Chapter 1: Introduction to Data Communications (Video Notes)

Learning Objectives

• Be aware of the three fundamental questions this book answers, including how the Internet operates, how to design networks, and how to manage and secure them.

• Be aware of the diverse applications of data communications networks across various industries and daily life.

• Be aware of how data communications fit within the discipline of Management Information Systems (MIS), highlighting its role in enabling business processes and information flow.

• Be familiar with the major components of and types of networks, from local to wide area.

• Understand the critical role of network layers in organizing communication functions and facilitating interoperability.

• Be familiar with the role of network standards in ensuring compatibility and fostering innovation.

• Be aware of evolving cyber security issues in an interconnected world.

• Be aware of three key trends in communications and network technologies that are shaping the future of connectivity.

1.1 Introduction

• High-speed data communications networks have revolutionized global connectivity, allowing people and information to interact in unprecedented ways using technologies like the Internet.

• The concept of "collapsing information lag" means that information can be accessed and communicated almost instantaneously, eliminating geographical barriers and enabling real-time collaboration and information sharing globally.

• Data communications and networking is a dynamic, global field, necessitating an understanding of varying political and regulatory environments, as legal and compliance requirements can differ significantly across regions.

1.2 Data Communications Networks

• Definition: Data communications refers to the process of moving digital computer information from one location to another through electrical signals (e.g., copper cables, radio waves) or optical signals (e.g., fiber optic cables). These pathways are commonly known as data communications networks.

• Three basic hardware components:

  • The server: A powerful computer that stores data, software, applications, or other resources and makes them accessible to network clients. Servers can specialize in various functions, such as file storage, web hosting, or email management.

  • The client: Typically an end-user device like a personal computer, smartphone, or tablet, which is the input-output hardware used by a user to access resources from a server over a communication circuit.

  • The circuit: The physical pathway or medium through which messages and data travel between network devices. This can include wired media (e.g., Ethernet cables, fiber optic cables) or wireless media (e.g., Wi-Fi, cellular networks).

• A network that operates without a dedicated central server, where all connected devices can act as both clients and servers to share resources directly with each other, is known as a peer-to-peer network.

1.2.1 Local Area Network (LAN) Example

• A Wireless Access Point (AP) effectively extends a wired network to allow wireless devices (e.g., laptops, smartphones) to connect to the network using radio frequencies.

• Client computers are typically connected to the network through physical switches using Ethernet cables (forming the circuit), allowing them to communicate with each other and with network servers.

• Routers are crucial devices that connect different networks together, such as connecting a local LAN to a larger Wide Area Network (WAN) like the Internet, by forwarding data packets between them.

• Servers within a LAN environment can include specialized devices such as a file server for centralized data storage, a Web server for hosting internal websites or applications, and a mail server for handling internal email communications.

1.2.2 LAN Terms

• LAN (Local Area Network): A network that connects computers and other devices within a relatively small, confined geographical area, such as an office building, school campus, or home. LANs typically offer high data transfer rates, often 100 ext{ Mbps} or faster.

• Wireless access point (AP): A networking device that allows Wi-Fi enabled devices to connect to a wired network using wireless communication standards (e.g., IEEE 802.11 ext{x}). It acts as a central hub for wireless clients.

• Router: A network device that forwards data packets between computer networks. It performs the traffic directing functions on the Internet by determining the best path for data to travel from its source to its destination.

1.2.3 Network Architecture Components (high-level view)

These components represent a hierarchical view of a typical large enterprise network:

• Enterprise Campus: The primary location of an organization, encompassing multiple buildings and departments.

• Building Access Layer (Local Area Network): Connects end-user devices within individual buildings to the network, providing network access.

• Distribution Layer: Aggregates traffic from multiple access layer devices and provides connectivity to higher-layer services, often including routing and policy enforcement.

• Building Backbone Network: High-speed network segment that connects distribution layer switches within a single building.

• Enterprise Edge: The boundary of the enterprise network, connecting the internal network to external networks like the Internet, partners, or remote sites.

• Core Layer Campus Backbone Network: The high-speed backbone that connects all buildings and various network segments within the entire campus, designed for high throughput and low latency.

• Wide Area Network (WAN) Access: Points where the enterprise network connects to an external WAN, often provided by a common carrier.

• Common Carrier: Telecommunications companies that provide public data transmission services, such as long-distance telephone services or Internet access.

• Data Center and Internet Access: Facilities housing shared computing resources and connections to the Internet, essential for hosting applications and data.

• Internet Service Provider (e.g., e-Commerce edge): Organizations that provide Internet access and services, often involving specialized infrastructure for high-volume transactions, like in e-commerce.

1.2.4 Server Types

• File server: A dedicated server that stores and manages data files and application software, making them accessible to multiple clients over the network. It handles requests for file retrieval, storage, and access control.

• Web server: A program that serves web content (HTML pages, images, videos, etc.) to client web browsers using HTTP (Hypertext Transfer Protocol). It processes requests from browsers and delivers the corresponding web resources.

• Mail server: A server specifically designed to handle and deliver email messages. It stores incoming emails for users, sends outgoing emails, and manages email accounts and related services (e.g., SMTP, IMAP, POP3 protocols).

1.2.5 Types of Networks

• Local Area Network (LAN): As defined earlier, a network covering a small geographical area, typically characterized by high data transfer rates, often 100 ext{ Mbps} (Megabits per second) or faster, used for connecting devices within a single location.

• Backbone Network (BN): A high-capacity, central network infrastructure that connects multiple LANs and other network segments within a larger campus or organization. It acts as a primary communication conduit, with typical speeds ranging from 100 ext{ Mbps} to 1000 ext{ Mbps} (1 Gigabit per second).

• Wide Area Network (WAN): A network that spans a large geographical distance, connecting LANs and BNs across cities, states, or even continents. Because of the vast distances and infrastructure costs, most organizations lease WAN services from common carriers rather than building their own.

1.2.6 Intranets and Extranets

• Intranet: An internal organizational network that utilizes Internet technologies (like Web browsers, TCP/IP, and HTTP) but is strictly for the exclusive use of internal employees. It functions like a private version of the Internet, often hosted on a separate Web server securely isolated from public Internet access, for sharing internal company information and applications.

• Extranet: An extension of an intranet that allows controlled access to selected internal databases or services for authorized external users, such as customers, suppliers, or partners, typically over the Internet. It provides a secure and managed way to collaborate and share information with trusted third parties while maintaining internal network security.

1.3 Network Models

• The fundamental purpose of any network is to efficiently and reliably transfer messages from a sender to a receiver. While networks can vary significantly in their underlying hardware and software, network models provide a structured way to organize these diverse functions through layers.

• Two important models that guide network design and understanding:

  • Open Systems Interconnection (OSI) model: A conceptual framework developed by the International Organization for Standardization (ISO).

  • Internet model: A more practical and widely adopted model that forms the basis of the Internet and most modern networks.

• The OSI model, released in 1984, is an important theoretical standard reference model, widely used for pedagogical purposes and for understanding general networking principles. However, it was not widely adopted commercially in North America due to early market dominance by other protocols; it found more practical implementation in some European networks.

• The Internet model is the current dominant model for network architecture. It is a simplified 5-layer structure (sometimes shown as 4 layers, merging Data Link and Physical for certain contexts) that, while not formally defined by a single, comprehensive standard body in the same way as OSI, is de facto standard due to its widespread implementation and success.

1.3.1 OSI Model (7 layers)

Each layer of the OSI model performs a specific set of functions and communicates with the layer directly above and below it. Data is encapsulated as it moves down the stack and decapsulated as it moves up.

• Layer 1: Physical Layer

  • Function: Responsible for the transmission and reception of raw unstructured data bits (0 ext{s} and 1 ext{s}) over a physical medium (e.g., copper wire, fiber optic cable, radio waves).

  • Details: Defines electrical, optical, mechanical, and functional specifications for activating, maintaining, and deactivating the physical link. Deals with voltage levels, physical data rates, cable types, connectors, and physical topology.

• Layer 2: Data Link Layer

  • Function: Manages the physical transmission circuit, ensuring a reliable, error-free link for the layers above it. It handles basic error detection and correction, and flow control.

  • Details: Divides data from the Network Layer into frames, adds source and destination physical (MAC) addresses, performs error checking using mechanisms like Cyclic Redundancy Check (CRC), and manages access to the physical medium (e.g., Carrier Sense Multiple Access/Collision Detection - CSMA/CD for Ethernet).

• Layer 3: Network Layer

  • Function: Performs routing of data packets across different networks and determines the logical paths for data to travel from the source to the destination host. Assigns logical addresses (e.g., IP addresses).

  • Details: Responsible for logical addressing (e.g., IP addresses), routing algorithms, and forwarding packets. It determines the best path for data, even if it has to traverse multiple network segments or routers.

• Layer 4: Transport Layer

  • Function: Provides end-to-end communication services between applications running on different hosts. It ensures reliable data transfer, manages segmentation and reassembly of data, and handles flow control at the segment level.

  • Details: Breaks upper-layer messages into smaller segments for transmission and reassembles them at the destination. It manages logical connections between applications (using port numbers), provides connection-oriented (TCP) or connectionless (UDP) services, and handles error recovery (retransmission) and flow control (buffering).

• Layer 5: Session Layer

  • Function: Establishes, manages, and terminates communication sessions between applications. It handles dialogue control and synchronization between communicating systems.

  • Details: Responsible for dialogue control (e.g., half-duplex, full-duplex), token management (preventing two parties from attempting the same critical operation simultaneously), and synchronization (inserting checkpoints in the data stream for recovery in case of failures).

• Layer 6: Presentation Layer

  • Function: Translates, encrypts, and compresses data for the Application Layer. It ensures that data is presented in a format that the receiving application can understand, regardless of the native format used by the sending application.

  • Details: Handles data representation (e.g., ASCII, EBCDIC), data encryption/decryption, and data compression/decompression to reduce the number of bits that need to be transmitted, thus improving efficiency and security.

• Layer 7: Application Layer

  • Function: Provides network services directly to end-user applications. It is the layer that interacts with software applications that receive and send data over the network.

  • Details: Offers various utilities and protocols for common services such as file transfer (FTP), email (SMTP, POP3, IMAP), remote login (Telnet, SSH), and web browsing (HTTP). It is the closest layer to the end-user.

1.3.2 The OSI vs Internet Model (groupings)

• OSI mapping (examples): The OSI model explicitly defines 7 distinct layers, offering a granular, theoretically complete framework.

  • Layer 7: Application Layer

  • Layer 6: Presentation Layer

  • Layer 5: Session Layer

  • Layer 4: Transport Layer

  • Layer 3: Network Layer

  • Layer 2: Data Link Layer

  • Layer 1: Physical Layer

• Internet Model (5 layers): The Internet model is a more pragmatic approach, combining some of the OSI layers for simplicity and practical implementation. Its layers are:

  • Layer 5: Application

  • Layer 4: Transport

  • Layer 3: Network

  • Layer 2: Data Link

  • Layer 1: Physical

• Common visual cues: The Internet model is often considered more tightly coupled because its layers were developed and evolved together with the Internet protocols, making it a more direct reflection of real-world implementations. The OSI model, while highly influential in teaching, is more cross-disciplinary and theoretical, aiming for a universal communication standard before widespread protocol development.

Summary of Layer Functions Comparison

1.3.4 The Internet Model – Five Layers (overview)

This model is foundational to how the Internet operates, with each layer performing essential services.

• Layer 1: Physical Layer

  • Function: Deals with the actual physical connection between network devices (sender and receiver). It encompasses the actual transmission of electrical or optical signals.

  • Details: Includes specifications for hardware devices (e.g., network interface cards, cables, connectors), transmission media attributes (e.g., copper wire, fiber optics, radio waves), voltage levels, data rates, and signaling methods used to transmit raw data bits.

• Layer 2: Data Link Layer

  • Function: Responsible for moving a message from one computer to the next along the direct path. It controls the Physical Layer and handles basic error detection and correction for transmisions over a single link.

  • Details: Frames messages to add structure for transmission, includes physical (MAC) addresses for local delivery, detects and often corrects errors that occur during physical transmission, and controls access to the shared network medium (e.g., Ethernet).

• Layer 3: Network Layer

  • Function: Primarily performs routing of data packets across potentially diverse networks to reach the destination. It is also responsible for logical addressing (IP addresses) that uniquely identify devices globally.

  • Details: Determines the most efficient path for packets to travel from source to destination, even if multiple hops through routers are required. It manages the Internet Protocol (IP) addressing scheme and enables communication between devices on different networks.

• Layer 4: Transport Layer

  • Function: Links application software to the network, providing end-to-end communication services. It establishes logical connections, breaks large messages into smaller packets for efficient transmission, and reassembles them at the destination.

  • Details: Manages two primary protocols: TCP (Transmission Control Protocol) for reliable, connection-oriented communication with error checking and retransmission, and UDP (User Datagram Protocol) for faster, connectionless communication where reliability is handled by the application or is less critical. It also uses port numbers to direct data to the correct application.

• Layer 5: Application Layer

  • Function: This is the top layer, directly interacting with the application software used by the network user. It defines the protocols that applications use to exchange data.

  • Details: Examples include HTTP for web browsing, SMTP and POP3/IMAP for email, FTP for file transfer, and DNS for converting domain names to IP addresses. It provides the interface through which users interact with network services.

1.3.5 Groups of Layers

• Hardware layer: In common practical discussions, the Data Link Layer and Physical Layer are often grouped together as the "hardware layer" because they are most closely tied to the physical network components, network interface cards (NICs), and the transmission medium itself.

• Internetwork layers: The Transport Layer and Network Layer are sometimes conceptually treated together as the "internetwork layer" because they are primarily responsible for ensuring end-to-end communication and routing across multiple, interconnected networks.

• Layer coupling: Decisions made and protocols implemented in one layer significantly influence and depend on the layers above and below it. For example, the maximum frame size supported by the Data Link Layer affects how the Transport Layer segments data. This interdependence highlights the need for careful design and consistent standards across the entire stack.

• Implication: While standards provide a rigid model, network practitioners often group layers for practical design, implementation, and troubleshooting. This pragmatic grouping allows for more efficient development while relying on underlying standards to ensure cross-layer compatibility and interoperability.

1.3.6 Message Transmission Using Layers

• Each computer participating in a network communication runs specialized software at every layer of the network model. Each layer operates according to a specific protocol, which is a formal set of rules that governs how data is formatted, transmitted, and received.

• As a message travels from an application (e.g., a web browser) down through the layers of the sending computer, each layer adds its own control information, known as a Protocol Data Unit (PDU), to the original data. This process is called encapsulation.

• PDUs per layer correspond to different terms (sometimes called packets at a general level):

  • Application Layer: When an application generates data, it might form an HTTP Request (for web browsing); the PDU at this layer is the application message itself.

  • Transport Layer: The Application Layer data is encapsulated within a TCP Segment (or UDP Datagram). TCP adds source/destination port numbers, sequence numbers, and checksums.

  • Network Layer: The TCP Segment is then encapsulated within an IP Packet. The Network Layer adds source/destination IP addresses and other routing information.

  • Data Link Layer: The IP Packet is encapsulated within an Ethernet Frame (or other data link layer frame type). This layer adds source/destination MAC addresses, error-checking codes (like Frame Check Sequence), and frame delimiters.

  • Physical Layer: Finally, the Ethernet Frame is converted into raw bits (electrical signals, light pulses) that are transmitted over the physical wire or wireless medium.

• Standard practice: This process of encapsulation means PDUs are progressively wrapped within headers and trailers of lower layers as they move down the protocol stack on the sending computer. At the destination, the reverse process, decapsulation, occurs, where each layer removes its corresponding PDU header/trailer and passes the data up to the next higher layer until the original application message is delivered.

• Visualization concept: This layered encapsulation and decapsulation can be effectively envisioned using the Russian nesting dolls analogy, where each doll (layer's PDU) contains the smaller doll (next layer's PDU).

1.3.7 Concepts: Application/Transport/Network/Data Link/Physical (example flow)

Let's trace the path of a simple web request (e.g., accessing a webpage):

• Application Layer: When a user types a URL (e.g., www.example.com) into a Web browser, the browser (the application) translates this request into a standardized HTTP Request message. This message specifies what resource is being requested from the web server.

• Transport Layer: The HTTP Request is passed down to the Transport Layer. Here, the TCP protocol breaks potentially large HTTP messages into smaller, manageable TCP segments. It adds a TCP header containing source and destination port numbers (e.g., port 80 for HTTP), sequence numbers for reassembly, and flags for connection management. TCP opens a logical connection to the server for reliable delivery.

• Network Layer: Each TCP segment is then passed to the Network Layer. The IP protocol encapsulates each TCP segment into an IP packet. An IP header is added, containing the source and destination IP addresses. The Network Layer's role is to select the most suitable "next hop" router to forward the packet closer to its final destination.

• Data Link Layer: The IP packet is handed down to the Data Link Layer, which prepares it for transmission over the local physical link. For an Ethernet network, an Ethernet frame is created, surrounding the IP packet. An Ethernet header includes the source and destination physical (MAC) addresses of the devices on the local segment (e.g., your computer's MAC and the router's MAC). An Ethernet trailer containing an error-checking code (Frame Check Sequence, or FCS) is also added.

• Physical Layer: Finally, the complete Ethernet frame is passed to the Physical Layer. This layer converts the digital data within the frame into electrical signals (over copper cable) or light pulses (over fiber optic cable) based on the medium being used. These signals are then transmitted bit by bit across the network cable to the next device, such as a switch or router.

1.4 Network Standards

• Purpose of standards: Network standards are crucial for ensuring that hardware and software components from different vendors can interoperate seamlessly. Without standards, network devices would be incompatible, leading to vendor lock-in and stifling innovation. Standards enable modular software and hardware development, allowing manufacturers to focus on specific components (e.g., developing a physical layer device) without needing to understand the entire network stack, ultimately promoting competition and reducing costs.

• Standards types:

  • De jure standards: These are official, formal standards developed by recognized international or national standards-making bodies. They often undergo a rigorous, multi-stage standardization process and are legally mandated or strongly recommended (e.g., IEEE 802.3 for Ethernet).

  • De facto standards: These are standards that emerge in the market through widespread adoption, intense competition, and broad vendor support, rather than being formally sanctioned. They become standard due to their popularity and utility (e.g., Microsoft Windows operating system, or PDF document format).

1.4.1 Standard-Making Organizations

These organizations play a vital role in defining the rules that govern global data communications.

• International Organization for Standardization (ISO):

  • Role: A non-governmental organization that develops and publishes international standards across a vast range of fields, including information and communication technologies. Its membership consists of national standards bodies from 170 countries, driving global standardization efforts.

• International Telecommunication Union (ITU) – Telecommunications Group (ITU-T):

  • Role: A specialized agency of the United Nations responsible for matters concerning information and communication technologies. ITU-T sets technical standards that define how global telecommunication networks operate, ensuring international connectivity and interoperability for public telecommunication networks.

• American National Standards Institute (ANSI):

  • Role: A private, non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. It acts as the coordinator for the U.S. national standards system, accrediting standards developers.

• Institute of Electrical and Electronics Engineers (IEEE) – Standards Association (IEEE-SA):

  • Role: A professional organization for electrical and electronics engineering. The IEEE-SA is a leading developer of globally recognized industry standards in a broad range of technologies, notably including many foundational networking standards like the IEEE 802 series for Local Area Networks (e.g., WiFi and Ethernet).

• Internet Engineering Task Force (IETF):

  • Role: A large, open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. It develops and promotes Internet standards (Request for Comments - RFCs) that govern much of how the Internet operates, such as TCP/IP, HTTP, and SMTP.

1.4.2 Some Common Data Communications Standards (by layer)

These are examples of widely adopted standards at different layers of the Internet model:

• Layer 5 – Application Layer:

  • HTTP (Hypertext Transfer Protocol), HTML (Hypertext Markup Language): Fundamental protocols/languages for the World Wide Web.

  • MPEG (Moving Picture Experts Group), H.323: Standards for encoding, compressing, and transmitting audio and video over networks.

  • SMTP (Simple Mail Transfer Protocol), IMAP (Internet Message Access Protocol), POP (Post Office Protocol): Essential protocols for sending, receiving, and managing email.

• Layer 4 – Transport Layer:

  • TCP (Transmission Control Protocol): The primary reliable, connection-oriented transport protocol used extensively across the Internet and LANs.

• Layer 3 – Network Layer:

  • IP (Internet Protocol): The foundational protocol for addressing and routing data across the Internet.

• Layer 2 – Data Link Layer:

  • Ethernet: The most prevalent wired LAN technology standard (IEEE 802.3).

  • Frame Relay: A legacy high-performance WAN protocol.

  • T1: A digital carrier system used for transmitting data at 1.544 ext{ Mbps}, often used as a WAN circuit.

• Layer 1 – Physical Layer:

  • RS-232C: A legacy standard for serial communication interfaces, often used for connecting modems or console ports.

  • Category 5 cable (Cat 5/5e/6): A type of twisted-pair cable for Ethernet networks, defining physical characteristics and performance.

  • V.92 modem: A standard for dial-up modems that allows maximum download speeds of 56 ext{ Kbps} over traditional telephone lines.

1.5 Future Trends

• Wireless LAN and BYOD (Bring Your Own Device):

  • The proliferation of wireless networks and personal mobile devices (smartphones, tablets) means that employees increasingly use their own devices for work. Employers face significant challenges in managing secure access to corporate applications and sensitive data while maintaining network performance and user experience.

• Internet of Things (IoT) and Network of Things (NoT):

  • This trend describes the evolution of the Internet from primarily connecting computers and people to connecting a vast array of physical objects that are embedded with sensors, software, and other technologies. These devices can collect and exchange data over a network, leading to pervasive connectivity, unprecedented data generation, and new applications in smart homes, cities, industries, and healthcare.

• Massively Online Experiences:

  • The increasing demand for highly interactive and immersive online experiences, such as massively multiplayer online games (MMOs), virtual reality (VR), augmented reality (AR) applications, and high-quality online education platforms. These applications place immense demands on network bandwidth, latency, and reliability, driving innovations in network infrastructure to support real-time, high-fidelity interaction for large numbers of users.

1.6 Implications for Cyber Security

• The widespread adoption of networking and the Internet has fundamentally altered almost every aspect of business and society, creating both immense opportunities and significant security challenges.

• As a wide variety of devices, from traditional computers to IoT sensors and personal mobile devices, connect to networks, the attack surface expands dramatically, leading to diverse security risks, including data breaches, denial-of-service attacks, and privacy violations.

• The growing demand for network services and capacity, coupled with the increasing value of data, makes secure storage, secure servers, and secure data transfer increasingly critical. Organizations must implement robust cyber security measures, including encryption, access controls, intrusion detection systems, and regular security audits, to protect sensitive information and maintain operational integrity in this interconnected world.

Notes on Key Concepts and Connections

• The OSI model serves as a comprehensive theoretical reference framework for understanding network communication functions, while the Internet model is the dominant practical implementation that underpins the global Internet.

• Encapsulation (PDU layering) is a fundamental concept that enables modular design and interoperability among different network components. However, this layering also introduces a certain degree of overhead due to the addition of multiple headers and trailers at each layer, slightly reducing the efficiency of raw data transmission.

• Standards are paramount because they enable vendor interoperability, allowing hardware and software from different manufacturers to work together. This also facilitates layered development, where different teams can focus on specific layers or components, fostering innovation and preventing vendor lock-in. The distinction between de jure (formal) and de facto (market-driven) standards influences their adoption and compliance.

• Real-world implications include significant challenges such as managing BYOD security (securing corporate data on personal devices), mitigating IoT risk management (addressing vulnerabilities in a vast and diverse landscape of connected devices), and the ongoing need for scalable cyber security controls as networks continue to expand globally and integrate new technologies.

• These foundational ideas are deeply tied to Management Information Systems (MIS). Networks are indispensable for data storage (e.g., cloud storage), data analysis and visualization (accessing remote databases), enabling operational automation (e.g., smart factories), and critically, for data protection. These capabilities are core IT functions that modern networks enable and enhance.

• The three fundamental questions that guide this book map directly to corresponding chapters: (1) Internet operation (covered in Chapters 1-5), (2) network design (explored in Chapters 6-10), and (3) network management and security (detailed in Chapters 11-12).

Formulas and numbers referenced in this section

• LAN speed examples: 100\text{ Mbps}

• BN/backbone speeds: typically 100\text{ Mbps}-1000\text{ Mbps}

• T1 line speed: 1.544\text{ Mbps}

• Modem reference: V.92 at 56\text{ Kbps}