Isochronous (Isoc)
Isochronous (Isoc) data is synchronous data transmitted without a clocking source. From the Greek isochronos, all bits are of equal importance and are anticipated to occur at regular intervals of time. Bits are sent continuously, with no start-stop bits for timing. Rather, timing is recovered from transitions in the data stream, with a whole number of bit-length intervals between characters. Bit integrity is preserved, with no modifications (i.e., bipolar conventions). Isochronous transmission is transparent and does not recognize control characters. While there are few commercial applications, some T-carrier nodes operate isochronously on the DS-1 links, syncing up with several lines operating at slightly different speeds. Isoc is often used in secure military applications that require encryption. Isochronous voice and video transmission also is supported by several recent LAN standards, such as Isochronous Ethernet (IsoEthernet), which will be discussed in Chapter 9.
Realtime, uncompressed voice communication is a type of isochronous data, as human conversation is not synchronized and can be presented in a continuous stream. If voice were synchronous, we would all talk at a precise and common rate of speed; not overtalking each other. Similarly, realtime, uncompressed video communication is isochronous, or stream-oriented.
Pleisiochronous
From the Greek pleion, meaning more, pleisiochronous communications involves careful synchronization of transmission systems of varying levels of bandwidth through the use of highly accurate clocking devices. The preferred approach involves a master clocking source such as a cesium clock. All lower order devices (switches and MUXs) slave off the master clock. Through such a technique, the T/E carrier hierarchy is developed, with a master clocking source serving to ensure that digital facilities of lesser capacities can be aggregated into facilities of higher capacities, with each set of data retaining its individual integrity.
Code Sets
Code sets or coding schemes, analogous to alphabets, are employed by all computer systems in order to create, store and exchange information. While code sets vary, they all rely on a specific combination of 1s and 0s, of a specific total length, in order to represent something of value, such as a letter, number or control character (e.g., carriage return, line feed, space, blank, and delete).
The first widely accepted standard coding scheme was Morse code, invented by Samuel Morse for use in telegraphy. Morse code used a series of dots and dashes to represent letters, numbers, and punctuation marks. In order to speed transmission, the most commonly used letters were represented by the fewest number of dots and dashes (e.g., E became •, T became –, A became • –).
Baudot Code (ITA #2)
Morse code was the primary communication code for many years, until Emile Baudot invented the Baudot Distributor in the 1870s. That device transmitted values in a 5-bit coding scheme over a line between two electromechanical devices that were synchronized. The Baudot Distributor soon gave way to the teletype, which also was based on the Baudot coding scheme, subsequently known as International Telegraph Alphabet (ITA) #2.
Baudot code (updated in 1930) is limited to 32 (25)characters. Considering that each bit has two possible states (1 or 0), 5 bits in sequence yield 25 (32) possible combinations. As 32 values is not sufficient to represent all 26 characters in the English alphabet, plus the 10 decimal digits, punctuation marks, and the space character, the shift key was used to shift between letters and other characters. Letters (LTRS) are represented by an arrow pointing down. Lower case (LTRS shift) means that all following characters are alpha characters (LTRS). Figures (FIGS) are represented by an arrow pointing up. When a FIGS character is recognized, all succeeding characters are recognized as FIGS numbers and special characters) [7-4] and [7-5].
Baudot employs asynchronous transmission, as start and stop bits separate characters. Error detection and correction requires human editing. It is, therefore, a human-to-human, rather than a machine-to-machine communication technique, with detected errors requiring retransmission.
Clearly, Baudot is a highly limited coding scheme. The limited range of expression (32 characters) is barely enough to accommodate the relatively simple English, French, and Spanish alphabets. For that matter, all letters must be in upper case. Additionally, the asynchronous requirement for start and stop bits make Baudot overhead-intensive. Finally, the error detection and correction technique is far less than desirable. As a result, Baudot currently is limited to use in very old teletypewriters.
As a footnote, 5-bit coding schemes are not necessarily overly limiting. For instance, a proprietary five-bit code is used in the airline reservation systems (e.g., American Airlines’ SABRE System and United Airlines’ APOLLO). Such applications involve a limited character set that is easily accommodated by a 5-bit code. In fact, a 7- or 8-bit coding scheme would be inefficient.
Extended Binary Coded Decimal Interchange Code (EBCDIC)
EBCDIC, developed by IBM in 1962, was the next standardized code to be used extensively. An improvement over earlier Binary Coded Decimal (BCD) (1950) and Extended Binary Coded Decimal (Extended BCD) (1951), EBCDIC was developed to allow different IBM computer systems to communicate based on a standard coding scheme. Although EBCDIC is standardized today, users can modify the coding scheme [7-4].
EBCDIC involves an 8-bit coding scheme, yielding 28 (256) possible combinations and, thereby, significantly increasing the range of expression. As a result, more complex languages can be supported, as can both upper and lower case letters, a full range of numbers (0 to 9) and all necessary punctuation marks. Equally importantly, if not more so, the 8-bit coding scheme supports a large number of control characters, which is critical in the coordination of communications between computers of real significance.
EBCDIC-based machines communicate on a synchronous basis, thereby improving on the speed on transmission. As start and stop bits do not surround each character, overhead is reduced and efficiency of transmission is improved. Further, a more complex, machine-to-machine error detection and correction technique yields improved performance in that regard-detected errors may require retransmission, although forward error correction is often employed, with the receiving system determining the errored bits and correcting for those errors.
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