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Data & Computer Communications MSCEG 425 Lecture 1 Introduction Network Models Fall 2007 Dr. L. Christofi Fall 2007 1 0. Overview In this lecture we will cover the following topics: 1. Introduction 1.1 1.2 1.3 1.4 1.5 Introduction to data communications and networks Networks The Internet Protocols & Standards Summary (part 1) 2. Network Models 2.1 2.2 2.3 2.4 2.5 2.6 Dr. L. Christofi 1.# Layered tasks OSI model Layers in the OSI model TCP/IP protocol suite Addressing Summary (part 2) Fall 2007 2 1.0 INTRODUCTION The term telecommunication means communication at a distance. The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data. Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable. Topics discussed in this section: Components Data Representation Data Flow Dr. L. Christofi Fall 2007 3 Components of data communication A data communication system has 5 components: - Message: info (data) to be communicated Sender: device that sends the data message Receiver: device that receives the message Medium: transmission medium (physical path) by which a message travels from the transmitter to the receiver - Protocol: set of rules that governs data communications Dr. L. Christofi 1.# Fall 2007 4 Data representation • • • • • • Text: bit pattern ASCII: 128 different symbols (7 bits) Extended ASCII: size of each pattern is 1 byte (8 bits) Unicode: 65.536 symbols (16 bits) ISO: 4.294.967.296 symbols (32 bits) Numbers: decimal numbers converted directly to binary • Images: divided into a matrix of pixels • Audio: representation of sound by an analog or a digital signal • Video: represented by an analog or digital signal Dr. L. Christofi Fall 2007 5 Direction of data flow Dr. L. Christofi 1.# Fall 2007 6 1.2 NETWORKS A network is a set of devices (often referred to as nodes) nodes) connected by communication links. links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network. Topics discussed in this section: Network Criteria Physical Structures Categories of Networks Dr. L. Christofi Fall 2007 7 Network criteria • Performance —Can be measured in may ways • transit time: amount of time required for a message to travel from one device to another • response time: time elapsed between an inquiry and a response • Number of users • Type of transmission medium • Hardware capabilities and software efficiency • Reliability —A measure of frequency of failure and the time needed to recover, network robustness • Security —Protecting of data from unauthorized users Dr. L. Christofi 1.# Fall 2007 8 Physical Structures Types of connections: point-to-point and multipoint Dr. L. Christofi Fall 2007 9 Physical Topology Physical topology refers to the way in which a network is laid out physically. Network topology is the geometric representation of the relationship of all the links and linking devices (nodes) Dr. L. Christofi 1.# Fall 2007 10 Topology categories Star Mesh Number of links = n*(n-1)/2 Number of links = n Ring Bus Number of links = n+1 Dr. L. Christofi Number of links = n Fall 2007 11 A hybrid topology A star backbone with three bus networks Dr. L. Christofi 1.# Fall 2007 12 Categories of networks • Local Area Networks (LANs) — Usually privately owned and links the devices in a single office, building or campus — Most common LAN topologies are bus, ring and star — Speeds of 100Mbps-1Gbps • Metropolitan Area Networks (MANs) — Designed to extend over a city — May be owned by a large private organization or a service provider (telephone company) • Wide Area Networks (WANs) — Provide long-distance transmission of data, voice, image and video info over large geographical areas that may comprise a country or a continent • Internetworks — When two or more networks are connected they become an internetwork or internet Dr. L. Christofi 13 Fall 2007 WAN examples A switched WAN and a point-to-point WAN Dr. L. Christofi 1.# Fall 2007 An heterogenous network made of 4 WANs and 2 LANs 14 An heterogeneous network made of four WANs and two LANs Dr. L. Christofi 15 Fall 2007 Comparison of LANs, MANs, & WANs LANs: 1 – 1000 Mbps MANs: 10 – 40 Gbps WANs: Tbps Dr. L. Christofi 1.# Fall 2007 16 1.3 THE INTERNET The Internet has revolutionized many aspects of our daily lives. It has affected the way we do business as well as the way we spend our leisure time. The Internet is a communication system that has brought a wealth of information to our fingertips and organized it for our use. Topics discussed in this section: A Brief History The Internet Today (ISPs) Dr. L. Christofi Fall 2007 17 A brief history • Mid-1960s — Standalone devices — ARPA (Advanced Research Projects Agency) was interested in finding a way to connect computers to share information — Backbones: None - Hosts: None • 1967 — ARPA presented its ideas for ARPANET — Backbones: None - Hosts: None • 1969 — The first physical network was constructed — Backbones: 50Kbps ARPANET - Hosts: 4 • 1972 — The first e-mail program was created by Ray Tomlinson of BBN — Backbones: 50Kbps ARPANET - Hosts: 23 • 1973 — Development began on the protocol later to be called TCP/IP (by Vint Cerf and Bob Kahn) — Backbones: 50Kbps ARPANET - Hosts: >23 Dr. L. Christofi 1.# Fall 2007 18 Hierarchical organization of Internet Dr. L. Christofi Fall 2007 19 The Internet today (ISPs) Dr. L. Christofi 1.# Fall 2007 20 1.4 PROTOCOLS AND STANDARDS In this section, we define two widely used terms: protocols and standards. standards. First, we define protocol, which is synonymous with rule. Then we discuss standards, which are agreedagreed-upon rules. Topics discussed in this section: Protocols Standards Standards Organizations Internet Standards Dr. L. Christofi Fall 2007 21 Protocols • A protocol is a set of rules that governs data communications • It defines what is communicated, how it is communicated and when it is communicated • Key elements of a protocol: —Syntax • Structure or format of data, meaning the order in which they are presented —Semantics • Refer to the meaning of each section of bits, how a particular pattern is interpreted and what action to be taken —Timing • Refers to when data should be sent and how fast can they be sent Dr. L. Christofi 1.# Fall 2007 22 Standards • Standards are essential in creating and maintaining an open and competitive market for equipment manufacturers • Required to guarantee national and international interoperability of data and telecommunications technology and processes • Categories of data communications standards — De facto: • Standards that have not been approved by an organizational body but have been adopted through widespread use, eg. model TCP/IP) — De jure: • Those that have been legislated by an official recognized body, eg. OSI model Dr. L. Christofi Fall 2007 23 Standards organizations • Standards creation committees — ISO (International Organization for Standardization) — ITU-T (International Telecommunications Union – Telecommunications Standards) • Initially known as CCITT (Consultative Committee for International Telegraphy and Telephony) — ANSI (American National Standards Institute) — IEEE (Institute of Electrical and Electronics Engineers) — EIA (Electronic Industries Association • Forums — Made up of representatives from interested corporations to speed acceptance and use of new technologies in the telecom industry • Regulatory Agencies — Governmental agencies: to protect public interest by regulating radio, TV and wire/cable communications Dr. L. Christofi 1.# Fall 2007 24 Internet standards • An Internet standard is a thoroughly tested specification used by those who work with the Internet • A specification begins with an Internet draft —Working document with no official status and a 6month lifetime —Upon recommendation from the Internet authorities a draft may be published as a Request for Comment (RFC) Dr. L. Christofi Fall 2007 25 1.5 SUMMARY (part 1) • • • • • • • • • • • • • • • • • • • • • Data communication is the transfer of data from one device to another via some form of transmission medium. A data communications system must transmit data to the correct destination in an accurate and timely manner. The five components that make up a data communications system are the message, sender, receiver, medium, and protocol. Text, numbers, images, audio, and video are different forms of information. Data flow between two devices can occur in one of three ways: simplex, half-duplex, or full-duplex. A network is a set of communication devices connected by media links. In a point-to-point connection, two and only two devices are connected by a dedicated link. In a multipoint connection, three or more devices share a link. Topology refers to the physical or logical arrangement of a network. Devices may be arranged in a mesh, star, bus, or ring topology. A network can be categorized as a local area network (LAN), a metropolitan-area network (MAN), or a wide area network (WAN). A LAN is a data communication system within a building, plant, or campus, or between nearby buildings. A MAN is a data communication system covering an area the size of a town or city. A WAN is a data communication system spanning states, countries, or the whole world. An internet is a network of networks. The Internet is a collection of many separate networks. TCP/IP is the protocol suite for the Internet. There are local, regional, national, and international Internet service providers (ISPs). A protocol is a set of rules that governs data communication; the key elements of a protocol are syntax, semantics, and timing. Standards are necessary to ensure that products from different manufacturers can work together as expected. The ISO, ITU-T, ANSI, IEEE, and EIA are some of the organizations involved in standards creation. Forums are special-interest groups that quickly evaluate and standardize new technologies. A Request for Comment (RFC) is an idea or concept that is a precursor to an Internet standard. Dr. L. Christofi 1.# Fall 2007 26 2.0 NETWORK MODELS • A network uses a combination of hardware and software to send data from one location to another —Hardware consists of the physical equioment that carries signals from one point of the network to another —The task of sending a piece of information from one point in the worls to another can be broken into several tasks, each performed by a separate software package • Each piece of software uses the services of another software package o do its job • At the lowest layer, a signal is sent from the source to the destination computer • In this part we will see a general idea of layers of a network and discus the functions of each one Dr. L. Christofi Fall 2007 27 2.1 LAYERED TASKS We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office. Topics discussed in this section: Sender, Receiver, and Carrier Hierarchy Dr. L. Christofi 1.# Fall 2007 28 Tasks involved in sending a letter • The task of transporting the letter between the sender and the receiver is done by the carrier. • The tasks must be done in the order given in the hierarchy. carrier Dr. L. Christofi Fall 2007 29 2.2 THE OSI MODEL Established in 1947, the International Standards Organization (ISO (ISO)) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) OSI) model. It was first introduced in the late 1970s. Topics discussed in this section: Layered Architecture Peer-to-Peer Processes Encapsulation Dr. L. Christofi 1.# Fall 2007 30 Seven layers of the OSI model Dr. L. Christofi Fall 2007 31 PeerPeer-toto-peer processes Dr. L. Christofi 1.# Fall 2007 32 Encapsulation • The process starts at layer 7 (application layer), then moves from layer to layer in descending, sequential order. • At each layer, a header is added to the data unit. • At layer 2, a trailer is added as well. • When the formatted data unit passes through the physical layer (layer 1) it is changed into an electromagnetic or optical signal and transported along a physical link • At the destination the reverse process is performed Dr. L. Christofi Fall 2007 33 2.3 LAYERS IN THE OSI MODEL In this section we briefly describe the functions of each layer in the OSI model. Topics discussed in this section: Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer Dr. L. Christofi 1.# Fall 2007 34 Physical layer Note The physical layer is responsible for movements of individual bits from one hop (node) to the next. Dr. L. Christofi Fall 2007 35 Data link layer Note The data link layer is responsible for moving frames from one hop (node) to the next. Dr. L. Christofi 1.# Fall 2007 36 HopHop-toto-hop delivery Dr. L. Christofi Fall 2007 37 Network layer Note The network layer is responsible for the delivery of individual packets from the source host to the destination host. Dr. L. Christofi 1.# Fall 2007 38 SourceSource-toto-destination delivery Dr. L. Christofi Fall 2007 39 Transport layer Note The transport layer is responsible for the delivery of a message from one process to another. Dr. L. Christofi 1.# Fall 2007 40 Reliable processprocess-toto-process delivery Dr. L. Christofi Fall 2007 41 Session layer Note The session layer is responsible for dialog control and synchronization. Dr. L. Christofi 1.# Fall 2007 42 Presentation layer Note The presentation layer is responsible for translation, compression, and encryption. Dr. L. Christofi Fall 2007 43 Application layer Note The application layer is responsible for providing services to the user. Dr. L. Christofi 1.# Fall 2007 44 Summary of layers Dr. L. Christofi Fall 2007 45 2.4 TCP/IP PROTOCOL SUITE The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: hosthost-toto-network, network, internet, internet, transport, transport, and application. application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, physical, data link, link, network, network, transport, transport, and application. application. Topics discussed in this section: Physical and Data Link Layer Network Layer Transport Layer Application Layer Dr. L. Christofi 1.# Fall 2007 46 TCP/IP layers Dr. L. Christofi 47 Fall 2007 TCP/IP and OSI model OSI Dr. L. Christofi 1.# TCP/IP model Fall 2007 48 2.5 ADDRESSING Four levels of addresses are used in an internet employing the TCP/IP protocols: physical, physical, logical, logical, port, port, and specific. specific. Topics discussed in this section: Physical Addresses Logical Addresses Port Addresses Specific Addresses Dr. L. Christofi Fall 2007 49 Addresses in TCP/IP Dr. L. Christofi 1.# Fall 2007 50 Physical & Logical address • Physical address — In computer networks a physical address means a MAC (Medium Access Control) address • Also known as Ethernet Hardware Address (EHA) or hardware address or adapter address — It is a number that acts like a name for a particular network adapter, eg. the network cards • Logical address — In computer networks, a logical address refers to a network layer address such as an IP address — An IP address (Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP) Dr. L. Christofi Fall 2007 51 Port & specific address • Port address — TCP and UDP are transport protocols used for communication between computers via ports — The port numbers are divided into three ranges. • The Well Known Ports are those in the range 0–1023. • The Registered Ports are those in the range 1024–49151. • The Dynamic and/or Private Ports are those in the range 49152–65535. These ports are not used by any defined application. • Specific address — This address is used by application processes Dr. L. Christofi 1.# Fall 2007 52 Relationship of layerslayers-addresses in TCP/IP Dr. L. Christofi Fall 2007 53 Example: Physical address In the following figure, a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver. Dr. L. Christofi 1.# Fall 2007 54 Example: Physical address As we will see in a later lecture, most LANs use a 48-bit (6byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below: 07:01:02:01:2C:4B A 6-byte (12 hexadecimal digits) physical address. Dr. L. Christofi Fall 2007 55 Example: IP addresses This figure shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection. Dr. L. Christofi 1.# Fall 2007 56 Example: Port addresses This figure shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination. Dr. L. Christofi Fall 2007 57 Note The physical addresses change from hop to hop, but the logical and port addresses usually remain the same. Dr. L. Christofi 1.# Fall 2007 58 2.6 SUMMARY (part 2) • The five-layer model provides guidelines for the development of universally compatible networking protocols. • The physical, data link, and network layers are the network support layers. • The application layer is the user support layer. • The transport layer links the network support layers and the user support layer. • The physical layer coordinates the functions required to transmit a bit stream over a physical medium. • The data link layer is responsible for delivering data units from one station to the next without errors. • The network layer is responsible for the source-to-destination delivery of a packet across multiple network links. • The transport layer is responsible for the process-to-process delivery of the entire message. • The application layer enables the users to access the network. Dr. L. Christofi Fall 2007 59 References • B.A. Forouzan, Data Communications and Networking, 4th edition, McGraw-Hill, 2007 • W. Stalling, Local and Metropolitan Area Networks, 6th edition, Prentice Hall, 2000 • W. Stallings, Data and Computer Communications, 7th edition, Prentice Hall, 2004 • F. Halsall, Data Communications, Computer Networks and Open Systems, 4th edition, Addison Wesley, 1995 Dr. L. Christofi 1.# Fall 2007 60