DATA COMMUNICATION (EC-603 (A)) - Unit 4 Short Notes

UNIT 4: DATA COMMUNICATION

4.1 Foundations of Data Communication

• Definition: The process of transmitting data from one device to another through a communication medium.

• Characteristics: Delivery, Accuracy, Timeliness, Jitter.

• Components: Sender, Receiver, Medium, Protocol.

• Transmission Modes:

  • Simplex: One-way communication (e.g., TV broadcast).

  • Half-Duplex: Two-way but not simultaneous (e.g., walkie-talkie).

  • Full-Duplex: Simultaneous two-way (e.g., telephone).

• Serial vs Parallel:

  • Serial: Bits sent sequentially over one channel (long distance, e.g., USB).

  • Parallel: Multiple bits sent simultaneously (short distance, e.g., printer port).

• Network Classification:

  • PAN (Personal Area Network): ~10 m (Bluetooth).

  • LAN (Local Area Network): ~1–5 km (office, Ethernet).

  • MAN (Metropolitan Area Network): ~5–50 km (city, DQDB).

  • WAN (Wide Area Network): Country/globe (Internet, ATM).

• Network Topologies:

  | Topology | Description | Advantages | Disadvantages | |----------|-------------|------------|---------------| | Bus | Single central cable | Simple, cheap | Single point of failure, collisions | | Star | All nodes connect to hub/switch | Easy to manage, robust | Hub failure breaks network | | Ring | Nodes form closed loop | No collisions, orderly | Single node failure breaks ring | | Mesh | Every node connected to others | High redundancy, fault-tolerant | Expensive, complex | | Tree | Hierarchical (bus + star) | Scalable, easy to debug | Root node failure affects all | | Hybrid | Combination of above | Flexible | Complex design |

[!TIP] Exam Focus: Topology comparison and transmission mode examples are frequent 7-mark questions.

4.2 Physical Layer Fundamentals

• Guided Media:

  • Twisted Pair: UTP/STP, up to 100 Mbps, susceptible to EMI.

  • Coaxial Cable: Thick/thin, higher bandwidth, better shielding.

  • Fiber Optic: Light pulses, very high speed (Gbps), low loss, immune to EMI.

• Unguided Media: Radio (Wi-Fi), Microwave (point-to-point), Infrared (short-range).

• Transmission Impairments:

  • Attenuation: Signal strength loss → use amplifiers/repeaters.

  • Distortion: Signal shape change → use equalization.

  • Noise: Unwanted energy (thermal, crosstalk) → shielding, filtering.

  • Interference: From other signals → frequency planning, spread spectrum.

• Line Encoding Schemes:

  | Scheme | Description | Example | |--------|-------------|---------| | Unipolar NRZ | 0=0V, 1=+V | DC component issue | | Polar NRZ-L | 0=+V, 1=-V | No DC if balanced | | Bipolar AMI | 0=0, 1 alternates ±V | No DC, error detection | | Manchester | Transition in middle (0=↑,1=↓) | Self-clocking, Ethernet | | Differential Manchester | Mid-bit transition always, presence/absence at start | Token Ring |

[!TIP] Common Pitfall: Confusing NRZ-L with NRZ-I. Manchester encoding includes clocking information.

4.3 Switching Techniques

• Circuit Switching: Dedicated path established (e.g., PSTN). Guaranteed bandwidth, inefficient for bursty traffic.

• Packet Switching:

  • Datagram (e.g., IP): Each packet independent, route may differ. No connection setup.

  • Virtual Circuit (e.g., Frame Relay): Path established before data, packets follow same route.

• Message Switching: Store-and-forward entire message. No dedicated path, but high delay.

• Comparison:

  | Feature | Circuit | Packet (Datagram) | Packet (VC) | Message | |---------|---------|-------------------|-------------|---------| | Path | Dedicated | Dynamic | Predefined | Store-forward | | Delay | Low (after setup) | Variable | Low | High | | Efficiency | Low | High | High | Medium | | Use | Telephony | Internet | Frame Relay | Email (historical) |

[!TIP] Exam Key: Differentiate VC vs Datagram by connection establishment and packet ordering.

4.4 Network Architecture Models

• OSI Reference Model (7 Layers):

  1. Physical: Bits over medium.

  2. Data Link: Frames, error control (HDLC, Ethernet).

  3. Network: Routing, IP addresses.

  4. Transport: End-to-end, TCP/UDP.

  5. Session: Dialog control.

  6. Presentation: Encryption, compression.

  7. Application: User interface (HTTP, FTP).

• TCP/IP Model (4 Layers):

  1. Link (Network Interface): Corresponds to OSI Physical + Data Link.

  2. Internet: OSI Network (IP).

  3. Transport: OSI Transport (TCP, UDP).

  4. Application: OSI Session + Presentation + Application.

• OSI vs TCP/IP:

  | Aspect | OSI | TCP/IP | |--------|-----|--------| | Layers | 7 | 4 | | Protocol Development | Theory-first | Implementation-first | | Communication | Horizontal (peer-to-peer) | Vertical (top-down) | | Examples | HDLC, X.25 | TCP, IP, HTTP |

[!TIP] Mnemonic: OSI layers: Please Do Not Throw Sausage Pizza Away.

4.5 Data Link Layer

5.1 Framing

• Purpose: Delimit frame boundaries for error detection and addressing.

• Methods:

  • Character-Oriented (BISYNC): Flag bytes (DLE, STX/ETX).

  • Bit-Oriented (HDLC): Flag sequence 01111110 with bit stuffing.

  • Length-Based: Length field in header.

• Role: Enables receiver to synchronize and identify frame start/end.

5.2 Error Detection and Correction

• Parity Check:

  • Single-bit: Even/odd parity for each byte. Detects single-bit errors.

  • Two-Dimensional: Block parity (row + column). Detects burst errors.

• Checksum: Sum of data blocks (1’s complement). Used in IP/TCP.

• Cyclic Redundancy Check (CRC):

  • Concept: Polynomial division modulo-2. Data as dividend, generator polynomial $G(x)$.

  • Steps:

    1. Append $r$ zeros (where $r$ = degree of $G(x)$).

    2. Divide by $G(x)$ using XOR.

    3. Remainder = CRC bits.

    4. Transmit data + CRC.

  • Example: For $$\displaystyle G(x) = x^3 + 1 $$ (binary 1001), data 1010 → CRC 001 → transmitted 1010001.

  • Detection: All single-bit, all double-bit, all odd-numbered errors, burst < $r+1$ bits.

• Longitudinal Redundancy Check (LRC):

  • Organize data in matrix, compute column-wise parity.

  • Example: Data block:

    
    1011001
    
    1100110
    
    0011011
    
    

    LRC = XOR of columns → 0100100. Transmit block + LRC row.

  • Detects burst errors affecting one column.

[!TIP] Exam Calculation: Practice CRC with polynomials like $$\displaystyle x^8 + x^2 + x + 1 $$ (100000111). Show division steps.

5.3 Error Control Protocols

• Automatic Repeat Request (ARQ): Receiver requests retransmission on error.

• Stop-and-Wait:

  • Sender sends one frame, waits for ACK.

  • Efficiency $$\displaystyle \eta = \frac{1}{1 + 2a} $$ where $$\displaystyle a = \frac{\text{propagation time}}{\text{frame transmission time}} $$.

  • Low utilization for long propagation delays.

• Sliding Window:

  • Go-Back-N: Sender can send up to $N$ frames without ACK. On error, retransmit from error frame onward.

  • Selective Repeat: Retransmit only erroneous frames. Requires larger window, buffering.

  • Efficiency: Higher than Stop-and-Wait, window size $N$ affects throughput.

5.4 Data Link Protocols

• High-Level Data Link Control (HDLC):

  • Frame Structure:

    
    Flag (01111110) | Address | Control | Info | FCS | Flag
    
    

  • Modes:

    • NRM (Normal Response): Primary/secondary stations.

    • ABM (Asynchronous Balanced): Balanced configuration (most common).

    • ARM (Asynchronous Response): Secondary can transmit anytime.

  • Features: Bit stuffing, 3-bit sequence numbers, supervisory frames (RR, RNR, REJ), unnumbered frames (SABM, DISC).

[!TIP] HDLC Control Field: Bits 1-2 indicate frame type (I-frame, S-frame, U-frame).

4.6 Local Area Networks (LAN)

• Ethernet (IEEE 802.3):

  • CSMA/CD: Carrier Sense Multiple Access with Collision Detection.

  • Operation: Listen before transmit, collision → backoff (binary exponential).

  • Frame: Preamble, Dest/Src MAC, Type, Data, FCS.

• Token Ring (IEEE 802.5):

  • Operation: Token passed sequentially. Station with token transmits.

  • Advantages: Deterministic access, no collisions.

  • Disadvantages: Token overhead, single point failure (ring break).

• Token Bus (IEEE 802.4):

  • Token passed in logical order on physical bus.

  • Comparison with Token Ring:

    | Feature | Token Ring | Token Bus | |---------|------------|-----------| | Topology | Ring | Bus | | Access | Physical order | Logical order | | Fault tolerance | Medium-dependent | Easier to add/remove nodes |

• FDDI (Fiber Distributed Data Interface):

  • Architecture: Dual counter-rotating rings (primary/secondary).

  • Fault Tolerance: Ring wraps on failure (single ring operation).

  • Speed: 100 Mbps over fiber.

  • Use: Backbone networks, high-speed LANs.

[!TIP] Token Passing: Token Ring uses physical order; Token Bus uses logical addressing order.

4.7 Metropolitan Area Networks (MAN)

• DQDB (Distributed Queue Dual Bus, IEEE 802.6):

  • Architecture: Two unidirectional buses (A and B). Each station queues requests.

  • Operation:

    • Upstream (bus A): Stations request slots on downstream (bus B).

    • Distributed Queue: Requests queued in order of arrival.

    • Slot Reuse: After destination reads, slot available for downstream.

  • Advantages: Fair access, supports isochronous traffic.

• SMDS (Switched Multi-megabit Data Service):

  • Connectionless, datagram service over MAN.

  • Comparison with DQDB:

    | Feature | DQDB | SMDS | |---------|------|------| | Access | Distributed queue | Stateless | | Service | Connection-oriented-like | Pure datagram | | Use | Integrated services | High-speed data |

4.8 Network Layer

• Routing Algorithms:

  • Distance Vector (Bellman-Ford):

    • Each node maintains distance vector (cost to destination).

    • Update: $$\displaystyle D_x(y) = \min_v \{ \text{cost}(x,v) + D_v(y) \} $$.

    • Example: Node A updates via neighbor B: $$\displaystyle D_A(C) = \min( \text{cost}(A,B)+D_B(C), ... ) $$.

    • Problems: Count-to-infinity, slow convergence.

  • Link State (Dijkstra):

    • Each node has complete map (LSA flood).

    • Algorithm: Shortest path first (SPF).

    • Steps:

      1. Initialize: $$\displaystyle N' = \{ \text{self} \} $$, $$\displaystyle D(v) = \text{cost}(self,v) $$.

      2. Find $w \notin N'$ with min $D(w)$, add to $N'$.

      3. Update $D(v)$ for neighbors: $$\displaystyle D(v) = \min(D(v), D(w) + \text{cost}(w,v)) $$.

      4. Repeat until all nodes in $N'$.

    • Advantage: Fast convergence, no count-to-infinity.

• Internet Protocol (IP):

  • Addressing: IPv4 (32-bit), IPv6 (128-bit).

  • Packet Format: Version, IHL, Type of Service, Total Length, Identification, Flags, Fragment Offset, TTL, Protocol, Header Checksum, Source/Dest IP, Options, Data.

[!TIP] Routing Comparison: Distance vector shares entire table periodically; link state shares only its own links.

4.9 Wide Area Networks and Protocols

• ATM (Asynchronous Transfer Mode):

  • Cell Structure: Fixed 53 bytes (5-byte header, 48-byte payload).

  • Switching: Virtual circuit (VPI/VCI). Cell relay, high speed (155 Mbps+).

  • Advantages: QoS support, low delay, scalability.

  • Disadvantages: Overhead from small cells, complex.

• Frame Relay:

  • Operation: Packet switching over virtual circuits. No error correction (rely on upper layers).

  • Efficiency: High throughput, low overhead (2–4 bytes header).

  • Limitations: No flow control, bursty traffic can cause congestion.

• X.25:

  • Early packet switching protocol. Three layers: Physical, Data Link (LAPB), Packet (virtual circuits).

  • Reliable but high overhead → obsolete for backbone.

• SONET (Synchronous Optical Network):

  • Architecture: Hierarchical (STS-1: 51.84 Mbps, STS-3: 155.52 Mbps).

  • Layers: Section, Line, Path.

  • Features: Synchronous multiplexing, ring protection (UPSR, BLSR).

4.10 Network Devices

[!TIP] Key Difference: Switch operates at Data Link (MAC), Router at Network (IP). Hub is physical layer, broadcasts to all ports.

4.11 Additional Topics and Standards

• Standards Organizations:

  • ISO: OSI model.

  • IEEE: 802.x (Ethernet, Token Ring).

  • ITU-T: X.25, ISDN.

  • ANSI: US standards (e.g., T1 lines).

• TCP/IP Protocol Suite:

  • Layers:

    • Application: HTTP, FTP, SMTP, DNS.

    • Transport: TCP (reliable), UDP (unreliable).

    • Internet: IP (routing), ICMP (errors), ARP (MAC resolution).

    • Link: Ethernet, PPP, Wi-Fi.

  • Importance: Foundation of Internet, interoperable, scalable.

• Internetworking: Connecting multiple networks using routers and standard protocols (IP).

[!TIP] TCP vs UDP: TCP = connection-oriented, flow control, congestion control; UDP = connectionless, no reliability.

Key Formulas & Concepts Boxed

• Stop-and-Wait Efficiency: $$\displaystyle \boxed{\eta = \frac{1}{1 + 2a}} $$, where $$\displaystyle a = \frac{\text{propagation time}}{\text{frame transmission time}} $$.

• CRC Generation: Data $D(x)$ appended with $r$ zeros, divide by $G(x)$, remainder $R(x)$ → transmitted $$\displaystyle D(x) \cdot x^r + R(x) $$.

• Dijkstra’s Algorithm: $$\displaystyle \boxed{D(v) = \min(D(v), D(w) + \text{cost}(w,v))} $$ for each neighbor $v$ of $w$.

• Ethernet Minimum Frame Size: $\boxed{512 \text{ bits}}$ (for 10 Mbps, 51.2 µs slot time).

• ATM Cell Rate: $\boxed{53 \text{ bytes/cell}}$ (5 header + 48 payload).

Frequently Asked Diagrams

1. OSI Model: Seven layers with peer communication.

2. HDLC Frame: Flag, Address, Control, Info, FCS, Flag.

3. Ethernet Frame: Preamble, Dest MAC, Src MAC, Type, Data, FCS.

4. FDDI Dual Ring: Primary (clockwise), Secondary (counter-clockwise).

5. DQDB Architecture: Two unidirectional buses with stations tapping both.

6. ATM Cell: 5-byte header (VPI/VCI, PT, CLP, HEC), 48-byte payload.

7. Sliding Window: Sender/receiver windows, sequence numbers.

[!TIP] Exam Strategy: For 7-mark questions, always include diagram + explanation. For calculations (CRC, routing), show step-by-step working. Compare using tables for clarity.