外文翻译电路交换网与vb调用数据库

外文原文Electric circuit exchange network and vb transfer database The result of the connection setup with circuit switching is the reservation of bandwidth all the way from the sender to the receiver. All packets follow this path. Among other properties, having all packets follow the same path means that they cannot arrive out of order. With packet switching there is no path , so different packets can follow different paths, depending on network conditions at the time they are sent. They may arrive out of order.Packet switching is more fault tolerant than circuit switching. In fact, that is why it was invented. If a switch goes down, all of the circuits using it are terminated and no more traffic can be sent on any of them. With Packet switching, packets can be routed around dead switches.Setting up a path in advance also opens up the possibility of reserving bandwidth in advance. If bandwidth is reserved, then when a packet arrives, it can be sent out immediately over the reserved bandwidth. With packet switching, no bandwidth is reserved, so packets may have to wait their turn to be forwarded.Having bandwidth reserved in advance means that no congestion can occur when a packet shows up (unless more packets show up than expected).On the other hand, when an attempt is made to establish a circuit, the attempt can fail due to congestion. Thus, congestion can occur at different times with circuit switching(at setup time) and packet switching(when packets are sent).If a circuit has been reserved for a particular user and there is no traffic to send, the bandwidth of that circuit is wasted. It cannot be used for other traffic. Packet switching does not waste bandwidth and thus is more efficient form a system-wide perspective. Understanding this trade-off is crucial for comprehending the difference between circuit switching and packet switching. The trade-off is between guaranteed service and wasting resources versus not guaranteeing service and not wasting resources.Packet switching uses store-and-forward transmission. A packet is accumulated. in a routers memory, then sent on to the next router. With circuit switching, the bits just flow through the wire continuously. The store-and-forward technique adds delay.Another difference is that circuit switching is completely transparent.The sender and receiver can use any bit rate, format, or framing method they want to. The carrier does not know or care. With packet switching, the carrier determines the basic parameters. A rough analogy is a road versus a railroad. In the former, the user determines the size, speed, and nature of the vehicle; in the latter, the carrier does. It is this transparency that allows voice, data, and fax to coexist within the phone system.A final difference between circuit and packet switching is the charging algorithm. With circuit switching, charging has historically been based on distance and time. For mobile phones, distance usually does not play a role, except for international calls, and time plays only a minor role(e.g., a calling plan with 2000 free minutes costs more than one with 1000 free minutes and sometimes night or weekend calls are cheaper than normal).With packet switching, connect time is not an issue, but the volume of traffic sometimes is. For home users, ISPs usually charge a flat monthly rate because it is less work for them and their customers can understand this model easily, but backbone carriers charge regional networks based on the volume of their traffic. The differences are summarized in Fig.2-40.ItemCircuit SwitchedPacket SwitchedCall setupRequiredNot neededDedicated physical pathYesNoEach packet follows the same routeYesNoPackets arrive in orderYesNoIs a switch crash fatalYesNoBandwidth availableFixedDynamicTime of possible congestionAt setup timeOn every packetPotentially wasted bandwidthYesNoStore-and-forward transmissionNoYesTransparencyYesNoChargingPer minutePer packetBoth circuit switching and packet switching are important enough that we will come back to them shortly and describe the various technologies used in detail.2.6 THE MOBILE TELEPHONE SYSTEMThe traditional telephone system(even if it some day gets multi-gigabit end-to-end fiber) will still not be able to satisfy a growing group of users :people on the go .People now expect to make phone calls from airplanes ,cars ,swimming pools ,and while jogging in the park .Within a few years they will also expect to send e-mail and surf the Web from all these locations and more consequently ,there is a tremendous amount of interest in wireless telephony .In the following sections we will study this topic in some detail.Wireless telephones come in two basic varieties :cordless phones and mobile phones(sometimes called cell phones),Cordless phones are devices consisting of a base station and a handset sold as a set for use within the home. These are never used for networking ,so we will not examine them further .Instead we will concentrate on the mobile system ,which is used for wide area voice and data communication.Mobile phones have gone through three distinct generations ,with different technologies:1.Analog voice2.Digital voice3.Digital voice and data(Internet ,e-mail ,etc.).Although most of our discussion will be about the technology of these systems ,it is interesting to note how political and tiny marketing decisions can have a huge impact .The first mobile system was devised in the U.S. by AT&T and mandated for the whole country by the FCC .As a result ,the entire U.S. had a single(analog) system and a mobile phone purchased in California also worked in New York .In contrast ,when mobile came to Europe ,every country devised its own system ,which resulted in a fiasco.Europe learned from its mistake and when digital came around, the government-run PTTs got together and standardized on a single system(GSM),so any European mobile phone will word anywhere in Europe .By then ,the U.S. had decided that government should not be in the standardization business ,so it left digital to the marketplace .This decision resulted in different equipment manufacturers producing different kinds of mobile phones .As a consequence ,the U.S. now has two major incompatible digital mobile phone systems in operation(plus one minor one).Despite an initial lead by the U.S. , mobile phone ownership and usage in Europe is now far greater than in the U.S. Having a single system for all of Europe is part of the reason ,but there is more .A second area where the U.S. and Europe differed is in the humble matter of phone numbers .In the U.S. mobile phones are mixed in with regular (fixed) telephones .Thus ,there is no way for a caller to see if ,say ,(212)234-5678 is a fixed telephone (cheap or free call) or a mobile phone (expensive call).To keep people from getting nervous about using the telephone ,the telephone companies decided to make the mobile phone owner pay for incoming calls .As a consequence ,many people hesitated to buy a mobile phone for fear of running up a big bill by just receiving calls .In Europe ,mobile phones have a special area code (analogous to 800 and 900 numbers) so they are instantly recognizable .Consequently , the usual rule of “caller pays” also applies to mobile phones in Europe (except for international calls where costs are split).A third issue that has had a large impact on adoption is the widespread use of prepaid mobile phones in Europe (up to 75% in some areas ).These can be purchased in many stores with no more formality than buying a radio .You pay and you go .They are preloaded with ,for example ,20 or 50 euro and can be recharged (using a secret PIN code ) when the balance drops to zero . As a consequence , practically every teenager and many small children in Europe have (usually prepaid ) mobile phones so their parents can locate them ,without the danger of the child running up a huge bill .If the mobile phone is used only occasionally ,its use is essentially free since there is no monthly charge or charge for incoming calls .2.6.1 First-Generation Mobile Phones :Analog Voice Enough about the politics and marketing aspects of mobile phones .Now let us look at the technology ,starting with the earliest system .Mobile radiotelephones were used sporadically for maritime and military communication during the early decades of the 20th century .In 1946, the first system for car-based telephones was set up in St.Louis .This system used a single large transmitter on top of a tall building and had a single channel ,used for both sending and receiver .Such systems ,known as push-to-talk systems ,were installed in several cities beginning in the late 1950s.CB-radio,taxis,and police cars on television programs often use this technology.In the 1960s,IMTS(Improved Mobile Telephone System) was installed .It ,too ,used a high-powered (200-watt) transmitter ,on top of a hill , but now had two frequencies ,one for sending and one for receiving , so the push-to-talk button was no longer needed . Since all communication from the mobile telephones went inbound on a different channel than the outbound signals ,the mobile users could not hear each other (unlike the push-to-talk system used in taxis). IMTS supported 23 channels spread out from 150 MHz to 450 MHz .Due to the small number of channels ,users often had to wait a long time before getting a dial tone .Also ,due to the large power of the hilltop transmitter ,adjacent systems had to be several hundred kilometers apart to avoid interference . All in all ,the limited capacity made the system impractical.In VB6.0 MSChart controls is one function formidable high-level graph tool, has the rich graph plan function, may demonstrate two-dimensional and the three dimensional good chart, the linear chart, the cake chart and so on the many kinds of commonly used graph. Recently I in order to in double struck the graph when some region will the database content which corresponded with this region controlled on in DataGrid demonstrates, encountered many difficulties, finally used one to solve the problem from definition variable SelectSeries, concrete method as follows:Supposes the database name is the student information , has one Access table the student result , its content is one class students test result, includes the student number, the name, the result 3 fields, the result field form is the character, the value is is superior , is good , center , the difference center one.Window Form1 includes one MSChart controls MC$score, the type for the two-dimensional cake chart, uses in to demonstrate each kind of result the student counts; One ADO controls AdScore uses in to connect the database; One DataGrid controls DgScore uses in by the form form demonstration database content.The work process is: Double strikes the cake chart some region, then DgScore demonstrates the corresponding result the student name list.Code as follows:Option ExplicitDim SelectedSeries as Integer from definition variable Dim Rs () as String withdraws the character string array which the record compendium uses Private Sub Form_Load () Establishes DataGrid controls the data pool DgScore.DataSource= AdScore Establishes ADO controls the connection character string and the initial record source, namely demonstration content AdScore.ConnectString= Provider=Microsoftc.oledb.3.51; Persist_Security Info=False; Data Source= student information AdScore.RecordSource= selet * from student result order by result AdScore.Refresh Supposes in advance withdraws the SQL sentence which the recording uses Rs (1) = select * from the student result where result = + Chr (34) + is superior + _Chr (34) + order the by result Rs (2) = select * from the student result where result = + Chr (34) + is good + _Chr (34) + order the by result Rs (3) = select * from student result where result = + Chr (34) + center + _Chr (34) + order by result Rs (4) = select * from student result where result = + Chr (34) + difference _Chr (34) + order by result End SubPrivate Sub MC$score_SeriesSelected (Series as Integer, MouseFlags as _Integer, Cancel as Integer)SelectedSeries=SeriesEnd SubPrivate Sub MC$score_PointSelected (Series as Integer, DataPoint as_Integer, MouseFlags asInteger, Cancel as Integer)SelectedSeries=SeriesEnd SubPrivate Sub MC$score_Db1Click () Changes ADO controls the record source and renovates Adoc1.RecordSource=Rs (SeletedSeries)Adoc1.RefreshEnd Sub- VB does for the rapid application development (RAD) the tool more and more obtains the development personnels approval and accepts. It to many API (for example ODBC API, SOCKET API and so on) the seal causes the programming to change is simple. At the same time, it supports under the integrated development environment visible, the event actuates, to be object-oriented and so on the programming characteristic. Under, we chat in VB transfer the memory process the realization method and its the matters needing attention.- We know, the VB database programming has permits the many kinds of method, for instance straight takes over the use of ODBC the API programming, this method is flexible, is highly effective, the programmer may realize to the database complex control; Also may use in VB the data object, like RDO (long-distance data object), DAO (data visit object), ADO (ActiveX data object), this method realizes conveniently, quickly, but flexible worse somewhat. As a result of memory process in realization data merit and so on seal, hideaway as well as code pre- translation, reduced network load, maintenance convenience, therefore does by many RDBMS and the programming tool the support. In VB each 类数 also provides to saves the process according to the object the support.- We as regularly explain its realization take ADO the step - 1. Foundation, debugging memory process. You may also be allowed to hang the procedure in the database outside other under the support carry on the memory process the foundation and the debugging work. In this example memory process code is as follows (uses in PUBS MS SQL the example storehouse):CREATE PROCEDURE myprocedurejob_id smallint,job_lvl tinyintASSELECT *FROM employeeWHERE job_id job_lvl- 2. Produces one new project in VB, the project has one window, one COMMAND (NAME: COMMAND1) button, one MSFlexGrid (NAME: MSFlexGrid1) controls.- 3. The foundation connects ADO connection;- 4. The foundation orders ADO command;- 5. The foundation parameter and establishes each parameter the attribute;- 6. Carries out ADO command;- 7. The logarithm according to carries on processing; MSFlexGrid demonstrates inquires data - 8. The release connects, withdrawal procedure.- Code as follows:States below the variable in the window:Dim cnn1 As ADODB.Connection connects Dim mycommand As ADODB.Command orders Dim parm_jobid As ADODB.Parameter parameter 1Dim parm_joblvl As ADODB.Parameter parameter 2Dim rstByQuery As ADODB.Recordset result collection Dim strCnn As String connection character string Joins the following code in the window LOAD event:Set cnn1 = New ADODB.Connection Produces one connects strCnn = DSN=MYDSN; uid=sa; pwd= Foundation system data pool MYDSN aims at the PUBS database cnn1.Open strCnn opens connects Joins the code as follows in window UNLOAD:cnn1.Close the closure connects Set cnn1 = Nothing the release connects In button code as follows:Dim i As integerDim j as integerSet parm_jobid = New ADODB.ParameterSet mycommand = New ADODB.Command parm_jobid.Name = name1 this line can be ommitedparm_jobid.Type = adInteger 中文译文 电路交换网与vb调用数据库 切换电路的连接设置的结果是保留了从发送端到接收端的包所通过的线路带宽。其它的特性这一就是所有的包都沿同一个线路意味着如果包传输超过了时序那它将不能到达接收端.由于没有路径用来进行包交换，所以要想不同的包沿不同的路径传送就是能依靠网络的条件了。这样包即便是超过了时序它们也有可能到达的。包交换技术比线路交换技术更能容错。事实上，这也是它被发明的理由！当一个交换开头被拉下，则所有使用它的线路将被切断并且其余的包将不能再被传送。此时如果在进行包交换，包将在已选择的路径上停止交换。预先建立一条路径也开创预先保留带宽的可能性。如果带宽被保留，那么当一个包到达时，通过保留的带宽它可以被立即发送出去。假如没有带宽被保留用于包交换，那么所有的包将不得不等待着被转发。预先保留带宽意味着当一个包出现时不会发生拥塞(除非出现的包比预期的还多)。在另一方面,当尝试建立一条电路时，由于拥塞，尝试将会失败。因此，在线路交换(在设置时)和包交换(在包发送时)的不同时间拥塞都可能发生。如果为特定的用户建立一条线路那么当这个用户没有数据传送时，则这些保留下来的带宽将被浪费而不能用于其它用户的传输。而包交换则不会浪费带宽，因此它在系统宽带方面的应用更有前景。理解电路交换和包交换的这种商业差别是极其重要的。这种商业差别是在质保服务和浪费资源相对不质保服务和不浪费资源的。包交换是利用存储器并向前发送的。包是先存储在路由器的内存里然后再发送到下一个路由器的。和电路交换一样信息位也是连续不断地通过导线传输的。存储转发技术增加了延迟时间。另一个不同是电路交换是完全透明的。发送者和接收者可以使用他们想要的任一比特速率，格式和帧同步.这通信公司不知道也不关心这的。而在包交换中载体决定了基本的参数的。用一个简单比喻它们就像一个是公路一个是铁路。在以前，用户决定介质的大小，速度和特性。而后来这些都由通信公司做了的。这些都是透明的它允许声音，数据和传真共同存在于电话系统里。电路交换和包交换的最后一个不同点是计费方式。电路交换的计费是按照传统的距离和时间的。比如移动电话除打国际电话外距离是不列入计算的，而时间也只是列入一个简单的计算(例:拔打2000分钟电话比1000分钟电话花费的多并且晚上或周末也比平时来得便宜)。时间对于包交换来说是没有问题的，但是传输流量却是个问题。ISPs为家庭用户的计费方式基于按一个月的流量的一半的，这是因为它们用得相对较少并且他们的顾客也很好理解这种计费方式。但是主干网络中心的收费是基于它们的流量的。这些不同列在图2-40里。项目电路交换包交换呼叫设置需要不需要专用物理线路是否每个包沿相同路径是否包的到达中有规则的是否是否会致命的崩溃是否带宽利用固定动态分配可能的时间堵塞在设置时在每个包时可能浪费带宽是否以存储转发进行发送否是透明性是否收费按分钟按包图240：比较电路交换网络和包交换网络的不同电路交换和包交换是两者都是非常重要的，我们将立即在详细的资料里提及并描述它们之间各种不同的技术。2.6 移动电话系统 传统的电话的系统将不能满足用户的大量增长（即有一天实现了终端到终端的光纤连接）。人类在进步，人们期望能在飞机上，汽车里，船上，足球场甚至在公园里跑步的时候也能打电话。在近几年人们更是期望能在更多的地方发电子邮件和上网冲浪。因此有很多的人关心无线电话技术。在下面的章节我们将用详细的资料来学习这些主题。 无线电话可以分为基本的两种：无绳电话和移动电话（有时也叫蜂窝电话）。无绳电话设备是由痤机和听筒组成它们被安装在用户的家里。它们从不被应用于网络，所以我们没有办法做进一步的检测。取而代之，我们将把焦点放在在声音和数据通信上有广泛应用的移动系统上。 移动电话经历了三个不同的发展阶段，每个阶段都有不同的技术：1模拟声音2数字声音3数字声音和数据（互联网，电子邮件，etc.）虽然我们主要讨论这系统技术，但是我们也有必要注意政府和市场对它的巨大影响。第一个移动电话系统是在美国由受美国国家联邦通讯委员会(FCC)委托的美国电话电报公司(AT&T)设计的。结果在整个美国就有了一个简单的(模拟)系统并且移动电话开始在加利福尼亚上市同时也在纽约得到应用。相反的，当移动电话来到欧洲时，由于每个国家都设计了自己的系统，所以结局很惨败。当数字技术到来时，欧洲从前面的失败中吸收教训，政府和邮政制定了简单系统(G