A performance comparison of h brid and conventional ... - CiteSeerX

A Performance Comparison of Hybrid and Conventional MAC Protocols for Wireless Networks* I. Chlamtac

A. Farag6

A. D. Myers

V. R. Syrotiuk

G. Zkruba

Center for Advanced Telecommunications Systems and Services (CATSS) Erik Jonsson School of Engineering and Computer Science The University of Texas at Dallas e-mail: chlamtac ,farago amyers s y r o t i u k , z a r u b a @ u t d a l l a s .edu Abstract - Ad hoc networks consist of a group of wireless nodes that dynamically form a multihop network via shared communication channels. For each node, channel access is managed by a media access control (MAC) protocol. MAC protocols can be classified into three broad categories: contention, allocation, and hybrid protocols that combine the contention and allocation access schemes. ADAPT is a hybrid protocol comprised of two component protocols - an allocation protocol that provides stable operation under strenuous network conditions (e.g., high load and nodal degree) , and a contention protocol that dynamically manages the available bandwidth. This paper presents a simulation study that compares the performance of several conventional and hybrid MAC protocols. We examine the relative performance of each protocol under equivalent network conditions, and show that the overall performance of the ADAPT protocol is superior. We also discuss the ability of each protocol to support delay sensitive applications, such as voice, video, and multimedia transmission.

I.

INTRODUCTION

Ad hoc networks consist of a group of wireless nodes that collectively form an instantaneous multihop network. Unlike cellular networks that rely on wired base stations, communication in an ad hoc network is peerto-peer, i.e., nodes directly communicate with each other using shared wireless channels. When direct communication is impossible, nodes rely on one another to forward information in a hop-by-hop fashion. Consequently, an ad hoc network must be self-organizing and exhibit distributed control. This combination of rapid deployment and dynamic reconfiguration establish ad hoc networks as an ideal solution for providing communication and information access in situations where traditional network infrastructure is unavailable or infeasible. The lack of centralized control forces an ad hoc node to make independent decisions based on its local information, such as its current set of neighbors. The ability to gather and share information (in the form of packets) ultimately relies on effective access to a shared wireless 'This work was supportedin part by the DOD USARO (Army Research Office) under contract No. DAAG55-97-1-0312

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channel, i.e., the successful transmission of a packet across a single-hop. In an ad hoc network, channel access is managed by a media access control (MAC) protocol. Most MAC protocols fall into three broad categories based on their strategy for determining channel access rights. Contention protocols utilize direct, asynchronous competition to determine access rights. Early examples, such as CSMA [8], were simple, best-effort protocols that suffered from hidden terminal interference and instability (i.e., throughput breakdown) at high network loads. More recent protocols, such as MACA [a], address hidden terminal interference by introducing handshake mechanisms, yet remain susceptible to instability. Allocation protocols, such as TDMA, deterministically assign each node a transmission schedule indicating in which of the synchronized slots a node may access the channel. These protocols avoid instability by providing guaranteed collision-free access to each node within a schedule. However, allocation protocols generally use rigid slot assignments, preventing adaptation to changing network conditions. Hybrid protocols combine contention and allocation access schemes, resulting in a family of protocols that share the characteristics of both. The FPRP [9] and CATA [6] protocols use contention protocols to dynamically compute transmission schedules. When compared to allocation protocols, both protocols are more flexible in terms of bandwidth management. However, under certain traffic loads or mobility rates, these hybrid protocols remain subject to instability. In [l],we introduced ADAPT, a new hybrid MAC protocol that avoids instability while remaining adaptive to changing network conditions. In this paper, we present an in depth simulation study comparing the performance of ADAPT with the Collision-Avoidance Time Allocation (CATA) [6] and IEEE 802.11 [lo] protocols under the same network conditions. We examine how the performance of each protocol is affected by variations in traffic load and node degree, and discuss their ability to support delay sensitive applications. The rest of this paper is organized as follows. In Section 11, we present an overview of the ADAPT, CATA,

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and IEEE 802.11 protocols, discussing their basic properties and operation. In Section 111, we first introduce the simulation test bed that was used, describing the various models and parameters chosen. We then examine the performance of each protocol, and comment on their relative performance in terms of channel utilization and channel access delay under various network loads and nodal degrees. Section IV provides a summary of our conclusions and completes the paper.

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11. PROTOCOL DESCRIPTIONS The ADAPT Protocol ADAPT has a time division multiple access (TDMA) allocation protocol as its basis as this provides the shortest possible collision-free schedule for a fully connected network (i.e., when all nodes are in the transmission range of each other). When the network is sparsely connected or the traffic load is low, ADAPT employs a collision-avoidance handshake to reclaim and reuse slots that might otherwise remain unused. In order to avoid instability, ADAPT preserves the slot assignments of its underlying TDMA protocol by, in effect, using a priority scheme. Figure 1 illustrates the frame and slot structure of the ADAPT protocol for a network consisting of N nodes. In accordance with the TDMA protocol, each node is assigned one unique slot in a frame for which it is considered the slot owner. In each slot, there is a sensing interval in which only the slot owner may contend for the channel by initiating a handshake. All other nodes must defer contention until after the sensing interval, provided that no handshake is overheard, i.e., no signal is detected in the sensing interval. This effectively gives a node priority to use its assigned slot. Let us consider the ADAPT protocol in more detail. For a given source node s and slot t , channel access is determined as follows. I f s is the owner o f t , then it initiates a handshake in the sensing interval by sending a request-to-send (RTS) control packet to its neighboring destination d. Node d then responds with a corresponding clear-to-send (CTS) control packet, and source node s will transmit its data packet in the remainder of the slot. On the other hand, i f s is not the owner of slot t , then it must first determine whether the owner wishes to make use of its slot. If a handshake is overheard in the sensing interval, then source node s will not contend for slot t in the current frame. However, if no handshake is detected, then s may contend for the slot by sending its own RTS after the sensing interval has expired. Of course, there may be several source nodes contending for slot t , giving rise to possible collisions among RTS control packets sent after the sensing interval. A receiver that detects a collision remains silent for the remainder of the slot, i.e., it does not respond with a CTS. Thus, a source node s that does not receive a CTS response must defer transmission of its data packet until its assigned slot, or some later slot determined by the backoff scheme (see [l]),whichever occurs first. Recep-

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Figure 1: Frame and slot structure of ADAPT tion of a CTS response allows source node s to send its data packet in the remainder of slot t .

The CATA Protocol The Collision-Avoidance Time Allocation (CATA) protocol [6] is a hybrid protocol that allows nodes to contend for and reserve time slots. Moreover, CATA supports unicast, multicast, and broadcast transmission services simultaneously. The contention and reservation scheme is based on a collision-avoidance handshake, and time slots are organized into a synchronous frame. Unlike the ADAPT protocol, CATA does not use an underlying allocation protocol t o assign transmission slots. Therefore, each slot is subdivided into five mini-slots. The first four mini-slots (labeled CMSl - CMS4) are used to secure and reserve time slots through the exchange of short control packets. The last mini-slot (labeled DMS) is used for the transmission of a data packet. For a given source node s and time slot t , the CATA protocol works as follows. Regardless of the packet type, s must first determine whether or not the current slot has been previously reserved. To reserve a slot, all nodes that previously received d a t a in slot t send a slot reservation (SR) control packet in CMS1. In addition, each source node that wishes t o maintain a reservation sends a RTS and not-to-send (NTS) control packets in CMS2 and CMS4, respectively. If no SR packet is detected in CMS1, then source node s contends for slot t by sending its own RTS in CMS2. Reception of a unicast RTS causes a node to respond with a corresponding CTS in CMS3, and source node s can transmit its data packet in the subsequent DMS. Reception of a multicast or broadcast RTS or detection of a clear channel in CMS2 causes a node to remain silent during CMS3 and CMS4; otherwise, it sends a NTS in CMS4 to indicate a potential problem for local multicast or broadcast transmissions. Detection of a clear channel in CMS4, allows source node s to transmit a multicast or broadcast packet in the subsequent DMS. Any unsuccessful attempt to use a slot in this manner is managed by a backoff scheme.

The IEEE 802.11 Protocol The IEEE 802.11 [lo] protocol is a pure contention protocol in which nodes directly compete for channel access

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using a combination of carrier sensing and a collisionavoidance handshake. Although the IEEE 802.11 protocol supports operation in a wireless local area network, for this study we focused on the distributed coordination function that facilitates operation in a mobile multihop wireless network. Channel access is determined as follows. A source node s first uses non-persistent carrier sensing to determine the current state of the channel. If the channel is idle (i.e., no signal is detected), s sends a RTS control packet to its neighboring destination d. If the RTS is received correctly, then d responds with a corresponding CTS. Upon reception of the CTS, source node s sends its data packet expecting an acknowledgment from node d . If either the CTS or acknowledgment are not received, then s schedules its data packet for retransmission according to a backoff scheme. Any node that receives either a RTS or CTS control packet for which it is not the intended destination, refrains from accessing the channel until after the current data exchange is completed. Thus, any node that successfully completes a RTS/CTS handshake is ensured collision-free transmission to its neighboring destination.

111. PERFORMANCE COMPARISON OF HYBRID AND CONVENTIONAL PROTOCOLS Simulation Model

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Figure 2: ADAPT channel utilization. formance, no other specific protocol layers were introduced. Furthermore, the additional features of the CATA protocol, i.e., the reservation scheme and support for multicast and broadcast packet transmissions, were disabled since the ADAPT and IEEE 802.11 protocols have no equivalent capabilities.

Simulation Results

Using a discrete event simulator, we modeled an ad hoc network consisting of 100 mobile users operating within a two-dimensional plane that measured 10 km per side. Each simulated node was equipped with a wireless radio device capable of transmitting at a data rate of 1 Mbps to a distance of 1 km. For simplification purposes, all communication was assumed to have taken place on a single perfect channel, and a free-space propagation model was employed with no capture effect. Node movement was simulated using the random walkbased mobility model developed by McDonald and Znati in [4]. Briefly, a node's movement is described by a sequence of random length mobility epochs. During an epoch, a node moves in a constant direction with a constant speed. At the end of an epoch, a node randomly chooses a new direction and a new speed. The epoch lengths are exponentially distributed with mean l/v. The speed of a node during an epoch is an independent, identically distributed, and uniform random variable with mean p and variance r 2 . Node direction is uniformly distributed over the range (0,2n). The mobility parameters chosen for this study were 7 = 1/60s,p = 5m/s, and T* 8.33m/s corresponding to pedestrian mobility characteristics. Network traffic was generated according to a Poisson arrival process with a mean of X packets per second, and uniformly distributed among the nodes. Each packet was 512 bytes in length, and was addressed to a random neighbor. Since the focus of this paper is on MAC layer per-

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For each protocol, we collected numerical data pertaining to two key performance metrics - channel utilization and average access delay. The channel utilization measures how efficiently a protocol manages the available bandwidth resources. The average access delay measures the average time taken by a protocol to successfully exchange a packet across a single hop. The results are shown in Figures 2 through 7 as a function of the average node degree and packet arrival rate (traffic load), measured in packets per second. Figure 2 depicts the channel utilization of the ADAPT protocol. As expected, the channel utilization of ADAPT exceeds the channel capacity in those cases when the nodal degree is relatively sparse, i.e., when

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the average node degree is relatively low ( 5 10). This increased utilization can be attributed to the ability of ADAPT to reclaim and reuse the increased number of idle TDMA slots via its contention mechanism. As the average node degree and traffic load are increased, the number of idle TDMA slots decreases. Consequently, the contention level for the few remaining idle slots increases, reducing the probability of successful transmission in an unassigned slot. As a result, the channel utilization is highly dependent upon the average node degree. However, even under high traffic loads and nodal degree, ADAPT remains stable (i.e., near full utilization) since every node is guaranteed access to at least one slot per frame. Figure 3 depicts the average access delay of the ADAPT protocol, also shown as a function of the average node degree and traffic load. As anticipated, the average access delay is monotonically non-decreasing with respect to increasing load and node degree. Nevertheless, the underlying TDMA allocation protocol prevents the access delay from exceeding 1 second which corresponds

to the frame length employed. Figure 4 shows the channel utilization of the CATA protocol. In comparison to ADAPT, both protocols have similar performance when the average node degree is relatively low ( 5 10). However, unlike the ADAPT protocol, CATA does not permanently assign slots to nodes. Consequently, CATA does not provide channel access guarantees, and is subject to instability under more strenuous network conditions. In fact, when the average node degree exceeds 30, the CATA protocol is incapable of achieving full utilization at any traffic load. Figure 5 shows the average access delay of the CATA protocol. Since it does not provide any channel access guarantees, the access delay of CATA is unbounded. Thus, as the protocol becomes increasingly unstable at higher traffic loads and nodal degrees, the average access delay begins to increase rapidly. In fact, at the maximum average degree the access delay of CATA is nearly three times that of the ADAPT protocol. Figure 6 presents the channel utilization of the IEEE 802.11 protocol. When the network load and nodal de-

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gree are low, the IEEE 802.11 protocol functions effectively, yet its maximum performance is less than that of both the ADAPT and CATA protocols. However, the overall utilization of the IEEE 802.11 protocol degrades more gradually with increasing nodal degree and traffic load. Moreover, the utilization of the IEEE 802.11 protocol remains stable. This can be attributed to the influence of carrier sensing since this reduces the overall contention level at the expense of increased packet delay [7]. Figure 7 presents the average access delay of the IEEE 802.11 protocol. Compared to the ADAPT and CATA protocols, the access delay of the IEEE 802.11 protocol is reduced when the traffic load and nodal degree are low. This is also attributed to reduced contention levels caused by the influence of carrier sensing. However, at the higher loads and nodal degrees, the presence of carrier sensing begins to have a negative affect on the measured access delay. In comparison to the CATA protocol, we can see that the IEEE 802.11 access delay surface is more convex since the access delays are slightly higher for the intermediate values. Moreover, at the highest traffic load and nodal degree, we find that the access delay of IEEE 802.11 is about 500 milliseconds more than the equivalent CATA delay.

Support for Delay Sensitive Applications In order to provide acceptable service quality, most delay sensitive applications, including voice, video, and other multimedia applications, require an end-to-end delay in the range of 200-500 milliseconds. Considering the average access delay of the ADAPT, CATA, and IEEE 802.11 protocols (Figures 3, 5, and 7, respectively), we find a wide variety of traffic load and node degree combinations that yield acceptable delays. However, the large number of irregularities present in the CATA and IEEE 802.11 surfaces suggests a high degree of delay variability. With respect to video and multimedia transmission, high levels of delay variance introduce increased levels of packet jitter which has a negative impact on the overall service quality experienced by the user. On the other hand, end-to-end delay is just one of many quality-of-service (QoS) requirements needed by delay sensitive applications. Many applications also have strict rate specifications that require the allocation of specific bandwidth resources. With its slot reservation capabilities, the CATA protocol is a more appropriate choice. However, the instability associated with the CATA protocol limits its effectiveness to networks that are either sparsely connected, or exhibit a low traffic load.

Further Discussion Despite its overall superior performance, there are some hidden costs associated with the ADAPT protocol. Although the synchronous time frame is effective, it adds additional complexity in the form of more sophisticated hardware. Moreover, synchronous protocols tend to be

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less robust in the face of mobility and channel fading [31.

IV.

CONCLUSIONS

In this paper, we presented a simulation study that compared the relative performance of the ADAPT, CATA, and IEEE 802.11 MAC protocols in the same ad hoc network scenarios. We analyzed the performance of each protocol with respect to its channel utilization and access delay, and demonstrated that the overall performance of the ADAPT protocol is superior. We also investigated the ability of these protocols to support delay sensitive applications. We found that, under a wide range of network conditions, all three protocols could satisfy the end-to-end delay requirements of these applications. Moreover, the slot reservation mechanism of the CATA protocol may satisfy the rate requirements of many such applications. Nevertheless, these protocols are better suited to delay insensitive traffic.

REFERENCES I. Chlamtac et al. “ADAPT: A Dynamically SelfAdjusting Media Access Control Protocol for Ad Hoc Networks,” Proceedangs of IEEE Globecom, Rio De Janeiro, Brazil, 1999. P. Karn. “MACA - A New Channel Access Protocol for Wireless Ad-Hoc Networks,” Proceedzngs of the ARRL/CRRL Amateur Radao Nanth Computer Networkang Conference, pp. 134-140, 1990. C. Lin and M. Gerla. “Real-time Support in Multihop Wireless Networks,” Wareless Networks, vol. 5, pp. 125-135, 1999. B. McDonald and T. Znati. “Link Availability Models for Wireless Ad Hoc Networks,” Proceedzngs of IEEE WCNC, New Orleans, LA, 1999. A. Muir and J.J. Garcia-Luna-Aceves. “A Channel Access Protocol for Multihop Wireless Networks with Multiple Channels,” Proceedzngs of IEEE ICC, New York, NY, 1998. Z. Tang and J.J. Garcia-Luna-Aceves. “A Protocol for Topology-Dependent Transmission Scheduling in Wireless Networks,” Proceedangs of IEEE WCNC, New Orleans, LA, 1999. A. Tanenbaum. Computer Networks, 3rd edition, Prentice Hall, Inc., 1996. F. Tobagi and L. Kleinrock. “Packet Switching in Radio Channels: Part I - Carrier Sense Multiple Access Models and Their Throughput Delay Characteristics,” IEEE Transactaons on Communacatzons, vol. 23, no. 12, pp. 1400-16, 1975. C. Zhu and S. Corson. “A Five-Phase Reservation Protocol (FPRP) for Mobile Ad Hoc Networks,” Proceedangs of IEEE INFOCOM, New York, NY, 1998. Wireless Medium Access Control and Physical Layer WG, IEEE Draft Standard P802.11 Wireless LAN. IEEE Standards Department, D3, Jan. 1996.

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