Transmission mode

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Transmission mode:

Transmission mode Since various mainframes operate in either half- or full-duplex, a communications program should permit users to switch between these two transmission modes. Depending on the settings of the PC and the destination machine, either zero, one, or two characters may be displayed for each character typed on the keyboard.

Terminal emulation:

Terminal emulation Most of the original communications programs first written for use with personal computers transmitted and received data on a line-by-line basis. These programs did not take advantage of the screen-display capability of the personal computer and simply emulated Teletype-style transmission .

Character transfer support :

Character transfer support IBM PC applications involving communications ordinarily transmit data in seven-bit ASCII. Memory-image files can, however, be transferred only in an 8-bit representation . Also, the IBM PC and its compatibles use an extended ASCII character set, with 8-bit characters used to obtain ASCII characters from 128 to 255. Programs compiled, word processing files and most image files consist of data bytes that are represented using these extended-ASCII characters, which would preclude file transfers between personal computers if the communications program did not support 8-bit ASCII transmission.

File transfer capability :

File transfer capability The transfer of ASCII or binary files requires the use of a file transfer protocol. Currently there are over 30 file transfer protocols in use, ranging from XMODEM and its derivatives to INDSFILE used to transfer data to and from IBM mainframes. Thus, it is important to consider the target computers that you anticipate communicating with and the file transfer protocol or protocols that they support. Doing so will enable you to compare the protocols supported by different programs against your specific file transfer requirements.

Stripping/converting characters:

Stripping/converting characters Some characters transmitted from a mainframe or information utility can interfere with the intended operation of a personal computer. Others can be a display nuisance. For example, an ASCII 127 character is sometimes used for timing delay.

Buffer control :

Buffer control Communications programs use a buffer, which is a portion of random-access memory, for temporary storage when transmitting and receiving data. As the buffer fills, a flow-control procedure incorporated into most programs prevents the buffer from overflowing and thus keeps received data from being lost. The common method of implementing flow control is for the program to transmit an XOFF character when its receive buffer is between one-half and two-thirds full. If the transmitting device recognizes the XOFF, it will temporarily stop transmitting, so that the communications program can process the data in its receive buffer.

Pacing capability:

Pacing capability Pacing control, intended to keep buffers from overflowing, can involve one of three methods. The host can slow down the transmission of files to a remote personal computer by pausing for a set amount of time between each transmitted line.

Performance efficiency:

Performance efficiency

Selectable data rate:

Selectable data rate For remote communications, most programs let the user select a variety of data-transfer rates up to 33 600 or even 56 000 bps. This range is normally sufficient for many users. The introduction of modems that can operate at data rates up to 56 000 kbps means, however, that those operating rate settings may not be suitable for obtaining the full use of compression performing modems .  

Data compression:

Data compression Several communications programs incorporate data-compression techniques. These range from simple blank suppression to sophisticated encoding schemes that reduce the actual quantity of data transferred by several hundred percent or more.

Error detection and correction:

Error detection and correction For file-transfer operations, the probability of an error occurring increases with the length of the file. The communications program should be able to detect the occurrence of an error, as well as to automatically correct the error without intervention. Typically, the program first detects that an error has occurred within a data block and then asks the other device to retransmit that block.

Programmable key capability:

Programmable key capability   Some communications programs have a rigidly structured macro language. Others let the user assign a string of information for transmission to what is known as a programmable key. In this way, the user may define values associated with certain command-key combinations as well as function keys on their computer’s keyboard.

Programmable macros:

Programmable macros A macro is a command that can be stored and executed with a single keystroke. With these programmable macros, the user can store passwords and other log-on sequences and execute them with a single keystroke.

Control character transfer:

Control character transfer The most common way to tell mainframes to terminate a previously initiated operation is to send a break signal. This signal should not be confused with, for example, the CtrlþBreak (control and break) key combination on the IBM PC keyboard, which terminates the current operation and return the user to command mode. Instead of transmitting a character, the break signal causes a stream of binary zeros to occur on a line for approximately one-tenth of a second , which tells the mainframe to stop what it is doing and return to its operating-system command level.

Disk directory access:

Disk directory access For file-transfer operations, an ability to check the contents of a floppy or hard disk without exiting to the operating system can be a valuable tool. Some communications programs not only let the user specify the disk drive and display files on the default drive (that is, the drive that the operating system will go to on a command that lacks a drive identifier) but also display the amount of storage available on the drive. This feature can be quite handy for down-line loading a file, since it indicates whether or not there is sufficient room to store the file.

Editing capability:

Editing capability The ability to edit documents from within a communications program varies widely. Some packages provide only a backspace key. Others let the user perform full text editing of messages and files.

Screen-dump capability:

Screen-dump capability Members of the IBM PC family and Apple Macintosh series have a built-in command to send a dump of the screen image to an attached printer .

File closing:

File closing Only a few programs close all open files to ensure that data are not lost if the transmission line drops during a file-transfer session. Some programs are quite sophisticated, not only closing and saving whatever information was received during a partial file transfer but also allowing the user to resume filetransfer at the point where the disconnection occurred.

Performance flexibility:

Performance flexibility From within a communications session or normal command mode, a number of features can make a program more flexible. Users with non-standard screens will appreciate the ability to select display, whereas such features as editing capability benefit all users.

Protocol setting:

Protocol setting This can mean as little as setting such parameters as the number of data bits, the type of parity, and the number of stop bits transmitted with each character or as much as specifying the type of error detection and correction to be used for transferring files. Programs offering a menu of schemes permit a wider scope of potential communications, since there is no universally standardized error-detection and correction mechanism used with PC operations.

Communications port selection:

Communications port selection Expansion slots for adapter cards can support several asynchronous communications ports. Although most users have only a single modem, a serial printer could be run through the second communications port.

Switchable printer ports:

Switchable printer ports For users with both a letter-quality and a dot-matrix printer attached to their microcomputers, the ability to switch printer ports through the communications program can be a convenient feature. Users might select the letter quality device to print electronic messages from the home office that are to be reproduced for distribution to other employees. The dot-matrix printer could rapidly list data that do not require high-quality printing.

Display-width selection:

Display-width selection If the program emulates a 132-column display terminal and the hardware includes a special graphics-display board, the program can make full use of the hardware. Some programs emulate ‘wide display’ terminals by means of a horizontal scrolling mechanism. The program should be compatible with any specialized hardware used to display the full wide-display terminal screen.

Journalization :

Journalization The journalization feature lets the user record communications sessions. Some programs can log all transactions to a disk file or printer, whereas other programs let the user journalize selectively (everything transmitted, everything received , or both).

File-viewing capability:

File-viewing capability This feature permits the user to view previously stored files. The rationale for having a file viewing capability while in a communications program are twofold. First, the user may wish to preview a program as it is down-line loaded from a mainframe or utility. Secondly, and perhaps more importantly, the user may want to up-line load a certain program but has to scan a few files first to make sure the right one is sent.

Exit to OS command level:

Exit to OS command level Just about every communications program provides a simple mechanism to exit the program and re-enter the personal computer’s operating system. If this feature is not offered, the operating system must be reinitialized and the program terminated each time another program is invoked, which may be awkward and inconvenient.

Lowercase/uppercase conversion:

Lowercase/uppercase conversion Some mainframe computers support only uppercase letters. To be able to communicate with such machines, a program must perform a lowercase-to uppercase conversion prior to transmitting data. If the program does not include this feature, an alternative is to use the ‘Caps Lock’ key to ensure that conversational-mode communications are in uppercase.

Line-feed control:

Line-feed control This option lets the user adjust the program’s actions based on the presence or absence of line feeds after carriage returns. If neither the communications program nor the computer at the other end of the data link adds line feeds each transmitted line during file transfer, the data received will be treated as one long, continuous line.

Security performance:

Security performance Safeguarding user data is a familiar concern, but few users realize that the copy protection they can find so irritating is merely a security precaution taken by software vendors.

Encryption capability:

Encryption capability A few communications programs incorporate a security-mode data-transfer feature. Typically, this feature uses modulo-2 addition, adding together the binary value of the data to be transferred and a binary key that the program generates in a byte-to-byte fashion.


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