Computer Networks (CY-403) - Unit 4 Short Notes

UNIT 4: COMPUTER NETWORKS – COMPREHENSIVE SHORT NOTES

(Aligned with RGPV Past Papers: 2019–2025)

I. NETWORK FUNDAMENTALS AND ARCHITECTURES

A. Network Types and Scopes

B. Network Topologies

[!TIP]

Exam Focus: Compare Star vs. Mesh (reliability vs. cost). Ring topology questions often involve Token Ring/FDDI.

C. Switching Techniques

1. Circuit Switching

   • Dedicated path established before data transfer (e.g., PSTN).

   • Pros: Guaranteed bandwidth, low delay.

   • Cons: Inefficient for bursty traffic, setup delay.

2. Packet Switching

   • Datagram (Connectionless): Each packet routed independently (e.g., IP).

   • Virtual Circuit (Connection-Oriented): Path established first (e.g., Frame Relay, ATM).

3. Message Switching

   • Store-and-forward of entire messages (obsolete).

4. Store-and-Forward vs. Cut-through

   • Store-and-Forward: Entire frame received, checked, then forwarded (error detection).

   • Cut-through: Forwarding starts after header received (lower latency, no error check).

[!TIP]

Past Question: "Explain circuit, packet, message switching" (Jun 2025). Contrast connection-oriented vs. connectionless (May 2024).

D. Reference Models

1. ISO-OSI Model (7 Layers)

Critique of OSI:

• Too complex, layered architecture not perfectly matched to real protocols.

• Some layers (e.g., Session, Presentation) merged in TCP/IP.

• Never fully implemented; TCP/IP became de facto standard.

2. TCP/IP Model (4 Layers)

Advantages over OSI: Simpler, protocol-independent, real-world implementation (Internet).

Disadvantages: Less rigorous layering, no clear separation of services.

3. OSI vs. TCP/IP Comparison

[!TIP]

Exam Question: "Compare OSI and TCP/IP based on layer functionalities and real-world implementation" (Jun 2025). Key: OSI is model, TCP/IP is protocol suite.

II. PHYSICAL LAYER

A. Transmission Media

B. Multiplexing Techniques

1. FDM

   • Different signals on different frequency bands (e.g., radio, cable TV).

   • Bandwidth = sum of individual channel bandwidths + guard bands.

2. TDM

   • Synchronous TDM: Fixed time slots; if a station has no data, slot goes idle.

   • Statistical TDM: Dynamic slot allocation based on demand (higher efficiency).

   • Frame Size (bits) = (Number of sources) × (Bits per source)

   • Data Rate = Frame size / Frame time

3. WDM

   • Optical version of FDM; multiple light wavelengths on single fiber.

C. Line Coding

[!TIP]

Past Question: "Code binary stream in Manchester/Differential Manchester" (Nov 2022). Remember: Manchester has transition at middle; Differential Manchester at start.

D. Channel Capacity

1. Nyquist Formula (Noiseless):

$$ \text{Max bit rate} = 2B \log_2(L) $$

• \(B\) = bandwidth (Hz), \(L\) = signal levels.

• Example: \(B=3\) kHz, \(L=4\) → \(2 \times 3000 \times \log_2(4) = 12,000\) bps.

2. Shannon's Theorem (Noisy):

$$ C = B \log_2(1 + \text{SNR}) $$

• \(C\) = channel capacity (bps), SNR = \(P_{\text{signal}}/P_{\text{noise}}\) (linear, not dB).

• Example: \(B=4\) kHz, SNR=1000 → \(C = 4000 \log_2(1001) \approx 40,000\) bps.

[!TIP]

Calculation Trap: SNR must be in linear scale (e.g., 30 dB → 1000). Use \(\boxed{C = B \log_2(1+\text{SNR})}\) for noisy channels.

III. DATA LINK LAYER

A. Functions & Services

• Framing: Encapsulate packets into frames (header/trailer).

• Physical Addressing: MAC (48-bit) – unicast, multicast, broadcast.

• Error Control: Detection (CRC) and correction (Hamming).

• Flow Control: Prevent fast sender overwhelming slow receiver.

• Access Control: MAC sublayer for shared media (e.g., CSMA/CD).

B. Error Detection & Correction

1. Parity: Single bit; detects odd number of errors; cannot correct.

2. CRC (Cyclic Redundancy Check):

   • Steps:

     1. Append \(r\) zeros to data (\(D(x) \times x^r\)).

     2. Divide by generator \(G(x)\) (degree \(r\)).

     3. Remainder \(R(x)\) is checksum (size \(r\)).

     4. Transmit \(D(x) \times x^r + R(x)\).

   • Verification: Divide received codeword by \(G(x)\); remainder 0 → no error.

   • Example: \(D(x)=x^5+x^4+x^2+1\), \(G(x)=x^3+x+1\) → Compute codeword.

3. Hamming Code: Error correction; \(m\) data bits, \(r\) parity bits where \(2^r \ge m+r+1\).

4. Checksum: Internet checksum (ones complement sum); used in IP/TCP/UDP.

[!TIP]

Past Question: "CRC computation" (Jun 2025). Always show polynomial division steps.

C. Framing & Transparency

• Bit Stuffing: Insert '0' after five consecutive '1's in HDLC.

• Byte Stuffing: Insert escape byte (e.g., 0x7D) before flag byte (0x7E) in PPP.

D. Flow & Error Control Protocols

1. Stop-and-Wait

   • Sender sends one frame, waits for ACK before next.

   • Efficiency = \(\frac{1}{1+2a}\), where \(a = \frac{\text{propagation delay}}{\text{transmission time}}\).

   • Utilization ≥ 50% → \(1 \ge 2a\) → \(a \le 0.5\).

2. Sliding Window

   • Go-Back-N (GBN):

     • Sender window \(N\), receiver window \(1\).

     • Timeout → retransmit all unACKed frames.

     • Utilization ≈ \(\frac{N}{1+2a}\) (for large \(N\)).

   • Selective Repeat (SR):

     • Both windows \(N\) (typically \(N \le 2^{k-1}\), \(k\) = seq bits).

     • Individual retransmission of lost frames.

     • More efficient than GBN but complex.

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

E. Multiple Access Protocols (MAC Sublayer)

1. Random Access (Contention-Based)

CSMA/CD Minimum Frame Size:

\[ > \text{Min size} \ge 2 \times \text{propagation delay} \times \text{bandwidth} > \]

Ensures collision detected before transmission ends.

2. Controlled Access

• Reservation: Reserve slots in advance.

• Polling: Master polls slaves.

• Token Passing: Token circulates; holder transmits (Token Ring, FDDI).

3. Comparison: ALOHA vs. CSMA

• Pure ALOHA: Max throughput 18.4% at \(G=0.5\).

• Slotted ALOHA: Max 36.8% at \(G=1\).

• CSMA/CD: Efficiency \(\approx \frac{1}{1+2a}\) (for large \(a\), near 100%).

[!TIP]

Past Question: "Compare Pure ALOHA, Slotted ALOHA, CSMA, CSMA/CD" (Jun 2025). Throughput formulas must be memorized.

F. LAN Standards (IEEE 802)

Ethernet Frame Format:

\[ > \text{[Preamble (7B) | SFD (1B) | Dest MAC (6B) | Src MAC (6B) | Type (2B) | Data (46–1500B) | FCS (4B)]} > \]

G. Bridging & Switching

1. Bridges (Data Link Layer)

   • Transparent Bridge: Learns MAC addresses from source field; forwards/filters based on MAC table.

   • Spanning Tree Protocol (STP): Prevents loops by blocking redundant links.

2. Switches (Multiport Bridges)

   • Build MAC table by examining source MAC of incoming frames.

   • Modes:

     • Store-and-Forward: Error-checked, higher latency.

     • Cut-through: Forward header immediately, lower latency.

3. Comparison: Hubs, Switches, Bridges, Routers, Gateways

H. Data Link Protocols

1. HDLC (High-Level Data Link Control)

   • Frame: Flag (0x7E) | Address | Control | Info | FCS | Flag.

   • Modes: NRM (primary-secondary), ARM, ABM (balanced).

2. PPP (Point-to-Point Protocol)

   • Phases: Link establishment (LCP), authentication (PAP/CHAP), network layer (NCP).

   • Frame: Flag | Address (0xFF) | Control (0x03) | Protocol | Data | FCS | Flag.

   • vs. SLIP: PPP has error detection, authentication, multiplexing; SLIP does not.

3. Frame Relay (Brief)

   • Architecture: DTE (user) – DCE (switch).

   • DLCI: Virtual circuit identifier (locally significant).

   • Congestion: FECN, BECN, DE bits.

IV. NETWORK LAYER

A. Functions & Design Issues

• Routing: Path selection (DVR, LSR).

• Forwarding: Move packet from input to output link.

• Congestion Control: Prevent oversubscription (leaky/token bucket).

• QoS: Guarantees (IntServ, DiffServ).

• Internetworking: Interconnect heterogeneous networks (routers).

• Addressing: Logical (IP) vs. physical (MAC).

• Fragmentation/Reassembly: MTU adaptation (IPv4).

B. IP Addressing

1. IPv4 Format

• 32 bits, dotted-decimal (e.g., 192.168.1.1).

• Network ID + Host ID.

2. Classful Addressing

3. Limitations of Classful

• Address waste: Class B network may have thousands of hosts but only hundreds needed.

• Routing table explosion: Every network needs entry; no aggregation.

4. CIDR (Classless Inter-Domain Routing)

• Notation: prefix/d (e.g., 192.168.1.0/24).

• Benefits: Efficient allocation, route aggregation (supernetting).

5. Subnetting

• Subnet Mask: 32-bit number; 1s for network+subnet, 0s for host.

• Subnet ID: Bits borrowed from host part.

• Steps to Design Subnets:

  1. Determine required subnets/hosts.

  2. Borrow bits: \(2^n \ge \text{subnets}\) (for \(n\) bits).

  3. New mask = default mask + \(n\) bits.

  4. Subnet increment = \(2^{\text{host bits}}\).

  5. IP range: First usable to last usable (exclude network/broadcast).

Example: Divide 192.168.10.0/24 into 4 subnets:

• Borrow 2 bits → /26 mask (255.255.255.192).

• Increment = 64.

• Subnets:

• 192.168.10.0/26 → 192.168.10.1–62, broadcast 63.

• 192.168.10.64/26 → 65–126, broadcast 127.

• 192.168.10.128/26 → 129–190, broadcast 191.

• 192.168.10.192/26 → 193–254, broadcast 255.

6. IPv6 (Brief)

• 128 bits, hexadecimal (e.g., 2001:0db8:85a3::8a2e:0370:7334).

• Header: Fixed 40 bytes; no checksum, options via extension headers.

• Autoconfiguration: SLAAC (Stateless Address Autoconfiguration).

• vs. IPv4: Vast address space, simplified header, built-in security (IPsec), no broadcast.

C. Routing Algorithms

1. Distance Vector Routing (DVR)

• Bellman-Ford Equation:

  \[ D_x(y) = \min_v \{ c(x,v) + D_v(y) \} \]

  • \(D_x(y)\): Cost from \(x\) to \(y\).

  • \(c(x,v)\): Cost to neighbor \(v\).

• Steps:

  1. Each node knows cost to direct neighbors.

  2. Exchange full routing table periodically.

  3. Update using Bellman-Ford.

• Problems:

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

  • Solutions: Split horizon, poison reverse, hold-down timers.

• RIP: DVR implementation; hop count metric (max 15 hops); periodic updates (30s).

2. Link State Routing (LSR)

• Dijkstra's Algorithm (Shortest Path First):

  1. Each node floods LSAs (Link State Advertisements).

  2. Build complete topology map (link state database).

  3. Run Dijkstra to compute shortest paths.

• Steps (Dijkstra):

  1. Mark source node as permanent, others temporary.

  2. Update costs to neighbors via permanent nodes.

  3. Select temporary node with smallest cost, make permanent.

  4. Repeat until all nodes permanent.

• OSPF: LSR implementation; areas, authentication, fast convergence.

3. DVR vs. LSR Comparison

D. ICMP (Internet Control Message Protocol)

• Role: Error reporting, diagnostics (Network layer).

• Common Messages:

  • Echo Request/Reply: ping.

  • Destination Unreachable: Port/host/network unreachable.

  • Time Exceeded: TTL expired (traceroute).

  • Redirect: Better next-hop.

  • Source Quench: Deprecated (congestion).

E. Address Resolution Protocols

1. ARP (Address Resolution Protocol)

   • Need: Map IP → MAC (for delivery on LAN).

   • Operation:

     • Broadcast ARP request: "Who has IP X? Tell Y."

     • Unicast ARP reply: "IP X is at MAC Z."

   • ARP Cache: Stores recent mappings (TTL ~ 20 min).

2. RARP (Reverse ARP)

   • Need: Diskless workstation gets IP from MAC (obsolete, replaced by BOOTP/DHCP).

   • Operation: Broadcast RARP request with MAC; RARP server replies with IP.

3. BOOTP/DHCP: Dynamic IP assignment; DHCP offers lease, configuration parameters.

F. Fragmentation & Reassembly

• Need: MTU varies (Ethernet 1500B, PPP 532B, etc.).

• IPv4 Fields:

  • Identification: Same for all fragments of a packet.

  • Flags: DF (Don't Fragment), MF (More Fragments).

  • Fragment Offset: Position in original packet (in 8-byte units).

• Reassembly: Only at destination; intermediate routers do not reassemble.

G. Congestion Control in Network Layer

1. Causes: Buffer overflow, slow processors, low bandwidth.

2. Principles:

   • Load Shedding: Drop packets when overloaded (last resort).

   • Traffic Shaping: Regulate flow (leaky bucket, token bucket).

   • Resource Reservation: IntServ (RSVP).

3. Techniques:

   • Leaky Bucket: Fixed rate output; smoothes bursty traffic.

   • Token Bucket: Tokens accumulate (rate \(r\), burst size \(b\)); allows bursts.

     \[ \text{Max burst} = b \times \text{packet size}, \quad \text{Avg rate} = r \]

   • Comparison:

     | Leaky Bucket | Token Bucket | |------------------|------------------| | Output fixed rate | Output variable (up to burst) | | No burst allowed | Allows controlled bursts | | Smoothing only | Smoothing + burst control |

4. QoS Approaches:

   • IntServ: Per-flow resource reservation (RSVP); not scalable.

   • DiffServ: Per-class marking (DSCP in IP header); scalable.

V. TRANSPORT LAYER

A. Transport Services

• Connection-Oriented: TCP (reliable, ordered, flow/congestion control).

• Connectionless: UDP (unreliable, no control).

• Multiplexing/Demultiplexing: Port numbers (16-bit).

• Service Primitives: LISTEN, CONNECT, SEND, RECEIVE, DISCONNECT.

B. UDP (User Datagram Protocol)

• Header:

  \[ \text{[Src Port (2B) | Dest Port (2B) | Length (2B) | Checksum (2B)]} \]

• Features:

  • Connectionless, unreliable (no ACK, retransmission).

  • No flow/congestion control.

  • Low overhead, low latency.

• Applications: DNS, VoIP, streaming, DHCP.

C. TCP (Transmission Control Protocol)

1. Header Format

2. Connection Management

• Three-Way Handshake (Establishment):

  1. Client → SYN (seq=x)

  2. Server → SYN-ACK (seq=y, ack=x+1)

  3. Client → ACK (ack=y+1)

• Four-Way Handshake (Termination):

  1. Client → FIN (seq=u)

  2. Server → ACK (ack=u+1)

  3. Server → FIN (seq=v)

  4. Client → ACK (ack=v+1)

• Why Graceful Termination? Ensure all data delivered, resources released; prevent data loss.

3. Flow Control

• Sliding window; receiver advertises rwnd in header.

• Sender must have LastByteSent - LastByteAcked ≤ min(cwnd, rwnd).

4. Congestion Control

• Slow Start: cwnd starts at 1 MSS; doubles each RTT until threshold ssthresh.

• Congestion Avoidance: cwnd increases by 1 MSS per RTT (additive increase).

• Fast Retransmit: 3 duplicate ACKs → retransmit missing segment.

• Fast Recovery: After fast retransmit, set ssthresh = cwnd/2, cwnd = ssthresh + 3, then additive increase.

• AIMD: Additive Increase, Multiplicative Decrease (on loss).

5. Retransmission Strategies

• Go-Back-N: TCP uses cumulative ACKs; loss → retransmit from lost segment onward.

• Selective Repeat: TCP with SACK (Selective ACK) option allows individual retransmission.

D. TCP vs. UDP Comparison

VI. APPLICATION LAYER

A. DNS (Domain Name System)

• Need: Hierarchical naming for scalability (flat namespace impossible).

• Components:

  • Resolvers: Stub (in host), recursive (in DNS server).

  • Name Servers: Root, TLD (.com, .org), Authoritative.

• Resolution Process:

  1. Resolver queries root server for TLD server.

  2. Queries TLD for authoritative server.

  3. Queries authoritative for IP.

  4. Caching at each level (TTL-based).

• Resource Records (RR):

  • A: IPv4 address.

  • AAAA: IPv6 address.

  • CNAME: Canonical name (alias).

  • MX: Mail exchange.

  • NS: Name server.

  • PTR: Pointer (reverse DNS).

B. Electronic Mail

1. Architecture:

   • UA (User Agent): Mail client (Outlook, Thunderbird).

   • MTA (Message Transfer Agent): Server-to-server transfer (SMTP).

   • MDA (Message Delivery Agent): Local delivery (e.g., procmail).

2. SMTP (Simple Mail Transfer Protocol)

   • Operation: Port 25; text-based commands.

   • Commands: HELO, MAIL FROM, RCPT TO, DATA, QUIT.

   • ESMTP Extensions: STARTTLS (encryption), SIZE (max message size).

   • Limitations: 7-bit ASCII only; binary data via MIME.

3. Retrieval Protocols:

   • POP3: Download-and-delete; simple, no folder support.

   • IMAP: Manipulate mail on server; supports folders, partial fetch.

4. MIME (Multipurpose Internet Mail Extensions):

   • Encodes non-ASCII (Base64, quoted-printable).

   • Multipart messages (text + attachments).

C. HTTP (Hypertext Transfer Protocol)

• Operation: Request-response (client → server).

• Methods: GET, POST, PUT, DELETE, HEAD.

• Headers: Request (Host, User-Agent), Response (Server, Content-Type).

• Status Codes:

  • 1xx: Informational.

  • 2xx: Success (200 OK).

  • 3xx: Redirection (301, 302).

  • 4xx: Client error (404 Not Found).

  • 5xx: Server error (500 Internal).

• Persistent vs. Non-Persistent:

  • Non-persistent (HTTP/1.0): One TCP connection per object.

  • Persistent (HTTP/1.1): Multiple requests/responses per connection (pipelining).

• HTTPS: HTTP over SSL/TLS (port 443); encryption, server authentication.

D. FTP (File Transfer Protocol)

• Architecture: Client-server; separate control (port 21) and data (port 20) connections.

• Modes:

  • Active: Server opens data connection to client (client sends PORT).

  • Passive: Client opens data connection (server sends PASV).

• Commands: USER, PASS, LIST, RETR, STOR, QUIT.

E. SNMP (Simple Network Management Protocol)

• Components:

  • Manager: Central console (e.g., snmpwalk).

  • Agent: Software on managed device (router, switch).

  • MIB (Management Information Base): Database of managed objects (OID tree).

• Operations: GET, GETNEXT, SET, TRAP (asynchronous alert).

• Versions:

  • SNMPv1: Community strings (no encryption).

  • SNMPv2c: Enhanced operations, still no encryption.

  • SNMPv3: User authentication, encryption (USM, VACM).

F. WWW (World Wide Web)

• URL: scheme://host:port/path?query#fragment.

• HTML: Markup language for web pages.

• Web Server: Serves static/dynamic content (Apache, Nginx).

• Browser: Renders HTML, executes JavaScript.

G. Cryptography (Basic)

1. Symmetric Encryption: Same key (e.g., AES, DES). Fast, key distribution problem.

2. Asymmetric Encryption: Public/private key (e.g., RSA). Slow, solves key distribution.

3. Digital Signature: Hash + sender's private key; verifies integrity, authentication, non-repudiation.

4. PKI (Public Key Infrastructure): CA issues certificates binding public key to identity.

5. Applications:

   • SSL/TLS: Handshake (asymmetric), session keys (symmetric).

   • IPsec: AH (authentication), ESP (encryption).

VII. ADDITIONAL TOPICS

A. Virtual LANs (VLANs)

• Need: Segmentation without physical separation; security, broadcast control, flexibility.

• Implementation Methods:

  • Port-based: Switch ports assigned to VLANs.

  • MAC-based: Based on source MAC address.

  • Protocol-based: Based on network layer protocol (e.g., IP, IPX).

• Tagging: IEEE 802.1Q

  • Inserts 4-byte VLAN tag into Ethernet frame (TPID=0x8100).

  • VID (VLAN ID): 12 bits (1–4094; 0 and 4095 reserved).

B. Broadband Access Technologies

C. Network Security Basics

1. Firewalls:

   • Packet Filtering: ACLs on router.

   • Stateful: Track connection state.

   • Application-Level (Proxy): Inspect application data.

2. VPNs:

   • IPsec: Tunnel/transport mode; AH/ESP.

   • SSL/TLS: Remote access VPNs (e.g., OpenVPN).

3. IDS (Intrusion Detection System):

   • NIDS: Network-based (sniff traffic).

   • HIDS: Host-based (log files).

   • Detection: Signature-based, anomaly-based.

D. Wireless Networks (802.11 Details)

• Modes:

  • Infrastructure: AP (access point) connects stations.

  • Ad-hoc (IBSS): Station-to-station.

• MAC: CSMA/CA + RTS/CTS

  • DIFS: Distributed Inter-Frame Space; sense channel.

  • RTS/CTS: Optional; reduces hidden terminal problem.

• Security:

  • WEP: RC4, static key, weak (IV reuse).

  • WPA/WPA2: TKIP (WPA), AES-CCMP (WPA2); 802.1X authentication.

E. Frame Relay (Brief)

• Architecture: DTE (user) – DCE (switch).

• DLCI: Data Link Connection Identifier (locally significant).

• Congestion Control:

  • FECN (Forward Explicit Congestion Notification): Set by switch in direction of congestion.

  • BECN (Backward ECN): Set opposite direction.

  • DE (Discard Eligible): Mark low-priority frames for discard.

VIII. CALCULATION-BASED PROBLEMS (EXAM FOCUS)

A. Nyquist & Shannon Capacity

1. Nyquist (Noiseless): \( \text{Max bit rate} = 2B \log_2(L) \)

   • Given: \(B=3\) kHz, \(L=4\) → \(2 \times 3000 \times 2 = 12,000\) bps.

2. Shannon (Noisy): \( C = B \log_2(1+\text{SNR}) \)

   • Given: \(B=4\) kHz, SNR=30 dB → SNR=1000 → \(C=4000 \log_2(1001) \approx 40,000\) bps.

B. Subnetting

Example: Design 4 subnets for 192.168.10.0/24, each with 16 hosts.

1. Hosts needed: 16 → need 4 bits for hosts (\(2^4=16\), but 2 reserved → 14 usable).

2. Subnets: Borrow 2 bits from host part → /26 mask (255.255.255.192).

3. Increment = \(2^{(8-2)} = 64\).

4. Subnets:

   • 192.168.10.0/26 → IPs: 1–62, broadcast 63.

   • 192.168.10.64/26 → 65–126, broadcast 127.

   • 192.168.10.128/26 → 129–190, broadcast 191.

   • 192.168.10.192/26 → 193–254, broadcast 255.

C. ALOHA Throughput

1. Pure ALOHA: \( S = G e^{-2G} \), Max \(S=0.184\) at \(G=0.5\).

2. Slotted ALOHA: \( S = G e^{-G} \), Max \(S=0.368\) at \(G=1\).

3. Given idle probability \(P_{\text{idle}}=0.1\):

   • \(P_{\text{idle}} = e^{-G}\) → \(G = -\ln(0.1) \approx 2.302\).

   • Throughput \(S = G e^{-G} = 2.302 \times 0.1 = 0.2302\).

D. CSMA/CD Minimum Packet Size

\[ \text{Min size} \ge 2 \times \text{propagation delay} \times \text{bandwidth} \]

• Given: Length=2 km, speed=\(2 \times 10^8\) m/s, bandwidth=\(10^7\) bps.

• Propagation delay = \(\frac{2000}{2 \times 10^8} = 10^{-4}\) sec.

• Min size = \(2 \times 10^{-4} \times 10^7 = 2000\) bits = 250 bytes.

E. TDM Frame Size & Data Rate

• Given: Channel 1: 190 kbps, Channel 2: 180 kbps.

• Frame size = sum of bits per channel = 190,000 + 180,000 = 370,000 bits.

• Frame time = 1 sec (assuming 1 sec per frame for simplicity).

• Data rate = 370 kbps.

F. CRC Computation

Example: \(D(x)=x^5+x^4+x^2+1\), \(G(x)=x^3+x+1\).

1. \(D(x) = 110101\) (bits for \(x^5\) to \(x^0\)).

2. Append 3 zeros → 110101000.

3. Divide by \(G(x)=1011\) (binary 1011).

4. Remainder \(R(x)\) → codeword = \(D(x) \times x^3 + R(x)\).

G. TCP Header Analysis (Hex Dump)

Dump: 05320017 00000001 00000000 500207FF 00000000

• Source Port: First 4 hex digits = 0532 = 1330 decimal.

• Dest Port: Next 4 = 0017 = 23 (Telnet).

• Seq Number: Next 8 = 00000001 = 1.

• Ack Number: Next 8 = 00000000 = 0 (no ACK).

• Header Length: First 4 bits of next word (50 hex = 0101 0000) → 0101 = 5 words = 20 bytes (no options).

• Flags: Next 6 bits (000111? Actually 50 hex = 0101 0000, so flags bits are next 6 bits after data offset: 000111? Let's parse properly:

  • 50 hex = 0101 0000 → Data offset = 5 (high 4 bits).

  • Next 6 bits (low 2 bits of first byte + next byte) = 00 0000? Actually standard:

    • Byte 12: 50 → high 4 bits = 5 (header length).

    • Low 4 bits + byte 13 = flags/reserved.

    • Here 50 = 0101 0000 → low 4 bits = 0000 (reserved), then next byte 07 = 0000 0111.

    • So flags = 00000111? That's not standard. Let's re-evaluate:

      TCP header fields in order:

      • Bytes 0–1: Source Port

      • Bytes 2–3: Dest Port

      • Bytes 4–7: Seq Number

      • Bytes 8–11: Ack Number

      • Byte 12: Data offset (4 bits) + Reserved (3 bits) + NS flag (1 bit)

      • Byte 13: Flags (8 bits: CWR, ECE, URG, ACK, PSH, RST, SYN, FIN)

      • Byte 14–15: Window

      • ...

      Given dump:

      05 32 00 17 00 00 00 01 00 00 00 00 50 02 07 FF 00 00 00 00

      • Source Port: 05 32 = 0x0532 = 1330.

      • Dest Port: 00 17 = 23.

      • Seq: 00 00 00 01 = 1.

      • Ack: 00 00 00 00 = 0.

      • Byte 12: 50 = 0x50 = 0101 0000 → Data offset = 5 (20 bytes), Reserved = 000, NS=0.

      • Byte 13: 02 = 0x02 = 0000 0010 → Only SYN flag set (bit 2).

      • Window: 07 FF = 0x07FF = 2047.

      So:

      • Source Port: 1330

      • Dest Port: 23

      • Seq: 1

      • Ack: 0

      • Header Length: 20 bytes

      • Flags: SYN (connection request)

      • Window Size: 2047

IX. SHORT NOTE TOPICS (FREQUENT 3–4 MARKS)

1. ARP, RARP, ICMP

2. PPP, SLIP, HDLC

3. FDM and TDM

• FDM: Frequency bands; guard bands; radio/cable TV.

• TDM: Time slots; synchronous (fixed slots) vs. statistical (dynamic).

4. Virtual LAN (VLAN)

• Need: Logical segmentation over single physical LAN.

• Tagging: 802.1Q (4-byte VLAN tag).

• Benefits: Security, broadcast control, flexibility.

5. UDP

• Connectionless, unreliable, no flow/congestion control.

• Header: 8 bytes (ports, length, checksum).

• Applications: DNS, VoIP, streaming.

6. Cryptography

• Symmetric: Same key (AES, DES).

• Asymmetric: Public/private (RSA).

• Digital Signature: Hash + private key.

• PKI: CA issues certificates.

7. HTTP

• Request-response; methods (GET/POST); status codes; persistent connections; HTTPS.

8. SNMP

• Manager-agent-MIB; operations (GET/SET/TRAP); versions (v1/v2c/v3).

9. FDDI

• Dual ring (counter-rotating); token passing; 100 Mbps; high reliability; ring maintenance (active monitor).

10. Token Ring

• Ring topology; token passing; 4/16 Mbps; frame format with token bit; beaconing for ring recovery.

11. SMTP

• Text-based; port 25; commands (HELO, MAIL FROM, RCPT TO, DATA); ESMTP extensions (STARTTLS).

12. DNS

• Hierarchical; resolvers, root/TLD/authoritative servers; iterative/recursive queries; RR types.

13. Congestion Control

• Causes: Buffer overflow, slow processors.

• Techniques: Load shedding, traffic shaping (leaky/token bucket), QoS (IntServ/DiffServ).

14. Leaky Bucket vs. Token Bucket

15. Pure vs. Slotted ALOHA

16. Persistent vs. Non-Persistent CSMA

• 1-persistent: Sense; if idle transmit; if busy, sense continuously.

• Non-persistent: Sense; if busy, wait random time.

• p-persistent: Slotted; transmit with prob. \(p\) if idle.

17. IEEE 802.3 vs. 802.4 vs. 802.5

18. Hubs, Switches, Bridges, Routers, Gateways

19. Remote Bridging Challenges

• Latency: Long distances increase propagation delay.

• Scalability: Large number of MAC entries; spanning tree issues.

• Loop Avoidance: STP may block links, reducing redundancy.

• Bandwidth: Limited WAN links vs. high LAN speeds.

20. TCP Connection Establishment & Release

• Establishment: 3-way handshake (SYN, SYN-ACK, ACK).

• Release: 4-way handshake (FIN, ACK, FIN, ACK); both sides close independently.

• Graceful Termination: Ensures all data delivered, resources freed; prevents data loss.

21. Link State vs. Distance Vector Routing

22. Classful vs. Classless Addressing (CIDR)

23. IPv4 vs. IPv6

24. Broadcast vs. Multicast Routing

• Broadcast: Send to all nodes in network; routers block (unless configured).

• Multicast: Send to group; uses IGMP (hosts) + multicast routing (PIM, DVMRP).

25. MLMA (Multi-Level Multi-Access)

• Combines multiple access methods at different levels (e.g., FDM + TDM).

• Used in broadband networks (e.g., cable modem: FDM for channels, TDM/TDMA within channel).

26. Service Primitives in Transport Layer

• Listen: Wait for connection request (server).

• Connect: Initiate connection (client).

• Send/Receive: Data transfer.

• Disconnect: Terminate connection.

27. Network Topologies (Star vs. Mesh)

• Star: Central hub; easy to manage, single point of failure.

• Mesh: Fully/partially connected; high reliability, redundant paths, expensive.

28. QoS Techniques

• IntServ: Per-flow reservation (RSVP); not scalable.

• DiffServ: Per-class marking (DSCP); scalable.

• Traffic Shaping: Leaky/token bucket.

• Priority Queuing: Class-based scheduling.

29. Email Architecture

• UA → MTA (SMTP) → MTA → MDA → UA (POP3/IMAP).

• SMTP: Push protocol (port 25).

• POP3/IMAP: Pull protocols (ports 110/143).

30. WWW

• URL: http://host:port/path?query#fragment.

• HTML: Markup language.

• HTTP: Request-response; persistent connections.

• Web Server/Browser: Client-server model.

X. DIAGRAM-BASED TOPICS (MENTAL PREPARATION)

1. ISO-OSI Model: Draw 7 layers with examples (HTTP→App, TCP→Transport, IP→Network, Ethernet→Data Link, RS-232→Physical).

2. TCP/IP Model: 4 layers (Application, Transport, Internet, Network Interface).

3. Ethernet Frame: Preamble (7B) + SFD (1B) + Dest MAC (6B) + Src MAC (6B) + Type (2B) + Data (46–1500B) + FCS (4B).

4. Token Ring Frame: Start delimiter, access control, frame control, Dest/Src MAC, data, FCS, end delimiter, status.

5. TCP Header: Source/Dest port, seq/ack, flags (6 bits), window, checksum, urgent pointer, options.

6. UDP Header: Src port, dest port, length, checksum.

7. IPv4 Header: Version/IHL, DSCP/ECN, Total length, Identification, Flags/Fragment offset, TTL, Protocol, Header checksum, Src/Dest IP, Options.

8. DNS Hierarchy: Root → TLD (.com) → Authoritative → Host.

9. Email Architecture: UA → MTA (SMTP) → MTA → MDA → UA (POP3/IMAP).

10. FTP Connections: Control (port 21) + Data (port 20 active; passive: client opens).

11. Topologies: Draw bus, star, ring, mesh.

12. Sliding Window (GBN/SR): Sender window \(N\), receiver window 1 (GBN) or \(N\) (SR); cumulative ACKs vs. selective ACKs.

13. Dijkstra's Algorithm: Step-by-step on graph; mark permanent nodes, update costs.

14. Bellman-Ford: Iterative updates; count-to-infinity example.

15. Subnetting Example: Show network address, borrowed bits, subnets, ranges.

Final Exam Strategy:

• 7-mark questions: Explain with diagrams, compare, give examples.

• 4-mark questions: Short notes; define, list key points, applications.

• Calculation problems: Show formula, substitute values, box final answer.

• Past Paper Focus: OSI/TCP/IP models, subnetting, routing algorithms (DVR/LSR), TCP connection, DNS, ARP/ICMP, LAN standards (802.3/4/5), CRC, ALOHA/CSMA, Nyquist/Shannon.

\boxed{\text{Revise diagrams and formulas daily. Practice past paper problems under time constraints.}}