DATA COMMUNICATION (EC-603 (A)) - Unit 1 Short Notes

UNIT 1: DATA COMMUNICATION

1.0 INTRODUCTION TO DATA COMMUNICATION

1.1 Definition, Components, and Characteristics

• Definition: The transmission of data (digital or analog) from a source to a destination through a communication medium.

• Core Components:

  1. Source (Sender): Generates data to be sent (e.g., computer, sensor).

  2. Transmitter: Converts data into transmittable signals (e.g., modem, network interface card).

  3. Transmission Medium: Physical path for signal travel (guided: cable; unguided: air).

  4. Receiver: Converts received signal back into data.

  5. Destination (Sink): Intended recipient of the data.

• Key Characteristics:

  • Delivery: Accuracy and completeness.

  • Timeliness: On-time delivery (real-time vs. store-and-forward).

  • Jitter: Variation in packet arrival time (critical for streaming).

  • Throughput: Actual data transfer rate (bps).

  • Reliability: Probability of error-free transmission.

1.2 Data Representation and Signals

• Analog Signals: Continuous waveforms (e.g., voice, sine wave). Represented by amplitude, frequency, phase.

• Digital Signals: Discrete, binary pulses (0s and 1s). Represented by voltage levels (e.g., 0V for 0, +5V for 1).

• Conversion:

  • Digital-to-Analog (Modulation): Modem. Techniques: ASK, FSK, PSK.

  • Analog-to-Digital (Sampling): PCM (Pulse Code Modulation). Nyquist Theorem: Sampling rate ≥ 2 × max frequency.

1.3 Transmission Modes

1.4 Transmission Directions

• Serial Transmission: Bits sent sequentially over a single path. Used for long distances (e.g., USB, SATA, network links). Lower cost, less error-prone.

• Parallel Transmission: Multiple bits sent simultaneously over multiple paths. Used for short distances (e.g., internal computer buses, printer ports). Higher speed, more crosstalk.

[!TIP] Exam Focus: Be prepared to draw waveforms for NRZ, RZ, etc., and differentiate serial vs. parallel with real-world examples.

2.0 NETWORK FUNDAMENTALS & TOPOLOGY

2.1 Network Topology Concepts

• Definition: The physical or logical arrangement of nodes (devices) and links (connections) in a network.

• Common Physical Topologies:

2.2 Network Classification (by geographical scope)

2.3 Internetworking

• Connecting multiple networks of different types (e.g., LAN to WAN) using routers or gateways.

• Requires common protocol suites (e.g., TCP/IP) and addressing schemes (e.g., IP addresses).

3.0 TRANSMISSION MEDIA & LINE ENCODING

3.1 Guided Media

1. Twisted Pair (UTP/STP): Two insulated copper wires twisted. UTP (Cat5e/6) for LANs; STP has shielding for noisy environments. Limited bandwidth (~100 MHz), susceptible to EMI.

2. Coaxial Cable: Central conductor, insulator, shield, jacket. Baseband (single signal, e.g., Ethernet) vs. Broadband (multiple signals, e.g., cable TV). Better shielding than TP, higher bandwidth (~500 MHz).

3. Fiber Optic: Glass/plastic core, cladding. Light pulses transmit data.

   • Single-mode: Small core, one light ray. Long distance, high bandwidth (100+ Gbps).

   • Multi-mode: Larger core, multiple rays. Shorter distance, cheaper.

   • Advantages: Immune to EMI, high security, low attenuation, high bandwidth.

3.2 Unguided Media (Wireless)

• Radio Waves: Omnidirectional, penetrate walls. Used for Wi-Fi, Bluetooth, cellular.

• Microwave: Directional, line-of-sight (LOS). Terrestrial (towers) & Satellite (GHz range). Requires repeaters.

• Infrared: Short range, LOS, cannot penetrate walls. Used for remote controls, IrDA.

• Satellite: Microwave-based; covers vast areas, high latency (~270 ms for GEO).

3.3 Transmission Impairments

3.4 Line Encoding Schemes

• Purpose: Convert binary data into digital signals for baseband transmission. Defines voltage/current transitions.

• Schemes:

[!TIP] Exam Tip: Be ready to encode a given bit sequence (e.g., 1011001) using NRZ-L, NRZ-I, AMI, and explain HDB3 substitution rules.

4.0 REFERENCE MODELS

4.1 OSI Seven-Layer Model

• Encapsulation: Data → (Presentation) → (Session) → (Transport: Segment) → (Network: Packet) → (Data Link: Frame) → (Physical: Bits).

• Layer-to-Layer Interaction: Each layer adds its header (and sometimes trailer) to the PDU from the layer above. The receiving peer layer removes the header/trailer.

4.2 TCP/IP Protocol Suite (Four-Layer Model)

• Comparison with OSI:

  • TCP/IP is practical, protocol-centric; OSI is theoretical, service-centric.

  • TCP/IP combines OSI's Session/Presentation/Application into one Application layer.

  • TCP/IP's Link layer maps to OSI's Data Link + Physical.

  • TCP/IP protocols (e.g., TCP, IP) were developed before the OSI model.

[!TIP] Common Pitfall: Don't confuse PDU names (Segment/ Packet/ Frame/ Bits) and devices operating at each layer. Remember: Router = Network Layer (IP), Switch = Data Link Layer (MAC).

5.0 SWITCHING TECHNIQUES

[!TIP] Key Comparison: Circuit Switching = Dedicated path (like a private train). Packet Switching (Datagram) = Independent letters (each with full address). Packet Switching (Virtual Circuit) = Pre-addressed envelopes with a route number.

6.0 ERROR DETECTION AND CORRECTION

6.1 Types of Errors

• Single-Bit Error: One bit flipped (e.g., 0→1).

• Burst Error: Contiguous sequence of bits in error (length = burst error length). More common in noisy channels.

6.2 Error Detection Mechanisms

• Concept: Add redundant bits (check bits) to data. Receiver uses same algorithm to check consistency.

[!TIP] CRC Example (Exam Critical): Given data 11010101 and CRC-8 polynomial x⁸ + x² + x + 1 → Generator 100000111. Show division steps. Box final CRC remainder.

6.3 Error Correction Fundamentals

• Forward Error Correction (FEC): Sender adds enough redundancy (e.g., Hamming code) so receiver can correct errors without retransmission. Used in real-time systems (satellite, deep space).

• Automatic Repeat Request (ARQ): Receiver detects error (via CRC) and requests retransmission. Requires feedback channel. Types: Stop-and-Wait, Go-Back-N, Selective Repeat.

7.0 DATA LINK LAYER PROTOCOLS

7.1 Framing

• Purpose: Define packet boundaries (start/end) for the Data Link layer. Separates incoming data into frames.

• Methods:

  1. Character-Oriented (Byte Stuffing): Uses special flag byte (e.g., 0x7E or ESC). If flag appears in data, insert escape character (ESC) before it. Receiver removes ESC. Used in PPP (for control frames) and old protocols (BISYNC).

  2. Bit-Oriented (Flag Stuffing): Uses special flag pattern (e.g., 01111110 for HDLC). If 5 consecutive 1s appear in data, insert a 0 bit (bit stuffing). Receiver removes stuffed 0 after 5 consecutive 1s.

• Role in Error Detection: Frame delimiters ensure the receiver knows where to apply CRC check. Stuffed bits/bytes are removed before CRC verification.

7.2 High-Level Data Link Control (HDLC)

• Features: Bit-oriented, full-duplex, synchronous, supports point-to-point and multidrop (secondary stations). Frame-based.

• Frame Structure:

  
  [Flag: 01111110] [Address] [Control] [Information (optional)] [FCS (CRC-16/32)] [Flag: 01111110]
  
  

  • Flag: 01111110 (start/end).

  • Address: Identifies secondary station (in multidrop).

  • Control: Frame type & sequence number (for ARQ).

  • FCS: Frame Check Sequence (CRC).

• Modes of Operation:

  • Normal Response Mode (NRM): Primary station controls token; secondary responds only when polled. (Used on multidrop lines).

  • Asynchronous Response Mode (ARM): Secondary can transmit without permission, but primary retains responsibility for line recovery.

  • Asynchronous Balanced Mode (ABM): Most common. Both stations are combined (peer-to-peer). Either can transmit anytime. Used in point-to-point links (e.g., PPP's HDLC-like framing).

7.3 Automatic Repeat Request (ARQ) Protocols

• Stop-and-Wait:

  • Sender sends one frame, stops and waits for ACK.

  • Efficiency = 1 / (1 + 2a) where a = Propagation Time / Transmission Time.

  • Problem: Very low utilization if propagation delay >> transmission time.

• Sliding Window:

  • Sender can send multiple frames before needing ACK. Window size = max unacknowledged frames.

  • Go-Back-N (GBN): Receiver discards out-of-order frames and requests retransmission of the frame where sequence broke. Sender re-sends all subsequent frames from error point. Simpler receiver, more retransmission.

  • Selective Repeat (SR): Receiver buffers out-of-order frames. Sender only retransmits specific errored frames. More complex receiver, less retransmission, higher efficiency.

  • Window Size Constraint: For GBN: Ws ≤ 2ⁿ - 1; for SR: Ws + Wr ≤ 2ⁿ (n = bits in sequence number).

7.4 Point-to-Point Protocol (PPP) - Brief

• Purpose: Data link protocol for direct connection between two nodes (e.g., dial-up, DSL).

• Phases: Link establishment (LCP), Authentication (PAP/CHAP), Network layer protocol (NCP), Link termination.

• Frame Format: Flag, Address (0xFF), Control (0x03), Protocol (identifies network layer packet), Data, FCS, Flag.

• Features: Supports multiple protocols (IP, IPX), authentication, error detection (CRC), but no error correction (assumes reliable underlying layer).

8.0 LOCAL AREA NETWORK (LAN) TECHNOLOGIES

8.1 Ethernet (IEEE 802.3)

• Access Method: CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

  • Procedure: Listen → If idle, Transmit → While transmitting, Listen for collision → If collision, Jam signal → Backoff (random time) → Retry.

  • Contention Slot Time: 512 bits (51.2 µs for 10 Mbps). Ensures collision detection across entire network.

  • Efficiency: High for light load, degrades with load. Modern switched Ethernet is full-duplex → no collisions.

• Frame Structure:

  
  [Preamble (7B)] [SFD (1B)] [Dest MAC (6B)] [Src MAC (6B)] [Type/Length (2B)] [Data (46-1500B)] [FCS (4B)]
  
  

  • Preamble + SFD: Clock synchronization.

  • Type: Indicates upper layer protocol (e.g., 0x0800 = IPv4).

  • FCS: CRC-32.

8.2 Token-Based LANs

• Token Ring (IEEE 802.5):

  • Architecture: Physical star, logical ring. Multistation Access Unit (MAU) connects nodes.

  • Operation: A token (3-byte frame: start, control, end) circulates. Station with token can transmit. After transmission, it releases a new token. Deterministic access.

  • Frame Format: Similar to Ethernet but with access control field.

• Token Bus (IEEE 802.4):

  • Architecture: Physical bus, logical ring. Stations ordered by logical address.

  • Operation: Token passed in logical order (not physical). Station with token transmits for max time (T_max), then passes token to next logical neighbor.

  • Comparison:

    | Feature | Token Ring | Token Bus | | :--- | :--- | :--- | | Physical Topology | Star | Bus | | Logical Topology | Ring | Ring | | Access Control | Token passing (physical order) | Token passing (logical order) | | Fault Tolerance | MAU can bypass failed station | Bus break disrupts entire ring | | Complexity | Higher (ring maintenance) | Simpler (like bus) |

8.3 Fiber Distributed Data Interface (FDDI)

• Architecture: Dual counter-rotating rings (primary & secondary). Data flows in opposite directions on each ring.

• Operation:

  • Normal: Data on primary ring, secondary idle (backup).

  • Fault: If primary ring breaks, stations wrap traffic to secondary ring, forming a single ring (fault tolerance).

• Features: High speed (100 Mbps), long distance (up to 200 km stations, 100 km between), uses token passing (like Token Ring), 4B/5B encoding.

8.4 Wireless LANs (IEEE 802.11) - Brief

• Architecture: Infrastructure mode (with Access Point) or Ad-hoc (peer-to-peer).

• Access Method: CSMA/CA (Collision Avoidance). Uses RTS/CTS (optional) and DIFS/SIFS inter-frame spaces to avoid collisions (since collision detection is hard in wireless).

• Key Standards: 802.11b/g/n (2.4 GHz), 802.11a/n/ac/ax (5 GHz). Wi-Fi 6 (802.11ax) introduces OFDMA.

9.0 METROPOLITAN AREA NETWORK (MAN) TECHNOLOGIES

9.1 Distributed Queue Dual Bus (DQDB) - IEEE 802.6

• Architecture: Two unidirectional buses (Bus A & Bus B) running in opposite directions. Each station connects to both buses.

• Access Control: Distributed Queue.

  • Each station maintains a request counter and a countdown counter.

  • To send on Bus A, station places a request on Bus B. All stations see this request.

  • Requests are queued in distributed fashion based on station position (closer to head of bus gets priority).

  • Station decrements its countdown counter when it sees a request ahead of it. When countdown reaches 0, it can transmit.

• Operation in MAN: Provides asynchronous, connectionless data service over city-wide fiber infrastructure. Supports both isochronous (voice/video) and non-isochronous traffic.

• Key Feature: Fairness through distributed queue; no central controller.

9.2 Switched Multi-megabit Data Service (SMDS)

• Connectionless, packet-switched service for MANs.

• Uses 53-byte cells (like ATM) over high-speed networks.

• Provides high throughput and low delay for bursty data.

• No call setup; each packet carries full destination address.

9.3 Comparison: DQDB vs. SMDS

10.0 WIDE AREA NETWORK (WAN) & CELL-BASED TECHNOLOGIES

10.1 Asynchronous Transfer Mode (ATM)

• Cell-Based Architecture: Fixed-size 53-byte cells (5-byte header, 48-byte payload).

  • Header Fields: VPI (Virtual Path Identifier), VCI (Virtual Circuit Identifier), PT (Payload Type), CLP (Cell Loss Priority), HEC (Header Error Control).

• Virtual Paths (VP) & Virtual Circuits (VC):

  • VC: End-to-end connection identifier (VCI) within a VP.

  • VP: Group of VCs sharing the same path; allows bulk management.

• Switching Mechanisms: Cell switching (similar to packet switching but fixed size). Fast hardware switching due to small, fixed cells. Supports PVCs (Permanent VCs) and SVCs (Switched VCs).

• Key Feature: QoS classes (CBR, VBR, ABR, UBR) for different traffic types (voice, video, data).

10.2 Frame Relay

• Operation: Simplified, high-performance packet-switched WAN protocol. Operates at Physical + Data Link layers.

• Frame Structure: Flag, Address (DLCI - Data Link Connection Identifier), Control, Information, FCS, Flag.

  • DLCI: Locally significant VC identifier.

• Efficiency: Minimal error detection (only FCS), no retransmission (assumes reliable underlying physical layer). Congestion bits (FECN, BECN) for flow control.

• Limitations: No flow control at data link layer, best-effort service (no guarantees), sensitive to congestion.

10.3 Synchronous Optical Network (SONET) / Synchronous Digital Hierarchy (SDH)

• Purpose: Standard for high-speed fiber optic transmission (starting at 51.84 Mbps - STS-1/OC-1).

• Key Concept: Synchronous byte-interleaved multiplexing of lower-rate signals (e.g., DS1, DS3).

• Frame Structure: STS-1 frame = 9 rows × 90 columns (810 bytes), transmitted every 125 µs. Section, Line, Path overhead bytes for management.

• Features: High reliability (automatic protection switching), built-in OAM (Operations, Administration, Maintenance), supports ATM over SONET.

10.4 X.25 Protocol Suite

• Legacy WAN protocol for packet-switched networks over unreliable links (e.g., analog phone lines).

• Three Layers: Physical (X.21), Data Link (LAPB - Link Access Procedure, Balanced), Packet Layer (virtual circuits).

• Key Feature: Extensive error correction and flow control at each node (store-and-forward). Very reliable but slow due to multiple error checks. Largely replaced by Frame Relay and ATM.

11.0 NETWORKING DEVICES & INTERNETWORKING

[!TIP] Mnemonic: Repeater/Hub (Physical), Bridge/Switch (Data Link), Router (Network), Gateway (Application). Switches break collision domains; Routers break broadcast domains.

12.0 ROUTING ALGORITHMS

12.1 Routing Fundamentals

• Goal: Find least-cost path from source to destination.

• Routing Table: Contains [Destination Network, Next Hop, Cost/Metric, Interface].

• Types:

  • Static: Manually configured. Simple, secure, but not adaptive.

  • Dynamic: Routers exchange information, automatically update tables. Distance Vector & Link State.

12.2 Distance Vector Routing (Bellman-Ford)

• Principle: Each router knows distance (cost) to each destination and the next-hop vector. Shares its entire table with direct neighbors periodically.

• Algorithm (Router X):

  1. Receive neighbor's distance vector D_Y.

  2. For each destination j: D_X(j) = min [ c(X,i) + D_i(j) ] for all neighbors i.

  3. Update own table and send new vector to neighbors.

• Example Protocol: RIP (Routing Information Protocol). Metric = hop count (max 15). Updates every 30 sec.

• Problems:

  • Count-to-Infinity: Slow convergence on link failure; metric increases gradually to infinity.

  • Routing Loops: Temporary loops before convergence.

  • Large Updates: Sends full table periodically (even if unchanged).

• Solutions: Split Horizon, Poison Reverse, Hold-down Timers.

12.3 Link State Routing (Dijkstra's)

• Principle: Each router has complete map of network topology (link costs). Runs Dijkstra's SPF (Shortest Path First) algorithm independently to compute shortest paths.

• Steps:

  1. Flooding: Each router broadcasts Link State Advertisements (LSAs) to all other routers (reliable flooding). LSAs describe router's links and costs.

  2. Database: All routers build identical Link State Database (LSDB).

  3. SPF Calculation: Each router runs Dijkstra on LSDB to compute shortest path tree with itself as root.

  4. Routing Table: Populated from shortest path tree.

• Example Protocol: OSPF (Open Shortest Path First). Uses areas, authentication, faster convergence.

• Comparison with Distance Vector:

[!TIP] Exam Focus: Be able to run Dijkstra's algorithm on a small graph (5-6 nodes). Know the step-by-step: initialize, find min-cost unvisited node, update neighbors, repeat. For Bellman-Ford, show the iteration table for a small network.