Gopher程序是一个很有用的工具，你可以用它来翻遍整个因特网来查找东西。
布告牌系统（BBS）
BBS是电子布告牌系统的缩写词，也是一种程序，可以让其他人拨号进入一台计算机，复制文件、留信息，或支付他们费用。
【Reading Material】
Serial Communications
The need to communicate between distant computers led to the use of the existing phone network for data transmission. Most phone lines were designed to transmit analogue information--voices， while the computers and their devices work in digital form--pulses. So， in order to use an analogue medium， a converter between the two systems is needed. This converter is the “modem”， which performs Modulation and Demodulation of transmitted data. It accepts serial binary pulses from a device， modulates some properly （amplitude， frequency， or phase） of an analogue signal in order to send the signal in an analogue medium， and performs the opposite process， enabling the analogue information to arrive as digital pulses at the computer or device on the other side of connection.
PCs have always provided the means to communicate with the outside world--via a serial communications port--but up until tile 1990s， it was a facility that was little used. The ability to access bulletin boards and to communicate via fax did attract some domestic users， but in general a modem was considered as a luxury item that could be justified only by business users. The tremendous increase in the popularity of the Internet has changed all that in recent years and nowadays the ability to access the World Wide Web and to communicate via email is regarded as essential by many PC users.
Modems
A modem allows a PC to connect to other computers and enables it to send and receive files of data over the telephone network. At one end it converts digital data into a series of analogue signals for transmission over telephone lines， at the other it does the opposite， converting an analogue signal into digital data.
Modems come in two types， internal， fitting into an expansion slot inside the PC’s system case or external， connected to tile PC via one of its serial ports （COM1 or COM2）.
Early modems were asynchronous devices， operating at slow rates of up to 18000bit/s in FSK modulation， using two frequencies for transmission and another two for receiving. Asynchronous data is not accompanied by any clock， and the transmitting and receiving modems know only the nominal data rate. To prevent slipping of the data relative to the modems’ clocks， this data is always grouped in very short blocks （characters） with framing bits （start and stop bits）. The most common code used for this is the seven-bitASCII code with even parity.
Synchronous modems operate at rates up to 56 K-bit/s in audio lines， using synchronous data. Synchronous data is accompanied by a clock signal and is almost always grouped in blocks. It is the responsibility of the data source to assemble those blocks with framing codes and any extra bits needed for error detecting and/or correcting according to one of many different protocols （BISYNC， SDLC， HDLC， etc.）. The data source and destination expect the modem to be transparent to this type of data； conversely， the modem can ignore the blocking of the data. The usual modulation methods are the phase modulation and integrated phase and amplitude.
Fax modems
Nearly all modems now include some sort of fax capability and usually come with bundled software which provides a PC with most of the functionality of a fax machine. Digital documents can be converted to analogue， ending up as an image file （if the receiver is another fax/modem）， or a printed document （if received by a facsimile machine）， incoming faxes received as image files are saved to the PC’s hard disk.
Fax-modems exploit the intelligence of the PC at their disposal to do things standalone fax machines can’t. For instance， faxes can be scheduled to be sent when the phone rates are cheaper. Also， since the data they receive is in digital form， it is immediately available on the PC for editing or retouching before printing. One of the common features in fax software is a cover-sheet facility which allows the definition of a fax cover-sheet. There’s often a quick-fax facility， too， which allows a single page fax to be created without the hassle of loading a word processor.
Group 3 fax/modems provide various levels of processing based upon their service class. Class 1 devices perform basic handshaking and data conversion and are the most flexible， because much of the work is done by the computer’s CPU. Class 2 devices establish and end the call and perform error checking. There are a variety of de facto Class 2 implementations and one Class 2.0 standard. As PCs have become more powerful， future service classes with more features are unlikely.
One problem with scanned images and received faxes is that they hog large amounts of disk space. Some bundled fax software includes an optical character recognition facility （OCR） which allows received faxes or scanned images to be converted from bitmap format to normal text. This not only reduces document sizes but also allows them to be edited in a word processor.
Voice modems
Voice modems are part of the current communications convergence trend--the merging of voice， data， fax， and even video--which is affecting all aspects of data communications. Consider the Internet， originally a file transfer system， which is now transmitting radio signals， real-time audio， telephone conversations and， for those who have the bandwidth， live video. Now， a number of modem manufacturers have produced modems which can answer phones and record voice messages.
Such multi-purpose modems perform as anything from a simple answering machine （recording messages on the hard disk） to a complete voicemail system with hundreds of boxes， message forwarding， and fax retrieval service. Incoming data or fax calls are automatically directed to the appropriate software module and voice calls passed through to the answering machine/voicemail software.
【New Words】
