Implicit MAC Acknowledgment: An Optimization to

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1 Implicit MAC Acknowledgment: An Optimization to Romit Roy Choudhury Abrita Chakravarty Tetsuro Ueda University of Illinois at Urbana-Champaign Infosys Technologies, Pune, India ATR Adaptive Communications Research Lab, Japan Abstract In the recent years, IEEE has become the de facto standard for medium access control in wireless LANs and ad hoc networks. One problem with is the high investment of bandwidth to control packets like RTS, CTS, and ACK. With the increasing trend in available data rates, this problem is becoming even more pronounced. In this paper, we propose an implicit MAC acknowledgment scheme that is capable of eliminating the ACK packet whenever the network load increases. Since ACK packets account for almost one-third of the total control overhead, we argue that such a scheme can be beneficial. We show that the benefits are both in terms of throughput and delay, and more importantly, takes effect when the network capacity is getting saturated. Simulation results, using the Qualnet simulator, confirm that the quantitative improvements are encouraging. I. INTRODUCTION The IEEE protocol [1] has become the MAC protocol standard for wireless LANs and multihop ad hoc networks. While the protocol addresses some of the key problems in wireless medium access control (like hidden terminal, and exposed terminal problems [2][3]), there is a growing concern over the large control overhead associated to More specifically, each data packet transmitted in is preceded by a handshake of RTS and CTS packets, and followed by an acknowledgment (ACK) packet. RTS/CTS packets are essential to resolve the hidden terminal problem, and therefore needs to be used for medium and large sized packets. Although it may appear that RTS,CTS, and ACK packets are comparatively small, and therefore do not pose a serious problem, a closer inspection of the physical layer parameters seems to suggest differently. Table I shows the number of time slots that is required to transmit a 512 Byte data packet in comparison to the RTS, CTS, and ACK control packets. The table corresponds to a channel data rate of 54 Mbps, when using /05/ c an OFDM modulation scheme for IEEE a. Observe from the table that the RTS packet, although 20 Bytes in size, takes one third of the time required to transmit the 512 Byte data packet. This happens because every packet is prepended with an initial preamble and physical layer header bits 1, and together they compose a large fraction of the final transmission. As a result, the total percentage of bandwidth invested on control packets accounts for nearly 46%. At higher data rates, the penalty of transmitting such control packets will become even more pronounced, leading to even poorer channel utilization. This paper aims to reduce the control overhead associated to by using an implicit acknowledgment scheme. The key idea is as follows. The MAC layer at the transmitter attempts to find out if it has multiple packets queued for its intended receiver. This information can be requested from the network layer which maintains a queue of packets to be transmitted to the neighboring receiver. If there are at least two packets to be transmitted to this receiver, then the transmitter can request the receiver to avoid transmitting an explicit ACK packet. Instead, when the RTS/CTS handshake is initiated for the second packet, the receiver can acknowledge the receipt of the first packet by piggybacking the CTS with an acknowledgment flag. The CTS for the third packet can similarly carry the acknowledgment of the second packet, and so on. When it is time for the MAC layer to transmit the last packet, it explicitly informs the receiver through the RTS, that the receiver must now send an explicit ACK packet. Clearly, as long as there are packets in the queue, explicit acknowledgments can be eliminated, resulting in a potential benefits. We discuss the details of our proposal later, showing how our proposed scheme can be effective at high data The physical layer preamble is required for receiver synchronization and the header includes information like the packet length, the data rate to be used, etc. The preamble and header is transmitted at the base rate of 1 Mbps

2 traffic. RTS CTS 512 Byte Data ACK 5 slots 4 slots 15 slots 4 slots TABLE I TRANSMISSION TIMES FOR A PACKETS The rest of this paper is organized as follows. Section II presents a brief overview of the IEEE protocol, and discusses some of the related research in the area of wireless medium access control. Section III presents the details of our proposal on implicit MAC-acknowledgment. In Section IV, we consider issues and variations to our protocol followed by a quantitative evaluation in Section V. Section VI discusses future work, and Section VII concludes the paper with a brief summary. II. RELATED WORK A. IEEE Distributed Coordinated Function (DCF) In the IEEE MAC protocol [1] for omnidirectional antennas, an exchange of RTS/CTS precedes DATA communication. Both RTS and CTS packets contain the proposed duration of data transmission. Nodes located in the vicinity of communicating nodes, that overhear either (or both) of these control packets, must themselves defer transmission for this proposed duration. This is called Virtual Carrier Sensing implemented by using a variable called the Network Allocation Vector (NAV). A node updates the value of the NAV with the duration field specified in the RTS or CTS. Thus the area covered by the transmission range of the sender and receiver is reserved for data transfer, to overcome the hidden terminal problem [3]. The IEEE MAC protocol uses a backoff interval to resolve channel contention. Before initiating transmission, a node S chooses a random backoff interval from a range [0, CW], where CW is called the Contention Window. Node S then decrements the backoff counter once every slot time. When the backoff counter reaches 0, node S transmits the packet. If the transmission from S collides with some other transmission (collision is detected by the absence of a CTS), S doubles its contention window, chooses a new backoff interval and attempts retransmission. The Contention Window is doubled on each collision until it reaches a maximum threshold, called. While in the backoff stage, if a node senses the channel as busy, it freezes its backoff counter. When the channel is once again idle for a duration called DIFS (DCF interframe spacing), the node continues counting down from its previous (frozen) value invokes the backoff procedure only after a channel has been sensed to be idle for DIFS duration. A shorter interframe space, SIFS, is used to separate transmissions belonging to a single dialog (i.e., a node performs physical carrier-sensing for SIFS duration before transmitting CTS, Data and ACK frames). B. Before and After Earlier to , initial ideas on ALOHA-based protocols [4], [5] were extended to multi-hop networks in MACA [3]. To handle the hidden terminal problems, the protocol proposed to use the RTS/CTS handshakes preceding a data transmission. MACA did not propose a MAClayer acknowledgment communication failures had to be handled by higher layers resulting in high performance penalties. However, MACA did not incur the overheads of explicit MAC-layer acknowledgments. Following MACA, Bhargavan et al. [2] proposed MACAW in which several interesting extensions were made to MACA. Explicit MAC-layer acknowledgment was one of the extensions. While performance improved, the control overhead for each data packet increased too. In this paper, we attempt to reduce the control overheads as in MACA, while retaining the benefits of MAC-layer acknowledgment of MACAW. In other words, our proposal on implicit acknowledgment utilizes the best of both MACA and MACAW. While the IEEE made changes to these schemes, it retained the explicit MAC-layer acknowledgment scheme, as described earlier. Later, several protocols have aimed to further improve on The DBTMA protocol [6] proposed to use multiple transceivers to transmit busy-tones while a node was either transmitting or receiving. DBTMA showed how CTS could be eliminated from the control packet handshakes. Although control overhead and collision probability was shown to reduce using such techniques, the protocol was complex and difficult to incorporate. Recently, there have been several efforts toward regulating the control overhead in In [7], authors have proposed a protocol that adaptively controls the RTS/CTS mechanism by using a CTSTimeout Counter. The authors observe that carrier sensing, in many occasions, can obviate the need for RTS/CTS, and therefore the latter should be used only when necessary. In [8], authors reveal yet

3 another problem with that prevents TCP from performing well. The authors show that carrier sensing can prevent a receiver from replying with a CTS to the sender. As a result, the sender continues to transmit multiple times, increasing its backoff counter exponentially before each transmission. This leads to wastage in channel resources, and particularly affects TCP throughput. Similar observations have been made when using directional antennas with in [9]. In an effort to completely shift to a different channel access technique, Tsatsanis et al. [10] proposed network assisted diversity protocols, where the possibility of exploiting corrupted packets has been explored. Put differently, the authors propose the idea of allowing multiple transmitters to collide multiple times (synchronously). From the vector of corrupted packets, the receiver then separates the individual packets, using known signal processing algorithm. The authors show that such approaches might be effective in eliminating the additional control overhead in like protocols. Several of these protocols are interesting and may successfully improve upon wireless medium access control. Our proposal in this paper complements these ideas, and therefore can further improve the performance. The focus in this paper is to improve with a simple modification, while retaining backward compatibility. Our scheme requires a cross-layer interaction with the network layer, and is therefore in coherence with the growing awareness on the benefits of cross-layer design and architecture. We discuss the details of our protocol next. III. PROTOCOL DESCRIPTION In this section, we discuss the main ideas of our proposal in detail, and later discuss the issues and optimizations. Consider a simple scenario in which node A transmits multiple DATA packets to node B. Packets are generated at some rate by the application at node A, and passed down to the transport layer, which in turn passes them to the network layer. The network layer passes packets one by one to the MAC layer (since the MAC layer is not expected to maintain a packet queue). Once the MAC layer has finished transmitting a packet successfully, it requests for the next outgoing packet to the network layer, and the process continues. We describe our protocol as a sequence of events at the MAC layer. Before initiating an RTS transmission to node B, the MAC layer at node A determines if there are other packets for B in the network layer s outgoing queue. This information can be obtained either by explicitly querying the network layer, or by having the network layer pass this information along with each packet. If at least one other packet for node B is in the network layer s queue, then node A resets a flag in the RTS, i.e., Send-ACK-Flag = False. This flag is not originally in the RTS packet, and we propose to add this 1 bit information. If there are no packets for node B in the network layer s queue, then Send-ACK-Flag = TRUE. On receiving the RTS from node A, node B records the value of the Send-ACK-Flag, and replies with the CTS. Later, on receiving the DATA packet from node A, node B transmits an ACK packet only if the the Send-ACK-Flag in the RTS was set to TRUE, otherwise it omits sending the ACK packet and records the outcome of the DATA packet reception. The outcome of the reception indicates whether the DATA packet was received successfully or not 2. Let us assume that node A had multiple packets for node B in its queue, and therefore had set the Send-ACK- Flag = TRUE within the RTS. As a consequence, node B had not replied with an ACK packet at the end of the dialog. Observe that at the end of the first dialog (i.e., RTS/CTS/DATA), node A is unaware whether the DATA packet was successfully received by node B. Now, node A must initiate RTS transmission for the next packet in the queue. On receiving the RTS packet from node A, node B piggybacks the acknowledgment for the previous DATA packet, on the CTS. If the outcome of the previous transmission was a reception-success, then node A transmits the new DATA packet. Otherwise node A transmits the previous DATA packet once again, and temporarily stores the new packet for the next round of channel access. When node B receives an old DATA packet, it always replies with an explicit acknowledgment packet. The key ideas are illustrated in Figure 1. Note that as long as there are packets for node B in the network layer queue, one ACK packet (for every DATA packet) needs to be less transmitted. Now consider the last packet that needs to be transmitted to B. Since the MAC layer at A realizes that there are no more packets to be transmitted to B, it sets Send-ACK-Flag = FALSE in the RTS. As a result, node B would send explicit acknowledgment (ACK) packet for the last packet, and the session Observe that if node B does not receive the DATA packet within a timeout duration specified in the standard, then the outcome is set to reception-failure. If the packet is received successfully, then node B passes the packet to its network layer, and remembers the outcome as a reception-success.

4 RTS Data Backoff RTS Data CTS ACK CTS ACK IEEE Timeline Implicit ACK For Previous Packet RTS Data Backoff RTS Data CTS CTS Our Proposal on Implicit Acknowledgment Fig. 1. Illustrating the implicit-ack improvement to would successfully terminate. In summary, when the sending rate at a transmitter is high or bursty, the outgoing queue at the transmitter is likely to have multiple packets for the same receiver. This is an opportunity to eliminate the ACK packet for every DATA packet, because the CTS of the next packet can be piggybacked with an acknowledgment for the previous packet. Clearly, the overhead of explicit acknowledgments can be eliminated leading to improvements in throughput and delay. In addition, there is always a non-zero probability that ACK packets are lost or corrupted when using , and the RTS/CTS/DATA/ACK dialog is unnecessarily initiated once again. With our scheme such possibilities are eliminated, resulting in encouraging performance improvements. Our proposal requires a few modifications to the IEEE protocol. We discuss these modifications in detail next. Buffering one packet at MAC layer Our proposal requires that a node needs to provision additional buffer space for one DATA packet for every distinct neighbor that it is currently transmitting to. The reason is as follows. When node A has transmitted a to node B, it must buffer this packet until it has initiated the next round of RTS/CTS (for ) to node B. If node A learns from B s CTS that the was received successfully at B, then node A flushes from its buffer. Otherwise node A re-transmits. Observe that with our scheme, a transmitter will need to buffer one packet for each distinct neighbor that it communicates to. In real life, a node may not transmit to many distinct neighbors, and therefore it need not provision buffers for too many packets. However, if there are indeed restrictions on buffer space 3, a node may choose to perform our scheme for only a subset of the neighbors. This is implementation-specific and can be decided upon by the network designer. Remembering outcomes of reception Similar to a transmitter, a receiver also needs to remember the outcome of reception from each distinct transmitter. However, this information can be stored using one bit for each distinct transmitter, and is therefore not a cause of concern. Observe that both packet buffers and reception outcomes are not hard state stored in the protocol. For example, in case of failures, the transmitter/receiver can conservatively assume that the old packet was not received successfully, and can initiate the process of retransmission. Packet Sizes and NAV Settings With our implicit ACK scheme, a problem arises when the transmitter needs to include the network allocation vector (NAV) duration in the RTS of a new packet. If an old data packet is waiting for a piggybacked acknowledgment, and if the old packet is different in size from the new packet, then it is unclear what the NAV value must be. A conservative approach could be to assign the larger of the two NAV values. However this can lead to channel wastage. In current wireless interfaces, this is not a problem

5 We propose the transmitter to include the NAV value in the RTS, as calculated from the size of the new packet. Observe that the receiver is aware of which packet, old or new, would follow the RTS/CTS handshake. Thus, the receiver updates the NAV in the CTS accordingly. On receiving the CTS, the transmitter recalculates the NAV (if necessary), and updates the data packet with this potentially new NAV. Clearly, this simple scheme results in no wastage of the wireless channel. Network Layer Queue of outgoing packets Pkt i+1 to B Pkt j+1 to C Pkt j to C Pkt i to B A B IV. DISCUSSION We discuss several issues and tradeoffs associated to our scheme on implicit acknowledgment. We propose optional parameters, variations, and optimizations to add flexibility to our basic scheme. From this point onward we only discuss the case in which the transmitter has multiple DATA packets, enqueued for a specific receiver, and therefore applies our implicit acknowledgment scheme. We do not concentrate on the other case (where only one packet for a receiver is enqueued) because the sequence of operations in such cases is identical to Latency When using , a DATA packet is transmitted multiple times until an acknowledgment is received for that packet. With our scheme, piggybacked acknowledgment for arrives along with the CTS for. It is possible that the RTS/CTS handshake for begins after a long duration from the point was trans- mitted. This can happen if packets for other neighbors are ahead of in the queue (See Figure 2). Moreover, the channel can be busy with other transmissions, and therefore might need to wait long before getting sent out. In such a case, if was not received successfully earlier, then the retransmission gets delayed. It might appear that the average latency of successful communication increases as a result of this. However, that is not true. Although retransmission of gets postponed in our scheme, observe that other packets get transmitted during this duration. In other words, for each packet that experiences a higher waiting time before retransmission, there is another packet that waits for a shorter duration before being transmitted. Moreover, elimination of explicit ACKs causes the dialogs to be shorter, thereby reducing the average end to end delay in communication. Of course, an issue with our scheme is that the time between transmission and retransmission is unpredictable and can lead to a larger variance. Applications that have specific timing constraints, or depend on the variance of communication latency, may prefer quick retransmission Fig. 2. Once is transmitted,, also intended for node B, must wait for and to be transmitted to node C. of the data packets, in the same spirit of To address this problem, we propose a simple adaptive optimization. A transmitter maintains a history of the previous piggybacked acknowledgments. If all piggybacked acknowledgments are negative, then the transmitter reverts back to original with explicit acknowledgments. Similarly, positive acknowledgments can prompt the transmitter to toggle back to our scheme. Based on network specific requirements, the value of and can be adjusted suitably. Accessing FIFO Queue The transmitter of a packet must look into the outgoing packet queue in order to decide whether the receiver must respond with explicit acknowledgments. With regular FIFO queue data-structures, this might not be easy. We propose to use a separate table that counts the number of packets in the queue for each unique receiver. The network layer can make this table accessible to the MAC layer. When the network layer enqueues a packet for a receiver, it increments the counter for. When a packet for is transmitted by the MAC layer, it decrements the corresponding counter. Using this mechanism, the MAC can tell at any given time, whether the FIFO queue contains a packet for any specific receiver. ACK Failures The benefits of eliminating explicit acknowledgments can be even more pronounced when the collision probability of ACK packets are considered. Recall that on a failure to receive an ACK, IEEE requires the transmitter to re-initiate a fresh dialog, starting from backoff, followed by an RTS/CTS handshake. When channel fluctuations are high, failures to receive ACKs (even after a successful data communication) can result in loss of throughput. With C

6 our proposal, such possibilities are eliminated, thereby increasing the throughput of the network. Energy Conservation Several proposals in the past have attempted to reduce energy consumption either by making modifications to the protocol, or by designing new MAC protocols [11],[12],[13]. However, in optimizing for power, many of these protocols tradeoff throughput performance, and sometimes protocol complexity. In this regard, our modification to can reduce power consumption while achieving significant performance gains. Since the ACK packets are not transmitted for each data packet, transmitters save on transmission energy, while receivers save on receiving energy. In addition, nodes in the vicinity of the communicating nodes no longer need to overhear these packets, adding to the total power conserved in the entire network. Piggybacking Overheads To be able to request implicit or explicit acknowledgments, a transmitter requires to include a boolean flag in the RTS. Similarly, the receiver needs to include a boolean acknowledgment flag in the CTS. Clearly, the boolean flags can be a single bit the overheads thereby can be negligible. V. PERFORMANCE EVALUATION We evaluated our proposal on the Qualnet simulator, version 3.1, and compared the performance with We used 11 Mbps data rates, and assumed 30 nodes in a bounded region of sqm. We used 12 randomly chosen source nodes to send CBR traffic over two hop flows. Aggregate Throughput (Mbps) Aggregate throughput in multi-hop networks (512 Byte packets) Proposed Scheme Sending Rate (Mbps) Fig. 3. The improvement in throughput in ad hoc scenarios Figure 3 shows the variation of aggregate throughput with increase in sending rate. Clearly, when the sending rate is low, the queue at the network layer does not contain more than one packet at a time, and therefore our scheme does not come into play. During this phase, our scheme performs exactly as However, as the load increases, packets intended for the same receiver get enqueued in the queue of the transmitter, and our scheme takes effect from that time on. The improvement in throughput results from the elimination of the explicit acknowledgment packets. Observe that when nodes are almost always backlogged, the improvement in throughput is almost 25%. This is encouraging, given the small network with only a few hop flows. The control overhead is certainly expected to be less severe if the size of DATA packets increase. We simulated CBR traffic with packet size of 1024 Bytes. Our results in Figure 4 show that the improvements are still conspicuous. Of course, the percentage improvements are less than scenarios in which packet sizes are 512 Bytes, but the improvements are not marginal. Aggregate Throughput (Mbps) Aggregate throughput in multi-hop networks (1024 Byte packets) Proposed Scheme Sending Rate (Mbps) Fig. 4. The improvement in throughput in ad hoc scenarios with larger packet size of 1024 Bytes Avg. End to End Delay (s) Average end to end delay in multi-hop networks Proposed Scheme Sending Rate (Mbps) Fig. 5. The improvement in end to end delay in ad hoc scenarios Figure 5 shows the variation of average end to end delay for the same scenarios used in Figure 3. As discussed earlier, end to end delay improves over primarily because transmission of the ACK can be eliminated, and thereby subsequent transmissions can be initiated ear-

7 lier. Of course, when the sending rates are lower than 0.5 Mbps, there is again no difference in delay because our scheme degenerates into the standard RTS/CTS/Data/ACK sequence of communication. We also observed the variation of throughput with packet sizes, as shown in Figure 6. Since the control overhead of RTS, CTS, and ACK packets become pronounced with smaller Data packets, the improvement in throughput becomes significant with our proposed scheme. This is because the fraction of the channel that is allocated to control packets grows larger with smaller data packets. Eliminating the ACK packets can alleviate this degradation, showing improvement in performance. Percentage Throughput Improvement(Mbps) Variation of througput with Packet Size % improvement over Packet Size (Bytes) Fig. 6. The improvement in throughput for smaller packet sizes VI. FUTURE WORK We have proposed a basic scheme which can effectively reduce the control overhead of , thereby improving on aggregate throughput and average end-to-end delay. However, it is possible that the variance of end-to-end delay increases because two consecutive packets can be scheduled with a long time-gap. One possibility to handle such an issue could be to maintain separate queues for each neighboring node. Now, a scheduling algorithm can be executed that prioritizes queues and does not schedule other queues if there is an unacknowledged packet for one queue. This would prevent a packet from being buffered for a long time in the MAC layer. In future, we plan to evaluate the impact of such a scheme on flows requiring QoS guarantees. Another case where our scheme can be beneficial is when used in conjunction with directional antennas. To avoid collision of the ACK packets, directional MAC protocols require the transmitter to carrier-sense conservatively before initiating transmission [14],[15],[9]. If the need for ACK packets are eliminated, directional communications can extract higher channel utilization by being more aggresive. We plan to propose modifications to directional MAC protocols in light of our implicit acknowledgment scheme. VII. CONCLUSION IEEE is a MAC protocol that prescribes the use of RTS/CTS to handle the hidden and exposed terminal problems in wireless LANs and ad hoc networks. We proposed optimizations to the IEEE protocol that can yield significant benefits, while incurring only a few bits of overhead in the RTS/CTS packets. Our proposal exploits the opportunity of back-to-back transmission of DATA packets, and proposes to use the second dialog to acknowledge the first, thereby removing the need for explicit acknowledgment (except for the last packet). We presented variations to our scheme that incorporates flexibility into the protocol. Simulation results show encouraging improvement in performance, leading us to investigate how our scheme can be even more effective when using directional antennas. REFERENCES [1] IEEE , Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, [2] V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, MACAW: A media access protocol for wireless LAN s, in Proceedings of ACM SIGCOMM, October 1994, vol. 24. [3] P. Karn, MACA - a new channel access method for packet radio, in Proceedings of 9th Computer Networking Conference, [4] J. Ward and R. Crompton, Improving the performance of slotted ALOHA packet radio networks with adaptive arrays, IEEE Transactions on Communications, vol. 40, no. 11, pp , [5] J. Zander, Slotted ALOHA multihop packet radio networks with directional antennas, Electronic Letters, vol. 26, no. 25, pp , [6] J. Deng and Z. Haas, Dual Busy Tone Multiple Access (DBTMA): A New Medium Access Control for Packet Radio Networks, IEEE Transactions on Communications, [7] H. Ju, I. Rubin, and Y. Kuan, An adaptive rts/cts control mechanism for ieee mac protocol, in Proceedings of IEEE VTC, [8] S. Xu and T. Saadawi, Does the IEEE MAC protocol work well in multihop wireless ad hoc networks, IEEE Communications Magazine, June [9] R. Roy Choudhury and N. H. Vaidya, Deafness: A MAC problem in ad hoc networks when using directional antennas, Tech. Rep., Proceedings of the IEEE International Conference on Network Protocols (ICNP), October [10] M. tsatsanis and R. Zhang, Network assisted diversity for random access wireless networks, IEEE Transactions on Signal Processing, vol. 48, no. 3, March [11] E. Jung and N. H. Vaidya, A power saving MAC protocol for wireless networks, in Proceedings of Infocom, [12] Tamer A. ElBatt, Srikanth V. Krishnamurthy, Dennis Connors, and Son Dao, Power management for throughput enhancement in wireless ad-hoc networks, in IEEE International Conference on Communications, 2000, pp

8 [13] Suresh Singh, Mike Woo, and C.S. Raghavendra, Power aware routing in mobile ad hoc networks, in Proceedings of MOBI- COM, 1998, pp [14] R. Roy Choudhury, X. Yang, N. H. Vaidya, and R. Ramanathan, Using directional antennas for medium access control in ad hoc networks, in Proceedings of ACM MOBICOM, September [15] R. Ramanathan, On the performance of ad hoc networks with beamforming antennas, in Proceedings of ACM MobiHoc, October 2001, pp

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