2012 IEEE 23rd International Symposium on Personal, Indoor and Mobile Radio Communications - (PIMRC) A Directional MAC Protocol with the DATA-frame Fragmentation and Short Busy Advertisement Signal for Mitigating the Directional Hidden Node Problem Sho Motegi, Hiroo Sekiya, Jing Ma, Kosuke Sanada and Shiro Sakata Graduate School of Advanced Integration Science, Chiba University, Chiba, 263 8522 Japan Email: z8t1580@students.chiba-u.jp Abstract This paper proposes a MAC protocol for ad hoc networks with directional antennas for mitigating the directional hidden-node problem. In the proposed protocol, it is possible to tell the communication situation to the directional hidden node by using DATA-frame fragmentations and short busy advertisement signals. As a result, the frame collisions induced by the directional hidden nodes can be reduced and the network throughput is enhanced compared with the conventional protocol. Simulation results show the validity and effectiveness of the proposed protocol. I. Introduction Wireless communication using a beam-forming of the directional antennas [2]-[5] enhances the spatial-reusability of the network. In [2], a MAC protocol for directional antenna networks was proposed. In [2], IEEE 802.11 with RTS/CTS handshakes is applied to directional antenna networks, which is called DMAC. In the DMAC protocol, network throughput improves because of the high spatial reusability. However, a new type of hidden node problem, which is called directional hidden-node problem, appears in the directional antenna networks. Directional hidden nodes are generated when a node cannot hear the RTS/CTS exchange because of the own communication to another direction. The directional-hidden-node problem limits the network throughput of the DMAC system. The mitigation of the directional hidden-node problem is one of the important problems for more enhancement of the directional-antenna-network throughput. In [6], a Short Busy Advertisement MAC (SBA-MAC) protocol was proposed for the special hidden-node problem reduction in omni-directional antenna networks. Hidden nodes, which were treated in [6], appears when the node cannot receive an RTS frame correctly because of own CTS frame transmission to another node. In [6], the DATA-frame fragmentation and the Short Busy Advertisement, (SBA) signal are used for avoiding data-frame collisions. The transmitter divides the DATA frame into several data fragments and inserts a small interval between two adjacent data-fragment transmissions. The receiver sends a SBA signal to tell neighbor nodes what the receiver is in ongoing communication during the data-fragment-reception intervals. All the nodes, which detect the SBA signal, defer their transmission opportunities even though they cannot receive the RTS and/or CTS frames. Therefore, hidden-node collisions are reduced in high node-density situation and heavy offered load environments, in particular. We find that appearance mechanism of hidden nodes pointed out in [6] is different from but similar to that of the directional hidden nodes. Therefore, it may be a good idea that the SBA-MAC concepts are applied to the protocol design for solving the directional hidden-node problem. In this paper, a MAC protocol for ad hoc networks with directional antennas is proposed for mitigating the directional hidden-node problem. In the proposed protocol, SBA-MAC protocol concept applied in a DMAC protocol. Therefore, the proposed protocol is named as SBA-DMAC. By using the DATA-frame fragmentation and the SBA signals, it is possible to tell the communication situation to the directional hidden node. As a result, the frame collisions induced by the directional hidden nodes can be reduced dramatically and the network throughput is enhanced. Simulation results show the validity and effectiveness of the SBA-DMAC protocol. II. Related works A. DMAC and directional hidden-node problem Wireless communications using a beamforming of the directional antennas enhance the spatial-reusability of the network [2]-[5]. A MAC protocol for directional antenna networks which is called DMAC, was proposed in [2]. The DMAC protocol is recognized as the basic MAC protocol for directional antenna systems. The network spatial reusability of the directional antenna networks is higher than that of the omni-directional antenna networks because all frames i.e. RTS/CTS/DATA/ACK are transmitted by using directional antennas. Therefore, the network throughput of directional-antenna networks can be improved compared with the omni-directional antenna networks. In the DMAC protocol, however, collisions induced by hidden nodes degrade the network throughput. Additionally, the directional hidden node problem, which never occurs in omni-directional-antenna networks, appears in directional antenna networks [4]. Figure 1 shows an example of the directional hidden-node problem. In Fig. 1 (a), the node H transmits a frame to the node I by using the directional 978-1-4673-2569-1/12/$31.00 2012 IEEE 409
Fig. 2. An example scenario in the omni-directional antenna networks. (a) the hidden-node collision in the 802.11 with RTS/CTS handshakes (b) the hidden-node-collision avoidance in the SBA-MAC protocol Fig. 1. An example scenario of the collision due to the directional hidden node problem in the directional antenna networks antenna. Meanwhile, the node S communicates with the node R with the directional antennas and the receiver node R transmits a CTS frame. In this case, the node H cannot hear the CTS frame because the node H beamforms toward the node I. When the node H would like to transmit a new frame to the node R, an RTS frame from the node H to the node R collides with the DATA frame from the node S to the node R as shown in Fig. 1 (b). This is because the node H is not aware of the ongoing communication between the nodes S and R due to the previous frame transmission to the node I. This is an occurrence mechanism of the directional hidden-node problem. This type of the hidden node problem is a dominant factor of the throughput degradation in directional antenna networks. B. MAC protocol using the DATA-frame fragmentation and short busy advertisement signal The MAC protocol with Short Busy Advertisement (SBA-MAC) was proposed in [6] for avoiding the frame collisions induced by hidden node in omni-directional antenna networks. In the SBA-MAC protocol, hidden-node collisions can be mitigated effectively because the receiver periodically sends SBA signals to tell neighbor nodes what the receiver is in ongoing communication. Figure 2 (a) shows a scenario of hidden-node collisions in the IEEE 802.11 with RTS/CTS handshakes, which was pointed out in [6]. In Fig. 2 (a), the node R starts transmitting a CTS frame for replying for the RTS-frame from the node S. Meanwhile, the node H, which is a hidden node of the node S, starts transmitting the RTS frame to the node R. The node H cannot receive the CTS frame from the node R because the node H is transmitting the RTS frame to the node R. In this case, the node H is not aware of the communication between the node S and R. When the node H retransmits an RTS frame to the node R as shown in Fig. 2 (a), the RTS frame collides with the DATA-frame from the node S. The SBA-MAC protocol is designed for avoiding this type of the collisions. Figure 2 (b) shows an example time series of the SBA-MAC protocol. In the SBA-MAC protocol, the node S divides the DATA frame into several fragments. Figure 3 shows a message sequence of the SBA-MAC protocol. After finishing the RTS/CTS exchanges, the transmitter divides the payload of the DATA frame into several fragments and inserts a 410
Fig. 3. An example of the message sequence in the SBA-MAC protocol small interval between two adjacent data-fragment as shown in Fig. 3. The interval is called Inter-Data-Frame Spacing or Inter-Data-Fragment Spacing (IDFS). The IDFS durations are needed for receiver transmission of the SBA signal. The IDFS duration includes T RT and T TR as shown in Fig. 3, which are duration of the switching time from the reception mode to the transmission one and that from the transmission mode to the reception one, respectively. When the node R successfully receives the first data fragment, the node R sends a SBA signal to the neighbor nodes for notifying the reception state of the receiver during the IDFS period. The time length of the SBA duration is long enough for other nodes to sense the SBA signal. Any node sensing the SBA signal should keep silent for a certain period, denoted as Inter-Frame Spacing due to short Busy advertisement (BIFS) [6], by setting the Network Allocation Vector (NAV). Therefore, the data-fragment transmissions can be protected and the hidden-node problem as shown in Fig. 2 (a) can be mitigated in the SBA-MAC protocol. In the SBA-MAC protocol [6], the larger the data-fragment number is, the more number of the IDFS should be inserted between two adjacent data-fragment transmissions. When the period for sending the SBA is short, it is possible to avoid frame collisions induced by the frame-collision mechanism as shown in Fig. 2 (a) with high probability. This factor enhances the network throughput. Excessive data-frame fragmentations, however, cause the overhead enlargement, which suppresses the network throughput. The overhead due to the unnecessary DATA-frame fragmentation may dominantly degrade the network throughput for light offered load networks, in particular. Therefore, it is one of the good strategies that the fragment number is adoptively changed according to the network environments. We find that the mechanism of the frame collision in Fig. 2 (a) is different from but similar to that of the directional hidden-node problem as shown in Fig. 1. It is thought that the SBA-MAC concept may be effective for mitigating the directional hidden-node problem. III. Protocol description In this paper, a MAC protocol for ad hoc networks with directional antennas is proposed. In the proposed protocol, the SBA-MAC-protocol concept is applied in the DMAC protocol [2]. Therefore, we named the proposed protocol as SBA-DMAC. Fig. 4. A flowchart for the transmitter in SBA-DMAC protocol A. Basic operations of the SBA-DMAC protocol Figure 4 shows a flowchart of the SBA-DMAC protocol for the transmitter. In the SBA-DMAC protocol, a transmitter transmits multiple data fragments continuously posterior to the RTS/CTS exchange. The DATA-frame-fragmentation method of the SBA-DMAC is the same as that of the SBA-MAC as shown in Fig. 3. When a node has a transmission frame, the node senses the channel in omni-directional mode. If the transmitter confirms that the channel is free, the node transmits the an RTS frame to the receiver using a directional antenna and sets a CTS-wait timer for preparing the CTS frame reception with beam-forming toward to the receiver [5]. In the SBA-DMAC protocol, information of both the time duration of the DATA frame and the predefined data-fragment number, which expresses F in Fig. 4, is are included in the RTS frame. If the transmitter cannot receive a CTS frame during the CTS-wait duration, it retransmits the an RTS frame by doubling the contention window (CW) value. When the transmitter receives a CTS frame successfully, the transmitter starts the first data-fragment transmission to the receiver with directional antenna and the transmitter transmits the data fragments continuously with IDFS intervals. In the SBA-DMAC protocol, the transmitter checks the SBA signal from the receiver after every data-fragment transmissions. When the SBA signal cannot be detected successfully, the transmitter stops data-fragment-transmission process for avoiding the unnecessary transmission-time wastage. In this case, the transmitter starts to prepare the retransmissions by doubling the CW value. When the transmitter finishes a last data-fragment transmission, the transmitter sets an ACK-wait timer for waiting the an ACK frame as a response. If the transmitter cannot receive the an ACK frame in the ACK-wait 411
Fig. 5. A flowchart for the receiver in SBA-DMAC protocol Fig. 6. An example of mitigating the directional hidden node problem in SBA-DMAC protocol duration, it retransmits an RTS frame by doubling the CW as shown in Fig. 4. When the transmitter can receive the an ACK frame, the transmitter recognizes that the frame transmission is succeeded. Figure 5 shows a flowchart of the SBA-DMAC protocol for the receiver. When a node receives an RTS frame, the receiver transmits a CTS frame to the transmitter with the time duration of the DATA frame and the data-fragment number. The data-fragment-length information is needed for advertising the directional hidden node of the transmitter. Data-fragment length is calculated from the DATA-frame length and the data-fragment number included in the RTS frame. When the receiver cannot receive any data fragment during the data-fragment-length period, the receiver escapes from the reception state. When the receiver receives a data fragment successfully, the receiver sends the SBA signal for notifying not only the success of fragment reception to the transmitter but also the ongoing communication situation to directional-hidden nodes. When the receiver receives the last data fragment, it replies an ACK frame to the transmitter. When a node receives an RTS frame but the node is not a receiver, the node defers its frame-transmission process toward the RTS-reception direction by setting the Directional Network Allocation Vector (DNAV) during the CTS-frame-transmission duration and the double of Short Inter Frame Spacing (SIFS) period. When a node receives a CTS frame without RTS-frame reception, the node defers its frame-transmission process toward the CTS-reception direction by setting the DNAV during one data-fragment transmission duration, which can be obtained from the CTS frame, and the (IDFS + SIFS) period. Additionally, when a node only detects a SBA signal, the node also defers its transmission toward the signal-detection direction by setting the DNAV. In concrete, if the node detects the first SBA signal at t = t 1, the node defers its transmission for half time length of the longest DATA-frame transmission time because the node has no information about the data-fragment length. If the node can detect a next SBA signal, at t = t 2, the node rests DNAV value to ( t 2 t 1 ), which is the data-fragment length. B. Hidden-node-collision reduction In the SBA-DMAC protocol, collisions due to the directional hidden node problem can be reduced by using the DATA-frame fragmentations and the SBA signals. Figure 6 shows an example of the directional-hidden-node-problem mitigation in the SBA-DMAC protocol under the same scenario as shown in Fig. 1. In the DMAC protocol as shown in Fig. 1, the node H is unaware of the communication between the node S and R because of the directional hidden node problem. In the SBA-DMAC protocol, because the node R sends the SBA signals periodically, the node H can detect the SBA signal of the node R before the node H starts frame transmission. By detecting the SBA signal, the node H accordingly defers its frame-transmission by setting the DNAV as shown in Fig. 6. As a result, the collisions due to the directional hidden node problem can be mitigated in the SBA-DMAC protocol when the proper fragment length is set. IV. Performance Evaluations The SBA-DMAC protocol is evaluated by simulations. Generally, directional transmissions have larger transmission range than omni-directional transmissions. Therefore, the directional beam-forming may potentially interfere with communications taking place far away. In this paper, however, we would like to focus on the gains from spatial reuse exclusively. Therefore, it is assumed that the transmission distance of the directional antenna is the same as that of the omni-directional antenna. In the simulation, it is also assumed that each node can comprehend all neighbor nodes and their directions. For investigating effects of the DATA-frame fragmentation and the SBA signal usage to directional hidden 412
TABLE I SIMULATION PARAMETERS UDP/IP header 28 bytes MAC header 20 bytes PHY header 6 bytes DATA-payload size 4095 bytes RTS size 20 bytes CTS size 14 bytes ACK size 14 bytes Channel bit rate 11 Mbps PHY header bit rate 1 Mbps Angle of antann beam π/2 Transmission range 150m Carrier sensing range 150m Distance of each node 100m Slot Time 20 µsec SIFS Time 10 µsec DIFS Time 50 µsec SBA Tx Time 5 µsec Minimum CW Size 31 Maximum CW Size 1023 Simulation time 5sec Retransmission limit 7 Fig. 8. Average throughput as a function of the offered load per flow for fixed data-fragmentation number Fig. 7. A network topology for simulations node problem, the adoptive variations of the data-fragment number is not considered and the fixed data-fragment number is used. The simulation parameters are given in Table I, which basically follow those in IEEE 802.11b standard [1]. Data-channel and Control-channel rates are 11 Mbps and 1 Mbps, respectively. Figure 7 shows a simulation network topology. Nodes are placed as 4 4 grid topology as shown in Fig. 7 and the node mobilities are not considered. In the simulations, adjacent nodes are included in the transmission range. For example, there are the nodes 2, 5 and 6 in the node-1 transmission range. The flow pattern is fixed and there are eight flows as shown in Fig. 7. The source of the flows generate DATA frames randomly in time with specified offered load. Figure 8 shows average throughputs per flow as functions of the offered load per flow for fixed data-fragmentation number. In this figure, the plots for F = 1 expresses the throughput for DMAC. The maximum data-fragment number is 15 because the PHY- and MAC-header should be in the first fragment. It is seen from Fig. 8 that the highest throughput can be obtained when the data-fragment number is 6. When the data-fragment number is lower than the optimal value, the network throughput degrades because a directional hidden node cannot detect a SBA signal prior to a new frame transmission. Conversely, the network throughput is also degraded when the frame-fragmentation number is larger than the optimal value. This is because unnecessary IDFS periods increase, which compresses the network throughput. It is confirmed from the Fig. 8 that there is a trade-off relationship between the hidden-node collision reduction and the overhead increase. Figure 9 shows throughputs per flow as functions of the offered load per flow for the proposed and conventional protocols. In this paper, the IEEE 802.11 with RTS/CTS handshakes (IEEE 802.11) [1], SBA-MAC [6], and DMAC [2] are investigated as conventional protocols. In this simulation, the SBA-MAC and SBA-DMAC protocol use the optimal data-fragment numbers, which are 9 and 6, respectively. It is seen from Fig. 9 that the throughputs of the SBA-MAC and SBA-DMAC are higher than that of the IEEE 802.11 and DMAC respectively. Figure 10 shows the DATA-frame collision probabilities, which are defined as the ratio of the DATA-frame collision number to the DATA-frame transmission one, as functions of the offered load per flow. In the SBA-based protocol, the DATA-frame transmission is in failure when at least one data-fragment is collided with other frames. It is seen from Fig. 10 that the DATA-frame collision probability of the SBA-DMAC is lower than that of the DMAC. This is because the frame collisions induced by directional hidden nodes are reduced effectively in the SBA-DMAC. Additionally, the DATA-frame collision probabilities of the DMAC and SBA-DMAC protocols are lower than those of the IEEE 802.11 and SBA-MAC protocols, respectively. This is because the usage of the directional 413
Fig. 9. Average throughput as a function of the offered load per flow with conventional protocols Fig. 10. DATA-frame collision probability as a function of the offered load per flow with conventional protocols antenna reduces the DATA-frame collision probability. It is also seen from Fig. 10 that the difference of DATA-frame collision probabilities between the IEEE 802.11 and DMAC protocol is smaller than that between the SBA-MAC and the SBA-DMAC protocols, respectively, when the offered load is high. In the DMAC protocol, the collisions due to the directional hidden-node problem progressively increase as the offered load increases. In the SBA-DMAC protocol, the usage of the SBA signals can effectively mitigate the directional hidden-node problem. Therefore, the reduction rate between the SBA-DMAC and SBA-MAC protocols is much higher than that between the DMAC and IEEE 802.11 at heavy offered load. As a result, the SBA-DMAC protocol can keep high network throughput at the heavy offered load as shown in Fig. 9. It is also seen form Fig. 9 that the throughput difference between the SBA-DMAC and DMAC protocols is higher than that between the IEEE 802.11 and SBA-MAC protocols, respectively, at heavy offered load, in particular. This is because the probability of the directional hidden node occurrence is higher than that of the hidden node occurrence pointed out in [6]. This result indicates that the application of the DATA-frame fragmentation and the SBA signal usage to the directional antenna networks are very effective for mitigating directional hidden-node problem, which is a dominant factor of the throughput degradation. V. Conclusion and future works This paper has proposed a MAC protocol for ad hoc networks with directional antenna networks for mitigating the directional hidden-node problem. In the proposed protocol, it is possible to tell the communication situation to the directional hidden node by using the DATA-frame fragmentation and the SBA signals. As a result, the frame collisions induced by the directional hidden nodes can be reduced dramatically and the network throughput is improved. Simulation results show the validity and effectiveness of the proposed SBA-DMAC protocol. Though the SBA-MAC protocol fixes the data-fragment length, it is possible for the proposed protocol to adjust the data-fragment length according to network environments by inserting the fragmentation information into the RTS and CTS frames. The data-fragment number should be determined by the node density and the traffic amount. The detection methods of network environment information and the adoptive variations of the fragment number are very important problem in the SBA-DMAC we should address in the future. Acknowledgment This research was partially supported by Scholarship Foundation and Grant-in-Aid for scientific research (No.23656249 and No.2336017) of JSPS, Japan. References [1] ANSI/IEEE Standard 802.11, Part 11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications, 1999. [2] Y.-B. Ko, V. Shankarkumar, and N. H. Vaidya, Medium access control protocols using directional antennas in ad hoc netwroks, in Proc. IEEE INFOCOM, vol 1, Tel-Aviv, Israel Mar. 2000, pp. 13-21. [3] Subramanian, A.P., Das, S.R., Addressing deafness and hidden terminal problem in directional antenna based wireless multi-hop networks, Wireless networks., vol.16, issue.6, pp.1557-1567, August 2010. [4] R. R. Choudhury, X. Yang, R. Ramanathan, and N. H. vaidya, On designing MAC protocols for wireless networks using directional antennas, in IEEE Trans. Mobile Computer, vol. 5, no.5, pp.477-491, May 2006. [5] Bazan. O and Jassemuddinn. M, On the design of opportunistic MAC protocols for multihop wireless networks with beamfoming antennas, in IEEE Trans. Mobile Computing, vol. 10, pp.305-319, March 2011. [6] H. Zhai and Y. Fang, A solution to hidden terminal problem over a single channel in wireless ad hoc networks, Proceedings - IEEE Militariy Communications Conference MILCOM, art no. 4086637, Oct. 2006. [7] Bazan. O and Jassemuddin, M, A survey on MAC protocol for wireless adhoc networks with beamforming antennas in Communications Surveys & Tutorials, IEEE, vol.pp, pp.1-24, April 2011. 414