Read an Excerpt
Chapter 1: Digital Transmission Fundamentals
Before talking about specific technologies, we need to first understand the fundamentals of digital transmission. When we speak about protocols, we talk about parameters, fields, and various values within these fields as if they were plain text. However, what is actually transmitted over wire, through airwaves, and through computer circuitry is nothing more than electrical current (or optical, depending on the medium). Transmission is the lowest form of communications within a network.
1.1 Analog to Digital
All transmissions within a network must be converted from binary code to electrical or optical signals. All information, including the information appended by protocols, must be sent at this lowest form of transmission. In some cases, additional information is appended at this layer to facilitate timing and error control.
1.1.1 ASCII and EBCDIC
Characters must be converted into binary numbers so that they can be converted into electrical current or optical transmission. There are two standards used for representing just plain text, without any formatting. It is important to understand that these standards are not used for the text that is generated by modernday word processors or desktop publishing systems. When text is word processed, additional information must be provided by the source so that the receiver knows how the text is to look (italics, bold, underlined, specific fonts, etc.). This formatting is not represented in these two standards, but in proprietary formats handled at the upper layers of protocols. Applications receive envelopes of data that include binary information regarding theformatting of the text.
The two standards that deal with plain text are the American Standard Code for Information Interchange (ASCII) and Extended Binary Coded Decimal Information Code (EBCDIC). ASCII was developed by the American National Standards Institute (ANSI), and EBCDIC was introduced by IBM and is used predominantly by their terminal equipment.
ASCII code is much like Morse code. There are 7-bit codes for each character (both upper- and lowercase), supporting 128 characters. Some of the codes represented do not appear on screens but are used as control characters. For example, EOT is the code for end of text, which is the same as end of transmission. Today, modern protocols provide the necessary control information, and the ASCII characters are now encapsulated within the protocol envelope.
The purpose of the code was for use in terminals, which have no processing capability. A terminal receives a serial bitstream of characters in ASCII code and displays those characters as they are received. Today, ASCII code is still used, but as mentioned above, the characters are now encapsulated within the protocol used to transmit the data. Table 1.1 illustrates the entire ASCII code set. Although numbers are represented in the ASCII code set, there is another method of representing numbers called Binary Coded Decimal (BCD).
The EBCDIC code set is almost identical to the ASCII code set, with the exception of an extra bit. EBCDIC supports more graphical characters than does ASCII. However, in today's networking environment, EBCDIC is rarely used. Modern desktop publishing applications have provided a new means for conveying how information is to be displayed. Table 1.2 shows the entire EBCDIC code set.
BCD is used today to represent numbers. Originally, it was a 6-bit code set used to represent alphanumeric characters. Today, it is a 4-bit set used to represent numbers only. You will find it used within many protocols where digits must be represented. Its advantage is in the number of bits to represent a number. The code set only covers digits 0 through 9, but these can be combined to support every number conceivable, with fewer bits than would be required with ASCII or EBCDIC. BCD is used in the Integrated Services Digital Network (ISDN), where telephone numbers must be represented, and in Signaling System #7, a protocol used by telephone companies to convey control information between telephone switches. Table 1.3 shows the entire BCD code set.
One last code set that is used in many telecommunications protocols is the International Alphabet Number S (IA5) code set. This is an International Telecommunication Union (ITU) standard and is very close to the ASCII code set...
