Comms 3 Reviewer: Introduction to Data Communications PDF

Comms 3 Reviewer: Introduction to Different sets of bit patterns have Data Communications been designed to represent text symbols. Each set is called a Telecommunication – means...

Comms 3 Reviewer: Introduction to Different sets of bit patterns have Data Communications been designed to represent text symbols. Each set is called a Telecommunication – means code, and the process of communication at a distance (tele is representing symbols is called Greek for "far"). coding. Data – refers to information presented in The prevalent coding system is whatever form is agreed upon by the called Unicode, which uses 32 parties creating and using the data. bits to represent a symbol or character. Data communications - exchange of data between two devices via some form of transmission medium. The American Standard Code for Delivery - the system must deliver Information Interchange (ASCII), data to the correct destination. developed some decades ago in the Data must be received by the United States, now constitutes the first intended device or user and only 127 characters in Unicode and is also by that device or user. referred to as Basic Latin. Accuracy - The system must Numbers - also represented by bit deliver the data accurately. Data patterns. However, a code such as ASCII that has been altered in is not used to represent numbers; the transmission and left uncorrected number is directly converted to a binary is unusable. number to simplify mathematical Timeliness - The system must operations. deliver data in a timely manner. Data delivered late is useless. Images - also represented by bit Jitter - refers to the variation in the patterns. In its simplest form, an image is packet arrival time. It is the composed of a matrix of pixels (picture uneven delay in the delivery of elements), where each pixel is a small audio or video packets. dot. - One method is called RGB, so called because each color is made COMPONENTS OF DATA of a combination of three primary COMMUNICATIONS SYSTEM colors: red, green, and blue. Message - is the information (data) to be - Another method is called YCM, in communicated. Popular forms of which a color is made of a information include text, numbers, combination of three other primary pictures, audio, and video. colors: yellow, cyan, and magenta. Sender - is the device that sends the data message. Audio - refers to the recording or broadcasting of sound or music. It is Receiver - device that receives the continuous, not discrete. message. Video - refers to the recording or Transmission medium - physical path broadcasting of a picture or movie. Video by which a message travels from sender can either be produced as a continuous to receiver. entity. Protocol - set of rules that govern data communications. DATA FLOW Simplex - the communication is DATA REPRESENTATION unidirectional, as on a one-way street. Text - represented as a bit pattern, a (transmit, receive). sequence of bits (0s or 1s). Half-Duplex - each station can both transmit and receive, but not at the same time. (Sending, receive, vice versa). Full-Duplex - (also called duplex), both CATEGORIES OF TOPOLOGY stations can transmit and receive Mesh - every device has a dedicated simultaneously. point-to-point link to every other device. -The term dedicated means that the link NETWORKS carries traffic only between the two devices it connects. - a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network. NETWORK CRITERIA Star Topology - each device has a dedicated point-to-point link only to a Performance - can be measured in central controller, usually called a hub. many ways, including transit time and response time. -depends on a number of factors, including the number of users 1. Transmit Time - amount of time required for a message to travel from one device to another. Bus Topology - is multipoint. One long 2. Response Time - elapsed time cable acts as a backbone to link all the between an inquiry and a devices in a network. response. Reliability - network reliability is measured by the frequency of failure, the time it takes a link to recover from a failure, and the network's robustness in a catastrophe. -A drop line is a connection running Security - issues include protecting data between the device and the main cable. from unauthorized access, protecting -A tap is a connector that either splices data from damage and development, and into the main cable or punctures the implementing policies and procedures for sheathing of a cable to create a contact recovery from breaches and data losses. with the metallic core. Ring Topology - each device has a TYPES OF CONNECTIONS dedicated point-to-point connection with only the two devices on either side of it. Point-to-Point - provides a dedicated link between two devices. Multipoint - (also called multidrop) connection is one in which more than two specific devices share a single link. Physical Topology - refers to the way in Hybrid Topology - A network can be which a network is laid out physically. hybrid. For example, we can have a main Topology – of a network is the geometric star topology with each branch representation of the relationship of all connecting several stations in a bus the links and linking devices (usually topology as shown. called nodes) to one another.  In the mid-1960s, mainframe computers in research organizations were standalone devices. In 1967, at an Association for Computing Machinery (ACM) meeting, ARPA presented its ideas for ARPANET, a small network of connected computers. By 1969, ARPANET was a reality. Four nodes, at the University of California at Los Angeles (UCLA), the University of California NETWORK MODELS at Santa Barbara (UCSB), Stanford Research Institute (SRI), and the University The OSI (Open Systems of Utah, were connected via the IMPs to form Interconnection) model defines a a network. seven-layer network Software called the Network Control The Internet model defines a five-layer Protocol (NCP) provided communication network. between the hosts. A LAN normally covers an area In 1972, Vint Cerf and Bob Kahn, both of whom were part of the core ARPANET group, less than 2 mi; a WAN can be collaborated on what they called the worldwide. Internetting Project1. A local area network (LAN) is usually Shortly thereafter, authorities made a privately owned and links the devices in decision to split TCP into two protocols: a single office, building, or campus Transmission Control Protocol (TCP) and Internetworking Protocol (lP). A wide area network (WAN) provides long distance transmission of data, image, audio, and video information over INTERNET TODAY large geographic areas that may comprise a country, a continent, or even Today most end users who want Internet the whole world. connection use the services of Internet service providers (lSPs). The switched WAN connects the end systems, which usually comprise a router There are international service providers, national service providers, (internetworking connecting device) that regional service providers, and local connects to another LAN or WAN. service providers. The point-to-point WAN is normally a line leased from a telephone or cable TV provider that connects a home computer International Internet Service Providers or a small LAN to an Internet service At the top of the hierarchy are the provider (lSP). international service providers that connect nations together. A metropolitan area network (MAN) is a network with a size between a LAN and a National Internet Service Providers WAN. It normally covers the area inside a town or a city. The national Internet service providers are backbone networks created and maintained by specialized companies. Brief History of Internet To provide connectivity between the end users, these backbone networks are An internet (note the lowercase letter i) is connected by complex switching stations two or more networks that can communicate (normally run by a third party) called network with each other. access points (NAPs). The most notable internet is called the Some national ISP networks are also Internet (uppercase letter I), a collaboration connected to one another by private of more than hundreds of thousands of switching stations called peering points. interconnected networks. These normally operate at a high data rate Those standards that have been legislated (up to 600 Mbps). by an officially recognized body are de jure standards. Regional Internet Service Providers STANDARD ORGANIZATIONS Regional internet service providers or regional ISPs are smaller ISPs that are International Organization for connected to one or more national ISPs. Standardization (ISO) – The ISO is a multinational body whose membership is They are at the third level of the hierarchy drawn mainly from the standards creation with a smaller data rate. committees of various governments throughout the world. Local Internet Service Providers International Telecommunication Union- Telecommunication Standards Sector Local Internet service providers provide (ITU-T) – The United Nations responded by direct service to the end users. forming, as part of its International Telecommunication Union (ITU), a committee, the Consultative Committee for PROTOCOLS AND STANDARDS International Telegraphy and Telephony (CCITT). First, we define protocol, which is synonymous with rule. American National Standards Institute (ANSI) – The American National Standards A protocol is a set of rules that govern data Institute is a completely private, nonprofit communications. corporation not affiliated with the U.S. federal A protocol defines what is communicated, government. how it is communicated, and when it is Institute of Electrical and Electronics communicated. Engineers (IEEE) - The Institute of Electrical The key elements of a protocol are syntax, and Electronics Engineers is the largest semantics, and timing. professional engineering society in the world. Syntax. The term syntax refers to the Electronic Industries Association (EIA) - structure or format of the data, meaning the Aligned with ANSI, the Electronic Industries order in which they are presented. Association is a nonprofit organization devoted to the promotion of electronics Semantics. The word semantics refers to manufacturing concerns. the meaning of each section of bits. Forums - To accommodate the need for Timing. The term timing refers to two working models and agreements and to characteristics: when data should be sent facilitate the standardization process, many and how fast they can be sent. special-interest groups have developed STANDARDS forums made up of representatives from interested corporations. Standards provide guidelines to manufacturers, vendors, government Regulatory Agencies – All communications agencies, and other service providers to technology is subject to regulation by ensure the kind of interconnectivity government agencies such as the Federal necessary in today's marketplace and in Communications Commission (FCC) in international communications. the United States. Data communication standards fall into two categories: de facto (meaning "by fact" or LESSON 2: NETWORK MODELS "by convention") and de jure (meaning "by law" or "by regulation"). The ISO standard that covers all aspects of networks communications is the Open De facto System Interconnection (OSI) model. Standards that have not been approved by Seven Layers of OSI Model an organized body but have been adopted as standards through widespread use are de Application facto standards. Presentation Session De jure Transport  Network Data Link The presentation layer is responsible for Physical translation, compression, and encryption. The physical layer is responsible for Presentation Layer is concerned to the movements of individual bits from one hop following: (node) to the next. Translation Physical Layer is concerned to the following: Compression o Physical characteristics of interfaces and Encryption medium. – mesh, hybrid o Representation of bits. – asp code o Data rate. – speed The application layer is responsible for o Synchronization of bits. providing services to the user. o Physical topology. o Line configuration. Application Layer is concerned to the o Transmission mode. – simplex, duplex, following: half-duplex Network Virtual Terminal File transfer, access, and The data link layer is responsible for moving management frames from one hop to the next. Mail services Data-Link Layer is concerned to the Directory services following: Framing Physical Addressing Summary of Layers Flow Control Error Control Access Control The network layer is responsible for the delivery of individual packets from the source host to the destination host. Network Layer is concerned to the following: Logical Addressing Routing TCP / IP PROTOCOL SUITE The transport layer is responsible for the Physical and Data-Link Layers delivery of a message from one process to At the physical and data link layers, TCPIIP another. does not define any specific protocol. Transport Layer is concerned to the It supports all the standard and proprietary following: protocols. Service-point Addressing Segmentation and reassembly Connection control Network Layers Flow Control Error Control At the network layer (or, more accurately, the internetwork layer), TCP/IP supports the Internetworking Protocol. The session layer is responsible for dialog The Internetworking Protocol (IP) is an unreliable and connectionless protocol-a control and synchronization. best-effort delivery service. Session Layer is concerned to the following: IP transports data in packets called datagrams, each of which is transported Dialog Control separately. Synchronization  Datagrams can travel along different routes presentation, and application layers in the and can arrive out of sequence or be OSI model. duplicated. IP, in turn, uses four supporting protocols: ARP, RARP, ICMP, and IGMP. The Address Resolution Protocol (ARP) is used to associate a logical address with a physical address. Addressing ARP is used to find the physical address of Four levels of addresses are used in an the node when its Internet address is known. internet employing the TCP/IP protocols: physical (link) addresses, logical (IP) The Reverse Address Resolution addresses, port addresses, and specific Protocol (RARP) allows a host to discover addresses. its Internet address when it knows only its physical address. The Internet Control Message Protocol (ICMP) is a mechanism used by hosts and gateways to send notification of datagram problems back to the sender. The Internet Group Message Protocol (IGMP) is used to facilitate the simultaneous transmission of a message to a group of recipients. LESSON 3: DATA CODING A data code is a standardized relationship between signaling elements and characters. It is also called character sets or character codes. Data Coding ( 3 types of characters ) Transport Layer 1. Data Link Control Characters – facilitate The User Datagram Protocol (UDP) is a the orderly flow of data. process-to-process protocol that adds only port addresses, checksum error control, and 2. Graphic Control Characters – arrange length information to the data from the upper the presentation of data at the receiving layer. terminal. The Transmission Control Protocol 3. Alphanumeric Characters – represent (TCP) provides full transport-layer services the data itself. to applications. TCP is a reliable stream transport protocol. Baudot Code At the sending end of each transmission, TCP divides a stream of data into smaller - is a five-bit (five-level) code invented by units called segments. Emile Baudot in 1874, and still used today in telex and radioteletype services. The Stream Control Transmission 2^5 = 32 Protocol (SCTP) provides support for newer - It has been designated as International applications such as voice over the Internet. Telegraph Alphabet number two (ITA2) by the International Telegraph and Telephone It is a transport layer protocol that Consultative Committee (CCITT). combines the best features of UDP and TCP. LTRS character (set includes uppercase character) and FIGS character (set includes control characters) act like the shift key on a typewriter to shift between letters and figures. Application Layer A problem with Baudot is that an error is interpreted as numerical until the next LTRS The application layer in TCPIIP is character occurs. equivalent to the combined session,  To reduce this problem, most modern ASCII code has seven bits so that both upper systems use a technique called unshift-on- and lower case could be printed. space (USOS), in which the receiver switches back to LTRS whenever a space character is received, so that an error can LESSON 4: BAR CODES cause at most one word to be garbled. A bar code is a series of vertical black bars A five-bit WRU stands for “Who are you?” and is separated by vertical white bars (called code has theused to request the identity of the other spaces). advantage station. of faster The widths of the bars and spaces along with data transfer BEL rings the bell on a teleprinter or beeps their reflective abilities represent binary 1s for a given the speaker in a computer. and 0s, and combination of bits identify bit rate, specific items. compared to Classification of Bar Codes: codes with ASCII Code more bits 1. Discrete Code. It has spaces or gaps - is the most common code for per between characters. communication between character. microcomputers. 2. Continuous Code. It does not include spaces between characters. ASCII stands for American Standard Code for Information Interchange. 3. 2D Code. It stores data in two dimensions. ASCII is a seven-bit code, so it has 128 possible combinations without shifting. Code 39 BEL: ring the bell (or beep the speaker) - is a bar code developed in 1974. BS: backspace - It is also called Code 3 of 9 and 3 of 9 Code. LF: line feed (advance to the next line) Code 39 consists of 36 unique codes ESC: escape representing the 10 digits and 26 FF: form feed (advance to the next page) uppercase letters. CR: carriage return (move to the beginning There are 7 additional codes used for of the line) special characters, and an exclusive start/stop character coded as an asterisk (*). DEL: delete Each Code 39 character contains 9 vertical elements (five bars and four spaces). The logic condition (1 or 0) of each element EBCDIC Code is encoded in the width of the bar or space. The extended binary-coded decimal A wide element, whether it be a bar or a interchange code (EBCDIC) is an eight-bit space, represents a logic 1, and a narrow fixed length character set developed in 1962 element represents a logic 0. by the International Business Machines Three of the nine elements in each Code 39 Corporation (IBM). character must be logic 1s, and the rest With 8 bits, 𝟐^𝟖 = 𝟐𝟓𝟔 codes are possible, must be logic 0s. although only 139 of the 256 codes are In addition, of the three logic 1s, two must be actually assigned characters. bars and one a space. The name binary coded decimal was Each character begins and ends with a black selected because the second hex character bar with alternating white bars in between. for all letter and digit codes contains only the hex values from 0 to 9, which have the same Since Code 39 is discrete code, all binary sequence as BCD codes. characters are separated with an intercharacter gap, which is usually one Question: character wide. Why can the Baudot code operate with five The asterisks at the beginning and end of bits while ASCII needs seven? the bar code are stop and start characters, Ans: Baudot code has only enough code respectively. which is 5-bit points to print uppercase, while Universal Product Code The National Association of Food Chains officially adopted the Universal Product Code (UPC) in 1974. UPC code is a continuous code since there are no intercharacter spaces. The two long bars on the outermost left- and right-hand sides of the label are called the start guard pattern and the stop guard pattern, respectively. The start and stop guard patterns consist of a 101 (bar-spacebar) sequence, which is used to frame the 12-digit UPC number. The left and right halves of the label are separated by a center guard pattern (01010). The first six digits of the UPC code are encoded on the left half of the label (left- hand characters), and the last six digits of the UPC code are encoded on the right half (right-hand characters). The first five right-hand characters are data characters, and the sixth right-hand character is a check character, which is used for error detection. The decimal value of the number system In UPC Character Set, there are two binary character is always printed to the left of the codes for each character. UPC label, and on most UPC labels the When a character appears in one of the first decimal value of the check character is six digits of the code, it uses left-hand code. printed on the right side of the UPC label. When the character appears in one of the last six digits, it uses a right-hand code. LESSON 5: ERROR CONTROL, CETECTION, AND CORRECTION Transmission Error - are caused by electrical interference from natural sources, such as lightning. 1. Single-bit errors are when only one bit within a given data string is in error. 2. A multiple-bit error is when two or more nonconsecutive bits within a given data string are in error. 3. A burst error is when two or more consecutive bits within a given data string are in error. ERROR DETECTION process of monitoring data transmission and determining when errors have occurred The purpose of error detection is not to prevent errors from occurring but to prevent undetected errors from occurring. REDUNDANCY CHECKING Duplicating each data unit for the purpose of detecting errors Vertical redundancy checking (VRC) is probably the simplest error-detection scheme and is generally referred to as character parity or simply parity. Checksum is another relatively simple form of redundancy error checking where each character has a numerical value assigned to it. LESSON 6: ERROR CONTROL: DETECTION & CORRECTION Longitudinal redundancy checking (LRC) is a redundancy error detection ERROR CORRECTION scheme that uses parity to determine if a A lost message is one that never arrives transmission error has occurred within a at the destination or one that arrives but is message and is therefore sometimes called message parity. damaged to the extent that it is unrecognizable. The group of characters that comprise a A damaged message is one that is message (i.e., THE CAT) is often called a recognized at the destination but contains block or frame of data. Therefore, the bit one or more transmission errors. sequence for the LRC is often called a block check sequence (BCS) or frame check sequence (FCS). Error-correcting codes include sufficient Probably the most reliable redundancy extraneous information along with each checking technique for error detection is a message to enable the receiver to determine convolutional coding scheme called cyclic when an error has occurred and which bit is redundancy checking (CRC). in error. With CRC, approximately 99.999% of all transmission errors are detected. In the United States, the most common CRC code is CRC-16. With CRC-16, 16 bits are used for the block check sequence. RETRANSMISSION when a receive station requests the transmit station to resend a message (or a portion of a message) when the message is received in error. ARQ (Automatic Repeat Request or Automatic Retransmission Request) - is probably the most reliable method of error correction, although it is not necessarily the most efficient. It can be shown statistically that messages between 256 and 512 characters long are the optimum size for ARQ error correction. Types of ARQ Discrete ARQ uses acknowledgments to indicate the successful or unsuccessful reception of data. The destination station responds with a positive acknowledgment when it receives an error-free message. The destination station responds with a negative acknowledgment when it receives a message containing errors to call for a retransmission. If the sending station does not receive an acknowledgment after a predetermined length of time (called a time-out), it retransmits the message. Called retransmission after time-out. Continuous ARQ - can be used when messages are divided into smaller blocks or frames that are sequentially numbered and transmitted in succession, without waiting for acknowledgments between blocks. This technique is sometimes called selective repeat, as it can be used to call for a retransmission of an entire message or only a portion of a message.