Basic Data Communications:

Transcription

1 Basic Data Communications: Academic Student Guide Published by ComputerPREP, Inc. Phoenix, Arizona ACL03-CNBD00-PR-211 Version 6.0P

2 Basic Data Communications Developers Meagan McLaughlin and Brent Capriotti Editors Jill McKenna, David Oberman, Denise Siino Publishers LeAnna Shank and Tina Strong Project Manager Karlene Salaiz TRADEMARKS ComputerPREP is a registered trademark of ComputerPREP, Inc. in the United States and other countries. Microsoft, Microsoft Internet Explorer logo, and Windows are either registered trademarks or trademarks of the Microsoft Corporation in the United States and/or other countries. All other product names and services identified throughout this book are trademarks or registered trademarks of their respective companies. They are used throughout this book in editorial fashion only. No such use, or the use of any trade name, is intended to convey endorsement or other affiliation with the book. Copyrights of any screen captures in this book are the property of the software s manufacturer. DISCLAIMER ComputerPREP, Inc. makes a sincere effort to ensure the accuracy of the material described herein; however, ComputerPREP, Inc. makes no warranty, express or implied, with respect to the quality, correctness, reliability, currentness, accuracy, or freedom from error of this document or the products it describes. ComputerPREP, Inc. makes no representation or warranty with respect to the contents hereof and specifically disclaims any implied warranties of fitness for any particular purpose. ComputerPREP, Inc. disclaims all liability for any direct, indirect, incidental, consequential, special, or exemplary damages resulting from the use of the information in this document or from the use of any products described in this document. Mention of any product does not constitute an endorsement by ComputerPREP, Inc. of that product. Data used in examples and sample data files are intended to be fictional. Any resemblance to real persons or companies is entirely coincidental. ComputerPREP makes every effort to ensure the accuracy of URLs referenced in all our materials, but we can not guarantee that all will be available throughout the life of the course. When this manual/disk was published, all URLs were checked for accuracy and completeness. However, due to the ever-changing nature of the Internet, some URLs may no longer be available or may have been re-directed. COPYRIGHT NOTICE This Guide is copyrighted and all rights are reserved by ComputerPREP, Inc. No part of this publication may be reproduced, transmitted, transcribed, stored in a retrieval system, or translated into any language or computer language, in any form or by any means, electronic, mechanical, magnetic, optical, chemical, manual, or otherwise, without the prior written permission of ComputerPREP, Inc., 410 North 44th Street, Suite 600, Phoenix, Arizona Copyright by ComputerPREP, Inc. All Rights Reserved ISBN:

3 iii Table of Contents Course Description...ix ComputerPREP Courseware...x Course Objectives...x Classroom Setup...xii Lesson 1: Overview Pre-Assessment Questions Data Networks Signals Transmission Basics Data Terminal Devices Lesson Summary Lesson 1 Review Lesson 2: Networks Pre-Assessment Questions Overview of Computer Networks System Network Architecture TCP/IP Lesson Summary Lesson 2 Review Lesson 3: Transmission Principles Pre-Assessment Questions Overview of Data Transmission DTE-to-DCE Interface Protocols DCE-to-DCE Protocols IEEE LAN Protocols Packet Technologies SONET Lesson Summary Lesson 3 Review Lesson 4: Communications Equipment Pre-Assessment Questions Overview of Communications Equipment Analog Communications Devices Modulation Digital Communications Devices Line Coding Multiplexers Lesson Summary Lesson 4 Review Course Assessment...Course Assessment-1 Glossary...Glossary-1 Index... Index-1

4 iv List of Figures Figure 1-1: Analog and digital signals Figure 1-2: Computers communicate using digital and analog telephone lines Figure 1-3: Voice transmission can use digital lines Figure 1-4: Digital signal encoded for transmission Figure 1-5: Network hosts and PCs are examples of DTE Figure 1-6: Modems and SCU/DSUs are DCE Figure 1-7: DCE connect DTE to the network Figure 1-8: An analog signal is like a sine wave Figure 1-9: A cycle is the distance from one peak of an analog wave to the next Figure 1-10: A bit is a single binary digit of a digital signal Figure 1-11: A bit interval is the amount of time occupied by one bit Figure 1-12: Digital transmission bandwidth measurements Figure 1-13: Analog signals are tolerant of errors Figure 1-14: Digital signals are less tolerant of errors than analog signals Figure 1-15: Analog networks are more complex than digital networks Figure 1-16: Digital signals are easy to discriminate from noise Figure 1-17: Digital signals are resistant to crosstalk Figure 1-18: Digital signals are reliable and cost-effective Figure 1-19: The PSTN includes local- and long-distance service providers Figure 1-20: The PSTN is accessible from any telephone Figure 1-21: Telephone call between LATAs Figure 1-22: Leased circuits connect remote offices Figure 1-23: Circuits have two transmission paths Figure 1-24: Half-duplex circuits are unidirectional Figure 1-25: Full-duplex circuits are bi-directional Figure 1-26: Dedicated circuits are non-switched, private lines Figure 1-27: Point-to-point circuit dedicated connection between two devices Figure 1-28: Multipoint circuit A single channel serving three or more users Figure 1-29: Multipoint circuit Figure 1-30: Switched circuit using the PSTN Figure 1-31: Full-time, point-to-point transmission via dedicated circuit Figure 1-32: Switched circuits handle occasional transmissions to various locations Figure 1-33: Switched circuits use different paths for each transmission Figure 1-34: Digital circuits are more reliable than analog circuits Figure 1-35: Dedicated and dial-up modems operate at the same speeds Figure 1-36: Throughput the amount of useful data transmitted within a given time Figure 1-37: Top throughput is rarely possible in dial-up networks Figure 1-38: Analog dedicated circuits are optimal for small, regular transmissions Figure 1-39: Multiplexed digital circuit replaces many dedicated analog circuits Figure 1-40: Network using T1 lines Figure 1-41: Network using a T3 line Figure 1-42: Six-channel FT1 connection Figure 1-43: Analog switched network using leased phone lines Figure 1-44: The voice network is a circuit-switching network Figure 1-45: Circuit switching versus packet switching Figure 1-46: Switched 56 network Figure 1-47: ISDN BRI versus ISDN PRI Figure 1-48: ISDN PRI offers 23 B channels and 1 D channel Figure 1-49: ISDN BRI provides 2 B channels and 1 D channel Figure 1-50: Switched Multimegabit Data Service (SMDS) Figure 1-51: X.25 transmission is like a regular phone call

5 v Figure 1-52: ATM versus X.25 and frame relay Figure 1-53: Data terminal devices and network operations work together Figure 1-54: Modems and CSU/DSUs connect DTE to the network Figure 1-55: Many computers (DTE) have internal modems (DCE) Figure 1-56: Mouse devices, keyboards, and printers are hardware components Figure 1-57: Terminals access applications from the host Figure 1-58: Host maintenance and network maintenance are separate Figure 1-59: The FEP manages communications between the host and terminals Figure 1-60: Com ports connect computers and modems Figure 1-61: Communications processor with several com ports Figure 1-62: Monitors and printers are output devices Figure 1-63: Modern monitors use LED or LCD Figure 1-64: Keyboards and mouse devices are input devices Figure 1-65:Batch terminals send data to host in a batch file Figure 1-66: Smarter terminals have more independent processing power Figure 1-67: The cluster controller manages terminals Figure 1-68:Dumb terminals are managed completely by the host Figure 1-69: Dumb terminals send keystrokes and display information from the host Figure 1-70: Dumb terminals use asynchronous communications Figure 1-71: Cluster controllers use synchronous communications Figure 1-72: NCs are dumb terminals used to connect to the Internet Figure 1-73: Windowing & multitasking provide multiple looks at multiple applications Figure 1-74: Smart terminals have onboard processors Figure 1-75: Hosts and smart terminals share processing tasks Figure 1-76: X-Window functionality is defined by software Figure 1-77: Host with X-Windows client software Figure 1-78: PCs perform many tasks offline Figure 1-79: PC with terminal emulation software Figure 1-80: Notebook or laptop computer Figure 1-81: Personal digital assistant (PDA) Figure 2-1: Computer network Figure 2-2: Host computers manage the terminals Figure 2-3: Distributed processing networks Figure 2-4: LANs linked to create a WAN Figure 2-5: A single host manages several terminals Figure 2-6: Systems Network Architecture (SNA) Figure 2-7: Dumb terminals and PCs use the mainframe for processing Figure 2-8:Internetworking refers to LANs linked by telecommunications circuits Figure 2-9: LANs Linked by Routers Figure 2-10: LANs linked by bridges Figure 2-11: TCP/IP interconnects dissimilar LANs Figure 2-12: Value-added network (VAN) Figure 2-13: SNA is an example of hierarchical network architecture Figure 2-14: SNA has a tree-like pattern with a single head and multiple branches Figure 2-15: Host computers have the most processing power Figure 2-16: Systems Network Architecture (SNA) Figure 2-17: CPs regulate communication between the host and cluster controller Figure 2-18: The cluster controller regulates communication with the terminals Figure 2-19: Communications processors use SDLC Figure 2-20: SNA recognizes people and programs as end users Figure 2-21: IN SNA, the programming determines a device s function Figure 2-22: SNA networks can be configured in many ways Figure 2-23: Starting and ending messages define a session

6 vi Figure 2-24: SSCP software manages all devices and functions on a network Figure 2-25: LU0 sessions run on a multitasking computer Figure 2-26: LU1 session Figure 2-27: LU2 session Figure 2-28: LU3 session Figure 2-29: LU4 session Figure 2-30: LU6 session Figure 2-31: LU6.2 session Figure 2-32: LU7 session Figure 2-33: Hardware and software create a physical unit (PU) Figure 2-34: PU5 is a mainframe running SSCP software Figure 2-35: PU4 is a communications processor, and PU2 is a cluster controller Figure 2-36: PU2.1 is an intelligent PC Figure 2-37: PU1 units are now obsolete Figure 2-38: LU6.2 session enabled by PU2.1 devices Figure 2-39: TCP/IP is the backbone of the Internet Figure 2-40: TCP/IP serves many platforms Figure 2-41: LANs link to create a WAN Figure 2-42: Upper levels encapsulate lower levels to form an internetwork Figure 2-43: Each TCP/IP layer works with its adjacent layers Figure 2-44: TCP/IP layers and functions Figure 2-45: Each TCP/IP layer handles a different function Figure 2-46:The router reads the MAC layer part of the IP address Figure 2-47: The network interface reads the port address Figure 2-48: IP addresses are 32-bit numbers divided into 8-bit segments Figure 2-49: Telnet is a terminal emulation program Figure 2-50: File transfer protocol (FTP) Figure 2-51: SNMP monitors networks for trouble Figure 2-52: Hypertext Transfer Protocol (HTTP) Figure 2-53: POP3 is used to receive from a server Figure3-1: Protocols are necessary for computers to communicate Figure 3-2: Steps must be completed in order for protocols to be used Figure 3-3: Protocol stacks show how protocols work together Figure 3-4: Layer 2 defines interaction with Layer 3 and Layer Figure 3-5: Each layer adds information to the capsule Figure 3-6: Peer layers allow the receiving device to read the capsule Figure 3-7: OSI encourages vendors to use uniform protocols Figure 3-8: OSI reference model Figure 3-9: DCE connects DTE to the network Figure 3-10: DTE are the end points of data transmissions Figure 3-11: DTE use DCE to communicate with the network Figure 3-12: OSI reference model Figure 3-13: The interface is the physical connection between DTE and DCE Figure 3-14: DTE/DCE interfaces in internal and external modems Figure 3-15: RS-232: DTE has a male connector and DCE has a female connector Figure 3-16: Limitations of RS Figure 3-17: RS-449 supports fast transmission speeds and long distances Figure 3-18: Software sends commands to the modem Figure 3-19: Modems and CSU/DSUs are two types of DCE Figure 3-20: Transmission is addressed at OSI Layers 1 and Figure 3-21: Protocols must be compatible for communication to occur Figure 3-22: Asynchronous transmission is also called start-stop transmission Figure 3-23: Asynchronous transmission counts bits as they are received

7 vii Figure 3-24: Start, stop, and parity bits are overhead Figure 3-25: Synchronous transmission uses clocks Figure 3-26: Asynchronous versus synchronous transmission Figure 3-27: Signaling messages have several functions Figure 3-28: Byte-oriented versus bit-oriented protocols Figure 3-29: Bit-oriented protocols use less overhead Figure 3-30: High-level Data Link Control (HDLC) frame Figure 3-31: DCE devices transmit over circuits Figure 3-32: Full-duplex transmission Figure 3-33: Half-duplex transmission Figure 3-34: Asynchronous transmission uses a parity bit for error-checking Figure 3-35: Odd parity sends an odd number of ones Figure 3-36: Even parity sends an even number of ones Figure 3-37: In even parity, an odd number of ones indicates an error Figure 3-38: Parity error-checking requests retransmission Figure 3-39: Checksum, or longitudinal redundancy check (LRC) Figure 3-40: CRC sends a unique number to the receiving DCE Figure 3-41: CRC: The receiving DCE requests retransmission when an error occurs Figure 3-42: DCE devices must use the same protocols and speeds Figure 3-43: Higher bps rates mean faster speeds Figure 3-44: Modems have fallback speeds to match slower modems Figure 3-45: Transmission is as fast as the slowest modem Figure 3-46: Transmission speeds of common modem standards Figure 3-47: IEEE 802.X committees Figure 3-48: IEEE 802.X protocols work at OSI Layers 1 and Figure 3-49: WANS involve the first three OSI layers Figure 3-50: Circuit switching versus packet switching Figure 3-51: The PAD places the data into packets Figure 3-52: Packet-switching network Figure 3-53: X.25 interfaces are similar to interfaces in the local loop Figure 3-54: RS-232 and V.35 are physical layer standards Figure 3-55: LAPB allows full-duplex communication Figure 3-56: X.25 network layer standards provide reliable communication Figure 3-57: The OSI model versus the SONET model Figure 3-58: SONET provides a backbone for DS3 and ATM Figure 3-59: Electrical signals are converted to optical signals Figure 3-60: SONET transmission speeds Figure 3-61: SONET mapping provides flexibility Figure 3-62: The payload envelope carries the signal Figure 3-63: A SONET frame holds 32 DS1 frames Figure 3-64: SONET overhead tracks each DS-1 frame Figure 4-1: Data communication systems transmit data from point A to point B Figure 4-2: Data communications components Figure 4-3: Open Systems Interconnection (OSI) reference model Figure 4-4: Modems connect remote computers via a phone line Figure 4-5: Modulation converts digital signals to analog signals Figure 4-6: Demodulation converts analog signals to digital signals Figure 4-7: Each modem has two modem interfaces Figure 4-8: Modems connect DTE to the network Figure 4-9: Modem protocol standards Figure 4-10: Equalization is the modem s ability to compensate for distortion Figure 4-11: Modems use configuration management Figure 4-12: Monitoring alerts the operator of faults

8 viii Figure 4-13: Special features require the same type of modem at both ends Figure 4-14: Special features may not function with different modems Figure 4-15: Morse code uses light as a carrier Figure 4-16: Modems transmit electricity as sine waves Figure 4-17: Frequency is the time required for a wave to go from peak to peak Figure 4-18: Out-of-phase signals with the same amplitudes and frequencies Figure 4-19: Modems send carrier signals to establish contact Figure 4-20: Modems modulate the carrier signal Figure 4-21: The receiving modem demodulates the signal Figure 4-22: Speed is measured in bits per second Figure 4-23: FSK modems can transmit at two different frequencies Figure 4-24: FSK assigns each frequency as a1 or a Figure 4-25: FSK transmits at 300 bps or less Figure 4-26: Differential phase shift keying uses four dibits Figure 4-27: DPSK modems alter the signal s phase Figure 4-28: Quadrature amplitude modulation (QAM) transmits at 9,600 bps Figure 4-29: V.32 uses twelve different phases Figure 4-30: V.32 QAM uses 16 combinations of phase and amplitude Figure 4-31: V.32 alters the carrier signal to the correct phase and amplitude Figure 4-32: QAM is twice as fast as DPSK Figure 4-33: Digital signals require greater bandwidth than analog signals Figure 4-34: CSUs and DSUs are commonly combined into CSU/DSUs Figure 4-35: Unipolar RZ line code Figure 4-36: RZ line code returns to 0 halfway through the bit interval Figure 4-37: NRZ line coding Figure 4-38: Bipolar line coding uses positive and negative values Figure 4-39: AMI line coding alternates positive and negative values Figure 4-40: Digital signals use synchronous communications Figure 4-41: Insufficient ones density Figure 4-42: Insufficient pulse density Figure 4-43: Diphase coding uses positive and negative values in every bit interval Figure 4-44: Binary N-Zero Substitution (BNZS) corrects insufficient pulse density Figure 4-45: Multiplexers combines signal into a single data stream Figure 4-46: Frequency-division multiplexing (FDM) Figure 4-47: Time-division multiplexing (TDM) Figure 4-48: FDM assigns each of the 24 T1 channels a time slot Figure 4-49: Statistical multiplexing does not require T1 circuits

9 ix Course Description Welcome to the Basic Data Communications course which will help prepare you for the Certified in Convergent Network Technologies (CCNT) exam, a program sponsored by the TIA (Telecommunications Industry Association). This course is aimed at preparation and review for the Basic Data Communications module of the CCNT exam, as well as professional development for Information Technology (IT) professionals. The course is designed to be used in a lecture-based classroom setting. The Basic Data Communications course provides an understanding of how data communication systems and their various software and hardware components work. This course has four lessons, and each lesson covers several topics. Following are the four lessons of the Basic Data Communications course, along with the topics covered in each lesson. Topics Covered Overview Data Networks Signals Transmission Basics Data Terminal Devices Networks Computer Networks System Network Architecture TCP/IP Transmission Principles Overview of Data Transmission DTE-to-DCE Interface Protocols DCE-to-DCE Protocols IEEE LAN Protocols Packet Technologies SONET Communication Equipment Overview of Communication Equipment Analog Communication Devices Modulation Digital Communication Lines Line Coding Multiplexing

10 x ComputerPREP Courseware This learning guide was developed for instructor-led training and will assist you during class. Along with comprehensive instructional text and objectives checklists, this learning guide also includes pre-assessment questions, tech terms, as well as lesson summaries and reviews. Each lesson in this course follows a regular structure, along with graphical cues to illustrate important terms and concepts. The structure of a typical module includes: Pre-Assessment Questions Each lesson includes pre-assessment questions to test the student s understanding of the key concepts presented in the lesson. Objectives Each lesson includes a list of objectives to set the stage for the rest of the lesson. Tech Terms Tech terms appear in bold in the narrative text for quick and easy access (technical terms are also included in the index and glossary). Lesson Summary The Lesson Summaries at the end of each lesson include: an Application Project to extend learning, a Skills Review of key concepts and objectives presented in the lesson, and Lesson Review Questions designed to test understanding. Glossary The Glossary contains a list of key terms defined throughout the course which can be used for self-study once the course has been completed. Table of Contents and Index The Table of Contents appears at the beginning of the course book and the Index appears at the end. These two allow for easy access to review key areas. Course Objectives Identify the key components in a data network. Identify the three categories of data communications. Identify the function of DTE and DCE as related to a data network. Define the features of analog signals Define the features of digital signals. Identify the advantages of digital signals over analog technology. Define the roles and responsibilities of transmission service providers. Define the features and functions of transmission circuits. Identify the features and benefits of the primary forms of transmission technology services. Define basic data equipment terms.

11 xi Define the features and functions of host computers. Differentiate between display and non-display terminals. Differentiate between smart versus dumb terminals. Identify the function of dumb terminals in a data communications network. Identify the function of smart terminals in a data communication network. Identify the key features and functions of computer networks. Differentiate between LAN and WAN network architectures. Define the features and functions of specified WAN architectures. Define the features of SNA. Identify the features and functions of hardware in an SNA network. Explain how SNA works. Define the role of TCP/IP and its relationship to the internet. Explain the function of TCP/IP layers. Explain how TCP/IP transports data. Identify common TCP/IP applications. Describe the role of a protocol in a data transmission network. Describe the role of the OSI model in a data transmission network. Describe the features and functions of DTE/DCE interface protocols. Describe the features and functions of DCE/DCE protocols. Describe the features and functions of transmission methods involved with DCE/DCE protocols. Describe the features and functions of transmission modes involved with DCE/DCE protocols. Describe the features and functions of error-checking methods involved with DCE/DCE protocols. Describe the features and functions of transmission speeds involved with DCE/DCE protocols. Describe the features of IEEE LAN protocols. Describe the features and functions of packet-switching technology.

12 xii Describe the features and functions of X.25. Describe the features and functions of SONET. Describe the components of a data communications system. Explain the role of analog communications devices in a data communications network. Describe the features and functions of modems. Define the key features and functions of modulation. Describe the features and functions of common modulation techniques. Explain the role of digital communications devices in a data communications network. Describe the features and functions of line coding. Explain the relationship between line coding and synchronization. Describe the features and functions of multiplexers. Classroom Setup Student computers are not required for this seminar course. However, if the instructor desires to supplement activities or quizzes electronically, computers addressing these needs will be required for each student. Otherwise, all supplemental material can be distributed as hardcopy documents and completed by students using a pen and paper.

13 1Lesson 1: Overview OBJECTIVES By the end of this lesson, you will be able to: Identify the key components in a data network. Identify the three categories of data communications. Identify the function of DTE and DCE as related to a data network. Define the features of analog signals. Define the features of digital signals. Identify the advantages of digital signals over analog technology. Define the roles and responsibilities of transmission service providers. Identify the features and benefits of primary forms of transmission technology services. Define basic data equipment terms. Define the features and functions of host computers. Differentiate between display and non-display terminals. Differentiate between smart and dumb terminals. Identify the function of dumb terminals in a data communications network. Identify the function of smart terminals in a data communications network. Define the features and functions of transmission circuits.

14 1-2 Basic Data Communications Pre-Assessment Questions 1. A modem is an example of which equipment category? a. MDE b. DCE c. DTE d. CSU 2. Bandwidth is synonymous with what characteristic of data communications? a. Speed b. Encoding c. Signal type d. Translation 3. Describe the function of X Window.

15 Lesson 1: Overview 1-3 Data Networks Historically, the telephone network has been analog. The voice is an analog sound. analog digital An electrical signal that varies continuously in amplitude and frequency. An electrical signal that varies in discrete steps represented by two electrical states: voltage present (1) and voltage absent (0). Figure 1-1: Analog and digital signals Computers are digital. Computer networks communicate by exchanging digital signals. The number of networked computers has increased dramatically in recent years, many of them connecting with other networks to make up what we call the Internet. Figure 1-2: Computers communicate using digital and analog telephone lines Many of these digital computer networks communicate with one another over telephone lines. Sometimes these lines are analog; sometimes they are digital. Now even voice transmission may be carried over digital lines. Figure 1-3: Voice transmission can use digital lines

16 1-4 Basic Data Communications Because a mix of analog and digital technology can make it difficult to understand how data is transmitted, we will divide all data communications into three large categories based on transmission speed: Low-speed data communications using analog transmission Medium-speed data communications using either analog or digital transmission High-speed data communications using strictly digital transmission Bandwidth is a common synonym for speed. bandwidth The information-carrying capacity of a communication channel. Analog bandwidth (Hertz) is the range of signal frequencies that can be transmitted by a communication channel or network. Digital bandwidth (bits per second) is the rate at which bits can be transmitted. Regardless of signal type or bandwidth, the original signal has to be translated or encoded for transmission. encoded Converted into a group of ones and zeros that is the binary representation of the numerical value; used to describe a numerical value. Figure 1-4: Digital signal encoded for transmission All data transmission originates or ends at data terminal equipment, more commonly referred to as DTE. Two common examples of DTE are personal computers and network hosts. data terminal equipment (DTE) User terminals ranging from a simple card-swipe terminal to a personal computer or super computer. Figure 1-5: Network hosts and PCs are examples of DTE

17 Lesson 1: Overview 1-5 Computers (DTE) operate using a specific, binary, digital code appropriate for their internal purposes. Codes for telecommunications circuits, both analog and digital, are different because they serve different functions. Consequently, specific equipment needs to sit between the DTE and the network to encode and decode the data. That equipment is called DCE, or data communication equipment. Two examples of DCE are modems and CSU/DSUs. data modem channel data communication equipment (DCE) Equipment used to connect DTE to the network. MOdulator/DEModulator. Device that converts digital signals to analog signals for transmission on analog telephone lines. service unit (CSU) A type of data communications equipment that connects data terminal equipment to a digital circuit, assuring a reliable connection between the DTE and the digital circuit. A CSU is always used with a DSU. service unit (DSU) A type of data communications equipment that connects data terminal equipment to a digital circuit, converting the DTE s signal to a signal acceptable to the digital circuit. A DSU is always used with a CSU. On the transmitting end, a CSU/DSU takes data from DTE, encodes it, and transmits it down the link. At the receiving end, another CSU/DSU equalizes the signal, filters it, and decodes it for interpretation by the DTE. Figure 1-6: Modems and SCU/DSUs are DCE This illustration shows some highly simplified examples of DTE and DCE usage. Figure 1-7: DCE connect DTE to the network

18 1-6 Basic Data Communications Signals The previous topic, Data Networks, discussed the key components in a data network and defined DTE and DCE. This topic provides an overview of signals transmitted over a data network, including analog and digital signals. Analog Signals A speaker's voice is an example of an analog signal. A voice chart is a line representing the continuously varying waveform of mechanical energy. As you can see, this line is very much like a sine wave. Figure 1-8: An analog signal is like a sine wave When an analog signal is transmitted over telephone lines, it requires transduction. transduction The conversion of energy from one form into another. With telephones, sound is transduced from the mechanical (sound pressure waves) into energy (either electrical or optical) and carried over a transmission circuit (either metallic wire or optical fiber). One full curve of the analog wave below, from one peak of the wave to the next peak, is a single cycle. Hertz, abbreviated Hz, refers to the number of cycles that a particular wave completes in a single second. Analog bandwidth is measured in Hertz. Hertz (Hz) The number of cycles that a particular wave completes in a single second. This is the unit of measurement for analog bandwidth. Figure 1-9: A cycle is the distance from one peak of an analog wave to the next

19 Lesson 1: Overview 1-7 Digital Signals Digital signals are represented by numbers. The numbers are encoded with a binary numbering system a system that uses only two digits, one and zero. Each binary digit in a signal is called a bit. binary A number system that uses only two digits: 0 and 1. bit Binary digit. A binary digit has a value of 0 or 1, and is often represented by the presence (1) or absence (0) of a voltage on a transmission medium. The bit is the basic unit of data communications and the smallest unit of information a computer can process. Figure 1-10: A bit is a single binary digit of a digital signal Binary numbering makes it easy to transmit a digital signal. A digital signal is a series of zeros and ones, ons and offs, high voltages and low voltages. It can be nothing else. In analog signals, both amplitude and frequency vary continuously. The digital signal's amplitude varies between only two values. Frequency is constant; there is only one frequency for a digital signal. This example of alternating 1s and 0s encodes a one (1) as a positive amplitude and a zero (0) as zero amplitude. Other encoding schemes will have other waveforms. Each bit occupies the same amount of time, referred to as the bit interval. Figure 1-11: A bit interval is the amount of time occupied by one bit Digital transmission bandwidth is measured in bits per second, or bps. Most transmission speeds are measured in kilobits (1,000 bits) or megabits (1 million bits), called Kbps and Mbps, respectively. Bps Bits per second. The number of bits sent and received every second. It is the unit of measurement for bandwidth or transmission rate for data communications.

20 1-8 Basic Data Communications kilobit One thousand bits. megabit One million bits. Figure 1-12: Digital transmission bandwidth measurements Each type of signal, analog and digital, has its advantages. Analog signals are more tolerant of errors than digital signals. If parts of words or sounds are lost or garbled during a voice transmission, the listener can usually compensate and still understand the speaker. Figure 1-13: Analog signals are tolerant of errors Digital signals, on the other hand, are much less tolerant of errors. For example, if a single bit is lost, the rest of the transmission may become gibberish to the receiving computer. Figure 1-14: Digital signals are less tolerant of errors than analog signals

21 Lesson 1: Overview 1-9 A digital signal is easier to measure than an analog signal. With a digital signal, each given bit is one of only two values, 0 or 1. This arrangement makes it relatively easy to develop network components that can recognize and process digital signals. Analog signals take on many values, requiring much more complex network components. Figure 1-15: Analog networks are more complex than digital networks Digital signals are easier to discriminate from distortion than analog signals. In digital transmission, repeaters strengthen (amplify) the signal, and filter out unwanted "noise." Even if the original signal is distorted, repeaters can perceive and clearly restore the signal for the next part of transmission. repeater A device that restores a degraded digital signal for continued transmission. Used with digital signals, it can remove the noise when regenerating the signal. Used with analog signals, both the signal and noise are amplified. Also known as a regenerator, or line regeneration equipment (LRE). Figure 1-16: Digital signals are easy to discriminate from noise In analog transmission, amplifiers strengthen the entire signal (including any distortion). Digital signals are highly resistant to crosstalk. An example of crosstalk is hearing a conversation from another transmission in the background of your own conversation. crosstalk An undesirable effect on one circuit or channel due to signals in another circuit or channel. Figure 1-17: Digital signals are resistant to crosstalk

22 1-10 Basic Data Communications The low level of the interfering signal is not often registered by the digital circuitry as a 1. So the interference usually results in a 0, and a long stream of digital zeros represents a silent channel. In an analog data transmission, such crosstalk can result in garble that an analog system may not be able to accurately interpret. Digital signals are more cost-effective than analog because they are more reliable, thereby reducing maintenance costs. As digital computer technology has become more prevalent, digital circuitry has become more affordable. Figure 1-18: Digital signals are reliable and cost-effective Transmission Basics This topic addresses some rudimentary transmission subjects, including service providers, circuits, and services. Transmission Service Providers Regardless of whether the transmission is analog or digital, voice or data, when most of us think of telecommunications networks, we think of the public switched telephone network (PSTN). This is the network that we use every day to make local and long-distance calls. There is, essentially, a single PSTN in the world. However, different companies own different parts of the PSTN. These include both local-service and longdistance providers. Figure 1-19: The PSTN includes local- and long-distance service providers

23 Lesson 1: Overview 1-11 In the United States, local service providers, or local exchange carriers (LECs), include: The seven Baby Bells: Ameritech, Bell Atlantic, Bell South, NYNEX, PacTel, Southwestern Bell, and Qwest. Large independent companies such as GTE. Numerous local access providers. The U.S. long-distance providers, or interexchange carriers (IXCs), include: AT&T Communications. MCI. Sprint. Other national long-distance carriers. Numerous smaller long-distance carriers that operate regionally. Anyone with a telephone can gain access to the switched services that these companies provide. In some cases, you may need an account with the service provider. Regardless of where you are and what service you use, you can probably dial a valid phone number, and the PSTN will connect you with that phone. Figure 1-20: The PSTN is accessible from any telephone The breakup of AT&T in 1984 resulted in the division of the United States into geographic boundaries called local access transport areas, or LATAs. A LATA may have one or more area codes. local access transport area (LATA) The geographical region defined by the Federal Communications Commission (FCC) in which a local services provider may offer telecommunications services.

24 1-12 Basic Data Communications Now, due to the Telecommunications Act of 1996, the original laws governing Regional Bell Operating Companies (RBOCs) and other local exchange carriers (LECs) have been relaxed. As a result (depending on local regulations), you can probably select any local-service provider or long-distance carrier to handle your call. This flexibility lessens the relevancy of LATA boundaries. RBOC Regional Bell Operating Company. local exchange carrier (LEC) A telco that provides local telephone service. If you want to call a location outside your LATA, the connections are more complex, particularly if your long-distance (interlata) provider is different from your local access provider. To begin the process, your LEC connects your call to a long-distance provider's point of presence (POP) in your LATA. point of presence (POP) A point of connection between an interexchange (IXC) carrier and a local carrier to pass the customer s long-distance call into the IXC s network, and vice versa. Next, the long-distance provider (IXC) transports your call across LATA boundaries to a POP in the LATA serving the phone number you are calling. The POP passes the call into the LEC network, which forwards the call to the called party s local SO. To complete the call, the local service provider (LEC) makes a connection to the phone number you are calling. Again, this presumes that the IXC is not the LEC. Figure 1-21: Telephone call between LATAs

25 Lesson 1: Overview 1-13 Keep in mind that the roles of LECs and IXCs will continue to change. In today's dynamic regulatory environment, companies have increasing opportunities to compete in areas that were previously off limits to them. For example, today IXCs can compete in the local calling market previously served only by LECs. As a result of these changes, terms such as LEC and IXC will probably have to be redefined or may disappear altogether. The network's flexibility is well-suited for voice communications. Most data communications, however, do not need this much flexibility. Reliability is more important. Two ways of obtaining reliable point-to-point data communications are: Leased dedicated (non-switched) circuits from a communications service provider. Leased specialized switched services from a communications service provider. What you use will depend on your budget and your specific needs. For example, you may want several offices in different LATAs to have access to the same computer. You may design the network so that a local telephone company in each city connects the local offices. Then a privately leased high-capacity circuit may provide a long-distance connection between the two groups of offices. Figure 1-22: Leased circuits connect remote offices The key to this or any other data communications network is that the various service providers have to work with your managers to make it all work together. The rest of this topic reviews the various rules and protocols that govern the many ways in which information can be transmitted.

26 1-14 Basic Data Communications Transmission Circuits After passing data through a modem or CSU/DSU, the network carries the data along an appropriate channel to its destination. A circuit typically has at least two transmission paths one for sending and one for receiving. Figure 1-23: Circuits have two transmission paths The circuit channels can be used in one of two ways: half duplex or full duplex. Half-duplex circuits are unidirectional. Both channels perform the same task at the same time. For example, either both channels are sending or both are receiving data. half-duplex Figure 1-24: Half-duplex circuits are unidirectional Two-way transmission in which terminals communicate with each other one at a time. Full-duplex transmission allows simultaneous transmission in both directions. Full-duplex circuits are bi-directional. In this case, the channels can perform different tasks at the same time. For example, one channel can send while the other is receiving.

27 Lesson 1: Overview 1-15 Figure 1-25: Full-duplex circuits are bi-directional In telecommunications, most statements about channels are actually referring to full-duplex circuits. full-duplex A simultaneous two-way and independent transmission. Half-duplex transmission is one-way only. Transmission circuits can be: Either analog or digital. Either non-switched or switched. non-switched switched circuit Circuit between two (or more) points that is not part of a switched network and is used only for communications within the organization that owns or leases the circuit. circuit Circuit designed to route calls through the network using some sort of addressing information, which could be anything from a telephone number to an IP address. The previous topic, Signals, dealt with the basic differences between analog and digital signals. So we will begin with non-switched circuits versus switched circuits. Non-switched, or private line, circuits are most often called dedicated circuits because non-switched circuits are dedicated to transporting information between fixed locations with no switching involved. dedicated circuit Circuits between two or more points that are not part of a switched network and are used only for communications within the organization that owns or leases the circuits.

28 1-16 Basic Data Communications Figure 1-26: Dedicated circuits are non-switched, private lines Following are the two types of dedicated circuits: Point-to-point Multipoint A point-to-point circuit is a dedicated connection between two devices. It is available to the users at both ends at all times. An example of a point-to-point circuit is a direct connection between a dumb terminal and the mainframe. point-to-point circuit Dedicated circuit connecting two devices with no switching. Figure 1-27: Point-to-point circuit dedicated connection between two devices A multipoint circuit is another type of dedicated or non-switched circuit. In a multipoint circuit, a single channel serves three or more sets of users. Therefore, the circuit is available to each user only some of the time. multipoint circuit Dedicated or non-switched circuit in which a single channel serves three or more sets of uses. Figure 1-28: Multipoint circuit A single channel serving three or more users An example of a multipoint circuit would be a store's credit card verification terminals (boxes with slots through which credit cards are slid). In this example, the microcomputer and all terminals are connected to their own modems. The modems are all connected to a line running to a "master modem" attached to a minicomputer.

29 Lesson 1: Overview 1-17 Figure 1-29: Multipoint circuit Now we will address switched circuits. It may be helpful to begin by defining switch. A switch is any device that enables an incoming circuit to be connected to any one of a number of outgoing circuits. Switched circuits using the PSTN are usually a mix of analog and digital technologies, as shown in this example. Switched circuits are interconnected circuits that establish a temporary connection between two or more points. switched circuit Circuit designed to route calls through the network using some sort of addressing information, which could be anything from a telephone number to an IP address. Figure 1-30: Switched circuit using the PSTN Transmission Services This discussion compares the advantages of circuit types. Data communications can use one of four types of telephone transmission circuit technology: Analog dedicated Digital dedicated Analog switched Digital switched

30 1-18 Basic Data Communications Each type has advantages and disadvantages. First we will consider the cost-effectiveness of dedicated circuits versus switched circuits. This example shows a full-time, point-to-point transmission via a dedicated circuit. Local or long-distance carriers provide dedicated circuits via SOs, but using separate facilities and equipment. Dedicated circuits are often provided by local or long-distance carriers via SOs, but using separate facilities and equipment. Figure 1-31: Full-time, point-to-point transmission via dedicated circuit On the other extreme, there is the switched circuit handling an occasional transmission to any one of several locations. In general, dedicated transmissions are more expensive than switched transmissions because users pay for a dedicated circuit as if they were using it full time. Figure 1-32: Switched circuits handle occasional transmissions to various locations Dedicated circuits are more reliable than switched because the transmission uses the exact same path every time. In addition, dedicated circuits are usually tested and come with some guarantee of reliability. With switched circuits, you never know the exact path that your transmission will take through the PSTN. In addition, switched circuits have to share PSTN resources with every other phone call. Thus when the network has more traffic than it can handle, it will not complete your call.

31 Lesson 1: Overview 1-19 Figure 1-33: Switched circuits use different paths for each transmission Digital circuits are usually more reliable than analog circuits because they handle noise better. This reliability translates into higher speeds. In addition, digital circuits allow more flexible network design. Digital multiplexing enables the simultaneous transmission of several calls over the same circuit. Figure 1-34: Digital circuits are more reliable than analog circuits We have compared the advantages of each circuit type. Now we will look at a detailed presentation of the services of each type of circuit-switching technology. Analog Dedicated Circuits Analog dedicated circuits are useful when a network requires a dedicated connection, but a digital line is either impractical or too expensive. In this example, the connection is through dedicated circuit modems. Because current modem technology has made line conditioning less relevant, dedicated modems now use the same speeds as modems on dial-up lines. line conditioning Any of several techniques used to allow communications circuits to transmit information farther and faster.

32 1-20 Basic Data Communications Figure 1-35: Dedicated and dial-up modems operate at the same speeds The actual amount of useful information transmitted within a given time is called throughput. Various data compression techniques enable information throughput to be faster than the actual transmission speed. For example, Kbps modems with a 4:1 compression ratio have a theoretical top throughput of 57.6 Kbps. However, keep in mind that this speed is rarely achieved in real networks. data compression Any of several ways of encoding data for transmission that reduce the total number of bits transmitted in a message. Figure 1-36: Throughput the amount of useful data transmitted within a given time One reason top throughput is rarely possible in dial-up networks is because the PSTN can generally support transmission only up to 19.2 Kbps. To ensure faster speeds at a dependable rate, you may want to consider using dedicated lines with special line conditioning.

33 Lesson 1: Overview 1-21 Figure 1-37: Top throughput is rarely possible in dial-up networks Analog dedicated circuits are advantageous when a network requires a dedicated connection, but a digital line is either impractical or too expensive. One example of this type of situation is a remote electrical meter tracking any sort of industrial or environmental data, such as the current status of a dam spillway; continually sending data back to the management company or environmental station. Because the data is so small and it occurs at regular intervals, there is no need for very fast, reliable communications. Figure 1-38: Analog dedicated circuits are optimal for small, regular transmissions In this example, low-speed (up to 28.8 Kbps) analog private lines may be very cost-effective. If the number of meters grows, a multiplexed digital circuit may be more economical than many dedicated analog circuits. Figure 1-39: Multiplexed digital circuit replaces many dedicated analog circuits

34 1-22 Basic Data Communications Dedicated Digital Circuits Because of their greater bandwidth, digital circuits are more common in larger, more complex networks. Network managers who want to establish digital dedicated circuit connections between premises can buy or lease various types of lines from a telephone company. A variety of different types of digital dedicated circuits are available today. The circuit you choose depends on your desired transmission speed and whether you need to multiplex. Some of the popular choices include: T1 T3 Fractional T1 (FT1) Digital data system (DDS) We will briefly address each choice. T1 Trunk carrier level 1. A DS1 digital carrier facility for transmitting Mbps over telephone wires, capable of supporting up to 24 channels. More generally, T1 refers to any Mbps digital stream. T3 A DS3 (44.76 signal) carrier system. fractional T1 FT1. A T1 carrier divided into channels that are multiples of the basic 64- Kbps channel, and an economical way of linking LANS into MANs or WANs. digital data system (DDS) A private line digital service, typically with data rates of 2,400, 4,800, 9,600, and 56,000 bits per second. T1 is a carrier system transporting Mbps of digital information, usually divided into 24 channels of 64 Kbps each for voice (using multiplexers) or data (usually using routers or cluster controllers) transmission. Formerly an interoffice trunk carrier between SOs, T1s now commonly form the basis of many private networks.

35 Lesson 1: Overview 1-23 Figure 1-40: Network using T1 lines T3 is a Mbps carrier system (equivalent to 28 T1s) that can be divided into 672 voice-grade channels. Although T3 was previously uncommon, demand is increasing, particularly among large corporations with complex communications needs. A common application is to interconnect LANs, as shown here. Most T3 transport is over optical fiber. Figure 1-41: Network using a T3 line As the name implies, fractional T1 is really a fraction of T1 service. Users lease bandwidth in increments of 64 Kbps (equivalent to a standard T1 channel) to fit their needs and budget. Each FT1 has the capability of full-duplex transmission. The following illustration shows an example of a six-channel FT1 (each channel operating at 64 Kbps) that might be used for CAD/CAM transfers. Figure 1-42: Six-channel FT1 connection Users use DDS (known as subrate circuits) when they need less than 64 Kbps. Leased DDS provides up to 56 Kbps for synchronous point-to-point or multipoint digital transmission. subrate circuit Any digital telecommunications service operating at speeds less than a single channel of a T1 circuit i.e., less than 64 Kbps.

36 1-24 Basic Data Communications Because analog modems now commonly transport (with data compression) up to 57.6 Kbps, DDS has become less commercially viable. One advantage is that a single DDS circuit can, for example, be multiplexed into five 9.6-Kbps channels. This arrangement is often less expensive than five dedicated analog circuits supporting 9.6-Kbps modems. Some multiplexers can even multiplex several voice channels onto 56-Kbps circuits. Analog Switched Services An analog switched network uses dial-up modems (as opposed to dedicated-line modems). A company with an analog switched network has decided to lease phone lines through the PSTN rather than use dedicated lines. When users want to gain access to remote computers, their modems (through controlling software) dial the receiving terminal's number, as in a standard telephone call. This method allows users more flexibility in call destination. Figure 1-43: Analog switched network using leased phone lines Digital Switched Services The switched digital services of the near future will have little in common with analog switching. Currently available digital services that can take advantage of digital switching include: Switched 56 Integrated Services Digital Network (ISDN) Switched Multimegabit Data Service (SMDS) X.25 Frame relay Asynchronous transfer mode (ATM) switched 56 A 56-Kbps digital service offered on a dial-up or switched basis for data communications.

37 Lesson 1: Overview 1-25 Integrated Switched Services Digital Network (ISDN) A collection of standards that defines interfaces for operation of digital switching equipment. Instead of using one analog telephone line, ISDN uses three digital channels. Each channel carries voice, video, data, images, or combinations of these. ISDN has two basic formats (for the United States): Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Multimegabit Data Service (SMDS) A technology for transmitting local area network traffic long distances through a switched network; a way of creating metropolitan area networks and wide area networks. Standardized as IEEE 802.6; also called DQDB. X.25 An OSI protocol for packet-switched networks. frame asynchronous relay Packet transmission technology using reduced frame overhead and advanced network hardware for rapid transmission of data. Can use either electrical or optical transmission media. transfer mode (ATM) An implementation of fast packet-switching technology (cell relay) that divides all communications streams into 53-byte cells. Switched 56 uses circuit-switching technology. SMDS, X.25, frame relay, and ATM use packet-switching technology. ISDN uses both circuit- and packetswitching technologies. Before we address each choice, we will briefly look at the difference between circuit switching and packet switching. circuit packet switching The switching process commonly used in telephone networks whereby a circuit between two users is opened on demand and maintained for their exclusive use for the duration of the transmission. switching A switching technique that divides a data stream into individual packets. Each packet is handled by network nodes independently of each other packet. Packet switching shares network resources efficiently because packets with various sources and destinations may travel over the same path through the network. The voice network is a circuit-switching network. A single, continuous circuit is set up for the duration of the call. All switches related to the call keep a continuous path open from the beginning of the call to the end of the call. Many data technologies use the voice network structure, sometimes with special hardware and software, to transmit circuit-switched data.

38 1-26 Basic Data Communications Figure 1-44: The voice network is a circuit-switching network In packet switching, information is stored in discrete units commonly called packets. (The technical term is protocol data unit {PDU.}) A packet from one call may share the same circuit as packets from other calls. packet protocol Group of bits, including control information and user data, which are transmitted over the network as a single entity. data unit (PDU) A unit of data containing both protocol-control information and user data from the layer above. Figure 1-45: Circuit switching versus packet switching Switched 56 is a data service similar to FT1 in that it offers a few channels. With switched 56, a circuit switch service lets users transmit data over a four-wire, digital synchronous network. It is also similar to ISDN in that it offers the flexibility of switched access. Switched 56 is not the same as the standard 56- Kbps modem commonly used today to transmit over the PSTN. Figure 1-46: Switched 56 network

39 Lesson 1: Overview 1-27 The key advantage of switched 56, as with any switched access, is that you pay only for the time connected, plus a little extra for access to the service. Switched 56 is useful as a backup to a dedicated T1 or FT1 connection or when a company has many data calls to multiple locations but too low a volume to justify dedicated services. ISDN is an all-digital switched service that allows the user to make simultaneous voice and data calls. Many of the newer telephone company switches are ISDN-capable. The two types of available ISDN are basic rate interface (BRI) and primary rate interface (PRI). basic primary rate interface (BRI) The user interface to the public ISDN, usually through Centrex. It contains two B channels at 64 Kbps and a D channel that operates at 16 Kbps. The BRI is the transmission path between the user s phone and the SO (switching office). The SO switches it to its eventual destination. rate interface (PRI) A user interface to the ISDN with 23 B channels and one D channel, all at 64 Kbps. It is primarily used to link private branch exchanges (PBXs) and to connect PBXs to the public switched telephone network. Figure 1-47: ISDN BRI versus ISDN PRI Primary rate interface (PRI) is similar to a T1 circuit, offering 23 voice/data channels (termed B channels) at 64 Kbps each. The 24th channel (or D channel) carries all the signaling information for the other 23. This arrangement enables telephone company switches to switch the other 23 channels. Figure 1-48: ISDN PRI offers 23 B channels and 1 D channel

40 1-28 Basic Data Communications Basic rate interface (BRI) offers a total bandwidth of 196 Kbps. Two 64-Kbps B channels are provided; they can be used for either switched voice or digital data communications. A third, 16-Kbps, D channel provides signaling between the telephone set and the switch. The remaining Kbps are provisioned for future use. Figure 1-49: ISDN BRI provides 2 B channels and 1 D channel Switched Multimegabit Data Service (SMDS) is a high-speed packet-switching technology (sometimes called fast packet ). Telephone companies offer SMDS to link separate LANs into a metropolitan area network, or MAN. SMDS MANs operate at speeds up to T3. SMDS requires special switches and currently is not easily obtainable. Switched Multimegabit Data Service (SMDS) A technology for transmitting local area network traffic long distances through a switched network; a way of creating metropolitan area networks and wide area networks. Standardized as IEEE 802.6; also called DQDB. Figure 1-50: Switched Multimegabit Data Service (SMDS) So far, you have studied circuit-switching technologies. X.25 is one of the oldest and most established packet-switching technologies. Many use the generic term packet-switching when talking specifically about X.25 and its related protocols.

41 Lesson 1: Overview 1-29 From the user s perspective, an X.25 transmission is almost like a regular phone call. It even uses the standard 64-Kbps voice circuit (up to 56 Kbps of user information). The user s data transmission device (DTD) sends a call address. The packet network sets up a virtual circuit and transports all of the call s packets along that route. virtual circuit (VC) A path through a network defined by a database of routing information. Routing information determines the path without dedicating an end-toend physical circuit to the packet. VCs may be set up on a call-by-call basis or may be permanent virtual circuits with the routing information stored between calls and the path guaranteed to be available. Figure 1-51: X.25 transmission is like a regular phone call A newer packet technology, frame relay, can transport user information at speeds up to Mbps. Frame relay also has less overhead than X.25 because it minimizes error checking. Finally, although X.25 requires protocol conversion at a PAD, frame relay is protocol-independent. This transparency gives frame relay the flexibility to carry data using a variety of protocols. overhead packet transparency The bits in a digital signal that do not carry user information. These bits typically perform housekeeping functions such as framing, error detection, and the transport of maintenance data from one digital terminal to another. assembler/disassembler (PAD) Hardware with specialized software capabilities that sits between a data device and a packet-switching network. Transmissions from the data device are encoded into packets specific to network operation. At the receiving end, packets are reassembled into a format appropriate for the receiving data device. Also called packet assembly/disassembly device. Acting without the intervention or knowledge of the user or other relevant entity on the network.

42 1-30 Basic Data Communications Asynchronous transfer mode (ATM) is a very high-bandwidth transmission technology designed to transport data and/or voice on optical fiber at speeds up to 600 Mbps. A unique feature of ATM is that it uses cells of uniform size. (X.25 and frame-relay packets vary in size according to user information and other parameters.) ATM s cells are always 53 octets (424 bits). This standardization helps to ensure a relatively uniform delay between cells, an attractive feature for network managers and users who may want to transport time-sensitive information such as voice over the packet network. cell Protocol data units, or packets, used by ATM or other cell relay protocols. Typically small packets of uniform size. Figure 1-52: ATM versus X.25 and frame relay Data Terminal Devices This topic addresses the main categories of network devices. Data terminal devices and network operations often belong to completely separate operations. Technical support for each rarely thinks about the other operation. However, because communications networks exist to support data processing users, it is important to understand the needs and demands of both. Figure 1-53: Data terminal devices and network operations work together This topic will briefly examine some of the issues concerning data terminal devices, including host computers, display and non-display terminals, and smart and dumb terminals. Before we look at the specific categories of terminal devices, it may be helpful to review some basic terminology.

43 Lesson 1: Overview 1-31 DTE stands for data terminal equipment. DTE can be anything from a simple card swipe terminal to a personal computer or even a supercomputer, and can include the following: Mainframes Minicomputers Batch terminals Cluster controllers Communication processors Personal computers Dumb display terminals Smart display terminals Non-display specialized terminals DCE stands for data communications equipment. The function of DCE is to connect DTE to the network. Two of the most common examples of data communications equipment are modems and CSU/DSUs. Figure 1-54: Modems and CSU/DSUs connect DTE to the network Occasionally, DCE may be integrated into DTE such as an internal modem in a computer. Such integration blurs the distinction between the two categories. Figure 1-55: Many computers (DTE) have internal modems (DCE)

44 1-32 Basic Data Communications Terminals are a common type of DTE that gives users access to a computer. Terminals may include many different hardware components such as mouse devices, keyboards and printers. Figure 1-56: Mouse devices, keyboards, and printers are hardware components In this topic, we will use the term DTE to discuss network communications. We will use the word terminal when talking about a device that provides a user with an interface to a host computer. The remainder of this topic focuses on terminals. Host computers are the first type of data terminal device we will consider. Host Computers Most hosts are mainframes or minicomputers. Host computers reside at the other end of the network from the user and terminal. Users at the remote terminals access the host to use its application programs, such as word processors or spreadsheets. mainframe minicomputer A large, powerful central computer that performs the processing for a network of remote terminals. A digital computer that is functionally intermediate between a microcomputer and a mainframe. Figure 1-57: Terminals access applications from the host

45 Lesson 1: Overview 1-33 Host maintenance is usually separate from network maintenance. This separation of responsibility can make problem resolution challenging. Figure 1-58: Host maintenance and network maintenance are separate The host computer rarely manages its own communications with terminals. This management is the job of the communications processor, often called a frontend processor (FEP). Some tasks that the FEP performs include data conversion, error analysis, and message handling. communications front-end processor A device in a terminal-to-host or SNA network connected directly to the mainframe host. It manages communications between cluster controllers and mainframe hosts and network traffic from remote terminals, leaving the host free to focus on other processing tasks. Also known as the frontend processor. processor (FEP) A device in a terminal-to-host or SNA network connected directly to the mainframe host. It manages communications between cluster controllers and mainframe hosts and network traffic from remote terminals, leaving the host free to focus on other processing tasks. Also known as the communications processor. Figure 1-59: The FEP manages communications between the host and terminals Communications ports are the physical connectors between the computer and the modem.

46 1-34 Basic Data Communications communication port A connector on a computer that allows the computer to exchange signals with another device such as a modem or a printer. Serial and parallel ports are two types of ports. In a personal computer, the serial port performs this task. A minicomputer usually contains several communications ports (serial ports, also referred to as com ports) to support multiple terminals. In mainframes, the communications processor provides the ports. Figure 1-60: Com ports connect computers and modems Minicomputer ports usually conform to some variation on the EIA 232 standard, commonly called the RS-232, RS-232c, or RS232d. Communications processors may follow the 232 or V.35 standard. The DCE used in these situations is usually a modem operating at 9,600, 14,400, or 28,000 bps. Computers connect in many different ways. Typically CSU/DSUs or modems provide the DCE connection between DTE. Routers or cluster controllers organize how data from terminals is placed onto the trunks. Older systems used multiplexers, but those are less common today. Most multiplexers are used only for voice connections over trunks. Figure 1-61: Communications processor with several com ports

47 Lesson 1: Overview 1-35 Display Terminals vs. Non-Display Terminals When most people refer to their computer's terminal, they are talking about the terminal's display, or monitor. Monitors, along with printers, are examples of output devices. monitor An output device used to display information during interaction with computers. output device Used to retrieve processed or stored information from the microcomputer. Figure 1-62: Monitors and printers are output devices Many monitors use cathode-ray tubes to present the user interface. Modern display terminals commonly use either light-emitting diode (LED) or liquid crystal display (LCD) technology. cathode-ray light-emitting liquid tube (CRT) The picture tube used in most display terminals. The display terminals are often called CRT terminals. diode (LED) A semi-conductor device that emits light, often used as an indicator light or as the light source in low-performance fiber-optic communication systems. crystal display (LCD) The technology often used for the display screen in portable computers, because it allows such screens to be small and light and use little power. Figure 1-63: Modern monitors use LED or LCD

48 1-36 Basic Data Communications Terminals also require an input device. The most common input device is the keyboard. Other widely used input devices include: Mouse devices Touch screens Trackballs Barcode readers Magnetic stripe readers The mouse, a popular device for personal computers, is now also common to terminals. input device A device for entering data into the microcomputer for storage or processing. Keyboards and mouse devices are examples of input devices. Figure 1-64: Keyboards and mouse devices are input devices Non-display terminals, as the name implies, are not able to display information. Many of today's non-display terminals are not permanently connected to the network. They are often portable handheld devices that perform a specialized function such as inventory control. At the end of the day, they are plugged into the network for data download and calibration. If you have signed for an overnight delivery package with a stylus onto a rubber or glass pad, you have used a non-display terminal. Another type of non-display terminal is the batch terminal. batch terminal A computer terminal that accepts input from several different jobs, then forwards all the input as a single batch to the mainframe for processing. Batch terminals allow more efficient use of the network communication resources. In batch processing, information is passed in bunches called batches. A user operating a batch terminal enters information; then, when all the information has been entered, sends it in a batch file to the host computer. In this way, the communication channel is used only when batch files are transmitted.

49 Lesson 1: Overview 1-37 Figure 1-65:Batch terminals send data to host in a batch file Smart vs. Dumb Terminals The terms "smart" and "dumb" are used to compare one type of terminal with another. Some smart terminals are considerably more powerful than dumb terminals, and some are slightly less so. Two criteria that determine smartness are the terminal s: Processing power. Degree of local terminal management. Processing power refers to a terminal's ability to perform computer-processing functions independently of other devices, including a host. The more independent processing power a terminal has, the smarter it is. Figure 1-66: Smarter terminals have more independent processing power Terminal management indicates the terminal's contribution to maintaining a reliable connection to the host. This measurement includes the terminal's ability to handle related functions such as running multiple applications simultaneously. Some terminals are managed completely by a host. Other terminals are at least partially managed by more local DTE, such as a cluster controller. Other terminals have extensive management capabilities built in.

50 1-38 Basic Data Communications cluster controller A device in a terminal-to-host computer that manages the functions of a group of display terminals and mediates communication between the terminals and the rest of the network. Figure 1-67: The cluster controller manages terminals A dumb terminal has no processing power and is managed completely from a host computer. A smart terminal has a high degree of processing power, some of which is devoted to managing the terminal. dumb smart terminal A computer terminal with limited capacity to manage either its own functions or the communication channel to the host computer. terminal A terminal with enough memory and processing power to manage its own communications with devices farther up the hierarchy. Figure 1-68:Dumb terminals are managed completely by the host

51 Lesson 1: Overview 1-39 Dumb Terminals The core functions of a dumb terminal attached to a host computer are: To send keystrokes to a host computer. To display information received from a host computer. Figure 1-69: Dumb terminals send keystrokes and display information from the host Most dumb terminals do have some capabilities of their own. Many current dumb terminals are equipped with microprocessors and have enough intelligence to provide some basic functions independent of a host, including: Simple text editing. Formatted data entry. Screen customizing features (foreground/background colors). Most dumb terminals are confined to the activity of sending or receiving characters one at a time from the host. This activity is called asynchronous communications. In asynchronous communications, each character typed at the keyboard is sent to the DCE and across the network as a single unit. asynchronous communications Start/stop. A method of data communications in which every 8-bit code is surrounded by start and stop bits. The start and stop bits allow the sending and receiving devices to use different clocks and not synchronize with each other. Figure 1-70: Dumb terminals use asynchronous communications Most asynchronous terminals use the ASCII character set when communicating with the DCE. To many people, dumb terminals are therefore synonymous with asynchronous terminals, or ASCII terminals.

52 1-40 Basic Data Communications American Standard Code for Information Exchange (ASCII) A simple unformatted form designed for communication between computers and between computers and applications. The most well-known dumb, asynchronous terminal is Digital Equipment Corporation's (DEC) VT100. Because the VT100 is so widely used, many people inaccurately use the terms ASCII terminal or asynchronous terminal when referring specifically to a VT100 or its standard, ANSI X3.64. However, DEC and other companies make many types of asynchronous terminals, so it is important to avoid such generalizations. Many dumb terminals are part of a cluster of terminals, connected to a cluster controller. Each terminal in a cluster communicates with the controller in much the same way that a standard dumb terminal communicates with the host. The cluster controller sends large amounts of data quickly and efficiently to the communications processor. This connection is usually synchronous. Synchronous communications send large blocks of data as single units. synchronous Operating at the same speed. All communicating devices in a synchronous network are constrained to operate at their nominal rates with no significant variation. Figure 1-71: Cluster controllers use synchronous communications Any dumb terminal can be a clustered terminal. IBM's systems network architecture (SNA) was designed with clustered terminals in mind. Members of IBM's family of 3270 terminals serve as classic examples of clustered terminals. Clustered terminals for other environments are likely to be incompatible with the 3270 specifications. However, they all operate on similar principles. A dumb terminal that may become popular in the future is the network computer (NC). These terminals are also known as computer shells because they have relatively little memory and cannot run applications on their own. Primary users will probably be corporate networks that are connected to the Internet.

53 Lesson 1: Overview 1-41 Figure 1-72: NCs are dumb terminals used to connect to the Internet Today s terminals sometimes look smarter than they used to. This improvement is primarily due to: Improved display technology. More sophisticated cluster controllers and host. Two examples of the new "smart" look are: Windowing Multitasking A terminal with windowing capabilities can display different aspects of the same program in separate windows. Windowing enables users to display multiple images (of the same program) on the monitor. windowing multitasking A display terminal's ability to display information about multiple aspects of an application in different regions of the display screen, or, when combined with multitasking, the ability to display multiple applications in different regions of the display screen. The ability to run several applications simultaneously. Multitasking is a function of both the computer s processor and its operating system. A terminal with multitasking capabilities enables the user to run more than one program at the same time. The user can look at one program while others are running in the background. A terminal with both windowing and multitasking can provide multiple looks at multiple applications.

54 Lesson 1: Overview 1-41 Figure 1-72: NCs are dumb terminals used to connect to the Internet Today s terminals sometimes look smarter than they used to. This improvement is primarily due to: Improved display technology. More sophisticated cluster controllers and host. Two examples of the new "smart" look are: Windowing Multitasking A terminal with windowing capabilities can display different aspects of the same program in separate windows. Windowing enables users to display multiple images (of the same program) on the monitor. windowing multitasking A display terminal's ability to display information about multiple aspects of an application in different regions of the display screen, or, when combined with multitasking, the ability to display multiple applications in different regions of the display screen. The ability to run several applications simultaneously. Multitasking is a function of both the computer s processor and its operating system. A terminal with multitasking capabilities enables the user to run more than one program at the same time. The user can look at one program while others are running in the background. A terminal with both windowing and multitasking can provide multiple looks at multiple applications.

55 Lesson 1: Overview 1-43 Figure 1-75: Hosts and smart terminals share processing tasks Dumb terminals can do very little on the screen before communicating with the host. This communication requires contact between the host and every terminal in the network. Large processor-intensive jobs have to wait until off-peak hours; the host is too busy during the day simply managing users' terminals. Smart terminals reduce the amount of terminal management required. X-Window is the most fully developed standard for smart terminals and has become the most common interface for UNIX machines. The X-Window distributed processing environment enables standard multitasking and windowing capabilities. It also allows powerful graphics displays, with a wide range of color choices, high resolution, and fast screen panning. Threedimensional graphics are common. X-Window A set of software-based intelligent terminal functions that allows a terminal to provide windowing and a graphical user interface. X-Window is used in a distributed processing architecture where the X-Window server software at the terminal manages all the complex user interface functions while the X-Window client, running on a different computer, passes information between the X-Window server and the application program. X-Window is hardware-independent, meaning that the X-Window terminal's function is defined by software. The hardware must still be able to handle the software's demands. Figure 1-76: X-Window functionality is defined by software X-Window reverses the usual "client-server" relationship. The host is the client, and the terminal is the server. This arrangement frees each side to be concerned

56 1-44 Basic Data Communications only with its function within an operation. The X-client can communicate detailed screen information with very low bandwidth. Figure 1-77: Host with X-Windows client software Personal computers are stand-alone computers. Three types of PCs are commonly used today. Each is distinguished by the type of operating system (OS) it uses: Microsoft Windows and/or DOS (IBM and IBM-compatible PCs) Apple System 7+ (Macintoshes and Power Macs) UNIX (workstations, by a variety of manufacturers) operating system (OS) The core foundation software that controls the basic functions of the computer, manages memory and controls the internal flow of information to and from the main processor. PCs can stand alone as desktop computers. With proper software, they can also connect to a host network and act as terminals. Depending on the software, the PC on a host network can: Handle more processing functions than standard terminals. Reduce host processing requirements. Exchange files with the host. These abilities result in less work for the host and lower bandwidth requirements for the network. Smart terminals reduce the host's terminal management load. PCs go one step further and perform most of their work offline from the host computer, thereby freeing the host to devote its resources elsewhere. Figure 1-78: PCs perform many tasks offline

57 Lesson 1: Overview 1-45 Terminal emulation allows a computer to communicate with a remote host in the same way that a real terminal does. PCs can be made as dumb as a network requires. Each terminal standard has specific keystroke and network conventions. Terminal emulation software enables PCs to imitate conventions such as those of DEC VT100. Figure 1-79: PC with terminal emulation software Notebook computers, or laptop computers, take the desktop PC to another dimension. They put the power and functionality of a desktop PC in a package about the size of a small-town telephone directory. Usually weighing less than pounds and shorter than three inches high, notebooks are portable PCs. Although notebooks tend to be more expensive than their desktop counterparts, due in large part to the expense of their display screens, they come similarly equipped. CD-ROM drives, DVD drives, floppy disk drives, large-capacity hard disk drives, network interface cards, and modems can all be found in notebook computers. Notebooks are powered by a rechargeable battery or AC adapter. The laptop display screen is usually either dual-scan or the more expensive active matrix. With the ability to connect a full-sized external monitor, keyboard and mouse, laptops can function as desktop computers. notebook computer A small, portable computer with much of the same equipment and functionality of a desktop PC. Also known as a laptop computer. Figure 1-80: Notebook or laptop computer

58 1-46 Basic Data Communications Another data terminal device that is rapidly gaining popularity is the personal digital assistant (PDA). PDAs are small, handheld devices that are used to store and retrieve personal or business information. They contain a small display monitor and many have a tiny keyboard. Some contain a touch-sensitive pad that accepts handwritten information that is input with a small plastic stylus. PDAs are most commonly used to store address and schedule information, but many include word-processing and capability. Virtually all PDAs can exchange data with software applications that reside on desktop or laptop computers. Recently, PDA manufacturers have begun to include cellular phone and/or paging technology. personal digital assistant (PDA) A small, handheld device primarily used to store address and schedule information. Figure 1-81: Personal digital assistant (PDA) Key Points About Terminals Terminals are either: Smart or dumb. Display or non-display. Asynchronous character-by-character communication works well in interactive situations such as keying specific commands to the host terminal. Cluster controllers and batch terminals tend to be synchronous; they exchange information with the host in large blocks. Smart display terminals do not follow the same standards as the rest of the terminals. They may be connected to the network via a LAN or may be emulating a specific terminal type.

59 Lesson 1: Overview 1-47 Lesson Summary Application project A company is ready to update its communications and data management systems, including hardware and software. Consider the advantages and disadvantages of the different options the company has. Be sure to include LECs and IECs, processing power (smart versus dumb terminals), DTE and DCE, analog versus digital transmission, and dedicated versus switched transmission. Pay special attention to the types of transmission technologies and their advantages and disadvantages. Skills review Following are the key points covered in this lesson: Historically, the telephone network has been analog. The human voice is an analog sound. Computer networks communicate by exchanging digital signals. All data transmission originates or ends at data terminal equipment, more commonly referred to as DTE. Two common examples of DTE are personal computers and network hosts. DCE, or data communication equipment, is specific equipment that sits between the DTE and the network to encode and decode the data. Two examples of DCE are modems and CSU/DSUs. Digital signals are represented by numbers encoded with a binary numbering system a system that uses only two digits, one and zero. Each binary digit in a signal is called a bit. Analog and digital signals each have advantages. Analog signals are more tolerant of errors than digital signals are. A digital signal is easier to measure and easier to discriminate from distortion than analog signals are. Digital signals are also more cost-effective than analog because they are more reliable, reducing maintenance costs. There is, essentially, a single PSTN, but different companies own different parts of the PSTN. These include both local-service and long-distance providers. You can probably select any local service provider or long distance carrier to handle your calls. The network's flexibility is suitable for voice communications. Most data communications, however, do not need this much flexibility. Reliability is more important. Two ways of obtaining reliable point-to-point data communications are: leased dedicated (non-switched) circuits from a communications service provider, and leased specialized switched services from a communications service provider. A circuit typically has at least two transmission paths one for sending and one for receiving. The circuit channels can be used in one of two ways: half duplex or full duplex.

60 1-48 Basic Data Communications Transmission circuits can be either analog or digital, and either nonswitched or switched. Non-switched circuits are most often called dedicated circuits because non-switched circuits are dedicated to transporting information between fixed locations with no switching involved. Switched circuits using the PSTN are usually a mix of analog and digital technologies. Switched circuits are interconnected circuits that establish a temporary connection between two or more points. Data communications can use one of four types of telephone transmission circuit technology: analog dedicated, digital dedicated, analog switched, and digital switched. Each type has advantages and disadvantages. Analog dedicated circuits are useful when a network requires a dedicated connection but a digital line is either impractical or too expensive. Because of their greater bandwidth, digital circuits are more common in larger, more complex networks. An analog switched network uses dial-up modems the company leases phone lines through the PSTN rather than using dedicated lines. Currently available digital services that can take advantage of digital switching include: switched 56, ISDN, SMDS, X.25, Frame relay, and ATM. DTE stands for data terminal equipment. DTE can be anything from a simple card swipe terminal to a personal computer or even a supercomputer. DCE stands for data communications equipment. The function of DCE is to connect DTE to the network. Two of the most common examples of data communications equipment are modems and CSU/DSUs. Most hosts are mainframes or minicomputers. Host computers reside at the other end of the network from the user and terminal. The host computer rarely manages its own communications with terminals. Such management is the job of the communications processor, often called a front-end processor (FEP). Many display terminals use cathode-ray tubes to present the user interface. Modern display terminals commonly use either light-emitting diode (LED) or liquid crystal display (LCD) technology. Display terminals also require an input device. Non-display terminals cannot display information, and are usually not permanently connected to the network. A dumb terminal has no processing power and is managed completely from a host computer. Dumb terminals send and receive characters one at a time from the host. This activity is called asynchronous communications. A smart terminal has a high degree of processing power, some of which is devoted to managing the terminal. A smart terminal has one or more onboard processors. Within a network, information processing can take place both at the terminal and at the controller/host. This arrangement, referred to as distributed processing, increases the system s overall processing efficiency.

61 Lesson 1: Overview 1-49 Now that you have completed this lesson, you should be able to: Identify the key components in a data network. Identify the three categories of data communications. Identify the function of DTE and DCE as related to a data network. Define the features of analog signals. Define the feature of digital signals. Identify the advantages of digital signals over analog technology. Define the roles and responsibilities of transmission service providers. Identify the features and benefits of primary forms of transmission technology services. Define basic data equipment terms. Define the features and functions of host computers. Differentiate between display and non-display terminals. Differentiate between smart and dumb terminals. Identify the function of dumb terminals in a data communications network. Identify the function of smart terminals in a data communications network. Define the features and functions of transmission circuits.

62 1-50 Basic Data Communications Lesson 1 Review 1. What is the inherent form of transmission for the human voice? 2. Bandwidth is synonymous with what characteristic of data communications? 3. A modem is an example of which equipment category? 4. In a sine wave, the period from one peak to the next peak is called a:. 5. How many frequencies occur in a digital signal? 6. Which device helps eliminate distortion in digital signals by filtering all non-digital values? 7. What service providers utilize the PTSN?

63 Lesson 1: Overview In the United States, was the result of the Telecommunications Act of 1996? 9. What is the best way to ensure reliable connections between resources? 10. What is a multipoint circuit? 11. What are the advantages of digital signals? 12. Why are dedicated circuits more reliable than switched circuits? 13. Which circuit type shares PSTN resources with other phone calls? 14. What is the actual amount of useful information transmitted within a given time?

64 1-52 Basic Data Communications 15. Which digital dedicated circuit allows users to lease bandwidth in increments of 64 Kbps? 16. Which digital service provides access to a separate, dedicated, circuitswitched network offered by telephone company providers? 17. What digital switched service has two 64-Kbps channels and a separate signaling channel? 18. Which packet-switching system operates on the bandwidth of a voice circuit and requires protocol conversion? 19. Which packet-switching protocol transports uniform-sized packets over a fiber network? 20. What are examples of data terminal equipment (DTE)? 21. What is a terminal?

65 Lesson 1: Overview What is another term for a host computer? 23. What is an example of a display terminal? 24. What is a smart terminal? 25. The process a dumb terminal uses to send one character at a time to the cluster controller is termed communications. 26. Do PCs perform most of their work online or offline? 27. When a PC acts as a DEC VT100, it is using terminal software.

66 1-54 Basic Data Communications