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Ryerson University
Information Technology Management
ITM 301
Franklyn Prescod

Chapter 1 Server (Host Computer) – stores data or software that can be accessed by the clients Client – the input-output hardware device at the user’s end of a communication circuit Circuit – the pathway through which the messages travel Telecommunications - Transmission of voice, video, and/or data Data Communications - Movement of computer information by means of electrical or optical transmission systems • Local Area Networks (LAN) - room, building – a group of PCs that share a circuit. • Backbone Networks (BN) - less than few kms – a high speed backbone linking together organizational LANs at various locations. • Metropolitan Area Networks (MAN) - (more than a few kms) – connects LANs and BNs across different locations – Often uses leased lines or other services used to transmit data. • Wide Area Networks (WANs) - (far greater than 10 kms) – Same as MAN except wider scale • Intranet – A LAN that uses the Internet technologies within an organization – Open only those inside the organization – Example: insurance related information provided to employees over an intranet • Extranet – A LAN that uses the Internet technologies across an organization including some external constituents – Open only those invited users outside the organization – Accessible through the Internet – Example: Suppliers and customers accessing inventory information in a company over an extranet Page 1 of 31 2 Important Multi-layer network models • Open Systems Interconnection Model – Created by International Standards Organization (ISO) as a framework for computer network standards in 1984 Based on 7 layers 1. Application Layer - set of utilities used by application programs 2. Presentation Layer - formats data for presentation to the user  provides data interfaces, data compression and translation between different data formats 3. Session Layer - initiates, maintains and terminates each logical session between sender and receiver 4. Transport Layer - deals with end-to-end issues such as segmenting the message for network transport, and maintaining the logical connections between sender and receiver 5. Network Layer - responsible for making routing decisions 6. Data Link Layer - deals with message delineation, error control and network medium access control 7. Physical Layer - defines how individual bits are formatted to be transmitted through the network • Internet Model – Created by DARPA originally in early 70’s – Developed to solve to the problem of internetworking – Layers allow simplicity of networking in some ways • Easy to develop new software that fits each layer • Relatively simple to change the software at any level • Matching layers communicate between different computers and computer platforms – Accomplished by standards that we all agree on – e.g., Physical layer at the sending computer must match up with the same layer in the receiving computer • Somewhat inefficient – Involves many software packages and packets – Packet overhead (slower transmission, processing time) – Interoperability (ability to exchange and use information) achieved at the expense of perfectly streamlined communication Based on 5 layers - Based on Transmission Control Protocol/ Internet Protocol (TCP/IP) suite 1. Application Layer - used by application program 2. Transport Layer - responsible for establishing end-to-end connections, translates domain names into numeric addresses and segments messages 3. Network Layer - responsible for making routing decisions (Chooses computer) and find address of that computer 4. Data Link Layer – responsible for moving a message from one computer to the next computer in the network path from the sender to the receiver. Performs 3 functions:  Controls the physical layer by deciding when to transmit messages over the media  Formats messages by indicating where they start and end  Detects and corrects errors that have occurred during transmission 5. Physical Layer – physical connection between the sender and receiver (Hardware, circuits) Page 2 of 31 Protocols - Sets of standardized rules to define how to communicate at each layer and how to interface with adjacent layers • Used by Network model layers Standards (Importance) - Provide a “fixed” way for hardware and/or software systems (different companies) to communicate – Help promote competition and decrease the price Types of Standards  Formal standards o Developed by an industry or government standards-making body  De-facto standards o Emerge in the marketplace and widely used o Lack official backing by a standards-making body  Standardization Process o Specification - Developing the nomenclature and identifying the problems to be addressed o Identification of choices - Identifying solutions to the problems and choose the “optimum” solution o Acceptance - Defining the solution, getting it recognized by industry so that a uniform solution is accepted  Major Standard Bodies o ISO (International Organization for Standardization)  Technical recommendations for data communication interfaces  Composed of each country’s national standards orgs.  Based in Geneva, Switzerland (www.iso.ch) o ITU-T (International Telecommunications Union –Telecom Group  Technical recommendations about telephone, telegraph and data communications interfaces  Composed of representatives from each country in UN  Based in Geneva, Switzerland (www.itu.int) o ANSI (American National Standards Institute)  Coordinating organization for US (not a standards- making body) Page 3 of 31  www.ansi.org o IEEE (Institute of Electrical and Electronic Engineers)  Professional society; also develops mostly LAN standards  standards.ieee.org o IETF (Internet Engineering Task Force)  Develops Internet standards  No official membership (anyone welcome)  www.ietf.org Emerging Trends:  Pervasive Networking - Networks will be everywhere  Convergence - Networks that were previously transmitted using separate networks will merge into a single, high speed, multimedia network in the near future Application Service Providers (ASPs) -Develop specific systems for companies to use; such as providing and operating a payroll system for a company that does not have one of its own  Information Utilities (Future of ASPs) - Providing a wide range of info services (email, web, payroll, etc.) (similar to electric or water utilities) Page 4 of 31 Chapter 2 Application Layer - Introduction • Application architecture • The way in which the functions of the application layer software are spread among the clients and servers on the network • Functions of Application Layer • Data storage - Storing of data generated by programs (e.g., files, records) • Data access logic - Processing required to access stored data (e.g., SQL) • Application logic - Business logic such as word processors, spreadsheets • Presentation logic - Presentation of info to user & acceptance of user commands Four Application Architectures: Host-based Architectures - Server performs almost all functions  Problems: Host becoming a bottleneck - All processing done by the host, which can severely limit network performance  Host upgrades typically expensive and “lumpy” o Available upgrades require large scale and often costly jumps in processing and memory o Network demand grows more incrementally than does the host capacity o May see poor fit (too much or too little) between host performance and network demand Client-based architectures - Client performs most functions Page 5 of 31  Data traffic must travel back and forth between server and client – Example: when the client program is making a database query, the ENTIRE database must travel to the client before the query can be processed – Often the large file sizes moving across the LAN can yield a poor result in network performance Client-server architectures - Functions shared between client and server • Advantages – More efficient because of distributed processing – Allow hardware and software from different vendors to be used together • Disadvantages – Difficulty in getting software from different vendors to work together smoothly – May require Middleware, a third category of software Peer-to-Peer architectures – computers are both clients and servers and thus share the work • All computers can serve as a client and a server • Increased popularity in the last decade due to the rise of P2P services such as Napster • Advantages: • Data can be stored anywhere on the network • Very resilient to failure • Disadvantages: • Finding data • Security Middleware - a standard way of translating between software from different vendors – Manages message transfers – Insulates network changes from the clients (e.g., adding a new server) Examples of standards for Middleware: – Distributed Computing Environment (DCE) – Common Object Request Broker Architecture (CORBA) – Open Database Connectivity (ODBC) Multi-tier Architectures • Involve more than two computers in distributing application program logic Page 6 of 31 – 2-tier architecture • Uses clients and servers in a balance, very popular approach in simple LANs – 3-tier architecture • 3 sets of computers involved – N-tier architecture • More than three sets of computers used, more typical across complex organizations • Allows load balancing across servers • Advantages – Better load balancing: • More evenly distributed processing. (e.g., application logic distributed between several servers.) – More scalable: • Only servers experiencing high demand need be upgraded • Disadvantages – Heavily loaded network: • More distributed processing necessitates more data exchanges – Difficult to program and test due to increased complexity Thin and Thick Clients -Classification depends on how much of the application logic resides on the client or server • Thin client - Little or no application logic on client • Becoming popular because easier to manage, (only the server application logic generally needs to be updated) • The best example: World Wide Web architecture (uses a two-tier, thin client architecture) • Thick client - All or most of the application logic resides on the client Criteria for Choosing Architecture • Infrastructure Cost – Cost of servers, clients, and circuits – Mainframes: very expensive; terminals, PCs: inexpensive • Development Cost – Mainly cost of software development – Software: expensive to develop; off-the-shelf software: inexpensive • Scalability – Ability to increase (or decrease) in computing capacity as network demand changes – Mainframes: not scalable; PCs: highly scalable Page 7 of 31 Uniform Resource Locators (URLs) - A formal way of identifying links to other documents HTML (Hypertext Markup Language) - A language used to create Web pages • Also developed at CERN (initially for text files) • Tags are embedded in HTML documents – include information on how to format the file • Extensions to HTML needed to format multimedia files • XML - Extensible Markup Language - A new markup language becoming popular E-Mail standards: SMTP (Simple Mail Transfer Protocol) - Main e-mail standard for  Originating user agent and the mail transfer agent  Between mail transfer agents  Originally written to handle only text files  Usually used in two-tier client-server architectures Post Office Protocol (POP) and Internet Mail Access Protocol (IMAP)  Main protocols used between the receiver user agent and mail transfer agent  Main difference: with IMAP, messages can be left at the server after downloading them to the client Other competing standards  Common Messaging Calls (CMC), X.400 Two-Tier E-mail Architecture  User agent is another word for an e-mail client application o Run on client computers o Send e-mail to e-mail servers o Download e-mail from mailboxes on those servers Page 8 of 31 o Examples: Eudora, Outlook, Netscape Messenger  Mail transfer agent is another word for the mail server application o Used by e-mail servers o Send e-mail between e-mail servers o Maintain individual mailboxes. Host Based e-mail Architectures • An old method used on UNIX based hosts • Similar to client-server architecture, except – Client PC replaced by a terminal (or terminal emulator) • Sends all keystrokes to the server • Display characters received from the server – All software resides on the server • Takes client keystrokes and understand user’s commands • Creates SMTP packets and sends them to next mail server • Useful when traveling in locations with poor internet facilities Multipurpose Internet Mail Extension (MIME) - A graphics capable mail transfer agent protocol (to send graphical information in addition to text) o SMTP was designed years ago for text transfer only – MIME software is included as part of an e-mail client – Translates graphical information into text allowing the graphic to be sent as part of an SMTP message (as a special attachment) – Receiver’s e-mail client then translates the MIME attachment from text back into graphical format – MIME example Listserv Discussion Groups - Mailing lists of users who join to discuss some special topic (e.g., cooking, typing, networking) File Transfer Protocol (FTP) - Enables sending and receiving files over the Internet • Requires an application program on the client computer and a FTP server program on a server • Commonly used today for uploading web pages • Many packages available using FTP – WS-FTP (a graphical FTP software) • FTP sites: Closed sites - Requires account name and password | Anonymous sites - Account name: anonymous; password: email address Telnet - Allows one computer to log into another computer – Remote login enabling full control of the host • Requires an application program on the client computer and a Telnet server program on the server – Client program emulates a “dumb” terminal off the server • Many packages available conforming Telnet – EWAN • Requires account name and password – Anonymous sites similar to FTP approach • Account name: anonymous; password: email address • Instant Messaging (IM) - A client-server program that allows real-time typed messages to be exchanged – Client needs an IM client software – Server needs an IM server package Page 9 of 31 • Some types allow voice and video packets to be sent – Like a telephone • Examples include AOL and ICQ • Two step process: – Telling IM server that you are online – Chatting Videoconferencing - Provides real time transmission of video and audio signals between two or more locations – Allows people to meet at the same time in different locations – Saves money and time by not having to move people around – Typically involves matched special purpose rooms with cameras and displays • Desktop videoconferencing – Low cost application linking small video cameras and microphones together over the Internet – No need for special rooms – Example: Net Meeting software on clients communicating through a common videoconference server • Common standards in use today – H.320 - Designed for room-to-room videoconferencing over high-speed phone lines – H.323 - Family of standards designed for desktop videoconferencing and just simple audio conferencing over Internet – MPEG-2 - Designed for faster connections such as LAN or privately owned WANs • Webcasting - Special type of uni-directional videoconferencing – Content is sent from the server to users • Process – Content created by developer – Downloaded as needed by the user – Played by a plug-in to a Web browser • No standards for webcasting yet – Defacto standards: products by RealNetworks Web Has 2 application software packages: a web browser on the client, and a web server on the server  They communicate using a HTTP  Most web pages written in HTML but also use other formats Page 10 of 31 Chapter 3 Physical Layer Overview • Includes network hardware and circuits • Network circuits: – physical media (e.g., cables) and – Special purposes devices (e.g., routers and hubs). • Types of Circuits – Physical circuits connect devices & include actual wires such as twisted pair wires – Logical circuits refer to the transmission characteristics of the circuit, such as a T-1 connection refers to 1.544 Mbps – Physical and logical circuits may be the same or different. For example, in multiplexing, one physical wire may carry several logical circuits. Analog data – Produced by telephones – Sound waves, which vary continuously over time, analogous to one’s voice – Can take on any value in a wide range of possibilities Digital data – Produced by computers, in binary form – information is represented as code in a series of ones and zeros – All digital data is either on or off, 0 or 1 Types of Transmission • Analog transmissions – Analog data transmitted in analog form – Examples of analog data being sent using analog transmissions are broadcast TV and radio • Digital transmissions (Baseband transmission) – Made of discrete square waves with a clear beginning and ending – Computer networks send digital data using digital transmissions • Data converted between analog and digital formats – Modem (modulator/demodulator): used when digital data is sent as an analog transmission – Codec (coder/decoder): used when analog data is sent via digital transmission Digital Transmission: Advantages • Produces fewer errors - Easier to detect and correct errors, since transmitted data is binary (1s and 0s, only two distinct values) – A weak square wave can easily be propagated again in perfect form, allowing more crisp transmission than analog • Permits higher maximum transmission rates - e.g., Optical fiber designed for digital transmission • More efficient - Possible to send more digital data through a given circuit, circuit can be “packed” • More secure - Easier to encrypt digital bit stream • Simpler to integrate voice, video and data - Easier mix and match V, V, D on the same circuit, since all signals made up of 0’s and 1’s Circuit Configuration • Basic physical layout of the circuit Page 11 of 31 • Configuration types: – Point-to-Point Configuration • Goes from one point to another • Sometimes called “dedicated circuits” • Used when computers generate enough data to fill the capacity of the circuit • Each computer has its own circuit to reach the other computer in the network (expensive) – Multipoint Configuration • Many computer connected on the same circuit • Sometimes called “shared circuit” • Used when each computer does not need to continuously use the entire capacity of the circuit • Cheaper (not as many wires) and simpler to wire • Only one computer can use the circuit at a time Selection of Data Flow Method – Simplex Method -If data required to flow in one direction only • e.g., From a remote sensor to a host computer – Half-Duplex Method – Take turns so that the data required flow in one direcectopn and then in the other • Terminal-to-host communication (send and wait type communications) – Full Duplex Method – Data flows in both direction • Client-server; host-to-host communication (peer-to-peer communications) • Capacity may be a factor too – Full-duplex uses half of the capacity for each direction Multiplexing - Breaking up a higher speed circuit into several slower (logical) circuits – Several devices can use it at the same time – Requires two multiplexer: one to combine; one to separate • Main advantage: cost – Fewer network circuits needed • Categories of multiplexing: – Frequency division multiplexing (FDM) - Makes a number of smaller channels from a larger frequency band by dividing the circuit “horizontally” – Time division multiplexing (TDM) - Dividing the circuit “vertically” Page 12 of 31 • TDM allows terminals to send data by taking turns • This example shows 4 terminals sharing a circuit, with each terminal sending one character at a time • Time on the circuit shared equally • Each terminal getting a specified timeslot whether needed or not • More efficient than FDM • Since TDM doesn’t use guardbands, entire capacity can be divided up between terminals – Statistical time division multiplexing (STDM) - Designed to make use of the idle time slots • In TDM, when terminals are not using the multiplexed circuit, timeslots for those terminals are idle • Uses non-dedicated time slots • Time slots used as needed by the different terminals • Complexities of STDM • Additional addressing information needed • Since source of a data sample is not identified by the time slot it occupies • Potential response time delays (when all terminals try to use the multiplexed circuit intensively) • Requires memory to store data (in case more data comes in than the outgoing circuit capacity can handle) - Wavelength division multiplexing (WDM) - Transmitting data at many different frequencies o Lasers or LEDs used to transmit on optical fibers o Previously single frequency on single fiber (typical transmission rate being around 622 Mbps) o Now multi frequencies on single fiber  n x 622+ Mbps o Dense WDM (DWDM) o Over a hundred channels per fiber o Each transmitting at a rate of 10 Gbps o Aggregate data rates in the low terabit range (Tbps) o Future versions of DWDM o Both per channel data rates and total number of channels continue to rise o Possibility of petabit (Pbps) aggregate rates Inverse Multiplexing (IMUX) - Shares the load by sending data over two or more lines • Bandwidth ON Demand Network Interoperability Group (BONDING) standard • Commonly used for videoconferencing applications • Six 64 kbps lines can be combined to create an aggregate line of 384 kbps for transmitting video Digital Subscriber Line (DSL) - Became popular as a way to increase data rates in the local loop. – Uses full physical capacity of twisted pair (copper) phone lines (up to 1 MHz) instead of using the 0-4000 KHz voice channel xDSL • Several versions of DSL – Depends on how the bandwidth allocated between the upstream and downstream channels – A for Asynchronous, H for High speed, etc • G.Lite - a form of ADSL Page 13 of 31 – Provides • a 4 Khz voice channel • 384 kbps upstream • 1.5 Mbps downstream (provided line conditions are optimal). Communications Media: Physical matter that carries transmission - Guided media - Transmission flows along a physical guide (media guides the signal across the network) Examples include twisted pair wiring, coaxial cable and fiber optic cable - Wireless media (radiated media) - No wave guide, the transmission flows through the air or space Examples include radio such as microwave and satellite, as well as infrared communications Twisted Pair (TP) Wires - Commonly used for telephones and LANs • Reduced electromagnetic interference – Via twisting two wires together (Usually several twists per inch) • TP cables have a number of pairs of wires – Telephone lines: two pairs (4 wires, usually only one pair is used by the telephone) – LAN cables: 4 pairs (8 wires) • Also used in telephone trunk lines (up to several thousand pairs) • Shielded twisted pair also exists, but is more expensive Coaxial Cable • Less prone to interference than TP due to shield • More expensive than TP, thus quickly disappearing • Used mostly for cable TV Fiber Optic Cable • Light created by an LED (light-emitting diode) or laser is sent down a thin glass or plastic fiber • Has extremely high capacity, ideal for broadband • Works well under harsh environments – Not fragile, nor brittle; Not heavy nor bulky – More resistant to corrosion, fire, water – Highly secure, know when is tapped • Fiber optic cable structure (from center): – Core (v. small, 5-50 microns, ~ the size of a single hair) – Cladding, which reflects the signal Protective outer jacket Types of Optical Fiber • Multimode (about 50 micron core) – Earliest fiber-optic systems – Signal spreads out over short distances (up to ~500m) – Inexpensive • Graded index multimode – Reduces the spreading problem by changing the refractive properties of the fiber to refocus the signal – Can be used over distances of up to about 1000 meters • Single mode (about 5 micron core) Page 14 of 31 – Transmits a single direct beam through the cable – Signal can be sent over many miles without spreading – Expensive (requires lasers; difficult to manufacture) Wireless Media  Radio o Wireless transmission of electrical waves through air o Each device has a radio transceiver with a specific frequency • Low power transmitters (few miles range) • Often attached to portables (Laptops, PDAs, cell phones) – Includes • AM and FM radios, Cellular phones • Wireless LANs (IEEE 802.11) and Bluetooth • Microwaves and Satellites, Low Earth Orbiting Satellites 2.) Infrared – “invisible” light waves with frequency below red light spectrum – Requires line of sight; generally subject to interference from heavy rain, smog, and fog – Used in remote control units such as for controlling the TV 3.) Microwave Radio • High frequency form of radio communications – Extremely short (micro) wavelength (1 cm to 1 m) – Requires line-of-sight • Performs same functions as cables – Often used for long distance, terrestrial transmissions (over 50 miles without repeaters) – No wiring and digging required – Requires large antennas (about 10 ft) and high towers • Possesses similar properties as light – Reflection, refraction, and focusing – Can be focused into narrow powerful beams for long distance – Some effect from water, rain and snow 4.) Satellite Communications – Special form of microwave communications – Signals travel at speed of light, yet long propagation delay – due to great distance between ground station and satellite Factors Used in Media Selection • Type of network - LAN, WAN, or Backbone • Cost - Always changing; depends on the distance • Transmission distance - Short: up to 300 m; medium: up to 500 m • Security - Wireless media is less secure • Error rates - Wireless media has the highest error rate (interference) • Transmission speeds - Constantly improving; Fiber has the highest Page 15 of 31 Digital Transmission of Digital Data • Computers produce binary data • Standards needed to ensure both sender and receiver understands this data – Codes: digital combinations of bits making up languages that computers use to represent letters, numbers, and symbols in a message – Signals: electrical or optical patterns that computers use to represent the coded bits (0 or 1) during transmission across media • ASCII: American Standard Code for Information Interchange – Originally used a 7-bit code (128 combinations), but an 8-bit version (256 combinations) is now in use – Found on PC computers • EBCDIC: Extended Binary Coded Decimal Interchange Code – An 8-bit code developed by IBM – Used mostly in mainframe computer environment Transmission Modes • Bits in a message can be sent on: – a single wire one after another (Serial transmission) – multiple wires simultaneously (Parallel transmission) • Serial Mode – Sends bit by bit over a single wire – Serial mode is slower than parallel mode – Can be used over longer distances since bits stay in the order they were sent • Parallel mode – Uses several wires, each wire sending one bit at the same time as the others • A parallel printer cable sends 8 bits together • Computer’s processor and motherboard also use parallel busses (8 bits, 16 bits, 32 bits) to move data around • Used for short distances (up to 6 meters) since bits sent in parallel mode tend to spread out over long distances Signaling of Bits • Digital Transmission – Signals sent as a series of “square waves” of either positive or negative voltage – Voltages vary between +3/-3 and +24/-24 depending on the circuit • Signaling (encoding) – Defines how the voltage levels will correspond to the bit values of 0 or 1 Page 16 of 31 – Examples: • Unipolar, Bipolar • RTZ, NRZ, Manchester – Data rate: describes how often the sender can transmit data • 64 Kbps  once every 1/64000 of a second Signaling (Encoding) Techniques • Unipolar signaling (Digital Transmission) – Use voltages either vary between 0 and a positive value or between 0 and some negative value • Bipolar signaling – Use both positive and negative voltages – Experiences fewer errors than unipolar signaling • Signals are more distinct (more difficult for interference to change polarity of a current) – Return to zero (RZ) • Signal returns to 0 voltage level after sending a bit – Non return to zero (NRZ) • Signals maintains its voltage at the end of a bit – Manchester encoding (used by Ethernet) • Used by Ethernet, most popular LAN technology • Defines a bit value by a mid-bit transition • A high to low voltage transition is a 0 and a low to high mid-bit transition defines a 1 • Data rates: 10 Mb/s, 100 Mb/s, 1 Gb/s • 10- Mb/s  one signal for every 1/10,000,000 of a second (10 million signals or bits every second) • Less susceptible to having errors go undetected • If there is no mid-bit voltage transition, then an error took place Analog Transmission of Digital Data • A well known example using phone lines to connect PCs to the Internet • PCs generate digital data • Local loop phone lines use analog transmission technology • Modems translate digital data into analog signals Telephone Network • Originally designed for human speech (analog communications) only • POTS (Plain Old Telephone Service) – Enables voice communications between two telephones –
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