Data Communications and Networking Study Guide

Fundamental Concepts of Data Communications

Communication is the act of sharing information. This sharing can occur locally or remotely. The term telecommunication, which encompasses telephony, telegraphy, and television, translates directly to communication at a distance. Data communications refers specifically to the exchange of data between two devices via some form of transmission medium.

Five Components of Data Communication

A data communications system is composed of five essential components:

1. Message: The information or data to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.

2. Sender: The device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.

3. Receiver: The device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.

4. Transmission Medium: The physical path by which a message travels from sender to receiver. Examples include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

5. Protocol: A set of rules that governs data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but will not be able to communicate effectively.

Four Fundamental Effectiveness Characteristics

The effectiveness of a data communications system depends on four fundamental characteristics:

1. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.

2. Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left

   uncorrected are unusable.

3. Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. In the case of real-time video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay.

4. Jitter: Jitter refers to the variation in packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, if video packets are sent every $30\,\text{ms}$, and some arrive with a $30\,\text{ms}$ delay while others arrive with a $40\,\text{ms}$ delay, an uneven quality in the video is the result.

Data Representation

Information today comes in various forms:

• Text: Represented as a bit pattern (a sequence of $0$\text{s} and $1$\text{s}). Sets of bit patterns called codes are designed to represent text symbols (e.g., ASCII, Unicode).

• Numbers: Represented directly using binary number systems to simplify mathematical operations.

• Images: Composed of a matrix of pixels. Each pixel is assigned a bit pattern determining its brightness and color.

• Audio: Refers to the recording or broadcasting of sound or music. Audio is continuous by nature.

• Video: Refers to the recording or broadcasting of a picture or movie. Video can be produced as a continuous signal or as a combination of discrete images (frames).

Data Flow Modes

Communication between two devices can occur in three distinct operational directions:

1. over Simplex: Communication is strictly unidirectional, like a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Keyboards and traditional monitors operate in simplex mode.

2. Half-Duplex: Each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa. Walkie-talkies operate in half-duplex mode.

3. Full-Duplex: Both stations can transmit and receive simultaneously. Signals going in one direction share the capacity of the link with signals going in the other direction. A telephone conversation is a full-duplex system.

Data Networks and Physical Topologies

A network is the interconnection of a set of devices capable of communication. A device in this context can be:

• A host (end system): such as a large computer, desktop, laptop, workstation, cellular phone, or security system.

• A connecting device: such as a router, switch, modem, etc.

Types of Connections

A network consists of two or more devices connected through links. A link is a communications pathway that transfers data from one device to another.

• Point-to-Point Connection: Provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices.

• Multipoint Connection (Multidrop): A connection in which more than two specific devices share a single link. The capacity of the channel is shared either spatially or temporally.

Physical Topologies

The term physical topology refers to the way in which a network is laid out physically. Two or more devices connect to a link; two or more links form a topology.

Mesh Topology

In a fully connected mesh topology, every device has a dedicated point-to-point link to every other device. "Dedicated" means that the link carries traffic only between the two devices it connects.

• Physical Link Calculation: For $N$ physical devices in a fully connected mesh network, the total number of physical duplex links required is:   $\text{Number of links} = \frac{N(N - 1)}{2}$

• I/O Port Calculation: Every device must have $N - 1$ input/output (I/O) ports to connect to all other devices.

• Advantages: Dedicated links eliminate traffic congestion; robust system (if one link fails, it does not affect the rest of the network); enhanced privacy and security; fault identification and fault isolation are easy.

• Disadvantages: High volume of cabling and excessive number of required I/O ports; expensive installation and hardware.

Star Topology

In a star topology, each device has a dedicated point-to-point link only to a central controller, usually referred to as a hub or switch. Devices do not directly send data to one another.

• Characteristics: Requires only $1$ link and $1$ port per device.

• Advantages: Less expensive than a mesh topology; easy to install and reconfigure; robust (if one link fails, only that device's link is affected).

• Disadvantages: Single point of failure (if the central hub or switch fails, the entire network goes down).

Bus Topology

A bus topology is a multipoint connection where one long cable acts as a central backbone to link all devices in a network.

• Components:

  • Drop line: The connection running between the device and the main cable.

  • Tap: A connector that splices into the main cable or punctures the outer sheath to create a contact.

  • Cable end: Terminators positioned at each end of the main bus cable to absorb signals and prevent signal reflection.

• Advantages: Ease of installation; uses significantly less cabling than mesh or star topologies.

• Disadvantages: Difficult reconnection and fault isolation; damage or a break in the main backbone cable stops all network transmission.

Ring Topology

In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its intended destination.

• Repeater: Each device in the ring incorporates a repeater. When a device receives a signal intended for another station, its repeater regenerates the bits and passes them along.

• Advantages: Easy to install and reconfigure; fault isolation is simplified.

• Disadvantages: Unidirectional traffic; a break in the ring (such as a disabled station) can disable the entire network.

Network Types and Switching

Networks are categorized according to their size, geographical coverage, and ownership.

Local Area Network (LAN)

A Local Area Network (LAN) is usually privately owned and connects devices in a single office, building, or campus.

• Scale: Can range from two PCs and a printer in a home office to an enterprise network spanning an entire company.

• Addressing: Each host in a LAN has a unique address/identifier that uniquely defines the host within that LAN.

• Evolution: Historically, LANs utilized a shared common cable (bus topology with cable taps and terminators). Modern LANs predominantly use a central switch connected to hosts via point-to-point links.

Wide Area Network (WAN)

A Wide Area Network (WAN) provides long-distance transmission of data, image, audio, and video information over large geographic areas that may comprise a country, a continent, or the entire world.

Key Differences Between LAN and WAN:

• Geographical Span: LAN is limited in size (office, building, campus); WAN covers a wide geographical region.

• Interconnected Elements: LAN interconnects host end-systems directly; WAN interconnects connecting devices such as switches, routers, or modems.

• Ownership: LAN is normally privately owned by the organization using it; WAN is created and operated by communication companies and leased to organizations.

WAN Architecture Categories:

1. Point-to-Point WAN: Connects two connecting devices across a dedicated transmission medium (e.g., a leased line connecting two routers).

2. Switched WAN: A network with more than two ends that connects multiple switches together to route traffic across a wide geographic region.

Internetworks and The Internet

An internetwork (or internet with a lowercase 'i') is formed when two or more individual networks are connected by internetworking devices such as routers.

• The Internet (uppercase 'I'): The specific worldwide internetwork composed of thousands of interconnected commercial, academic, and government networks.

Hierarchy of The Internet Today:

• Backbones: High-capacity communication trunks operated by large telecommunications service providers.

• Provider Networks: Intermediate networks operated by Internet Service Providers (ISPs) that purchase service from backbones.

• Customer Networks: End-user networks (home, business, campus) that connect to provider networks.

• Peering Points: Internet Exchange Points (IXPs) where multiple provider networks directly connect and exchange traffic.

Switching Technologies

An internet is a switched network in which switches connect two or more links together. A switch forwards data from one network to another as required. The two primary switching paradigms are:

Circuit-Switched Networks

A dedicated connection (circuit) is established between two nodes before communication begins. The capacity of the channel is reserved exclusively for the entire duration of the communication session.

• Traditional voice telephone networks are classic examples of circuit-switched networks.

• Employs low-capacity lines from end devices to switches, and high-capacity lines between switches.

Packet-Switched Networks

Data is divided into discrete chunks called packets. Each packet travels independently through the network. Routers store and forward packets using internal queues.

• No dedicated resource reservation occurs; packets dynamically share high-capacity trunk lines.

Protocol Layering and Network Models

When communication between devices is simple, a single protocol suffices. When communication is complex, the process is divided into distinct layers, creating a protocol layering architecture.

Principles of Protocol Layering

There are two fundamental principles governing protocol layering:

1. Bidirectional Communication Principle:    If bidirectional communication is required, each layer must be designed to perform two opposite tasks—one in each direction (e.g., listening and talking, encryption and decryption, sending and receiving).

2. Peer-Layer Identical Objects Principle:    The two objects under each layer at both communication sites should be identical. For example, at Layer 3 on both sites, the object handled is plaintext; at Layer 2, it is ciphertext; at Layer 1, it is a mail envelope.

Logical Connections vs. Physical Transmission

• Logical Connections: An imaginary horizontal layer-to-layer connection where peer layers appear to communicate directly with each other using peer protocols.

• Physical Transmission: In reality, data moves vertically down through the layers at the sending host, across the physical medium, and vertically up through the layers at the receiving host.

The TCP/IP Protocol Suite

The TCP/IP protocol suite was developed prior to the OSI model. It is a hierarchical protocol suite made up of interactive modules. The 5-layer architecture includes:

1. Physical Layer (Layer 1): Responsible for the movements of individual bits from one node to the next across a physical link. Converts digital bits into electrical, optical, or electromagnetic signals.

2. Data Link Layer (Layer 2): Responsible for moving frames from one hop (node) to the next. Handles physical addressing (MAC addresses), framing, flow control, error control, and access control.

3. Network Layer (Layer 3): Responsible for the delivery of individual packets from the original source host to the final destination host. Handles logical addressing (IP addresses) and routing.

4. Transport Layer (Layer 4): Responsible for process-to-process delivery of the entire message. Handles service-point addressing (port numbers), segmentation and reassembly, connection control, flow control, and error control.

5. Application Layer (Layer 5): Responsible for providing network services directly to the end-user (human or software program).

Delivery Scope of TCP/IP Layers

• Data Link Layer: Performs hop-to-hop (node-to-node) delivery across a single physical link.

• Network Layer: Performs source-to-destination (host-to-host) delivery across multiple intermediate routers and networks.

• Transport Layer: Performs process-to-process delivery between specific software applications running on host computers.

Data Encapsulation and Protocol Data Units (PDUs)

As data flows down the protocol stack at the sender, each layer adds control information called a header (and at Layer 2, a trailer). This process is encapsulation. At the destination, headers and trailers are stripped away in reverse order (decapsulation).

• Application Layer: Generates a Message.

• Transport Layer: Adds header $H_4$ to form a Segment or User Datagram.

• Network Layer: Adds header $H_3$ to form a Packet or Datagram.

• Data Link Layer: Adds header $H_2$ and trailer $T_2$ to form a Frame.

• Physical Layer: Converts the frame into a sequence of Bits transmitted over the medium.

The OSI Reference Model

The Open Systems Interconnection (OSI) model was created by the International Organization for Standardization (ISO) as a 7-layer theoretical reference framework:

1. Layer 7: Application - Provides services to users (network virtual terminal, file transfer/access/management [FTAM], mail services [X.400], directory services [X.500]).

2. Layer 6: Presentation - Translation, encryption/decryption, data compression.

3. Layer 5: Session - Dialog control, session management, synchronization (checkpointing).

4. Layer 4: Transport - Process-to-process message delivery, flow/error control.

5. Layer 3: Network - Host-to-host routing and logical addressing.

6. Layer 2: Data Link - Hop-to-hop framing, MAC addressing.

7. Layer 1: Physical - Bit transmission over a physical medium.

Reasons for OSI Model's Lack of Commercial Success:

1. Late Development: OSI standards were finalized after TCP/IP was already fully implemented and widely deployed.

2. Incomplete Specification: The presentation and session layers were never fully defined or practically implemented in software.

3. Performance Issues: Early OSI trial implementations exhibited lower performance compared to TCP/IP suites.

Physical Layer Signals and Performance

Analog and Digital Signals

Data must be converted into physical signals to be transmitted across a medium.

• Analog Data & Signals: Represent continuous information that varies smoothly over time (e.g., human voice).

• Digital Data & Signals: Represent information that has discrete states (e.g., binary values $0$ and $1$).

Composite Signals and Bandwidth

A single-frequency sine wave cannot carry information. Real-world communication relies on composite signals composed of multiple simple sine waves.

• Bandwidth ($B$): The range of frequencies contained in a composite signal. It is calculated as the difference between the highest frequency ($f_h$) and the lowest frequency ($f_l$):   $B = f_h - f_l$

Bandwidth Worked Example (Example 3.10)

Problem: A periodic signal is decomposed into five sine waves with frequencies of $100\,\text{Hz}$, $300\,\text{Hz}$, $500\,\text{Hz}$, $700\,\text{Hz}$, and $900\,\text{Hz}$. What is its bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of $10\,\text{V}$.

Solution:

Let $f_h$ be the highest frequency, $f_l$ the lowest frequency, and $B$ the bandwidth:

• Lowest frequency $f_l = 100\,\text{Hz}$

• Highest frequency $f_h = 900\,\text{Hz}$

$B = f_h - f_l = 900 - 100 = 800\,\text{Hz}$

The frequency spectrum contains five vertical lines located at $100\,\text{Hz}$, $300\,\text{Hz}$, $500\,\text{Hz}$, $700\,\text{Hz}$, and $900\,\text{Hz}$, each with a peak amplitude height of $10\,\text{V}$.

Digital Signals and Bit Rate

Most digital signals are nonperiodic; thus, period and frequency are not appropriate characteristics. Instead, digital signals are described by bit rate.

• Bit Rate: The number of bits sent in $1\,\text{s}$, expressed in bits per second (bps).

• Signal Levels: A digital signal can have more than two levels. If a signal has $L$ levels, the number of bits represented per signal level ($r$) is:   $r = \log_2(L)$

  • A 2-level digital signal ($L = 2$) carries $1\,\text{bit}$ per level.

  • A 4-level digital signal ($L = 4$) carries $2\,\text{bits}$ per level.

High-Definition Television (HDTV) Bit Rate Example

Problem: Calculate the raw bit rate required to broadcast High-Definition Television (HDTV).

Parameters:

• Aspect ratio: $16:9$

• Screen resolution: $1920 \times 1080$ pixels

• Refresh rate: $30$ frames per second

• Color depth: $24\,\text{bits}$ per color pixel

Calculation: $\text{Bit Rate} = 1920 \times 1080\,\frac{\text{pixels}}{\text{frame}} \times 30\,\frac{\text{frames}}{\text{s}} \times 24\,\frac{\text{bits}}{\text{pixel}}$

$\text{Bit Rate} = 2,073,600 \times 30 \times 24 = 1,492,992,000\,\text{bps} \approx 1.4928\,\text{Gbps}$

TV stations reduce this raw bit rate down to $20\,\text{Mbps}$ to $40\,\text{Mbps}$ through compression before transmission.

Transmission Impairments and Network Performance

Causes of Transmission Impairment

Signals travel through imperfect transmission media. Consequently, the signal at the beginning of the medium is not identical to the signal at the end of the medium.

1. Attenuation: Loss of signal energy due to resistance in the medium. To compensate for attenuation, amplifiers are introduced along the path to increase signal amplitude.

2. Distortion: Alteration of signal shape. Occurs in composite signals because different frequency components propagate at different speeds through a medium, arriving at the receiver out of phase.

3. Noise: External unwanted signals (thermal noise, induced noise, crosstalk, or impulse noise) corrupting the signal.

Signal-to-Noise Ratio (SNR)

The ratio of signal power to noise power determines signal clarity:

• High SNR: Signal power is significantly higher than noise power; data is easily recovered.

• Low SNR: Noise power is comparable to signal power; severe data corruption occurs.

Network Performance Metrics

1. Bandwidth

Network bandwidth can be expressed in two ways:

• Bandwidth in Hertz ($\text{Hz}$): Range of composite frequencies passed by a channel.

• Bandwidth in bits per second ($\text{bps}$): Maximum data transmission rate capability of a channel.

2. Throughput

Throughput is a measure of how fast data can actually be sent through a network. Although a link may have a bandwidth of $B\,\text{bps}$, the throughput $T\,\text{bps}$ is always less than $B$ due to overhead and network congestion ($T < B$).

3. Latency (Delay)

Latency or delay defines how long it takes for an entire message to completely arrive at the destination from the time the first bit leaves the source host.

$\text{Latency} = \text{Propagation Time} + \text{Transmission Time} + \text{Queuing Time} + \text{Processing Delay}$

• Propagation Time: Time required for a bit to travel from source to destination over the physical medium.   $\text{Propagation Time} = \frac{\text{Distance}}{\text{Propagation Speed}}$

• Transmission Time: Time required to push all bits of a message into the link.   $\text{Transmission Time} = \frac{\text{Message Size}}{\text{Bandwidth}}$

• Queuing Time: Time spent by a packet waiting in intermediate router queues before processing.

• Processing Delay: Time intermediate devices (routers/switches) take to process packet headers, check errors, and make routing decisions.

4. Bandwidth-Delay Product

The bandwidth-delay product (BDP) defines the maximum number of bits that can fill a transmission link at any given moment. It can be envisioned as the volume of a pipe where bandwidth represents cross-sectional area and delay represents pipe length.

$\text{BDP} = \text{Bandwidth} \times \text{Delay}$

Case 1: Low Bandwidth, High Delay

• Bandwidth: $1\,\text{bps}$

• Delay: $5\,\text{s}$

• $\text{BDP} = 1\,\text{bps} \times 5\,\text{s} = 5\,\text{bits}$

Case 2: High Bandwidth, High Delay

• Bandwidth: $5\,\text{bps}$

• Delay: $5\,\text{s}$

• $\text{BDP} = 5\,\text{bps} \times 5\,\text{s} = 25\,\text{bits}$

5. Jitter

Jitter is the variation or fluctuation in packet delay (delay variation).

• Jitter is critical for time-sensitive real-time applications such as voice and video streaming.

• Example: If consecutive packets encounter delays of $20\,\text{ms}$, $45\,\text{ms}$, and $40\,\text{ms}$, the real-time application endures jitter, producing uneven playback quality.