Computer Networks (CY-403) - Unit 1 Short Notes

1.0 FUNDAMENTAL CONCEPTS & NETWORK MODELS

1.1 Definition & Components of a Computer Network

• Computer Network: Interconnected autonomous computing devices (nodes) that share resources and communicate using standardized protocols.

• Components:

  • Nodes: Hosts (end systems), routers, switches, hubs.

  • Links: Guided (copper/fiber cables) and unguided (wireless) media.

  • Protocols: Rules for communication (e.g., TCP, IP, HTTP).

  • Standards: IEEE, IETF, ISO ensure interoperability.

1.2 Network Types & Topologies

• Network Types by Scale:

  | Type | Coverage | Example | |------|----------|---------| | PAN | <10 m | Bluetooth, Zigbee | | LAN | Room/Building | Ethernet, Wi-Fi | | MAN | City | Cable TV network | | WAN | Country/Global | Internet | | Internetwork | Multiple WANs | The Internet |

• Physical Topologies: Bus, Star, Ring, Mesh, Tree, Hybrid.

• Logical Topologies: How data flows (e.g., Ethernet logical bus, Token Ring logical ring).

1.3 ISO-OSI Reference Model (7-Layer)

• Layers & Functions:

  | Layer | Function | Protocols/Devices | |-------|----------|------------------| | 7. Application | Network services to apps | HTTP, FTP, SMTP | | 6. Presentation | Translation, encryption, compression | SSL/TLS | | 5. Session | Dialog control, synchronization | RPC, NetBIOS | | 4. Transport | End-to-end delivery, reliability | TCP (reliable), UDP (unreliable) | | 3. Network | Routing, logical addressing | IP, ICMP, routers | | 2. Data Link | Framing, MAC, error control | Ethernet, PPP, switches | | 1. Physical | Bits over medium | RJ45, fiber, repeaters |

• Peer-to-Peer Communication: Same layer on different nodes exchange PDUs (Protocol Data Units).

• Critique/Limitations:

  • Too complex, theoretical.

  • Session and presentation layers often merged with application in practice.

  • TCP/IP model gained real-world adoption.

1.4 TCP/IP Protocol Suite (4/5-Layer Model)

• Layers:

  • Network Access (Link): Physical + Data Link (Ethernet, Wi-Fi).

  • Internet: IP, ICMP, ARP (routing, addressing).

  • Transport: TCP, UDP (end-to-end).

  • Application: HTTP, DNS, SMTP (user-facing).

• Comparison with OSI:

  | Aspect | OSI | TCP/IP | |--------|-----|--------| | Layers | 7 | 4 or 5 | | Protocols | Theoretical | Practical (de facto) | | Session/Presentation | Separate | Merged into Application | | Network Layer | Connection-oriented (X.25) | Connectionless (IP) | | Transport Layer | Both CO & CL | Both (TCP & UDP) | | Adoption | Reference model | Internet standard |

1.5 Connection-Oriented vs. Connectionless Services

• Connection-Oriented:

  • Setup phase (handshake), guaranteed delivery, sequencing, flow control.

  • Example: TCP (three-way handshake, ACKs, retransmissions).

• Connectionless:

  • No setup, best-effort delivery, no guarantees.

  • Example: UDP, IP.

• Key Differences:

  | Feature | Connection-Oriented | Connectionless | |---------|---------------------|----------------| | Reliability | High (ACKs, retransmission) | Low (no ACKs) | | Overhead | High (headers, control) | Low | | Ordering | Guaranteed | Not guaranteed | | Use Case | File transfer, web (HTTP/TCP) | DNS, VoIP, streaming |

1.6 Service Primitives

• Request: Service user → service provider (e.g., "send data").

• Indication: Service provider → service user (e.g., "data arrived").

• Response: Service user → service provider (e.g., "data accepted").

• Confirm: Service provider → service user (e.g., "send complete").

• Used for communication between adjacent OSI layers.

[!TIP] Exam Focus: OSI vs TCP/IP mapping is a ** perennial 7-mark question**. Remember TCP/IP's Application layer encompasses OSI's Application, Presentation, and Session.

2.0 PHYSICAL LAYER & TRANSMISSION MEDIA

2.1 Guided Transmission Media

2.2 Unguided Transmission Media

• Radio: Omnidirectional, penetrates walls (Wi-Fi, Bluetooth).

• Microwave: Directional, line-of-sight, satellite links.

• Infrared: Short range, line-of-sight (remote controls).

• Satellite: Global coverage, high propagation delay (~270 ms).

2.3 Line Coding (Digital-to-Digital Conversion)

• Types:

  • Unipolar: All positive (e.g., NRZ-L). Has DC component.

  • Polar: Positive/negative (NRZ-L, NRZ-I). DC balance issues.

  • Bipolar: AMI (alternate mark inversion). No DC, but synchronization issues with long zeros.

  • Manchester: Mid-bit transition for clock, used in Ethernet (10BASE-T). Bandwidth doubles.

  • Differential Manchester: Transition at start for clock, inversion for data.

• Problems:

  • Synchronization: Need clock recovery (solved by Manchester, bipolar with stuffing).

  • DC component: Affects AC-coupled lines (solved by bipolar, Manchester).

  • Baseline wander: Long sequences of 0s/1s shift baseline (solved by bipolar with stuffing).

2.4 Multiplexing Techniques

• FDM: Different frequency bands (radio, cable TV).

• TDM:

  • Synchronous TDM: Fixed time slots per channel, inefficient if channel idle.

  • Statistical TDM: Dynamic slot allocation, needs addressing.

  • TDM Frame Size Calculation:

    \[ \text{Frame size (bits)} = \sum_{i=1}^{n} \text{slots per channel}_i \]

    \[ \text{Data rate} = \text{Frame size} \times \text{Frame rate} \]

    Example: 3 channels at 100 kbps each, 1 bit per slot → frame size = 3 bits, frame rate = 100,000 frames/s → data rate = 300 kbps.

2.5 Transmission Parameters & Limits

• Nyquist Formula (Noiseless Channel):

  \[ R_{\text{max}} = 2B \log_2 M \]

  \(B\) = bandwidth (Hz), \(M\) = signal levels.

  Example: \(B = 3 \text{ kHz}, M = 4\) → \(R_{\text{max}} = 2 \times 3000 \times \log_2 4 = 12,000 \text{ bps}\). Increasing \(M\) increases rate but requires higher SNR.

• Shannon's Theorem (Noisy Channel):

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

  \(C\) = channel capacity (bps), SNR in linear scale (not dB).

  Example: \(B = 4 \text{ kHz}, \text{SNR} = 1000\) → \(C = 4000 \log_2(1001) \approx 39,868 \text{ bps}\).

• Relationship: Both bandwidth and SNR increase capacity; Nyquist is upper bound for noiseless, Shannon for noisy.

[!TIP] Common Pitfall: In Shannon's formula, SNR must be linear (e.g., 30 dB = 1000 linear). Nyquist assumes no noise.

3.0 DATA LINK LAYER

3.1 Functions

• Framing, physical addressing (MAC), error control, flow control, link management.

3.2 Framing & Bit/Byte Stuffing

• Character-Oriented (Byte Stuffing):

  • Flag: 0x7E (01111110), Escape: 0x7D (01111101).

  • Stuff escape before flag or escape in data.

  • Example: Data 0x7E 0x7D → 0x7D 0x7E 0x7D 0x7D.

• Bit-Oriented (Bit Stuffing):

  • Flag: 01111110.

  • Insert 0 after five consecutive 1s in data.

  • Example: Data 01111110 → 011111010 (stuff after five 1s).

• Bit Stuffing Calculation: Given bit stream, scan left to right, insert 0 after every five consecutive 1s.

3.3 Error Detection & Correction

• Error Detection:

  • Parity Check: Single-bit (1D) or 2D matrix. Detects odd number of errors.

  • Checksum (Internet Checksum):

    1. Sum 16-bit words using ones complement arithmetic.

    2. Take ones complement of sum.

    3. Example: Words 0x1234, 0x5678 → sum 0x68AC → checksum 0x9753.

  • CRC (Cyclic Redundancy Check):

    1. Represent data \(D(x)\) and generator \(G(x)\) as binary.

    2. Append \(r\) zeros (\(r = \deg G(x)\)) to \(D(x)\).

    3. Divide \(D(x) \cdot x^r\) by \(G(x)\) using XOR (mod 2).

    4. Remainder \(R(x)\) is CRC; codeword = \(D(x) \cdot x^r + R(x)\).

    Example: \(D(x) = x^9 + x^8 + x^6 + x^5 + x^3 + x^2 + x^1 + 1\) (1101011011), \(G(x) = x^4 + x + 1\) (10011).

    Steps:

    
    11010110110000 ÷ 10011
    
    → Remainder: 1100 (4 bits)
    
    → Codeword: 11010110111100
    
    

• Error Correction:

  • Hamming Code: Add parity bits at positions \(2^i\). Minimum Hamming distance 3 → single-bit correction.

  • Example: For 4 data bits, need 3 parity bits (positions 1,2,4). Compute parity for overlapping groups.

[!TIP] CRC Calculation: Always use polynomial division (XOR). Generator must have \(x+1\) factor (ends with 1). Remainder length = degree of \(G(x)\).

3.4 Data Link Protocols

• Stop-and-Wait:

  • Sender transmits one frame, waits for ACK.

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

  • Inefficient for long propagation delays.

• Sliding Window:

  • Go-Back-N (GBN):

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

    • Cumulative ACKs. Timeout → retransmit all unACKed frames.

    • Utilization \(U \approx \min(1, \frac{N}{1+2a})\).

  • Selective Repeat (SR):

    • Both windows \(N \leq 2^{m-1}\) (where \(m\) = sequence number bits).

    • Individual ACK/NACK. Retransmit only lost frames.

    • Higher efficiency, complex receiver buffering.

  • Piggybacking: ACK carried in data frames to save bandwidth.

• Link Utilization:

  • GBN: \(U = \frac{N}{1+2a}\) (if \(N \leq 2a+1\)).

  • SR: \(U = \frac{N}{1+2a}\) (with \(N \leq 2^{m-1}\)).

3.5 Specific Protocols

• HDLC (High-Level Data Link Control):

  • Frame: Flag 01111110, Address, Control, Info, FCS, Flag.

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

• PPP (Point-to-Point Protocol):

  • Phases: Link establishment (LCP), authentication (optional), network layer (NCP).

  • Frame: Flag, Address (0xFF), Control (0x03), Protocol (e.g., 0x0021 for IP), Data, FCS, Flag.

  • Replaces SLIP (no error detection, no multiplexing).

• SLIP (Serial Line IP):

  • Simple, frame delimiter END (0xC0), escape ESC (0xDB).

  • Limitations: No error detection, no protocol type field, only IP.

[!TIP] GBN vs SR: GBN simpler but wastes bandwidth on single error; SR efficient but requires larger buffers and sequence numbers.

4.0 MEDIUM ACCESS CONTROL (MAC) SUBLAYER

4.1 Channel Allocation Problem

• Static: FDM, TDM, WDM, CDMA. Fixed allocation, inefficient for bursty traffic.

• Dynamic: Needed for bursty traffic (e.g., Ethernet, Wi-Fi).

4.2 Multiple Access Protocols

• ALOHA:

  • Pure ALOHA: Transmit anytime. Vulnerable period \(2T\).

    \[ \text{Throughput } S = G e^{-2G}, \quad \text{Max } 18.4\% \text{ at } G=0.5 \]

  • Slotted ALOHA: Transmit only at slot boundaries. Vulnerable period \(T\).

    \[ S = G e^{-G}, \quad \text{Max } 36.8\% \text{ at } G=1 \]

• CSMA (Carrier Sense Multiple Access):

  • 1-persistent: Sense idle → transmit immediately. High collision risk.

  • Non-persistent: Sense idle → transmit; if busy, random wait. Lower collision, higher delay.

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

• CSMA/CD (Collision Detection):

  • Used in Ethernet. Jam signal on collision, binary exponential backoff.

  • Minimum Frame Size: \(2 \times \text{propagation delay} \times \text{bandwidth}\).

    \[ \text{Min frame size} = 2 \times \frac{\text{distance}}{\text{signal speed}} \times \text{bandwidth} \]

  • Efficiency \(\eta = \frac{1}{1 + 2a}\) for long cables.

• CSMA/CA (Collision Avoidance):

  • Used in Wi-Fi (802.11). RTS/CTS, DIFS, SIFS, NAV (virtual carrier sense).

4.3 IEEE 802 Standards for LANs

4.4 Broadband & Baseband

• Baseband: Digital signal, entire bandwidth for one channel (Ethernet).

• Broadband: Analog signal, multiple channels via FDM (cable TV).

4.5 Virtual LANs (VLANs)

• Concept: Logical segmentation of physical LAN.

• Advantages: Security, broadcast control, flexibility.

• Implementation:

  • Port-based: Assign switch ports to VLANs.

  • MAC-based: Assign based on MAC address.

  • Tagging (IEEE 802.1Q): 4-byte tag inserted in Ethernet frame with VLAN ID.

5.0 NETWORK LAYER

5.1 Functions & Design Issues

• Forwarding (per-packet), routing (path selection), congestion control, QoS, internetworking.

5.2 IP Addressing (IPv4)

• Classful Addressing:

  | Class | Range | Default Mask | Hosts | |-------|-------|--------------|-------| | A | 0.0.0.0 – 127.255.255.255 | 255.0.0.0 | 16M | | B | 128.0.0.0 – 191.255.255.255 | 255.255.0.0 | 64K | | C | 192.0.0.0 – 223.255.255.255 | 255.255.255.0 | 254 | | D | 224.0.0.0 – 239.255.255.255 | – | Multicast | | E | 240.0.0.0 – 255.255.255.255 | – | Experimental |

• Limitations: Address wastage (class C for small networks), routing table explosion (many small networks).

• CIDR & Classless Addressing:

  • Notation: a.b.c.d/n (e.g., 192.168.1.0/24).

  • Address blocks: Size \(2^{32-n}\).

  • All-zeros/all-ones subnets historically reserved; now usable.

• Subnetting:

  • Steps:

    1. Determine bits to borrow: \(2^{\text{borrow}} \geq \text{subnets needed}\).

    2. New subnet mask: default mask + borrowed bits.

    3. Block size = \(2^{32-n}\).

    4. Subnet addresses: increment by block size.

    5. For each subnet:

       • Network address: base address.

       • First host: network + 1.

       • Last host: network + block size – 2.

       • Broadcast: network + block size – 1.

  Example: 192.168.10.0/24 into 4 subnets:

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

  • Block size = 64.

  • Subnets: 192.168.10.0/26 (hosts 1–62, bcst 63), 192.168.10.64/26 (hosts 65–126, bcst 127), etc.

• VLSM (Variable Length Subnet Mask):

  • Allocate subnets of different sizes. Start with largest requirement, then next.

  Example: ISP has 190.100.0.0/16. Allocate:

  • 64 customers × 256 addresses → /24 subnets (64 of them).

  • 128 customers × 128 addresses → /25 subnets (128 of them).

  • 128 customers × 64 addresses → /26 subnets (128 of them).

  Calculate remaining addresses.

5.3 Address Resolution Protocols

• ARP (Address Resolution Protocol):

  • Resolves IP → MAC. Broadcast request, unicast reply.

  • Cache entries with TTL.

• RARP (Reverse ARP):

  • Diskless workstations get IP from MAC via server.

• Proxy ARP: Router answers ARP for another host (makes remote network appear local).

• Gratuitous ARP: Host announces its IP/MAC (detect duplicates, update caches).

5.4 ICMP (Internet Control Message Protocol)

• Role: Network layer error reporting and diagnostics.

• Error Messages:

  • Destination Unreachable, Time Exceeded (TTL=0), Parameter Problem.

• Query Messages:

  • Echo Request/Reply (ping), Timestamp, Address Mask Request.

• Used by traceroute (TTL exceeded messages).

5.5 Routing Algorithms

• Optimality Principle: Optimal path from source to dest is also optimal for all intermediate nodes.

• Shortest Path Routing (Dijkstra):

  • Steps:

    1. Initialize: Source node cost 0, others ∞; tree = {source}.

    2. Find node not in tree with smallest cost.

    3. Add node to tree, update costs of neighbors via this node.

    4. Repeat until all nodes in tree.

  • Example: Graph with nodes J, A, I, H, K and delays. Compute shortest paths from J.

  • Limitations: Single metric (e.g., delay), requires complete topology knowledge.

• Distance Vector Routing (Bellman-Ford):

  • Each router sends its distance vector to neighbors periodically.

  • Update: \(D_x(y) = \min_{v \in \text{neighbors}} [c(x,v) + D_v(y)]\).

  • Count-to-Infinity: Bad news propagates slowly. Solved by Split Horizon (don't send route back to source) and Poisoned Reverse (send ∞ metric back).

  • Convergence: Slow, limited scalability (RIP uses DVR).

• Link State Routing (LSR):

  • Each router discovers neighbors (Hello packets), floods LSPs (link state packets).

  • Build complete topology, run Dijkstra locally.

  • Convergence: Fast, more overhead (OSPF uses LSR).

• Comparison:

  | Feature | DVR | LSR | |---------|-----|-----| | Convergence | Slow (count-to-infinity) | Fast | | Overhead | Periodic full updates | Flooding LSPs on change | | Scalability | Limited (RIP max 15 hops) | Better (OSPF areas) | | Knowledge | Only neighbor distances | Complete topology |

5.6 Congestion Control in Network Layer

• Causes: Too many sources, slow processors, low bandwidth.

• Load Shedding: Discard packets based on policy (random, priority, source).

• Choke Packets: Source reduces traffic upon receiving choke packet from congested router.

• QoS Techniques:

  • Leaky Bucket: Constant output rate, smooths bursts.

  • Token Bucket: Tokens accumulate (rate \(r\), capacity \(b\)); burst up to \(b\) tokens allowed. More flexible.

    \[ \text{Max burst duration} = \frac{b}{r} \]

  • Resource Reservation:

    • IntServ: Per-flow reservation (RSVP).

    • DiffServ: Per-class (PHB: Expedited Forwarding, Assured Forwarding).

5.7 IPv6 (Next Generation IP)

• Motivation: Address exhaustion (128-bit), header simplification, built-in security (IPsec), autoconfiguration.

• Header Format (Fixed 40 bytes):

  • Version (4), Traffic Class (8), Flow Label (20), Payload Length (16), Next Header (8), Hop Limit (8), Source/Dest Address (128).

  • No checksum, no options (extension headers instead).

• Address Representation: 8 groups of 4 hex digits (e.g., 2001:0db8:85a3::8a2e:0370:7334).

• Autoconfiguration: Stateless (SLAAC via router advertisements) or stateful (DHCPv6).

• Transition Strategies:

  • Dual Stack: Run IPv4 & IPv6.

  • Tunneling: Encapsulate IPv6 in IPv4.

  • Translation: NAT-PT (deprecated).

[!TIP] Subnetting: Always draw the subnet mask in binary to visualize borrowed bits. For VLSM, allocate largest subnets first to avoid fragmentation.

6.0 TRANSPORT LAYER

6.1 Services & Functions

• Process-to-process delivery (ports), segmentation/reassembly, connection control, flow control, error control, congestion control.

6.2 Transport Layer Protocols: TCP vs UDP

• UDP (User Datagram Protocol):

  • Header: Source Port (16), Dest Port (16), Length (16), Checksum (16).

  • Characteristics: Connectionless, unreliable, no flow/congestion control, low overhead.

  • Use Cases: DNS, VoIP, streaming, DHCP.

• TCP (Transmission Control Protocol):

  • Header fields:

    • Source/Dest Port (16 each)

    • Sequence Number (32)

    • Acknowledgment Number (32)

    • Flags (6 bits): URG, ACK, PSH, RST, SYN, FIN

    • Window Size (16)

    • Checksum (16)

    • Urgent Pointer (16)

    • Options (variable)

  • Characteristics: Connection-oriented, reliable, full-duplex, flow/congestion control.

6.3 TCP Connection Management

• Connection Establishment (Three-Way Handshake):

  1. Client → Server: SYN (seq=x, SYN=1).

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

  3. Client → Server: ACK (seq=x+1, ack=y+1, ACK=1).

• Connection Termination (Four-Way Handshake):

  1. A → B: FIN (seq=u, FIN=1).

  2. B → A: ACK (ack=u+1, ACK=1).

  3. B → A: FIN (seq=v, FIN=1).

  4. A → B: ACK (ack=v+1, ACK=1).

• Why Graceful Termination? Ensure all data delivered; handle delayed packets.

  • TIME_WAIT: 2MSL wait (Maximum Segment Lifetime × 2) to handle delayed packets and prevent old duplicates.

  • Half-Close: One direction closed (FIN sent, but other can still send).

  • Simultaneous Close: Both sides send FIN simultaneously.

6.4 Flow Control in Transport Layer

• Sliding Window: Receiver advertises window size (in header). Sender cannot send more than window size unacknowledged bytes.

• Buffering at sender (retransmission buffer) and receiver (out-of-order buffer).

6.5 Congestion Control in Transport Layer

• Need: Prevent network collapse, fair resource sharing.

• TCP Mechanisms:

  • Slow Start: Initial cwnd=1 MSS, double each RTT until threshold \(ssthresh\).

  • Congestion Avoidance: After \(ssthresh\), increase cwnd by 1 MSS per RTT (additive increase).

  • Fast Retransmit: After 3 duplicate ACKs, retransmit lost segment without waiting for timeout.

  • Fast Recovery: After fast retransmit, set \(ssthresh = cwnd/2\), cwnd = \(ssthresh + 3\), then on each dup ACK cwnd++, on new ACK set cwnd = \(ssthresh\).

  • AIMD: Additive Increase, Multiplicative Decrease on loss.

• Load Shedding: Discard packets when buffer full (e.g., tail drop).

[!TIP] TCP States: Remember the state diagram: CLOSED → LISTEN → SYN_SENT → SYN_RECEIVED → ESTABLISHED → FIN_WAIT_1 → FIN_WAIT_2 → TIME_WAIT → CLOSED. TIME_WAIT prevents old duplicates.

7.0 APPLICATION LAYER & APPLICATION PROTOCOLS

7.1 Domain Name System (DNS)

• Role: Hierarchical, distributed database mapping domain names → IP addresses.

• Components:

  • Resolvers: Client-side library/daemon.

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

  • Zones: Administrative domains (e.g., example.com).

• Resolution Process:

  • Recursive Query: Resolver asks server, server does full lookup (or referral).

  • Iterative Query: Server returns closest known answer (referral to another server).

  • Caching: Responses cached with TTL (Time-to-Live).

• Resource Records (RR):

  • A (IPv4 address), AAAA (IPv6), NS (name server), CNAME (canonical name), MX (mail exchange), PTR (reverse lookup).

7.2 World Wide Web & HTTP

• HTTP (Hypertext Transfer Protocol):

  • HTTP/1.0: Non-persistent (new connection per request).

  • HTTP/1.1: Persistent connections (keep-alive), pipelining.

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

  • Status Codes:

    • 1xx: Informational (100 Continue)

    • 2xx: Success (200 OK)

    • 3xx: Redirection (301 Moved Permanently)

    • 4xx: Client Error (404 Not Found)

    • 5xx: Server Error (500 Internal Server Error)

  • Statelessness: No memory of previous requests; Cookies for state.

• HTTPS: HTTP over SSL/TLS (port 443).

7.3 Electronic Mail

• Architecture:

  • MUA (Mail User Agent): Outlook, Thunderbird.

  • MTA (Mail Transfer Agent): Sendmail, Postfix (SMTP).

  • MDA (Mail Delivery Agent): Procmail (local delivery).

• SMTP (Simple Mail Transfer Protocol):

  • Push protocol, port 25, text-based (7-bit ASCII).

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

  • Uses CRLF line endings.

• POP3 / IMAP:

  • POP3 (port 110): Download-and-delete, simple.

  • IMAP (port 143): Keep mail on server, folders, concurrent access.

• MIME (Multipurpose Internet Mail Extensions):

  • Extends SMTP for non-ASCII via encoding (base64, quoted-printable).

  • Headers: Content-Type, Content-Transfer-Encoding.

7.4 File Transfer

• FTP (File Transfer Protocol):

  • Two connections:

    • Control (port 21, persistent): Commands/responses.

    • Data (port 20 active, or passive mode): Actual file transfer.

  • Active Mode: Server connects to client data port.

  • Passive Mode: Client connects to server data port (firewall-friendly).

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

7.5 Network Management

• SNMP (Simple Network Management Protocol):

  • Architecture: Manager (NMS), Agent (on device), MIB (database of objects).

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

  • MIB: Hierarchical tree of managed objects (OIDs).

7.6 Other Application Layer Concepts

• Peer-to-Peer (P2P) vs Client-Server:

  • P2P: Decentralized, peers share resources (BitTorrent).

  • Client-Server: Centralized server (web, email).

• Socket: Endpoint = IP address + port number (e.g., 192.168.1.1:80).

8.0 NETWORK DEVICES & INTERCONNECTION

8.1 Devices by OSI Layer

• Switch vs Bridge: Switch is multi-port bridge, hardware-based, higher port density.

• Router vs Gateway: Router connects networks (IP), gateway connects dissimilar networks (protocol conversion).

8.2 Virtual LANs (VLANs)

• Concept: Logical segmentation of physical LAN.

• Advantages: Security (isolate groups), broadcast control, flexibility (move users without rewiring).

• Implementation:

  • Port-based: Assign switch ports to VLANs.

  • MAC-based: Assign based on MAC address.

  • Tagging (IEEE 802.1Q): 4-byte tag inserted in Ethernet frame; VLAN ID in tag.

9.0 SPECIAL TOPICS & SHORT NOTE POTENTIAL

9.1 Cryptography (Basics)

• Symmetric: Same key for encryption/decryption (AES, DES). Fast, key distribution problem.

• Asymmetric: Public/private key pair (RSA). Slow, solves key distribution.

• Digital Signatures: Hash(message) + encrypt with private key → authenticity/integrity.

9.2 Network Topologies

• Star: Central hub/switch. Advantages: Easy management, single failure point. Disadvantages: Hub failure brings down network.

• Mesh: Full connectivity. Advantages: High reliability, multiple paths. Disadvantages: Expensive, complex.

9.3 Broadcast & Multicast Routing

• Broadcast: Send to all nodes (e.g., ARP). Flooding, reverse path forwarding.

• Multicast: Send to group (e.g., video streaming). Tree construction (source-based or group-shared).

9.4 Frame Relay & ATM

• Frame Relay: Packet switching, virtual circuits (PVC/SVC), no error correction (rely on higher layers).

• ATM (Asynchronous Transfer Mode): Cell switching (53-byte cells: 5 header + 48 payload), QoS, virtual circuits.

9.5 FDDI (Fiber Distributed Data Interface)

• Dual ring, token passing, 100 Mbps, fault tolerance (ring wrap on failure).

9.6 MLMA (Multiple Access with Collision Avoidance)

• Concept: Sense channel, avoid collisions (e.g., CSMA/CA in Wi-Fi).

9.7 Persistent vs Non-Persistent CSMA

• 1-Persistent: Sense idle → transmit immediately. High collision risk under heavy load.

• Non-Persistent: Sense idle → transmit; if busy, random backoff. Lower collision, higher average delay.

9.8 Remote Bridging

• Connect distant LANs via bridge over WAN (e.g., leased line).

• Challenges:

  • Latency: Propagation delay affects collision detection (CSMA/CD fails if delay > frame transmission time).

  • Loop Prevention: Spanning Tree Protocol (STP) needed.

  • Cost: Leased lines expensive.

[!TIP] Special Topics: These are often 4-mark short notes. Focus on one key advantage/disadvantage or working principle. For remote bridging, emphasize latency issue for CSMA/CD.