comparison omnidirectional directional mac

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A Comparison Study of Omnidirectional and DirectionalMAC Protocols for Ad hoc Networks

Zhuochuan Huang Chien-Chung ShenDepartment of Computer and Information Sciences

University of Delaware

Newark, DE 19716

Abstract— Traditional MAC protocols used in ad hoc networksemploy omnidirectional antennas. Recently, directional antennashave emerged as an alternative due to their capability of spatialreuse, low probability of detection, robustness to jamming, andother beneficial features. In this paper, we conducted a compar-ison study of existing directional and omnidirectional MAC pro-tocols by contrasting their features and evaluating their perfor-mance under various network load and topology. Specifically wepresented rationale for the better performance of some directionalantenna based MAC protocols by using the metric of effective spa-

tial reuse, which is also evidenced by the simulation study.

I. INTRODUCTION

Ad hoc networks consist of mobile nodes, which dynamicallyand spontaneously create and maintain the network topologyvia wireless communication. Carrier Sense Medium Access(CSMA) protocol, when applied to ad hoc networks, causesthe hidden terminal and the exposed terminal problems [1].MACA (Medium Access Collision Avoidance) [2] relievesthese problems by utilizing the RTS and CTS (Request-To-Send and Clear-To-Send) frames. MACAW [3] further refinesMACA with more optional control frames. FAMA (Floor Ac-quisition Multiple Access) [4] combines non-persistent carrier

sensing and the RTS/CTS scheme together. The IEEE 802.11MAC DCF specification [5] is a variation of the CSMA/CA(CSMA with Collision Avoidance) protocol that also supportsboth the carrier sensing and the virtual sensing (RTS/CTSscheme). DBTMA (Dual Busy Tone Multiple Access) [6] isalso an RTS/CTS-based MAC protocol, where it splits a singlechannel and uses a pair of transmit and receive busy tones.

The MAC protocols stated above all assume the usage of om-nidirectional antennas. The directional antenna based MACprotocols1, however, are capable of transmitting only in cer-tain narrower azimuth and thus significantly reduce the chanceof collision and increase the effective network capacity. Re-cently, there have been a few directional MAC protocols pro-posed for ad hoc networks. For instance, [7] applies directional

antenna to the IEEE 802.11 MAC DCF specification by send-ing the RTS, data, and ACK frames directionally and succeedsin achieving better network throughput. However, it relied onextra location tracking support. To avoid this problem, [8]

Prepared through collaborative participation in the Communications andNetworks Consortium sponsored by the U. S. Army Research Laboratory un-der the Collaborative Technology Alliance Program, Cooperative AgreementDAAD19-01-2-0011. The U. S. Government is authorized to reproduce anddistribute reprints for Government purposes notwithstanding any copyright no-tation thereon.1In this paper, we will refer to it as “directional MAC protocol.” Similarly,

MAC protocols based on omnidirectional antenna is referred to as “omnidirec-tional MAC protocols.”

utilized the exchange of omnidirectional RTS/CTS frames be-tween communicating nodes in order for them to identify thedirections of each other. However, it did not have the benefitof reserving the channel directionally. A directional version of DBTMA was described in [9], which demonstrated improvedperformance and comparable effectiveness of spatial reuse withother directional MAC protocols.

In this paper, we performed a comparative study of these MACprotocols via simulations. Specifically, we explained the ratio-

nale for the improved performance of directional MAC proto-cols by evaluating the effectiveness of their spatial reuse. Theremainder of this paper is organized as follows. Section IIbriefly reviews omnidirectional and directional MAC proto-cols. Section III compares the features of these MAC protocolsand measures the effectivenessof their spatial reuse. Section IVpresents results and observations of the simulations. Section Vconcludes this paper with future work.

I I . OVERVIEW OF MAC PROTOCOLS

A. Omnidirectional MAC Protocols

1) IEEE 802.11 MAC DCF: In the IEEE 802.11 MAC DCF,

when the frame size exceeds certain threshold, the RTS/CTSscheme is used in addition to carrier sensing. When a nodehears an RTS or a CTS frame, it will set the NAV (Network Allocation Vector) to defer itself from access until the end of the corresponding data transmission to avoid collision with theACK frame. When the virtual sensing is used, the hidden ter-minal problem is relieved while the exposed terminal problememerges; otherwise the hidden terminal problem exists, whilethe exposed terminal problem is not significant.

2) MACA and MACAW: MACA uses only the virtual sensing.The subtle difference between MACA’s RTS/CTS scheme andthat of the IEEE 802.11 MAC DCF is that, when a node over-hears an RTS frame, instead of deferring until the end of thedata transmission, it only defers until the corresponding CTS

frame is expected to be received. The consequence is that theexposed terminal problem, in addition to the hidden terminalproblem, is also solved. MACAW basically follows the sameRTS/CTS scheme as MACA, while it introduces more optionalcontrol frames and a different backoff algorithm.

3) FAMA: Like IEEE 802.11 MAC, FAMA exploits both car-rier sensing and virtual sensing. It combines non-persistent car-rier sensing with the virtual sensing scheme of MACA, whichis referred to as FAMA-NTR (FAMA Non-persistent TransmitRequest). MACA is deemed as a variant of FAMA without thecarrier sensing.



4) DBTMA: DBTMA distinguishes itself from other MACprotocols in two aspects: it splits a single channel into two sub-channels and it uses a pair of transmission and receiving busytones to accomplish the virtual sensing.

B. Directional Antenna Model

We adopt the switched beam directional antenna [10] model asdepicted in Figure 1, which is similar to what is assumed in [8].Each node is equipped with a directional antenna consisting of N antenna elements which are deployed into non-overlappingfixed sectors each spanning an angle of 360/ N degrees. Whenbeing transmitted, a signal will be propagated in exactly one orall of the sectors, which corresponds to unicast and broadcast,respectively. Signals will be sensed in all sectors and the an-tenna is capable of recognizing the sector with the maximumgain. When receiving, exactly one sector, which usually isthe one chosen by the sensing process, will collect the signals.Though the node may be mobile, we assume that the orientationof each sector remains the same. Unicast and broadcast havethe same transmission power range, which is also assumed in

[7] and [8].

(a) Unicast (b) Broadcast(a) Unicast (b) Broadcast

Fig. 1. Top-view of ideal azimuth patterns for 8-sector directional antennas

C. Directional MAC Protocols

1) MAC/DA1: [7] is one of the first efforts to adapt the IEEE802.11 MAC DCF scheme for directional antennas. Its key fea-ture is the usage of directional RTS frame. On one hand, it nar-rows the area in which an unintended receiver can overhear theRTS frame and thus significantly relieves the exposed terminalproblem. On the other hand, by recording the directions fromwhich the CTS frames are recently overheard and then block-ing the antenna elements in the corresponding sectors, a nodeis further allowed to transmit in the directions that will not col-lide with other data transmissions, in addition to relieving thehidden terminal problem.2 The prerequisite of transmitting adirectional RTS/data frame is the knowledge of the direction of the intended receiver, which is referred to as the location track-ing problem. The solution that MAC/DA1 suggests is to equipevery node with GPS support and rely on a beacon protocol fornodes to exchange location information periodically.

2) MAC/DA2: [8] exploits the ability of the receiver to deter-mine the direction of the arriving frame in order for the trans-mitting and the receiving nodes to learn each other’s direction.

2Recognizing that using directional RTS frame all the time may increase thechance of collisions between control frames, MAC/DA1 also presents a variantscheme that also includes omnidirectional RTS frame under certain conditions.Since these two schemes have similar mechanism and performance, for sim-plicity we refer to the one that only uses directional RTS frame.

In contracts to MAC/DA1, it accomplishes location tracking inan on-demand manner, instead of in a pro-active manner. How-ever, since it uses omnidirectional antennas to transmit both theRTS and CTS frames, it does not have the benefits of using di-rectional RTS frame as in MAC/DA1.

3) MAC/DA2ACK: MAC/DA2 is basically a modificationof the IEEE 802.11 MAC DCF specification with directional

antenna support. However, it does not include the ACKframe. For comparison purpose, we implemented a version of MAC/DA2 with ACK frame, termed MAC/DA2ACK .

4) DBTMA/DA: [9] describes a directional version of DBTMA. Similar to DBTMA, it splits a single channel intotwo sub-channels and uses the busy-tones. However, it usesdirectional rather than omnidirectional busy-tones. By usingdirectional transmitting busy tones, it shares the similar fea-ture of the directional RTS frame scheme of MAC/DA1 in thatit reserves the network capacity in a finer grain and thus re-lieves the exposed terminal problem. In the meantime, by us-ing directional receiving busy tones, it realizes a similar func-tionality of blocking the corresponding antenna element in thedirection from which omnidirectional CTS frame is received in

MAC/DA1. Since DBTMA/DA does not rely on the directionalRTS frame to solve the exposed terminal problem or to ex-pand effective network capacity, location tracking is only usedfor transmitting data frame directionally. Therefore, we coulduse an on-demand location tracking mechanism. We adopted asimilar 0 as in MAC/DA2 to transmit RTS frames omnidirec-tionally and CTS frames directionally.

III. COMPARISON OF MAC PROTOCOLS

Table I3 compares the MAC protocols described above, whichare grouped into three families rooted from the IEEE 802.11MAC DCF, MACA, and DBTMA4.

AE

F

B

C

D

AE

F

B

C

D

Fig. 2. Spatial reuse example

The effectiveness of a MAC protocol to achieve spatial reuseis crucial to expand effective network capacity and improve itsperformance in ad hoc networks. We use the following exam-ple, as shown in Figure 2, to analyze the different capabilitiesin spatial reuse among the MAC protocols in Table I. Whennode A is transmitting data to node B, the shaded area is usu-ally reserved in some way such that other nodes within thisarea are deferred from transmitting and/or receiving. Here weconsider whether node C and node E, which are located within

3BEB is Binary Exponential Backoff; MILD is Multiple Increase Linear De-crease backoff; OMNI is Omnidirectional; DIR is Directional; 80211 refers toIEEE 802.11 MAC DCF.4Since FAMA includes both MACA and FAMA-NTR, it is marked as either

with the carrier sensing or without. 80211 and its derivatives largely dependon the size of the data frame to decide whether to use either only the carriersensing or both the carrier sensing and the virtual sensing.



0

200

400

600

800

1000

1200

1400

T h r o u g h p u t ( k b / s )

Saturation−point Throughput of Various MAC Protocols in Nine Cases

t1−s1 t1−s2 t1−s3 t2−s1 t2−s2 t3−s1 t3−s2 t3−s3 t4−s1

80211MACA

DBTMADBTMA/DA

MAC/DA1MAC/DA2

MAC/DA2ACK

0

0.2

0.4

0.6

0.8

1

1.2

1.4

E n d − t o − E n d D e l a y ( s )

Saturation−point End−to−End Delay of Various MAC Protocols in Nine Cases

t1−s1 t1−s2 t1−s3 t2−s1 t2−s2 t3−s1 t3−s2 t3−s3 t4−s1

80211MACA

DBTMADBTMA/DA

MAC/DA1MAC/DA2

MAC/DA2ACK

Fig. 6. Average Saturation-point Throughput and End-to-end Delay as a func-tion of per-flow traffic load for all nine scenarios

(c) All directional antenna based MAC protocols are gener-ally better than 80211 and better than MACA when i is large.MACA is expected to have better performance when a datatransmitter is not blocked by overhearing a CTS frame. There-fore, the smaller i(T, R) is, the better MACA performs, as sup-ported by Table III.

We further evaluated the performance of multi-hop flows in a6 x 6 mesh topology, as depicted in Figure 4, with twelve in-terleaved five-hop flows: (1 → 31), (2 → 32), (3 → 33),

(4 → 34), (5 → 35), (6 → 36); (1 → 6), (7 → 12), (13 →18), (19 → 24), (25 → 30), and (31 → 36). The resultsare shown in Figure 7. It shows that, most directional MACprotocols (DBTMA/DA, MAC/DA1 and MAC/DA2ACK) andMACA have better performance than 80211 when traffic load ishigh; MAC/DA2 improves throughput marginally when ACKframe is used. 80211 family MAC protocols exhibits signifi-cantly longer delay than others, which suggests that using ACKframe leads to better throughput at the cost of longer delay.

V. CONCLUSIONS

We have performed a comparative study of the directional andomnidirectional MAC protocols for ad hoc networks. In par-

ticular, we studied their effectiveness in achieving spatial reuseby using an exemplary scenario. We found that two directionalMAC protocols, DBTMA/DA and MAC/DA1 are more effec-tive in spatial reuse than others, especially when the traffic loadand node density are high. We have further conducted sim-ulation studies to evaluate their performance with interleavedCBR traffic flows on a 6 x 6 topology. The results evidencedthe analysis of effective spatial reuse and illustrates the relativeperformance of these protocols. We also observed that MACprotocols derived from the IEEE 802.11 MAC suffer from longend-to-end delay as the result of using the ACK frame, which,however, marginally increases the throughput.

0

20

40

60

80

100

120

140

0 50 100 150 200 250

T h r o u g h p u t p e r F l o w ( k

b p s )

Traffic Load per Flow (kbps)

Throughput of Various MAC Protocols with 12 CBR Flow(s)

80211MACA

DBTMADBTMA/DA

MAC/DA1MAC/DA2

MAC/DA2ACK

0

1

2

3

4

5

6

7

0 50 100 150 200 250

A v e r a g e E n d t o E n d D e l a y ( s )

Traffic Load per Flow (kbps)

End to End Delay of Various MAC Protocols With 12 CBR Flow(s)

80211MACA

DBTMADBTMA/DA

MAC/DA1MAC/DA2

MAC/DA2ACK

Fig. 7. Average Throughput and End-to-end Delay as a function of per-flowtraffic load for twelve CBR flows in 6 x 6 topology

REFERENCES

[1] F. A. Tobagi and L. Kleinrock, “Packet Switching in Radio Channels:Part II the Hidden Terminal Problem in Carrier Sense Multiple-Accessand the Busy-Tone Solution,” IEEE Trans. on Commun., pp. 1417–33,1975.

[2] Phil Karn, “MACA - A New Channel Access Method for Packet Radio,”in ARRL/CRRL Amateur Radio 9th Computer Networking Conference,1990, pp. 134–40.

[3] Vaduvur Bharghavan, Alan Demers, Scott Shenker, and Lixia Zhang,“MACAW: A Media Access Protocol for Wireless LAN’s,” in ACM SIG-COMM , London, UK, Aug. 31–Sep. 9 1994, pp. 212–25.

[4] Chane L. Fullmer and J. J. Garcia-Luna-Aceves, “Floor Acquisition Mul-tiple Access (FAMA) for Packet-Radio Networks,” in ACM SIGCOMM ,Cambridge, MA, Aug. 28–Sep. 1 1995, pp. 262–73.

[5] IEEE 802.11 Working Group, “Wireless LAN Medium Access Control(MAC) and Physical Layer (PHY) specifications,” 1999.

[6] J. Deng and Z. Haas, “Dual Busy Tone Multiple Access (DBTMA): ANew Medium Access Control for Packet Radio Networks,” in ICUPC ,Florence, Italy, Oct. 5–9 1998.

[7] Young-Bae Ko, Vinaychandra Shankarkumar, and Nitin H. Vaidya,“Medium Access Control Protocols Using Directional Antennas in AdHoc Networks,” in IEEE INFOCOM , Tel Aviv, Israel, Mar. 26–30 2000,pp. 13–21.

[8] Asis Nasipuri, Shengchun Ye, and Robert E. Hiromoto, “A MAC Proto-col for Mobile Ad Hoc Networks Using Directional Antennas,” in IEEE WCNC , Chicago, IL, Sep. 26–29 2000.

[9] Zhuochuan Huang, Chien-Chung Shen, Chavalit Srisathapornphat, and

Chaiporn Jaikaeo, “A Busy-Tone Based Directional MAC Protocol forAd Hoc Networks,” in IEEE MILCOM , Anaheim, CA, Oct. 7–10 2002.[10] Per H. Lehne and Magne Pettersen, “An Overview of Smart Antenna

Technology for Mobile Communications Systems,” IEEE Communica-tions Surveys, vol. 2, no. 4, 1999.

[11] Qualnet, “http://www.qualnet.com/products/QualNet,” 2002.

∗The views and conclusions contained in this document are those of the au-thors and should not be interpreted as representing the official policies, eitherexpressed or implied, of the Army Research Laboratory or the U. S. Govern-ment.

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