On the Performance of Ad Hoc Routing Protocols under a Peer-to-Peer Application Leonardo B.Oliveira,Isabela G.Siqueira?,
Antonio A.F.Loureiro
Computer Science Department
Federal University of Minas Gerais,ICEx
Av.Ant?o nio Carlos,6627-Pampulha
Belo Horizonte,Minas Gerais,Brazil CEP:31270-010
Telephone:+55313499-5860,Fax:+55313499-5858
1Introduction
The recently introduced Peer-to-Peer(P2P)paradigm[1]is the basis for both Mobile Ad hoc Networks(MANETs)[2]and popular Internet P2P applications (e.g.SETI@home,Napster,Gnutella,Freenet).One of the most signi?cant characteristics of the P2P paradigm is the fact that central units,which are responsible for managing and meeting the needs of the network,are non-existent.In this model,nodes have equivalent functionalities and provision capabilities and,as a consequence,are called“peer”entities.Every peer is able to send and reply to request messages originated from each other.This shows the dual interface of these peers,since they might play the role of servers and clients simultaneously.That is the reason why they are also named“servents”(servers/clients).
Similar to the architecture on which they are based,MANETs and P2P appli-cations have recently attracted both research community and media attention. The growth of computing resources for mobile devices has been the key con-tributing factor for the focus on mobile ad hoc networks.Moreover,the launch of new applications–such as rescue team management in disaster situations or the exchange of information in battle?elds[3,4]–generates an increase in demand for networks without previous infra-structure.The spread out of P2P applications,on the other hand,can be attributed to their success as content sharing and distributed processing platforms[5,4]–where parallel applications run on available peers.
Based on the same paradigm,both P2P application networks–composed by a set of servers implementing a P2P application–,and MANETs have common characteristics and functionalities.In essence,both are self-organizing networks,have dynamic topology,and are responsible for routing queries in a distributed environment.Figure1(a)and Figure1(b)exhibit a MANET and a P2P Application diagram,respectively.
Because nodes in mobile ad hoc networks usually have low computing capac-ity and,therefore,are unable to play the role of servers all the time,–or even supply many clients simultaneously–a P2P application appears to be a powerful tool to spread information on this type of scenario.In other words, since a P2P application network does not possess a unique service provider at a certain time,but many servents that play this role,the assignment of distributed network tasks among nodes prevents them to become overloaded. In addition,we envision that some applications enabled by MANETs(e.g.res-cue team communication in disaster situations and exchange of information in battle?elds[4,3])will have each instance working in cooperation with the others(i.e.,sending and replying to queries like peers).For instance,a rescue team participant might require information about nearest neighbor location.
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Transmission Range
Out of range
Direction and velocity
Ad Hoc Connection
(a)A MANET
diagram (b)A P2P application diagram [6]
Fig.1.MANET and P2P application diagrams
That is true a central server could be responsible for store information,but this approach not only would be more expensive (this would require more hops and constant location updates),but also less resilient –a single point o failure is not desirable in rescue team situations and servers would be target of attacks in battle ?eld contexts.A novel diagram of a P2P application running over a MANET is shown in Figure
2.
Fig.2.A diagram of a P2P Application over a MANET
The main purpose of this work is to learn about the performance of ad hoc routing algorithms in a scenario in which a P2P application runs over a MANET.In order to accomplish the desired goals,we have conducted sim-ulation experiments of some well-known ad hoc routing protocols.The results help clarifying the di?erences between P2P applications and client/server ones showing the usability of such networks,and discovering possible improvements in MANET routing.
The Destination-Sequenced Distance-Vector Routing (DSDV)[7],the Dy-
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namic Source Routing Protocol(DSR)[8],and the Ad hoc On Demand Dis-tance Vector(AODV)[9]are the protocols evaluated in this work.The reasons for the choice are twofold.First,the nature of those algorithms is distinct. While DSDV is pro-active,DSR and AODV are reactive–though the last employs methods of the two formers.Second,the three have already been ex-haustively tested and validated[10].The results point out that each one of these protocols performed well in some scenarios yet had drawbacks in others. This con?rms the importance of considering characteristics of both application and network in order to have the best integrated solution.
The rest of this paper is organized as follows.Section2discusses the related work and Section3presents a comparison between MANETs and P2P ap-plication networks.Section4brie?y describes the routing protocols used in this work.In Section5,the P2P application is discussed.The simulation sce-narios are described in Section6and the performance evaluation metrics in Section7.Section8discusses the simulation results.Section9provides a dis-cussion about the P2P application in comparison with the client/server results. Finally,Section10presents our conclusions.
2Related Work
Only recently research community realized the synergy between MANETs and P2P networks.
Schollmeier et al.[6]and Borg[4]discuss similarities and di?erences of MANETs and P2P networks.The former focus mainly in routing aspects and the latter discusses content discovery,security,quality of service,etc.
Kortuem presented Proem[11,12],a middleware platform for developing and deploying P2P applications tailored to PANs(Personal Area Networks),a special class of MANETs.
Hu et al.[13]propose Dynamic P2P Source Routing(DPSR),a routing ad hoc protocol that integrates strategies used by DSR routing protocol and Pastry P2P protocol[14]to improve scalability.
Papadopouli and Schulzrinne[15,16]and Klemm et al.[17]present P2P data sharing systems tailored to MANETs namely Seven Degrees of Separation (7DS)and Optimized Routing Independent Overlay Network(ORION),respec-tively.7DS focus on enabling the exchange of data among peers not directly connected to the Internet by exploring peer mobility,while ORIOM concen-trates on?le sharing applications by setting up overlay routes on demand.
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Franciscani et al.[18]concentrate on minimizing the impact of the highly dynamic topology,obtained through the combination of P2P networks and MANETs,may have over network resources by proposing algorithms for con-?guration of these networks.In the algorithms,three combinations of neigh-borhood assignment are compared:1)regular,where P2P neighborhood corre-sponds to the physical neighborhood;2)random,where authors try to achieve the small-world[19,20]phenomenon by picking each neighbor at random among online peers;3)and hybrid,where links are built following a hierarchy and each peer communicates through an intermediate.
Ding and Bhargava[21]perform a theoretical comparison between P2P sys-tems over MANETs(broadcast over broadcast;broadcast;DHT over broad-cast;DHT over DHT;and DHT)and presents important results in O-notation. Nevertheless,they do not evaluate real P2P systems and do not taking into account practical aspects(e.g.,mobility and channel error).
3Comparison between MANETs and P2P Application Networks P2P applications and mobile ad hoc networks have several aspects in com-mon[22,23,11,4].Both MANETs and P2P application networks lack managing and centralizing units,since the network is established as soon as the partici-pants opt to interact with one another.The decision to connect to the network can be taken at distinct moments,so variance is constantly introduced in the environment.
Another similarity is their dynamic topology,which is a result of the constant changes in connections used by peers.In mobile ad hoc networks these alter-ations are mainly caused by node mobility.That is,as a node moves,it might leave the transmission range area of its current neighbors and has its links broken as a consequence.Thus,in order to reestablish contact with peer enti-ties,the peers must set new connections.Conversely,what causes the dynamic topology of P2P application networks is the low availability of peers.In this scenario applications are executed mostly over?xed networks and the main reason for link breakage is not the mobility of nodes,but the short session duration.
Curiously,because P2P applications are usually built over a network which is based on the client/server model,their networks present some characteris-tics that di?er from the P2P paradigm.MANETs,on the other hand,have their own communication mechanism and,therefore,are more faithful to the distributed model.
As previously mentioned,in the P2P architecture peers can communicate with
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one another without intervention of any centralized access point.Paradoxi-cally,P2P applications are,in fact,clients of services provided by external servers–such as DHCP(Dynamic Host Con?guration Protocol),DNS(Do-main Name Service),and Web servers.In MANETs,requests are really han-dled by any network participant.Another evidence that MANETs are more in conformity with the P2P paradigm than P2P application networks is the fact that in the former,the peers are only a single-hop away from their neighbors, whereas in the last,the neighbors are just logic ones and might be geographi-cally many hops apart.Typical di?erences between both technologies[22]are described in Table1.
Item MANET
Motivation for creating the network
create a physical infra-
structure to provide con-
nectivity
?xed medium and direct
Connection con?dence low(wireless connec-
tions)
any Internet point
Structure physical structure corre-
sponds to logical struc-
ture
only reactive algorithms
possible,reliable algo-
rithms not implemented
yet
Peer behavior mobile
virtual,multiple unicasts
4.1DSDV
The DSDV[7]is a variation of the Distance Vector Routing protocol modi?ed for ad hoc networks.The changes were performed in order to reduce looping properties that would be present in the original protocol.DSDV is a hop-by-hop routing and pro-active protocol that provides each node a routing table that lists the next-hop information for each reachable destination.Thus,it requires periodic broadcasting of routing updates and triggered beacon mes-sages,which leads to an increase in routing overhead.
4.2DSR
DSR[8]employs an on-demand approach regarding route discovery and main-tenance processes.The key di?erence of the protocol from other on-demand routing protocols is the fact that it adopts the source routing strategy-as its own name indicates.That is,the complete path from the source to des-tination is carried in each packet.Such path is discovered through routing query broadcasts.DSR also provides each node a route cache for decreasing the number of control messages sent.In order to update its respective caches, every intermediate node makes use of the source route information available in the packet it forwards.
The main advantage of the approach adopted by DSR is that no additional mechanism is necessary to detect routing loops.The disadvantage,clearly,is the overhead caused by the introduction of source routing information in the header of the data packet.
4.3AODV
The AODV[9]is a reactive protocol which combines both DSR and DSDV characteristics.It borrows the basic route discovery and route-maintenance of DSR as well as hop-by-hop routing,sequence numbers and beacons of DSDV. When a source node desires to establish a communication session,it initiates a route discovery process to locate the destination node,by generating a“route request”message,which might be replied by the intermediate nodes in the path to destination or the destination node itself.At the time of arrival,the “route reply”message contains the whole path to destination.To handle the case in which a route does not exist,or the query or reply packets are lost, the source node rebroadcasts the query packet if no reply is received by the source after a time-out.
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5Description of the P2P Implemented Protocol
In order to achieve the previously described purposes,it was required to im-plement a P2P application in the simulator.The adoption of special strategies was entailed,which would be dispensable in a client/server architecture.This is due to the P2P decentralization and dynamic nature and also to the role played by the servents in the P2P application network,which alternates from server to client and from client to server.
The implemented protocol is mainly based on Gnutella protocol,which is used for peer-to-peer communication in Gnutella decentralized?le-sharing system in the Internet.The main reason for choosing Gnutella protocol is the sim-plicity of its communication model.Since it was developed neither for best performance nor for best scalability,it is very suitable for evaluating net-work performance.Furthermore,Gnutella protocol is regarded as being able to adapt very well to dynamically changing peer populations,which is a very important characteristic.
Although Gnutella was taken as a reference,the protocol was altered for the simulator environment and also for ad hoc networks.The strategies adopted are described below.
5.1Joining the Network
A peer desiring to join the P2P application network starts by sending a broadcast-send message through the network in order to elect its“neighbors”, which may be used for message?ooding.The initial replies will settle virtual connections between the new servent and each answerer.Each one of these connections is maintained by an entry in a neighbors list which has prede?ned maximum size.It is important to notice that virtual neighbors are not equiv-alent to physical neighbors,although this is likely to occur at the beginning since answers of near hosts tend to arrive more quickly.
5.2Content Discovery
The fact that a P2P application network does not possess a server that cen-tralizes information complicates the task of locating data.In a client/server architecture,this is not a problem since the client knows the server address in advance.The strategy widely adopted,which has also been used in this work,is sending a query-send message through the network order to gather
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information.This message contains the required?le identi?cation–its name –and the identi?cation of the peer that is consulting,the“query-source”. The transmission of queries in the P2P application network is carried out through controlled?ooding.The servent that receives a query message will forward it in case the?le wanted is not stored in its node.The process goes on until the information source is found or the message is dropped due to a TTL (Time-to-Live)expiration.Whenever a source is located,i.e.,when a“query-hit”event occurs,the peer that owns the?le wanted(“?le-source”)sends a reply to the“query-source”peer validating its availability for?le transfer.
5.3Content Dissemination
After receiving the?rst reply,the“query-source”servent establishes an end-to-end communication with the“?le-source”.The?le is fragmented into small pieces and each piece is sent inside a pull-data message from the“?le-source”to the“query-source”.The service for transferring data is datagram,typical of wireless environments.
5.4Controlled Flooding
Each peer of the network maintains a cache in order to avoid duplicate query processing.This is possible since the query message is uniquely identi?ed by the pair(query-id,query-source).
The P2P message header has a TTL?eld to prevent a message being forwarded in?nitely in the P2P application network.The idea is similar to TTL?eld of the Internet Protocol(IP).The next hop of a node in P2P,though,might be another node which is not directly connected.
5.5Neighborhood Control
P2P application networks have a dynamic behavior,as mentioned before.Peers can leave or join the network at anytime they necessitate.This implies the employment of a special control scheme for maintaining an up-to-date list of neighbors.To solve the problem in the implemented protocol,all peers have to send periodical ping messages to their neighbors to check if they are still “alive”.When no answer is detected,i.e.,when a pong message is not received, the related peer is removed from the neighbor list and a broadcast-send message is sent to?nd another neighbor.
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5.6Message Size
Messages that circulate among peers may have a variable size and the maxi-mum value is210bytes,based on the Gnutella Protocol.Table2presents the message types sent by the peers with their respective functions and sizes.
Message Type Size(Bytes)
broadcast-send23
Answer a broadcast-send
ping23
Answer a ping
query-send26
Retransmit a query originated
by another peer
query-reply26
Require the transfer of a?le
pull-request210(maximum)
Table2
Messages transmitted in P2P Application Network
6Simulation Scenarios
In order to evaluate the accomplish the purposes of this work,we have con-ducted simulation experiments using the Network Simulator(ns-2)[24]and its CMU wireless and mobility extension[25].We have considered a set of default settings.Some of them were varied throughout the simulation experi-ments.The variations can give?ndings on the performance of the algorithms for diverse scenarios.
The default settings taken into consideration are the following.It was con-structed a200×200m2topology composed of40mobile nodes,12of which implementing a single instance of the P2P application.
The mobility scheme employed was the random way point(since it is frequently used for individual movements[10]).We have chosen as default settings a pause-time of50seconds and a maximum speed of0.5m/s.
The transmission range of all nodes was set to50m.The radio propagation
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model chosen was the Shadowing Propagation Model with a rate of95%of correct reception within the range area.The IEEE802.11was the protocol used in the MAC layer,with2Mbits/s of bandwidth.The radio interface chosen was the914MHz Lucent WaveLAN.The total simulation time for all scenarios was set to be300s.
In respect to P2P application parameters,the maximum size of the neighbors list,for neighborhood control,was set to3.Also,the initial number of?les per peer was set to be10.The choice of the initial?le names as well as their sizes follow the normal distribution model.The average?le size is adjusted to10KB.This reduced value was estimated taking into account the low bandwidth of mobile scenarios,as well as memory and energy constraints of mobile devices.
For controlled?ooding,the TTL of the query-send messages were set to3–large enough for queries to reach most of the peers in the simulated scenario. During the simulation,10searches are scheduled for each peer.The scheduling time is uniformly distributed and at each time the search is carried out only if the peer is part of the P2P network at the moment.
The choice of the?le to be searched follow the normal distribution model. The ping messages were sent with a default rate of6per minute and the pong messages were waited for no longer than10s.The broadcast-send interval time was2s.
In the simulation,nodes start with100Joules of energy each.The power loss for transmission was set to0.330Watts and0.230Watts for reception[26].
During the simulation,both the peer entrance time and the exit time were uniformly distributed in order to simulate the dynamic topology of the P2P application network.
Each simulation was run33times,with di?erent seeds for the random number generator,on ns-2.1b8a[24]and its CMU wireless and mobility extension[25]. The results represent the average values on the33runs.
7Evaluation Metrics
The performance evaluation of ad hoc routing protocols supporting a P2P application examined four metrics:workload,mobility,network density and peer quantity.The metrics were chosen considering its signi?cance for each evaluation parameter.
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7.1Workload
It shows the workload introduced into the network by the P2P application. Its increase might place undesirable changes in network performance,such as latency,packet dropping and control overhead.The term latency suggests the amount of time spent for a speci?c event to happen,such as a query-hit or the reception of a response.The overhead was measured only in terms of packets, since the cost to access the medium to transmit a packet is signi?cantly more expensive than the cost of adding a few extra bytes to an existing packet. As a consequence,the network may not provide a good service for the appli-cation.We have chosen to vary the total number of queries generated by P2P peers(1,10,100,and1000)as well as the average size of the?les transferred through the network(1,10,100,and100KB)with the aim of investigating the protocols scalability.The results,for low,medium and high workload,are pre-sented in Section8.The following metrics were evaluated:number of initiated ?le transfers,throughput,percentage of queries not responded,delivery rate, energy consumption,and routing overhead associated to the ad hoc network.
7.2Mobility
It represents the speed and pause time applied to the ad hoc nodes.Simulation experiments considering mobility were conducted,and the protocol capability in adapting to distinct speed(0.1,0.5,2.5,and10m/s)and pause time(0, 60,120,180,240,and300s)of node values was analyzed.The metrics chosen were path length,connectivity among application peers,and latency.
7.3Network Density
It a?ects greatly the performance of the ad hoc network,and,therefore,is an important point of analysis.Simulation experiments considering di?erent values of transmission range(1,10,100,and100m)and number of nodes (0,20,40,60,and80)that populate the network were performed.In the last case,the number of peers was maintained at30%of the total number of nodes.Percentage of queries not responded,path length,latency,routing overhead,connectivity among application peers,and delivery rate were the metrics employed to evaluate the three protocols.
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7.4Peer Quantity
In order to investigate the in?uence of the amount of peers over the protocols, the number of nodes was left unmodi?ed and the number of peers was varied (10,20,30,and40).This type of analysis is important because it demon-strates the scalability of the ad hoc routing protocols taking into account the number of application instances that run over the network.This is essential for selecting the best algorithm in case of deploying a P2P application.The metrics chosen were routing overhead,latency,path length,throughput,and energy consumption.
8Simulation Results
This section presents the results according to the four metrics described above.
8.1Workload
The three routing protocols introduced distinguishing amounts of overhead when the number of queries by a node was varied.As shown in Figure3(a), the DSDV exhibits the most overhead,followed by AODV and then DSR.The former,for one query,introduced ten times more control packets than DSR. Comparing DSDV to AODV and DSR on-demand protocols,the considerable increase in overhead obtained was due to route update messages that are constantly triggered by DSDV.Although DSDV produced more overhead,it demonstrated to have a steady behavior considering workload increase.The others,in contrast,did not suggest to be as scalable–the DSR overhead, speci?cally,duplicated from one extreme of the x axis to the other.
DSDV was the protocol which consumed the greatest amount of energy for lower,medium,and higher loads,as depicted in Figure3(b).This is a result of the higher number of control packets sent and received by the nodes. Figure4(a)depicts the fraction of messages delivered to the application,as the shared?le sizes were incremented.For all protocols,the curves assumed almost identical shapes.It can be noticed that while for1KB?les the delivery rate is higher than90%,for1000KB practically all packets were dropped.This is due mainly to the low bandwidth available in the ad hoc network.
The results for initiated?le transfers indicate that for all protocols the best performance is achieved when the average?le size is10KB.This metric is
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N u m b e r o f P a c k e t s Number of Queries (a)Routing Overhead
C o n s u m e d E n e r g y (J )Number of Queries (b)Energy Consumption
Fig.3.Number of Queries Variation
D e l i v e r y R a t e (%)File Size (KB)
(a)Delivery Rate for Application N u m b e r o f P a c k e t s File Size (KB)(b)pull-data Messages Received
Fig.4.File Size Variation
represented by the number of pull-request messages received in Figure 4(b),which can also be considered a result of throughput.
On the whole,DSDV performed better for extremely high loads.It obtained the lowest number of queries not responded,the highest throughput and more ?les successfully transferred.From Figure 4(b)it is clear that DSR and AODV did not support the application requirements,in contrast to DSDV.This is due to the huge congestion generated,which caused di?culties for them to ?nd routes on demand.
8.2Mobility
Figures 5(b)and 5(a)show the behavior of the connections among peers.Is is noticed that none of the protocols had a remarkable performance compared to the others.For lower mobility,the average number of neighbors and the
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amount of ping messages sent were reduced,while the number of broadcast-send messages and the number of queries not responded grew for all protocols.That is,DSDV,DSR and AODV produced more information unavailability and worse P2P connectivity.
N u m b e r o f N e i g h b o r s Pause Time (s)
(a)Neighbors
Q u e r i e s n o t R e s p o n d e d (%)Pause Time (s)(b)Queries not Responded
Fig.5.Pause Time Variation
As the mobility was incremented,surprisingly,the connectivity degree rose (see Figure 5(a)).This apparently anomalous behavior was mainly caused by the partitioning of the network.When the mobility is low,the network might isolate peers during the whole simulation,whereas in a higher mobility scenario these partitions are eliminated because of a peer movement.As a result,for high pause-time values,i.e,for low mobility,the number of queries not responded is
also high.
Paradoxically,Figure 6(a)demonstrates that the increase in speed did not have signi?cant in?uence after 2.5m/s.It is important to observe,though,that the DSDV and AODV curves stabilized earlier than the DSR curve.
H o p s Speed (m/s)
(a)Hops for Finding Information L a t e n c y (s )Speed (m/s)(b)Latency for Query-Hit
Fig.6.Speed Variation
Figure 6(b)shows the time elapsed for a query-hit to happen after the query-
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send message was sent.Both DSDV and AODV protocols had similar behav-iors and showed to be insensitive to the node speed,whereas DSR was very sensitive to the node speed.
DSR was the protocol which presented the highest number of hops and latency,when mobility was increased.The term hops suggests the average amount of hops for a query to reach an information source.Due to its source routing nature,in case nodes move at high speeds,a route generated might become outdated,even at the time when the packet is traversing the network from the source to the destination.As a result,more time and hops are consumed with routing.
8.3Network Density
Concerning routing overhead,as shown in Figure 7(a),DSDV was badly af-fected by the increase in the number of the network nodes,as it requires periodic routing updates and broadcasting of triggered beacon messages.In contrast,this scenario modi?cation did not in?uence the other two protocols,which indicated to scale gracefully.Curiously,this performance declination did not appear when the transmission range was extended.
N u m b e r o f P a c k e t s Number of Nodes
(a)Routing Overhead L a t e n c y (s )Number of Nodes
(b)Latency for Receiving a query-
reply
Fig.7.Nodes Variation
The three protocols behaved equivalently for both number of nodes and trans-mission range variation with respect to connectivity among application peers.The protocols had their latency intensi?ed for a denser network,as presented in Figure 7(b).Particularly,DSDV appears to be less scalable regarding this metric due to its routing overhead,as previously highlighted.
Regarding both number of queries not responded and network delivery rate,DSDV,DSR,and AODV performances were similar.The former protocol,
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despite producing more routing overhead,managed to maintain the same de-livery rate for a denser network.Figure 8(a)shows the results obtained for the delivery rate.
In respect to path length,it was observed that this metric is very dependable on network density,as shown in Figure 8(b).The highest average of hops and forwarded packets was o?ered by DSR and DSDV protocols,for denser and less dense networks,respectively.The former result can be easily explained,as DSR does not take into account path optimality when routes are generated.The last,though,can be considered a positive result,since the others obtained nearly zero average hops.In other words,this result means that DSDV is the only protocol that really delivers packets and provides support to the P2P application layer in less dense scenarios.Regarding the curve shapes of the three protocols,a change in the behavior could be detected.At this point,the number of hops falls suddenly,as a consequence of the proximity of the desired information.That is,when the number of existent nodes in the network is higher,it is more likely that the required information is stored on a near or easily reachable node.Furthermore,when the transmission range is expanded,the packets predictably tend to arrive in the destination with less hops.
D e l i v e r y R a t e (%)Number of Nodes
(a)Delivery Rate for Application H o p s Transmission Range (m)(b)Hops for Finding Information
Fig.8.Nodes and Transmission Range Variation
8.4Peers
Figure 9(a)indicates that the DSR protocol needs more hops to ?nd informa-tion than the others (nearly 2times more hops than AODV,in the worst case),in agreement with the previously described results.Nevertheless,the shape of the curves is similar for the three protocols.When the network is populated with less instances of P2P applications,the desired information tends to be found in a fewer number of P2P hops.Also in this case,the amount of P2P neighbors of a peer is lower,since the number of reachable peers is lower as well.As a result,the network is likely to become partitioned,and in the rare
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cases in which the information is found,it will be located in one or two hops apart.The growth in the number of peers,by contrast,may expand the route lengths of the P2P application layer,allowing information to be found in a greater amount of P2P hops,and obviously the same for network hops.After a certain point in the increase of peers,the number of neighbors reached its maximum value and no more in?uence was detected.
H o p s Number of Peers (a)Hops for Finding Information
C o n s u m e d E n e r g y (J )
Number of peers (b)Energy Consumption
Fig.9.Peers Variation
All protocols were a?ected equivalently by the throughput.AODV was re-sponsible for the best performance concerning routing overhead,while DSDV,as usual,generated more routing control packets.AODV also achieved better results respecting time.DSR,in contrast,presented the highest latency not only because it does not provide an optimal path,but also due to the fact that packets to be transmitted are held in its bu?er until the path to destination is found.
Finally,Figure 9(b)presents the energy consumption.It is possible to observe that the shape of the curves for all the protocols evaluated was similar,con-sidering the increasing of peers.AODV,however,provided less consumption (0.35J less,approximately),since it possess the lower overhead.
9Comparison between P2P and Client/Server Applications
In this section a performance comparison between applications based on P2P and client/server paradigms is presented.The results concerning the client/server application were obtained mainly from [10].
First,regarding mobility,P2P and client/server applications exhibit consid-erable di?erences.Unlike the client/server model,in which the shortest path was obtained by DSR and DSDV in the simulated application,AODV was the protocol that presented the best performance,delivering the queries with the
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lowest hops average.This result shows that the merge between hop-by-hop routing of DSDV and route-discovery of DSR is less a?ected by the mobility property when a P2P application is considered.
Second,in both paradigms DSDV demonstrated to have the highest and stead-iest overhead.However,the discrepancy between it and the other protocols is higher with the P2P application execution.It happens due to the growth in the topology dynamism caused by this type of application.Many link break-ages occur and,as a result,a great amount of update messages need to be triggered.Despite the fact that AODV still has more overhead than DSR, the separation between the performance of both has decreased60%at most, comparing with the results of a client/server application.
In respect to the delivery rate,the network performance supporting a P2P application presented a worse result.With the increase in the amount of nodes, the rate achieved at most80%,whereas in client/server applications,it was achieved nearly100%.
The most interesting result,possibly,was obtained with the mobility varia-tion.Contrary to the scenarios that run applications based on a client/server model,the results achieved in this work reveal that P2P applications are not suitable for low mobility scenarios.First of all,it is important to emphasize that this kind of distributed applications are built to function in high dynamic scenarios,so it is reasonable that they present a low performance in more sta-ble scenarios.Furthermore,when a peer moves,despite being likely to lose physical connections in the ad hoc network level,it might not only maintain its P2P application links,but also establish others.And since the peers are highly dependable on their set of neighboring peers to communicate with the rest of the P2P application network,an increase in their amount of neighbors becomes an advantage.
10Conclusion and Future Work
In the last few years,Mobile Ad hoc Networks and Peer-to-Peer applications have started to be deployed,leading to a greater interest in the network com-munity due to their distinct characteristics from traditional networks.Both MANETs and P2P applications have several points in common since they are based on the same model.Therefore,it is natural to study both MANETs and P2P applications together.In this paper we conducted a detailed study of a Gnutella-like application running over a Mobile Ad hoc Network where three di?erent routing protocols were considered.It is interesting to notice that each of the protocols that was analyzed performed well in some scenarios for some metrics yet had drawbacks in others.This conclusion shows the importance of
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identifying more precisely characteristics of the P2P application itself(work-load,peer quantity)and characteristics of the network and mobile devices (mobility,network density)before committing to a particular protocol. Future work will focus on improving performance of P2P applications over MANETs.Some of the strategies,which may be adopted are:modify existent ad hoc protocols or even propose other ones,use multicast protocols to dis-seminate information,and develop a middleware so that one layer–network or application–can take better advantage of the services of the other layers. References
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实验一 常用仪器的使用 一、实验目的 （1） 了解双踪示波器、函数信号发生器、数字万用表的原理框图和主要技术指标。 （2） 掌握用双踪示波器测量信号的幅度、频率和相位。 （3） 掌握万用表的正确使用方法。 二、实验仪器 （1） 双踪示波器; （2） 低频信号发生器； （3） 数字式（或指针式）万用表。 三、实验原理 在电子技术实验里，测试和定量分析电路的静态和动态的工作状况时，最常用的电子仪器有：示波器、低频信号发生器、直流稳压电源、晶体管毫伏表、数字式（或指针式）万用表等。它们之间的连接方式如下图所示。 输出信号 图1-1电子技术实验中测量仪器、仪表连接框图 示波器：用来观察电路中各点的波形，以监视电路是否正常工作，同时还用于测量波形的周期、幅度、相位差及观察电路的特性曲线等。 函数信号发生器：为电路提供各种频率和幅度的输入信号。 直流稳压电源：为电路提供电源。 数字式（或指针式）万用表：用于测量电路的静态工作点和直流信号的值等。 四、实验内容及步骤： 熟悉仪器（仪器使用简单步骤见附录） 1.学会正确使用函数信号发生器 2.学会正确使用数字示波器 3.熟悉并学会使用数字式万用表 4.熟悉模拟电路实验箱 五、实验步骤
1、使用函数信号发生器输出频率的调节方法 （1）使用Sine按键，波形图标变为正弦信号，并在状态区左侧出现“Sine”字样。按Sine → 频率/周期→ 频率，设置频率参数值。配合面上的“频率调节”旋钮可使信号发生器输出频率在1HZ~10MHZ的范围改变。 屏幕中显示的频率为上电时的默认值，或者是预先选定的频率。在更改参数时，如果当前频率值对于新波形是有效的，则继续使用当前值。若要设置波形周期，则再次按频率/ 周期软键，以切换到周期软键（当前选项为反色显示）。 使用数字键盘，输入所需的频率值。直接输入所选参数值，然后选择频率所需单位，按下对应于所需单位的软键。也可以使用左右键选择需要修改的参数值的数位，使用旋钮改变该数位值的大小。 （2）根据手册通过设置频率/周期、幅值/高电平、偏移/低电平、相位，可以得到不同参数值的正弦波。 2、双踪示波器的使用 (1)使用前的检查与校准 （2）交流信号电压幅值的测量 使低频信号发生器信号频率为1kHz、信号幅度为5V，适当选择示波器灵敏度选择开关“V/div”的位置，使示波器屏上能观察到完整、稳定的正弦波，则此时上纵向坐标表示每格的电压伏特数，根据被测波形在纵向高度所占格数便可读出电压的数值，置于表1-1 中要求的位置并测出其结果记入表中。 注意：若使用10：1 探头电缆时，应将探头本身的衰减量考虑进去。 (3) 交流信号频率值的测量 将示波器扫描速率中的“微调”置于校准位置，在预先校正好的条件下，此时扫描速率开关“t/div”的刻度值表示屏幕横向坐标每格所表示的时间值。根据被测信号波形在横向所占的格数直接读出信号的周期，若要测量频率只需将被测的周期求倒数即为频率值。按表1-5 所示频率，由信号发生器输出信号，用示波器测出其周期，再计算频率，并将所测结果与已知频率比较。
检测机构一览表 序号机构名称备注 1. C-001 信息产业部第四研究所（电子工业安全与电磁兼容检测中心） 2. C-002 信息产业部第三研究所（国家广播电视产品检测中心） 3. C-003 上海市电子仪表标准计量测试所 4. C-004 中国赛宝实验室 5. C-005 广州日用电器检测所 6. C-006 中国家用电器研究院 7. C-007 机械工业电线电缆质量检测中心(北京) 8. C-008 上海电缆研究所 9. C-009 上海电器设备检测所 10. C-010 成都电气检验所 11. C-011 上海电气器具检验测试所 12. C-012 国家电光源质量监督检验中心（北京） 13. C-013 国家灯具质量监督检验中心（上海） 14. C-014 中国泰尔实验室 15. C-015 信息产业部通信计量中心 16. C-016 上海市劳动保护科学研究所特种电器检测站 17. C-017 上海出入境检验检疫局电器检测室 18. C-018 浙江检验检疫技术中心 19. C-019 江苏检验检疫机电产品检测中心 20. C-020 深圳出入境检验检疫局工业品检测技术中心 21. C-021 沈阳出入境检验检疫局电子电器产品检测中心 22. C-022 深圳电子产品质量检测中心 23. C-023 广州出入境检验检疫局电气安全实验室 24. C-024 (C-205) (Q-012) 国家质量技术监督局广州电气安全检验所 25. C-026 浙江方圆检测股份有限公司 26. C-027 (SL-036) 福建省中心检验所 27. C-028 上海市产品质量监督检验所 28. C-029 四川省产品质量监督检验所 29. C-030 成都市产品质量监督检验所 30. C-031 深圳市计量质量检测研究院 31. C-032 (SL-007) 大连市产品质量监督检验所 32. C-033 山东省计量科学研究所
一、常见仪器的分类 一般根据仪器的主要用途的不同，可将常见化学实验仪器分为下列8类： （一）计量类 用于量度质量、体积、温度、密度等的仪器。这类仪器中多为玻璃量器。主要有滴定管、移液管、量筒、量杯等。 （二）反应类 用于发生化学反应的仪器，也包括一部分可加热的仪器。这类仪器中多为玻璃或瓷质烧器。主要有试管、烧瓶、蒸发皿、坩埚等。（三）容器类 用于盛装或贮存固体、液体、气体等各种化学试剂的试剂瓶等。（四）分离类 用于进行过滤、分液、萃取、蒸发、灼烧、结晶、分馏等分离提纯操作的仪器。主要有漏斗、分液漏斗、蒸发皿、烧瓶、冷凝器、坩埚、烧杯等。 （五）固体夹持类 用于固定、夹持各种仪器的用品或仪器。主要有铁夹、铁圈、铁架台、漏斗架等。 （六）加热类 用于加热的用品或仪器。主要有试管、烧杯、烧瓶、蒸发皿、坩埚等。 （七）配套类
用于组装、连接仪器时所用的玻璃管、玻璃阀、橡胶管、橡胶塞等用品或仪器。 （八）其它类 不便归属上述各类的其它仪器或用品。 二、中学化学常见仪器的名称和使用 （一）计量仪器 1．量杯 量杯属量出式（符号Ex）量器，它用于量度从量器中排出液体的体积。排出液体的体积为该液体在量器内时从刻度值读取的体积数。 量杯有2种型式。面对分度表时，量杯倾液嘴向右，便于左手操作，称为左执式量杯。倾液嘴向左，则称为右执式量杯。250 mL以内的量杯均为左执式，500 mL以上者，则属于右执式。 2．温度计 温度计是用于测量温度的仪器。其种类很多，有数码式温度计，热敏温度计痔。而实验室中常用为玻璃液体温度。 温度计可根据用途和测量精度分为标准温度计和实用温度计2类。标准温度汁的精度高，它主要用于校正其它温度计。实用温度计是指所供实际测温用的温度计，主要有实验用温度计、工业温度计、气象温度计、医用温度计等。中学常用棒式工业温度汁。其中酒精温度计的量程为100℃，水银温度计用200℃和360℃2种量程规格。 使用注意事项
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一般过去式 时间状语：yesterday just now (刚刚) the day before three days ag0 a week ago in 1880 last month last year 1. I was in the classroom yesterday. I was not in the classroom yesterday. Were you in the classroom yesterday. 2. They went to see the film the day before. Did they go to see the film the day before. They did go to see the film the day before. 3. The man beat his wife yesterday. The man didn’t beat his wife yesterday. 4. I was a high student three years ago. 5. She became a teacher in 2009. 6. They began to study english a week ago 7. My mother brought a book from Canada last year. 8.My parents build a house to me four years ago . 9.He was husband ago. She was a cooker last mouth. My father was in the Xinjiang half a year ago. 10.My grandfather was a famer six years ago. 11.He burned in 1991
第27卷 第5期 武汉理工大学学报?信息与管理工程版 Vol .27No .52005年10月 JOURNAL OFWUT (I N FORMATI O N &MANAGE MENT ENGI N EER I N G ) Oct .2005文章编号:1007-144X (2005)05-0116-04 收稿日期:2005-05-15. 作者简介:孟芳兵(1977-),男,陕西宝鸡人,武汉理工大学信息工程学院硕士研究生.基金项目:教育部重点科技攻关资助项目(03120). 基于移动Ad Hoc 网络的OL SR 路由改进协议 孟芳兵 (武汉理工大学信息工程学院,湖北武汉430070) 摘 要:提出了一种改进的OLSR 路由协议,改进后的协议能根据本地网络拓扑结构自适应改变HE LLO 消息和TC 消息的发送频率。仿真结果表明,改进后的协议在网络吞吐量和数据传送成功率等性能上有明显提高。 关键词:移动Ad Hoc 网络;路由协议;OLSR;自适应改进中图法分类号:TP393 文献标识码:A 1 引 言 移动Ad Hoc 网络的概念出现于 20世纪70年代,近年来由于便携式设备在移动性、储存能力和无线通信能力等方面的迅速发展,使得不需要固定基站支持的Ad Hoc 网络得到了广泛应用。Ad Hoc 网可以在不能利用或不便利用现有网络 基础设施的情况下提供一种通信支撑环境,因此对于军用网有着特殊意义。另外它可用于偏远地区的援救系统,或者是需要快速扩展覆盖范围的网络,如大型建筑工地。在大型会议中与会者也可利用笔记本电脑或掌上电脑在Ad Hoc 网络环境下互相传递信息。 图1描述了一个简单的移动Ad Hoc 自治网络。 图1 移动Ad Hoc 网络示意图 可见,移动Ad Hoc 网络是一种无基站支持的多跳无线网络,具有高度动态变化的拓扑结构并且网络中各节点可任意移动,各移动终端利用无线技术传送数据包。由于无线传输覆盖范围的限制,网络中任意2个节点间可能不存在直接的通 信链路,因此分组需要网络中间节点的中转才能实现通信,这使网络中各节点具有别于常规网络的重要特点,即在Ad Hoc 网中,每个用户终端(每个节点)兼备路由器和主机2种功能。因此对路由的研究在移动Ad Hoc 网络中显得尤为重要。 2 OL SR 路由协议 OLSR 路由协议 [1] 是由I ETF MANET (Mobile Ad Hoc Net w ork )工作组提出的一种表驱动式的 链路状态路由协议 [2] ,节点之间需要周期性地交 换各种控制信息,通过分布式计算来更新和建立自己的网络拓扑图,被邻节点选为多点中继站MPR (Multi point Relay )的节点需要周期性地向网络广播控制信息。控制信息中包含了把它选为MPR 的那些节点的信息(称为MPR Select or ),只有MPR 节点被用作路由选择节点,非MPR 节点不参与路由计算。OLSR 还利用MPR 节点有效地广播控制信息,非MPR 节点不需要转发控制信息。 OLSR 主要采用HE LLO 分组和T C 分组方式 控制分组。HELLO 用于建立1个节点的邻居表,其中包括邻居节点的地址以及本节点到邻居节点的延迟或开销,OLSR 采用周期性地广播HE LLO 分组来侦听邻居节点的状态,同时HELLO 分组用于计算该节点的MPR,HELLO 分组只在邻居节点范围内广播,不能被转发;与HELLO 分组方式相反,T C 分组必须被广播到全网,在TC 分组中包
泰尔终端实验室（CTTL-terminals）成立于2014年元月1日，行政隶属于工业和信息化部电信研究院（CATR）。泰尔终端实验室是在电信研究院相关检验机构不断发展和融合的基础上诞生的，现在的实验室是由工业和信息化部电信研究院所属的电信传输研究所（RITT）、通信计量中心（TMC）通过业务重组和资源优化整合而重新成立的。泰尔终端实验室是集信息通信技术发展研究，信息通信产品标准、测试方法、通信计量标准、计量方法研究，以及国内外产品的测试、验证、技术评估、测试仪表计量以及通信软件的评估、验证为一体的高科技组织。实验室现有员工400余人，博士后1人，博士20人，具有硕士以上学位者超过百人。实验室面积超过5万平方米，仪器仪表设备资产在7亿元人民币。实验室定位以电子信息通信产业为主要领域，以市场需求为导向，向政府组织、行业组织、运营企业、产品设计制造业以及社会公众提供全方位技术支撑和服务，实验室的主要任务是电子信息通信科学新技术发展研究、相关标准研究与制定、技术可行性和标准符合性技术实验和测试。实验室建立了以ISO/IEC 17025 为标准的质量体系，并通过了中国合格评定国家认可委员会（CNAS）以及德国国家认可机构有限公司（DakkS）等国内外认证组织的认证。实验室现在已经获得PTCRB、GCF、CCF、CTIA、WIFI alliance、SIG、USB IF、NFC forum等行业组织的授权，也获得了Orange、Vodafone、中国移动、中国电信及中国联通等运营商的授权，同时也是CCC、FCC、CE等授权实验室。实验室专注从事终端产品及相关领域的检测与认证工作，其检测能力覆盖了各类电子信息通信终端、多媒体通信设备、电子产品及芯片、汽车电子，测试能力包括：一致性、电磁兼容、天线、生物电磁学、电气安全、信息安全、软件验证、短距离通信的相关接口、节能环保、有害物质检测等。实验室已成为中国面向国内外的综合性、规模化电子信息通信设备检验和试验的基地。 China telecommunication Technology Labs-Terminals(CTTL-terminals) was founded on Jan. 1st, 2014, under the supervision of China Academy of Telecommunication Research of MIIT (CATR). Based on the development and merging of related testing bodies of CATR, the present CTTL-terminals was reestablished by an amalgamation of the Research Institute of Telecommunication Transmission (RITT) and the Telecommunication and Metrology Center(TMC), by means of business reengineering as well as optimization and integration of resources. CTTL-terminals is a high-tech organization being engaged in research on the following fields: development of information and communication technology(ICT), standards and testing methods for information and communication products, telecommunication metrolog y standards and methods; as well as in missions of testing, verification, technology
化学实验教学中心功能 根据学校教务处和设备处等相关部门的指导性意见，配合学院发展的需要，原材料科学与化学工程学院的中心实验室更名为化学实验教学中心，并设立三个分实验室：基础化学实验室、专业化学实验室、理化检测中心，共同承担了教学、科研、地方服务的任务。各分实验室的主要功能分述如下： 基础化学实验室： 本分室主要承担校级公共平台（生命、环境、医学等专业）化学实验类课程、院内化学专业和应用化学专业的基础化学实验类课程的教学工作，包含大学化学、普通化学、无机及分析化学、无机化学、分析化学、有机化学、物理化学、仪器分析等实验课程。 其中，大学化学实验课程面向07年学校大类平台招生改革而设置，给自然科学大类的一年级学生开设。普通化学实验课程为工科类一年级学生开设。无机及分析化学实验、有机化学实验为生命类和医学类一年级学生开设。这四门实验课程所开设的实验项目以无机化学和分析化学相关的基础项目为主，包含少量的简易综合性和趣味性实验，以培养学生宽基础的知识体系和实验能力，并辅以激发学生的学习兴趣。 无机化学、分析化学、有机化学、物理化学的实验课程为学院内本科生的基础课程。同时承担基础实验教学、开放实验和学生毕业论文设计实验，其开出的实验项目分为基础性、综合性和设计性三个层次，部分包含选做实验，根据不同的教学情况进行选择。分述如下：无机化学实验是化学专业学生的第一门必修的、独立的基础实验课。它是以实验为手段来研究无机化学中的重要理论、典型元素及其化合物的变化规律，以及相应的仪器、装置、基本操作的一门课程。着力于培养学生具有宽广的基础知识和熟练的基本技能、能够适应未来社会发展需要的专业人才。教学内容着眼于为学生今后的学习发展奠定基础。学生在学习无机化学专业理论知识的同时，通过实验研究活动，学习和掌握无机化学专业的基本实验技术，研究元素的单质及其化合物的重要性质，熟悉重要无机化合物的制备方法；加深理解和掌握无机化学基本理论和基础知识；比较牢固地掌握化学实验的基本知识和操作技能；培养学生严谨的科学态度；培养学生准确观察化学反应现象，处理实验数据的能力，达到训练学生基本理论知识的综合应用能力；培养学生分离、分析与鉴别物质，合成、制备物质及将所学知识与生产实际结合起来的能力。 通过分析化学实验的学习，学生可以掌握定量化学分析及可见吸光光度分析和部分仪器分析实验的基本知识、基本操作和典型的分析方法；通过实验加深对有关理论的理解，并能灵活运用所学的理论知识指导实验设计与操作；确立“量”的概念、“相对误差”的概念和“有效数字”的概念；培养严谨的科学作风和良好的实验素养，激发实验兴趣和探索精神，提高分析问题和解决实际问题的能力。 有机化学实验是化学专业本科的一门基础实验课程。其主要目的是：通过有机化学实验验证、巩固和深入理解所学的有机化学理论知识；通过实验，使学生正确地掌握基础化学实验的基本操作方法和技能技巧，培养学生独立工作和独立思考的能力，养成严谨的科学态度和良好的科学思维方法。为后续课程的学习、为培养合格的化学教学工作者和化学化工技术人才打下扎实的基础。 通过物理化学实验课程的学习，使学生巩固物理化学理论课中所学习的基本概念、基本理论；掌握通用仪器的基本操作，掌握物理化学中常用的基本实验方法和实验技能，为学生今后做专业基础实验，专业实验和毕业论文打下坚实的基础。
民勤县第三中学化学实验室简介 民勤三中现有化学实验室1个，其中仪器室1个，准备室一个，学生实验室2个。各室的水电到位，布局合理，有防火、防盗、换气设备，双人双锁管理，仪器设备按省级标准配置齐全。实验室完全达到两名学生一组进行分组实验的条件。 实验室设有演示台，供电到位，实验室的演示台和学生实验桌采用防酸碱阻燃面板，长、宽、高及材质符合要求。教师演示台有电源总控制设备，集中控制学生实验电源，配有触电保护器。化学实验室演示台和学生实验桌旁设置水槽，实验室内悬挂有名言警句。仪器室与实验室毗邻设置，配备满足仪器存放的仪器橱。对危险品和毒品有完整的管理和领用制度，配有准备台、工具箱（常用类）等。所有仪器按配备标准顺序上架、入柜。教学演示实验及学生分组实验能全部开出。此外，学校还经常及时增补教学实验材料及仪器设备，满足教师演示及学生分组实验的需求。所有实验室采光良好，灯管垂直黑板安装；保持自然通风。 实验室实物流水账、管理明细账记录规范；仪器存放、分类、编号、贴签入柜，摆放科学有序，存取方便；药品、仪器分室存放。 实验教学坚持做到期初有计划、期末有总结，各项制度、实验员工作职责都张贴在墙上，以便经常对照学习。实验的周计划，每学期初就公布上墙，做到合理安排，一周工作早知道。学生进入实验室必须遵守学生实验规则，实验结束要填好实验记录单。仪器损坏按有关制度赔偿。各室登记台帐，既有电子台账，又有纸质台账，做到帐、
物、卡相符，并及时做好新增仪器的登记和仪器的报损工作。实验室工作以教学为中心，以提高教学质量为目的，加强实验教学环节。保证紧密配合教学工作。服务教学一线，结合新课程要求，开足开齐演示实验、学生实验和探究性实验，演示实验开出率达100%，分组实验的开出率达85%以上。
●I wonder if it’s because I have been at school for so long that I’ve grown so crazy about going home. ●It is because she wasn’t well that she fell far behind her classmates this semester. ●I can well remember that there was a time when I took it for granted that friends should do everything for me. ●In order to make a difference to society, they spent almost all of their spare time in raising money for the charity. ●It’s no pleasure eating at school any longer because the food is not so tasty as that at home. ●He happened to be hit by a new idea when he was walking along the riverbank. ●I wonder if I can cope with stressful situations in life independently. ●It is because I take things for granted that I make so many mistakes. ●The treasure is so rare that a growing number of people are looking for it. ●He picks on the weak mn in order that we may pay attention to him. ●It’s no pleasure being disturbed whena I settle down to my work. ●I can well remember that when I was a child, I always made mistakes on purpose for fun. ●It’s no pleasure accompany her hanging out on the street on such a rainy day. ●I can well remember that there was a time when I threw my whole self into study in order to live up to my parents’ expectation and enter my dream university. ●I can well remember that she stuck with me all the time and helped me regain my confidence during my tough time five years ago. ●It is because he makes it a priority to study that he always gets good grades. ●I wonder if we should abandon this idea because there is no point in doing so. ●I wonder if it was because I ate ice-cream that I had an upset student this morning. ●It is because she refused to die that she became incredibly successful. ●She is so considerate that many of us turn to her for comfort. ●I can well remember that once I underestimated the power of words and hurt my friend. ●He works extremely hard in order to live up to his expectations. ●I happened to see a butterfly settle on the beautiful flower. ●It’s no pleasure making fun of others. ●It was the first time in the new semester that I had burned the midnight oil to study. ●It’s no pleasure taking everything into account when you long to have the relaxing life. ●I wonder if it was because he abandoned himself to despair that he was killed in a car accident when he was driving. ●Jack is always picking on younger children in order to show off his power. ●It is because he always burns the midnight oil that he oversleeps sometimes. ●I happened to find some pictures to do with my grandfather when I was going through the drawer. ●It was because I didn’t dare look at the failure face to face that I failed again. ●I tell my friend that failure is not scary in order that she can rebound from failure. ●I throw my whole self to study in order to pass the final exam. ●It was the first time that I had made a speech in public and enjoyed the thunder of applause. ●Alice happened to be on the street when a UFO landed right in front of her. ●It was the first time that I had kept myself open and talked sincerely with my parents. ●It was a beautiful sunny day. The weather was so comfortable that I settled myself into the
TYE—2000KN压力试验机操作规程 一．一般规定： （1）本机由本室指定的试验人员进行操作，非本室人员未经许可，不得随意摆弄，以防事故发生。 （2）试验操作人员在试验前必须对本机进行检查，看贮油是否足够，油管接头是否松动，如油不足应加油，松动的油管接头应拧紧。（3）每次试验完毕，应及时关闭油泵，切断电源后应清理试台上的渣物，并盖好防尘罩。 二．操作方法： （1）首次对试样的最大荷载，应有所估计，然后根据所估计的最大荷载，合理选择并悬挂应力弹簧，校正指针回零。 （2）将试件放在下压板中心，顺时针旋动上压板手轮直至上压板接触试件。 （3）接通电源开动油泵，关闭加油阀，打开送油阀，按要求速度加荷，至试件破坏后，立即徐徐松开回油阀，关闭送油阀，读取从动指针所指荷载值并记录之（大弹簧读取外圈值，小簧读取内圈值）。（4）反时针转动上压板手轮，取出已破坏的试件。
ZBSX-92型振筛机操作规程 一. 一般规定： 使用人员在使用前，仔细阅读使用说明书，了解本机的有关事项，方可使用。 二. 操作步骤： 2.1接通电源。 2.2先将装有试样的套筛固定在振筛机上； 2.3将旋扭转至要求定时位，即可开始工作，若中途停机，应切断电源开关，让定时片中自行停止位置，不可强行扭回。 2.4将筛好的试样分别称重。 三. 注意事项： 1.1开机前，加足润滑油至螺孔面。 3.2转动旋扭时，不应过快，过猛，以免损坏零件。 3.3每隔6个月更换机油一次，使用时注意避免盖板上脦根导杆变形。
101型电热恒温干燥箱 一. 一般规定 使用人员在使用前，仔细阅读使用说明书，了解本机的有关事项，方可使用。 二. 操作方法 1. 打开电源开关。 2. 将温控仪设定所需温度，温控仪绿灯灭，此时箱内开始升温，同时打开鼓风机开关，使鼓风机工作，并注意鼓风机运转状况是否正常。 3. 恒温时，可关闭辅助加热开关，只留一组工作，以免功率过大，影响恒温波动度。 三. 注意事项 1. 本箱装置时必须用导线连接通地。 2. 本箱非防爆型，故带腐蚀性及易燃性物品禁止放入箱内干燥，
高中英语~词性~句子成分~语法构成 第一章节：英语句子中的词性 1.名词：n. 名词是指事物的名称，在句子中主要作主语.宾语.表语.同位语。 2.形容词;adj. 形容词是指对名词进行修饰~限定~描述~的成份，主要作定语.表语.。形容词在汉语中是（的）.其标志是: ous. Al .ful .ive。. 3.动词：vt. 动词是指主语发出的一个动作，一般用来作谓语。 4.副词：adv. 副词是指表示动作发生的地点. 时间. 条件. 方式. 原因. 目的. 结果.伴随让步. 一般用来修饰动词. 形容词。副词在汉语中是（地）.其标志是：ly。 5.代词：pron. 代词是指用来代替名词的词，名词所能担任的作用，代词也同样.代词主要用来作主语. 宾语. 表语. 同位语。 6.介词：prep.介词是指表示动词和名次关系的词，例如：in on at of about with for to。其特征：
介词后的动词要用—ing形式。介词加代词时，代词要用宾格。例如：give up her（him）这种形式是正确的，而give up she（he）这种形式是错误的。 7.冠词：冠词是指修饰名词，表名词泛指或特指。冠词有a an the 。 8.叹词：叹词表示一种语气。例如：OH. Ya 等 9.连词：连词是指连接两个并列的成分，这两个并列的成分可以是两个词也可以是两个句子。例如：and but or so 。 10.数词：数词是指表示数量关系词，一般分为基数词和序数词 第二章节：英语句子成分 主语：动作的发出者，一般放在动词前或句首。由名词. 代词. 数词. 不定时. 动名词. 或从句充当。 谓语：指主语发出来的动作，只能由动词充当，一般紧跟在主语后面。 宾语：指动作的承受着，一般由代词. 名词. 数词. 不定时. 动名词. 或从句充当. 介词后面的成分也叫介词宾语。 定语：只对名词起限定修饰的成分，一般由形容
M A: Has the case been closed yet? B: No, the magistrate still needs to decide the outcome. magistrate n.地方行政官，地方法官，治安官 A: I am unable to read the small print in the book. B: It seems you need to magnify it. magnify vt.１．放大，扩大；２．夸大，夸张 A: That was a terrible storm. B: Indeed, but it is too early to determine the magnitude of the damage. magnitude n.１．重要性，重大；２．巨大，广大 A: A young fair maiden like you shouldn’t be single. B: That is because I am a young fair independent maiden. maiden n.少女，年轻姑娘，未婚女子 a.首次的，初次的 A: You look majestic sitting on that high chair. B: Yes, I am pretending to be the king! majestic a.雄伟的，壮丽的，庄严的，高贵的 A: Please cook me dinner now. B: Yes, your majesty, I’m at your service. majesty n.１．［Ｍ－］陛下（对帝王，王后的尊称）；２．雄伟，壮丽，庄严 A: Doctor, I traveled to Africa and I think I caught malaria. B: Did you take any medicine as a precaution? malaria n.疟疾 A: I hate you! B: Why are you so full of malice? malice n.恶意，怨恨 A: I’m afraid that the test results have come back and your lump is malignant. B: That means it’s serious, doesn’t it, doctor? malignant a.１．恶性的，致命的；２．恶意的，恶毒的 A: I’m going shopping in the mall this afternoon, want to join me? B: No, thanks, I have plans already. mall n.（由许多商店组成的）购物中心 A: That child looks very unhealthy. B: Yes, he does not have enough to eat. He is suffering from malnutrition.
一、常见仪器得分类 一般根据仪器得主要用途得不同,可将常见化学实验仪器分为下列8类: （一)计量类 用于量度质量、体积、温度、密度等得仪器。这类仪器中多为玻璃量器。主要有滴定管、移液管、量筒、量杯等． (二)反应类 用于发生化学反应得仪器,也包括一部分可加热得仪器。这类仪器中多为玻璃或瓷质烧器.主要有试管、烧瓶、蒸发皿、坩埚等。（三）容器类 用于盛装或贮存固体、液体、气体等各种化学试剂得试剂瓶等。 （四）分离类 用于进行过滤、分液、萃取、蒸发、灼烧、结晶、分馏等分离提纯操作得仪器.主要有漏斗、分液漏斗、蒸发皿、烧瓶、冷凝器、坩埚、烧杯等。 （五)固体夹持类 用于固定、夹持各种仪器得用品或仪器。主要有铁夹、铁圈、铁架台、漏斗架等. (六)加热类 用于加热得用品或仪器。主要有试管、烧杯、烧瓶、蒸发皿、坩埚等。
(七)配套类 用于组装、连接仪器时所用得玻璃管、玻璃阀、橡胶管、橡胶塞等用品或仪器。 (八)其它类 不便归属上述各类得其它仪器或用品。 二、中学化学常见仪器得名称与使用 (一)计量仪器 1。量杯 量杯属量出式(符号Ex）量器,它用于量度从量器中排出液体得体积．排出液体得体积为该液体在量器内时从刻度值读取得体积数。量杯有2种型式。面对分度表时,量杯倾液嘴向右,便于左手操作，称为左执式量杯。倾液嘴向左，则称为右执式量杯。250 mＬ以内得量杯均为左执式,500mL以上者，则属于右执式。 2。温度计 温度计就是用于测量温度得仪器。其种类很多,有数码式温度计，热敏温度计痔。而实验室中常用为玻璃液体温度。 温度计可根据用途与测量精度分为标准温度计与实用温度计2类.标准温度汁得精度高，它主要用于校正其它温度计。实用温度计就是指所供实际测温用得温度计,主要有实验用温度计、工业温度计、气象温度计、医用温度计等。中学常用棒式工业温度汁。其中酒精温度计得量程为100℃,水银温度计用２00℃与3６0℃２种量程规格。 使用注意事项
意见应以事实为根据. 3 来自辞典例句 192. The bombers swooped ( down ) onthe air base. 轰炸机 突袭 空军基地. 来自辞典例句 193. He mounted their engines on a rubber base. 他把他们的发动机装在一个橡胶垫座上. 14 来自辞典例句 194. The column stands on a narrow base. 柱子竖立在狭窄的地基上. 14 来自辞典例句 195. When one stretched it, it looked like grey flakes on the carvas base. 你要是把它摊直, 看上去就象好一些灰色的粉片落在帆布底子上. 18 来自辞典例句 196. Economic growth and human well - being depend on the natural resource base that supports all living systems. 经济增长和人类的福利依赖于支持所有生命系统的自然资源. 12 1 来自辞典例句 197. The base was just a smudge onthe untouched hundred - mile coast of Manila Bay. 那基地只是马尼拉湾一百英里长安然无恙的海岸线上一个硝烟滚滚的污点. 6 来自辞典例句 198. You can't base an operation on the presumption that miracles are going to happen. 你不能把行动计划建筑在可能出现奇迹的假想基础上.