Tie-Based Undewate Acoustic Routing fo Applications with Reliability and Delay Constaints Li-Chung Kuo Depatment of Electical Engineeing State Univesity of New Yok at Buffalo Buffalo, New Yok 14260 Email: lkuo2@buffalo.edu Tommaso Melodia Depatment of Electical Engineeing State Univesity of New Yok at Buffalo Buffalo, New Yok 14260 Email: tmelodia@buffalo.edu Abstact UndeWate Acoustic Senso Netwoks (UW-ASNs) ae expeiencing a apid gowth, due to thei high elevance to commecial and militay applications such as oceanogaphic data collection, pollution monitoing, offshoe exploation, disaste pevention, and tactical suveillance. Howeve, the design of efficient communication potocols fo undewate senso netwoks is still an open eseach poblem because of the unique chaacteistics of the undewate acoustic communication channel such as limited bandwidth, high and vaiable popagation delays, and significant multipath and scatteing. In this pape, we intoduce a tie-based distibuted outing algoithm. The objective of the poposed algoithm is to educe the enegy consumption though adequate selection of the next hop subject to equiements on the end-to-end packet eo ate and delay. The potocol is based on lightweight message exchange, and the pefomance tagets ae achieved though the coopeation of tansmitte and available next hops. Index Tems Undewate acoustic senso netwoks, Routing algoithm, Coss-laye design. I. INTRODUCTION In ecent yeas, UndeWate Acoustic Senso Netwoks [1] (UW-ASNs) have expeienced a apid gowth, due to thei high elevance to commecial and militay applications such as oceanogaphic data collection, pollution monitoing, offshoe exploation, disaste pevention, and tactical suveillance. Howeve, cuently available undewate acoustic technology suppots only low-data-ate and delay-toleant applications. State-of-the-at typical expeimental point-to-point acoustic modems use signaling schemes that can achieve data ates lowe than 20 kbit/s with a link distance of 1km, while commecially available modems povide even lowe data ate wavefoms [2][3]. In addition, the ecent availability of inexpensive hadwae such as CMOS cameas and micophones able to ubiquitously captue multimedia content fom the envionment is enabling so-called Wieless Multimedia Senso Netwoks [4], i.e., wieless systems designed to etieve video and audio steams, still images, and scala senso data fom the envionment. Similaly, multimedia undewate senso netwoks would enable new applications fo undewate multimedia suveillance, undesea exploations, video-assisted navigation and envionmental monitoing. Howeve, these applications equie moe flexible potocol design to accommodate heteogeneous taffic demands in tems of bandwidth, end-to-end packet eo ate and delay. To suppot such taffic demands, in this pape we popose a new coss-laye outing potocol to flexibly educe the enegy consumption by selecting enegy-efficient next hops subject to taget packet eo ate and delay bounds unde the unique challenges posed by the undewate envionment. The dawbacks of existing teestial outing solutions fo undewate netwoks [1] ae well-undestood. Theefoe, in this aticle, we popose a tie-based outing potocol, whee the netwok topology is patitioned into ties, and each individual node, in detemining a next hop towads a suface sink, is limited to selecting nodes that belong to its uppe tie. In geneal, a tieed topology educes the complexity of outing potocols, since, much like geogaphical outing, it povides each node with a set of coodinates that indicate a diection towads the sink (physical coodinates in geogaphical outing, vitual coodinates in ou poposed scheme). In addition to that, the vitual position can be used to estimate the end-to-end delay. To counte the effect of the caie sensing delay caused by the high popagation delay, a Medium Access Contol (MAC) potocol integated with ou poposed outing potocol is also intoduced. The poposed low-complexity solution can be used to enable undewate monitoing applications with delay and eliability equiements. The emainde of this pape is oganized as follows. In Section II, we intoduce the communication achitectue fo undewate senso netwoks and the undewate popagation model. In Section III, we descibe how to constuct a tieed topology and a Medium Access Contol (MAC) potocol to be integated with ou poposed outing potocol. In Section IV, we intoduce the poposed outing potocol. In Section V, we assess the pefomance of the poposed solutions though simulation expeiments. Finally, in Section VI, we daw the main conclusions. II. PRELIMINARIES In this section, we fist intoduce the communication achitectue of thee-dimensional undewate senso netwoks. Then, we biefly descibe the unique chaacteistics of undewate acoustic popagation.
Fig. 1. The Achitectue fo 3D Undewate Senso Netwoks. A. Netwok Achitectue In thee-dimensional undewate senso netwoks, nodes ae deployed at diffeent depths to obseve a given phenomenon and epot to suface stations, as shown in Fig. 1. Each senso is anchoed to the bottom of the ocean and equipped with a floating buoy. The depth of each senso can be egulated by adjusting the length of the wie that connects the senso to the ancho. Undewate sensos ae able to elay infomation to the suface station via multi-hop paths. Existing deployment stategies fo undewate senso netwoks, as discussed in [5], guaantee that the netwok topology be always connected. Theefoe, we assume that at least one path fom evey senso to the suface station always exists, and that highe senso density inceases the numbe of possible paths. Moeove, one o moe suface stations ae deployed on the suface of the ocean. Each suface station is equipped with an acoustic tansceive, and it may be able to handle multiple paallel communications with the undewate sensos and suface stations. B. Undewate Popagation Model Undewate acoustic popagation [6] is substantially diffeent fom its RF countepat [7]. Specifically, undewate acoustic communications ae mainly influenced by tansmission loss, multipath, Dopple spead, and high popagation delay. The tansmission loss T L(d, f) [db] that a naowband acoustic signal at fequency f [khz] expeiences along a distance d [m] can be descibed by the Uick model [6]: T L(d, f) = χ Log(d) + α(f) d + A. (1) In (1), the fist tem accounts fo geometic speading. The second tem accounts fo medium absoption, whee α(f) [db/m] epesents an absoption coefficient. The last tem, expessed by the quantity A [db], is the so-called tansmission anomaly. Moe details can be found in [8]. III. TOPOLOGY CONTROL AND MAC PROTOCOL As discussed above, since in undewate communications the tansmission loss inceases moe than linealy with distance, elay tansmission should be consideed. Theefoe, a Fig. 2. The channel capacity with diffeent tansmitte-eceive distance when the caie fequency is 10 khz, the caie bandwidth is 5 khz and the tansmit powe is 10 W. well-designed topology contol stategy can potentially educe the end-to-end powe consumption [9]. Fo this eason, we popose to patition the topology into ties, and each individual node, in detemining a next hop towads a suface sink, is limited to selecting nodes that belong to its uppe tie. In geneal, a tieed topology educes the complexity of implementing a outing potocol, since it povides each node with a set of coodinates that indicate a diection towads the sink (physical coodinates in geogaphical outing, vitual coodinates in ou poposed scheme). In addition to that, the vitual position can be used to estimate the end-to-end delay. Accoding to the tie whee the node is located, a node can calculate the numbe of hops sepaating it fom the destination. We theefoe heeby descibe a pocedue to be used at the suface station to constuct a tieed topology, i.e., to select the adius of each tie. Based on the channel capacity expession between the tansmitte and eceive, we intoduce a pocedue to patition nodes into diffeent ties stating fom the suface station. In the outing pocedue, a node is then limited to selecting nodes that belong to its uppe tie to educe the powe consumption and guaantee the taget end-to-end delay bounds. Then, a new MAC potocol integated with ou poposed outing potocol is intoduced. The inteaction between the MAC and outing potocols takes advantage of the tieed topology. A. Topology Constuction The channel capacity can be expessed as C(d) = f log 2 (1 + P T L 1 (d,f) N 0 ) [kbit/s], (2) whee f [khz] is the caie bandwidth, P [W] is the tansmit powe, and N 0 [W] is the aveage ambient noise [10]. Figue 2 shows the channel capacity fo a typical acoustic link with
Fig. 3. Ceating ties fom the sink. vaying tansmitte-eceive distance. The caie fequency f is set to 10 khz, f is 5 khz and P is 10 W. The channel capacity deceases with a highe tansmitteeceive distance and thus we can detemine the tansmission ange (which detemines the ange of each tie ), which we efe to as d max, fo a taget channel capacity. Fo example, in Fig. 2, when the tansmitte-eceive distance is 1000 m, the channel capacity is 48.32 kbit/s. Theefoe, if the equied channel capacity is 48.32 kbit/s, the tansmitte-eceive distance cannot exceed 1000 m. In pactice, the achievable data ate will be lowe than the channel capacity because of the effect of intefeence. Based on the taget channel capacity, we can detemine the value of d max and accodingly patition nodes into ties. The nodes in the ange of the sink with adius d max constitute the fist tie. Nodes in the ange of at least 2 fist tie nodes with adius d max constitute the second tie, and so on. Note that this pocedue guaantees the existence of at least two paths between the souce and the destination, which guaantees a backup outing path in case of failues. We could incease 2 to moe if necessay. The tie stuctue is illustated in Fig. 3. With a tieed topology, the numbe of hops h between a souce and destination depends only on what tie the souce is located at. B. Medium Access Contol Potocol To take advantage of the tieed topology, we popose a new Medium Access Contol (MAC) potocol integated with ou poposed outing potocol as shown in Fig. 4. The tansmitte at tie h tansmits a Request to Send (RTS) packet, which includes the tie of the tansmitte, to indicate that it has packets to send. Only idle nodes at tie h 1 will espond with a Clea to Send (CTS) packet, which includes the intefeence measued at the node and the aveage packet queueing delay. Since the popagation delay is high in undewate, we do not use caie sensing and all idle h 1 tie nodes send a CTS immediately afte eceiving the RTS. Consequently, the tansmitte will choose the node with minimum equied tansmit powe as its next hop among those that satisfy the equiements on the end-to-end packet eo ate and delay. The tansmitte will announce the selected next hop by sending an Intent to Send (ITS) packet. Afte tansmitting the ITS, the tansmitte tansmits the packet immediately. We conside a CDMA envionment [11], whee RTS, CTS and ITS ae tansmitted using a common speading code which is known Fig. 4. The MAC potocol used to excute the poposed outing potocol. by all nodes. The data packet is tansmitted using a tansmitteassigned speading code, whee the paametes that will be used by the tansmitte to geneate the assigned speading code fo the data packet ae included in the ITS. A CTS may collide with anothe CTS. Howeve, the pobability of collision is small as illustated in Fig. 4. In the figue, the distance between Node T 1 and Node R 1 is d, and the distance between Node T 1 and Node R 2 is d + d. Node T 1 will stat eceiving CT S 1 at 2 d, and finish eceiving CT S 1 + T CT S. If Node T 1 stats eceiving CT S 2 at 2 (d+ d) at 2 d, a collision would happen when 2 (d+ d) < 2 d + T CT S, i.e., d < T CT S 2. If T CT S is 1 ms and the sound velocity is 1500 m/s, a collision happens only when d is smalle than 0.75 m. In fact, d max in ou simulation is 500 m, which is much lage than 0.75 m. Theefoe, the collision pobability fo CTS packets is vey small. The analysis above povides the ationale fo not using a caie sense mechanism in undewate and in geneal on popagation media affected by high popagation delay. An RTS may also collide with an RTS fom anothe tansmitte. Howeve, if the RTS collides at R 1 in Fig. 4, the pobability that it also collides with the RTS at R 2 is vey small. Afte R 2 eceives the RTS, it will espond with a CTS. Theefoe, T 1 can choose R 2 as its next hop, and the communication would not be inteupted. IV. ROUTING PROTOCOL In this section, we intoduce ou end-to-end delay guaanteed distibuted outing algoithm. In the poblem fomulation, a node i at tie h needs to tansmit a packet m to a idle node j at tie h 1. Pi max is the maximum tansmit powe dictated by hadwae constaints at node i. The intefeence at
node j, I j, and the aveage queueing delay at node j, Q j, ae included in the CTSs fom j. Moeove, E elec is the distanceindependent enegy to tansmit one bit, whee we assume that the enegy pe bit needed by tansmitte electonics and by eceive electonics is the same. The tansmit powe and the chip ate ae P ij [W] and [chip/s], espectively. Theefoe, the enegy to tansmit one bit fom node i to node j is 2 E elec + P ij c /, whee c [chip/bit] is the speading code length. The poposed algoithm allows each node to distibutively select the optimal next hop and the optimal tansmit powe, with the objective of minimizing the enegy consumption. Thee constains ae included in the poposed algoithm to meet the delay-sensitive application equiements: 1) The tansmit powe should not exceed the maximum tansmit powe. Theefoe, P ij Pi max ; 2) The end-to-end packet eo ate should be lowe than an application-dependent theshold P ERe2e max. BER ij, which epesents the bit eo ate on link (i, j), is a function of the tansmission powe and the intefeence at node j, Φ(P ij, I j ). The packet eo ate on link (i, j), P ER ij = 1 (1 BER L D ij ), whee L D [bit] is the packet size. Since the numbe of hops between node i and the destination is h, the end-to-end packet eo ate is 1 (1 P ER ij ) h and it should be lowe than P ERe2e max. Thus, 1 (1 P ER ij ) h P ERe2e max. Note that coupted packets would be dopped. Theefoe, the packet must be coectly fowaded fom the souce to node i. If the numbe of hops between the souce and node i is h, 1 1 h (1 P ER ij ) h should be lowe than P ERe2e max ; 3) The end-to-end packet delay should be lowe than an application-dependent theshold T max. We calculate T MAC = T RT S + 2 dmax + T CT S + T IT S as the delay time between the beginning of RTS and the end of ITS, as shown in Fig. 4. Since node i does not have infomation about the uppe tie nodes of node j, we deive the wost-case delay fom node j to the destination as T wost = (h 1) (Q j +T MAC + L D c + dmax ). (3) The distance between node j and the selected tie h 2 node is d max, and we assume that uppe tie nodes have the same queueing delay Q j. Theefoe, the numbe of hops between node j and destination is h 1, and the wost delay time would be (h 1) (Q j + T MAC + L D c + dmax ). To guaantee that the end-to-end packet delay will not exceed the application-dependent delay bound, T IT S + L D c + d ij + T wost T max (t m i,now t m 0 ), (4) whee t m i,now and tm 0 ae the aival time of packet m at node i and the time packet m was geneated, espectively. The poposed algoithm does not etansmit coupted packets at the link laye. Besides, it time-stamps packets when they ae geneated by a souce so that they can be discaded when they expie. Finally, note that the space of solutions to the above poblem is fo all pactical puposes vey limited and the poblem can be solved by enumeation - no specialized solve is needed. P: End-to-end Delay Guaanteed Routing T IT S + L D c Given: Find: Minimize: Subject to: i, j, h, Pi max, I j, Q j j, Pij E ij = 2 E elec + Pij c P ij P max i ; (5) 1 (1 P ER ij ) h P ER max e2e ; (6) + d ij + T wost T max (t m i,now t m 0 ). (7) V. PERFORMANCE EVALUATION We have developed a discete-event object-oiented packetlevel simulato to assess the pefomance of the poposed coss-laye potocol. The physical-laye undewate acoustic link module models the undewate acoustic signal popagation channel with path loss, multipath, and undewate delays. The undewate acoustic link module geneates bit eo ate cuves in tems of input paametes such as the link distance, the numbes of tansmit/eceive elements, total tansmit powe and acoustic noise level. We consideed a CDMA envionment, with fixed length speading code length 7. The othe simulation paametes ae the same as descibed in [12]. We evaluate the pefomance of ou poposed outing potocol in a thee-dimensional shallow wate envionment. In addition, we compae it with the Geedy Routing Scheme (GRS) [13]. The GRS is based on geogaphical distance. We set the knowledge ange [9] of GRS the same as the tie adius d max of ou poposed algoithm, and set d max to 500 m. The 802.11 caie sense multiple access with collision avoidance potocol (CSMA/CA) with RTS/CTS exchange is used with GRS. Note that ou integated MAC potocol does not employ caie sense, and thee is no collision avoidance mechanism. It educes the end-to-end delay caused by the high caie sensing delay in undewate since the slot time in the 802.11 backoff mechanism is set to 0.18 s and the contention windows CW min and CW max ae set to 32 and 1024 [11]. Note that all figues ae obtained by aveaging ove multiple topologies and epot 95% confidence intevals. We set the chip ate to 100 kcps, the speading code length c to 7, the maximum tansmission powe P max to 10 W, the data packet size to 250 Bytes, RTS, CTS, and ITS size to 10 Bytes. We do not conside o account fo a physical-laye peamble in this pape. Still, we believe that the compaison among competing potocols is fai since all would equally need a peamble. In addition, we conside an initial node enegy of 1000 J, a packet inte-aival time of 5 s, a maximum numbe of etansmissions
Fig. 5. Aveage Used Enegy pe Successfully Received Packet vs. Numbe of Relay Nodes. Fig. 6. Aveage End-to-end Delay pe Successfully Received Packet vs. Numbe of Relay Nodes. equal to 4, an end-to-end packet eo ate theshold of 0.05, an end-to-end packet delay theshold of 8 s, and a queue size of 10 kbytes. All nodes ae andomly deployed in a 3D shallow wate scenaio with volume of 1.5x1.5x1 km 3. The numbe of souce nodes is 50. Taffic packets ae tansmitted to any of the 4 suface stations. In Fig. 5, ou poposed outing algoithm is shown to consideably educe the enegy consumption by selecting suitable tansmit powe compaed with GRS. Moeove, when the numbe of elay nodes inceases, the numbe of uppe tie nodes inceases. Theefoe, afte the tansmitte tansmits an RTS, moe idle uppe tie nodes will espond with CTSs and consume moe enegy. Howeve, the enegy consumption Fig. 7. Aveage Packet Dopping Pobability vs. Numbe of Relay Nodes. only inceases when the numbe of elay nodes is lage. Fig. 6 shows the aveage delay of successfully eceived packets. GRS does not select a backup outing path. Thus, if the next hop is busy, the tansmitte must wait and tansmit an RTS again. This also inceases the packet dopping pobability, as shown in Fig. 7. Fo example, when the numbe of elay nodes is 100, some souce nodes may select the same elay node as thei next hop. The packet delay time and the numbe of etansmissions incease. Ou poposed outing potocol will select an optimal next hop fom the idle uppe tie nodes. Theefoe, GRS has highe end-to-end delay time and packet dopping pobability compaed with ou poposed algoithm. VI. CONCLUSIONS We poposed, discussed and analyzed a outing potocol fo undewate acoustic senso netwoks. Ou poposed outing potocol adequately selects the next hop among idle uppe tie nodes. The end-to-end delay is educed by avoiding the high popagation delay caused by etansmissions. Moeove, in a coss-laye fashion, ou poposed algoithm selects optimal tansmit powe though the coopeation of tansmitte and eceive to achieve the desied level of eliability and data ate accoding to application needs and channel condition. Ou poposed outing potocol was shown to consistently outpefom GRS in tems of enegy consumption, aveage endto-end delay and packet dopping pobability. ACKNOWLEDGMENT This wok was suppoted by the National Science Foundation unde gant CNS-1055945. REFERENCES [1] I. F. Akyildiz, D. Pompili, and T. Melodia, Undewate Acoustic Senso Netwoks: Reseach Challenges, Ad Hoc Netwoks (Elsevie), vol. 3, no. 3, pp. 257 279, May 2005. [2] G. Palou and M. Stojanovic, Undewate Acoustic MIMO OFDM: An Expeimental Analysis, in Poc. IEEE Oceans 09 Confeence, Biloxi, MS, USA, Octobe 2009.
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