Data Communications Networking Devices - 4th Ed.pdf

42 _______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSmanipulated with appropriate computer instructions. Although only 36 charactersare shown for illustrative purposes,a BCD code is capable of containing a set of 2 6or 64 different characters.EBCDIC codeIn addition to transmitting letters,numerals,and punctuation marks,aconsiderable number of control characters may be required to promote linediscipline. These control characters may be used to switch on and off devices whichare connected to the communications line,control the actual transmission of data,manipulate message formats,and perform additional functions. Thus,an extendedcharacter set is usually required for data communications. One such character set isEBCDIC code. The extended binary coded decimal interchange code EBCDIC)is an extension of the BCD system and uses 8 bits for character representation.This code permits 2 8 or 256 unique characters to be represented,althoughcurrently a lesser number is assigned meanings. This code is primarily used fortransmission by byte-oriented computers,where a byte is normally a grouping ofeight consecutive binary digits operated on as a unit by the computer. The use ofthis code by computers may alleviate the necessity of the computer performingcode conversion if the connected terminals operate with the same character set.One of the more interesting examples of computer terminology is the use of theterms bytes and octets. In the early days of computer development machines wereconstructed using different groupings of bits that were operated upon as an entityand referred to as a byte. At various times during the 1960s groupings of 4,6,8,12and 16 bits operated upon as an entity were referred to as bytes. Owing to thisambiguity,the term octet became commonly used in communications to refer to an8-bit byte. However,since virtually all modern computers use 8-bit bytes,thisauthor will use the term byte to refer to a grouping of 8 bits.Several subsets of EBCDIC exist that have been tailored for use with certaindevices. As an example,IBM 3270 type terminal products would not use a paperfeed and its character representation is omitted in the EBCDIC character subsetused to operate that type of device,as indicated in Table 1.9.ASCII codeAs a result of the proliferation of data transmission codes,several attempts todevelop standardized codes for data transmission were made. One such code is theAmerican Standard Code for Information Interchange ASCII). This 7-level codeis based upon a 7-bit code developed by the International Standards OrganizationISO) and permits 128 possible combinations or character assignments,to include96 graphic characters that are printable or displayable and 32 control characters toinclude device control and information transfer control characters. Table 1.10 liststhe ASCII character set while Table 1.11 lists the ASCII control characters byposition and their meaning. A more detailed explanation of these controlcharacters is contained in the section covering protocols in this chapter.