Published on

  • Be the first to comment

  • Be the first to like this

No Downloads
Total Views
On Slideshare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide


  1. 1. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252 Measurement and Modeling of the congestion-controlled traffic over CSMA based multihop Wireless Mesh Networks Venkatesh.Donepudi, vahiduddinshariffAbstract wireless mesh network is a well suited network deployment [2, 4, 5]. Wireless Meshfuture technology. Its properties like low cost, Network consists of three types of nodes. Meshhigh bandwidth and significant progress has been Points(MP), Mesh Routers(MR), andMeshmade in understandingthe behavior of TCP and Clients(MC). In WMN, some Access Points(APs)congestion-controlled traffic over CSMA based have wired connections known as meshmultihop wireless networks. Despite these points(MPs). APs that don’t have wired connectionsadvances, however, no prior work identified arecalled Mesh Routers(MRs), they can form multi-severe throughput imbalances in the basic hop communication with theMPs. Both MPs andscenario of mesh networks, in which a one-hop MR form the backhaul network, which is used toflow contends with a two-hop flow for gateway forwardtraffic between the nodes. MP and MR areaccess. In this paper, we demonstrate via real usually equipped with multiple wireless interfacesnetwork measurements, testbed experiments, and built on either same or different wirelessan analytical model that starvation exists in such technologies and theyestablish a permanenta scenario; i.e., the one-hop flow receives most of infrastructure. New MP and MR are easily added,the bandwidth, while the two-hop flow starves. sincethe connection is wireless connection [5].Our analytical model yields a solution consisting Compared to conventional wirelessof a simple contention window policy that can be routers, a mesh router can achieve the same radioimplemented via standard mechanisms defined in range with much lower transmission power throughIEEE 802.11e. Despite its simplicity, we multi-hop communications.Mesh router anddemonstrate through analysis, experiments, and conventionalrouters are usually built based on asimulations that the policy has a powerful effect similar hardware platform [5]. MPs and MRshaveon network-wide behavior, shifting the network’s the similar design but MPs are connected to wiredqueuing points, mitigating problematic MAC networks. Mesh Clients(MC) usually have only oneand transport behavior, and ensuring that TCP wireless interface and hardware as well asflows obtain a fair share of the gateway softwareare much simpler than MPs or MRs. MCsbandwidth, irrespective of their spatial location. do not have routing capabilities andworks as Mesh Router(MR) and as a router [4, 5]. Due to highIntroduction mobility of meshclients they can leave and join the Wireless Mesh Networks(WMN) are next Mesh Router(MR) at any time [5].There is angeneration wireless networks with theirpromising increasing need towards portable and mobileand low-cost technology. It provides high-speed computers or workstations with the development ofInternet access forfuture broadband applications. wireless technology and Internet. Wireless networksWMNs are flexible, mobile, reliable and need to provide communications between mobilescalablewireless networks. WMNs reduce the initial terminals, in addition, access to high speed wiredinvestment and deployment time compared to networks needs to be provided too. Wirelesstraditional broadband Internet access technologies[1]. In WMNs802.11 based multi-hopcommunication is used for delivering fast services toend-users,WMNs are not used for any fixed Accesspoint networks likeWireless LocalAreaNetworks(WLAN).WMNs are extremely reliablewith its mesh connection.If any node fails or drop ofpacket in the network occurs, it uses another routeasactivce route.Wireless mesh networks (WMN) is anemerging technology. Existing definitions are simplybased on Mobile Ad hoc Networks (MANETs),which arevariants of WLAN. WMNs are consideredas special MANETS [3,5]. Wireless MeshNetworks(WMNs) are self-organized, self-configured, ease and can rapidity change with the Figure 1: Wireless Mesh Networks 242 | P a g e
  2. 2. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252Local Area Networks(WLANs), which provides 802.11, which is based on binary slotted exponentialbetter flexibility and convenience than their wired backoff. In this also uses Markov chain andcounterpart, are being developed to provide high considers the frame retry limits to analyze thebandwidth access for users in a limited geographical saturated throughput; therefore, a more exact modelarea. IEEE Project 802 recommends an international isproposed in this thesis. On the other hand, with thestandard 802.11 for WLANs. The standards include prosperity of Internet, Transport Controldetailed specifications both for MediumAccess Protocol(TCP), which is the widely used reliableControl(MAC) Layer and Physical(PHY) Layer. In protocol in the Internet, is supposed to work well inWLANs, the physical media, which is shared by heterogeneous environment. Since the WLAN MACallstations and has limited connection range, has has its own characteristics, such as MAC Layersignificant differences when compared to wired ACK frame, MAC retransmissions, which aremedia. The design ofWLAN MAC protocol is different from traditional wireless medium, thefurther complicated by the presence of hidden performance evaluation and enhancement will beterminal and capture effects. Currently, the IEEE somewhat different from the research before. The802.11 WLAN standards include a basic medium performance of TCP over WLAN is being studiedaccess protocol Distributed Coordination recently; however, none of these give a TCPFunction(DCF) and an optional Point Coordination performance enhancement based on the WLANFunction(PCF). In 802.11, the DCF is the MAC layer solutions. In fact, when TCP runs overfundamental access method used to support WLAN, where a shared channel is used for multipleasynchronous data transfer on a best effort basis. As access, the forward TCP data and the backward TCPspecified in the standards, the DCF must be ACK will compete the channel, which may causesupported by all the stations in a basic service collisions and degrade the overall performance.set(BSS). The DCF protocol is based on Carrier Meanwhile, 802.11 has been standardized and anySense Multiple Access with Collision proposed enhancement scheme must keep backwardAvoidance(CSMA/CA). CSMA/CD is not used compatibility with 802.11, i.e., it can work withbecause a station is unable to listen to the channel 802.11 without introducing performancefor collision while transmitting. In 802.11 CS is degradation.performed both at physical layer, which is alsoreferred to as physical carrier sensing, and at the Large-scale mesh network deployments areMAC layer, which is known as virtual carrier planned and underway in cities across the world.sensing. The PCF in the 802.11 is a polling-based According to In-Stat, the market will grow from 248protocol, which is designed to support collision free cities in 2005 to 1,500 cities in 2010, becoming aand real time services. This paper focuses on the $1B industry in mesh access point sales alone. Theperformance analysis and modeling of DCF in prevailing architecture for large-scale deployments802.11 WLAN. There are two techniques used for is a two-tier architecture in which an access tierpacket transmitting in DCF. The default one is a connects end users’ PCs and mobile devices to meshtwo-way handshaking mechanism, also known as nodes and a backhaul tier forwards traffic to andbasic access method. A positive MAC from high-speed gateway nodes. Directionalacknowledgement(ACK) is transmitted by the antennas terminating at the gateway are also used todestination station to confirm the successful expand coverage and capacity. We have deployedpackettransmission. The other optional one is a four- and are operating UrbanMesh,1 a two-tier meshway handshaking mechanism, which uses request-to- network serving a user population of nearly 2,000send/clear-to-send(RTS/CTS) technique to reserve users in a 3 km2 urban community. As mostthe channel before data transmission. This technique deployments are in the planning or early deploymenthas been introduced to reduce the performance stages, to the best of our knowledge, this is thedegradation due to hidden terminals. However, the highest density urban mesh network operating todrawback of using the RTS/CTS mechanism is date. Unfortunately, we have observed that underincreased overhead for short data frames. The heavy load, flow starvation will occur in which one-modeling of 802.11 has been a research focus since hop flows obtain high throughput whereasthestandards has been proposed. In this,the effect of competing multi-hop flows obtain near zerocapture and hidden terminal and paper gives the throughput. The phenomena occurs under twotheoretical throughput limit of 802.11 based on a p- hardware/software platforms as well as inpersistent variant. However, none of these captures simulation. Clearly, for mesh networks to bethe effect of the Contention Window(CW) and successful, it is critical that network resources arebinary slotted exponential back-off procedure used distributed fairly among users, irrespective of theirby DCF in 802.11. Unlike those ones, we use spatial location. In this paper, we (i) performMarkov process to analyze the saturated throughput extensive measurements in UrbanMesh toof 802.11 and show that the Markov analysis works characterize starvation, (ii) studystarvation’swell. We believe that the Markov chain analysis protocol origins, (iii) develop an analytical model tomethod is fit for examining the performance of IEEE capture the interaction of medium access and 243 | P a g e
  3. 3. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252congestion control that leads to starvation and (iv) solves the starvation problem for TCP upstream anddesign, justify, and evaluate a counter-starvation downstream traffic. We extend our investigation to a broader set of scenarios using simulations and show Policy in which nodes one-hop away from that our solution enables TCP flows to fairly sharethe gateway increase their minimum contention the gateway bandwidth in more generalwindow. In particular, our contributions are as scenarios.Wireless mesh backhaul nodes arefollows. First, we experimentally demonstrate the predominantly deployed on single-story residencesexistence of starvation in TCP wireless mesh with the exception of two schools, two businesses,networks. Moreover, we design a set of experiments and a public library within the neighborhood. Theto isolate the factors that cause starvation. We current mesh infrastructure is composed of 18identify that only a one-hop TCP flow coupled with backhaul nodes which coordinate to share thea two-hop TCP flow is sufficient to induce Internet bandwidth from a single fiber that isstarvation. Moreover, we demonstrate that starvation burstable to 100 Mbps. Fig. 1 depicts the spatialis not merely a hidden-terminal effect by showing distribution of the wireless mesh backhaul tier andthat starvation does not occur if the TCP flows are the connectivity map. In the figure, nodes arereplaced by UDP flows and IEEE 802.11’s depicted as connected if a direct transmission canRTS/CTS handshake is used to counter hidden occur between the two backhaul nodes. All links areterminals; yet, the use of RTS/CTS is irrelevant for omni-directional with the exception of a directionalTCP flow starvation. Second, we describe the link(shown in black) which serves as an additionalprotocol origins of starvation as a compounding point of capacity for the network. Each mesh nodeeffect of three factors. First,the medium access serves access nodes or clients wirelessly within itsprotocol induces bi-stability in which pairs of nodes immediate proximity of approximately 200-300 m inalternate in capturing systemresources. Second, the radius. The radius is limited primarily by heavy treesystem’s nested congestion control loops utilizing foliage and densely packed homes with lot sizesthe wireless medium make it more likely that outer averaging only 510 𝑚2 . At the time of ourloops (multi-hop flows) are disrupted rather than experiments, there are nearly 2,000 users in theinner loops (single-hop flows). Third, and most network in an area of nearly 3 𝐾𝑚2 .,i.e.,critically, the system incurs a high penalty when approximately 600 users and 6 backhaul nodes perswitching between the two states of the bi-stable 𝐾𝑚2 ., which makes wireless mesh one of the mostsystem. In particular, a state is normally exited when dense mesh networks operating to date; foreither a flow ―runs out of ACKs‖ and thus cannot comparison, the St. Cloud mesh network currentlytransmit data because it cannot receive feedback, or serves 120 users per 𝐾𝑚2 .. There are 4,760 residentswhen a DATA packet is dropped at themedium per 𝐾𝑚2 . in the served neighborhood; as a point ofaccess layer. Third, we develop an analytical model reference, the average population density of the 20to both study starvation and to drive the solution to largest U.S. cities is 2,800 residents per 𝐾𝑚2 ..counter starvation. The model omits many Planned and deployed commercial mesh networksintricacies of the system (TCP slow start, fading also employ this two-tier architecture in whichchannels, channel coherence time, etc.) and instead Internet gateways feed multiple multi-hop wirelessfocuses on the minimal elements needed such that paths and directional links are used to provide high-starvation manifests. Namely, the model uses a speed ―short-cuts‖ to the gateway for capacitydiscrete-time Markov chain embedded over improvement. The process is then duplicated everycontinuous time to capture a fixed end-to-end 3-10 𝐾𝑚2 . for larger coverage areas.congestion window, a carrier sense protocol with orwithout RTS/CTS, and all end-point and Wireless mesh networks (WMNs) are beingintermediate queues. The model yields a Counter- deployed to provide low-cost high-bandwidthStarvation Policy in which all of the gateway’s Internet access on a large scale. In a WMN,neighbors should increase their minimum contention stationary wireless mesh nodes (or routers) providewindow to a value significantly greater than that of network connectivity for mobile wireless meshother nodes.2 The model also characterizes why the clients. Multiple wireless mesh nodes arepolicy is effective in that it forces all queueing to interconnected to form a wireless backbone. Severaloccur at the gateway’s one-hop neighbors rather than mesh nodes also serve as the gateways (GW), whichelsewhere. Because these nodes have a perfect connect to the Internet. The mesh architecture, inchannel view of both the gateway and their which all mesh nodes participate in creating anneighbors that are two hops away from the gateway, infrastructure for decentralized data transmission,bi-stability is eliminated such that the subsequent makes a WMN a stable and economically viablepenalties are not incurred. Finally, we validate the solution for providing universal Internet access. InCounter-Starvation Policy by re-deploying most WMN usage scenarios, the availableaanageable set of MirrorMesh nodes on-site bandwidth must be utilized in a fair manner by all(mirroring a subset of the wireless mesh nodes). The mesh nodes. Unfortunately, the carrier senseresults demonstrate that our solution completely multiple access with collision avoidance 244 | P a g e
  4. 4. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252(CSMA/CA) medium access control (MAC) three-hop scenarios where a node is at most two orprotocol in IEEE 802.11 can lead to unfairness in three hops from the GW. Specifically, we considermulti-hop data transmission in WMNs. In addition, the interference model where some adjacent nodesin the transport layer, the transmission control are within carrier sensing range but are not withinprotocol (TCP) does not account for fairness among transmission range of each other. This is a commonusers and sometimes may lead to starvation of some situation when a WMN is employed to providenodes depending upon the location of nodes and network connectivity in dense environments suchtheir distance from the GW. An undesirable as a university campus. The contributions of thiscondition is the flow starvation which can cause paper are as follows: 1) We propose a simplifiedsome flows to attain extremely low data transfer Markov chain model to analyze WMNs with linearrates compared to other flows. Starvation in wireless topology and numerically compute the degree ofnetworks has been observed in some earlier studies. unfairness between nodes. 2) We propose a fairThe poor performance of CSMA/CA-based MAC binary exponential backoff (FBEB) algorithm toprotocols has been considered through simulations reduce the extent of starvation. Our proposedand analytical modeling. It has been shown that algorithm uses the intuition that, to improve fairness,when senders observe different channel conditions mesh nodes that have successfully transmitted a datacompared to theirneighbors, some of them can packet should not be permitted to eagerly transmitencounter flow starvation. The existence of more data packets. By delaying the transmission ofunfairness in single hop networks, where any two successive packets we are able to reduce the degreecommunicating nodes are within transmission range of starvation. This effect is achieved by adapting theof each other, was reported in [4]. The work in [5] contention window for transmissions in the IEEEclassified various two-flow scenarios into twelve 802.11 protocol. 3) Our analytical results show thatclasses and investigated theshort-term and long-term solving the exposed terminal problem alone isperformance in each class. Similar studies for multi- insufficient for combating starvation in WMNs. 4)hop networks can be found in [6]–[8]. The work in Simulation results also show that our proposed[9] identified bi-stability as a cause for starvation in FBEBalgorithm can improve the data rate ofWMNs. In a linear topology, bi-stability occurs starving nodes by a factor of 7 in some cases.when nodes closer to the GW enter a phase of Fairness is achieved by limiting the aggressive usesuccessful packet transmissions that forces other of the channel; the overall loss in throughput acrossnodes to wait for an extended period of time. The all nodes was observed to be about 20%. The loss inwork in [9] is closely related to ourwork in this overall throughput is, however, acceptable if fairnesspaper. They analytically determined the existence of is a key parameter in WMNs. We also show throughstarvation in a multi-hop network with linear our simulations that our proposed FBEB algorithmtopology, which they called the basic topology. outperforms other schemes [9], [10] under the 2-hopAnalytical modeling was carried out for the two- carrier sense assumption. Our work presumes anode scenario but not for the three-node scenario. linear topology (or a chain) for mesh nodes. This isNo consideration was given to possible solutions to an elementary structure and understandingthe exposed terminal problem. Furthermore, they thebehavior of nodes arranged in such a topologyconsidered the interference model where nodes that provides insight into the behavior of larger networks,are two hops away are outside each other’s which could be studied as a collection of chains.transmission range and carrier sensing range. Also,another work which is closely related to our work is 2. Related WorkIdle Sense [10], which adjusts the contention 2.1 Wireless Networkswindow of the IEEE 802.11 MAC protocol to Multi-hop wireless ad hoc networks offerachieve fairness in wireless local area networks communications capability to mobile hosts without(WLANs). Idle Sense, however, is suitable for single requiring a fixed infrastructure. In such a network,hop WLANs only and does not consider the effect of packets can traverse multiple intermediate nodes endata flows with multiple hops.In [11] and [12], it has route from the source to the destination. An examplebeen argued that modifications to the transport layer of a multi-hop wireless network is shown in Figurecan provide fairness despite unfair MAC protocols. 2. This example shows four nodes (labeled A, B, C,Multi-channel protocols have also been considered D) in a simple chain" network topology. Many otherto mitigate starvation [13], as well as rate control topologies are possible, though we consider only thetechniques [14], multi-path solutions [15], and simple chain network in this paper. The topologyisoptimization-based approaches [16]. All these called multi-hop because (for example) a packetmethods are relevant but we believe that solutions at destined from A to D must use B and C asthe MAC layer are central to intermediate routers. Each forwarding step is calledperformanceenhancement and we suggest a hop. The overall performance achieved within aimprovements at the MAC layer to limit starvation. multi-hop wireless network depends on the MediumIn this paper, we establish the existence of, and Access Control (MAC) protocol and transportanalyze the extent of, starvation in two-hop and protocol used. For mobile nodes, performance also 245 | P a g e
  5. 5. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252depends on mobility patterns and the ad hoc routing network using end-to-end measurements. Router-protocol used. A popular MAC protocol for wireless based mechanisms, such as AQM, make changes tonetworks is the IEEE 802.11 MAC [2]. This protocol the routers so that they notify senders whenrequires each node to sense the channel before congestion is occurring but before packets aresending a frame. The protocol is called CSMA/CA dropped.(Carrier Sense Multiple Access with CollisionAvoidance). To address the hidden node problem, 2.2.1 End-to-End Approachesthe 802.11 MAC uses a Request-To-Send (RTS) and TCP/Vegas:Clear-To-Send (CTS) handshake. A node sends an Vegas have mainly showed that it canRTS control frame to indicate that it has a frame to achieve 40%-70% better throughput than Reno.send. Upon receiving the RTS, the intended receiver However, this throughput is achieved not byreturns a CTS control frames if it is okay to receive aggressive retransmissions, but of efficient use of thethe data frame. In this fashion, a packet traverses one available bandwidth. Vegas proposed threehop at a time toward the destination. important techniques to improves throughput and reduce losses. (1) New Retransmission Mechanism: Vegas estimate the RTT and variance using a fine- grained timer. Vegas read and record the system clock each time a segment is sent. This helps to estimate when to retransmit a lost packet. Also Vegas only decreases the congestion window if the retransmitted segment was previously sent after the last decrease. Whereas, in Reno it isFigure 2 –Multi-hop wireless Network possible to decrease the congestion windowTCP (Transport Control Protocol) more than once for losses that occurred during one RTT interval. TCP performance in multi-hop wireless (2) Congestion avoidance mechanism:Thesimplenetworks is poor [5, 12, 16]. TCP throughput often idea that Vegas exploits is thatnumber ofbytesdecreases dramatically with the number of hops in transit is directlyproportional to the expectedtraversed by a flow, regardless of the MAC protocol throughput, and therefore, as the window sizeused [5, 12]. The primary reason is link-layer packet increases the bytes in transit increases resultinglosses caused by contention between data packets in increased throughput of the connection. Thetraveling in the same direction, and collisions goal of Vegas is to maintain the ―right‖ amountbetween data packets and TCP ACK packets of extra data in the network. Vegas congestiontraveling in opposite directions. Several methods avoidance actions are based on changes in thehave been proposed to remedy these problems. Fu et estimated amount of extra data in the network,al. [5] propose a link-layer version of RED [4] to and not only on the dropped packets.signal the TCP sender about impending congestion, Expectedthroughput =and an adaptive pacing algorithm to distribute TCP windowSize/BaseRTTBaseRTT is set to thedata packets evenly across a multi-hop chain. minimum of all measured round trip times.Combined, these algorithms improve throughput by WindowSize is the size of the current congestion5%-30%. As another example, Cordeiro et al. [3] windowModified Slow-Start Mechanism: Vegaspropose disjoint routes for forward TCP packets and uses rate control during slow-start, using a ratebackward TCP ACKs so that contention is reduced. defined by thecurrent window size and theThey report throughput improvements of 90%. BaseRTT.Several multi-channel MAC protocols have beenproposed to improve the overall ad hoc network 2.2.2 Router-based approachescapacity [6, 7, 11, 13, 15]. We do not consider multi- Random Early Detection: Random Earlychannel Detection (RED) is an AQM mechanism that seeksapproaches in this paper, but we have studied them to reduce the long-term average queue length inin prior work [9] routers. In RED, as each packet arrives, routers compute a weighted average queue length that is2.2 TCP Protocols used to determine when to notify end systems of There have been two main approaches to incipient congestion. ―Marking‖ a packet performsdetecting congestion before router buffers overflow: congestion notification in RED.end-to-end methods and router-based mechanisms.End-to-end methods are focused on making changes While TCP/Vegas also use delay-basedto TCP Reno. Most of these approaches try to detect congestion control in an effort to increase TCPand react to congestion earlier than TCP Reno (i.e., throughput due to reduced number of packet lossesbefore packet loss occurs) by monitoring the and timeouts, and a reduced level of congestion over 246 | P a g e
  6. 6. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252the path. In contrast, TCP-LP uses one-way delay Another disadvantage of this approach is that themeasurements vs. round-trip delays. Moreover, the end-to-end semantics of TCP acknowledgments iskey difference between TCP-LP and RTT-based 5 violated, since acknowledgments to packets can nowcongestion control protocols is in their primary reach the source even before the packets actuallyobjective. While the former aim to achieve fair-share reach the mobile host. Also, since this protocolrate allocations, TCP-LP aims to utilize only excess maintains a significant amount of state at the basebandwidth. station per TCP connection, handoff procedures tend to be complicated and slow.• Link-layer protocols: There have been severalproposals for reliable link-layer protocols. The two • The Snoop Protocol: The snoop protocolmain classes of techniques employed by these introduces a module, called the snoop agent, at theprotocols are: error correction (using techniques base station. The agent monitors every packet thatsuch as forward error correction (FEC)), and passes through the TCP connection in bothretransmission of lost packets in response to directions and maintains a cache ofTCP segmentsautomatic repeat request (ARQ) messages. The link- sent across the link that have not yet beenlayer protocols for the digital cellular systems in the acknowledged by the receiver. A packet loss isU.S. — both CDMA [10] and TDMA [17] — detectedby the arrival of a small number of duplicateprimarily use ARQ techniques. While the TDMA acknowledgments from the receiver or by a localprotocol guarantees reliable, in-order delivery of timeout. Thesnoop agent retransmits the lost packetlink-layer frames, the CDMA protocol only makes a if it has it cached and suppresses the duplicatelimited attempt and leaves it to the (reliable) acknowledgments. In ourclassification of thetransport layer to recover from errors in the worst protocols, the snoop protocol is a link-layer protocolcase. The AIRMAIL protocol [1] employs a that takes advantage of the knowledgeof the higher-combination of FEC and ARQ techniques for loss layer transport protocol (TCP). The main advantagerecovery. The main advantage of employing a link- of this approach is that it suppresses duplicatelayer protocol for loss recovery is that it fits acknowledgments for TCP segments lost andnaturally into the layered structure of network retransmitted locally, thereby avoiding unnecessaryprotocols. The link-layer protocol operates fast retransmissions and congestion controlindependently of higher-layer protocols (which invocations by the sender. The per-connection statemakes it applicable to a wide range of scenarios), maintained by thesnoop agent at the base station isand consequently, does not maintain any per- soft, and is not essential for correctness. Like otherconnection state. The main concern about link-layer link-layer solutions, thesnoop approach could alsoprotocols is the possibility of adverse effect on suffer from not being able to completely shield thecertain transport-layer protocols such as TCP. We sender from wireless losses.investigate this in detail in our experiments. • Selective Acknowledgments: Since standard TCP• Indirect-TCP (I-TCP) protocol: This was one of uses a cumulative acknowledgment scheme, it oftenthe early protocols to use the split-connection does notprovide the sender with sufficientapproach.It involves splitting each TCP connection information to recover quickly from multiple packetbetween a sender and receiver into two separate losses within a single z transmission window.connections at thebase station — one TCP Several studies [6] have shown that TCP enhancedconnection between the sender and the base station, with selective acknowledgmentsperforms better thanand the other between the base stationand the standard TCP in such situations. SACKs were addedreceiver. In our classification of protocols, ITCP is a as an option to TCP by RFC 1072 [9]. However,split-connection solution that uses regular TCPfor its disagreements over the use of SACKs prevented theconnection over wireless link. I-TCP, like other specification from being adopted, and the SACKsplit-connection proposals, attempts to separate loss option was removed from later TCP RFCs. Recently,recovery over the wireless link from that across the there has been renewed interest in adding SACKs towireline network, thereby shielding the original TCP TCP. Two of the more interesting proposals are thesender from the wireless link. However, as our TCP SACKs Internet Draft [14] and the SMARTexperiments indicate the choice of TCP over the scheme [12].The Internet Draft proposes that eachwireless link results in several performance acknowledgment contain information about up toproblems. Since TCP is not well-tuned for the lossy three non-contiguous blocks of data that have beenlink, the TCP sender of the wireless connection often received successfully. Each block of data istimes out, causing the original sender to stall. In described by its starting and ending sequenceaddition, every packet incurs the overhead of going number. Due to the limited number of blocks.through TCP protocol processing twice at the basestation (as compared to zero times for a non-split- 3. Analytical Modeling of Flow Starvationconnection approach), although extra copies are In this, we present an analytical model toavoided by an efficient kernel implementation. study the effect of flow starvation among nodes in 247 | P a g e
  7. 7. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252WMNs with a linear topology.We first define the 3.2 States and Transition Probabilitiesstate space and then describe how to determine the We model the state of each node using astate transition probabilities. tuple that consists of the number of packets in each queue: a separate queue is maintained for data3.1 System Model packets from a particular flow and a separate queue We consider a linear topology such that is utilized for ACKs corresponding to a particulareach node is within the transmission range of its flow. Furthermore, the state of each node alsoneighboring nodes (see Fig. 3). For nodes (or includes the size of its contention window. Let N =wireless mesh routers) that are two hops away from {1, 2, . . . ,|N|} denote the set of nodes (i.e., wirelesseach other, they are within the carrier sensing range mesh routers and GW). In the linear topology, nodebut not within each other’s transmission range. We |N| is GW, node n ∈ {1, 2, . . . ,|N| − 1} is theconsider the Transmission Control Protocol at the (|N| − n) 𝑡ℎ hop downstream neighbor of GW. Thus,transport layer. Nodes that send data packets receive node 1 is the farthestnode from GW in terms of theTCPacknowledgments (ACKs). The number of data number of hops. Let vectorc = (CW1,CW2, . . .packets sent by a node before receiving an ACK is ,CW|N|), where CWn is the contentionwindow sizeless than or equal to the TCP congestion window of node n ∈ N. Let F denote the set of flows.A flowparameter, cgw, which we assume to be fixed for f ∈ F corresponds to a source and destinationease of analysis. We assume that nodes are always pair.For n ∈ N, we let vectorbacklogged. A source node can attempt to send data 𝑞 𝑛𝑑 =( 𝑄 𝑛𝑑,1 , 𝑄 𝑛,2 , … . , 𝑄 𝑛𝑑,|𝑓| ), where 𝑄 𝑛,𝑓 denotes the 𝑑 𝑑packets as long as the number of unacknowledged number of data packets stored innode n for flow f ∈packets is less than cgw. F. Similarly, for n ∈ N, we letvector 𝑎 𝑎 𝑞 𝑛𝑎 =( 𝑄 𝑛,1 , 𝑄 𝑛,2 , … . , 𝑄 𝑛,|𝑓 | ), where 𝑄 𝑛,𝑓 denotesthe 𝑎 𝑎 number of ACK packets stored in node n for flow f ∈ F.The system state is an aggregate state that is composed ofthe states of individual nodes. The system state is defined as s = (c,𝑞 𝑛𝑑 ,𝑞 𝑛𝑎 ,∀ n ∈ N).(1) The term "TCP starvation" describes the phenomena of the output queue being filled with larger volumes of UDP, causing TCP connections to have packets tail dropped. Tail drop does not distinguish between packets in any way, including whether they are TCP or UDP, or whether the flowFigure 3. Upstream data transmission in a linear uses a lot of bandwidth or just a little bandwidth.topology with three wireless mesh routers {1, 2,3} TCP connections can be starved for bandwidthand a gateway (GW) {4}. because the UDP flows behave poorly in terms of congestion control. Flow-Based WRED (FRED), Any intermediate node may either have which is also based on RED, specifically addressesdata packets to send in the upstream direction (from the issues related to TCP starvation.node to GW) or ACKs to send in the downstreamdirection (from GW to the node). In the data link 4. Implementation and simulationlayer, each node listens to the channel before 4.1 Implementationattempting a packet transmission. If the channel is Implementing NS2 within the Adaptivebusy, the node continuously listens to the channel Transport Protocol requires some effort, even withuntil it senses that the channel is idle. The node then the aid of a reliable protocol such as TCP.randomly selects a slot among the next CW slots Bandwidth estimation, managing timers, and(where CW is the contention window size) and providing message-oriented, rather than stream-transmits a packet in that slot if the channel is idle. oriented semantics, must be provided on top of thePacket collision can happen when two neighboring underlying kernel protocol. In addition, TCP’snodes transmit a packet in the same slot. When congestion-control scheme represents a potentialpacket collision occurs, the corresponding nodes obstacle to our mechanisms for flow control anddouble their contention window and start the sending assigning priorities to messages. ATP, the completeprocess again. If the contention window reaches its implementation of NS2 which we describemaximum value, it is reset and the packet to be sent here, runs at user-level over TCP or UDP, andis being dropped. consists of C++ code. 248 | P a g e
  8. 8. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252 A. Reliable transport subsystem ATP is a reliable protocol, so it must run on top of a reliabledatagram protocol or a reliable stream protocol. At first glance, TCP would seem to be perfectly adequate, since it hasbeen highly optimised to perform well both in local-area networks and wide-area networks. TCP has also been adapted to cope with the peculiarities of wireless networks, such as high error rates and packet losses [8], [9]. However, implementing ATP on TCP requires considering anumber of alternatives. Transmitting an entire message at once using TCP may result in the message being buffered in the kernel (if there is sufficient buffer space), preventing an application from deferring the send operation or aborting it. Once TCP has copied data into the kernel, it is not easy to determine how much of it has been sent. A final difficulty lies in deciding whether to use multiple streams to connect thefigure 2(a) sender to the receive r, and, if so, how to allocate messages to streams. Using a single TCP stream will result in all message transmissions being serialised, so that a high-priority message may have to wait for a low-priority message. Using multiple TCP streams may result in unpredictable competition for bandwidth, since TCP is a greedy protocol and most common implementations of TCP do not coordinate congestion control between streams [10]. In our initial version of ATP, we chose to allow message transmission over either TCP, or a reliable datagram protocol on top of UDP, which we will refer to as SPP (Sequenced Packet Protocol). The TCP implementation is the simpler of the two, and runs over a single TCP connection, but sends messages in fixed size segments (1 MTU), rather than sending an entire message at a time. Padding is required for small messages to enable the receiver to figure 2(b) detect segment boundaries. It has the natural advantage of being TCP-friendly. The SPP implementation performs its own buffering, retransmissions and duplicates suppression at user level. Since it uses UDP datagrams, it requires no padding to distinguish message boundaries, and is therefore more efficient for transmitting small messages. Unlike TCP, SPP is not optimised for WAN use, and has not been thoroughly tested to determine its fairness or behavior under high error rates. It is robust to the errors we have seen in our 802.11b network, and to packet drops caused by queue overflows in the sender’s networkstack. The ATP experiments in this paper use the versionrunning over SPP. For the purposes of comparison, we have also implemented a version of NAI using multiple TCP connections, one per priority level, which we refer to as MTCP (multi- stream TCP, described in more detail in Section V). Figure 2(c) We conducted some experiments to assess the behavior of concurrent TCP streams and determine if this design was suitable for an NAI 249 | P a g e
  9. 9. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252implementation. The graphs in Figure 2 illustrate estimate the total bandwidth available on it, rathersome cases of competition between concurrent TCP than deriving separate estimatesfor each connectionstreams which result in an unequal division of or destination host. Our current estimatorassumesbandwidth. We conducted experiments to measure that traffic from protocols other than ATPhow three concurrent TCP flows compete for constitutes a negligible fraction of the total traffic,bandwidth over a link with a capacity of 256 KB/s. but this restriction would be removed in a kernelEach flow sends 32 KB messages in a loop for 120 version of ATP. A further important difference fromseconds. One of the three senders was delayed for a wide-area bandwidth estimation is a side-effect ofconstant amount of time between the completion of ATP semantics. Since ATP incorporatespriorities forone send call and the start of the next, to investigate messages, an inaccurate estimate can cause aprioritythe effect of TCP’s slow-start restart [11] for idle inversion. The difficulty is that the device driverconnections. The expected outcome of these tests may buffer datagrams when it is unable to transmitwould be for the two undelayed senders to receive a as fast as the incoming rate. If an over-optimisticroughly equal share of bandwidth,and the delayed bandwidth estimate causes the kernel to buffersender to receive a smaller share, decreased datagrams from low-priority messages, these willinproportion to the size of the delay. However, as hold up the transmission of datagrams from high-Figures 2(a)and 2(b) show, the shares of bandwidth priority messages until the send queue is free of low-are not constant, andat some points the delayed priority datagrams. It is impractical to removesender outperforms the undelayedsenders! In datagrams from the send queue, but some operatingaddition to looking at sequence number plots, systems allow the length of the queue to be read bywealsocalculated the times taken for individual send applications (for instance, by FreeBSD’s ifmiboperations bythe delayed sender. Figure 2(b) shows feature). The ATP bandwidth estimator incorporatesa sequence number plot a heuristic to ―back off‖ and reduce its estimate where the delayed sender waits for 150 ms when it detects a backlog in the send queue.between send operations;measuring the timebetween completion of successivesends gives an The bandwidth estimation algorithm: Aaverage of 0.42 seconds, but a range from 0.22to straightforwardscheme for bandwidth estimation is1.09, almost a five-fold variation (a minimum of to count how many send operations, and of what0.22 is possible because the send can buffer data in sizes, complete over an interval, and divide to findthe kernel and return fast, overlapping with the the estimate. This can be inaccurate because thedelay). This compares to an expected mean of 0.375 kernel buffers data, both at the protocol level (in theseconds if all the streams received equal bandwidth. case of TCP – UDP does no buffering), and at theFigure 2(c) shows the region of this test that the send network device driver. Additionally, the timeoperation taking 1.09 seconds lies in, with vertical required to derive an estimate depends on thebars indicating the completion times of sends. This amount of data being sent. Blocking on a sendtype of unpredictable and uncontrollable contention operation for a large message will delay the estimateeffect argues that NAI over multiple TCP streams until the send completesmay ultimately be inferior to other options, intcurbw; int staleness = 0;particularly in settings where small and infrequent intpolldelay; // configurable, >= 0high-priority messages are intermixed with lower- bandwidth_estimate(used, backlog) {priority bulk data transfers. used = maximum(used, filter(used)); if (backlog > MAXIMUM_BACKLOG) {B. Bandwidth estimator curbw = used; staleness = 0; Estimating bandwidth is a necessary return (used, used - backlog*MTU);component of an adaptive transport protocol, since }both the application and the protocol itself rely on else if (used <curbw) {this value in order to adapt appropriately to network int probe = PROBE_SIZE;changes. ATP requires a bandwidth estimate to fully staleness++;implement callback timers, or else a message can probe *= staleness / polldelay;never be reported as undeliverable before its curbw = used;callback timer expires. Much work has been devoted return (curbw, curbw + probe);to the general problem of estimating bandwidth for }flows in a wide-area network. Wide-area bandwidth else {estimation schemes must arrive at an estimate of staleness = 0;limiting bandwidth, which lies at some link along return (curbw, curbw + PROBE_SIZE);the path between a sender and receiver. In contrast, }we assume that the rate of communication is }principally limited by the bandwidth on the wireless Figure 3: The bandwidth estimation algorithm.link. Since all communication between the mobile Every second, ATP invokes the bandwidth estimatorhost and remote hosts must be over this link, we can to determine a new estimate (this is a tuple of the 250 | P a g e
  10. 10. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252internal estimate and the estimate advertised to the instance, an application using ATPmay use theapplication) The bandwidth used over the preceding bandwidth estimate to determine its mode ofsecond is averaged with the values for the four operation,and not change into a mode using morepreceding seconds, then the new estimate is bandwidth unlessthe estimate goes up. In this casegenerated, depending on the backlog and staleness the surplus bandwidthwould never be exploited.of the current estimate .MAXIMUM BACKLOG is ATP breaks this deadlock in twoways. Firstly, in theset high enough to avoid transient backlogs (10 in trivial case where the application is unwillingour implementation), and PROBE SIZE is half the to send any data because it believes the bandwidthminimum of the send queue capacity and the current iszero, ATP polls the remote host to determine whenestimate. bandwidthrises above zero, making a simple packet- pair estimate [12].Secondly, ATP probes the Alternatively, the bandwidth used by a TCP bandwidth by speculatively increasingthe estimate itconnection can be derived from TCP’s round-trip gives the application. The intent behindtime estimate and the send window size. However, thismechanism is that the application will eventuallythis value reflects how much data has been sent on decide the estimateis high enough and attempt tothe connection, and not the potential capacity of the exploit it by sending moredata. Figure 3 shows theconnection. ATP derives its estimate by polling the probe mechanism as part of the bandwidthestimatenetwork card for the amount of data sent and routine.received over the course of each second. Thisquantity is then used as a predictor of the bandwidth References:over the next second. Since the bandwidth reported [1] A. Aggarwal, S. Savage, and T. Anderson,by the network card depends on the amount of data ― Understanding the Performance of TCPwhich ATP tries to send, this simple estimate is Pacing ", Proceedings of IEEE INFOCOM,inaccurate if the true bandwidth is higher than the Tel Aviv, Israel,March 2000.send rate. Accordingly, the bandwidth estimator uses [2] ANSI/IEEE Standard 802.11b, ―Part 11:a probing scheme to speculatively increase the Wireless LAN Medium Access Controlestimate. Estimation relies on three statistics: the (MAC) and PhysicalLayer (PHY)observed bandwidth, the length of the network Speci_cations: Higher-Speed Physicalinterface send queue (backlog), and the staleness of Layer Extension in the 2.4 GHz band",its current estimate. Staleness measures the number 1999.of seconds since the last point at which the estimate [3] C. Cordeiro, S. Das, and D. Agrawal,changed, or since there was a ―genuine decrease‖ in ―COPAS:Dynamic Contention-Balancingavailable bandwidth. A genuine decrease can be to Enhance the Performance of TCP overdistinguished from a decrease in the count of bytes Multi-hop Wireless Networks",transmitted by detecting that the length of the send Proceedings of IC3N02, Miami, FL,queue has increased (in fact, we check that it October 2002.exceeds a threshold of 10 packets; the maximum [4] S. Floyd and V. Jacobson, Random Earlylength allowed is 50 in FreeBSD). Figure 3 shows Detection Gateways for Congestionpseudocode for the bandwidth estimation algorithm. Avoidance", IEEE Transactions onIt runs once every second, and computes two values Networking, Vol. 1, No. 4,pp. 397-413,based on the statistics obtained over the previous August 1993.second curbw and available. The available value is [5] Z. Fu, P. Zerfos, H. Luo, S. Lu, L. Zhang,the estimate of available bandwidth on the wireless and M. Gerla, ―The Impact of Multi-hoplink, which is suppliedto the application. The curbw WirelessChannel on TCP Throughput andvalue is the amount of bandwidth which ATP should Loss", Proceedings of IEEE INFOCOM03,restrict itself to using over the next second.This is an San Francisco, CA,April 2003.internal ATP statistic, which may be lower [6] W. Hung, K. Law, and A. Leon-Garcia, ―Athanavailable if the network interface send queue is Dynamic Multi-Channel MAC for Ad Hocnonemptyand there is a consequent risk of priority LAN",Proceedings of 21st Biennialinversion. The available value may also be higher if Symposium on Communications, Kingston,the estimator is probing thebandwidth. An averaging ON, Canada, June 2002.filter with a window size of 5 is usedto smooth the [7] N. Jain, S. Das, and A. Nasipuri, ―Aestimates in order to make them less sensitive MultichannelCSMA MAC Protocol withtotransient spikes.If the application has queued more Receiver-Based Chan- nel Selection formessages than can be sentin a single second, then the Multi-hop Wireless Networks",Proceedingsestimator will automatically detectincreases in of the IEEE ICCCN01, Phoenix, AZ,available bandwidth, since the per-second usage October 2001.willincrease as the bandwidth increases. However, ifthe network isunderutilised, then an increase may [8] J. Ke, ―Towards a Rate-Based TCPnot be detected without anadditional mechanism. For Protocol forthe Web", Proceedings of 251 | P a g e
  11. 11. Venkatesh.Donepudi, vahiduddinshariff / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 3, Issue 2, March -April 2013, pp.242-252 MASCOTS2000, San Francisco, CA, pp. 36-45, October 2000. [9] T. Kuang and C. Williamson, ―A Bidirectional Multi-Channel MAC Protocol for Improving TCP Performance on Multi- Hop Wireless Ad Hoc Networks", submitted for publication, 2004. [10] T. Kuang, F. Xiao, and C. Williamson, ―Diagnosing Wireless TCP Performance vahiduddinsheriff Completed his Problems: A Case Study", Proceedings M.Tech in CSE in VignanUnivrsity. Working as of SCS SPECTS Conference, Montreal, Asst.Professor in CSE Department ,CR Reddy PQ, pp. 176-185, July 2003. college of Engineering and Technology,Eluru. [11] J. So and N. Vaidya, ―A Multi-channel Having 2 years of Teaching Experience. MAC Protocol for Ad Hoc Wireless Networks", TechnicalReport, Dept. of Computer Science, University of Illinois at Urbana-Champaign, January 2001. [12] K. Tan and M. Gerla, ―Fair Sharing of MAC under TCP in Wireless Ad Hoc Networks", Proceedingsof IEEE MMT99, Venice, Italy, October 1999. [13] A. Tzamaloukas and J. Garcia-Luna- Aceves, ―A Receiver-Initiated Collision- Avoidance Protocol for Multi-Channel Networks", Proceedings of IEEE INFOCOM01, Anchorage, USA, April 2001. [14] VINT Group, ―Network Simulator ns-2", available at http://www.isi.edu/nsnam/ns. [15] S. Wu, Y. Tseng, C. Liu, and J. Sheu, ―A Multi Channel MAC Protocol with Power Control for Multi-Hop Mobile Ad Hoc Networks", The Computer Journal, Vol. 45, No. 1, pp. 101-110, 2002. [16] S. Xu and T. Saddawi, ―Does the IEEE 802.11 MAC Protocol Work Well in Multi- hop Wireless Ad Hoc Networks?", IEEE Communications Magazine, June 2001.Authors: Venkatesh.Donepudicompleted hisM.Tech in IT in VignanUniversity.He is workingas Asst.Professor in CSE Department,Jagans Collegeof Engineering and Technology, choutapalem,Nellore. Having 1.5 Years of Teaching Experience. 252 | P a g e