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11.2 Data Communications, OSI Model & Protocols

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PRC Electronics Engineering Licensure Examination Study Guide Exam

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1 Chapter 1: Introduction & Board Exam Strategy

1.1 PRC ECE Licensure Exam Overview & Regulation 1.2 Table of Specifications & Exam Strategy

2 Chapter 2: Mathematics Foundations

2.1 College & Advanced Algebra 2.2 Trigonometry & Solid Mensuration 2.3 Analytic Geometry & Differential Calculus 2.4 Integral Calculus & Applications

3 Chapter 3: Advanced Mathematics for ECE

3.1 Differential Equations & Circuit Transients 3.2 Complex Numbers & Laplace Transforms 3.3 Fourier Series, Fourier Transforms & Z-Transforms 3.4 Vector Analysis, Matrices & Probability

4 Chapter 4: GEAS - Engineering Sciences & Economics

4.1 Engineering Physics & Materials Chemistry 4.2 Engineering Mechanics & Strength of Materials 4.3 Engineering Economics & Financial Analysis 4.4 Computer Fundamentals & C/C++ Programming

5 Chapter 5: GEAS - Management, Materials, Environment & CAD

5.1 Engineering Management & Technopreneurship 101 5.2 Materials Science & Engineering 5.3 Environment Science & Engineering 5.4 Computer-Aided Drawing (CAD) & Engineering Graphics

6 Chapter 6: GEAS - ECE Laws, Ethics & Contracts

6.1 Republic Act No. 9292 (Electronics Engineering Law of 2004) 6.2 ECE Code of Ethics & Professional Conduct 6.3 Contracts, Specifications & National Building Code (PD 1096)

7 Chapter 7: Electronics Engineering - Circuits & Semiconductor Diodes

7.1 DC & AC Circuit Analysis & Network Theorems 7.2 Electromagnetics & Passive Components 7.3 Semiconductor Physics & Diode Circuits 7.4 Power Supply Design & Voltage Regulators

8 Chapter 8: Electronics Engineering - Transistors & Analog Circuits

8.1 BJT Operation, Biasing & Small-Signal Amplifiers 8.2 FET Characteristics, Biasing & Amplifier Circuits 8.3 Multistage Amplifiers, Frequency Response & Power Amplifiers 8.4 Operational Amplifiers & Active Filter Design

9 Chapter 9: Electronics Engineering - Digital Electronics & Control Systems

9.1 Feedback Topologies & Oscillator Circuits 9.2 Boolean Algebra, Logic Gates & Combinational Circuits 9.3 Sequential Logic, Flip-Flops & Microprocessor Architecture 9.4 Industrial Power Electronics & Control System Theory

10 Chapter 10: EST - Communications Fundamentals & Transmission Lines

10.1 Signal Fundamentals, Decibel Calculations & Noise Analysis 10.2 Amplitude Modulation (AM) Principles & Receivers 10.3 Frequency Modulation (FM), Phase Modulation & Discriminators 10.4 Transmission Line Parameters, SWR & Smith Chart

11 Chapter 11: EST - Digital Communications, Wireless & Optical Systems

11.1 Pulse & Digital Modulation Systems 11.2 Data Communications, OSI Model & Protocols 11.3 Antenna Principles & Electromagnetic Wave Propagation 11.4 Optical Fiber, Microwave, Satellite & Philippine NTC Regulations

11.2 Data Communications, OSI Model & Protocols

Key Takeaways Explain This Topic AI

The ISO 7-Layer OSI Reference Model organizes network functions from Layer 1 Physical up to Layer 7 Application.

Transmission Control Protocol (TCP) provides connection-oriented, reliable byte-stream transport with flow control, whereas UDP provides connectionless, low-overhead datagram transport.

Cyclic Redundancy Check (CRC) utilizes polynomial binary division modulo-2 to detect burst transmission errors.

IPv4 Classless Inter-Domain Routing (CIDR) notation defines network prefix length, where a /24 subnet mask (255.255.255.0) yields 254 usable host IP addresses.

Data link ARQ protocols (Stop-and-Wait, Go-Back-N, Selective Repeat) manage frame retransmission to ensure error-free link delivery.

Last updated: July 2026

11.2 Data Communications, OSI Model & Protocols

Quick Answer: Modern data networks structure communication tasks into functional layers. Key concepts heavily tested on the PRC ECE board exam include the 7 layers of the OSI model, TCP vs. UDP protocol characteristics, Cyclic Redundancy Check (CRC) polynomial division modulo-2, Automatic Repeat Request (ARQ) flow control, and IPv4 CIDR subnetting math ( 2 32 − n − 2 2^{32-n} - 2 2 32 − n − 2 usable host addresses per subnet).

Introduction to Data Communications & Network Architectures

Data communications refers to the exchange of digitized information between two or more nodes via a transmission medium governed by agreed-upon communication rules designated as protocols . To reduce system complexity, modern networking protocols are organized as a vertical stack of functional layers, where each layer provides specific services to the layer immediately above it while hiding internal implementation details.

The OSI 7-Layer Reference Model

Promulgated by the International Organization for Standardization (ISO) in 1984 under standard ISO/IEC 7498-1 , the Open Systems Interconnection (OSI) model defines a 7-layer framework for network intercommunication. ┌─────────────────────────────────────────────────────────────┐

  • │ Layer 7: Application (HTTP, FTP, SMTP, DNS, SSH) │
  • ├─────────────────────────────────────────────────────────────┤
  • │ Layer 6: Presentation (Data Formatting, Encryption, JPEG) │
  • ├─────────────────────────────────────────────────────────────┤
  • │ Layer 5: Session (Dialog Control, RPC, NetBIOS) │
  • ├─────────────────────────────────────────────────────────────┤
  • │ Layer 4: Transport (TCP, UDP, Segment/Datagram Flow) │
  • ├─────────────────────────────────────────────────────────────┤
  • │ Layer 3: Network (IPv4, IPv6, ICMP, OSPF - Routing) │
  • ├─────────────────────────────────────────────────────────────┤
  • │ Layer 2: Data Link (Ethernet, MAC/LLC, Switches, CRC) │
  • ├─────────────────────────────────────────────────────────────┤
  • │ Layer 1: Physical (RS-232, RJ-45, Cables, Bit Encoding) │
  • └─────────────────────────────────────────────────────────────┘
  • Functional Overview of the 7 Layers

Layer 1 - Physical Layer: Transmits raw, unstructured bitstreams over physical mediums (copper wires, fiber optics, radio frequencies). Defines electrical voltage levels, bit timings, connector pinouts (e.g., RS-232, RJ-45), and physical data rates.

Layer 2 - Data Link Layer: Handles physical node-to-node framing, Media Access Control (MAC) addressing, error detection via CRC, and link-level flow control. Divided into two sublayers: Logical Link Control (LLC) (IEEE 802.2) and Media Access Control (MAC) (IEEE 802.3 Ethernet).

Layer 3 - Network Layer: Manages logical network addressing (IPv4/IPv6 addresses), packet routing across interconnected networks (routers), and packet fragmentation.

Layer 4 - Transport Layer: Manages end-to-end process-to-process communication, connection establishment, error recovery, segment sequencing, and port addressing. Key protocols: TCP and UDP.

Layer 5 - Session Layer: Establishes, maintains, synchronizes, and terminates session dialogs between endpoints (e.g., Remote Procedure Calls - RPC, NetBIOS).

Layer 6 - Presentation Layer: Translates data formats between application syntax and standard wire network syntax. Handles data compression, character encoding (ASCII, Unicode), and encryption/decryption (SSL/TLS).

Layer 7 - Application Layer: Provides direct network services to software applications (browsers, email clients). Examples: HTTP/HTTPS, FTP, SMTP, DNS, SNMP.

OSI Model vs. TCP/IP Architecture Comparison

While OSI serves as the formal theoretical reference model, the practical internet architecture is built upon the 4-layer (or 5-layer) TCP/IP Protocol Suite .

  • OSI Model Layer TCP/IP Architecture Layer Core Protocols & Services
  • 7. Application ; 6. Presentation ; 5. Session Application Layer HTTP, HTTPS, FTP, SMTP, DNS, SSH, DHCP, SNMP
  • 4. Transport Transport Layer TCP, UDP, SCTP
  • 3. Network Internet Layer IPv4, IPv6, ICMP, ARP, OSPF, BGP

2. Data Link ; 1. Physical Network Access Layer (or Link/Physical) Ethernet (802.3), Wi-Fi (802.11), Frame Relay, PPP

Transport Layer Protocols: TCP vs. UDP

The Transport Layer provides process-to-process data delivery using 16-bit Port Numbers (allowing 65 , 536 65,536 65 , 536 distinct logical ports). TCP 3-Way Handshake UDP Connectionless

  • Client Server Client Server
  • │ │ │ │
  • │──── SYN (Seq=x) ───────>│ │──── Datagram ──────────>│
  • │<─── SYN-ACK (Ack=x+1) ──│ │ (No Handshake/Ack) │
  • │──── ACK (Ack=y+1) ─────>│ │ │
  • Transmission Control Protocol (TCP)

Connection-Oriented: Establishes a reliable connection using a 3-Way Handshake ( SYN → \rightarrow → SYN-ACK → \rightarrow → ACK ).

Reliability: Guarantees ordered data delivery using sequence numbers, positive acknowledgments ( ACK ), and automatic retransmission of lost segments.

Flow Control & Congestion Control: Implements a dynamic Sliding Window Protocol to prevent receiver buffer overflow and congestion window mechanisms (Slow Start, Congestion Avoidance).

Header Size: 20 bytes minimum (up to 60 bytes with options).

User Datagram Protocol (UDP)

Connectionless: Transmits independent datagrams without establishing prior connections or performing handshakes.

Unreliable & Low Overhead: Provides no sequence numbering, no acknowledgments, and no retransmission. Unordered datagrams may be lost or duplicated.

Header Size: Minimal 8-byte fixed header (Source Port, Destination Port, Length, Checksum).

Use Cases: Real-time applications prioritizing speed over reliability: VoIP, video streaming, DNS queries, online gaming.

Error Detection & Control: CRC Polynomial Division

Cyclic Redundancy Check (CRC) is a powerful error-detecting mechanism operating at Layer 2. A block of data bits is treated as coefficients of a binary polynomial and divided modulo-2 by a agreed-upon generator polynomial G ( X ) G(X) G ( X ) .

Mathematical Principles

Given an k k k -bit message polynomial M ( X ) M(X) M ( X ) and an ( n + 1 ) (n+1) ( n + 1 ) -bit generator polynomial G ( X ) G(X) G ( X ) of degree n n n .

Append n n n zero bits to the message stream ( M ( X ) ⋅ X n M(X) \cdot X^n M ( X ) ⋅ X n ).

Divide M ( X ) ⋅ X n M(X) \cdot X^n M ( X ) ⋅ X n by G ( X ) G(X) G ( X ) using binary modulo-2 division (exclusive-OR / XOR operations without carry/borrow).

The remainder R ( X ) R(X) R ( X ) of length n n n bits is appended to the message as the CRC checksum field.

The receiver divides the received frame by G ( X ) G(X) G ( X ) . A remainder of zero indicates no errors occurred during transmission.

Step-by-Step CRC Calculation Example

Problem: Compute the 3-bit CRC remainder for a binary message M = 110101 M = 110101 M = 110101 using generator polynomial G ( X ) = X 3 + X + 1 G(X) = X^3 + X + 1 G ( X ) = X 3 + X + 1 (binary divisor 1011 ).

Solution:

Determine Generator Degree ( n n n ): G ( X ) = 1 X 3 + 0 X 2 + 1 X 1 + 1 X 0 → G(X) = 1X^3 + 0X^2 + 1X^1 + 1X^0 \rightarrow G ( X ) = 1 X 3 + 0 X 2 + 1 X 1 + 1 X 0 → Divisor = 1011 ( n = 3 n = 3 n = 3 bits).

Append n = 3 n=3 n = 3 Zeros to Message: Augmented Dividend = 110101000 .

  • Perform Modulo-2 Division (XOR Subtraction): 111001 <-- Quotient (Discarded)
  • ───────────────
  • 1011 ┘ 110101000
  • 1011
  • ────
  • 1100
  • 1011
  • ────
  • 1111
  • 1011
  • ────
  • 1000
  • 0000
  • ────
  • 1000
  • 0000
  • ────
  • 1000
  • 1011
  • ────
  • 011 <-- 3-Bit CRC Remainder R(X)

Final Transmitted Frame: Transmitted bits = Message + CRC = 110101011 .

Automatic Repeat Request (ARQ) Mechanisms

When a receiver detects a corrupted frame via CRC failure, ARQ protocols manage retransmission:

Stop-and-Wait ARQ: Transmitter sends 1 frame and pauses, waiting for an ACK before sending the next frame. Simple but inefficient over long propagation delay paths.

Go-Back-N ARQ: Transmitter sends multiple frames up to a window size W W W . If frame k k k is lost or corrupted, receiver rejects all subsequent frames ( k + 1 , k + 2 , … k+1, k+2, \dots k + 1 , k + 2 , … ), forcing the transmitter to retransmit frame k k k and all subsequent frames in the window.

Selective Repeat ARQ: Transmitter retransmits only the specific missing or corrupted frame. Receiver maintains a buffer to store out-of-order frames. Most bandwidth-efficient method.

IPv4 Addressing & CIDR Subnetting Guide

An IPv4 address is a 32-bit binary number represented in dotted-decimal notation (four 8-bit octets separated by dots, e.g., 192.168.1.1 ).

Classless Inter-Domain Routing (CIDR)

CIDR notation appends a slash followed by the prefix length n n n indicating how many leading bits constitute the Network ID (e.g., 192.168.1.0/24 ). Total IP Addresses = 2 32 − n \text{Total IP Addresses} = 2^{32-n} Total IP Addresses = 2 32 − n Usable Host IP Addresses = 2 32 − n − 2 \text{Usable Host IP Addresses} = 2^{32-n} - 2 Usable Host IP Addresses = 2 32 − n − 2

(Two addresses are reserved: the Network Address with all host bits set to 0, and the Directed Broadcast Address with all host bits set to 1).

Subnetting Reference Table

  • CIDR Prefix Subnet Mask Total IPs Usable Host IPs Common Application
  • /24 255.255.255.0 256 254 Standard LAN Network
  • /26 255.255.255.192 64 62 Departmental Subnet
  • /27 255.255.255.224 32 30 Small Subnet
  • /28 255.255.255.240 16 14 Small Server Farm
  • /30 255.255.255.252 4 2 Point-to-Point WAN Link
  • Step-by-Step Subnetting Calculation Example

Problem: Given the IP address 192.168.10.130/27 , determine (a) the Subnet Mask, (b) the Network ID, (c) the Broadcast Address, and (d) the range of usable host IP addresses.

Solution:

Subnet Mask: /27 prefix means 27 ones followed by 5 zeros → \rightarrow → 11111111.11111111.11111111.11100000 = 255.255.255.224 .

Block Size (Magic Number): 256 − 224 = 32 256 - 224 = 32 256 − 224 = 32 (or 2 5 = 32 2^5 = 32 2 5 = 32 ). Subnet multiples: 0, 32, 64, 96, 128, 160...

Locate Subnet for Host IP .130: The host IP .130 lies between 128 and 159.

  • Network ID: 192.168.10.128
  • Broadcast Address: 192.168.10.159 (one less than next subnet 160)

Usable Host Range: 192.168.10.129 to 192.168.10.158 (30 usable hosts).