TP-CRAHN: A Transport Protocol for Cognitive Radio Ad-hoc Networks

Transcription

1 TP-CRAHN: A Transport Protocol for Cogntve Rado Ad-hoc Networks Kaushk R. Chowdhury Broadband Wreless Networkng Lab Georga Insttute of Technology Atlanta, GA 3332, USA Emal: kaushkc@ece.gatech.edu Marco D Felce 1 Department of Computer Scence Unversty of Bologna Bologna, Italy Emal: dfelce@cs.unbo.t Ian F. Akyldz Broadband Wreless Networkng Lab Georga Insttute of Technology Atlanta, GA 3332, USA Emal: an@ece.gatech.edu Abstract Exstng research n transport protocols for wreless ad-hoc networks has focused on relable end-to-end packet delvery under uncertan channel condtons, route falures due to node moblty and lnk congeston. In a cogntve rado (CR) envronment, there are several key challenges that must be addressed apart from the above concerns. The ntermttent spectrum sensng undertaken by the CR users, the actvty of the lcensed users of the spectrum, large-scale bandwdth varaton based on spectrum avalablty, and the channel swtchng process need to be consdered n the transport protocol desgn. In ths paper, a wndow-based Transport Protocol for CR Ad-Hoc Networks, TP-CRAHN, s proposed that dstngushes each of these events by a combnaton of explct feedback from the ntermedate nodes and the destnaton. Ths s acheved by adaptng the classcal TCP rate control algorthm runnng at the source to closely nteract wth the physcal layer channel nformaton, the lnk layer functons of spectrum sensng and buffer management, and a predctve moblty framework that s developed at the network layer. To the best of our knowledge, ths s the frst work on the transport layer to specfcally address the concerns of the CR ad-hoc networks and our approach s thoroughly valdated by smulaton experments. I. INTRODUCTION A wreless ad-hoc network s comprsed of nodes that forward data packets n a decentralzed manner over multple hops to the destnaton. The ncreasng deployment of such networks n mltary applcatons, vehcular survellance, dsaster relef, commercal messagng, among others have led to a growng congeston and spectrum scarcty n the unlcensed 2.4GHz ISM band. The emergng feld of cogntve rado (CR) networks attempts to allevate the problem of spectrum shortage n the ISM band by opportunstcally transmttng on other vacant portons of the spectrum, such as frequences lcensed for televson broadcast and publc servces [1]. Whle the moblty of the ntermedate nodes and the nherent uncertanty n the wreless channel state are the key factors that affect the relable end-to-end delvery of data n classcal ad-hoc networks, several addtonal challenges exst n a CR envronment. The perodc spectrum sensng, channel swtchng operatons, and the awareness of the actvty of the prmary users (PUs) are some of the features that must be ntegrated nto the protocol desgn [2]. In ths paper, we propose a wndow-based, TCP-lke spectrum-aware transport 1 Ths work was conducted durng hs stay at the BWN lab n 28. layer protocol for CR ad-hoc networks, called TP-CRAHN, that dstngushes between these dfferent condtons n order to undertake state-dependent recovery actons. As the transport protocol usually runs at the end nodes (source and destnaton), t has lmted knowledge of the condtons of the ntermedate nodes. Unknown to the source, the route may be dsconnected due to node moblty. Also, packet losses may be wrongly attrbuted to network congeston rather than bad channel condtons at the lnk layer. Classcal TCP suffers from some of the above ssues and efforts have been made to address them for wreless scenaros n [9] [11]. However, these protocols for classcal wreless ad-hoc networks do not consder the cases that may arse n CR adhoc networks. As an example, n a classcal wreless ad-hoc network, packets may ncur a longer round trp tme (RTT) owng to network congeston or due to a temporary route outage. In CR ad-hoc networks, a smlar effect on the packet RTT may be caused f an ntermedate node on the route s engaged n spectrum sensng and hence, unable to forward packets. Also, the sudden appearance of a prmary user may force the CR nodes n ts vcnty to lmt ther transmsson leadng to an ncrease n the RTT. In such cases, the network s parttoned untl a new channel s dentfed and coordnated wth the nodes on the path. The duraton of the perodc spectrum sensng decdes, n part, the end-to-end performance - a shorter sensng tme may result n hgher throughput but may affect the transport layer severely f a PU s msdetected. Whle several works have focussed on spectrum sensng algorthms n the last few years [1], the ntegraton of the channel nformaton collected at the nodes and the performance study of these approaches from the vewpont of an end-to-end protocol remans an open challenge. TCP, n general, s a well researched area and several theoretcal models exst that explan and predct ts behavor n wreless networks [12]. It s also mplemented at the transport layer for commercally avalable devces. In addton, the adhoc network may ferry user traffc to and from the external nfrastructure network, recevng confguraton commands from remote statons. TCP s the de-facto standard n the wred world and a measure of compatblty s useful from the network management perspectve. Hence, the goal of TP-CRAHN s to retan the wndow-based approach of the /9/$ IEEE 2482

2 S Fg. 1. B Spectrum Sensng Spectrum Swtchng D cwnd B ssthresh tme A mult-hop CR ad-hoc network and the forced cwnd scalng If the source S does not lmt ts transmsson rate for the sensng duraton, packets queue up at node 1 and may soon cause a buffer overflow. Apart from the above consderatons, the sensng nterval also plays a crtcal role n decdng the optmal end-to-end throughput, as ponted out n [1]. If the sensng tme s longer, the CR user montors the channel rather than forward the data packets whle a very short sensng nterval ncreases the rsk of nterferng wth the actvty of a PU [8]. Thus, the transport layer has to balance ths tradeoff so that the throughput of the connecton s mantaned at the desred level and the PU nterference s mnmzed. Fg. 2. Effect of changng channel bandwdth on cwnd classcal TCP, and at the same tme ntroduce novel changes that allow ts applcablty n CR ad-hoc networks. The rest of ths paper s organzed as follows. In Secton II, we motvate the need of a new transport protocol for CR networks. The network archtecture s gven n Secton III. In Secton IV, we descrbe our transport layer protocol n detal. We undertake a thorough performance evaluaton n Secton V, and fnally, Secton VI concludes our work. II. MOTIVATION In ths secton, we dscuss the problems wth the exstng mplementatons of transport protocols based on TCP n CR ad-hoc networks, n whch, nodes are equpped wth a sngle rado transcever. The features of the CR network that we study are: () spectrum sensng () effect of prmary user (PU) actvty, and () spectrum change. A. Spectrum Sensng State CR users perodcally montor the current channel over a pre-decded sensng duraton for the occurrence of PUs before usng t for transmsson. Durng ths nterval, the nodes are not actvely nvolved n transmttng data packets and the mult-hop network s vrtually dsconnected at the node that s engaged n spectrum sensng. As an example, for the mult-hop network shown n Fgure 1, let S and D are the source and destnaton nodes, respectvely. When node 2 senses the spectrum, the path forms two connected segments, from (S 1) and (3 D) gvng the followng cases: As the acknowledgments (ACKs) from the destnaton can no longer reach the source, the RTT of the transmtted segments ncreases and ths may trgger a retransmsson tmeout (RTO), even n the absence of true congeston. B. Effect of PU Actvty On detectng the presence of a PU, ether durng spectrum sensng or an ongong data transfer, the CR users cease ther operaton on the affected channel and search for a dfferent vacant porton of the spectrum. Whle the spectrum sensng on the current channel s perodc and has a well defned nterval, the tme taken to () search for a set of avalable channels on dfferent spectrum bands, and () coordnate wth the next hop neghbors to fnd a mutually acceptable channel n ths set, s of an uncertan duraton. The path to the destnaton s dsconnected untl the new channel s successfully found. Moreover, unlke spectrum sensng, ths duraton s not known to the source n advance. Thus, the transport protocol needs to dfferentate ths state from other causes of route dsconnectons wth the help of an explct feedback from the nodes affected by the PU actvty. The spectrum sensng tme and the delay nduced by PU arrvals also nfluence the way n whch notfcatons about route falures should be used n the CR ad-hoc transport protocols. In classcal ad-hoc networks, ATCP [9] and TCP- EFLN [5] react to the route dsrupton after t happens by an explct notfcaton n the form of the Internet Control Message Protocol (ICMP) message at the IP layer. In Fgure 1, f the node 4 generates the ICMP message, t must traverse through the ntermedate hops before the source S s reached. However, ths packet may suffer a large wat perod as the nodes n the path (S 3) complete ther sensng schedules or negotate a new channel on detecton of a PU. A classcal scheme for for reducng the packet losses by routng layer feedback s proposed n [13]. However, ths method uses cached routes and does not nvolve new route dscovery. For CRAHNs, the changng spectrum envronment may not guarantee the feasblty of the cached route. Thus, we beleve that a predctve framework s needed so that that source can restrct ts sendng rate n advance of the route falure and lmt the number of transmtted packets, when the notfcaton of falure s actually receved. C. Spectrum Change State A key concern n CR networks s the effcent utlzaton of the spectrum resource, as the opportunty for transmsson n the lcensed bands s avalable for a lmted tme. The lcensed channels may have a large varaton n bandwdth, especally 2483

3 as nodes swtch from one spectrum band to the other. In Fgure 2, we study through smulaton how classcal TCP ncreases the cwnd as t probes for the addtonal bandwdth avalable on a sngle lnk. There are three dfferent channel bandwdths possble- 2/3 Mbps, 4/3 Mbps, and 2 Mbps. The vertcal bars denote the bandwdth avalable to the node and at any gven tme, ths s the upper lmt that can be utlzed by the TCP connecton. Ths gves three dstnct levels of bandwdth avalablty wth tme. On each channel, the PU s modeled as a Posson arrval, wth an on tme (α =4s) and off tme (β =5s). When the PU arrves, the CR user swtches to a dfferent channel, and consequently TCP must adjust to the new avalable bandwdth. From the fgure, we observe that the cwnd s unable to correctly track the avalable bandwdth. Moreover, the spectrum opportunty s often lost before the cwnd has ncreased to half the segments that may be supported on the new channel. A smlar concluson s drawn n [1], where TCP cannot effectvely adapt to bref reductons n capacty, f the end-to-end delay s large. We beleve that the cwnd n TCP must be scaled approprately to meet the new channel condtons, as shown n the transton from the operatng pont B to the pont B n Fgure 1. Estmatng ths new operatng pont s a challenge and lnk layer metrcs, that determne the effectve bandwdth, must also be consdered apart from the raw bandwdth. Bandwdth estmaton technques have been proposed n [3] [7], that do not requre nformaton from the ntermedate nodes, but also do not respond mmedately to the avalable spectrum. III. NETWORK ARCHITECTURE The nodes formng the CR ad-hoc network have a sngle rado transcever, that can be tuned to any channel n the lcensed spectrum. We assume ψ spectrum bands are present wth n(x) channels n a gven band x. The channels of ths spectrum band are denoted by ξp x,p=1,...,n(x). In general, two channels n dfferent spectrum bands may have dssmlar raw channel bandwdth,.e. ξp x =ξq y. In addton, we assume the statstcal knowledge of the PU arrval (α) and departure rate (β) for each channel are known, so that an ntal estmate of the channel sensng tme can be calculated. We use CSMA/CA at the medum access control (MAC) layer, that has a pre-decded common control channel (CCC) for coordnaton of the spectrum band and channel durng data transfer. We also use a prorty queue, Q p at the MAC layer for the TP-CRAHN control packets, whch may also be drawn from ntermedate postons n Q p. In a CR network, nodes mantan a lst of unoccuped channels (other than the current one n use) that may belong to dfferent spectrum bands. In our work, we assume that ths set of channels s dentfed through spectrum sensng, undertaken durng the backoff nterval followng a packet transmsson or recepton at the lnk layer. On the current operatonal channel, however, t s mportant to have an accurate dea of the PU actvty. For ths, we do not rely on probablstc sensng tmes. Rather, nodes sense ther current channel for the sensng tme t s at regular ntervals at the cost of contnued network connecton [8]. We now descrbe our protocol and dscuss ts operaton n a CR network. IV. TP-CRAHN: A TRANSPORT PROTOCOL FOR CR AD-HOC NETWORKS TP-CRAHN comprses of the followng 6 states, as shown by the state dagram n Fgure 3. They are () Connecton Establshment, () Normal, () Spectrum Sensng, (v) Spectrum Change, (v) Moblty Predcted, and (v) Route Falure. Each of these states addresses a partcular CR network condton and we descrbe them n detal as follows. A. Connecton Establshment TP-CRAHN modfes the three-way handshake n TCP newreno so that the source can obtan the sensng schedules of the nodes n the routng path. Frst, the source sends out a synchronzaton (SYN) packet to the destnaton. An ntermedate node, say, n the routng path appends the followng nformaton to the SYN packet: () ts ID, () a tmestamp, and () the tuple {t 1,t2,ts }. Here, t1 s the tme left before the node starts the next round of spectrum sensng, measured from the tmestamp. t 2 s the constant duraton between two successve spectrum sensng events, and t s s the tme taken to complete the sensng n the current cycle. On recevng the SYN packet, the recever sends a SYN- ACK message to the source. The sensng nformaton collected for each node s pggybacked over the SYN-ACK and thus, the source knows when a node n the path shall undertake spectrum sensng and ts duraton. The fnal ACK s then sent by the source to the destnaton completng the handshake. We note that the calculaton of the sensng tme t s by a node s undertaken locally. Based on the bandwdth of the channel (W ), the external sgnal to nose rato (γ), and the probabltes of the on perod (P on ) and the off perod (P off ), a framework to calculate ths tme s gven as follows [8], t s = 1 W γ 2 [Q 1 (P f )+(γ +1)Q 1 ( P off P f P on )] 2 (1) Equaton (1) gves the sensng tme t s that mnmzes the probablty of mssed prmary user detecton P f,.e., ncorrectly statng the channel s vacant when ndeed there s an actve PU and Q s the standard Q functon. The sensng tmes collected from the nodes are the prelmnary values whch are dynamcally updated by TP-CRAHN, as descrbed n Secton IV-C. State Transtons: On successful handshake, the source and destnaton are synchronzed and the Normal state s entered. B. Normal State The normal state n TP-CRAHN s the default state and resembles the classcal functonng of the classcal TCP newreno protocol. Our protocol enters ths state when () no node n the path s currently engaged n spectrum sensng, () there are no connecton breaks due to PU arrvals, and () no mpendng route falure s sgnaled. Thus, the path to the destnaton remans connected and ACKs sent by the latter are 2484

4 ECN/ ACK Fg. 3. Moblty Predcted ACK/ Tmeout Spectrum Sensng Perodc Sensng ACK/ Tmeout ECN/ ACK PU Detect. EPN PU Detect. EPN Connecton Est. Normal Spectrum Change Synchronzed ACK/ Tmeout CHN Network Layer Informaton ICMP ICMP ICMP Route Falure Fnte state machne model of our transport protocol receved at the source. The dfferences between TP-CRAHN n the normal state and the classcal TCP are as follows: 1) Explct Congeston Notfcaton: The congeston control algorthm n classcal TCP operates n two phases, namely, the slow start and the congeston avodance. In the slow start, the congeston wndow cwnd s ntally set to 1 (Maxmum Sze Segment or MSS) and doubled for each ncomng ACK. As TCP probes for the avalable bandwdth, the cwnd ncreases exponentally untl the threshold, ssthresh, s reached. It then enters the collson avodance phase, where the cwnd s ncremented by 1 MSS for every acknowledged packet. Durng network congeston, ndcated by the RTO tmeout events, TCP reduces the cwnd to 1 and the new threshold s set to the value ssthresh 2. Whle the above ACK based self-clockng mechansm that ncreases the cwnd s retaned n TP-CRAHN, the congeston event s sgnaled through an explct feedback congeston notfcaton (ECN) generated by the affected node. The congeston s detected at the node by comparng the current buffer usage for the gven flow wth a pre-decded threshold value B f con. The ECN s sent n two ways to guarantee ts tmely delvery. Frst, a packet s sent from the affected node to the source drectly. In addton, the ECN s pggybacked to the destnaton over the data packets and then sent to the source through the ACK. Ths s done as the remander of the connecton from the affected node to the destnaton may suffer delay from a temporary dsrupton caused by channel sensng or swtchng. When an ECN s receved by the source, TP-CRAHN frst evaluates f t s stll relevant to the network congeston state by checkng the tme lag from ts generaton at the affected node to ts recepton. If ths tme s wthn the tme lag threshold L max and no pror acton has been taken for an earler ECN from the same node, for the detected congeston event, TP- CRAHN reduces the cwnd to 1 and cuts the ssthresh by half. In our work, we set L max =1.5 RT T, as any further delay suggests that the path was temporarly dsconnected due to a sensng or channel swtchng event. In ether case, the transmsson rate at the source s reduced, as we shall see later n the protocol descrpton. 2) Feedback Through the ACK: The ntermedate nodes of the path pggyback the followng lnk-layer nformaton over the data packets to the destnaton, whch s then sent back to the source through the ACK. Resdual buffer space (B f ): Consder a node that has B u bytes currently of unoccuped buffer space. Let the number of flows passng through t be n f. The far share of the resdual buffer space per flow s, B f = Bu. n f Observed lnk bandwdth (W,+1 ): Each node mantans a weghted average of the observed bandwdth on the lnk formed wth ts next hop,.e. {, +1}, durng the normal state. Ths s obtaned from the lnk layer as the rato of the acknowledged data bts to the tme taken for ths transfer between the nodes and +1. Total lnk latency (L T,+1 ): Let L,+1 be the sum of the () tme taken by a packet of the current flow to move to the head of the queue () the tme for contendng the access to the channel and fnally, () the transmsson tme measured at node wth respect to the next hop +1. The total lnk latency s now defned consderng the bdrectonal lnk latences, L T,+1 = L,+1 + L +1,. Apart from these felds that are updated every tme by the nodes, the ACK also carres the ECN notfcaton whenever a node experences congeston and the moblty predcted flag (MF), when a possble route outage s dentfed. State Transtons: The ECN message and the ACKs regulate the cwnd. When a node n the path performs sensng, TP- CRAHN transtons nto the Spectrum Sensng state. If PU actvty s reported to the source through an Explct Pause Notfcaton (EPN), t enters the Spectrum Change state and resumes the usual operaton on recevng the nformaton about the new channel through the Channel (CHN) message. Possble route dsruptons may be sgnaled by the nformaton contaned n the ACKs leadng to the temporary Moblty Predcted state, where the cwnd s restrcted to ssthesh. If the ICMP message s receved, TP-CRAHN enters nto the Route Falure state and stops transmsson. C. Spectrum Sensng State We descrbe how TP-CRAHN adapts to spectrum sensng through () flow control, that prevents buffer overflow for the ntermedate nodes durng sensng and () regulatng the sensng tme to meet the specfed throughput demands. 1) Flow Control: When a node undertakes spectrum sensng, the path gets vrtually dsconnected for a fnte duraton. At ths tme, the goal of TP-CRAHN s to adapt the flow control mechansm n TCP, so that the node 1, pror to the sensng node, s not overwhelmed wth ncomng data packets. If another node j has an overlappng sensng schedule, TP- CRAHN uses the resdual buffer space of the prevous hop of the node closest to the source durng the perod of overlap, say. When the sensng tme of the closest node s completed, the buffer space of node j 1 s used n the ewnd computatons. We recall that, n classcal TCP, the maxmum number of bytes of unacknowledged data allowed at the sender s 2485

5 Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subject matter experts for publcaton n the IEEE INFOCOM 29 proceedngs. Sensng Tme (sec) Mssed Detecton Probablty (Pf) SNR=-25dB SNR=-22.5dB SNR=-2dB Gradent of the Sensng Tme SNR=-25dB SNR=-22.5dB SNR=-2dB Mssed Detecton Probablty (Pf) Fg. 4. The sensng duraton for ncreasng error probablty and the gradent of the curves are shown n and, respectvely, for dfferent SNR ranges the mnmum of the current congeston wndow, cwnd, and the receve wndow advertsed by the destnaton, rwnd. The rwnd represents the free space n the recever s buffer that can accommodate addtonal transmtted packets. Durng the sensng duraton, no ACKs are receved by the source and hence the rwnd remans unchanged. Ths also results n a constant cwnd as TCP s self clocked and does not ncrease n the absence of the recever ACK. The effectve wndow, ewnd at the sender s modfed to nclude an estmate of the free buffer space, B f 1, at the prevous hop node 1 as follows, ewnd = mn{cwnd, rwnd, B f 1 } (2) As the packets fll up the buffer n the node 1, the remanng free buffer space needs to be progressvely reduced, so that the effectve wndow can be computed from equaton (2). The ntermedate node, unlke the destnaton, does not send back the ACKs wth the new advertsed receve wndow and hence, ths s estmated as follows: From the last receved ACK, we know the free buffer space for the gven flow, at the node 1, sb f 1. The approxmate tme for successfully transmttng a packet over the lnk 2, 1 can be calculated at the source as L 2, 1 = LT 1, 2 2, where L T 1, 2 s the bdrectonal lnk latency pggybacked over the ACK. Thus, the space avalable B f 1 n the node 1 s decremented at ntervals of L 2, 1, when node s engaged n sensng. We note that whle the rate of decrease of the buffer space s not exact, the node 1 s oblvous to ths sender-sde adaptaton. It can stll force the source to reduce ts sendng rate through the congeston notfcaton. If ts buffer s reachng the overflow lmt, the congeston condton wll be sgnaled and the cwnd wll be reduced to 1 at the source as a response. If any of the ntermedate nodes on the path from the source to the node 1 detect congeston, the ECN packet s sent by them and the cwnd, used n equaton (2), s then reduced to 1. We note that the effectve wndow ewnd remans at 1, as long as the path remans dsconnected, as the cwnd cannot be ncreased wthout the arrval of the ACK. 2) Sensng Tme Regulaton: In Secton II-A, we stated that f there s no PU actvty on a gven channel, the comparatvely large sensng tmes degrade the end-to-end throughput. To address ths, TP-CRAHN conservatvely reduces t s for the nodes that see lmted PU actvty. Ths calculaton s carred out at the transport layer as t s aware of the observed (τ o ) and the desred (τ d ) throughputs, respectvely. The two nputs needed by our protocol are - () the value, δ s, by whch the sensng tme should be decreased, and () the node at whch ths reducton must be undertaken: Sensng Tme Decrease: Fgures 4 and 4 gve the optmal sensng tme and the gradent of the curve for the sensng tme, respectvely, n order to mantan a gven mssed detecton probablty (P f ) for dfferent SNR ranges and a bandwdth of 2MHz, as per the analytcal formulaton n [8]. We observe that the t s s large when the target error probablty s very low. Moreover, there s also a large fall n the t s for a fnte change n the P f, as shown by the gradent curve (Fgure 4), when the P f s small. Ths means that the t s can be reduced by a greater margn n the ntal stage, when t has a comparatvely hgher duraton, wthout mpactng the error sgnfcantly. Also ths margn must tself be lowered as the value of t s gets progressvely reduced. The ntutve reasonng s as follows: A node frst sets t s = t s max correspondng to the low error probablty of P f =.1. Ifthe number of spectrum changes that occur over tme, s small n proporton to the number of total changes along the entre path, we assume that the node s stuated n a regon wth lmted PU actvty. Thus, the perodc sensng tme t s may be reduced at ths node. If the current t s at the node s large, ts reducton s consequently hgher, as the probablty of error s not affected proportonally. However, as the t s value falls, the reducton gets progressvely smaller untl t s = ts mn s reached, correspondng to the lmtng error probablty, P f =.5. We can now formulate the steps for obtanng the new sensng duraton t s (new) from the old value ts as follows, t = ts max 2 γ t s γ t s max (3) = dts dp f ts =t s,p f =func(t s ) (4) = dts dp f ts =t s max,p f =.1 (5) δ s = γ t s γ t s max t (6) t s (new) = t s δ s (7) The default decrement value of the sensng tme, t,s taken as half of the maxmum value, t s max, needed to mantan the error probablty at.1, as shown n (3). Ths s later scaled by a factor n the range [, 1] to get the true decrement δ s.in(4), we calculate the value of the gradent γ t s to the sensng curve at the current sensng duraton t s, at node. The correspondng value of P f s obtaned from the current t s from Fgure 4, whch s n turn, a numercal plot of equaton (1). The maxmum gradent of the sensng curve γ t s max s gven n (5) and s computed at t s = ts max. The normalzed gradent, γ t s γ t, at the current tuple gven by {t s s,p f } s used as the max scalng factor to gve the true decrement δ s n (6). Fnally, the sensng tme s adjusted to the new value t s (new) n (7). When successve mssed detecton events occur, the node ncreases the sensng duraton n the steps { 1 2 ts 3 max,

6 ξ x p 1 PU c, 1 +1 c,+1 1 (Old Channel ξ x p) Channel Lst Channelξ y q ACK (New Channel ξ y q) Probe L T, 1 (tme) Fg. 5. The PU nterference scenaro and the lnk layer total delay estmaton are shown t s max,t s max}, n that order. Whenever the sensng tme s changed, the node sends back the new value to the sources of the flows passng through t by pggybackng over the ACK. Node Selecton: In order to dentfy a specfc node for adjustng the sensng tme, TP-CRAHN ranks the nodes n the path based on the number of tmes the operatonal channel was changed due to PU actvty. It keeps a count of the CHN messages sent by each node of the path, whch reveals the number of tmes the connecton was paused whle a new-channel was beng coordnated. Intutvely, the node that generated the hghest proporton of the CHN message also experenced the maxmum number of PU detecton events and thus, must be located n a regon of frequent PU actvty. Such a node needs to retan a hgher sensng duraton. Let the total number of tmes the spectrum change occurs at a gven node, and that consderng all the nodes of the path be gven by η and η T, respectvely. We defne the probablty of the node beng susceptble to PU actvty, S as the rato S = η η T. Let the set of n nodes along the route have ther PU actvty susceptblty gven by the set S = {S 1,...,S n }. Recallng that τ d and τ o are the desred and observed throughputs, the source executes the followng algorthm to determne the node q and adjust ts sensng tme to the new value t s q(new): PROCEDURE:Sense-Adjust Input: τ d, τ o, S Output: q, t s q(new) f τ d >τ o then q =arg mn{s }, =1,...,n f t s q(old) >t s mn & S <S max then t s q(new) = t s q(old) δ s end end We explan the algorthm as follows: If the desred throughput (τ d ) s greater than the observed throughput (τ o ), then TP- CRAHN fnds the node q wth the mnmum PU susceptblty, S,=1,...,n. Two condtons are checked for the node q- () the current sensng tme at the node t s q must be greater than the mnmum allowed sensng tme t s mn and (), the PU susceptblty must be below the lmtng threshold S max, consdered as.3 n our work. Ths ensures that only nodes that have are relatvely undsturbed by PU actvty over tme are chosen for reducton of the sensng duraton by the value δ s, as descrbed n equaton (7). Ths new value of the sensng nterval s sent to the ntermedate node by the source. State Transtons: On recevng the EPN message, the Sensng state s nterrupted and our protocol mmedately transtons to the Spectrum Change state, or else, t reverts back to the Normal state on the completon of the sensng duraton. The transtons to the Moblty Predcted and the Route Falure state are smlar to the descrpton of the Normal state. D. Spectrum Change State In the deal case, the effectve bandwdth of the TCP connecton s dependent on several factors, such as contenton delays and channel errors at the lnk layer, apart from the raw bandwdth of the channel. In ths secton we show how TP- CRAHN scales ts cwnd rapdly, say from pont B to a dfferent value B, n Fgure 1, accountng for these factors, so that the avalable spectrum resource s most effcently utlzed. Consder three nodes gven by 1, and +1 on the current path and the channels used by the lnks { 1,} and {, +1} be c 1, and c,+1, respectvely (Fgure 5). If the PU s on the channel ξ x p and ether c 1, = ξ x p or c,+1 = ξ x p, the node must search for a new channel to prevent nterference to tself and to the PU, respectvely. At ths stage, t sends an explct pause notfcaton (EPN) to the source, whch n turn, freezes the protocol state and wats for a new channel CHN message to resume the transmsson. We consder the case where TP- CRAHN adjusts to a sngle affected lnk { 1,} and then extend the analyss for the case when both the prevous and next hop lnks need a channel change. The set of avalable channels s known at node, as descrbed n Secton III. The preferred lst of channels, from ths avalable set, s sent by ths node to the prevous hop 1 (Fgure 5 ). The node 1 chooses a channel from ths set, say ξ x q. It then sends back a lnk layer ACK to node to nform the node of ts choce, ξ x q. All the coordnaton up to ths pont occurs on the old channel. A second set of Probe and ACK messages are then exchanged on the channel to be swtched, ξ x q, as a confrmaton and also to approxmately estmate the new lnk transmsson delay tmes L, 1 and L 1,.Ifthe probe and ACK packets are of the sze P probe and P ACK, respectvely, the observed lnk bandwdth W, 1 s, W, 1 = P probe + P ACK L, 1 + L 1, (8) The CHN message contans n t the bdrectonal lnk layer packet delay over the newly dentfed channel, (L T, 1 = L, 1 +L 1, ), that s used by the source to calculate W, 1 from equaton (8). From Secton IV-B2, we recall that the ACKs forwarded over the ntermedate hops also carry the total bdrectonal lnk latency, L T, 1, correspondng to the earler used channel. On recevng the CHN message, the source frst estmates the new RTT usng () the earler observed RTT durng the last normal state of the protocol and () adjustng for the new bdrectonal lnk delay, L T, 1, RT T = RT T + L T, 1 L T, 1 (9) 2487

7 For the gven path of n nodes, let W b be the old observed bottleneck bandwdth, before the channel change. After the channel change, the new bottleneck bandwdth s dentfed as W b, where W b = mn{w l,l+1 },l=1,...,n 1. The updated estmate of the bandwdth W, 1 s used n ths calculaton from equaton (8). If the rato of the old bottleneck bandwdth to the new s wthn the allowed range of [1 ϖ, 1+ϖ],.e. W b W b [1 ϖ, 1+ϖ], then no scalng of the earler cwnd s needed, where ϖ =.2. If t les beyond ths range, then we calculate the new value of the cwnd as follows, cwnd = α c W b RT T (1) The factor α c =.8 n equaton (1) s used to adjust the cwnd to a value slghtly lower than the predcted bandwdth to prevent the rsk of over-estmatng the cwnd [4]. Over tme, the cwnd converges to the optmal value around ths range. In the event that the channels of both the upstream and downstream lnks are changed, the bdrectonal lnk latences, L T, 1 and L T,+1 are used n the equatons (9) and (1). State Transtons: The Spectrum Change state s entered as soon as an EPN message s receved. It reverts back to the Normal state when the new channel nformaton s receved n the CHN message or enters nto the Route Falure state on the recept of an ICMP message. Exstng sensng schedules are gnored as long as the protocol stays n the current state. E. Moblty Predcted State In order to address the problem of delayed route falure notfcaton (Secton II), we develop a moblty predcton framework based on Kalman flter based estmaton [6], that uses the receved sgnal strength (RSS) nformaton from the lnk layer. The nodes of the path montor the connectvty to ther next hop downstream node by measurng the RSS of the ACKs and the perodc beacon messages. At each epoch, the predcton value s compared wth the mnmum RSS requred for recever operaton. If the condton of possble lnk falure s predcted n the next epoch, the destnaton s nformed, whch then sets the moblty flag (MF) n the outgong ACKs. The source responds to ths by lmtng the cwnd to the ssthresh and the congeston avodance phase s never ntated. The am of ths adjustment, cwnd ssthresh, s to lmt the number of packets njected nto the route whch has a possblty of an outage, as the CR specfc functon of the nodes may delay the arrval of the actual lnk falure notfcaton. If no ICMP message s receved at the source subsequently, sgnalng that a route falure has ndeed occurred or the ncomng ACKs do not have the MF flag sent, the moblty predcton state s cancelled and TP-CRAHN reverts back to the normal state, where the cwnd s no longer bounded. State Transtons: TP-CRAHN regards the Moblty Predcted state as a transent or vrtual state, n whch the cwnd s restrcted to the ssthresh and the current operaton ether n the Normal or the Spectrum Sensng state s contnued. F. Route Falure State The node sends a destnaton unreachable message n the form of an ICMP packet f () the next hop node +1 s not reachable based on lnk layer re-tres, () there s no ongong spectrum sensng based on the last known schedule, and () no EPN message s receved at node sgnalng a temporary path dsconnecton due to PU actvty. At ths stage, the source stops transmsson and a fresh connecton needs to be formed over the new route by TP-CRAHN. State Transtons: The Route Falure state s the termnal state of the current cycle and a fresh TCP connecton must be establshed when a new route s formed. The protocol enters ths state on recevng the ICMP message and t takes precedence over all the others states. V. PERFORMANCE EVALUATION In ths secton, we study the behavor of TP-CRAHN under the scenaros of () spectrum sensng, () spectrum change wth PU actvty, and () node moblty. To the best of our knowledge, there s no exstng transport layer protocol that s desgned consderng the CR specfc functons, and protocols devsed for classcal ad-hoc networks cannot be compared farly wth our work. Rather, we use TCP newreno (henceforth referred to as TCP) as a benchmark and focus on how TP-CRAHN adjusts to each of the above CR scenaros through a stage-wse mplementaton of ts modules. We extend the NS-2 smulator wth mult-channel extensons and a channel swtchng tme of 5ms s used. In order to study the effect of the cwnd scalng, we consder 5 channels, C = {c 1,...,c 5 }, havng varyng raw channel bandwdth gven by {B, B K,B K, B K,B K}, respectvely, where B = 2 Mbps, K=2. In addton, a prorty queue s mplemented at the lnk layer, as descrbed n Secton III, and the allowed retres for the feedback packets n TP-CRAHN s rased to 2. Out of 1 nodes any source-destnaton par s chosen formng a chan topology and we vary the number of flows n the path. A parallel chan s then created, that uses the same channel selecton as our test chan topology. The correspondng nodes of the two chans are placed wthn transmsson range of each other and ths provdes the lnk contenton for the bandwdth calculaton. The transmsson ranges of the PU and the CR users are 3 m and 12 m, respectvely and the desred throughput τ d = 8 Kbps. A. Spectrum Sensng The evaluaton of TP-CRAHN durng spectrum sensng s carred out n two parts - () by observng the mprovement n throughput resultng from the change n the ewnd (Secton IV-C1), and () the beneft of reducton of the sensng duraton n the absence of PU actvty (Secton IV-C2). Fgures 6 and 6 show the end-to-end throughput for varyng network loads as the sensng tme of the nodes s ncreased to the maxmum value t s = t s max =.3, for a constant data transmsson tme T p. In the frst set of experments, we dsable the dynamc adjustment of the sensng tme. We observe that TP-CRAHN outperforms TCP sgnfcantly as t 2488

8 Ths full text paper was peer revewed at the drecton of IEEE Communcatons Socety subject matter experts for publcaton n the IEEE INFOCOM 29 proceedngs. TP-CRAHN, Tp=2 6 TCP, Tp=2 TP-CRAHN, Tp=3 TCP, Tp= Sensng Tme (sec) TP-CRAHN, Tp=2 TCP, Tp=2 TP-CRAHN, Tp=3 TCP, Tp= Sensng Tme (sec) TCP Congeston Wndow (# segments) TP-CRAHN Smulaton Tme (sec) TCP (c) Fg. 6. The effect of spectrum sensng on the throughput s shown for 1 and 5 flows n and, respectvely. Varaton of the congeston wndow wth tme s gven n (c) Sensng Tme=.5s (mn) Sensng Tme=.5s (max) Sensng Tme=var Sensng Tme=.5s (mn) Sensng Tme=.5s (max) Sensng Tme=var Throughput Sensng Tme (sec) Average PU ON Tme (sec) Average PU ON Tme (sec) Smulaton Tme (sec) (c) Fg. 7. The effect of dynamcally changng the sensng duraton on throughput s shown for 1 and 5 flows n and respectvely. A study of the throughput as a functon of the varyng sensng tme s gven n (c) TP-CRAHN, Toff=3s TCP, Toff=3s TP-CRAHN, Toff=6s TCP, Toff=6s Average PU ON Tme (sec) TP-CRAHN, Toff=3s TCP, Toff=3s TP-CRAHN, Toff=6s TCP, Toff=6s Average PU ON Tme (sec) Bandwdth Utlzaton Effcency (%) Bandwdth Dfference Factor (K) (c) TCP TP CRAHN TCP Congeston Wndow (# segments) TP-CRAHN TCP Smulaton Tme (sec) Fg. 8. The effect of the bandwdth scalng adjustment on throughput s shown for 1, and5 flows n, and, respectvely. The bandwdth utlzaton effcency and the cwnd scalng are shown n (c) and (d), respectvely. (d) does not stop transmttng when the path gets dsconnected, but transmts at a reduced rate to prevent a buffer overflow. Ths ensures a hgher throughput n TP-CRAHN, as TCP suffers an RTO tmeout whenever a node n the path undertakes sensng. Ths phenomenon s also seen n Fgure 6(c), where the cwnd almost never reduces to 1 for the case t s =.15,T p =3, unless there s congeston n the network (at sm. tme = 7 s). Compared to ths, TCP undergoes repeated tmeouts durng the sensng and the cwnd s forced to the slow start phase n the absence of true congeston. For the study of the dynamcally changng sensng duraton, we frst defne the PU operaton as follows: We contnuously vary the on tme (T on ) of the PU on the x-axs, and measure the throughput for dfferent PU off tmes, T off,for1 and 5 flows as shown n Fgures 7 and 7, respectvely. We frst assgn a a low PU susceptblty of.1 to a randomly chosen number of nodes S low n the path, and for the remanng nodes, formng the set S hgh, we set an ntal hgh susceptblty value of.95. Ths ensures that TP-CRAHN changes the sensng duraton from the maxmum value t s max =.28 only for the nodes present n the set S low. When the sensng tme s vared dynamcally by TP-CRAHN, (shown by sensng tme = var), we observe that the throughput mproves sgnfcantly for both the cases of 1 and 5 flows (Fgure 7 and 7). The varaton of the cwnd for throughput as a functon of the changng sensng tme s shown n Fgure 7(c).The sensng tme falls n a non-lnear manner (Secton IV-C2), and n the absence of PU actvty, ths mproves the throughput. For successve PU mssed detectons, the sensng tme s scaled to 1 2 ts max, then 3 4 ts max and fnally to the maxmum value t s max. For each ncrease n t s, though the throughput s partally reduced, the mutual nterference wth the PU s also mtgated. 2489

9 Avg Pred. Error (db) var=.3 var=.6 var= Tme Interval (ms) TCP Congeston Wndow (# segments) TP-CRAHN TCP Smulaton Tme (sec) Fg. 9. The Kalman flter accuracy and the varaton n the cwnd wth tme are shown n and, respectvely B. Spectrum Change and PU Actvty From Secton IV-D, we recall that when PU actvty s detected, TP-CRAHN stops the source from transmttng and coordnates the use of a new channel. The source then modfes ts cwnd, f the new channel on the affected lnk sgnfcantly changes the bottleneck bandwdth avalable to the connecton. We study the performance mprovement n TP-CRAHN by consderng the throughput, the bandwdth effcency (rato of the avalable bandwdth to average used bandwdth of the bottleneck lnk), and the varaton n the cwnd. A PU s placed on each of the 5 possble channels so that the channel (and hence, the bandwdth) change often and ts effect s clearly demonstrated. Fgures 8 and 8 gve the throughput for 1 and 5 flows, respectvely, when PUs exst on the channel. We observe that the throughput mprovement n TP-CRAHN ncreases wth hgher PU actvty, formally T on T on+toff defned as σ =. For lower values of σ,.e. when T on s small, the channels are readly avalable and the gan n TP-CRAHN s due to the scalng of the cwnd alone. For hgher values of σ, when T on s large, t takes longer for the affected node to fnd a vacant channel. Ths delay domnates the network performance and by explctly specfyng the source to pause ts transmsson, TP-CRAHN prevents packet loss and mproves the throughput. The effect of cwnd scalng results n hgher bandwdth effcency n TP-CRAHN, as seen n Fgure 8(c). Moreover, the performance mproves when the dfference n the raw bandwdths of the avalable channels (gven by ncreasng the factor K) s hgher, mplyng that the forced scalng of the cwnd s effectve n fully utlzng the spectrum resource. The varaton of the cwnd aganst tme n Fgure 8(d) shows that TP-CRAHN responds to the changed bandwdth mmedately and after the scalng, the cwnd ncreases n most of the cases, wthout causng congeston. C. Moblty Predcton The error between the true sgnal strength (RSS) and the value predcted by the Kalman flter s shown n Fgure 9 as a functon of the epoch nterval used between two successve calculatons. In our chan topology, nodes move wth an average speed of 1 m/s wth locatons gven by the random waypont model. The varance (var) n the RSS n db, due to external nose affects the predcton accuracy as shown n the fgure. We observe two cases n Fgure 9, the frst of whch shows a route dsconnecton at tme = 7 s. The conservatve reducton n the cwnd resulted n fewer packets that were lost n the unusable route, as compared to TCP. An ncorrect predcton that restrcted the cwnd to the ssthresh tll the tmeout perod s shown at tme= 15 s. VI. CONCLUSIONS Our proposed transport protocol, TP-CRAHN, ntegrates as an end-to-end metrc, the spectrum sensng and swtchng functonaltes n a CR network, apart from the classcal concerns of congeston, flow control and connecton losses due to node moblty. By relyng on updates from the ntermedate nodes and the destnaton feedback, the source mantans nformaton about the network state and responds approprately by adjustng ts transmsson rate. Future research n ths drecton would nvolve proposng an mproved predctve framework to reduce the dependence on ntermedate feedback by the nodes. In addton, the qualty of servce demands of the flows wll be ntegrated n the TP-CRAHN protocol. ACKNOWLEDGEMENT Ths materal s based upon work supported by the US Natonal Scence Foundaton under Grant No: CNS Marco D Felce was supported by Italan Mur funds for the PRIN-26 project NADIR. REFERENCES [1] I. F. Akyldz, W. Y. Lee, M. C. Vuran, and S. 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