Data And Computer Communications

1. Data Communication Fundamentals: Signals, Media, and Encoding

The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point.

Signals and Transmission. Data communication relies on electromagnetic signals, which can be analog (continuous) or digital (discrete). These signals are transmitted through various media, including guided (twisted pair, coaxial cable, optical fiber) and unguided (wireless) options. The characteristics of the signal, such as frequency, amplitude, and phase, and the properties of the medium, such as bandwidth and attenuation, determine the quality and efficiency of data transmission.

Encoding Techniques. To transmit information, data must be encoded into a signal suitable for the chosen medium. Digital data can be represented by digital signals (e.g., NRZ, Manchester) or analog signals (e.g., ASK, FSK, PSK). Similarly, analog data can be represented by analog signals (e.g., AM, FM, PM) or digital signals (e.g., PCM). The choice of encoding technique affects the signal's bandwidth, synchronization, and error detection capabilities.

Transmission Impairments. Signals are subject to impairments during transmission, including attenuation (signal loss), distortion (alteration of signal shape), and noise (unwanted signals). These impairments can degrade signal quality and introduce errors. Understanding these impairments is crucial for designing effective communication systems.

2. Networking Basics: LANs, WANs, and the Internet

Effective and efficient data communication and networking facilities are vital to any enterprise.

Local Area Networks (LANs). LANs connect devices within a limited area, such as a building or campus. They are characterized by high data rates, low error rates, and shared media. Common LAN topologies include bus, ring, star, and tree. LANs are essential for connecting personal computers, workstations, and servers within an organization.

Wide Area Networks (WANs). WANs cover larger geographical areas and typically involve the use of public or leased communication lines. WANs are used to connect geographically dispersed LANs and to provide access to the Internet. Common WAN technologies include circuit switching, packet switching, frame relay, and ATM.

The Internet. The Internet is a global network of interconnected networks. It uses the TCP/IP protocol suite to enable communication between diverse systems. The Internet has become a dominant force in both business and personal life, providing access to information, services, and applications.

3. Protocol Architectures: TCP/IP and OSI Models

To destroy communication completely, there must be no rules in common between transmitter and receiver—neither of alphabet nor of syntax.

Layered Approach. A protocol architecture organizes communication functions into a series of layers, each performing a specific set of tasks. This modular approach simplifies design, implementation, and maintenance. Each layer relies on the services of the layer below it and provides services to the layer above it.

TCP/IP Model. The TCP/IP protocol suite is the foundation of the Internet. It consists of five layers: physical, network access, internet, transport, and application. The internet layer uses IP to provide routing across multiple networks, and the transport layer uses TCP or UDP to provide reliable or unreliable end-to-end data transfer.

OSI Model. The Open Systems Interconnection (OSI) model is a seven-layer reference model for network protocols. It includes the physical, data link, network, transport, session, presentation, and application layers. Although the OSI model is not widely implemented, it provides a useful framework for understanding network functions.

4. Data Link Control: Ensuring Reliable Transmission

The receiver must be able to determine when a signal begins to arrive and when it ends.

Synchronization. Data link control protocols manage the exchange of data between two directly connected devices. A key function is synchronization, which ensures that the receiver can accurately interpret the incoming bit stream. Asynchronous transmission uses start and stop bits for each character, while synchronous transmission uses a clock signal and framing.

Error Control. Data link control protocols also provide error control mechanisms, including error detection and correction. Error detection techniques, such as parity checks and cyclic redundancy checks (CRCs), are used to identify errors. Error correction techniques, such as forward error correction (FEC), can correct some errors without retransmission.

Flow Control. Flow control mechanisms prevent a sender from overwhelming a receiver with data. Stop-and-wait flow control requires an acknowledgment for each frame, while sliding-window flow control allows multiple frames to be in transit simultaneously.

5. Multiplexing and Spread Spectrum: Efficient Use of Resources

Transmission services remain the most costly component of a communications budget for most businesses.

Multiplexing. Multiplexing allows multiple data streams to share a single transmission medium. Frequency division multiplexing (FDM) divides the available bandwidth into multiple frequency bands, each carrying a separate signal. Time division multiplexing (TDM) divides the transmission time into slots, each allocated to a different source. Statistical TDM dynamically allocates time slots based on demand.

Spread Spectrum. Spread spectrum techniques increase the bandwidth of a signal to improve its resistance to interference and jamming. Frequency-hopping spread spectrum (FHSS) rapidly switches the signal across multiple frequencies. Direct sequence spread spectrum (DSSS) uses a spreading code to increase the signal's bandwidth.

Code Division Multiple Access (CDMA). CDMA is a multiple access technique that uses spread spectrum to allow multiple users to share the same bandwidth simultaneously. Each user is assigned a unique code, which allows the receiver to separate the signals from different users.

6. Switching Technologies: Circuit, Packet, and ATM

The Internet, the Web, and associated applications have emerged as dominant features of both the business and personal world.

Circuit Switching. Circuit switching establishes a dedicated path between two stations for the duration of a communication. It is commonly used in telephone networks. Circuit switching is inefficient for bursty data traffic because resources are dedicated even when no data are being transmitted.

Packet Switching. Packet switching transmits data in small blocks called packets. Each packet contains addressing information and is routed independently through the network. Packet switching is more efficient than circuit switching for bursty data traffic. Datagram packet switching treats each packet independently, while virtual circuit packet switching establishes a preplanned route for all packets.

Asynchronous Transfer Mode (ATM). ATM is a high-speed packet-switching technology that uses fixed-size packets called cells. ATM is designed to support a wide range of traffic types, including voice, video, and data. ATM provides quality of service (QoS) guarantees for different types of traffic.

7. Routing and Congestion Control: Navigating the Network

The fundamental purpose of a communications system is the exchange of data between two parties.

Routing. Routing algorithms determine the path that packets take through a network. Routing algorithms can be classified based on performance criteria (e.g., minimum hop, least cost), information source (local, adjacent, all nodes), and decision time (packet, session). Common routing algorithms include Dijkstra's algorithm and the Bellman-Ford algorithm.

Congestion Control. Congestion occurs when the network is overloaded with traffic. Congestion control mechanisms are used to prevent or alleviate congestion. Techniques include backpressure, choke packets, implicit congestion signaling (e.g., TCP), and explicit congestion signaling (e.g., FECN, BECN).

Traffic Management. Traffic management techniques are used to provide different levels of service to different types of traffic. These techniques include traffic policing, traffic shaping, and quality of service (QoS) mechanisms.

8. Wireless Networks: Cellular and LAN Technologies

There has been a trend toward ever-increasing mobility for decades, liberating workers from the confines of the physical enterprise.

Cellular Networks. Cellular networks divide a geographical area into cells, each served by a base station. Frequency reuse allows the same frequencies to be used in non-adjacent cells. Cellular networks use a variety of techniques to manage mobility, handoffs, and interference.

Wireless LANs. Wireless LANs use radio waves or infrared light to transmit data over short distances. Common wireless LAN technologies include infrared, spread spectrum, and narrowband microwave. Wireless LANs are used to provide mobility, flexibility, and ease of installation.

IEEE 802.11. The IEEE 802.11 standard defines a set of services and physical layer options for wireless LANs. The standard includes specifications for medium access control (MAC), authentication, key management, and data transfer privacy.

9. Internet Protocols: IP, IPv6, and ICMP

The Internet evolved from the ARPANET, which was developed in 1969 by the Advanced Research Projects Agency (ARPA) of the U.S. Department of Defense.

Internet Protocol (IP). IP is the foundation of the Internet protocol suite. It provides a connectionless, best-effort service for transmitting data packets across networks. IP includes addressing, routing, and fragmentation functions.

IPv6. IPv6 is the next-generation version of IP. It uses 128-bit addresses, provides improved option mechanisms, and supports address autoconfiguration. IPv6 is designed to address the limitations of IPv4 and to support the growing demands of the Internet.

Internet Control Message Protocol (ICMP). ICMP is an integral part of IP. It provides a means for routers and hosts to exchange control and error messages. ICMP is used for network management, troubleshooting, and congestion control.

10. Transport Protocols: TCP and UDP

Regardless of the nature of the applications that are exchanging data, there is usually a requirement that data be exchanged reliably.

Transmission Control Protocol (TCP). TCP is a connection-oriented, reliable transport protocol. It provides flow control, error control, and ordered delivery of data. TCP is used by many applications that require reliable data transfer, such as file transfer, electronic mail, and Web access.

User Datagram Protocol (UDP). UDP is a connectionless, unreliable transport protocol. It provides a simple mechanism for sending data without the overhead of connection management and error control. UDP is used by applications that do not require reliable data transfer, such as streaming audio and video.

TCP Congestion Control. TCP uses a variety of techniques to manage congestion in the Internet, including slow start, congestion avoidance, fast retransmit, and fast recovery. These techniques allow TCP to adapt to changing network conditions and to avoid overwhelming the network with traffic.

11. Network Security: Protecting Data and Systems

The sender of data may wish to be assured that only the intended receiver actually receives the data.

Confidentiality. Confidentiality is achieved through encryption, which transforms data into an unreadable format. Symmetric encryption uses a single key for both encryption and decryption, while public-key encryption uses two keys, one for encryption and one for decryption.

Message Authentication. Message authentication ensures that a message has not been altered and that it came from the alleged source. Message authentication codes (MACs) and hash functions are used to generate authentication tags that are appended to messages.

Secure Sockets Layer (SSL) and Transport Layer Security (TLS). SSL and TLS are protocols that provide secure communication over TCP. They use encryption and authentication to protect data transmitted between Web browsers and Web servers.

IP Security (IPSec). IPSec is a set of protocols that provides security at the IP layer. IPSec can be used to create virtual private networks (VPNs) and to secure communication between hosts and routers.

12. Internet Applications: E-mail, Web, and Multimedia

The Internet, the Web, and associated applications have emerged as dominant features of both the business and personal world.

Electronic Mail (SMTP and MIME). The Simple Mail Transfer Protocol (SMTP) is used to transfer e-mail messages between hosts. The Multipurpose Internet Mail Extensions (MIME) standard extends SMTP to support multimedia content, such as images, audio, and video.

World Wide Web (HTTP). The Hypertext Transfer Protocol (HTTP) is the foundation of the World Wide Web. It is used to transfer Web pages and other resources between Web browsers and Web servers. HTTP is a stateless, request/response protocol.

Internet Directory Service (DNS). The Domain Name System (DNS) is a directory lookup service that maps domain names to IP addresses. DNS is a distributed, hierarchical database that is essential for the functioning of the Internet.

Multimedia (SIP and RTP). The Session Initiation Protocol (SIP) is used to establish, modify, and terminate multimedia sessions over IP networks. The Real-Time Transport Protocol (RTP) is used to transmit real-time data, such as audio and video, over IP networks.

Last updated: January 17, 2025