without interfering with one another. For instance, in the example depicted in Fig. 3, nodes A, C, and E have packets to transmit to B, D, and F, respectively (the packet arrival times are indicated by vertical arrows on the time axis). The possible channel states with single channel CSMA and a scheme that uses three channels with a the same total aggregate capacity, are illustrated. In this scenario, two advantages of using multiple channels are observed: (a) for the case of single channel, node E has to wait for accessing the channel, whereas for 3-channels the need for backoff is removed, (b) with 3 channels the possibility of collisions is reduced. Here, we utilize the fact that with multiple channels, the interference is distributed over channels as well as over space. If the global interference distribution on all channels was available at a transmitting node, it could choose the channel that has minimum interference at the receiving node. Even in the absence of this global information, a transmitting node using multichannel CSMA can benefit by determining the different levels of the carrier signal on all channels at its location. We propose a channel selection algorithm that chooses the “best” channel for transmission
at the transmitting node. This implies that when a node has a packet to transmit, it senses the carrier on all the channels, and selects the channel with the lowest sensed power. Assuming omnidirectional transmitting antennas, the channel having the least power is also the channel that is being used at the farthest point from the transmitter, and therefore a good choice. Though this does not guarantee that the same channel has the least intereference at the destination as well, it is likely to be the best channel at that location. Also, by always selecting the channel with the least interference, the scheme indirectly distributes the radio interference equally over all channels. This makes the average interference level in any one channel low. The details of the protocol is presented below as a variation of the IEEE 802.11 basic CSMA/CA channel access protocol. Multiple channels are used with a signal powerbased channel selection algorithm. 1. Each node monitors the N channels continuously, whenever it is not transmitting. It measures the total received signal strength (TRSS) in the channels and detects if they are above or below its sensing threshold (ST). The channels for which the TRSS is below the ST, are marked as IDLE. The time at which the TRSS dropped below ST is noted for each channel. These channels are put on a free channel list. The rest of the channels are marked as BUSY. 2. At the start of a protocol cycle, i.e., when a packet arrives from the traffic generator: (a) If the free channel list is empty, the node waits for the first channel to be IDLE. Then it waits for a period called the Long Interframe Space (LongIFS), and it waits further for a random access backoff period before transmitting the packet. It is required that the channel remains IDLE during this period. (b) If the free channel list is not empty, the node checks the TRSS measured in all the channels in the list and selects the channel which has the minimum value. 3. Before actually transmitting the packet the node checks to see whether the TRSS on the chosen channel has remained below ST for at least a LongIFS period. (a) If not, the node initiates a backoff delay after the LongIFS. (b) If yes, then the node initiates transmission immediately, without further delay. 4. Any backoff is canceled immediately if the TRSS on the chosen channel goes above the ST at any time during the backoff period. When TRSS again goes below ST a new backoff delay is scheduled.
5 Simulation Model To evaluate the performance of the proposed MAC protocol, we use an event driven simulator which contains details of an indoor radio propagation model, multipath fading, and parameterized radio receiver characteristics. The IEEE 802.11 MAC layer specifications for single-channel operation along with appropriate modifications for implementing the proposed multichannel CSMA scheme are included in the program. The simulator assumes a network consisting of n2 stationary wireless nodes that are placed in a n n square grid. The transmitter power, the receiver sensivity for carrier sensing, the minimum signal-tointerference (SIR) required at the reciver to correctly detect a signal, and the grid spacing, are user-specified parameters which jointly decide the radio range. Following the radiopropagation model, the path loss is varied according to a piecewise log-log function. This implies that the dB path loss varies linearly for each of a sequence of linear ranges of distances from a transmitter. The slope for the path loss line for each linear range decreases with increasing distance from the transmitter. Multpath fading is simulated by generating a seperate random loss component for each packet. This fading loss is independent for every packet and is modeled as a Rayleigh distributed random variable. The simulator generates packet arrivals at the nodes according to a Poisson process. The MAC protocol uses the radio signal power measurements at every node location, which is done by combining the radio signals from all transmitters at that time. The power of the signal received from a particular source is calculated separately as the desired signal strength. The sum of the signal powers received from all other transmitters active at that time constitute the interference signal. The radio signal distribution is updated at every epoch when a change takes place, which happens either at the commencement or the end of transmission of a packet. If the calculated SIR at the receiver is found to be above the minimum required threshold for the entire packet duration, the packet is assumed to be received correctly. Packet count statistics are reported after the simulation is run for a certain period of time.
6 Simulation Results We performed simulations on a number of different example scenarios. These include specific topologies with selected source and destination combinations and radio ranges1. For each scenario, simulations are run using the single channel CSMA/CA protocol (same as IEEE 802.11) 1 The radio range is determined by the maximum distance from a transmitting node at which a receiver can correctly receive a signal in the absense of any interference, i.e. considering the receiver noise only.
and the proposed multichannel protocol. Some of the important and fixed parameters used for the all the simulations are shown in Table 1. These are discussed in the following subsections. Table 1. Parameter values used in simulations Parameter Grid size Carrier sense threshold (ST) Noise floor Minimum SIR Packet size Total bandwidth
Values used 200 m -40 dBm -90 dBm 20 dB 1000 bytes 1 Mb/sec
6.1 Scenario 1: the hidden terminal effect In this scenario we take a grid of 25 wireless nodes as shown in Fig. 4 where the nodes 10, 14, 17, and 7 are transmitting packets sequentially to the nodes 11, 13, 2, and 22, respectively. The packet arrival processes at the source nodes are independent of one another. We use a transmitter power of 35 dBm for each of the transmitters in this example, for which the radio ranges are marked by the dotted circles. Considering the interference effects during packet transmissions, we define the following two types of conversations:
Type 0 conversations: consisting of the links 10 ! and 14 ! 13. These links have the com‘mon characteristics that each of the destinations (11 and 13) are within range of two other transmitters (17 and 7), who are not within range of the Type 0 sources. Hence the Type 0 conversations are expected to suffer heavy losses due to the hidden terminal effect as well as normal radio channel losses and interference.
Type 1 conversations: consisting of the links 17 ! and 7 ! 2. The sources for these links are outside of the radio ranges of all transmitting nodes in the network. However, the combined interference from all the sources will cause some packet errors.
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is significantly small for Type 0 conversations, due to the hidden terminal effect. The proposed multichannel protocol, on the other hand, suffers fewer losses due to the hidden terminal effect.
6.2 Scenario 2: the exposed terminal problem We now turn to the scenario shown in Fig. 6, where the source nodes 11, 17, 13, and 7, can all hear one another, although their corresponding destinations, nodes 10, 22, 14 and 2, are not within range of any transmitting node other than their respective sources. This scenario is devised to particluarly highlight the effect of exposed terminals. The results, shown in Fig. 7, indicate that the multichannel CSMA MAC performs better for this situation as well. There is no difference between the throughputs of Type 0 and Type 1 links. The results suggest that the improvement of throughput is significant when the number of channels is increased from 1 (CSMA/CA) to 2. However, the throughput does not change much between the cases N = 2 and N = 3.
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6.3 Scenario 3: general case Finally, we try to evaluate the performance of the multichannel CSMA MAC protocol for the more general situation of a wireless network with a large number of nodes and an unrestricted traffic pattern. We consider a network with 225 nodes placed in a 15 15 uniform grid. With a transmitter power of 50 dBm, the radio range of each wireless transmitter contains a maximum of 12 neighbors as shown in Fig. 8. We assume that new packets are generated at every node according to an independent Poisson process. For each packet, a random destination is selected from the set of possible neighbors of that node. Thus, we have a situation where packet transmissions are distributed uniformly throughout the network with random source and destination pairs. Note that this scenario captures a typical behavior in an ad hoc network. In such a network, the source and destination may not be neighbors and data packets must be routed via multiple hops. But considering each hop in isolation, data transmissions are between neighboring nodes. The current scenario abstracts out this behavior for a stationary net-
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work. We have not studied mobile networks in this work, but we expect that mobility will affect all protocols (single channel or multichannel) equally. Fig. 9 shows the variation of the total throughput of the network with the total offered load for several different MAC protocols. The offered load is varied by varying the Poisson arrival rate of packets. In this example we compare the throughput performance of the following MAC protocols: 1. Single channel CSMA/CA. 2. A multichannel MAC where channels are chosen at random from the set of free channels sensed by a transmitting node (MC-RC). 3. The proposed multichannel MAC with signal power based channel selection (MC-SP). It can be seen that the peak throughput is highest for the MC-SP protocol with 5 channels (N = 5). The MC-RC protocol has no improvement over the single channel MAC for the chosen set of parameters. With the proposed MC-SP scheme, the throughput increases dramatically from N = 1 to 3. However, the improvement with N = 5 is less pronounced. The following analysis reveals the reason. In Fig. 10 we show the breakdown of the transmitted packets for one of the simulation runs in Fig. 9. Note that a transmitted packet can be unsuccessful either due to collision at the receiver or because the receiver is busy (i.e., busy transmitting a packet or receiving from another source, which cannot be interrupted). Observe that even by using a random channel to transmit packets, the number of collisions is reduced by the use of multiple channels, though the number of packet losses due to the “destination busy” condition is increased. For MC-RC N = 3, these effects
Figure 9. Throughput performance of the 225 node network configuration with parameters as given in Table 1.
cancel each other and the overall throughput remains unmodified. Packet losses due to the “destination busy” condition increase proportionately with increasing number of channels. This is because the packet transmission and reception times depend on the bandwidth per channel. With the MC-SP MAC, however, the reduction in the number of packet collisions far exceeds the increase in the counts of destination busy situations, when compared to single channel CSMA/CA. Hence the overall throughput is also higher for MC-SP.
7 Conclusions We proposed a multichannel MAC protocol with distributed channel selection to reduce channel contention in packet radio networks. The proposed scheme allows nodes to sense the carrier powers on all channels and employs a protocol similar to CSMA/CA on the channel which has the lowest power. Simulation experiments have demonstrated that this medium access method has a higher throughput in comparison to the CSMA/CA protocol using a single channel, due to a lower probability of packet collisions. Dividing the bandwidth into multiple channels, however, increases the packet transmission and reception times, affecting the overall throughput negatively. Hence, the net gain in throughput by using the proposed scheme depends on the number of channels used and other parameters that affect
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the probability of collisions in the network. The successful reception of a packet depends on the SIR at the receiver, and hence a MAC protocol that utilizes the carrier powers on the channels at the receiver site will certainly give better results. This could be done, for instance, by using an RTS/CTS exchange and allowing the destination to inform the source about the best channel sensed at its location using the CTS packet. The RTS/CTS exchange may also make it possible for nodes to share channel state information amongst its neighbors and cooperate in channel selection. However, the overhead costs and merits for such schemes need to be studied.
Acknowledgment
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[12]
[13]
[14]
[15]
This work is partially supported by AFOSR grant no. F49260-96-1-0472, NSF CAREER award no. ACI9733836 and NSF grant no. ANI-9973147.
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