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JWBK083-05 JWBK083-Glisic February 23, 2006 3:39 Char Count= 0 ADAPTIVE MAC FOR WLAN WITH ADAPTIVE ANTENNAS 155 [21] Circular adaptive antenna array, beamwidth 64 ◦ ,8dBgain (improvement over 802.11) 25 nodes (grid) 225 nodes (grid) No PC Global PC Local PC No PC Global PC Local PC (PC, power control) 1.3× 1.7× 2.1× 2.6× 4.75× 5.25× [22] Ideal adaptive antenna 20 nodes, no nulling (improvement over omni case) Packet transmission is directional at sender/receiver Protocol Beamwidth O, omnidirectional (20 nodes, degree = 7.5) D, directional 90 ◦ 60 ◦ 30 ◦ 10 ◦ ORTS/DCTS 35 % 57 % 100 % 142 % DRTS/DCTS 64 % 107 % 143 % 186 % DRTS/OCTS 28 % 43 % n/a 57 % ORTS/OCTS 29 % 50 % 86 % 121 % STDMA n/a 400% n/a 400 % [23] No mobility Omni RX directional DVCS DVCS-Ideal TX omnidirectional TX, RX directional 400 kbps 800 kbps 1.4 Mbps 2.2 Mbps Six-element circular antenna array (10 fixed patterns, no adaptation) 45 ◦ beamwidth, 100 nodes, 1500 m 2 2-ray propagation model, no nulling JWBK083-05 JWBK083-Glisic February 23, 2006 3:39 Char Count= 0 156 ADAPTIVE MEDIUM ACCESS CONTROL b a c d a has a packet for c b has a packet for d Node d mistakenly forms a beam towards a because a′s signal is stronger than b’s signal at d Figure 5.5 False beamforming. (Reproduced by permission of IEEE [27].) optimization is a single-entry cache scheme which works as follows: r If a node beamforms incorrectly in a given timeslot, it remembers that direction in a single-entry cache. r In the next slot, if the maximum signal strength is again in the direction recorded in the single-entry cache, then the node ignores that direction and beamforms towards the second strongest signal. If the node receives a packet correctly (i.e. it was the intended recipient), it does not change the cache. If it receives a packet incorrectly, it updates the cache with this new direction. r If there is no packet in a slot from the direction recorded in the cache, the cache is reset. The Smart-802.11b protocol is based on the 802.11b standard. As in the case of the Smart- Aloha protocol, transmitters beamform towards their receivers and transmit a short sender- tone to initiate communication. However, unlike Smart-Aloha, the transmitter does not immediately follow the tone with a packet. Instead, it waits for a receiver-tone and only then transmits its packet. After transmission of a packet, it waits for the receipt of an ACK. If there is no ACK, it enters backoff as in 802.11b. Figure 5.6 presents a state diagram of tone-based protocol. The behavior of the protocol in various states can be summarized as follows. 5.2.1.1 Idle In case a node has no packet to send, it will remain in the idle state and set its antenna to operate in the omnidirectional mode. If it receives a sender-tone from some other node, it will move into the data receive wait state. On the other hand, if it wishes to send data, it will beamform in the direction of the receiver. It chooses a random number [0–CW] and sets the CW (contention window) timer 1. When the CW timer expires, it sends a sender-tone in the direction of the receiver and moves to the ACK wait state. If, before the CW timer expires, the node receives a sender-tone from another node, it will freeze its CW timer and move to data receive wait state. 5.2.1.2 Data receive wait A node will move to this state in the event it receives a sender-tone. The node will beamform towards the sender and then randomly defer transmitting the receiver-tone by choosing a JWBK083-05 JWBK083-Glisic February 23, 2006 3:39 Char Count= 0 ADAPTIVE MAC FOR WLAN WITH ADAPTIVE ANTENNAS 157 While in backoff receive sender-tone Back off ACK Wait Idle Data receive wait Valid ACK received Send sender-tone and wait for receiver-tone Receive sender-tone (freeze CW timer and service sender-tone) Data received and valid, send ACK OR Data receive time expires If data received is invalid then update cache ACK timer expires, move to back off CW timer expires, send sender-tone Receive receiver-tone, send data Figure 5.6 State diagram of the Smart-802.11b protocol. random waiting period of [0–32] ×20 μs. The reason for deferring the reply is to mini- mize the chance of several receiver-tones colliding at sender 2. After transmitting a receiver- tone, the node remains in this state for 2τ (twice the maximum propagation delay + tone transmission time). If it does not hear a transmission, it returns to the idle state. If it hears the start of a transmission, it remains in this state and receives the packet. It then discards the packet if the packet was meant for some other node If, however, the packet was meant for it, then it sends an ACK. 5.2.1.3 Ack wait If the sender node receives a receiver-tone before the tone RTT timer goes off (which is twice the tone transmission time plus propagation delay), it will transmit the data packet. Reception of a valid ACK will move the node to the idle state, and if packets are there in the queue then it will schedule the one at the head of the queue. The node will move to the backoff state under two conditions: (1) a receiver-tone did not arrive; (2) an ACK was not received following transmission of the data packet. 5.2.1.4 Backoff The node computes a random backoff interval (as in 802.11) and remains in backoff for this time period (it also resets its antenna to omnidirectional mode). If, however, a sender-tone is received, it freezes the backoff timer and enters the data receive wait state. If the node is in backoff, upon expiration of the timer, it retransmits the sender-tone, increments the retransmit counter and enters the ACK wait state. A packet is discarded after the retransmit counter exceeds Max Retransmit = 7, as in the IEEE 802.11 standard. The reception of a data packet by a node may be interfered with by transmissions of sender-tones, receiver-tones or other data packets (since the protocol does not take care of JWBK083-05 JWBK083-Glisic February 23, 2006 3:39 Char Count= 0 158 ADAPTIVE MEDIUM ACCESS CONTROL hidden terminals). A node engaged in receiving a data packet can dynamically form nulls towards new interferers, but this process takes some time (we model this time as the length of a sender-tone). Thus, the data packet will have errors due to this interference. This error is mitigated by relying on FEC codes as used in IEEE 802.11e, where (224, 208) shortened RS codes are used. In 802.11e, an MAC packet is split into blocks of 208 octets and each block is separately coded using an RS encoder.A (48, 32) RS code, which is also a shortened RS code, is used for the MAC header, and CRC-32 is used for the FCS. Performance example – the simulation parameters are: Background noise + ambient noise = 143 dB Propagation model free space Bandwidth 1000 kHz Min frequency 2402 MHz Data rate 2000 kbps Carrier sensing threshold + 3dB Minimum SINR 9 dB Bit error based on BPSK modulation curve Maximum radio range 250 m Packet size 16 kb Simulation time 200 s Single hop: number of nodes 20, area 100 × 100 m Multihop: number of nodes 100, area 1500 × 1500 The existing 802.11b implementation in OPNET is modified to create Smart-802.11b. The modifications included adding the two tones (sender and receiver) as well as changing the FEC to the 802.11e specification. The performance of the protocol is presented for a single-hop case with 20 nodes and a five-hop case with 100 nodes using of 16 KB packets. The 16 antenna elements (for an effec- tive beamwidth of 400) were used. Figure 5.7 presents the aggregate one-hop throughput as a function of arrival rate for the one-hop case. One can see that 802.11bachieves a maximum throughput of 1 Mbps while Smart-802.11b achieves a high of 8.5 Mbps and Smart-Aloha achieves a high of approximately 10.5 Mbps. In fact, the throughput of Smart-802.11b and Smart-Aloha increases with arrival rate because of good spatial reuse of the channel. Figure 5.8 plots the aggregate throughput of the protocol for the 100-node five-hop case; 802.11b reaches a maximum throughput of well below 0.5 Mbs while Smart- 802.11b reaches a maximum of 50 Mbs and Smart-Aloha reaches a maximum throughput of 60 Mbs. Again, the better spatial reuse of the channel given the directivity of the antenna is the reason for this performance improvement. 5.3 MAC FOR WIRELESS SENSOR NETWORKS This sectiondiscussesan MACprotocol designed forwireless sensor networks (S-MAC). As will be discussed in Chapter 14, wireless sensor networks use battery-operated computing and sensing devices. A network of these devices will collaborate for a common application such as environmental monitoring. Sensor networks are expected to be deployed in an ad hoc fashion, with nodes remaining largely inactive for long time, but becoming suddenly ac- tive when something is detected. These characteristics of sensor networks and applications JWBK083-05 JWBK083-Glisic February 23, 2006 3:39 Char Count= 0 MAC FOR WIRELESS SENSOR NETWORKS 159 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Sending rate (kbs) 0 2000 4000 6000 8000 10000 12000 Aggregate throughput (kbs) 802.11b Smart-802.11b (16 elements) Smart-ALOHA (16 elements) Figure 5.7 Single-hop case with 20 nodes. (Reproduced by permission of IEEE [27].) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Sending rate (kbs) 0 10000 20000 30000 40000 50000 60000 Aggregate throughput (kbs) 802.11b Smart-802.11b (16 elements) Smart-ALOHA (16 elements) Figure 5.8 Five-hop case with 100 nodes. (Reproduced by permission of IEEE [27].) motivate an MAC that is different from traditional wireless MACs such as IEEE 802.11, described in previous sections, in several ways. Energy conservation and self-configuration are primary goals, while per-node fairness and latency are less important. S-MAC uses a few novel techniques to reduce energy consumption and support self-configuration. It enables low-duty-cycle operation in a multihop network. Nodes form virtual clusters based on common sleep schedules to reduce control overhead and enable traffic-adaptive JWBK083-05 JWBK083-Glisic February 23, 2006 3:39 Char Count= 0 160 ADAPTIVE MEDIUM ACCESS CONTROL wake-up. S-MAC uses in-channel signaling to avoid overhearing unnecessary traffic. Fi- nally, S-MAC applies message passing to reduce contention latency for applications that require in-network data processing. Woo and Culler [28] examined different configurations of carrier sense multiple access (CSMA) and proposed an adaptive rate control mechanism, whose main goal is to achieve fair bandwidth allocation to all nodes in a multihop network. There is also some work on the low-duty-cycle operation of nodes, which are closely related to S-MAC. The first example is Piconet [29], which is an architecture designed for low-power ad hoc wireless networks. Piconet also puts nodes into periodic sleep for energy conservation. However, there is no coordination and synchronization among nodes about their sleep and listen time. The scheme to enable the communications among neighboring nodes is to let a node broadcast its address when it wakes up from sleeping. If a sender wants to talk to a neighbor, it must keep listening until it receives the neighbor’s broadcast. In contrast, S-MAC tries to coordinate and synchronize neighbors’ sleep schedules to reduce latency and control overhead. Perhaps the power-save (PS) mode in IEEE 802.11 DCF is the most related work to the low-duty-cycle operation in S-MAC. Nodes in PS mode periodically listen and sleep, just like that in S-MAC. The sleep schedules of all nodes in the network are synchronized together. The main difference from S-MAC is that the PS mode in 802.11 is designed for a single-hop network, where all nodes can hear each other, simplifying the synchronization. As observed by Woo and Culler [28], in multihop operation, the 802.11 PS mode may have problems in clock synchronization, neighbor discovery and network partitioning. In fact, the802.11 MAC in generalis designed for a single-hopnetwork, and thereare questions about its performance in multihop networks [30]. In comparison, S-MAC is designed for multihop networks, and does not assume that all nodes are synchronized together. Finally, although 802.11 defines PS mode, it provides very limited policy about when to sleep, whereas in S-MAC, a complete system is defined. Tseng et al. [31] proposed three sleep schemes to improve the PS mode in the IEEE 802.11 for its operation in multihop networks. Among them the one named periodically fully awake interval is the closest to the scheme of periodic listen and sleep in S-MAC. However, their scheme does not synchronize the sleep schedules of any neighboring nodes. The control overhead and latency can be large. For example, to send a broadcast packet, the sender has to explicitly wake up each individual neighbor before it sends out the actual packet. Without synchronization, each node has to send beacons more frequently to prevent long-term clock drift. 5.3.1 S-MAC protocol design S-MAC includes approaches to reducing energy consumption from all the sources of energy waste such as: (a) idle listening; (b) collision; and (c) overhearing and control overhead. Before describing the components in S-MAC, we first summarize assumptions about the wireless sensor network and its applications. Sensor networks will consist of large numbers of nodes to take advantage of short-range, multihop communications to conserve energy (see Chapter 14). Most communications will occur between nodes as peers, rather than to a single base station. In-network processing is critical to network lifetime, and implies that data will be processed as whole messages in a store-and-forward fashion. Packet or fragment-level interleaving from multiple sources only JWBK083-05 JWBK083-Glisic February 23, 2006 3:39 Char Count= 0 MAC FOR WIRELESS SENSOR NETWORKS 161 C DAB Figure 5.9 Neighboring nodes A and B have different schedules. They synchronize with nodes C and D respectively. increases overall latency. Finally, we expect that applications will have long idle periods and can tolerate latency on the order of network messaging time. 5.3.2 Periodic listen and sleep As stated above, in many sensor network applications, nodes are idle for a long time if no sensing event happens. Given the fact that the data rate is very low during this period, it is not necessary to keep nodes listening all the time. S-MAC reduces the listen time by putting nodes into periodic sleep state. Each node sleeps for some time, and then wakes up and listens to see if any other node wants to talk to it. During sleeping, the node turns off its radio, and sets a timer (alarm clock) to wake itself later. A complete cycle of listen and sleep is called a frame. The listen interval is normally fixed according to physical-layer and MAC-layer parameters, such as the radio bandwidth and the contention window size. The duty cycle is defined as the ratio of the listen interval to the frame length. The sleep interval can be changed according to different application requirements, which actually changes the duty cycle. For simplicity, these values are the same for all nodes. All nodes are free to choose their own listen/sleep schedules. However, to reduce control overhead, we prefer neighboring nodes to synchronize together. That is, they listen at the same time and go to sleep at the same time. It should be noticed that not all neighboring nodes can synchronize together in a multihop network. Two neighboring nodes A and B may have different schedules if they must synchronize with different nodes, C, and D, respectively, as shown in Figure 5.9. Nodes exchange their schedules by periodically broadcasting a SYNC packet to their immediate neighbors. A node talks to its neighbors at their scheduled listen time, thus ensuring that all neighboring nodes can communicate even if they have different schedules. In Figure 5.9, for example, if node A wants to talk to node B, it waits until B is listening. The period for a node to send a SYNC packet is called the synchronization period. One characteristic of S-MAC is that it forms nodes into a flat, peer-to-peer topology. Unlike clustering protocols, S-MAC does not require coordination through cluster heads. Instead, nodes form virtual clusters around common schedules, but communicate directly with peers. One advantage of this loose coordination is that it can be more robust to topology change than cluster-based approaches. The downside of the scheme is the increased latency due to the periodic sleeping. Furthermore, the delay can accumulate on each hop. Later on, a technique that is able to significantly reduce such latency will be presented. 5.3.3 Collision avoidance If multiple neighbors want to talk to a node at the same time, they will try to send when the node starts listening. In this case, they need to contend for the medium. Among con- tention protocols, the 802.11 does a very good job on collision avoidance. S-MAC follows JWBK083-05 JWBK083-Glisic February 23, 2006 3:39 Char Count= 0 162 ADAPTIVE MEDIUM ACCESS CONTROL similar procedures, including virtual and physical carrier sense, and the RTS/CTS (request to send/clear to send) exchange for the hidden terminal problem [32]. There is a duration field in each transmitted packet that indicates how long the remaining transmission will be. If a node receives a packet destined to another node, it knows how long to keep silent from this field. The node records this value in a variable called the network allocation vector (NAV) [33] and sets a timer for it. Every time the timer fires, the node decrements its NAV until it reaches zero. Before initiating a transmission, a node first looks at its NAV. If its value is not zero, the node determines that the medium is busy. This is called ‘virtual carrier sense’. Physical carrier sense is performed at the physical layer by listening to the channel for possible transmissions. Carrier senses time is randomized within a contention window to avoid collisions and starvations. The medium is determined as free if both virtual and physical carrier senses indicates that it is free. All senders perform carrier sense before initiating a transmission. If a node fails to get the medium, it goes to sleep and wakes up when the receiver is free and listening again. Broadcast packets are sent without using RTS/CTS. Unicast packets follow the sequence of RTS/CTS/DATA/ACK between the sender and the receiver. After the successful exchange of RTS and CTS, thetwo nodes will use theirnormal sleep time for data packet transmission. They do not follow their sleep schedules until they finish the transmission. With the low- duty-cycle operation and the contention mechanism during each listen interval, S-MAC effectivelyaddresses the energy waste due to idle listeningand collisions. In the nextsection, details of the periodic sleep coordinated among neighboring nodes will be presented. Two techniques will be presented that further reduce the energy waste due to overhearing and control overhead. 5.3.4 Coordinated sleeping Periodic sleeping effectively reduces energy waste on idle listening. In S-MAC, nodes coordinate their sleep schedules rather than randomly sleep on their own. This section details the procedures that all nodes follow to set-up and maintain their schedules. It also presents a technique to reduce latency due to the periodic sleep on each node. 5.3.5 Choosing and maintaining schedules Before each node starts its periodic listen and sleep, it needs to choose a schedule and exchangeit with itsneighbors. Each nodemaintains aschedule tablethat stores theschedules of all its known neighbors. It follows the steps below to choose its schedule and establish its schedule table. (1) A node first listens for a fixed amount of time, which is at least the synchronization period. If it does not hear a schedule from another node, it immediately chooses its own schedule and starts to follow it. Meanwhile, the node tries to announce the schedule by broadcasting a SYNC packet. Broadcasting a SYNC packet follows the normal contention procedure. The randomized carrier sense time reduces the chance of collisions on SYNC packets. (2) If the node receives a schedule from a neighbor before choosing or announcing its own schedule, it follows that schedule by setting its schedule to be the same. Then the node will try to announce its schedule at its next scheduled listen time. JWBK083-05 JWBK083-Glisic February 23, 2006 3:39 Char Count= 0 MAC FOR WIRELESS SENSOR NETWORKS 163 (3) There are two cases where a node receives a different schedule after it chooses and announces its own schedule. If the node has no other neighbors, it will discard its current schedule and follow the new one. If the node already follows a schedule with one or more neighbors, it adopts both schedules by waking up at the listen intervals of the two schedules. To illustrate this algorithm, consider a network where all nodes can hear each other. The node that starts first will pick up a schedule first, and its broadcast will synchronize all its peers on its schedule. If two or more nodes start first at the same time, they will finish initial listening at the same time, and will choose the same schedule independently. No matter which node sends out its SYNC packet first (wins the contention), it will synchronize the rest of the nodes. However, two nodes may independently assign schedules if they cannot hear each other in a multihop network. In this case, those nodes on the border of two schedules will adopt both. For example, nodes A and B in Figure 5.9 will wake up at the listen time of both schedules. In this way, when a border node sends a broadcast packet, it only needs to send it once. The disadvantage is that these border nodes have less time to sleep and consume more energy than others. Another option is to let a border node adopt only one schedule – the one it receives first. Since it knows that some other neighbors follow another schedule, it can still talk to them. However, for broadcasting, it needs to send twice to the two different schedules. The advantage is that the border nodes have the same simple pattern of periodic listen and sleep as other nodes. It is expected that nodes only rarely see multiple schedules, since each node tries to follow an existing schedule before choosing an independent one. However, a new node may still fail to discover an existing neighbor for several reasons. The SYNC packet from the neighbor could be corrupted by collisions or interference. The neighbor may have delayed sending a SYNC packet due to the busy medium. If the new node is on the border of two schedules, it may only discover the first one if the two schedules do not overlap. To prevent the case that two neighbors miss each other forever when they follow com- pletely different schedules, S-MAC introduces periodic neighbor discovery, i.e. each node periodically listens for the whole synchronization period. The frequency with which a node performs neighbor discovery depends on the number of neighbors it has. If a node does not have any neighbors, it performs neighbor discovery more aggressively than in the case where it has many neighbors. Since the energy cost is high during the neighbor discovery, it should not be performed too often. In a typical implementation, the synchronization period is 10 s, and a node performs neighbor discovery every 2 min if it has at least one neighbor. 5.3.6 Maintaining synchronization Since neighboring nodes coordinate their sleep schedules, the clock drift on each node can cause synchronization errors. Two techniques can be used to make it robust to such errors: (1) all exchanged timestamps are relative rather than absolute; and (2) the listen period is significantly longer than clock drift rates. For example, the listen time of 0.5 s is more than 10 times longer than typical clock drift rates. Compared with TDMA schemes with very short time slots, S-MAC requires much looser time synchronization. Although the long listen time can tolerate fairly large clock drift, neighboring nodes still need to periodically update each other with their schedules to prevent long-term clock drift. The synchronization JWBK083-05 JWBK083-Glisic February 23, 2006 3:39 Char Count= 0 164 ADAPTIVE MEDIUM ACCESS CONTROL Receiver Sender 1 Sender 2 Sender 3 Listen for SYNC for RTS for CTS Sleep Sleep Send data Send data CS CS CS CS TX SYNC TX SYNC Got CTS TX RTS TX RTS Got CTS Figure 5.10 Timing relationship between a receiver and different senders. CS stands for carrier sense. period can be quite long. The measurements show that the clock drift between two nodes does not exceed 0.2 ms/s. As mentioned earlier, schedule updating is accomplished by sending a SYNC packet. The SYNC packet isvery short, and includes the address ofthe sender and the time of its next sleep. The next sleep time is relative to the moment that the sender starts transmitting the SYNC packet. When a receiver gets the time from the SYNC packet, it subtracts the packet transmission time and uses the new value to adjust its timer. In order for a node to receive both SYNC packets and data packets, its listen interval is divided into two parts. The first one is for SYNC packets, and the second one is for data packets, as shown in Figure 5.10. Each part has a contention window with many time slots for senders to perform carrier sense. For example, if a sender wants to send a SYNC packet, it starts carrier sense when the receiver begins listening. It randomly selects a time slot to finish its carrier sense. If it has not detected any transmission by the end of that time slot, it wins the contention and starts sending its SYNC packet. The same procedure is followed when sending data packets. Figure 5.10 shows the timing relationship of three possible situations that a sender transmits to a receiver. Sender 1 only sends a SYNC packet. Sender 2 only sends a unicast data packet. Sender 3 sends both a SYNC and a data packet. 5.3.7 Adaptive listening The scheme of periodic listen and sleep is able to significantly reduce the time spent on idle listening when traffic load is light. However, when a sensing event indeed happens, it is desirable that the sensing data can be passed through the network without too much delay. When each node strictly follows its sleep schedule, there is a potential delay on each hop, [...]... 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JWBK0 83- 05 JWBK0 83- Glisic February 23, 2006 3: 39 Char Count= 0 MAC FOR WIRELESS SENSOR NETWORKS 1 63 (3) There are two cases where a node receives a different. interfering JWBK0 83- 05 JWBK0 83- Glisic February 23, 2006 3: 39 Char Count= 0 MAC FOR AD HOC NETWORKS 171 1 2 3 4 5 6 7 8 9 10 12 11 13 14 Communication Link Interfering Link Sensing Link Figure 5.14 A wireless. detected. These characteristics of sensor networks and applications JWBK0 83- 05 JWBK0 83- Glisic February 23, 2006 3: 39 Char Count= 0 MAC FOR WIRELESS SENSOR NETWORKS 159 0 200 400 600 800 1000 1200