【正文】 other cable connects the modem to the telephone wall jack. 外部調(diào)制解調(diào)器除了電腦 ,通常由電纜連接到計(jì)算機(jī)的串行端口。電話電纜連接 墻上電話端口 。 電話端連接到固定電話端口 247 A wireless modem may be internal, external,a PC Card. 一個(gè)無線調(diào)制解調(diào)器可能是 內(nèi)置的 、 外置的 的、 或者 一個(gè) 電腦芯片 。相反 ,無線調(diào)制解調(diào)器通過 無線電波 發(fā)送和接收。 Although still the most popular type of connection service,dialup service is quite slow, and many users find it inadequate to meet their munication needs. 雖然仍然是最 受歡迎 的連接服務(wù) ,但是撥號(hào)服務(wù)相當(dāng)慢 ,很多用戶發(fā)現(xiàn)它不足以滿足需求。 These linesknown as T1, T2, T3, and T4 linessupport all digital munications, do not require conventional modems, and provide very high capacity. 這些線已知為 T T T T4，這些 線 路 支持所有數(shù)字通信 ,不需要傳統(tǒng)的調(diào)制解調(diào)器 ,并提供非常高的 傳輸 能力 Unfortunately, this type of connection is very expensive. 不幸的是 ,這類 線路的連接費(fèi)用非常昂貴 For example,T1 lines provide a speed of mbps (over 26 times as fast as a conventional modem) and cost several thousand dollars. 例如 ,T1線路提供的速度為 mbps(速度 超過 26倍常規(guī)調(diào)制解調(diào)器 )，但 成本 卻是 數(shù)千美元。但如果能夠 負(fù)擔(dān) 這筆費(fèi)用 ,高速線路顯然能比普通線路提供更快捷的服務(wù) 。對(duì)于一個(gè)比較典型的用戶連接的成本和速度 。 ADSL(不對(duì)稱數(shù)字用戶線 )是一種 使用 最廣泛的 DSL。 Cable modems use existing television cables to provide highspeed connections as fast as a T1 or DSL connection, at a lower cost. Although cable connections reach 90 percent of the homes in America, all cable nanies do not support cable modems. Industry observers, however. Predict 100 percent availability within the next few years. 電纜調(diào)制解調(diào)器使用現(xiàn)有的電視電纜 ，以較低的成本 提供高速互聯(lián)網(wǎng)連接速度 T1 或 DSL連接 ,。 行業(yè) 認(rèn)識(shí)預(yù)測(cè)，幾年內(nèi)覆蓋率就可達(dá)到 100% 248 Satellite/air connection services use satellites and the air to download(downlink) or send data to users at a rate seven times faster than dialup connections. While older satellite services could not upload (uplink)or send data to satellites and had to rely on slow dialup connections,newer twoway satellite connections are now available. While slower than DSL and cable modems, satellite/air connections are available almost anywhere that a satellitereceiving disk can be aimed at the southern skies. 衛(wèi)星 /無線 連接服務(wù)使用衛(wèi)星和 無線 下載 (下行 ),或?qū)?shù)據(jù)發(fā)送給用戶速率比撥號(hào)連接快 7倍。雖然低于 DSL和電纜調(diào)制解調(diào)器、衛(wèi)星 /無線 連接 幾乎可以在任何地方使用 Cellular services offer an alternative for mobile devices and laptops. Using 3G cellular works, devices such as cell phones and appropriately equipped laptop puters can download data from the Inter at 400 to 700 kbps. Uploading data, however, is a much slower 50 kbps. At present,the availability of this service is limited to specific metropolitan areas. In the future, however, this service is expected to be more widespread and to operate at speeds parable to DSL. 蜂窩服務(wù)替代移動(dòng)設(shè)備和筆記本電腦。 但是 上傳數(shù)據(jù)是非常慢 ，只有 50 kbps。在未來 ,然而 ,這種服務(wù) 相對(duì)于 DSL， 應(yīng)用將更快，更廣泛 。這些因素包括帶寬和協(xié)議。 實(shí)際上 ,帶寬意味著可以在固定時(shí)間內(nèi)傳輸多少數(shù)據(jù)量 。 但是 ,為了流暢的 傳輸視頻和音頻 ,則需要比較高的帶寬 。 Voiceband, also known as low bandwidth, is used for standard telephone munication. Microputers with standard modems and dialup service use this bandwidth. While effective for transmitting text documents, it is too slow for many types of transmission, including highquality audio and video. Typical speeds are 56 to 96 kbps. 音頻帶寬 ,也稱為低帶寬 ,是用于電話溝通 的標(biāo)準(zhǔn)形式 。 能有效的傳輸文檔，但是太慢了，不能滿足 ,包括高質(zhì)量的音頻和視頻傳輸 。 Medium band is the bandwidth used in special leased lines to connect miniputers and mainframes as well as to transmit data over long distances. Unlike voice band and broadband, medium band is not typically used by individuals. “中型帶寬 用于特殊租用線路 ， 連接小型機(jī) 、大型機(jī)以及長距離傳輸數(shù)據(jù)。 Broadband is the bandwidth used for highcapacity transmissions. Microputers with DSL, cable, and satellite connections as well as other more specialized highspeed devices use this bandwidth. It is capable of effectively meeting most of today39。微型計(jì)算機(jī)與 DSL,有線電視和衛(wèi)星連接以及其他更專門的高速設(shè)備使用這個(gè)帶寬。速度通常是 mbps,當(dāng)然也可以更快 PROTOCOLS For data transmission to be successful, sending and receiving devices must follow a set of munication rules for the exchange of information. These rules for exchanging data between puters are known as protocols. 對(duì)于數(shù)據(jù)傳輸成功 ,發(fā)送和接收設(shè)備必須遵循一套通信規(guī)則。 The standard protocol for the Inter is TCP/IP (transmission control protocol/Inter protocol). The essential features of this protocol involve (1) identifying sending and receiving devices and (2) reformatting information for transmission across the Inter. 互聯(lián)網(wǎng)的標(biāo)準(zhǔn)協(xié)議是 TCP / IP(傳輸控制協(xié)議 /互聯(lián)網(wǎng)協(xié)議 )。 Identification: Every puter on the Inter has a unique numeric address called an IP address (Inter protocol address). Similar to the way a postal service uses addresses to deliver mail, the Inter uses IP addresses to deliver and to locate Web sites. Because these numeric addresses are difficult for people to remember and use, a system was developed to automatically convert textbased addres