DATA COMMUNICATION (EC-603 (A)) - Unit 5 Short Notes

1.0 FUNDAMENTALS OF DATA COMMUNICATION

Core Components: Source → Transmitter → Transmission Medium → Receiver → Destination.
Characteristics:

• Delivery: Correct destination, timing.

• Accuracy: Error-free data.

• Timeliness: Within specified delay.

• Jitter: Variation in packet delay.
  Data Flow Modes:

• Simplex: One-way (e.g., keyboard → CPU).

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

• Full-duplex: Simultaneous two-way (e.g., telephone).
  Standards Organizations:

• ISO: OSI model.

• ITU-T: Telecom standards (e.g., X.25).

• IEEE: LAN/MAN standards (802.x).

• ANSI: US standards.

2.0 TRANSMISSION MEDIA & IMPAIRMENTS

Guided Media:

• Single-mode: Long distance, laser source.

• Multi-mode: Short distance, LED source.

Unguided Media: Radio waves (omnidirectional), Microwaves (directional, parabolic), Infrared (short-range, line-of-sight).

Transmission Impairments:

1. Attenuation: Signal strength loss → Mitigation: Amplifiers/repeaters.

2. Noise:

   • Thermal (random electron motion).

   • Induced (from external sources).

   • Crosstalk (adjacent wire interference) → Mitigation: Shielding, twisted pairs.

3. Distortion: Signal shape change → Mitigation: Equalizers.

3.0 SIGNAL ENCODING & LINE CODING

Digital-to-Digital Encoding:

Analog-to-Digital (PCM):

• Sampling Theorem (Nyquist): $$\displaystyle f_s \ge 2B $$, where $B$ = max frequency in Hz.

• Quantization → Encoding.

Digital-to-Analog:

• ASK: Carrier amplitude varies.

• FSK: Carrier frequency varies.

• PSK: Carrier phase varies (e.g., BPSK, QPSK).

• QAM: Amplitude + phase variation (e.g., 16-QAM).

Analog-to-Analog: AM, FM, PM (for radio/TV).

4.0 NETWORK TOPOLOGIES & CLASSIFICATION

Physical Topologies:

• Bus: Single cable, terminators at ends (e.g., 10BASE5).

• Star: Central hub/switch, point-to-point links.

• Ring: Closed loop, token passing (e.g., Token Ring).

• Mesh: Fully interconnected (high reliability, high cost).

• Tree: Hierarchical bus/star combination.

• Hybrid: Mix of two or more.

Logical Topology: Path data takes (e.g., Ethernet logical bus, physical star).

Network Classification:

5.0 OSI REFERENCE MODEL

Seven Layers:

Key Processes:

• Encapsulation: Data → Segment → Packet → Frame → Bits.

• Decapsulation: Reverse at receiver.

• Peer-to-Peer: Same-layer entities communicate via headers.

• Service Primitives: Request, Indication, Response, Confirm.

6.0 SWITCHING TECHNIQUES

7.0 DATA LINK LAYER: FRAMING & ERROR CONTROL

Framing Methods:

• Character-oriented: Flag (DLE, STX/ETX) + byte stuffing.

• Bit-oriented: Flag (01111110) + bit stuffing (HDLC).

• Length-based: Length field in header.

Error Detection:

CRC Example (Polynomial $$\displaystyle x^8 + x^2 + x + 1 $$ → 100000111):

Data: 11010101 → Append 8 zeros → 1101010100000000

Divide by 100000111 → Remainder = 01110010 (CRC-8).

Transmit: 11010101 01110010.

LRC Example (Data block):

10100101
11001010
00111100

Row parity: Append 0 to each row (even parity).

Column parity: 01111011 (bottom row).

LRC code = 01111011.

Error Correction:

• FEC: Add redundancy (e.g., Hamming code).

• ARQ:

  • Stop-and-Wait: Send frame, wait for ACK → Efficiency $$\displaystyle \eta = \frac{1}{1+2a} $$, $$\displaystyle a = \frac{t_{prop}}{t_{trans}} $$.

  • Go-Back-N: Sliding window, retransmit from error frame.

  • Selective Repeat: Retransmit only erroneous frames.

8.0 DATA LINK LAYER PROTOCOLS

HDLC (High-Level Data Link Control):

• Frame Format:

  Flag (01111110) | Address | Control | Data | FCS (CRC) | Flag.

• Control Field:

  • I-frame (01): Info + seq/ack (flow/error control).

  • S-frame (00): Control only (ACK/NAK).

  • U-frame (11): Management (setup/teardown).

• Modes:

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

  • ABM (Asynchronous Balanced Mode): Balanced (peer-to-peer).

  • ARM (Asynchronous Response Mode): Secondary can transmit without poll.

• Features: Full-duplex, error control (ARQ), flow control.

Sliding Window Protocol:

• Sender window $$\displaystyle W_s $$, receiver window $$\displaystyle W_r $$.

• Efficiency: $$\displaystyle \eta = \frac{W}{1+2a} $$ for $$\displaystyle W \le 2^m $$ ($m$ = seq bits).

• Piggybacking: ACK carried in data frames (bidirectional).

Multiple Access:

• ALOHA: Pure (vulnerable period $$\displaystyle 2t_{trans} $$), Slotted (synchronized).

• CSMA/CD: Carrier Sense, Collision Detect (Ethernet).

9.0 LOCAL AREA NETWORK (LAN) TECHNOLOGIES

Ethernet (IEEE 802.3):

• CSMA/CD: Listen before talk, abort on collision.

• Frame Format: Preamble | Dest MAC | Src MAC | Type | Data | Pad | FCS.

• Physical Layers: 10BASE-T (UTP), 10BASE-F (fiber).

Token Bus (IEEE 802.4):

• Physical bus, logical ring (token passed by station ID).

• Advantages: Deterministic access.

• Disadvantages: Complex token management, station failure issues.

Token Ring (IEEE 802.5):

• Physical star (MAU), logical ring.

• Token passing, priority bits, reservation.

• Frame format: SD | AC | FC | DA | SA | Data | FCS | ED | FS.

• Active monitor: Manages ring (token generation).

FDDI (Fiber Distributed Data Interface):

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

• Fault tolerance: Ring wrap on failure.

• Speed: 100 Mbps, token passing.

• Stations: Single-attached (one ring), dual-attached (both rings).

Comparison: Token Bus vs Token Ring

10.0 METROPOLITAN AREA NETWORKS (MAN)

DQDB (Distributed Queue Dual Bus) – IEEE 802.6:

• Architecture: Two unidirectional buses (eastbound/westbound).

• Access: Distributed queueing (request on one bus, transmit on other).

• Slot reuse: Empty slots reclaimed.

• Use: MAN backbone (e.g., city-wide fiber).

SMDS (Switched Multi-megabit Data Service):

• Connectionless datagram service (like Ethernet over MAN).

• Comparison:

  | Feature | DQDB | SMDS | |-------------------|-----------------------------------|-----------------------------------| | Access | Distributed queue | Contention-based (slotted) | | Service | Connection-oriented (VC) | Connectionless (datagram) | | Complexity | High (queue management) | Low (similar to Ethernet) |

11.0 WIDE AREA NETWORK (WAN) TECHNOLOGIES

X.25:

• Three layers: Physical (X.21), Data Link (LAPB), Packet (PLP).

• Packet-switched, error correction at each node (slow).

Frame Relay:

• Simplified X.25 (no error correction, only congestion detection).

• Virtual Circuits: PVC (pre-configured), SVC (dynamic).

• Frame Format: Flag | DLCI | Control | Data | FCS | Flag.

• Congestion: DE (Discard Eligible) bit, FECN/BECN.

ATM (Asynchronous Transfer Mode):

• Cell-based: 53-byte cells (5-byte header, 48-byte payload).

• Fixed length: Hardware switching (high speed).

• Layers: Physical (copper/fiber), ATM (cell relay), AAL (adaptation to higher layers).

• Switching: VPI/VCI lookup.

SONET (Synchronous Optical Network):

• Synchronous TDM: STS-1 (51.84 Mbps), STS-3 (155.52 Mbps).

• Frame: 9 rows × 90 columns (STS-1), 3× overhead.

• Ring topology: Self-healing (dual ring).

• Comparison with ATM: SONET is circuit-oriented, ATM is cell-based; SONET provides transport for ATM.

12.0 NETWORK LAYER & ROUTING

Routing Fundamentals:

• Routing table: Destination → Next hop, metric, interface.

• Metrics: Hop count, delay, bandwidth.

Distance Vector Routing:

• Algorithm: Bellman-Ford: $$\displaystyle D_x(y) = \min_v \{ c(x,v) + D_v(y) \} $$.

• Updates: Periodic (e.g., every 30 sec in RIP) or triggered.

• Problems:

  • Count-to-infinity: Slow convergence on link failure.

  • Split horizon: Do not advertise route back to source.

• Example: RIP (max hop count 15).

Link State Routing:

• Algorithm: Dijkstra’s (shortest path tree):

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

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

  3. Update $D(v)$ for neighbors: $$\displaystyle D(v) = \min[D(v), D(w) + c(w,v)] $$.

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

• LSA (Link State Advertisement): Flood topology info.

• Database: Each router has full topology map.

• Example: OSPF (fast convergence, more overhead).

Comparison: Distance Vector vs Link State

13.0 INTERNETWORKING DEVICES

Switch Types:

• Store-and-forward: Entire frame received, check CRC → high latency, error-free.

• Cut-through: Forward as soon as dest MAC known → low latency, no error check.

14.0 TRANSPORT LAYER & TCP/IP PROTOCOL SUITE

TCP/IP Model:

TCP Features:

• Connection-oriented (3-way handshake).

• Flow control: Sliding window (receiver advertises window).

• Congestion control: Slow start, congestion avoidance, fast retransmit/recovery.

• Reliability: Seq/ack numbers, retransmission timeout.

UDP Features:

• Connectionless, no retransmission, minimal headers (8 bytes).

• Used for DNS, VoIP, streaming.

Encapsulation:

Application data → TCP segment (header + data) → IP packet (header + segment) → Frame (header + packet + trailer) → Bits.

Addressing:

• IP address (32-bit IPv4, 128-bit IPv6) + subnet mask.

• Port number (16-bit) + IP = socket address.

• Well-known ports: HTTP (80), FTP (21), DNS (53).

15.0 ADDITIONAL & EMERGING TOPICS

Network Security Fundamentals:

• Confidentiality: Encryption (AES, RSA).

• Integrity: Hashing (SHA), digital signatures.

• Availability: DoS protection, redundancy.

Quality of Service (QoS):

• Mechanisms: Prioritization (DiffServ), traffic shaping, reservation (IntServ).

• Metrics: Bandwidth, delay, jitter, packet loss.

Wireless LANs (IEEE 802.11):

• CSMA/CA: Collision Avoidance (RTS/CTS).

• Modes: Infrastructure (AP), Ad-hoc (peer-to-peer).

• Security: WEP (weak), WPA/WPA2 (AES).

Network Management (SNMP):

• Components: Manager, Agent, MIB (Management Info Base).

• Operations: Get, Set, Trap (asynchronous notification).

\boxed{\text{KEY EXAM FORMULAS & CONCEPTS}}

1. Nyquist Sampling Rate: $$\displaystyle f_s \ge 2B $$.

2. Stop-and-Wait Efficiency: $$\displaystyle \eta = \frac{1}{1+2a} $$, $$\displaystyle a = \frac{t_{prop}}{t_{trans}} $$.

3. CRC Generation: Append $n$ zeros (for $n$-bit CRC), divide by generator polynomial $G(x)$, remainder = CRC.

4. Dijkstra’s Algorithm: Shortest path tree from source using link costs.

5. Sliding Window Efficiency: $$\displaystyle \eta = \frac{W}{1+2a} $$ for $$\displaystyle W \le 2^m $$.

6. ATM Cell: 53 bytes = 5 header + 48 payload.

7. SONET STS-1 Rate: 51.84 Mbps = 810 bytes/frame × 8000 frames/sec.

[!TIP] Common Pitfalls

• CRC: Forget to append zeros before division; polynomial degree = number of CRC bits.

• LRC: Only detects errors if entire column/row corrupted; cannot correct.

• Routing: Distance vector uses Bellman-Ford (next-hop), link state uses Dijkstra (full topology).

• HDLC: I-frames carry data/seq, S-frames control, U-frames management.

• Switching: Circuit = dedicated path; packet = statistical multiplexing.

• OSI: Transport = end-to-end; Network = routing; Data Link = hop-to-hop.