Globecom 2013 Workshop - Broadband Wireless Access
Fighting Against Access Collision And Hidden Node Problem in Broadcast Scheme Of Wireless Ad Hoc Networks Xianbo Chen, Lih-Feng Tsaur
Hazem H. Refai
Xiaomin Ma
Broadcom Corporation San Diego, CA 92127 Email: {xianbo, lftsaur}@broadcom.com
The University of Oklahoma Norman, OK 73019 Email:
[email protected]
Oral Roberts University Tulsa, OK 74171 Email:
[email protected]
Abstract— Broadcast service is an integral part of wireless networks, through which important control and signaling information are typically exchanged. However, its multi-receiver nature determines that broadcast suffers more from access collision and hidden node problem. In wireless ad hoc networks, the phenomenon becomes even worse due to the absence of a central coordinator. Existing media access control (MAC) protocols in wireless ad hoc networks were primarily designed for unicast service and the potential broadcast support is not satisfying. In this paper, we introduce a MAC layer protocol, namely Parallel Signaling and Persistently Synchronous MAC (PS2 MAC) that can significantly alleviate both access collision and hidden node problem. A novel relay technique is deployed in PS2 MAC so that a superior broadcast performance, such as throughput and packet delivery ratio, is achieved. The conclusion is supported with simulation analysis.
I. INTRODUCTION Broadcast service is an integral part of wireless networks, through which important control and signaling information are typically exchanged. However, its multi-receiver nature determines that broadcast suffers more from access collision and hidden node problem. In wireless ad hoc networks, the phenomenon becomes even worse due to the absence of a central coordinator [1]. The MAC layer protocol, which cooperates with the physical layer and coordinates nodes to share the common medium efficiently and fairly, plays a critical role to resolve the above two issues. Existing MAC protocols, for example IEEE 802.11 [2], in wireless ad hoc networks were primarily designed for unicast service and the potential broadcast support is not satisfying. Previous research on MAC protocol design and enhancement has been conducted [3]–[8]. Authors in [3], [4] use the time division multiplexing (TDMA) specifically for vehicular networks. In [5], BlackBurst consisting of pulses of energy jams the channel to achieve successful contention; however the hidden node problem is not addressed. Dual busy tone multiple access (DBTMA) in [6] utilizes two tone signals to prevent exposed/hidden nodes from initiating new transmission. The busy tone technique is uncomplicated, although three sets of transceivers are required to implement DBTMA. Contentiontone protocol (CTP) is proposed in [7], yet the hidden node 978-1-4799-2851-4/13/$31.00 ©2013IEEE
problem is not addressed. The BSMA algorithm in [8] is required to have a ”very good” capture capability in order to correctly interpret the clear-to-send packet. Authors in [8] directly apply unicast request-to-send/clear-to-send into broadcast. This may make the source believe the transmission is going well while many receivers are suffering from collision or interference due to hidden node problem. In this paper, we introduce a MAC layer protocol, namely Parallel Signaling and Persistently Synchronous MAC (PS2 MAC), which combines the synchronicity of TDMA and the flexibility of random access. The concept of parallel processing is realized by utilizing separate data channel and signaling channel so that a high bandwidth efficiency is achievable. On the signaling channel, any signal sensing techniques can be adopted. We propose a novel relay technique. This technique can significantly alleviate hidden node problem. Through simulation analysis, we show that PS2 MAC is capable of achieving a superior broadcast performance, such as throughput and packet delivery ratio (PDR) in wireless ad hoc networks. This paper is organized as follows. In section II, the concepts of access collision and hidden node problem in wireless communication are briefly introduced. Section III presents a detailed description of PS2 MAC’s basic mechanism. Broadcast performance comparison between IEEE 802.11 and PS2 MAC is carried out in Section IV after the system parameters and analytical environment are defined and clarified. In Section V, several important aspects regarding the implementation of PS2 MAC are studied. Finally, the conclusion is drawn in Section VI. II. ACCESS C OLLISION
AND
H IDDEN N ODE P ROBLEM
Access Collision. In wireless ad hoc networks, each node autonomously decides when and how to transmit a packet at a given time. Therefore, neighboring nodes may transmit at the same time and access collisions occur on the receivers. Because the on-the-fly collision detection is expensively impossible to implement, a smart mechanism must be deployed to distributively coordinate nodes to minimize access collision.
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A
Fig. 1.
B
At the beginning of a frame, each node sets its status to active if its packet queue is nonempty, or to inactive otherwise. pc is a constant called contention probability. The function rand generates a random number in [0, 1] each time being called. In each signaling slot, all nodes execute the procedure shown as the following pseudo-code. if my status is active and rand > pc signal in this contention sub-slot; else listen to this contention sub-slot; endif
C
The most basic hidden node problem topology frames
Inter-frame space
Data channel
time axis
Signaling channel
if I heard signal in this contention sub-slot set my status to inactive; signal in this relay sub-slot; endif
n signaling slots per frame contention sub-slot
Fig. 2.
relay sub-slot
2 sub-slots per slot
PS2 MAC structure
Hidden Node Problem. The most basic network topology with hidden node problem is shown in Fig. 1. Node B is in the transmission ranges of both A and C, but A and C cannot hear each other, signifying that both A and C are B’s potential hidden nodes and a collision will happen on B if A and C transmit simultaneously. In order to achieve high performance in wireless ad hoc networks, these two fundamental causes of the low performance must be effectively addressed. Meanwhile the simplicity and the cost of protocol implementation should be considered. III. PS2 MAC A. Basic Mechansim The protocol structure of PS2 MAC is shown in Fig. 2. In compliance with PS2 MAC, each node has two half-duplex transceivers that utilize different amounts of bandwidth and work on different channels (frequencies). The data channel use the wide bandwidth and hosts the data packets. The signaling channel runs any narrow-band signal sensing techniques, for example the tone technique [5]–[7]. The details of the signal sensing technique is out of the scope of this paper. In time domain, both channels are divided into frames with equal length. An inter-frame space exists between any two neighboring frames, giving transceivers adequate turnaround time. All nodes in the network are synchronized so that they have the same knowledge of the start of each frame. The synchronization is achievable by means of various techniques. The global positioning system (GPS) is one of the choices and has been adopted for MAC designs in [3], [4]. Each frame of the signaling channel is divided into n signaling slots. Each signaling slot is further divided into two sub-slots, namely contention sub-slot and relay sub-slot. Nodes transmit data packets on the data channel at the start of a frame when they believe that they have won the access of this frame. The knowledge of winning the frame access is determined by the nodes’ cooperation on the signaling channel, which is described next.
if I did not signal in this contention sub-slot listen to this relay sub-slot; endif if I heard signal in this relay sub-slot set my status to inactive; endif After the nth signaling slot, any node whose status is still active wins the competition and can transmit its data packet in the immediate frame on the data channel. PS2 MAC makes access decisions in the signaling channel in parallel with the transaction of data transmission in the data channel. Nodes generate and relay signals to compete for next data frame while the current data frame transmission is in transaction. A node’s intention of accessing the data channel is announced in contention sub-slots and is further spread in relay sub-slots. Intuitively, it is easy to see that the access collision is addressed by signaling in contention sub-slots and that the hidden node problem is addressed by signaling in relay sub-slots. It is easily verifiable that if all nodes in Fig. 1 follow the procedure detailed above, it is highly likely that the access collision and hidden node problem will be resolved. B. Technical Discrimination 1) Contention Tone and Busy Tone: PS2 MAC and DBTMA are different. First, PS2 MAC uses signal sensing technique to contend for next data frame, where the tone technique is one of the options. DBTMA specifically uses tone signal to notify the current channel occupancy by a node. Second, PS2 MAC’s signaling transmission starts and ends before the corresponding data is transmitted. DBTMA’s tone transmission and data transmission go on simultaneously. Third, DBTMA transmits tone continuously to block other nodes from transmission, while PS2 MAC transmits signaling discretely according to the random value pc in each slot. 2) IEEE 802.11’s NAV And PS2 MAC’s Tone Relay: In IEEE 802.11, network allocation vector (NAV) is a virtual carriersense mechanism, maintaining a prediction of future traffic on
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Globecom 2013 Workshop - Broadband Wireless Access TABLE I
partitioning methodology, it is evident that the intersection of any two such groups is null and that no hidden nodes exist in one group. If a group has no interconnection with other groups, it is an isolated group and is free from the influence of hidden node problem. When any interconnection exists between any two groups, they affect each other through hidden node problem. The above definition and description imply that each of such groups is independently an object that can be investigated to see how hidden nodes (from its interconnected groups) affect its performance. In this paper, a simulation environment, illustrated in Fig. 3, is therefore constructed, where the impact of hidden node problem to a group is investigated. There are two types of nodes and a pair of concentric circles (solid-line circles) in this environment.
P ROTOCOL PARAMETERS Parameters
Value
Packet Payload, Lp
8184 bits
MAC header, HM AC
272 bits
PHY header, HP HY
128 µs
Data rate, Rd
1 M bit/s
IEEE 802.11 SIFS
28 µs
IEEE 802.11 slot time
50 µs
IEEE 802.11 minimum contention window size
32
PS2 MAC inter-frame space, LIF S
28 µs
PS2 MAC sub-slot time,
50 µs
PS2 MAC contention probability, pc
0.35 •
the medium based on duration information that is announced in RTS/CTS frames prior to the actual exchange of data [2]. At a first glance, both NAV and PS2 MAC’s signaling relay technique have a similar functionality—to try to reserve the wireless medium for the initiators. Actually, they are different. First, NAV needs correct decoding—logical carrier sensing. PS2 MAC needs only signal sensing —physical carrier sensing. Secondly, NAV is used only in unicast. PS2 MAC’s signal sensing technique is applicable for both unicast and broadcast. Thirdly, NAV is in-band technique but PS2MAC is out-of-band technique.
•
IV. PS2 MAC B ROADCAST P ERFORMANCE A NALYSIS In this section, the broadcast performance of PS2 MAC is investigated by simulation analysis. A. Parameters and Assumptions The values of protocol parameters applied in this paper are listed in TABLE I. The number of signaling slots per frame n is calculated as H +Lp + LIF S HP HY + M AC Rd (1) n= 2
Inner circle and one-hop nodes. The inner circle has a radius of Rtx /2 and all the first type nodes (grey color nodes) are radian-evenly situated on the inner circle. Because the distance between any two of these nodes is evidently less than or equal to Rtx the diameter of the inner circle, all of these nodes can hear each other and constitute a group. They are named one-hop nodes. In this paper, these one-hop nodes as a whole compose the target group, around which hidden nodes are added and on which the performance is measured. Outer circle and hidden nodes. The outer circle has a radius of Rtx , on which all the second type nodes (dark color nodes) lie radian-evenly. For the target group, these nodes form hidden nodes. For example, in Fig. 3, the transmission (the dashed-line circle) of the rightmost node reaches only part of all the one-hop nodes. Therefore collisions happen on these one-hop nodes when this hidden node and any other one-hop node transmit simultaneously.
By having such an environment, we can benefit from the following methodological advantages in our research.
where Lp is the payload length; HP HY and HM AC are header lengths of the physical layer and the MAC layer respectively; Rd is the data rate; LIF S is the length of the PS2 MAC interframe space; and is the sub-slot time. We assume that nodes are in saturation condition [1], [7] for the sake of tracking the analytical data and evaluating the extreme performance of the protocols. We also assume that all nodes have the same transmission range Rtx and zero bit error rate, exists. These assumptions simplify the analysis in the paper while introducing no negative impact to the conclusion.
•
•
•
B. Environment In a large and randomly distributed ad hoc network, nodes can be partitioned into several groups according to their ”hearing capability” such that all nodes within a group can hear exactly the same subset of nodes [9]. According to this
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•
The simplicity of deploying two categories of nodes evenly enables us to setup the environment conveniently, given any number of one-hop nodes and hidden nodes. Tractability and scalability are feasible. The performance of networks with hidden nodes varies greatly from different instances of nodes’ positional distribution even when the number of nodes keeps constant. So it is not advisable to analyze by scattering all nodes in an uncontrolled manner within an area. Instead, the evenly circular distribution keeps the outcome stable given the same input and makes the analysis tractable and scalable. Inheritability. Because all one-hop nodes are treated as an entity, our previous research data from [1] can be utilized. By adding different numbers of hidden nodes around this entity, we can conveniently observe how its performance varies, which can be seen in the next sub-section. This also makes our research work continue in a gradual and progressive manner. Certain protocol’s (802.11 and PS2 MAC in this paper) capability of fighting against access collision and hidden
Globecom 2013 Workshop - Broadband Wireless Access
.
Transmission coverage of the rightmost hidden node
Rtx
Rtx/2 Outer circle
Fig. 3.
Rtx
1 0.9 0.8
: One-hop node : Hidden node
Normalized throughput
Inner circle
An illustration of the simulation environment
0.7
PS2MAC, w/o hidden nodes
0.6
PS MAC, w/ 1 hidden node
2 2
PS MAC, w/ 4 hidden node 802.11, w/o hidden nodes 802.11, w/ 1 hidden node 802.11, w/ 4 hidden nodes
0.5 0.4 0.3 0.2
node problem can be easily observed in such an environment by changing either the number of one-hop nodes or the number of hidden nodes.
0.1 0 10
20
30
40 50 60 Number of one−hop nodes
C. Performance Comparison Fig. 4.
In this subsection, we compare following broadcast performance indices of the one-hop network between PS2 MAC and IEEE 802.11 [2] in the given environment. • Normalized Throughput is defined as the fraction of bandwidth which is used to transmit one-hop nodes’ payload bits successfully. It is always less than one due to the existence of packet overhead. Evidently, PS2 MAC’s maximum throughput is HP HY +
HM AC +Lp Rd
+ LIF S
90
1 0.9
2
All PS MAC PDR curves overlap completely
0.8
(2)
In this paper, Smax is 95% according to TABLE I. • Packet Delivery Ratio is defined as the ratio of the number of packets successfully received to the total number of packets transmitted by one-hop nodes. Obviously, the maximum PDR is 100%. The simulation platform reported in [1] is used to show IEEE 802.11 broadcast performance and to develop the simulation program for PS2 MAC. Also, the research data of onehop 802.11 broadcast network is inherited from [1]. Three scenarios of the aforementioned environment are investigated. First is the case of no hidden nodes. Second has only one hidden node. In the third scenario, four hidden nodes evenly surrounds the one-hop network. Fig. 4 and Fig. 5 show the throughput and packet delivery ratio over the number of one-hop nodes for both PS2 MAC and IEEE 802.11. We have the following observations from these figures. PS2 MAC consistently outputs high performance and superior stability around the throughput 95% and on the packet delivery ratio 100% in all scenarios while the number of onehop nodes increases. PS2 MAC’s three PDR curves always completely overlap at the maximum 100%, which contributes to a highly reliable service. It also indicate that PS2 MAC can resolve all the access collisions and the hidden node problem in the given environment. Contrasted with the nice stability of PDR, the throughput curves have some interesting variations, which are discussed as follows. When the number of hidden nodes is zero, the throughput persists at the maximum 95% regardless the increase of one-hop nodes, which indicates the channel is fully utilized without extra overhead besides
80
Normalized throughput of PS2 MAC and IEEE 802.11 broadcast
Packet Delivery Ratio
Smax =
Lp
70
0.7 0.6
PS2MAC, w/o hidden nodes
0.5
PS MAC, w/ 1 hidden node
2 2
PS MAC, w/ 4 hidden node 802.11, w/o hidden nodes 802.11, w/ 1 hidden node 802.11, w/ 4 hidden nodes
0.4 0.3 0.2 0.1 0 10
Fig. 5.
20
30
40 50 60 Number of one−hop nodes
70
80
90
Packet delivery ratio of PS2 MAC and IEEE 802.11 broadcast
the protocol overhead. When the number of hidden nodes becomes one, the throughput curve is around 86.7% first. This is because part of the bandwidth is used by the hidden node, during which none of the one-hop nodes transmits. It does not conflict with the previous conclusion of that all access collision and hidden node problem are resolved. Otherwise, the PDR cannot always stays at 100%. Moreover, the curve asymptotically approaches up to the maximum 95%, which is because the more one-hop nodes there are, the less chance the hidden node has to access the channel. The outcome trend is similar when there are four hidden nodes. The difference is that the whole curve is lower than that of the case of only one hidden node because more bandwidth is used by more hidden nodes. However, the performance of IEEE 802.11 is far worse than the PS2 MAC’s when the number of nodes increases. In the no-hidden-node scenario, the maximum values of throughput and PDR are 69.7% and 58% respectively. The throughput drops to 13.7% and the packet delivery ratio drops to 2.9% at the point of ninety one-hop nodes. When hidden nodes exist, either throughput or the packet delivery ratio experiences catastrophic degradation. In the one-hidden-node case, it can
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only achieve 6.3% throughput and 5.9% PDR when there are only ten one-hop nodes. The throughput goes down to 1.7% and the PDR drops to 0.5% at the point of ninety one-hop nodes. In the four-hidden-node scenario, its performance never leaves zero, which indicates no packets can be transmitted successfully. PS2 MAC supports a consistently high packet delivery ratio, which is extremely beneficial for many applications, such as topology discovery of routing protocols, safety-critical messaging in vehicular ad hoc networks, and muti-media multicast service, etc. Despite of PS2 MAC’s superior performance, it for sure has its limited capacity like other protocols. Namely, PS2 MAC’s performance will drop if the number of nodes increases up to a certain threshold. Fortunately, a desired capacity can be obtained by statically or dynamically tuning multiple factors, such as frame length, contention probability, slot length, etc. V. C ONSIDERATION
ON I MPLEMENTATION
VI. CONCLUSION PS MAC, combining the synchronicity of time division multiplexing access (TDMA) and the flexibility of random access, is introduced in this paper. A comparison with IEEE 802.11 in the same environment demonstrates the superior performance of PS2 MAC. Becuase PS2 MAC’s two key points of implementation are feasible, it is our belief that a systematic MAC solution based on PS2 MAC can be further customized to efficiently serve broadcast, multicast, and unicast in a specific ad hoc network, for example, vehicular ad hoc networks. Likewise, this solution can be further developed to provide QoS in MAC layer by fine tuning the configuration of various parameters and adding additional features, e.g. priority. These will be the focus of our subsequent research, where we also plan to relax the assumptions employed in this paper to certain level so that a more realistic performance of PS2 MAC can be evaluated and presented. 2
Some implementation aspects of PS2 MAC are discussed in this section to support a feasible protocol design. It is believed that PS2 MAC is implementable and extensible. A. Two Key Points 1) Additional Hardware and Bandwidth Requirement: The requirement of additional hardware and bandwidth to implement tone transmission and detection scheme has been investigated and discussed [6]. The conclusion is that the performance gain is high enough to offset those additional consumptions, which, practically, may not be negligible [6]. The same conclusion can be drawn based our research outcome in this paper. It is also true that the signal sensing related transmission and detection circuit is way much simpler than a traditional data transceiver circuit. Dual-transceiver based design is also required in [4]. 2) Synchronization Techniques: Certain synchronization mechanism must be implemented to achieve slot delimitation among nodes for PS2 MAC, as required by [3], [4]. GPS is one of the candidates. The cost of a basic GPS head is dropping continuously and GPS functionality is becoming available on mobile devices. Therefore the cost and complexity of retrieving timing information from GPS is not an issue. On the other hand, GPS may not be the only candidate to provide sync functionality to PS2 MAC nodes. Other synchronization protocols may be conceived and used later to fulfill this purpose without a consistent timing feed [3]. B. More Features 1) Frame Size Adjustment and Multi-packet Packing: PS2 MAC’s frame length is calculated as HM AC + Lp + LIF S (3) Rd In this paper, it is fixed at 8612 microseconds, where 8184-bit of payload is sent (see TABLE I). In this case, if a packet is less than 8184 bits, bandwidth waste happens because part of the frame is unused. There are two possible ways to tackle this Lf = HP HY +
issue. One is to adjust the frame length to an optimal value according to the network characteristics and traffic type. The other one uses packet packing technique to combine multiple packets together to fit in one frame so that high efficiency may be achieved. These two ways can be used collectively or separately. 2) Adaptive Contention Probability: In this paper, the contention probability pc is fixed at 0.35 (see TABLE I). Practically, it is possible to tune its value dynamically according to multiple factors, such as node status, traffic load, traffic priority, accumulative delay, level of QoS, etc. For example, if the delay that a delay-sensitive packet has experienced reaches certain threshold, its pc may be increased in order to gain priority of channel access to meet its delay requirement. Therefore a high QoS service may be provided if certain algorithm of adaptive pc is implemented.
R EFERENCES [1] X. Chen, H. H. Refai, and X. Ma, “Saturation Performance of IEEE 802.11 Broadcast Scheme in Ad Hoc Wireless LANs,” in Proc. of the 66th IEEE Vehicular Technology Conference (VTC2007-Fall), 2007. [2] IEEE-802.11, Part 11: wireless LAN medium access control (MAC) and physical layer (PHY) specifications. IEEE, 1999. [3] K. Sjoberg, E. Uhlemann, and E. Strom, “Delay and interference comparison of csma and self-organizing tdma when used in vanets,” in Wireless Communications and Mobile Computing Conference (IWCMC), 2011 7th International, 2011. [4] H. Omar, W. Zhuang, and L. Li, “Vemac: A tdma-based mac protocol for reliable broadcast in vanets,” Mobile Computing, IEEE Transactions on, vol. PP, no. 99, pp. 1–1, 2012. [5] J. L. Sobrinho and A. S. Krishnakumar, “Real-Time traffic over the IEEE 802.11 medium access control layer,” Bell Labs Tech. J., 1996. [6] Z. J. Haas and J. Deng, “Dual busy tone multiple access (DBTMA)a multiple access control scheme for ad hoc networks,” IEEE IEEE Transactions on Communications, vol. 50, no. 6, pp. 975–985, 2002. [7] J. W. Tantra and F. Chuan Heng, “Achieving near maximum throughput in IEEE 802.11 WLANs with contention tone,” IEEE Communications Letters, vol. 10, no. 9, pp. 658–660, 2006, 1089-7798. [8] K. Tang and M. Gerla, “Random access MAC for efficient broadcast support in ad hoc networks,” in Proc. of IEEE Wireless Communications and Networking Conference (WCNC2000), 2000. [9] F. Tobagi and L. Kleinrock, “Packet Switching in Radio Channels: Part II–The Hidden Terminal Problem in Carrier Sense Multiple-Access and the Busy-Tone Solution,” IEEE Transactions on Communications, vol. 23, no. 12, pp. 1417–1433, 1975.
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