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Glossary | |
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| Analogue loopback is a technique for testing transmission equipment. It isolates faults to the analogue signal receiving or transmitting circuitry by echoing back a received signal and comparing it with the original signal. |
| Automatic Repeat Request. A general term for error correction protocols which feature error detection and automatic re-transmission of defective blocks of data. |
| American Standard Code for Information Interchange. The widely accepted standard 7-bit character code used for data communications. |
| A data transmission scheme that handles data on a character-by-character basis without clock synchronisation. The character code normally includes a start bit which indicates the beginning of a data character, 5-8 data bits, an optional parity bit and one, one-and-a-half or two stop bits. |
| Some modems are capable of automatically answering a dialled-in call from another suitable modem and setting up the data link. Modems of this type are essential for use in setting up host systems such as bulletin boards which are left unattended. |
| Auto-dial modems can originate telephone calls without the use of a telephone device. This may be carried out under operator or software control. |
| The baud rate is an indication of the speed at which data is passed between two communicating devices, a higher baud rate equating to a faster transfer rate. In most cases, the bit rate is the same as the baud rate so that when the word length is 10 bits, each word representing one character, then 30 characters/second can be transferred via a 300 baud connection. |
| A bulletin board is a computer service, usually running on a microcomputer, that accepts calls via an auto-answer modem and allows callers to leave messages, transfer files etc. The software that manages the service can be left unattended to control access to the various facilities that may be available. |
| A continuous signal frequency capable of being modulated or impressed with a second data-carrying signal. |
| International telecommunications body, the International Telegraph and Telephone Consultative Committee (from the French Comite Consultatif International Telegraphique et Telephonique). Now renamed the ITU-T. |
| This term is used to describe software that allows a micro or other computer to emulate a terminal device. Packages that are available range from simple, dumb terminal emulators to more sophisticated packages which provide facilities such as automatic and selective storage of incoming data, automatic dialling of sequences of telephone numbers at particular times, error correction protocols, security etc. |
Data Communications Equipment (DCE)
| The CCITT uses the term DCE to describe communications equipment such as modems, multiplexers etc. |
| A method whereby data can be reduced in size for storage or transmission purposes. |
| A signal defined by RS232. It is a signal that passes from the DCE to the DTE to show that the modem has established a valid connection with the remote system. |
| A signal defined by RS232. It is a signal that passes from the DCE to the DTE to show that the modem is ready to communicate. |
| This is the term used by the CCITT to describe terminal equipment in a communications environment. A DTE might actually be a terminal or it could be a computer running some communications software. |
| A signal defined by RS232. It is a signal that passes from the DTE to the DCE to show that the terminal is ready to use the communications equipment. |
| Digital loopback is a technique for testing the digital processing circuitry of a communications device. It may be initiated either locally or remotely via the communications link. The device being tested will echo back a received test message which is then compared with the original message. |
| The term used to describe the process of transferring data or programs from a remote information/data service to a user. |
| Echo cancellation is used by higher-speed modems to allow for the isolation and filtering out of unwanted signal energy in the receive path caused by echoes from the transmitted signal. |
| The standard application interface to allow commands similar to those used by data modems to control all aspects of communication with a Group 3 facsimile terminal. The host DTE software performs all call set-up and management functions, including image handling. See fax operation. |
| An extended version of the Class 1 standard, in which more of the call set-up and page transmission management functions are performed by the modem itself. The host DTE still performs the image handling functions. See fax operation. |
| A mechanism that starts and stops the flow of data to compensate for differences in the flow of data into and out of a modem or other device. |
| Operation of data communications link where transmissions are possible in both directions simultaneously. |
| International standard for fax communication over ordinary telephone lines. Describes the protocol to be used between two communicating fax units. It does not describe the interface between the fax unit and any host DTE. |
| Operation of data communications link where transmissions are possible in both directions but only in one direction at a time. |
| Hardware or software handshaking is a pre-defined exchange of signals or control characters between two devices or nodes that sets up the conditions for data transfer. |
| This is a general term which is used to describe a computer which accepts data calls from other remote computers or terminals via a suitable transmission medium, usually the public telephone network. In their simplest form they may offer electronic messaging and notice board facilities e.g. bulletin boards. At the other end of the spectrum there are some very powerful information services which offer complex search and retrieval facilities for a whole range of specialist information areas. |
| The International Telecommunication Union - Telecommunications group. An international body that governs telecommunications. Formerly known as the CCITT. |
| A device which both MOdulates and DEModulates data. Modulation is the process of converting digital computer data into a form suitable for transmission over the analogue speech telephone system. Demodulation is the reverse of this, the conversion of incoming analogue data into digital form suitable for use by a computer. |
| Used to describe the state of a modem or a telephone when it is using the telephone line. |
| Used to describe the state of a modem or a telephone when it is not using the telephone line. |
| Private Branch Exchange is the term used to describe the control equipment which manages internal telephone systems such as British Telecom's Herald and Monarch. |
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A parity bit is an extra bit that is sometimes added to a character as a simple form of error check. It is best illustrated by example. The binary form of the ASCII character 'A' is 01000001. You can see that there are two '1' bits in this character. With EVEN parity added this character would become 010000010 i.e. after the addition of EVEN parity the number of 1 bits is still EVEN. If ODD parity was used instead, the character would become 010000011.
The idea is that the parity bit maintains the 'evenness' or 'oddness' of the number of 1 bits in a character. Thus when a character with EVEN parity is transmitted over a communication line it should still have an EVEN number of 1 bits at the receiver. If an error occurs during transmission which changes a 1 bit to a 0 (or vice versa) the total number of 1 bits will be no longer be EVEN and the receiver can detect the error. You can see from this that it is very important for the sender and receiver to be configured for the same type of parity. Parity checking in this way is not very effective. For example, if two bits in a character are corrupted then the parity will be correct at the receiver. |
| A protocol is a set of rules which both the sending and receiving equipment obey during communication. |
| PSTN is the abbreviation used to describe the voice telephone network. It stands for Public Switched Telephone Network, the word "switched" referring to the physical switching of electronic circuits which takes place when you make a call. |
| RS232 is the name given to a standard which defines the electrical and physical characteristics of a serial interface. The standard originated in America but is now widely accepted throughout the world. |
| A method of controlling the flow of data between two systems by means of hardware signals. Hardware flow control is the preferred means of regulating the flow of data between a modem and a computer. It operates by using the clear to send (CTS) and request to send (RTS) signals on the RS232 interface to start or stop the flow of data in each direction. In order to operate correctly hardware flow control requires a suitably wired connecting cable. |
| A serial interface is a device which converts parallel data into a serial form which may be transmitted over a two- or three-wire circuit. They also carry out the reverse operation of converting serial data into parallel form. Most modems, some printers and a few other devices require serial input and a serial interface allows the connection of such devices to terminals and computers. |
| This is an extra bit added to a character before it is transmitted so that the receiving equipment recognises the start of the character. |
| During data transfer, 1 or 2 extra bits may be added onto the end of each character by the RS232 interface so that the receiving equipment can detect the end of the character. These are the stop bits and, depending on the quiescent state of the line, may be 0's or 1's. The generation of the stop bits at the sender and then their removal at the receiving equipment, is totally transparent to the user and is carried out by the RS232 interfaces. |
| The term originally used to describe a teletypewriter and keyboard which were used to communicate with the central processing unit (CPU) of a computer. The definition now includes any device (including a personal computer running suitable software) which displays and sends serial data. |
| Universal Asynchronous Receiver/Transmitter. The device contained within all DTE and DCE equipment that performs the transfer of serial data to or from the DCE or DTE. High speed communication requires a superior UART to maintain reliable performance. The 16550 UART is commonly used in high speed serial cards on PC compatible computers. |
| The total number of bits which are recognised as comprising a unit of data transfer, a word. This need not be the same as the word length within the computer; extra formatting bits are added during data transfer (e.g. stop bits, start bits, parity). |
| A simple form of data transfer protocol which is used to control the flow of data. If the receiving system wishes to stop the transmission, for example, to save the data it has already received into a disc file, then it sends an XOFF signal to the host. When the sender receives an XOFF signal, it ceases transmission and will not resume until it receives an XON signal from the receiver. XOFF is usually the ASCII character 19 (CTRL-S) and XON is ASCII 17 (CTRL-Q). |
| A relatively simple form of file transfer protocol, developed by Ward Christiensen, that allows the transfer of any type of file, either ASCII or binary, without conversion. The protocol involves breaking the data into blocks and adding calculated checksums which can be verified by the receiver. It also caters for flow control in that the receiver can request re-transmission of blocks that have been received with errors. |
| An improved form of the Xmodem protocol with an ability to send multiple files in a single transfer. |
| Zmodem is a second generation streaming protocol for text and binary file transmission between applications running on microcomputers or mainframes. It provides increased transmission throughput and reliability compared to Xmodem. It should be regarded as the file transfer protocol of choice and is especially suitable for sending files to an unattended remote computer. |
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