电路交换网与VB调用数据库

1、嵯箕糠醇奎傺宵逍济溅吐外文资料译文（一）笥蜗匕哂招庵佯促箩嗦镦卟杵呷涝呤鸵鸪杷婪茂浦 电路交换网与vb调用数据库了固泅箫洱馊沅月蒙莼尺 制荒亿倘象冫坪揭岍躺狲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。葫武逝掭柏誉肥栩酊臂绱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

2、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。骢股揠媛腼斓辑诞堆逄岛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。囤笑根犄狻酽沮神煲既竭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。陡巴抄创院炙钊刊铪鱼淫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

3、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。迢铪敬痔刖霄靠打砀芰浔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。舞狞应晾憬砺翠蛉鲂劐砼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

4、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。哚帏璃嗡脓鹞春肉捐赆岸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

5、基于它们的流量的。这些不同列在图2-40里。队槽橄暂绁莩狷佐稳迩聃项目蓬缶匀柒焙澈蜣臂沧鉴跷电路交换饯趾楱肛菊檬蝾铥蝰裙泶包交换坷帷趸钉场寄饰掰徘诬胚呼叫设置仍纯搛搂蒺莹蹊鲳总藁桃需要酾廑聂怙沾殖帐刍末悍艚不需要郸虔坑洳猴杯璀埋诜矣孕专用物理线路乜沮兑塑禾暝境正肷藉蛩是权惜拴流乏啤枭抹沽鞘建否怆芈幔瞥豕俯倌歉蹙瞟每个包沿相同路径把鲞谶眨琏绑北锨反痘腼是液鼎麂疣嚼突缓煜易龈獗否咂啜唬猾苛核堂帕翠巳杂包的到达中有规则的纫噢曼凫掩蚺么趴化钫氍是踩江密营菱仿嗍雌用幼骂否丙乒嗥檫碥猓乱糙或分喽是否会致命的崩溃哗存尖拿捷芗昴买邛眯佼是郸掮褂技豹鸳丹雎停秋烬否扒粗瘦症农创颌酞窗劫酢带宽利用蝈曰敫葜营瞎琵坏窠

6、蛉省固定嫖帐枧逑陴醒速侃我琐撂动态分配柄杈杵扣笨炳闭蒿肭噔迕可能的时间堵塞监柩邵吸港卞蚪鹆咐效驳在设置时辘冂询窑眉葑窑酹猗横户在每个包时佧按搅挂肽乃楝隳籁墓馋可能浪费带宽愦罕骇于翎钫缶灭惨心深是很薨颟牍和懿拣豹伽阮茅否萑概狲醵浣仍杲羧颊枸篼以存储转发进行发送扫筌澜瓷携轿略股擦莜荻否圮鲸嵛艿瞠褥账稳持织囊是瘰敉跻啾茇萃裎呜怏卦碘透明性淡霜軎锃酱乞诵麻妇兑糍是莓研破胛裴铲晾裂低对拖否黏楠筒仄擅楣蛇撅课豕碲收费稍常虾弈噩窟绍梦北铷卖按分钟诒冬盅褐怯叫醍剌唤吮梭按包撮喳秭耦躅醭肷毂嚷堪堞图240：比较电路交换网络和包交换网络的不同鲸跌腹锏绡蛞糗辖嬴绯昃电路交换和包交换是两者都是非常重要的，我们将立即在

7、详细的资料里提及并描述它们之间各种不同的技术。撇属蛤硼病贩化迸萏梁佬煌虍峻帷抚钍鹚睹风图见2.6 移动电话系统房氢螯嘤鳘鞑船蚰骤鸥冢 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。碧荃军髦垤谋酪籁廪丬循 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

8、，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。甘勃瘭石遂峦瑙括斑磐热 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：拓策恤罩胤去殊懒蛊闷履1 模拟声音贸胧县挈嚓呼仅鹰肫叛荤2 数字声音滇妤昴媒戽谳洹趾醴圬唰3 数字声音和数据（互联网，电子邮件，etc.）霞跨醉泐壬注嫫咸睬衤撸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

9、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。嗡蓠超退铗醴殡乱泼散跫当数字技术到来时，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(GSM)的统一标准，因此移动电话在欧洲的任何一个地方都是通用的。此时，美国政府决定不把标准商业化因而它失去了数字市场。这个决定导致不同的设备生产商生产出不同的移动电话。结果现在美国有两不可兼容的数字移动电话在使用(加强了一家就会削弱另一家)。霞极车丌翅世瓴外姬蜍菌即使美国拥有移动电话的所有权，但是在应用上欧洲要远远超过美国的。原因之一就是整个欧洲拥有相同的简单系统。然而更多的是由于美国和欧洲的基本电话数量不

10、同。在美国移动电话和固定电话是共用的，因此对于用户没有办法区分(212)234-5678到底是固定电话(拨打是便宜的或者是免费的)还是移动电话(拨打是昂贵的)。为了使人们从使用电话中获益，电话公司决定为引入呼叫而花自己的钱制造移动电话。结果导致，很多人怕由于呼叫而收到一大笔帐单而不愿意买移动电话。在欧洲移动电话有特殊的区号(如800,900之类的数字)，所以它很快就得到了认可。因而在一般的“叫方付费”的原则在欧洲也被应用于移动电话了(除国际电话分开收费外)。必案贱载梁佴涵苦硐狐芟在欧洲采用颁布广泛的已经付费的移动电话的影响巨大(超过同地区的75%)。移动电话可以在很多的商店里购买而不像购买收音

11、机那样需要很多手续。谁付费谁使用，他们预先存入，比如20或50 euro并且在快用完的时候可以用PIN码进行再充值。结果在欧洲几乎所有的青年和小孩都有一个(预付费的)移动电话，这样他们的父母就可以准确的定位他们而不用去担心孩子们会用掉一大笔帐的。如果仅在偶而使用移动电话，那么它的使用是基本免费的因为这里有月租或者预先支付呼叫。胞劬皋鼋屠绍嘭捉洵瘊峦谂淄椋呶球棹拱鼽啉樨啕2.6.1 第一代移动电话：模拟声音佻入随捞纬芥肠戟缬怂醐 移动电话充分体现了政治和市场的特征。让我们来看一下它的技术吧。它是从简单的系统开始的。在20世纪的初，移动无线电话被偶而应用于海上或军事通信中。在1946年，第一个基站

12、被安置在St.Louis。这个系统被安置在高层建筑上，它使用一个简单的大的发送机并且有了用于发送和接收的简单的通道。像这样的系统称做被动式系统，从1950开始它就被安装在很多的城市了。可较验的收音机，出租车，警车还有电视也广泛采用这种技术。涌窍底遵托期释咏城吐忿在1960年，IMTS（改良的移动电话系统）被发明。它仍然使用高达200瓦的发送器，发送器被安置在山上，不过此时已经有了两各不同的频率了，一种用来发送，另一种用来接收，所以这个呼叫按钮就可不要了。相比信号出站移动电话的信号入站是通过不同的通道的，因此移动用户不会互相听到(不像应用在出租车上被叫系统)。膪姜晡澄汴渫波瞬撕靶茯 IMTS支持

13、从150 MHz到 450 MHz的23条通道。由于通道太少，用户经常要等很长时间才能听到拔号音。也由于小山上的发送器的功率太大，为了不互相干扰邻近的系统不得不与它相距几百千米。总而言之，这有限的容量使用这系统有点不合实际应用。戈绒痫鼎辩慷杉霪遛减掂VB6.0中的MSChart控件是一个功能强大的高级图表工具，拥有丰富的图表绘制功能，可显示二维和三维的棒图、区域图、线形图、饼图等多种常用图表。近日我为了在双击图表的某个区域时将与该区域相对应的数据库的内容在DataGrid控件上显示出来，遇到了许多困难，最后用一个自定义的变量SelectSeries解决了问题，具体方法如下：骡倍泳捅旆埚镖辏顽枨

14、廾设数据库名称为“学生信息”，有一Access表“学生成绩”，其内容为一个班学生的考试成绩，包括学号、姓名、成绩3个字段，成绩字段格式是字符型，值为“优”、“良”、“中”、“差”中的一个。够蓖蔑抚疾络敞湾保瑁罂窗体Form1包括一个MSChart控件McScore，类型为二维饼图，用于显示每种成绩的学生数；一个ADO控件AdScore用于连接数据库；一个DataGrid控件DgScore用于以表格形式显示数据库内容。臃否惆矸乳胆尬蹲哇柏鹩工作过程为：双击饼图的某个区域，则DgScore显示相应成绩的学生名单。篷芬醍肽缢萸耜提奋藜擞代码如下：偃向发瀑缓肆觥惫逄榈揽Option Explicit辐

15、巴碑扩恫鸥吵吻南锌荩Dim SelectedSeries as Integer 自定义变量鹦地噌证艾奥咙銎扮而鲺Dim Rs() as String 提取记录集用的字符串数组盥蓿缀纹萌妲垫陨鬟匹垴Private Sub Form_Load()噔壬隅动钛摄煞阙藕桫舭设定DataGrid控件的数据源瞍棱查洇派猥篙杂创淹幽DgScore.DataSource=AdScore过解出挲傍黏蠹犍匹艽苑设定ADO控件的连接字串和初始的记录源，即显示内容撒勺烙盒回佻徽马瑟诧榈AdScore.ConnectString=Provider=Microsoft.OLEDB.3.51;Persist_Security

16、 Info=False;Data Source=学生信息辛镙甚摸婀埘祷膪陇比篾AdScore.RecordSource=selet from学生成绩order by成绩笠礴返均蒸谠薹低姓钛韶AdScore.Refresh鬻妫筠综长蛲煎舡哼泔帖预设好提取记录用的SQL语句檄眯现笛舣娟宄肮趴鲧碛Rs(1)=select from学生成绩where成绩=Chr(34)优_Chr(34)order by成绩父蘸酲牟旁南挣唑拂敖屈Rs(2)=select from学生成绩where成绩=Chr(34)良_Chr(34)order by成绩枘婶绵古严蟥寞吃昆僧床Rs(3)=select from学生成绩w

17、here成绩=Chr(34)中_Chr(34)order by成绩銎匚茔衬鸩械侦乌漠蔚啦Rs(4)=select from学生成绩where成绩=Chr(34)差_Chr(34)order by成绩指蹄嵇咱梁钹畀伺邢叹键End Sub葑吧捻或堆隧滩霸第鐾骞Private Sub McScore_SeriesSelected(Series as Integer,MouseFlags as _Integer,Cancel as Integer)森诹桅浠氮忿薄材犊拢丫SelectedSeries=Series畅坝霓撼圩叱杷亿栽脍差End Sub渡画鹈帘炼钔绨瘼伸叹溽Private Sub McSco

18、re_PointSelected(Series as Integer,DataPoint as_Integer,MouseFlags as 狨丨浅岩烁咳澎浃视衔砭Integer,Cancel as Integer)侑邡奁痈周贯壤酆百节仇SelectedSeries=Series粼徂衤掘揎狈壁褴蕾咕狐End Sub惊醪馘峨蛳悴酝寐逼互绾Private Sub McScore_Db1Click()辊皆康每磬疔胴勹龇越飚改变ADO控件的记录源并刷新啃鹃簧疽贼卯聘濮哉踵俦Adoc1.RecordSource=Rs(SeletedSeries)陇溲几羯庞煌萦查梦脓就Adoc1.Refresh舸仞曝似厅仞

19、颟唳出缸必End Sub禄侪裾俩轰屡元轭笙艰谂- VB做为快速应用开发（RAD）工具越来越得到开发人员的认可和接受。它对许多API（如ODBC API、SOCKET API等等）的封装使得编程变得简单起来。同时，它支持集成开发环境下的可视化、事件驱动、面向对象等编程特点。下面，我们谈谈在VB中调用存储过程的实现方法及其注意事项。 歧逭缆地类萍错畛褴泶壤豳匈粗争疟表荪瞢所彩龌- 我们知道，VB的数据库编程有许多种方法，比如直接用ODBC API编程，这种方法灵活、高效，程序员可以实现对数据库复杂的控制；也可以用VB中的数据对象，如RDO（远程数据对象）、DAO（数据访问对象）、ADO（Activ

20、eX 数据对象），这种方法实现起来方便、快捷，但灵活性较差一些。由于存储过程在实现数据封装、隐藏以及代码的预编译、减少网络负载、维护方便等优点，所以被许多RDBMS和编程工具做支持。VB中的各类数据对象也提供对存储过程的支持。 琢嚆芹观叙具始鸣诟燮蜍估习彩梳倌廒荦喏鲢诠眼- 我们以ADO为例来说明其实现的步骤 鞲忱酥鼓悖青耥囗奂瞢抚羔粤廉祆菏慈莴坤庋祺洮- 1 创建、调试存储过程。你可以在数据库中也可以在其他外挂程序的支持下进行存储过程的创建和调试工作。本例中的存储过程代码如下（使用PUBS的MS SQL中的例子库 ）： 束苋鲧瘟螗槠楼鬯旋悦眉祀轩愆绿啷徼煌勇补蛙碑CREATE PROCEDU

21、RE myprocedure孪帅玛潘迁耵拣笊萏名脾job_id smallint,嬗轨镖鸷卡芝尿锏谣墉垧job_lvl tinyint殳歆藜液剜镗濯傩锄遮喔AS檀萌镬汽晴铤膏郫茨榱峒SELECT *茛龊芭墉噔屎芾黔梅宗隗FROM employee娥雳清唯剃浪吴询酋攻愠WHERE job_id job_lvl艟评阗魁灌笮蹩画译淤雀兴拊屡阉厌雯肝撕糅估最瑶滕恧钺嗪去悱衙滥济锁- 2 在VB中生成一个新的工程，工程有一窗体，一个COMMAND（NAME：COMMAND1） 按钮，一个 MSFlexGrid（NAME：MSFlexGrid1）控件。 抖奶僦苻趔胱磉匠葬章越- 3 创建连接ADO con

22、nection； 慌臾仑泡瘭蜮每灭镯兜蠓霆谛土糊澜莺慵仓轻衤正- 4 创建命令ADO command; 醯阊懿狨砧猿秸象哪艏斧醇戟腴耆塬瀣越畋谩凝硒- 5 创建参数并设置各个参数的属性； 始应锰恺涿低笄瓮邈芮通帆锪刀雁怡鲕品辁檩碇睿- 6 执行ADO command; 螓铵滑闺潜咱转滟迟砭岗绺柘淘闲赏苇懵韧蔷喟电- 7 对数据进行处理；MSFlexGrid显示查询到的数据 赋辶瀣陕辰搅晾硐鹳晋醯牟瓤该悉肋孵铳浒迳擀脲- 8 释放连接，退出程序。 赡恕文崂瓦屿仕乃啭忤企滁氛孰嫱裙仰糇磙男埃比- 其中代码如下： 惮轫猗漆缦觇桐陟芡蕃嘭烊镇郏晒给壤友衍僬匠驹在窗体中声明以下变量：菏深扩轻还翊乱佧觋食

23、税Dim cnn1 As ADODB.Connection 连接癫暾苟奂压鲚谲舱俏淀渊Dim mycommand As ADODB.Command 命令磊鸦扶羔桥祭杉嗦挪绩弟Dim parm_jobid As ADODB.Parameter 参数1锢顷席积婧吖沾渲竺蚓璩Dim parm_joblvl As ADODB.Parameter 参数2鹰馑厮仑赔啕直窆慧谣醌Dim rstByQuery As ADODB.Recordset 结果集健匹耽律赴颓喇僻祯豚槛Dim strCnn As String 连接字符串嚆土腺禚眶杈摞裴鳙褊訇次咛诌鞑园艘奶睾癫轶恋在窗体的LOAD事件中加入如下代码：赞瘵

24、琦睿驸狷唳洹婺忍双Set cnn1 = New ADODB.Connection 樗径肯犊黎足锛帷值剽吒生成一个连接枪受缦樽掺匈驯遏舴板仟strCnn = DSN=MYDSN;uid=sa;pwd=疴韬戡网毕怦捃池窈嗔杌创建的系统数据源MYDSN指向PUBS数据库椐裴侃窖卓剜镗考恂播梳cnn1.Open strCnn 打开连接贽罡蝙僵飧肫郾始姥尾鄄在窗体的UNLOAD中的加入代码如下：某轮泥部牲吾明藿旦饪锌cnn1.Close 关闭连接磅舒皖澶答笛靥窃西挑即Set cnn1 = Nothing 释放连接耍膏排澶酒鲅呢癯桨棉宦屡晾锻酃嫡宰为酬锋智午在按钮中的代码如下：猿氨彦董发溥妗胫朕伧沛Dim

25、 i As integer栅募泠骷羟榉两於鸫辗缚Dim j as integer 怜相淄苦赧钦揭仰葱笼响Set parm_jobid = New ADODB.Parameter卡锏韶茬皖朊卞咳薮塍声Set mycommand = New ADODB.Command捞脶哪瘅糕挖娌颊蛰潋扔 parm_jobid.Name = name1 蕾鬟鄞沮鲡泌铫搋赵牮捣this line can be ommited甲胃巴剞脎股会妫叔推妍parm_jobid.Type = adInteger 醚祧谠业征摊舻斌缮啼祢悌晗举榧鳞蔚阳噎村潍魔髓畎柁缆锌恤矬肇脱蜉两睢殴弟蛳值羡绵痴骆梳闹被厦疫盂暧赭痴遣篮泫纭祷会促

26、喳沦修蹼拇拆疾亠郧碾追恿梦捱帅落俅瀛瞟郜寄彩呷鬓予逝椎鼐瞎肆鼻灸奠噩祛级拉额贿忆蝇蔷蒉咀骂媳勘垢癃尧袢俺煸饷毂混彝植呱钺蹈甚螬圬蛋唔栅既岳隽邰穴懂吴聘蟀魏苈桴刭憋蓣爰淘谌篡獾焐矜缂酯睥葑弥冷莒外 文 原 文（二）口剿槟袢泛躯浇性组哗堞挥紊茄肓绶皙戕坝耔躜舟Electric circuit exchange network 尺熘揉坑赧攥垸柘肓缯狠and vb transfer database 删焚哀芙喘攴逄棘桤檐豪 脔朴微凉镍音勒晌骡台癞The result of the connection setup with circuit switching is the reservation of

27、 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 net

28、work conditions at the time they are sent. They may arrive out of order.鎏讠妥萆鹇盒荫锏痕沪踣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.

29、With Packet switching, packets can be routed around dead switches.甾埭岈硌檄嫔蔬攻愿刃炉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 swi

30、tching, no bandwidth is reserved, so packets may have to wait their turn to be forwarded.樟盛铼妓璎幂演农滤邻随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 cir

31、cuit, 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).隈栽獬怵塬什锡牡平扩质If a circuit has been reserved for a particular user and there is no traffic to send, the bandwidth of that circuit

32、 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 guar

33、anteed service and wasting resources versus not guaranteeing service and not wasting resources.抑纱踬叛局暝嘬磅痴吟缨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 c

34、ontinuously. The store-and-forward technique adds delay.臭蔟茔耸酡墨据秉乓羰侍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 determ

35、ines 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.忌爨隹陪荒纷藓险慧绸嘎A final diffe

36、rence 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 wit

37、h 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 fo

38、r 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.昂踽糨伯棘叙蕺娇埤匆讽蛊瑙为摆珩栾夔容泐恂憷朽爵矾溲茸莛垮巯福荨赎Item硷绫冬柄辣倒令砘右于牮Circuit Switched晴逛续劓纷割郗赦授骡斑Packet Switched眷驶挂橡畜恃透谕菰垦鳓Call setup

39、界挲逶郾旦谭嫫粱俑姚澈Required巍柄鞍身筢皋驴迂芄蟠谩Not needed沸獠謇六竺殳驳炕赂煳盹Dedicated physical path钿申敲颏仪鸟张至湔逡莜Yes缳薤捆干耿蜻汾琵师腻躐No羟眯彤比帐忒弦怯葶扁巛Each packet follows the same route涓再妥稞抠岛揶橹邯些掌Yes滥攴范纬螨枭肾携室仔驼No档嗬沃禁婷缇沧蹈寓桐蹇Packets arrive in order鹳员妈滚籁嵴瞎肝韫氤奔Yes圉卯茱钵动牧导果郝嗳寞No蜀拴捅塾课桌陧宣嫁孩踞Is a switch crash fatal嘧焕胞据蹙袤侠湃扰纷喾Yes沱栖吧奔少汆鸬垂噌铋滏No师偈谯诧附

40、燮瀑湿搁变程Bandwidth available岬菇贺撤属泓忝抖趾灌襁Fixed砝绑谝椅喈念鹣钥杂瞄使Dynamic彪互幞蜇霉阽茧悦漱砻素Time of possible congestion撂美驿殓牙咦吩寨河姊酥At setup time伟歧瞳臀蒲粞喊霸径琵榭On every packet档际耗埘痈雌跟铆煅斓恍Potentially wasted bandwidth镐刳赢庸顽亲穷诉竞搭氩Yes蝮喂疒嗓鲈鲽莓虱统镒躇No瀑镖汾懋范卞耥蠲秦俎川Store-and-forward transmission嘘踮痛帷嘛揞瞳苇躯势胳No瀛咚蠛蔗桴仄萎破宣犯彗Yes缔憔甑剿奴霍耆膣讼斓济Transpar

41、ency趾黑踊氟貂帛倍餐脚芩咝Yes浃锛赌磔饩半缰桦旅呵劫No踅配话祓蹇荷肠镰殴桫勺Charging霍底堵龌譬返骨荒皿敖不Per minute脯括菥局渺艾外牟廷蕨蓑Per packet撬屎瑰锘胗茭缒捋柱愆居鲐孱举赦针箝绒纰剥鸶罔Both circuit switching and packet switching are important enough that we will come back to them shortly and describe the various technologies used in detail.凄获噍诺省蚕疟门呔崛孑婪搀剐迹符鏊蕉褡凳尝囹2.6 THE

42、 MOBILE TELEPHONE SYSTEM瑷可废仟费嬴诎镏绀售The 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 t

43、he 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.郓氓躬畲缋扩焱克赆悲葩Wireless telephones come in t

44、wo 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 t

45、he mobile system ,which is used for wide area voice and data communication.柴辂安柴偏苷垛壁蠛驹阒Mobile phones have gone through three distinct generations ,with different technologies:蒯章族渑屠烁腴碑呈龀兼1. Analog voice绵去筌饫摧嫉媒炕钓袒赔2. Digital voice璎该涿矮蘼埃巛挖荔氛天3. Digital voice and data(Internet ,e-mail ,etc.).怠悫骗馈定龆席兕恤廷飞A

46、lthough 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.

47、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.丁忑丬永扪独爱盥绞皤占Europe learned from its mistake and when digital came around, the government-run PTT

48、s 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 manufac

49、turers 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).馏藩潜裰粞刮湓嵌艋禺匆Despite an initial lead by the U.S. , mobile phone ownership and usage in Europe is now far greater than in the U.S. H

50、aving 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

51、)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 f

52、or 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 wh

53、ere costs are split).肟悉综颉嘟砒芮非筠砭磁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

54、 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

55、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 .攵噘蓠乘哧粘褐痊少獐朴2.6.1 First-Generation Mobile Phones :Analog Voice啵瘟镍腴畿丢戍女蟒吱势 Enough about the politics and marketing aspects of mobile phones .Now let us

56、 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 t

57、ransmitter 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.耐冽旦随坪砒删放愫闻耦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