End-to-end Model for a Flow in the Internet Muthaiah Venkatachalam

格式：pdf
大小：439.82 KB
文档页数：16

End-to-end Model for a Flow in the InternetMuthaiah Venkatachalam Jasleen Kaur Harrick M.VinTechnical Report TR-01-32Department of Computer SciencesUniversity of Texas at AustinTAY2.124,Austin,TX78712-1188,USAAug1,2001Abstract—We develop an end-to-end model for packet inter-arrival times ofﬂows in the Internet.Our model illustrates that under asymptotic conditions of high network utilization,(1)packet inter-arrival times ofﬂows become heavy-tailed after they traverse a large number of hops,and(2)the aggregate of suchﬂows is a long-range dependent self-similar process.We validate our model through extensive simulations and derive the region of applicability—with respect to non-asymptotic settings of network utilization,heterogeneity in trafﬁc composition,and number of hops that aﬂow traverses—of our model.Weﬁnd that forﬂows that contribute signiﬁcant amount(10%)to the cumulative bit rate of the aggregate,the observations yielded by the model hold at relatively low levels of network utilization(30-40%),small number of network hops(4-6),and moderate levels of heterogeneity in packet sizes and bandwidth requirements ofﬂows.1IntroductionSimplicity of implementation is central to the design of scalable,high-performance networks.Hence,high-performance routers have traditionally used First-In-First-Out(FIFO)packet scheduling algorithm.FIFO scheduling,unfor-tunately,does not provide isolation acrossﬂows;bursty trafﬁc arrival from aﬂow can affect signiﬁcantly the performance—delay,jitter,and loss—observed by otherﬂows.A number of analytical models have been proposed for characterizing delay and jitter performance ofﬂows in FIFO networks[5,17,16,20,23,22,25].However,primarily due to the complexity of the analysis involved,most of these studies do not provide closed-form characterization of end-to-end delay and jitter properties(see Section9 for a detailed discussion on related work).The aim of this paper is to provide a closed-form end-to-end analysis forﬂows in FIFO networks.For the remainder of this paper,we refer to aﬂow as a trafﬁc stream,all packets of which traverse the same network path from the source to the destination.A wide variety of trafﬁc streams—such as label-switched paths(LSP)in Multi-protocol Label Switching(MPLS)networks,Virtual Private Network(VPN)tunnels,and Virtual Circuits(VC)in ATM networks—ﬁt this deﬁnition of aﬂow.Even in the current Internet,TCP,UDP and other best-effort trafﬁc streams can be approximated asﬂows as long as route changes are not frequent.We model the inter-arrival time between packets at the destination for individualﬂows under asymptotic condi-tions of network utilization and number of hops.Our model yields the following key insights.The variance in inter-arrival times for individualﬂows tends to inﬁnity in the asymptotic case,indicating that ﬂows become heavy-tailed after traversing a network of FIFO routers.The aggregation ofﬂows becomes a long-range dependent self-similar process.A particularly interesting and deceptively simple type ofﬂow is one that is shaped to a constant bit-rate(CBR)—with constant packet inter-arrival times and packet sizes—at the source.It is widely believed that(1)shapingﬂows to CBR at the source and(2)ensuring that the bandwidth available at each node to the CBR aggregate is at least as large as the cumulative rate requirement of the CBRﬂows results in a satisfactory end-to-end delay and jitter.The above philosophy,in fact,is the basis of the Virtual Leased Line service model proposed in the context of Differentiated Services(DiffServ)networks[13,19].We demonstrate that,at high network utilization,even CBRﬂows become heavy tailed after traversing a few hops.Hence,shapingﬂows to CBR beneﬁts only in networks that operate at low levels of utilization.We further show that at low network utilization,shaping sources to constant packet sizes is sufﬁcient to reduce the asymptotic burstiness;shaping sources to constant inter-arrival times is not necessary.We validate our model through extensive simulations and derive the region of applicability—with respect to non-asymptotic network and trafﬁc parameters—of the model.Weﬁnd that although the model predicts the behavior of ﬂows in the asymptotic case,forﬂows that contribute signiﬁcant amount(10%)to the cumulative bit rate of the aggregate,the observations yielded by the model hold at relatively low levels of network utilization(30-40%),small number of network hops(4-6),and moderate levels of heterogeneity in the packet sizes and bandwidth requirements of theﬂows.We argue that,in many instances,aﬂow—such as one that represents a VPN tunnel between two sites of an enterprise—does occupy signiﬁcant fraction(10-20%)of the bit rate allocated for VPN services at routers. Our simulation results predict that suchﬂows will become heavy-tailed at relatively low levels of network utilization and after traversing a small number of network hops;further,all of the micro-ﬂows that get multiplexed onto such tunnels will themselves become heavy-tailed.Finally,we point out several important implications of carrying heavy-tailedﬂows and long-range dependent self-similar aggregates in a network.We argue that a client receiving a heavy-tailed multimediaﬂow either perceives poor performance or requires large amount of buffers to smooth out the heavy tails.Long-range dependence of trafﬁc aggregates,on the other hand,has several implications on network queuing delays and packet losses.We conclude that operating a network so as to maintain its trafﬁc outside the heavy-tailed and long-range dependence domain is highly desirable.The rest of the paper is organized as follows.We deﬁne related concepts in Section2.In Section3,we formulate the problem of end-to-end analysis of packet inter-arrival times ofﬂows.In Sections4and5,we present the models for individualﬂows andﬂow aggregates,respectively.We discuss the beneﬁts,if any,of shaping sources to CBR in Section6.In Section7,we describe our simulation setup and present simulation results.We discuss the impact of heavy-tailedness and long-range dependence on the design of networks in Section8.We discuss the related work in Section9,and summarize our conclusions in Section10.2BackgroundIn this section,weﬁrst summarize the concepts of self-similarity,long-range dependence and heavy-tailedﬂow and then discuss the tests used to detect the presence of these properties in a time series.2.1ConceptsSelf-similarity:Consider a time series.is self-similar iff,That is,is identically distributed as with a scaling factor,where and is known as the Hurst parameter.Let deﬁne increments on Y(t):.If has stationary increments,then it can be shown that,where.When,it can beshown that the variance of dies out at a sub-linear rate with increase in:Self-similar processes are often long-range dependent–we deﬁne this concept next.Long-range Dependence:Long-range dependence of a process refers to the property that its auto-covariance function does not sum up to aﬁnite quantity.LetThen is long-range dependent iff:The most commonly encountered form of such long-range dependent in communication networks is hy-perbolic and is given by:where and is a constant.This form of corresponds to that of a second-order self-similar process[26].Heavy-tailed Flows:Aﬂow is said to be heavy-tailed iff(1)where the random variable denotes the packet inter-arrival time of theﬂow and is a slowly varying function as.Slow variation can be stated as:A source is said to be heavy-tailed with parameter if stabilizes to after some value of....R 1R 2X X X 12R HFigure 1:Network Model3.Variance-time test:This test determines if a processis self-similar and long-range dependent.The logarithm of the variance of the aggregated (averaged)process is plotted versus the logarithm of theaggregation level,.A least-squares line is ﬁtted on the data,and the slope ()provides an estimate of .If the slope is greater than ,then ,indicating that the data is self-similar and long-range dependent.3Problem FormulationConsider a network that carries ﬂows between source-destination pairs.In this paper,a ﬂow is considered to be any trafﬁc stream,all packets of which traverse the same path,from the source to the destination,through the network.We assume all ﬂows in the network are independent of each other.This is a reasonable assumption in large-scalenetworks with diverse routing.Letdenote the routers along a path of length through this network.We consider networks where each routeremploys FIFO scheduling to arbitrate access to the link bandwidth.Very large router buffers are assumed which guarantee no loss of packets.Let be the utilization of the output linkof router.Divide the time-axis into very small slots.Consider a ﬂow (referred to as tagged ﬂow )that traverses the above path.Let the random variable represent the inter-packet separation (IPS)of this ﬂow as its packets leave router(see Figure 1).Let represent the inter-packet separation of the ﬂow at the source.We assume that the mean and the variance for the inter-packet separation of the tagged ﬂow at the source are ﬁnite.Let the random variable denote the time it takes to service the cross-trafﬁc that enters router during the time-slot after the arrival of a packet of ﬂow .Then the following relationship exists between ,and :(2)where,represents the index of the last time-slot after the arrival of a tagged packet but prior to the arrival of the next tagged packet,at which the queue becomes empty.We assume that is independent of –which is a reasonable assumption in a network with diverse routing and a large number of input links carrying independent cross-trafﬁc.Note that.For simplicity of analysis we assume that the cross-trafﬁc characteristics are uniform over all routers.Henceforth,for simplicity of exposition,we shall therefore denote as and as .In the steadystate,let.Then .Let .Let denote the auto-covariance in the cross-trafﬁc that arrives at time instances separated by time-slots,that isNotice that with the assumed independence of ﬂows,the auto-covariance of the cross-trafﬁc at a particular lag is approximately the average of the auto-covariances at the same lag of the various cross-trafﬁc ﬂows present.Further,for most ﬂows,the auto-covariance is a decreasing function of .Hence,if the cross-trafﬁc aggregates a large number of ﬂows,then it is safe to assume that is a strictly monotonically decreasing function of .The following lemma states how this assumption leads to a property of that we use in our analysis.Lemma1If is a strictly monotonically decreasing function,then is a sub-linear function in. Proof:cannot be super-linear due to the monotonically decreasing property.Suppose it is linear,that is,let for some.Differencing both sides w.r.t.,we obtain:,whichviolates the strictly monotonically decreasing property.The third term in(6)simpliﬁes to:(9) From Equations(6),(7),(8)and(9),we have:(10) From Equations(10)and(4),we have:(11) For the limiting distribution,we then haveThus,the variance of the taggedﬂow grows in an unbounded manner as it traverses a large number of hops in the network with utilization arbitrarily close to one.From Equations(3)and(5),we getThus,we see thatThus,the limiting distribution of the inter-packet separation of the taggedﬂow has aﬁnite mean and inﬁnite variance under asymptotic conditions.Lemma2shows that when the marginal distribution of such a random variable is well deﬁned,it is heavy-tailed.Lemma2If and then,where and is slowly varying.Proof:Considerwhere,is the probability density function for.Observe that,for,the inter-packet separation represented in time slots:(13) Since and,we have(14) From(13)and(14),we can conclude:(15) In[4](p.331,Th.8.1.2)put and use the converse implication.i.e,let,where,is slowly varying.From(16),,and,it can be shown that we must have .Thus,(17)5Aggregation of FlowsIn[18,27]it has been shown that the superposition of heavy-tailed on-offﬂows,with strictly alternating off-and on-periods,converges to Fractional Brownian Motion(FBM)as the number ofﬂows grows large.Our model shows that individualﬂows become heavy-tailed under asymptotic conditions.The cross-trafﬁc,which is a superposition of suchﬂows,therefore converges to FBM.FBM is a self-similar process with Hurst parameter From(18)and(12)we have:(19) For a CBRﬂow,and.Therefore,for CBRﬂows,we derive the following observations from (19):When is large,theﬁrst term in(19)dominates the right hand side.affects the asymptotic inter-arrival time only if is small.Hence,shaping sources to CBR is beneﬁcial only in networks that operate at low levels of utilization.does not play a role in the asymptotic burstiness as long as it isﬁnite,but does.It follows that at low network utilization,shaping sources to constant packet sizes is sufﬁcient to reduce the asymptotic burstiness;shaping sources to constant bit rate is not necessary.Cross traffic entering the tagged pathCross traffic leaving the tagged path ...(a)Network Topology:(b)Router ArchitectureFigure2:Simulation environment:network topology and router architecture7Experimental Validation and AnalysisWe conduct a simulation study of networks with trafﬁc inputs,with two objectives:(1)to validate under asymp-totic conditions,the model developed in Sections4and5;and(2)to identify the set of non-asymptotic network settings(for various trafﬁc compositions)where the predicted heavy-tailedness of individualﬂow becomes visible. To conduct these experiments we have developed a network simulator using CSIM[2].7.1Simulation Environment7.1.1Network TopologyFor our experiments,we consider a linear,multi-hop network topology(see Figure2(a)).This network model is fairly general and has been used in literature[11,15,16,28].Let denote a linear,multi-hop network topology with routers,and let()denote the router in the topology.Given such a topology,we are interested in the end-to-end performance of the taggedﬂow—aﬂow that enters the network topology at router and traverses the multi-hop network topology.While traversing the network,the taggedﬂow interacts with aggregates of ﬂows(referred to as the cross trafﬁc)that enter and depart the network at each router along the path.We model each router in this network as having input ports()and output ports().The network topology consists of routers such that,for all(),the output port of routeris connected to the input port of router.The taggedﬂow enters the network through port of router. Through each port of router,aggregates enter the network,and of each of these aggregates are routed to output port(see Figure2(b)).In addition,the taggedﬂow entering input port of is routed to its output port.Thus,for each router,the trafﬁc routed to the output port consists of:(1)The tagged trafﬁc (entering the router from port);and(2)of theﬂows entering from input ports.All of the remaining trafﬁc entering through the input ports is routed to output ports,.The above topology ensures that:(1)the taggedﬂow that enters the network at router is routed all the way through the multi-hop network,and(2)the cross trafﬁc entering the network at router()interferes with the transmission of the tagged trafﬁc for a single hop,and leaves the network at router.This topology facilitates experimentation with different compositions of the cross trafﬁc and different network depths.We have conducted experiments for ranging from to.We present results for experiments with;these results also hold for higher values of.7.1.2Trafﬁc SourcesWe consider two kinds of trafﬁcﬂows.CBRﬂows:CBR is one of the more popular trafﬁc models.Flows with CBR transmissions have been used commonly while transporting voice and video across packet networks,in ATM networks,and in the Virtual Leased Line service in DiffServ networks.Non-CBRﬂows:Most trafﬁc sources in the current Internet are not CBR in nature.In fact,the following empirical evidence suggests that a sub-exponential distribution of packet inter-arrival times may model sources in the current Internet well.–WWW trafﬁc forms a major portion of trafﬁc on the Internet.The inactive off times that form the tail ofthe inter-arrival times of the WWW trafﬁc are shown to have a sub-exponential nature[7].–telnet packet inter-arrival times have been shown to have a sub-exponential distribution[21].For these trafﬁc sources,is chosen to be,in order to ensure that the variance of the inter-arrival times at the source isﬁnite.We thus ensure that the input trafﬁc to the network is not heavy-tailed.7.1.3Modeling Heterogeneity in Cross TrafﬁcThe extent to which cross trafﬁc entering each router affects the characteristics of taggedﬂows depends on the burstiness of the cross trafﬁc.Burstiness in cross-trafﬁc results from:(1)super-positioning ofﬂows,(2)trafﬁc distortions that result fromﬂows traversing through multiple routers in a network,and(3)heterogeneity in the average inter-arrival times across variousﬂows.To reasonably approximate trafﬁc distortions,we model the cross trafﬁc entering at each router in the network as consisting of an aggregate of two types ofﬂows:(1)ﬂows that are at the beginning of their routes or have traversed through a small(or)number of routers,and(2)ﬂows that are at the end of their routes or have traversed through a large()number of hops.This model closely approximates the current Internet where each backbone router is a small number of hops away from some set of hosts while being far away from some others.To capture heterogeneity,we consider two classes of cross-trafﬁcﬂows—ﬂows with large and small average packet inter-arrival times at the source.We quantify the heterogeneity in the cross-trafﬁcﬂows in terms of inter-arrival time ratio(IATR),which refers to the ratio of the average packet inter-arrival times at the source for these two classes.We derive the twoﬂow classes in three ways.1.CBRﬂows with heterogeneous packet sizes:We consider CBRﬂows with same bandwidth requirement butwith packet sizes chosen uniformly from two intervals.2.CBRﬂows with heterogeneous bit rates:We considerﬂows withﬁxed packet sizes but with bandwidth re-quirements chosen uniformly from two intervals.3.Flows with heterogeneous sub-exponential inter-arrival times:The average inter-arrival times of theﬂows arechosen uniformly from two intervals.Packet sizes are chosen according to an empirically-derived distribu-tion[1].7.2Model Validation Under Near-asymptotic ConditionsTo validate the model presented in Sections4and5,we simulate a network setting where the taggedﬂow traverses a hop topology with Mbps links operating at utilization.We select a taggedﬂow with100Byte packet size and4Mbps bandwidth requirement.We consider cross trafﬁc with an IATR of.Figure3shows the plots of the inter-packet times of the taggedﬂow after it has traversed the network with the three trafﬁc settings.The slope(i.e.,the value of)of the selected region on all of the plots is between and.Figure4plots the function for the inter-arrival times of the taggedﬂow obtained for the three trafﬁc settings.Theﬁgures illustrate that the plots stabilize to a value between and,indicating that the values of−3.4−3.2−3−2.8−2.6−2.4−2.2−2−1.8−1.6−1.4−6.5−6−5.5−5−4.5−4−3.5−3−2.5log(x)l o g P (I A T > x )(a)CBR:heterogeneous packetFigure 5:Variance-time plot for cross trafﬁc2468101214164050607080# h o p sUtilizationIATR=1IATR=10IATR=100IATR=500IATR=10002468101214165055606570758595#h o p sUtilizationIATR=1IATR=10IATR=100IATR=500IATR=1000# h o p sUtilization(a)CBR:Heterogeneous packet sizes (b)CBR:Heterogeneous bit rates (c)Sub-exponential-EmpiricalFigure 6:Region of applicability of the model with heterogeneity of cross-trafﬁc2468101214161820405060708090100# h o p sUtilizationtagged bit-rate = 10%tagged bit-rate = 1%tagged bit-rate = 0.1%24681012141618205060708090100# h o p sUtilizationtagged bit-rate = 10%tagged bit-rate = 1%24681012141618202030405060708090100# h o p sUtilizationtagged bit-rate = 10%tagged bit-rate = 5%tagged bit-rate = 1%(a)CBR:Heterogeneous packet sizes (b)CBR:Heterogeneous bit rates (c)Sub-exponential-EmpiricalFigure 7:Region of applicability of the model with different bit-rates of tagged ﬂoware between and as the model predicts.These graphs demonstrate the heavy-tailedness of the packet inter-arrival times of trafﬁc,and thereby validate the predictions of our model.Figure 5depicts the variance-time plots of the cross-trafﬁc,for all the three trafﬁc settings.The slope of all ofthe plots is greater than(for comparison,a line with slope of is drawn),indicating long-range dependence of aggregates.7.3Non-asymptotic Region of ApplicabilityWe now use the following dimensions of experimental settings to identify the non-asymptotic region of applicability of the model in practice.Network Utilization:To identify the network utilization levels above which ﬂows exhibit heavy-tailed behavior,we conduct experiments with link utilization varying fromto .Network Depth:The greater the number of hops a ﬂow traverses,the larger is the likelihood for its inter-arrivaltime to approach the limiting distribution,.We evaluate the effect of increasing the number of hops—from to —on the validity of the model for individual ﬂows.IATR:The greater the heterogeneity in the cross-trafﬁc,the greater is its interference with the tagged ﬂow.Weevaluate the effect of this parameter by varying the IATR of the cross-trafﬁc from to.We conduct experiments with network settings deﬁned by each combination of speciﬁc parameter values for utiliza-tion,network depth,and IATR.Any network setting that yields a heavy-tailed tagged ﬂow (with )at thedestination is said to belong to the region of applicability of the model.We vary the IATR of the cross-trafﬁc for each of the three different cross-trafﬁc settings.For the three respective trafﬁc settings,Figures 6(a),6(b)and 6(c)plot the minimum utilization level versus the minimum number of router hops at that utilization necessary for observing for the tagged ﬂow.In each of these experiments,thetagged ﬂow occupiesof the total trafﬁc sharing a link.For each value of IATR,the area above and to the right of its corresponding curve identiﬁes the region of applicability of the model.The following conclusions can be drawn from these ﬁgures:1.The greater the heterogeneity in the cross-trafﬁc,the larger is the region of applicability of the model (i.e.,tagged ﬂows show heavy-tailed behavior with for a larger set of network settings).2.For same value of IATR,heterogeneity in packet-sizes results in a slightly larger region of applicability of the model than heterogeneity in bit-rates.3.More importantly,these ﬁgures demonstrate that ﬂows can become heavy-tailed in large heterogeneous net-works running at relatively low utilization levels (e.g.,),or after traversing as few as hops for networks running at high utilization levels.The above results hold for the case when the taggedﬂow occupies10%of the link bandwidth.To investigate the region of applicability of the model when the tagged trafﬁc occupies smaller fractions of total link bandwidth,we conduct the following sets of experiments.Weﬁx the IATR of the cross-trafﬁc to a value of.We vary the contribution of the taggedﬂow to the cumulative bit rate ofﬂows from to.For each of these values, we plot the minimum utilization level versus the minimum number of router hops at that utilization necessary for observing for the taggedﬂow.Figures7(a),7(b)and7(c)illustrate that when the taggedﬂow contributes only a small fraction of the total bit rate of aggregate,only at very high levels of utilization and only after theﬂow has traversed a large number of hops.In fact,when the taggedﬂow contributes just of the bit rate of the aggregate,even after traversing a20hop,highly loaded network,the value of does not belong to the desired interval.8Implications for Network Design8.1Impact of Heavy-tailsHeavy-tailed behavior ofﬂows can be contagious.Consider for instance,a persistent HTTP session in which a client waits for the server response prior to issuing subsequent requests.If the response trafﬁc streams from the server is heavy-tailed,future request generation from the client to the server also becomes heavy-tailed.Heavy-tailed behavior thus spreads from the server trafﬁc streams to the client trafﬁc streams.To cope with the heavy tailed distribution of inter-arrival times of packets from servers to clients,multimedia clients either have to maintain a play-out buffer large enough to smooth out the variation in inter-arrival times,or have to play out incomplete data.The latter deteriorates client performance,whereas the former requires provisioning of very large buffers to smooth out the heavy tails.A client may,therefore,be willing to pay more for a network service that does not result inﬂows becoming heavy-tailed.At the server end,managing heavy-tailed behavior of incoming ﬂows requires careful buffer management and request service procedures.8.2Impact of Long-range DependenceSeveral researchers have studied the impact of long-range dependence on network queues using analytical models and simulations(see[8,9,12]and the references there-in).In[12],it has been found that if the input trafﬁc to a network of queues has a large variance in service times,then the rate of approach to steady state can be so slow that the steady state performance may not be of engineering interest.This raises interesting questions on stability in the presence of such trafﬁc.For network queues with a long-range dependent input,sharp increase in queuing delays at fairly low levels of utilization and slow decay of queue lengths have been observed in[8,9].This implies that an incremental improvement in loss performance requires a signiﬁcant increase in buffer size.These raise important questions on source trafﬁc control and buffer/link dimensioning for satisfactory loss performance.Observe that long-range dependence can occur even if the network backbones are not highly utilized.It is often the case that edge routers run at high levels of utilization—hence,aﬂow may become heavy-tailed even after crossing just the edge routers.Aggregation of such heavy-tailedﬂows in the backbone would induce long-range dependence, even if the backbone network is not highly utilized.In summary,long-range dependent trafﬁc has severe implications on queuing performance.Hence,operating a network so as to maintain trafﬁc outside the long-range dependence domain is highly desirable.8.3Routing of Flow AggregatesRecall thatﬂows in our model could themselves be aggregates of micro-ﬂows that traverse the same path from the source(ingress edge router)to the destination(egress edge router)—examples include LSPs in MPLS networks,VPN tunnels,and VPs in ATM Networks.As mentioned in Section1,suchﬂows can easily occupy a signiﬁcant fraction of the cumulative bit-rate(for example,around).Our simulation results predict that suchﬂows(that is,aggregatesof micro-ﬂows)would become heavy-tailed at relatively low levels of network utilization and after traversing a small number of network hops.It can easily be shown that,as a result,individual micro-ﬂows constituting theseﬂows also become heavy-tailed.To prevent suchﬂows from becoming heavy-tailed,it may be desirable to create a large-capacity virtual tun-nel(that may aggregate multiple micro-ﬂows)between a source-destination pair as a collection of multiple lower bandwidth tunnels(each carrying a subset of micro-ﬂows)that traverse through different network paths.The lower the bandwidth requirement of each of these tunnels,the smaller is the possibility ofﬂows carried on these tunnels becoming heavy-tailed.8.4Considerations for Deploying VLL Service in DiffServ NetworksIn a differentiated services network,the Virtual Leased Line(VLL)service will utilize the Expedited Forwarding (EF)per-hop-behavior(PHB).It has been suggested that a network could provide the low end-to-end delay requested by the VLL service either(1)by providing bandwidth guarantees—possibly through the use of fair queuing algo-rithms for bandwidth differentiation acrossﬂow aggregates—to the class ofﬂows requesting the EF PHB;or(2)by providing higher priority toﬂows requesting EF PHB.If a fair queuing algorithm is used for allocating link bandwidth amongﬂow aggregates,then even if the class of trafﬁc requesting EF PHB occupies a small fraction(e.g.,10-30%)of the total link bandwidth,the overall link utilization can be quite high(due to the presence ofﬂows from other classes as well as best-effort TCPﬂows—which can utilize all of the available link bandwidth).Consequently,individual CBRﬂows requesting the VLL service could become heavy-tailed and the total VLL trafﬁc could become long-range dependent.If,on the other hand,EF trafﬁc is provided higher priority over otherﬂow aggregates,then since EF PHB occupies only a small fraction of the link bandwidth,individual CBRﬂows requesting VLL service are not likely to become heavy-tailed—provided that individualﬂows do not occupy signiﬁcant fractions of the bandwidth allocated to the VLL service class.Hence,the CBR aggregates are likely to remain outside the domain of long-range dependence. This argument suggests that,from the perspective of EF trafﬁc,priority queuing is a better implementation option than fair queuing.Unfortunately,however,with priority queuing,otherﬂow aggregates as well as best-effort TCP ﬂows perceive the network as one with dynamically changing bandwidth.This may affect the throughput and fairness observed by otherﬂows.This may also introduce long-range dependence in the lower priority trafﬁc if the utilization perceived by the lower priority trafﬁc becomes high due to starvation.9Related WorkModels for inter-arrival time(Jitter)forﬂows in general and CBRﬂows in particular in the ATM environment (homogeneous on the packet size)can be found in[5,17,16,20,22,23,25].In[17],the authors model a general renewal source with some background trafﬁc and derive expressions for the packet inter-arrival time(IAT)of the renewal source after one hop.The IAT of the tagged renewal stream is expressed as the difference in queue lengths seen by two consecutive packets of the sameﬂow.Expressions are derived for the IAT in the frequency domain under correlated and uncorrelated background trafﬁc.In[16],the authors model the same network for multiple nodes.In[22],the authors model CBRﬂows with a aggregated CBR background for one hop of the network and extend it to the end-to-end case in[25]under high utilization.The main disadvantage of these models is that they are very complicated,computationally intensive and do not yield any insights into the nature of CBRﬂows.Further, these studies do not discuss the possibility of self-similarity and long-range dependence in such networks.Our work is different from that of[17,16,22,25]in the following ways:Firstly,our analysis yields closed form results for the inter-arrival times ofﬂows under high utilization.Secondly,it does not make any assumptions about the nature of the cross-trafﬁc entering the network,unlike[17,16]where the authors evaluate their model for speciﬁc cross-trafﬁc models such as Markov Modulated processes.。

全新版大学英语第二册答案习题习题Unit5-8

全新版大学英语第二册答案习题习题Unit5-8Unit 5Part II Text AText Organization1.PartsParagraphsMain IdeasPart OneParas 1-2Michael faced the most challenging competition in his pole-vaulting career. Part TwoParas 3-5Michael' s childhood was marked with dreams and tough training.Part ThreeParas 6-12Michael topped his personal best, won the championship and set a new world record.Part FourPara 13What was most unusual about Michael's victory was that he was blind.2. 1) It also has the element of flying, and the thought of flying as high as a two-story building isa mere fantasy to anyone watching such an event. As long as Michael could remember he had always dreamed of flying. 2) All of Michael's vaults today seemed to be the reward for his hard work.V ocabularyI. 1. 1) startled3) tension5) sweating7) vain9) anxiety11) emotions13) In my mind's eye15) mere2) core4) tense6) stretched out8) On one occasion 10) detail12) ashamed of14) recurring2. 1) Mrs. White's birthday coincides with her husband's, iiovi2) The writer was assisted in the preparation of this article by other members of her family.3) It has been a week of alternate sunshine and rain.4) Politics and philosophy had been his lifelong passions, although he studied economics at university.5) She grew tense as she waited for her first TV interview.3. 1) Jessica's victory in the World Championships, which has aroused the media's attention, again bears out the fact that she is one of the best gymnasts in the world. And no one can deny that it is a delight to watch her performing various skills with natural grace.2) With so many relatives along with friends coming to their wedding to congratulate them on their marriage, the young couple felt like the happiest people in the world.3) Frank had made full preparations for his oral presentation. However, when he did begin to speak, he was so overwhelmed by the intensity of the situation that he couldn't find a way to relax himself and made numerous mistakes.--------II. Collocation1) Mike, a Green, made the suggestion that a large park be built near the community.2) In a letter to his daughter, Mr. Smith expressed his wishthat she (should) continue her educa-tion to acquire still another degree.3) There is no reason to hold the belief that humans have no direct moral responsibility to safe-guard the welfare of animals.4) Children need to feel safe about the world they grow up in, and it is unwise to give them the idea that everything they come into contact with might be a threat.5) Anxiety can result from the notion that life has not treated us fairly.6) Nobody believed his claim that he was innocent.III. Words with Multiple Meanings1.1 work out in the gym for one hour every morning.2. Florence has worked as a cleaner at the factory for five years.3. The wounded man worked his way across the field on his hands and knees.4. The safe load for a truck of this type works out at about twenty-five tons.5. It is difficult to understand how human minds work.6. To my disappointment, the manager's plan of promoting the new products doesn't work at all.7. The teacher has a lot of experience of working with children who don't know how to learn.8. The medicine began to work one hour after the child took it.Structure1. (Answers may vary.)1) It was his dream and hard work that made it possible for him to set a world record in the pole vault.2) It was nine years ago that I began to learn English.3) It is my parents who have exerted the greatest influence on me.4) It was in Central Park that I met John.2. 1) Obviously what the speaker wanted to emphasize was the impact of these findings rather thanthe process that led to these findings.2) It seems that he is never bothered about what people would think about his behavior.3) The CEO never hesitates to let his employees know what he is planning for the company.4) The scientist will show the audience what a tele-operated robot can do for a family.Comprehensive txercisesI. Cloze(A)1. In my mind's eye3. sweating5. tense7. preparation9. media(B)1. what3. an5. will7. got9. in11. support13. After15. face2. detail4. relax6. fantasy8. congratulate 10. emotion2. for4. rarely/never6. longed/wished8. Then10. life12. to14. run16. butII. TranslationPsychologists have used numerous facts to bear out the argument/statement/claim that in order to recover speedily from negative emotion, you should allow yourself to cry. You needn't/ don't have to be ashamed of crying. Anxiety and sorrow can flow out of the body along with tears. Consider the case of /Take Donna. Her son unfortunately died in a car accident. The inten-sity of the blow made her unable to cry. She said, "It was not until two weeks later that I began to cry. And then I felt as if a big stone had been lifted from my shoulders. It was the tears that brought me back to earth."Part III TextBComprehension Checkl.b 3. b 5. c2. c 4. b 6. atranslation(#J& Appendix III)Language Practice1. aid3. inclined5. horrible7. dismiss9. impatient11. optimistic13. ease15. install17. shortcomings 19. bumped2 traits4. in spite of6. penetrated8. has suffered from 10. progressive12. to a degree14. hold on to16. in good health 18. interval(s)20. miserablePart IV Theme-Related Language Learning Tasks■■Model paperThe Change in Mr. ManwaringThere is no doubt about it. Hal Manwaring has definitely changed. And changed for the better. He still limps, of course, still has trouble getting in and out of his car and climbing stairs. He still needs his stick to get about. Everyone feels sorry for him, naturally. They always have, ever since he first started suffering from his disease all those years ago. But, to be honest, in recent years people started to avoid him, he had become so bitter he hadn't a good word to say about anyone or anything. Understandable, you might say, but that didn't make him anybetter company.Then, all of a sudden, ever since that big storm we had last month, he picked himself up andnow has a cheerful word and a smile for everyone. Where once you would see him sitting hunched up on his porch for hours on end, now you see him hopping about all over town. He has even takenup helping to teach on the adult literacy programme. Just what happened he won't say, but whateverit was it certainly did him a power of good. (189 words)Unit 6Part II Text AText Organization1.PartsParagraphsMain IdeasPart OneParas 1-3Despite her friend's advice, the author, unable to resist the temp-tation, falls for superwomen stories again.Part TwoParas 4-7Her encounter with a superwoman and its impact on her.Part ThreeParas 8-11She tells about what prevents her from becoming a superwoman herself. Part FourParas 12-14She analyzes why she is still fascinated by modern superwomen tales.Part FivePara 15She comes to the conclusion that to her, admiring a heroine is something worth doing.2. The author explains how difficult it is to be a superwoman mainly by d. personal experiences And give facts to support your choice.1) She is unable to get her act together.2) She can almost expect something to go wrong as soon asshe sets to work.3) Anything might happen that would take up much more of time she puts aside for a certain job.4) She usually feels exhausted after a busy day.V ocabularyI. 1. 1) incredible3) hits home2) remainder 4) investment5) by nature 6) lap7) decorate 8) harmony9) move on to 10) bulletin11) at intervals 12) client13) theory 14) item15) military2. 1) Though Roger is absolutely convinced that the plan will work, I am still skeptical of/about it-2) Carol's worrying about her daughter has much to do with her being hooked on the Internet recently.3) Alison's so disorganized— I wish she'd get her act together.4) Despite her resolution not to believe such modern fairy tales any more, she couldn't resist the idea of admiring superheroines.5) Mention of the controversy over the matter would still undo her.3.1) Don't be taken in by his charming manner; he is hard and cruel by nature and rarely shows pity for the poor, rusrnc2) The rescue team is racing the clock to salvage (打捞) the bodies of the sailors trapped in the sunken submarine (潜水艇) with sophisticated equipment.3) The original draft of the declaration strongly reproaches some big powers for possessing large numbers of nuclear weapons and consuming too much of the earth's natural re-sources. 2. as well/too4. also6. too8. AlsoII. Synonyms1. also3. too5. as well/too7. alsoIII. Usage1. I used to hate jogging but I'm actually growing to like it now.2. We didn't have the chance to see her — she was too busy.3. It seems she was coming to understand it better as she grew older.4. How did you manage to become a television anchorwoman?5. Gradually I came to like the proposal she brought forward in the group discussion.6. You're becoming more and more like your mother.Structure1. 1) If only it were that simple!2) We could go out this afternoon if only it would stop raining.3) If only she hadn't told David about that, everything would have been all right.4) If only she hadn't come into the room at that particularmoment.2. 1) Tom is/was too intelligent for his class so he is/was not learning anything there.2) The task is/was far too difficult for a nine-year-old.3) There are/were too many people for such a small room.4) The noise became too much for me so I went and complained.Comprehensive ExercisesI. Cloze(A)1. incredible3. care for5. decorate7. put pen to paper9. piece of cake 11. despite(B)l.One3. having5. wrong7. with9. to11. in13. true15. Eventually 17. later2. corporation4. set aside6. move on8. draft10. get our act together12. charmed2. when4. However6. not8. if10. reads12. should14. up16. plusII. TranslationBarbara has dreamed of becoming a CEO for a long time. To achieve her heart's desire, she applied for a job in many a multi-national corporation, but failed to get it. However, nothing seems to be able to undo her. With a loan from a local bank she opened a restaurant not long ago. While doing business she is having two young children to care for. Also, she is working at/studying for an MBA degree. Despite all this she manages to get her act together. Nevertheless, even to her, racing the clock is by no means a piece of cake. It's a very exhausting job.Part III TextBComprehension Checkl.b 3.d 5. d 7. a2. a 4. b 6. cTranslation(#J& Appendix III)Language Practice1. set out3. temporary5. weighed down7. plentiful9. pleaded11. in control of 13. expense15. reaction17. at every opportunity 19. soak up2. renewed4. sought after6. quit8. takes all the credit 10. died of12. party14. lives for16. semester18. stir20. take ... back.Part IV Theme-Related Language Learning TasksModel paperWomen Hold up Half the Sky"Men are born free but everywhere they are in chains," wrote the French philosopher Rousseau. Being a man he forgot to add that the chains that hold women back are often stronger and heavier. It is true that women are no longer tied to the home, and have entered fully into the world of work. Nevertheless, childbirth and greater responsibilities for childcare can put women at a disadvantage in competition with men. The situation is made all the more difficult by discrimination against women in selection and promotion in certain professions.There are a number of ways to remedy this situation. Improving women's confidence aboutwhat they can achieve is one. Destroying male prejudices is another. But the answer lies not just in changing ideas. Material support in the form of improved childcare facilities is needed, asis greaterprotection from the law against sexual discrimination. (152 words)Unit 7Part II Text AText Organization 1.PartsParagraphsMain IdeasPart OneParas 1-3Massive borrowing from other languages is a major feature of the English language.Part TwoParas 4-16Tells about the history of the English language from the Indo-European parent language to modern English.Part ThreeParas 17-19Tolerance, love of freedom, and respect for the rights of oth-ers —these qualities in the English-speaking people explain the richness of their language.2.Paras 4-9The introduction of the Indo-European language —the parent language of En-riyioj won-gkshParas 10-11Germanic tribes came to settle in Britain and brought Anglo-Saxon words —Old EnglishPara 12The Christian religion enriched English with words from Greek and LatinPara 13The Vikings from Scandinavia came with words from Old NorsePara 14The Norman Conquest — French influencePara 15The European Renaissance and the printing press brought many new words from Latin and GreekPara 16The American revolution — the emergence of a new variety — American En-glishV ocabularyI. 1. 1) independent 2) Strictly speaking3) drifted 4) resembles5) virtually 6) invaded7) conquered 8) fascinating9) snack 10) will be put into practice 11) classics ..12) source13) climate 14) surrendered15) were ... aroused2. 1) In the United States, private cars are an absolute necessity rather than a luxury.2) Everyone thinks that the new member is a valuable addition to the football team.3) I am afraid the fire will get out of control if the firemen do not arrive within ten minutes.4) As all flights to the United States have been cancelled, you have no alternative but to go via Vancouver to get to Seattle.5) The government has declared all beef imports will be banned for the next six months for fear of the spread of mad cow disease.3. 1) Through systematic research, scientists have invented many drugs that are said to help us live longer, but the cause of aging remains, to a very real extent, one of life's mysteries. 2) The two parties have reached an agreement to establish a joint venture in Beijing, but before a formalcontract is signed, some clauses in the draft agreement need to be modi-fied/modifying so that no misunderstanding will arise as to their interpretation. 3) Our ancestors have passed onto us the wisdom that to ensure the long-lasting prosperity of a country, we must show tolerance for the errors made by those who strikeout revolution-ary paths. These people, with their new ideas and practice, will enrich the national experi-ence.II. Synonyms1. a) wish2. a) skin3. a) rear/raise4. a) royalb) wishb) hide/skinb) raiseb) kingly/royalc) wantc) hidec) rear/raise c) sovereignd) wish/want d) skind) raised) royal/kinglyIII. Usage 1.Indeed 3. Frankly5. To my knowledge 7. Nevertheless9. insteadi2. though4. Moreover6. however8. Yet10. in other wordsStructurel1) We are literally crawling. There must be some traffic accident ahead.2) Thomas sounds very knowledgeable about traditional Chinese medicine. He must have had some special training.3) The draft document is complete in virtually every detail. It must have taken him months to prepare it.4) Cathy must have missed the train. She should have arrived here two hours ago.5) What? It cost you one thousand dollars to fill a hollow tooth? You must be exaggerating!2. 1) Are we supposed to deposit our bags before entering the supermarket?2) The spaceship is supposed to land on Mars at 10:40 this morning.3) As a car driver, you are supposed to know how to changea tire.4) The nurses in the hospital are supposed to take good care of the patients.5) We are supposed to speak only in English when we discuss text organization.Comprehensive fxercisesI. Cloze(A)1. fascinating3. invented5. ban7. out of control9. Fortunately11. sources(B)1) early3) found5) source7) in9) how11) given/delivered13) by15) without2.tolerance4.addition6.corrupt8.establishing10.Massive12.enrich2) those4)must6) further8) began10)in12) than14)so16)commonII. TranslationThough how the English language came into existence remains a mystery, linguists/language scientists now tend to believe that English and most other European languages have descended from a common source; the Indo-European parent language. English was first spoken by the Anglo-Saxons who invaded England in the fifth century. They passed onto us the basic vocabu-lary of English. In its over 15 centuries of development, English has borrowed from other lan-guagesmassively, and such borrowing has greatly enriched its vocabulary. As settlers landed in America and established the United States as an independent nation, a new variety was added to the English language: American English. Though some people worry that the language is running out of control, many native speakers of English take pride in the tolerance of their language.Part III TextBComprehension Check1. c2. c3. c4. d5. b6. d Translation(#J& Appendix III)Language Practice1. give way to2. predict3. substantial4. integrate5. in transition6. aspect7. professionals8. genuine9. economic10. to name a few11. authority12. dominate13. crude14. trend15. shift16. had contributed to17. unique18. exceeding19. rid yourself of20. statusPart IV Theme-Related Language Learning TasksModel paperEstimates of English Speakers from 1950 to 2050This graph shows the changes of the estimated numbers of English speakers from 1950 to 2050. English speakers are divided into three groups, i.e., LI speakers, L2 speakers and EFL speak-ers. According to the graph, it is estimated that the number of LI speakers was a little over 200 million in 1950, a figure that will increase to around 450 million by 2050. L2 speakers will triple during the same period, from less than 200 million to around 600 million. EFL speakers will double in number over the period, starting from about 400 million in 1950 and reaching to over 900 million in 2050.Although the numbers of English speakers in all three groups tend to increase, their patterns of increase are different. LI speakers increase steadily over the entire period, but not as rapidly as L2 speakers. This rapid growth is predicted to lead to L2 speakers outnumbering LI speakers by the year 2010. Trends for the number of EFL speakers follow a somewhat different path. While the figures remained stable during the 1950s they then began to experience an increasingly sharp rise thatis expected to level off and remain stable from 2020. (195 words)Unit 8Part II Text A2. 1) environmental luxuries vs. environmental necessities2) sensible environmentalism vs. sentimental environmentalism3) man vs. natureV ocabularyI. 1. 1) universal 2) current3) worked his way 4) deny5) melt 6) came through7) threat 8) combat9) in the name of 10) raged11) vote 12) concerns13) in part 14) reality15) urgent2. 1) There has been much debate on the new teaching method invented by a young education- ist.2) Most kinds of cancer are fatal without proper treatment.3) John has submitted an application for retirement at the age of 56 on the grounds of ill health.4) Can you draw a distinction between these two approaches? They sound so similar to me.5) Cars should stop at red traffic lights. Similarly, bicycles should stop, too.3. 1) Ecologists constantly call for people to stop cutting trees for the sake of man's own future.As a matter of fact, more and more people have realized that man's fate is closely bound up with his environment.2) We distinguish the excellent manager from the common one by observing his human rela-tions skills to accommodate all kinds of conflicting interests within his company. The excellent manager can always create a lively atmosphere through various proposals and suggestions while the common one can do nothing more than complain about everything.3) It goes against the grain of contemporary concept of ecology ( 生态学) to overuse our natural resources regardless of serious ecological consequences. While it is ridiculous to worship the earth to the point of excess, to ignore the fundamentalprinciples of ecologi-cal balance is dangerous too.II. Confusable Words1. 1) sensible3) sentimental5) sensible2. 1) conserve3) conserve2) sensitive4) sensitive6) sentimental 2) preserves 4) preservingIII. Word Formationage-old 古老的air force 空军daughter-in-law 儿媳妇first-rate 一流的greenhouse 温室half brother 同母异父兄弟；同父异母兄弟ice-cream 冰激凌lifelike 逼真的light year 光年salesperson 售货员；推销员self-centered 以自己为中心的，自私的threefold 三倍overuse 过度使用stone-still 石头似的一动不动worldwide 全世界范围的；在全世界范围的Structure1. 1) Now that he has begun a new life in a foreign country, everything seems so different and he isunderstandably excited.2) The things I've worked so hard for appear to me so shallow and worthless now that I've achieved them.3) Now that the warmer months are coming, why don't you take the opportunity to travel around Siberia?4) Now that she knew the whole truth she realized how stupid she had been not to suspect that her boyfriend was actually a spy.2. 1) When Mount Qomolangma was remeasured in 1987 bya more accurate method, it was foundto be 24 meters higher than previously believed.2) The central heating system has proved less effective than originally hoped for.3) The new electronic dictionary is sold at a much lower price than predicted.4) Totally fed up with the noise in the surroundings of the hotel, Prof. Sherrington said he'd leave as soon as he finished his work and wouldn't stay there a moment longer than necessary.Comprehensive ExercisesI. Cloze(A)1. concern3. excess5. consequences7. distinguish9. distinctions (B)1. between3. conserving5. concern7. own9. use11. as13. lower2. environment4. regardless6. fatal8. urgent10. fundamental2. than4. less6. that8. more10. mankind/man 12. which14. differencesII. TranslationIt is relatively more difficult to call for poorly educated people to conserve resources be-cause they tend to view resources as nothing more than handy materials, which they can never overuse to the point of excess. They are seldom aware of the fatal consequences of the ruin of nature. In contrast, most educated people bind their own well-being to that of nature and try to preserve nature on the grounds of self-preservation. Though it may be difficult to distinguish a good proposal from a bad one in our current debate on environmental issues, one contemporary commentator has argued that protecting the environment in the name of man should be our guiding principle.Part III Text BComprehension Check1. a 3. d 5. d2. b 4. a 6. cTranslation(#JE Appendix III)Language Practice1. nothing but3. get away from 5. Pollute7. assumptions9. has rusted11. dump13. substance15. triumph17. hardens19. Uttered2. patches4. spoil6. explosion8. enables10. washed up12. stained14. destruction16. pinched18. For all20. mankindPart IV Theme-Related Language Learning TasksModel paperMy Thoughts on Environmental ProtectionWhen faced with all kinds of environmental problems, many people seem to believe that we have to choose either economic prosperity or environmental protection. In my view, this is a false choice. I think that economic development and environmental protection can and must coexist.It is true that development has brought with it a number ofenvironmental problems, such as air pollution, water shortages and acid rain, but at the same time modern industry and commerce have greatly improved our lives. Few would wish to live without the benefits of television, automobiles, computers, etc. To stop developing our industry and commerce for the sake of nature alone is pure sentimentalism.Of course, we can't allow economic growth to go on at the expense of the environment, either. When a healthy environment is gone, everything that our life depends on is gone, too. So, we must protect the environment—not for the sake of the environment alone—but for the sake of man. (168 words)。

Unit 3话题七选五专项练习2022-2023学年高一英语必修第一册单元重难点易错题精练人教版

Unit 3话题七选五专项练习2022-2023学年高一英语必修第一册单元重难点易错题精练人教版Unit 3单元话题七选五专项练习(名校最新真题)（2022·全国·高一课时练习）The Growing Popularity of CyclingThey say once you learn how to ride a bike, you never forget. Most of us learnt when we were kids, and many still strap (系好) on our helmets, put our feet on the pedals , and shoot off for a ride around the streets. In recent times, cycling has seen a boost in popularity. ____1____First, let’s look at the hea lth benefits. Cycling can improve your cardiovascular (心血管的) health and burm excess body fat. Strengthening your leg muscles in your calves (小腿肚) and thighs (大腿). ____2____ A long ride in the countryside could help you to clear your mind, or de-stress. But it’s not only about your body and mind— there are also some plus sides for your wallet’s health.Cycling can be a great way to commute (通勤) or get about town. ____3____ If you do drive into work, sometimes finding somewhere to park can be a nightmare, whereas finding somewhere to chain your bike up can be much simpler. Then, on the train, there are changes at stations and other commuters speaking loudly on their phones to deal with. And if you’re worried about safety, there are designated cycle lanes in many major cities. Wearing high-visibility clothing is another way to improve your safety when cycling on the roads.____4____ It can help you to reduce your carbon footprint. Not pumping out fumes into the atmosphere is better for the planet and your own environment.So, whether people want to cycle for their health, getting about town, their wallet, the environment, or a combination of all of them—____5____A．The popularity of cycling is on the rise.B．By cycling, you save money on fuel or train fares.C．Finally, cycling can also be good for the environment.D．Cycling except for transport, also can be used for fitness.E．And cycling’s benefits aren’t just physical, but mental as well.F．What makes it so appealing to people to straddle their bikes once again?G．I feel very relaxed by cycling, but the body was able to quickly reach the amount of exercise!（2022·浙江·杭十四中高一期末）Danielle Waterman, an English woman rugby (橄榄球) player is interviewed and talks about her experience and opinions about the sport.Having two older brothers taught me a lot about survival. ___6___ We’d play rugby together and if I wanted to play against my brothers, who I looked up to, I had to be good enough. It made me determined.I was lucky to have supportive parents, especially my mother, who was my rolemodel. ___7___ It’s different now, with lots of amazing women in British sport as positive examples to young girls. The situation for women’s sport has improved, but there’s a long way to go. The regularity of male sport allows the public to be familiar with teams and players. ___8___A few years ago, I got hurt in my knee and was told I couldn’t play again. ___9___ That’s when I really recognized the challenges I was facing mentally, as well as coping with my physical pain. No matte r how much I trained, I wouldn’t improve unless I was happier and more confident.I realized I needed to get help for my mental health. Without that support, I wouldn’t have played again.___10___ When I was injured, my immediate reaction was, ‘I need to keep going.’ I’ve built up strong will but it’s basically because I love what I do. When I play rugby, I work with other people, deal with failure and overcome difficulties - all these make me open-minded and determined. Rugby develops confidence in me around achieving and self-belief.A．That’s what women’s sport needs.B．Sometimes I did feel different to other girls.C．I never thought of myself as different from them.D．Playing rugby is my passion, and it enriches me.E．However, I decided to insist on and to get to the Olympics.F．As well as the physical side, the sport also leads me to a happy life.G．There weren’t many female role models in sport when I was growing up.（2022·湖北·十堰市教育科学研究院高一期末）The Importance of Getting Kids to Be ActiveEveryone knows that kids should be physically active and need to exercise regularly to be physically fit. ____11____, regular physical activity is considered by most experts to be an important part of a healthy lifestyle. However, that doesn’t mean every child needs to spend four or five hours a day exercising.How Much Exercise Do Kids Need?It is recommended that kids should be physically active for at least 60 minutes perday. ____12____. For example, if your eight-year-old played soccer for 20 minutes during PE at school and then played basketball with his friends for 40 minutes after school, he would meet the recommendation of 60 minutes of physical activity for that day.Exercise and Calories____13____. It can help them build self-respect, sleep better, have more energy, and reduce depression. And as most people know, along with a healthy diet, regular exercise is the best way to lose weight and prevent childhood obesity (肥胖). Since your child is not likely to get his exercise using exercise equipment, it can be hard to always tell how many calories he is burning while exercising. ____14____, as long as your child is getting his 60 minutes or more of physical activity each day and maintaining a healthy diet.Youth Exercise and FitnessRemember that kids, even teens, don’t usually stick with exercise programs. ____15____. Instead, kids do better with lifestyle exercise programs, such as active free play and organized team sports.A．Fortunately, it doesn’t matterB．Regular exercise is good for kidsC．So they will be able to burn even more caloriesD．That’s why you don’t see many kids in health clubsE．Whether they are overweight or at a healthy weightF．If your child is very active and is still gaining weightG．But that doesn’t have to be 60 minutes of continuous activity（2022·重庆·高一阶段练习）Since the start of the COVID-19 pandemic, many people have been stuck indoors. Online classes and working from home mean a lot of sitting around on your butt. We already know that if we don’t move much, we get fatter, we get heart problems and we may get diabetes. ____16____.A study of 937 Brazilian adults during the pandemic found that about one-third of them sat around for more than 10 hours per day. It reported that people who sat for too long often showed more symptoms of depression.____17____. It found that each extra hour of sitting time at ages 8, 10 and 12 led to higher depression scores by age 18. Eighteen-year-olds who sat most often had depression scores 28 percent higher than those who were more active.____18____. But according to Mats Hallgren from the Karolinska Institute in Sweden, “sitting for too long can slow blood flow in the body, including blood flow to the brain.” It might also affect the growth of neurons in the brain. And couch potatoes might do other unhealthy things while sitting, such as snacking on junk food. All these may lead to depression.The good news? ____19____. Aaron Kandola from University College London, UK, said “all types” of movement might help, even light activity. “It is great to exercise and play sports,” said Kandola. “But anything you can do to avoid sitting for too long will help with mood, stress and thinking clearly.” ____20____.A．Scientists are not sure yet about the link between too much sitting and depressionB．But new studies have shown that it could hurt our mental health tooC．Sitting for a long time may cause more damage to children than adultsD．It’s easy to overcomeE．The key message is just to move more, more oftenF．Another study looked at childrenG．Sitting for too long affects us in many ways（2022·辽宁·高一期中）If you have watched the amazing Beijing 2022 Winter Olympics, chances are that you have fallen in love with some winter sports and you’re already making sure your equipment is ready forthem. ____21____ However, there’s another el ement to get ready for the slopes (坡)—your body. Even if you exercise regularly, skiing, snowshoeing or skating challenges your muscles in ways other activities don’t.Exercising in the cold decreases your body temperature. ____22____ Your heart and lungs also need to work harder to warm air before it enters your body. Depending on the sport, certain muscles may also be used more frequently compared to activities in other seasons. The good news is that most winter sports involve heart endurance (耐力), muscle strength and muscle endurance. ____23________24____ Skiing and snowboarding are great winter sports that provide a total body workout. However, it can be hard on the body the first time you hit the slopes. As anyone who has spent time skiing or snowboarding can prove, after a long day on the slopes, the body becomes extremely tired. And that is when injuries are more likely to occur. ____25____ Before the winter season, a regular exercise program should include exercises that get the blood running while challenging the entire body, such as stair climbing or running. The intensity should vary from 20—45 minutes at least 3 to 5 days a week. As winter nears, adding a cardio (有氧的) workout longer in duration but lower in intensity can help increase endurance.A．Maybe you haven’t bought all the equipment.B．To avoid this, you need to strengthen your body and improve endurance.C．Your metabolism (新陈代谢) increases to warm you up.D．You should take advantage of this opportunity to have a rest.E．Probably, you have got your skis tuned and skates sharpened.F．All of them are areas that can be trained before the winter season.G．But there are still some other things to think about.（2022·河南·新野县第六高级中学高一期中）A Beginner’s Guide to Snowboarding Snowboarding is one of the most rewarding sports to learn and a great way to enjoy the winter. Not only is snowboarding fun, but it’s good for you, too. ___26___ Here are some tips if you want to learn to snowboard. These basics are very important, because you don’t want to develop bad habits.The most important thing to pay attention to is your equipment. ___27___ However, it’s important to consider that higher-end equipment can give you much more control, which generally results in less falling.Next, you need to pick a good day. Your ideal day would be clear and sunny with a good amount of soft snow for extra padding (垫状物) when you take a fall. ___28___ Lots of snow is great for those who know what they’re doing, but not so ideal for beginners.When starting, edge control is everything. Always keep the forward / downward-facing edge of your board in mind, and don’t let it move down to catch the snow. ___29___ Skiers have four edges and two poles, and their legs face forward. Snowboarders, on the other hand, have two edges and no poles, and their legs face sideways.When you’re first coming down the mountain, it’s important to learn how to stop, control your speed and make long, proper turns. It may feel strange, but watch how quickly your body adapts to this new environment. Ju st remember, soon you’ll be at one with the board. That’s said, you may be sore (酸痛的) the next day. This is normal. ___30___ Spend some time in a hot tub (浴缸) if you have really sore muscles.Most importantly, remember to have fun and that nothing happens overnight.A．Just make sure you give yourself enough rest.B．Check out the weather reports as you plan your trip.C．If you have a fall, there won’t be as much impact on your body.D．Your local snowboard shop will tell you everything you need to know.E．You can burn up to 400 or 500 calories an hour, and you work your entire body.F．It’s also worth mentioning that skiing and snowboarding are completely different.G．You may think you need just basic equipment as you’re not going to make a living by this.（2022·北京房山·高一期中）The music is on. The figure skater glides, jumps and spins. One turn, two turns, three turns. ___31___Why? People often feel dizzy and lose balance after spinning around. ___32___ The eyes reach their limit before our head finishes a full turn, so they turn back to a new starting point,said Scientific American. When our eyes repeat this, we get dizzy.Figure skaters don’t have any super power. ___33___ Their secret is lots and lots of practice, according to Science Focus magazine. As they train more, their brains get used to the feeling.___34___ One method is called “opposing eye movement”. It’s like the eye movement when we watch a moving train pass in front of us while our heads stay still. Skaters can train themselves to engage this eye movement when they spin. Scientific American said this can offset the dizzy feeling.___35___ We feel dizzier when our brain feels changes in speed. Skaters can also pick one spot to stare at as they stop turning. “Some skaters will design a dance move at the end of a long spin.” said Science Focus. This provides a little break while the dizziness passes.A．No pain, no gain.B．They get dizzy, too.C．Their movements are so smooth and graceful.D．Other methods include keeping a uniform speed.E．Skaters also have methods to help them feel less dizzy.F．It seems they never get dizzy no matter how many times they spin.G．This is because when we move our head during a spin, our eyes start to move in the opposite direction.（2022·四川省绵阳南山中学高一期中）The reasons skiers and snowboarders wear goggles or glasses might seem a little obvious, especially if you’ve ever hit the slopes yourself. Sunlight reflected off the snow can make it hard to see, and UV rays can even damage your eyesight. Special colored lenses also create contrast, so your whole path won’t look like one flat expanse of whiteness.____36____Olympic speed skaters, meanwhile, are racing inside on smooth ice. ____37____According to NBC Olympics, glasses can increase visibility on indoor tracks,too. ____38____ And while the ice may look smooth from a distance, those sharp blades can kick up ice chips that could pose problems if they end up in your eyes. In the event of a crash, shatter-proof glasses also protect skaters’ eyes from errant blades and body parts.____39____ Shot-track speed skaters (who race around a roughly 11-meter track) can go as fast as 30 miles per hour or more, and long-track speed skaters (whose track is 400 meters long) sometimes hit about 35 miles per hour. The wind resistance generated by such speed- in a chilly arena—is enough to make any skater’s eyes start streaming with tears. Picture leaning your head out the window of a car that’s going around 35 miles per hour on a cold day: You’d probably want to be wearing glasses, too.That said, glasses aren’t a requirement. ____40____ Belgium’s Stijn Desmet, for example, has raced in Beijing 2022 Olympic Winter Games without eyewear, as has China’s Zhang Chutong.A．How do they wear glasses?B．So why do they wear glasses?C．Some skiers are afraid of the sunlight.D．Some skaters’ lenses are tinted to help with that.E．You’ll occasionally see speed skaters with uncovered visages.F．Not to mention that eyewear shields your eyes from any snow you displace.G．But the biggest reason speed skaters opt for eyewear just might be their speed.参考答案：1．F 2．E 3．B 4．C 5．A【导语】本文是一篇说明文。

2023届河北省衡水中学高三上学期四调考试英语试题(含答案解析)

2023届河北省衡水中学高三上学期四调考试英语试题学校:___________姓名：___________班级：___________考号：___________一、阅读理解Monthly subscription educational boxes for students can be a great way to develop an interest in a particular subject.If you want to learn something at home while having fun,you can have educational boxes delivered monthly straight to your house.Spangler BoxesThe Spangler boxes feature a range of hands-on scientific experiments designed by Steve Spangler,who previously hosted a popular science education program on News for Kids.Projects focus on STEM-science,technology,engineering,and math.Examples of projects include exploring lava(岩浆)science,the sound waves,and physical laws.Finders Seekers BoxesAfter receiving a Finders Seekers box,you'll explore characters of a new city and culture every month while solving puzzles.Think of Finders Seekers as an escape room challenge that takes place in your living room.Clues and online research tools are available to help in case you get stuck solving the mystery.Each puzzle takes approximately two to four hours to complete.Creation BoxesCreation teaches students electronics,digital tips and high-level problem-solving.Each box teaches a lesson and is progressively more challenging.A sampling of the projects includes a mood lamp,an electronic memory game,and a distance detector using an ultrasonic sensor(超声传感器).Kiwi Doodle BoxesKiwi Doodle Boxes are among the most popular boxes on the market.They feature a variety of themes customized to students aged from8to18.Doodle focuses on art and design with projects such as a felt botanic garden,an artificial leather sample set,and handmade soap. Doodle projects introduce students to new crafting materials and techniques and all craft supplies are included in the box.1．What did Steve Spangler use to be?A．A scientist.B．An educator.C．A TV host.D．An engineer.2．Which of the following is included in Finders Seekers Boxes?A．Features of cities.B．Online instructions. C．Historical mysteries.D．Escape room challenges.3．If you are interested in handicraft art,which box would you subscribe? A．Creation Boxes.B．Spangler Science Boxes. C．Finders Seekers Boxes.D．Kiwi Doodle Boxes.A famous springboard driver was defeated by his competitor in the finals of a world championship,offering congratulations to the winner and waving a tearful goodbye to the audience.It is common in the athletic world.Sportsmen spend the best part of their lives on hard training and competitions.When it becomes evident that they are on the decline they still make great efforts to give their best so as to bring a satisfactory end to their career.Chances to compete for the championship are few and far between.Life is short.Still shorter is the time for an athlete trying to win games.An athletic contestant really feels that there are so few opportunities and time flies.He treasures every minute,makes full use of it and tries to grasp any chance coming his way.He gets as much as he gives,winning honors not only for himself but also for his country.To participate and to win——that is the Olympic spirit.It finds expression in the weak daring to challenge the strong,and the strong searching forever better performance.Ever better—the ideal always makes a sportsman forward.He will do everything he can for it, never relax,never give up.It is said that none of the competitors can avoid being defeated—even the best is bound to be defeated by someone still stronger.This is the rule of sports—thousands of losers set off one winner who in turn will eventually be replaced by someone on the honor list.However,he is always trying to do the best he can.When the time comes and he knows he can’t,he will step down happily to give place to the younger winner, aware contentedly of the fact that he has done his best for the“ever better”records of the Olympic Games.He will say proudly that he has not lived his youth in vain.4．What is the function of the first paragraph?A．To explain a phenomenon.B．To describe a competition.C．To lead in the topic of this passage.D．To express worries about sportsmen.5．According to the passage,what kind of ideal pushes an athlete forward?A．He wants to win people’s respect.B．He wants to win honor for his nation.C．He wants to make greater achievements than ever.D．He wants to earn more and more money and fame.6．What can be learnt from the rule of sports?A．Whatever difficulties you have met,never give up.B．Even the strongest athlete will be defeated one day.C．You can’t live in the sports field without any effort or progress.D．You are the winner if you are brave enough to challenge the strong.7．What is the passage mainly about?A．Sportsmen’s spirit.B．Sportsmen’s life. C．Sportsmen’s honor.D．Sportsmen’s pride.“A lot of the time when climate change is discussed in mainstream media,people are asking‘can humans overcome this?’,or‘what technology can solve this?’.It’s high time we recognized that animals also have to adapt to various changes.We know some animals change their skin colors to escape from natural enemies or due to environment pollution,”says Ryding.“The climate change that we have created is putting a lot of pressure on them, and some species try to adapt by shapeshifting(变形).”Strong shapeshifting has particularly been reported in birds.Several species of Australian parrot have shown,on average,a4%—10%increase in beak(鸟喙)size since 1871,and this is positively linked with the summer temperature each year.North American dark-eyed juncos,a type of small songbird,had a link between increased beak size and short-term temperature extremes in cold environments.There have also been reported changes in mammalian species.Researchers have reported tail length increases in wood mice and leg size increases in masked shrews(鼩鼱).“The increases in some body parts size we see so far are quite small—less than10%—so the changes are unlikely to be immediately noticeable,”says Ryding.“However,some body parts such as ears are predicted to increase.”Ryding intends to investigate shapeshifting in Australian birds firsthand by3D scanningmuseum bird specimens from the past100years.It will give her team a better understanding of which birds are changing their body parts and why.“Shapeshifting does not mean that animals are coping with climate change and that all is‘fine’,”says Ryding.“It just means they are adapting to survive it—but we’re not sure what the other ecological consequences of these changes are,or indeed that all species are capable of changing and surviving.”8．What may cause animals’shapeshifting according to Ryding?A．Human hunting.B．Climate change.C．Natural enemies.D．Polluted surroundings.9．What method does the author mainly use to develop paragraph2?A．Giving examples.B．Cause-effect analysis. C．Making comparison.D．Process analysis.10．What can we know from Ryding’s words in the last paragraph?A．Animals can well adapt to changes and survive.B．Influence of animals’shapeshifting is uncertain.C．Rdying will research into bird museums in Australia.D．All adaptations of animals to climate change are beneficial.11．In which section of a newspaper can we find the text?A．Technology.B．Health care.C．Environment.D．Education.For thousands of years,Chinese writers have travelled all over the country to take down notes about the geographical conditions of each city.Among them,well-known Chinese geographer and writer Li Daoyuan,in the Northern Wei Dynasty(386-534),composed his book,Commentary on the Water Classics,after studying the original literary version,Water Classics.He later expanded the river records to1,252from the original137.The book is now being studied again by Professor Li Xiaojie and his team from Fudan University.They have been using drawing software and3D modelling to recreate the waterway situations on a map based on the book description.So far,they have completed research on four rivers.“Ancient people knew really well how to apply the power of nature to technical considerations,”Li said,giving the example of Qianjin’e,one of the most famous ancient water conservation projects in Luoyang,Henan.In order to lead the river into the city for irrigation(灌溉)in ancient Luoyang,the officials built a canal branch by separating anorthwest-southeast river.However,the canal water wasn't enough to support the citizens in dry seasons.To solve that,on the northern side of the canal branch,the officials built a reservoir(水库)and a channel to lead the water to the canal branch,where the waterways would converge and flow together to the city.In Commentary on the Water Classics,a total of2,800cities are recorded with details. Still,the process of recreation takes much effort.After doing a lot of text analysis and fieldwork,the team has gradually created the model with3D modelling software based on repeated deductions(推论).For Professor Li,the book is not only a record of the natural landscape over1,000years ago,but also a detailed description of humanity and culture and a treasure for today’s reference.12．What can we learn about the book Commentary on the Water Classics?A．It has been out of date.B．It explains3D modelling.C．It keeps records of137rivers.D．It is based on previous studies. 13．What does the underlined word“converge”in paragraph3mean?A．Join.B．Pass.C．Cross.D．Begin. 14．What does paragraph4mainly tell us about model recreation?A．Its major problems.B．Its detailed analysis.C．Its complex process.D．Its successful application. 15．Which of the following can be the best title for the text?A．3D Technology Fuels Modern ResearchB．Ancient Classics Inspire Modern ResearchC．An Effective Approach to Model RecreationD．A Famous Writer of Chinese Ancient Classics二、七选五If you travel to nearly any Chinese city in the summertime,you will see people,mostly men wearing T-shirts,sitting out on the sidewalks on low chairs in the shade,playing a game with large round disks of wood.____16____Xiangqi dates back as early as the Warring States period.While experts have different opinions as to who created Xiangqi and how it is linked to Western chess,there is no doubt that by the Tang Dynasty Xiangqi was a populargame in China.____17____And in the20th century both Sun Yat-sen and Zhou Enlai were big fans of the game.And,though it may look quite a bit different than Western chess,Xiangqi is actually quite similar in what the pieces can do and how the game is played.Like Western chess,the object of the game is to capture the other player’s“king”.____18____Unlike Western chess,instead of placing the pieces in squares,the pieces are placed on the intersections of lines.Another big difference between Western chess and Xiangqi is that there is a large empty space in the middle of the board,which the elephants cannot cross.____19____This space refers to an area in China where Liu Bang and Xiang Yu had alife-or-death war.The war was so influential that“the Chu River and the Han Boundary”has become a metaphor(隐喻)referring to any boundary between two opposing armies.When the Chinese people today see this name on the board,the battle scenes come to their mind,as though they are surrounded by thick smoke,able to hear the beating of war drums.____20____A．Actually,in Xiangqi he is just a general.B．During the Qing Dynasty its popularity grew.C．It is called“the Chu River and the Han Boundary”.D．The game is called Xiangqi,a game of ordinary people.E．There are many differences between Xiangqi and Western chess.F．An intense battle is about to take place on this small chessboard.G．Several sayings related to Xiangqi are in common use in China today.三、完形填空Baby Miloszek was born with a heart defect.His condition is too___21___to be treated in Poland,but doctors at Stanford University Medical Center are able to take his case.___22___,they require80percent of the payment up front,a sum the family simply could not ___23___.That’s when Maria Andrejczyk___24___in to help!Just a week after___25___her first Olympic medal ever and returning from Japan, Maria posted a___26___on a social media platform announcing that she would be auctioning(拍卖)off her silver medal to raise money for Miloszek’s surgery.Days later,a Polish convenience store called Zabka___27___the medal for$125,000,which was more than Maria had___28___to get.Not only that,but when her___29___ broke,more people began donating to the fundraiser.Finally,they pulled in more than enough cash to____30____Miloszek’s trip to America!And the____31____doesn’t end there. Zabka announced that they would let Maria keep her____32____after all!“We were very touched by the extremely____33____gesture of our Olympian,so we decided to support the____34____for Miloszek,”the company wrote.“We decided that the silver medal from Tokyo will stay with Maria!We are glad that we were able to____35____.”21．A．particular B．optimistic C．serious D．common 22．A．Undoubtedly B．Unfortunately C．Additionally D．Absolutely 23．A．expect B．permit C．ensure D．afford 24．A．crowded B．turned C．brought D．stepped 25．A．earning B．declaring C．battling D．defeating 26．A．parcel B．message C．card D．list 27．A．sold B．returned C．purchased D．saved 28．A．expected B．needed C．accepted D．commanded 29．A．health B．dream C．heart D．story 30．A．transport B．provide C．cover D．cost 31．A．kindness B．news C．sympathy D．gratitude 32．A．secret B．reputation C．money D．medal 33．A．flexible B．noble C．gentle D．awkward 34．A．recovery B．request C．fundraiser D．award 35．A．contribute B．present C．fight D．change四、用单词的适当形式完成短文阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。

端到端流程运行机制

端到端流程运行机制End-to-end process execution mechanism is a crucial aspect in ensuring the smooth flow of operations within a system. It involves the seamless integration of various stages in a workflow, from the initiation of a task to its completion. This mechanism plays a significant role in enhancing efficiency, reducing errors, and improving overall performance.端到端流程执行机制在确保系统内运营流程的顺畅进行中起着至关重要的作用。

它涉及将工作流的各个阶段从任务的发起到完成无缝集成在一起。

这种机制在提高效率、减少错误和改善整体性能方面发挥着重要作用。

One key aspect of the end-to-end process execution mechanism is the clear definition of roles and responsibilities within the workflow. This ensures that each individual or team involved in the process knows exactly what is expected of them and what their specific duties are. By clearly defining roles and responsibilities, the likelihood of tasks being duplicated or overlooked is significantly reduced, leading to a more streamlined and efficient process.端到端流程执行机制的一个关键方面是对工作流中角色和责任的明确定义。

下载提示

1、下载文档前请自行甄别文档内容的完整性，平台不提供额外的编辑、内容补充、找答案等附加服务。
2、"仅部分预览"的文档,不可在线预览部分如存在完整性等问题,可反馈申请退款(可完整预览的文档不适用该条件!)。
3、如文档侵犯您的权益，请联系客服反馈,我们会尽快为您处理(人工客服工作时间：9:00-18:30)。

End-to-end Model for a Flow in the Internet Muthaiah Venkatachalam

合集下载

全新版大学英语第二册答案习题习题Unit5-8

Unit 3话题七选五专项练习2022-2023学年高一英语必修第一册单元重难点易错题精练人教版

2023届河北省衡水中学高三上学期四调考试英语试题(含答案解析)

端到端流程运行机制

文档推荐

最新文档