【正文】 簽名： 日期： 2 外文資料譯文（一） 電路交換網(wǎng)與 vb 調(diào)用數(shù)據(jù)庫 切換電路的連接設(shè)置的結(jié)果是保留了從發(fā)送端到接收端的包所通過的線路帶寬。假如沒有帶寬被保留用于包交換，那么所有的包將不得不等待著被轉(zhuǎn)發(fā)。 包交換是利用存儲器并向前發(fā)送的。在以前，用戶決定介質(zhì)的大小，速度和特性。但是主干網(wǎng)絡(luò)中心的收費是基于它們的流量的。 4 無線電話可以分為基本的兩種：無繩電話和移動電話（有時也叫蜂窩電話）。相反的，當(dāng)移動電話來到歐洲時，由于每個國家都設(shè)計了自己的系統(tǒng)，所以結(jié)局很慘敗。在美國移動電話和固定電話是共用的，因此對于用戶沒有辦法區(qū)分 (212)2345678到底是固定電話 (撥打是便宜的或者是免費的 )還是移動電話 (撥打是昂貴的 )。結(jié)果在歐洲幾乎所有的青年和小孩都有一個 (預(yù)付費的 )移動電話，這樣他們的父母就可以準(zhǔn)確的定位他們而不用去擔(dān)心孩子們會用掉一大筆帳的。像這樣的系統(tǒng)稱做被動式系統(tǒng)，從 1950 開始它就被安裝在很多的城市了?？偠灾@有限的容量使用這系統(tǒng)有點不合實際應(yīng)用。它對許多 API（如 ODBC API、 SOCKET API等等）的封裝使得編程變得簡單起來。本例中的存儲過程代碼如下（使用 PUBS 的MS SQL 中的例子庫 ）： CREATE PROCEDURE myprocedure job_id smallint, 8 job_lvl tinyint AS SELECT * FROM employee WHERE job_id job_id AND job_lvl job_lvl 2． 在 VB中生成一個新的工程，工程有一窗體，一個 COMMAND（ NAME： COMMAND1） 按鈕，一個 MSFlexGrid（ NAME： MSFlexGrid1）控件。 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(., a calling plan with 2020 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 works based on the volume of their traffic. The differences are summarized in . Item Circuit Switched Packet Switched Call setup Required Not needed Dedicated physical path Yes No Each packet follows the same route Yes No Packets arrive in order Yes No Is a switch crash fatal Yes No Bandwidth available Fixed Dynamic 12 Time of possible congestion At setup time On every packet Potentially wasted bandwidth Yes No Storeandforward transmission No Yes Transparency Yes No Charging Per minute Per packet Both circuit switching and packet switching are important enough that we will e back to them shortly and describe the various technologies used in detail. THE MOBILE TELEPHONE SYSTEM The traditional telephone system(even if it some day gets multigigabit endtoend 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 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 e 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 working ,so we will not examine them further .Instead we will concentrate on the mobile system ,which is used for wide area voice and data munication. Mobile phones have gone through three distinct generations ,with different technologies: 1. Analog voice 2. Digital voice 3. Digital voice and data(Inter , ,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 . by ATamp。 Establishes ADO controls the connection character string and the initial record source, namely demonstration content = Provider=。 18 5. The foundation parameter and establishes each parameter the attribute。 result collection Dim strCnn As String 39。 the release connects In button code as follows: 19 Dim i As integer Dim j as integer Set parm_jobid = New Set mymand = New \39。 opens connects Joins the code as follows in window UNLOAD: 39。 parameter 1 Dim parm_joblvl As 39。 Also may use in VB the data object, like RDO (longdistance data object), 17 DAO (data visit object), ADO (ActiveX data object), this method realizes conveniently, quickly, but flexible worse somewhat. As a result of mem