电子邮件让用户能撰写信函并将其发送给一个人或一个群。邮件应用程序的另一个功能是允许用户去阅读已经收到的信函。电子邮件如此成功，所以有很多因特网用户依靠它进行正常的通信。虽然现在有很多电子邮件系统，但采用TCP/IP使得邮件发送更可靠，这是因为TCP/IP不依靠中间的计算机去转发报文。TCP/IP邮件发送系统是发送方机器与接收方机器直接进行通信的。因而发送者知道，一旦报文本离开本地机器，就能被目的站点成功接收到。
在因特网上有两种电子邮件协议，一是简单邮件传送协议SMTP，它接收送达的邮件并从中复制报文到相应的邮箱。另一个邮件协议是邮局协议POP3，它从远程邮箱中取出电子邮件并将其存储在用户的本地机器上，以便以后阅读。
（3）文本传送协议（FTP）
FTP是TCP/IP协议栈中的应用协议，用来在网络节点间传送文件。
虽然用户有时用电子邮件传送文本，但邮件主要用来传送短的文本报文。TCP/IP协议包含一种文件传送应用程序，它允许用户发送和接收任意规模的数据或程序文件。例如，用文件传输程序，可以将含有卫星图像的大型数据库，用 Pascal 或 C++语言写的程序或一本英语词典从一台机器复制到另一台机器上。该系统提供一种对授权用户的检查办法，甚至可阻止所有的访问。与邮件一样，文件传输跨TCP/IP协议下的互联网是可靠的，因为所涉及的两台机器直接通信，在一条通路上进行复制，而不依靠中间机器。
（4）远程通信（Telnet）
它是TCP/IP协议栈中的标准终端仿真协议。Telnet用于远程终端连接，它允许用户登录到远程系统上，并像连接到本地系统那样使用资源。远程登录使一台计算机上的用户连到远程机器上并建立一个交互登录会话。远程登录将用户屏幕窗口直接连接到远程机器上，于是，用户键盘上的每一个击键都会发送到远程机器上，而远程计算机在该用户窗口上输出的每一个字符都会显示出来。当远程登录会话终止时，该应用程序将返回到本地系统。
【Reading Material】
Web Services
Web Services Interfaces are defined using XML in the form of a “schema” that describes the request and response data formats， types， and relationships. XML and the schema definition language are both standards published by the overseers of Internet standards， the W3C. Defining a schema in order to use XML and Web Services is not required， but it is a best practice since the schema definition “binds”the data contract of the service interface. It helps to explicitly define what the service will and will not do， so that developers of applications that use the service will know what to expect.
Web services technology supports increased operational efficiencies and improved services by allowing multiple applications to interoperate XML， the language of Web Services， is simple and non-proprietary. It is adaptable， extensible and supported by industry-developed standards—Web Services and XML messaging standards help IT managers resolve technology decisions by devolving application issues from infrastructure， and the limits otherwise imposed by proprietary protocols and features.. This layer of abstraction also allows applications and the middleware platform to embrace new technology and the current and future standards of multiple vendors. Using the interface， software developers can concentrate on creating the best business solution， without concern for the complexities of the underlying middleware.
The key use of Web Services technology within the enterprise is for Enterprise Application Integration （EAI）. The problem mentioned above， involving connecting your Website to both your loyalty system and reservation system， is an example of an EAI problem. The problem gets worse when combined with your colleagues CRM project down the hall because they need to connect to those systems as well. As you will often find， multiple groups within IT are all individually trying to discover， analyze and design ways to connect to different legacy systems and data sources at the same time， creating redundancy， overlap， and reducing tile chance for success.
There are several ways to use Web Services to solve this problem. The first is to enable the existing systems with Web Services technology by either updating the application， or by placing ail EAI “adapter” in front of the application to make it look like a Web Service. The choice of rewriting the application is usually not financially viable so the adapter approach will be the focus here.
There are two types of adapters， the first is an extension of the application itself that can be custom built but is often based on utilities provided by the vendor of the platform upon which the application resides. Companies such as SAP People Soft， Oracle， as well as specialist third party EAI providers， supply adapters for common Enterprise Resource Planning （ERP） systems and database applications. These application-side adapters usually require some modification to the existing application to be enabled or plugged in.
The second method is to create an outboard application broker that resides on its own platform and requires little or no application modifications. The broker can sit in between multiple back-end systems where it provides a consolidated， consistent and normalized view of those systems and data sources. For many EAI problems in the travel distribution and customer service arena， this is the preferred approach. Applications can then avail themselves of flexible services such as “Air-Booking” where the only difference for loyalty bookings is a change to the Form of Payment field. In this way， you’re Website and your CRM systems do not need to have an awareness of the subtleties and intricacies of the individual back-end systems. Rather， they can enjoy the use of higher-level services defined in a neutral and explicit fashion via XML schemas mid accessed using a standard protocol， SOAP.
【New Words】
schema
计划
middleware
中间设备，中间件
extensible
可扩展的
infrastructure
下部构造，基础下部组织
EAI
电子联合公司
modification
更改，修改，修正
back-end
后端
awareness
知道，意识到
6.6 B-ISDN
Today’s telecommunication networks are characterized by specialization. This means that for every individual telecommunication service at least one network exists that transports this service. A few examples of existing public networks are described below：
A telex network transports telex information i.e. messages of characters， transported at very low speed. The characters are coded based on a specific 5-bit code.
POTS （plain old telephone service） are transported via the public switched telephone network （PSTN）. This ubiquitous network offers the customersr classical two-way voice conversation.
Computer data are transported in the public domain either by a packet switched data network based on X.25 protocols or in a much bruited number of countries by a circuit switched data network based on X.21 protocols.