Data Communication and Computer Networks

Fundamentals of Data Communication

• Definition: Exchange of data between two devices via a transmission medium using combined hardware and software systems.
• Fundamental Effectiveness Criteria:
  • Delivery: Data must reach only the intended destination device or user.
  • Accuracy: Data must be delivered without uncorrected errors.
  • Timeliness: Data must arrive within a usable timeframe without significant delay (real-time transmission).
  • Jitter: The variation in packet arrival times, causing uneven delay in multimedia streams.
• Five Core Components:
  • Message: The information to be communicated (text, numbers, audio, video, pictures).
  • Sender: Device that transmits the data.
  • Receiver: Device that accepts the transmitted data.
  • Transmission Medium: Physical pathway conveying the signal (e.g., twisted-pair, coaxial cable, fiber-optic cable, radio waves).
  • Protocol: Mutually agreed-upon set of rules governing data transmission.
• Data Transmission Modes:
  • Simplex: Unidirectional transmission where one device only transmits and the other only receives (e.g., keyboard and monitor).
  • Half-Duplex: Bidirectional transmission where both devices can send and receive, but only one at a time (e.g., walkie-talkies).
  • Full-Duplex: Simultaneous bidirectional transmission where channel capacity is shared between both directions (e.g., telephone networks).

Network Structures and Topologies

• Evaluation Criteria:
  • Performance: Evaluated via transit time and response time; influenced by user load, medium, hardware capacity, and software efficiency.
  • Reliability: Assessed by failure frequency, recovery duration, and system robustness under catastrophe.
  • Security: Enforces protection against unauthorized access, corruption, and operational data loss.
• Connection Link Types:
  • Point-to-Point: Dedicated link reserving total channel capacity entirely for two connected devices.
  • Multipoint (Multidrop): A single link shared by more than two devices, divided either spatially (simultaneous sharing) or temporally (timeshared turns).
• Network Topologies:
  • Mesh: Every device possesses a dedicated point-to-point link to every other device; requires $\frac{n(n - 1)}{2}$ duplex links and $n - 1$ I/O ports per device. High fault isolation, privacy, and redundancy, but costly and cabling-heavy.
  • Star: Devices link directly to a central controller (hub) with point-to-point lines; relays traffic through the hub. Easy to install and isolate faults, but creates a single point of failure at the hub.
  • Bus: Multipoint architecture where a single central cable acts as a backbone connecting stations via drop lines and taps. Prone to signal reflection degradation, tap limits, and network-wide failure if the backbone breaks.
  • Ring: Devices link in a loop with point-to-point connections to their two immediate neighbors, using repeaters to regenerate signals unidirectionally. Simple reconfiguration and fault detection, but a break can disable the entire ring.
• Network Geographic Categories:
  • Local Area Network (LAN): Privately owned networks covering a building or campus up to a few kilometers; typically operates between $10\,\text{Mbps}$ and $10\,\text{Gbps}$ with microsecond delays.
  • Metropolitan Area Network (MAN): Covers city-wide spans; examples include cable television networks and IEEE 802.16 wireless standards.
  • Wide Area Network (WAN): Spans countries or continents; separates application hosts from communication subnets (transmission lines and packet-switched/store-and-forward switching elements or satellite systems).

Signals, Impairments, and Data Rates

• Signal Classification:
  • Analog: Continuous waves passing through an infinite spectrum of intensity levels over time.
  • Digital: Discrete signals possessing a limited count of defined voltage levels.
• Periodic Analog Signals (Sine Wave Characteristics):
  • Peak Amplitude: Absolute value of highest signal intensity, measured in volts.
  • Period ($T$) & Frequency ($f$): $T$ is the time (in seconds) for $1$ cycle; $f$ is cycles per second (in $\text{Hz}$):     $f = \frac{1}{T}$$T = \frac{1}{f}$
  • Phase: Waveform position relative to time $0$ ($0^\circ$, $90^\circ$, or $180^\circ$ shifts).
  • Wavelength ($\lambda$): Distance traveled in one period through a medium with propagation speed $c$:     $\lambda = \frac{c}{f}$
  • Bandwidth: The difference between the highest and lowest frequencies contained within a composite signal.
• Digital Signal Metrics:
  • Levels & Bits: A digital signal with $L$ discrete levels conveys $\log_2(L)$ bits per level.
  • Bit Rate: Count of bits transmitted per second ($\text{bps}$).
• Transmission Impairments:
  • Attenuation: Loss of signal energy converted to heat over distance; measured in Decibels ($\text{dB}$):     $\text{dB} = 10\log_{10}\left(\frac{P_2}{P_1}\right)$
  • Distortion: Alteration of a composite signal's shape due to unequal component propagation speeds and differing arrival phases.
  • Noise: Unwanted external signal additions, including thermal noise (electron agitation), induced noise (motors), crosstalk (wire coupling), and impulse noise (voltage spikes).
  • Signal-to-Noise Ratio (SNR):$\text{SNR} = \frac{\text{Average Signal Power}}{\text{Average Noise Power}}$$\text{SNR}_{\text{dB}} = 10\log_{10}(\text{SNR})$
• Channel Data Rate Limits:
  • Nyquist Bit Rate (Noiseless Channel):$\text{BitRate} = 2 \times \text{Bandwidth} \times \log_2(L)$
  • Shannon Capacity (Noisy Channel):$\text{Capacity} = \text{Bandwidth} \times \log_2(1 + \text{SNR})$

Digital-to-Analog Modulation and Multiplexing

• Modulation Techniques:
  • Amplitude Shift Keying (ASK): Carrier amplitude varies to represent binary states; frequency and phase remain fixed. Common as Binary ASK or On-Off Keying (OOK).
  • Frequency Shift Keying (FSK): Modulates between distinct carrier frequencies ($f_1$ and $f_2$) while amplitude and phase stay fixed.
  • Phase Shift Keying (PSK): Shifts carrier phase (e.g., $0^\circ$ for bit 0, $180^\circ$ for bit 1 in BPSK); most bandwidth-efficient method.
  • Signal Rate (Baud): Ratio of bit rate ($N$) to data elements carried per signal element ($r$):     $S = \frac{N}{r} = \frac{N}{\log_2(L)}$
• Multiplexing Techniques:
  • Frequency-Division Multiplexing (FDM): Analog technique combining multiple signals modulated at distinct carrier frequencies separated by unused frequency bands (guard bands).
  • Wavelength-Division Multiplexing (WDM): Analog technique combining narrow optical beams of differing wavelengths ($\lambda_1, \lambda_2, \dots$) through a prism over a fiber-optic cable (e.g., SONET).
  • Time-Division Multiplexing (TDM): Digital technique where multiple connections share the entire link bandwidth sequentially by allocating individual time slots.

Transmission Media

• Guided Media:
  • Twisted-Pair Cable: Two insulated copper conductors twisted around each other to equalize and cancel external electromagnetic interference and crosstalk; used in telephone local loops, DSL, 10Base-T, and 100Base-T.
  • Coaxial Cable: Central copper conductor surrounded by an insulating layer, braided outer conductor shield, and plastic cover; used in cable TV (RG-59) and early Ethernet.
  • Fiber-Optic Cable: Glass or plastic core surrounded by a lower-density cladding and protective Kevlar jacket; propagates signals via total internal reflection when the angle of incidence exceeds the critical angle ($I > \text{critical angle}$). Supports speeds up to $1600\,\text{Gbps}$, reaches $50\,\text{km}$ without regeneration, and is immune to electromagnetic noise.
• Unguided (Wireless) Media:
  • Ground Propagation: Low-frequency radio waves ($< 2\,\text{MHz}$) follow the planetary curvature through the lower atmosphere.
  • Sky Propagation: Medium-frequency waves ($2\text{--}30\,\text{MHz}$) bounce off the ionosphere for long-range communications.
  • Line-of-Sight Propagation: High-frequency waves ($> 30\,\text{MHz}$) transmit straight from antenna to antenna.
  • Radio Waves ($3\,\text{kHz}\text{--}1\,\text{GHz}$): Omnidirectional propagation; penetrates walls; suited for multicasting (AM/FM radio, cordless phones).
  • Microwaves ($1\text{--}300\,\text{GHz}$): Unidirectional, narrow line-of-sight propagation; cannot penetrate walls; uses parabolic dish and horn antennas.
  • Infrared ($300\,\text{GHz}\text{--}400\,\text{THz}$): Short-range, line-of-sight communication; blocked by walls (preventing cross-room interference); standardized by IrDA ($75\,\text{kbps}$ up to $8\,\text{m}$, newer standards up to $4\,\text{Mbps}$).

Switched Networks

• Circuit-Switched Networks: Establish a dedicated physical transmission path consisting of connected channels across intermediate switches. Operates across three distinct phases: connection setup, data transfer, and connection teardown.
• Datagram Networks: Connectionless packet-switched approach operating at the network layer via routers. Each packet (datagram) is treated as an independent entity routed dynamically without resource reservation; packets may arrive out of order or be dropped.
• Virtual-Circuit Networks: Connection-oriented hybrid packet-switched approach implemented at the data link layer. Requires setup and teardown phases; all packets follow a pre-established virtual circuit carrying a local jurisdiction address identifier.

Transmission Modes and Error Detection

• Data Transmission Modes:
  • Parallel Transmission: Transmits groups of $n$ bits simultaneously across $n$ separate physical wires per clock tick; fast but expensive, limiting use to short distances.
  • Serial Transmission: Transmits bits sequentially over a single physical channel:
  • Asynchronous: Data sent byte-by-byte framed by a start bit ($0$), one or more stop bits ($1$), and variable idle gaps; sender and receiver resynchronize on each byte.
  • Synchronous: Continuous bit stream grouped into frames without start/stop bits or gaps; receiver relies entirely on shared timing and clock synchronization.
  • Isochronous: Ensures constant bit arrival rates with zero variable delay, specifically designed for real-time video and audio.
• Cyclic Redundancy Check (CRC):
  • Linear cyclic block code where a dataword of $k$ bits is augmented with $n - k$ zeros and divided by an agreed $(n - k + 1)$-bit divisor using modulo-2 arithmetic (XOR operations).
  • The resulting remainder is appended to the dataword to form an $n$-bit codeword.
  • The receiver divides the received codeword by the same divisor; a zero syndrome ($000$) indicates an error-free transmission, whereas a non-zero syndrome signals corruption.

• Simple Parity-Check Code:
  • Generates an $n$-bit codeword from a $k$-bit dataword where $n = k + 1$.
  • Appends a parity bit to make the count of $1\text{s}$ even (or odd).
  • Minimum Hamming distance $d_{\min} = 2$; detects single-bit errors but cannot correct errors.

High-Level Data Link Control (HDLC)

• Operational Configurations & Modes:
  • Normal Response Mode (NRM): Unbalanced architecture with one primary station initiating commands and one or more secondary stations issuing responses.
  • Asynchronous Balanced Mode (ABM): Balanced point-to-point architecture where both stations act as combined peers capable of initiating commands and responses.
• Frame Categories:
  • Information Frames (I-Frames): Transport user payload and control data; facilitate acknowledgment piggybacking.
  • Supervisory Frames (S-Frames): Carry flow and error control commands without information payloads:
  • Receive Ready (RR, code 00): Positive acknowledgment.
  • Receive Not Ready (RNR, code 10): Acknowledges receipt while signaling receiver congestion.
  • Reject (REJ, code 01): Negative acknowledgment for Go-Back-N ARQ.
  • Selective Reject (SREJ, code 11): Negative acknowledgment for Selective Repeat ARQ.
  • Unnumbered Frames (U-Frames): Transport link and session management commands.
• Frame Format:

• Flag Field: 8-bit delimiter pattern (01111110) at start and finish.
• Address Field: Identifies the secondary station.
• Control Field: 1 or 2 bytes defining frame type, sequence numbers ($N(S)$), acknowledgment numbers ($N(R)$), and Poll/Final ($P/F$) bit.
• FCS Field: Frame Check Sequence using CRC for error detection.

Network Architecture Models

• OSI 7-Layer Reference Model:
  • Layer 1 (Physical): Mechanical, electrical, encoding, and transmission rate interface specifications.
  • Layer 2 (Data Link): Framing, physical MAC addressing, link flow control, error control (trailers), and medium access control.
  • Layer 3 (Network): End-to-end packet delivery across networks, logical addressing, and routing.
  • Layer 4 (Transport): Process-to-process message delivery, port addressing, segmentation, reassembly, connection control, end-to-end flow/error control.
  • Layer 5 (Session): Dialog management and checkpoint synchronization.
  • Layer 6 (Presentation): Data translation, character encoding interoperability, encryption, decryption, and data compression.
  • Layer 7 (Application): Direct user/network service interfaces (e.g., file transfers, remote login, mail).
• TCP/IP Protocol Suite:
  • Four core layers: Host-to-network, Internet (IP), Transport (TCP, UDP, SCTP), and Application (HTTP, SMTP, FTP, TELNET, DNS).

• Model Comparisons:
  • OSI clearly differentiates between services, interfaces, and protocols; protocols are better hidden and replaced.
  • OSI supports both connectionless and connection-oriented communication in the network layer, but only connection-oriented in the transport layer.
  • TCP/IP supports only connectionless communication in the network layer (IP), but provides a choice of connectionless (UDP) or connection-oriented (TCP) at the transport layer.

Network Interconnection Devices

• Repeater: Physical layer (Layer 1) device that regenerates weakened electrical or optical signals bit-for-bit to connect segments of the same LAN.
• Hub: Multiport Layer 1 device acting as a central connecting point that regenerates and broadcasts incoming signals to all outgoing ports except the port of origin.
• Bridge / Link Layer Switch: Operates at Layers 1 and 2; inspects physical (MAC) destination addresses in frame headers and selectively filters or forwards frames based on internal port-to-address tables.
• Router: Three-layer device (Layers 1, 2, and 3) routing packets across independent networks (LANs and WANs) based on logical IP addresses using dynamic routing tables.
• Gateway: Operates across all layers (up to Layer 7) to interpret and translate protocols between disparate network architectures.

Network Addressing Schemes

• Ethernet MAC Physical Address:
  • $48\text{-bit}$ ($6\text{-byte}$) address represented in hexadecimal format separated by colons (e.g., 06:01:02:01:2C:4B).
  • Unicast: The least significant bit of the first byte is $0$ (transmitted first on the line).
  • Multicast: The least significant bit of the first byte is $1$.
  • Broadcast: All $48$ bits set to $1$ (FF:FF:FF:FF:FF:FF).
• IPv4 Logical Address:
  • $32\text{-bit}$ ($4\text{-byte}$) address represented in dotted-decimal format (values $0\text{--}255$ per byte), yielding an address space of $2^{32} = 4,294,967,296$.
  • Classful Addressing Ranges:
  • Class A: First byte $0\text{--}127$ (starts with 0); $128$ blocks of $16,777,216$ addresses for large organizations.
  • Class B: First byte $128\text{--}191$ (starts with 10); $16,384$ blocks of $65,536$ addresses for midsize networks.
  • Class C: First byte $192\text{--}223$ (starts with 110); $2,097,152$ blocks of $256$ addresses for small networks.
  • Class D: First byte $224\text{--}239$ (starts with 1110); single block of $268,435,456$ addresses for multicasting.
  • Class E: First byte $240\text{--}255$ (starts with 1111); reserved addresses.

Application Layer Protocols

• TELNET (Terminal Network):
  • Establishes a remote connection via terminal emulation.
  • Client converts keystrokes to Network Virtual Terminal (NVT) format; remote server uses a pseudoterminal driver to pass NVT inputs into operating system applications.
• Electronic Mail Protocols:
  • User Agent (UA): Composes, reads, replies to, forwards, and manages mail in mailboxes.
  • Simple Mail Transfer Protocol (SMTP): Message Transfer Agent protocol exchanging commands (e.g., HELO, MAIL FROM, RCPT TO, DATA, QUIT) and 3-digit numeric responses between MTAs over TCP.
  • Post Office Protocol version 3 (POP3): Access protocol using TCP port $110$ to retrieve mail; operates in delete mode (removes from server) or keep mode (retains on server).
  • Internet Mail Access Protocol version 4 (IMAP4): Advanced retrieval protocol supporting partial message inspection, searching, and server-side folder hierarchies.
• File Transfer Protocol (FTP):
  • Employs two separate TCP connections: a control connection on port $21$ (persists through session for commands/responses) and a data connection on port $20$ (opens and closes per file transfer).

• Transmission Modes: Stream mode (default byte sequence), Block mode (prepended with a 3-byte descriptor header), and Compressed mode (run-length encoding).
  • Hypertext Transfer Protocol (HTTP):
• Stateless request/response protocol operating on TCP port $80$.
• Core Methods: GET (request resource), HEAD (request headers only), POST (send client data to server), PUT (upload document to server), TRACE, CONNECT, OPTIONS.
• Status Code Classes: 1xx (Informational), 2xx (Success, e.g., 200 OK), 3xx (Redirection), 4xx (Client Error), 5xx (Server Error).