Communication Concepts Ch. 1.pptx

Communicat ion Concepts Ch. 1 Communication Concepts This first chapter deals with the fundamentals of data communications We primarily probe and discuss the factors that affect all communications from a “big picture” perspective, so that when the details are presented in later chapters, you wil...

Communicat ion Concepts Ch. 1 Communication Concepts This first chapter deals with the fundamentals of data communications We primarily probe and discuss the factors that affect all communications from a “big picture” perspective, so that when the details are presented in later chapters, you will know which niche is being filled A portion of this chapter —the discussion of data organization sometimes called a coding —is almost a technical history in itself Elements In any communication there must be a source and one or more destinations The purpose of communication is to transmit data from the source to the destination In data communications, the media we use are: electrical conductors , light pipes , and electromagnetic transmission Data communications topologies are organized as oneto-one , one-to-many , or many-to-many Members of a network may have defined relationships: they may have no ranking or they may be ranked Modes In data communications, there are three modes of transmission: simplex, half-duplex, and duplex These terms may be used to describe all types of communications circuitry or modes of transmission, whether they are point-topoint, multi-drop, or networked A communications channel may consist of a communications circuit, which can be either a hardware configuration or a “virtual” circuit NOTES The differences between a “mode” and a “circuit” are rather arbitrary However, the reader needs to be aware that not all channels are hardware, and even though a channel is physically capable of operating in a certain mode, it does not mean that the channel is being utilized in that mode As an example, a duplex channel could be operated in half-duplex mode In this mode, communication occurs only in one direction —never in the opposite direction; in figure 1-2 it is from Station A to Station B. The circuit that provided this mode of operation was originally called simplex , but this led to confusion with more current telephony terminology NOTES “Unidirectional” is a more appropriate name for this mode of transmission and using the name for this circuit would be much more descriptive; however, old habits are hard to change, therefore we will use the term simplex in this text so the reader will not be confused when referencing technical data Half-duplex communication functions much like meaningful human conversation does, that is, one speaker at a time and one listener In duplex mode, communication can travel in both directions simultaneously: from A to B and from B to A at the same time Digital Signals A digital signal is defined as one with defined discrete states; no other state can exist Binary digital signals have only two states: a binary digit is either a “1” or a “0.” One of the bit patterns to be organized has to represent text, for in order for data organization to yield anything usable to humans, it is necessary to store and present information in a human-readable form American Standard Code for Information Interchange is an example of a text coding A standard signal is one that has the approval of the users and/or a standardization agency; it specifies a way to organize data This book focuses, in many respects, on how data is organized for different functional tasks Analog Standard Signals An analog signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal In short, an analog signal is a model representing quantities at any value between a specified set of upper and lower limits The 4–20 mA signal is one of the most commonly used industrial communications standards and it is used in the two-wire current loop that connects devices in an instrument circuit The reader needs to become familiar with the terms: voltage, current, resistance, impedance, capacitive reactance, and inductive reactance Definitions Voltage is the difference in electrical potential that is necessary prior to any work being performed Impedance is the opposition a conductor offers to a changing current value, as well as the steady state resistance A “standard signal” could be called a coding and in fact is referred to as such in the The ISA50 committee established a current loop as a standard because it has low impedance, it has a greater immunity to noise than a high-impedance voltage circuit, and it can power the loop instruments Standard signals allow users to pick and choose among vendors with the confidence that the inputs and outputs will be compatible Definitions The standard signals also allow a manufacturer to build instruments for a larger group of users than if each instrument had its own defined set of signals Digital Standard Signals However, in the measurement and control areas, no open, all-digital standard signal has been established as an alternative to the analog 4– 20 mA signal described in the last section The international standard for an industrial fieldbus defines eight different fieldbuses, four of which— including PROFIBUS PA and FOUNDATION Fieldbus—are primarily used in process control Process measurement and control has used and still uses several digital communications standards —such as EIA/TIA 232 or EIA/TIA 485 —that detail placing data on the media, which is a requirement if one is going to signal via an electrical channel Data Organization: Communications Codes Communications codes are the components of data organization that are designed for interface with humans IBM designed one of the first digital communications codes: the IBM 4 of 8 Code which allowed error detection on a characterby-character basis Several months later, IBM released its own code, an extended version of the 4 of 8 Code: EBCDIC, which is short for “Extended Binary Coded Decimal Interchange Code” The IBM 4 of 8 Code THE IBM 4 OF 8 CODE IS ILLUSTRATED IN TABLE 1 IN THIS CODE, THERE ARE FOUR 1S AND FOUR 0S —THAT IS 8 BITS, OR AN OCTET —FOR EACH CHARACTER THIS ARRANGEMENT WAS USED TO DETECT CHARACTER ERRORS, YET IT CARRIED A LARGE OVERHEAD; THAT IS, THE RATIO OF BIT PATTERNS USED FOR ERROR DETECTION TO THOSE FOR DATA TRANSMISSION IS HIGH International Telegraphic Alphabet #5 Also known as ASCII, the International Telegraphic Alphabet #5 does not make use of letter frequencies, as did the Morse code, the ITA2 Telegraph Code, and the IBM 4 of 8 Code It does, however, use a numerically arranged code, in which the alphabet is in ascending order, which permits sorting operations and the like ASCII is a 7-bit code Its arrangement is well thought out; particularly in light of the technology available at the time it was designed It was usually transmitted as an 8-bit signal with the “Most Significant Bit” reserved for parity—an error detection scheme International Telegraphic Alphabet #5 The parity bit was used for a number of years; however, the technological advantages to using an 8-bit data byte eliminated the need to use the B7 bit for parity Today when parity is used, another bit is added to the 8 bits as the parity bit, making the signal 9 bits in length The difference between uppercase and lowercase is bit Extended Binary Coded Decimal Interchange Code As described previously, the Extended Binary Coded Decimal Interchange Code was developed in the early 1960s by IBM from its 4 of 8 Code and it is proprietary to them Most computers using ASCII transmit 8 bits and use a ninth bit for parity, if parity is used In EBCDIC, the blank spaces are used for special or graphic characters particular to the device using them EBCDIC did not require a parity bit for error detection but instead used a different errordetection scheme—CRCC Unicode Character coding by electrical means originally became established with the Morse code, in which letter frequencies and a form of compression were used for efficiency Fixed-length character representations, of which ASCII and EBCDIC are examples, came about in the early 1960s and are still in wide use today However ASCII was designed for English and, as communications are now global, there are many languages that require a larger number than 128 patterns The coding begins at U+0000 with standard ASCII characters, and then continues with Greek, Cyrillic, Hebrew, Arabic, Indic, and other scripts Data Organization: Error Coding How we organize our data has a lot to do with how we recognize and correct transmission errors For the purposes of this discussion, we will assume that the data to be transmitted is correct; the matter of erroneous data is not within the scope of this book After the data has been taken off of the medium, we can only detect errors if the data was organized in a way to detect errors prior to transmission Parity Using 7 bits of an 8-bit structure, as ASCII does, will leave 1 bit extra and the parity bit is added to the 8-bit character, making it 9 bits Parity means to count the number of 1s in a character The agency determining the system’s operating specifications will have decided on which parity to use: “odd” or “even.” Block Parity The vertical character format mentioned previously is based on the punched cards used in early data entry and programming Because punched cards contained 80 columns and historically most data was on these cards, transmissions were in an 80-character “block.” Two framing characters were added at the start of the data block and one framing character was added at the end of the data block An 84th character, called the block parity character, was added to the end of the transmitted block To determine if parity was met for all the characters in the block, parity was determined horizontally for the Parity row by counting the 1s in the 83 counted columns and the result was compared to the vertical parity bit for the block parity character Error Correction In addition to having bits devoted to assisting with error detection, what means could be used to correct an error, once one was detected? The answer is normally an automatic retransmission query , known as an ARQ This method had significant ramifications in many applications since the transmission device had to store at least the last transmitted block We normally speak of packet transmission in modern data communications, although the difference between a block and a packet is now more one of semantics than of practice Cyclic Redundancy Checks Any time data is placed on a medium , magnetic or optical rotating media, and fiber cable), there is a probability of error; thus, an error detection scheme must be used Most modern devices use a far more efficient scheme of error detection than parity checking Though ASCII was designed for a vertical and horizontal blockchecking code, it can also be used with a cyclic redundancy check scheme that creates a check character A CRC character is the number 8-bit check character developed by the CRC A cyclic code divides the text bits with a binary polynomial , resulting in a check character Checksum Any of a number of error-detection codes may be called a checksum Many times the CRC-16 or CRC-CCITT is called a checksum; however, they are actually cyclic codes, whereas a checksum was originally designed as a linear code The efficiency of a checksum in detecting errors is not as high as with a cyclic code, yet the circuitry to produce the checksum is less complex Some of these codes are cyclic, some are linear, some are quasi-cyclic, and some are polynomial ARQ What hasn’t yet been explained is what happens when an error is detected The following is a simplified discussion of a generalized automatic retransmission query , the ARQ method of error correction Before beginning our discussion, it is important to note the change in terminology as multiple terms are used to refer to the data being transmitted Prior to HDLC, data sent was defined by control characters and was referred to as data blocks record) Since 1990, the terminology has been changed again ; the Layer 2 octets are called frames and the arrangement of data in Layers 3 and above is referred to as packet data units ARQ THE FRAME IS RECEIVED Forward Error Correction Although the ARQ method of error correction discussed above detects an error and then retransmits the portion of the message that was in error, what happens when there is simplex transmission? The ARQ method will not receive an ACK/NAK because the circuit is unidirectional, so ARQ cannot be used on a simplex type circuit FEC requires that the communications network’s pattern of errors be analyzed and an error-correcting algorithm be designed to fit the identified error patterns A thorough discussion of error-correcting algorithmic codes and associated theory is beyond the scope of this text Data Organization: Protocol Concepts WE USE PROTOCOLS IN PERSONAL INTERACTIONS ALL THE TIME WHEN YOU ARE INTRODUCED TO SOMEONE IN THE UNITED STATES, THE PROTOCOL IS TO SHAKE HANDS, UNLESS THERE IS A CEREMONY REQUIRING ANOTHER FORM OF PROTOCOL OR YOU ARE BEING INTRODUCED TO A LARGE NUMBER OF PEOPLE THERE IS A PHYSIOLOGICAL REASON FOR THIS PROTOCOL: HUMANS ARE EFFECTIVELY HALF-DUPLEX—THEY TYPICALLY CANNOT TALK AND LISTEN VERY WELL AT THE SAME TIME Terminologies ASYNCHRONOUS GENERALLY MEANS THAT SOMETHING MAY OCCUR AT ANY TIME AND ITS OCCURRENCE IS NOT TIED TO ANY OTHER EVENT THE OLD “STARTSTOP” TELETYPEWRITER SIGNAL (SEE APPENDIX IN TODAY’S VERNACULAR, ANY START-STOP SIGNAL IS ASSUMED TO BE ASYNCHRONOUS SINCE SYNCHRONOUS TRANSMISSION USES BIT TIMING IS IN SYNCHRONY WITH THE CHANGE IN BIT VALUE ), EACH BIT OF DATA MUST BE ACCOUNTED FOR BAUD RATE IS THE LINE MODULATION RATE; THAT IS, THE NUMBER OF SIGNAL CHANGES PER SECOND THAT ARE PLACED ON THE COMMUNICATION MEDIUM IN A GIVEN TIME BAUD PER SECOND IS A TERM THAT IS USED TO DESCRIBE A CHANGE IN A TRANSMISSION MEDIUM SIGNALING STATE, NOT A LINE SPEED Protocols AS ELECTRONIC MESSAGE HANDLING BECAME THE NORM, IT BECAME IMPERATIVE TO BUILD AS MUCH OF THE COMMUNICATIONS LINK CONTROL INTO THE TERMINALS AS POSSIBLE ALL DATA COMMUNICATIONS EFFORTS INVOLVE PROTOCOLS OF ONE KIND OR ANOTHER COMMUNICATIONS PROTOCOLS ARE EITHER CHARACTER-BASED OR BITORIENTED Character-Based Protocol One example of a character-based protocol is Binary Synchronous Communication The IBM Bi-Sync protocol, one of the first and most widely used of the proprietary protocols, was developed to link the IBM 3270 line of terminals to IBM computers in a synchronous manner The Bi-Sync protocol is character-based; control depends on certain character combinations, rather than on bit patterns Bi-Sync normally uses duplex transmission Duplex operation would not have been faster, except for eliminating the turnaround time, because the transmitter could do nothing else until it received a response to its transmitted block ACK1 and ACK2 signals were used to differentiate between ACKs Character-Based Protocol The primary benefit in this case was that transmission could take place without line turnaround or without halting transmission until an ACK was received It should not be forgotten that when BiSync was developed , the cost of data storage was approximately one US dollar per bit in 1980 dollars—a significant expense Bit-Oriented Protocol Bit-oriented protocols use a concept called framing, where there are bit patterns before and after the information frame In framing, there is a binary pattern that is the start delimiter There are also binary patterns that indicate the addressing and what type of frame it is , as well as some method of sequence numbering followed by the user data It is very similar in both structure and format to other bit-oriented protocols: High Level Data Link Control , Advanced Data Communications Control Procedure , and IBM’s Synchronous Data Link Control The protocol only allows the frame to have this pattern at its start and end The receiver protocol, upon detection of five consecutive 1s, knows to remove the 0 that follows It is that simple BitOriented Protocol FIGURE 1-12 ILLUSTRATES 0 INSERTION AND REMOVAL Protocol Summary WE HAVE LOOKED AT TWO PROTOCOLS, ONE CHARACTER-BASED AND ONE BIT-ORIENTED THE BIT-ORIENTED PROTOCOLS DO NOT DEPEND ON THE DATA STREAM CONTENTS; THEY OPERATE INDEPENDENTLY FRAMING THE DATA USUALLY MEANS HAVING SOME FORM OF START DELIMITER , SOME SORT OF ADDRESSING AND CONTROL PROCESS, THE ACTUAL USER DATA, AN ERROR CHECK ON THE FRAME, AND A STOP DELIMITER IN THE END, THE USER DATA IS A SERIES OF ORGANIZED 1S AND 0S AND A BITORIENTED PROTOCOL ONLY KNOWS HOW MANY BITS IT REQUIRES IN A GIVEN PROTOCOL, NOT WHAT THEY REPRESENT Summary In this chapter we have reviewed the organization of data, as organized into characters and as organized to detect errors in transmission The first data transmission code devoted more overhead to the error-detection scheme than to the data transmitted More than anything, this chapter has served to introduce you to the foundational concepts of data communications and how communications are organized into modes of transmission, into character codes, and into protocols