Wireless and computing system

 

⭐ 1. Hidden, Exposed, Near & Far Terminal Problems

Hidden Terminal Problem

  • Two nodes (A & C) cannot hear each other but both send to B.

  • Their signals collide at B.

Solution: RTS/CTS mechanism (IEEE 802.11), directional antennas.

Exposed Terminal Problem

  • Node C hears B’s transmission to A and assumes medium is busy.

  • But C could safely transmit to D (since A & D are out of range).

Solution: RTS/CTS, directional antennas.

Near–Far Problem

  • Nodes A and B transmit with same power, but B is closer to C, so B’s signal overpowers A.

  • A’s packet never reaches.

Solution: Power control & signal balancing.

⭐ 2. OSPF – Working + Messages

OSPF (Open Shortest Path First)

  • Link-State routing protocol.

  • Each router shares its link information with all others.

  • Builds the Shortest Path Tree using Dijkstra Algorithm.

  • Fast convergence, supports VLSM & CIDR.

OSPF Messages

  1. Hello – Discover neighbors

  2. DBD (Database Description) – Summary of LSAs

  3. LSR (Link State Request) – Ask for missing LSAs

  4. LSU (Link State Update) – Send LSAs

  5. LSAck – Acknowledge update

⭐ 3. BGP – Characteristics

Border Gateway Protocol

  • Used between Autonomous Systems (AS).

  • Path-vector protocol.

  • Provides loop-free, scalable routing.

Characteristics

  • Uses AS-PATH to avoid loops

  • Supports policy-based routing

  • Highly scalable

  • Slow convergence

  • Works between ISPs (eBGP) and inside an AS (iBGP)

⭐ 4. I-TCP, S-TCP, M-TCP

I-TCP (Indirect TCP)

  • Splits TCP connection into two:

    1. Fixed network ↔ FA

    2. FA ↔ Mobile device

  • Wireless errors hidden from fixed network.

S-TCP (Snooping TCP)

  • FA “snoops” packets.

  • Locally retransmits lost wireless packets.

  • End-to-end semantics preserved.

M-TCP (Mobile TCP)

  • Freezes TCP window during disconnections.

  • Avoids sender timeout on long breaks.

  • Good for mobility.

⭐ 5. Mobile IP – All Key Definitions

  • Mobile Node (MN): Device that moves network to network.

  • Home Network: Original network of MN.

  • Home Agent (HA): Router in home network storing MN’s location.

  • Foreign Network: Network MN visits.

  • Foreign Agent (FA): Router in foreign network assisting MN.

  • Care-of Address (CoA): Temporary address in foreign network.

  • Correspondent Node (CN): Any device communicating with MN.

⭐ 6. AODV, DSDV, DSR, ZRP

AODV

  • On-demand.

  • Route discovery using RREQ/RREP.

  • Sequence numbers avoid loops.

DSDV

  • Table-driven (proactive).

  • Updates routing table regularly.

  • Solves “count to infinity”.

DSR

  • On-demand.

  • Uses source routing (packet contains full path).

ZRP

  • Hybrid.

  • Zone-based routing (proactive inside zone, reactive outside).

⭐ 7. GSM Architecture + Handover Types

GSM Architecture

  1. MS (Mobile Station) – phone + SIM

  2. BSS (BTS + BSC) – radio management

  3. NSS (MSC, HLR, VLR, AUC, EIR) – switching, authentication

  4. OSS – network management

Types of Handover

  1. Intra-BTS – same tower

  2. Inter-BTS (intra-BSC) – different BTS

  3. Inter-BSC (intra-MSC) – different BSC

  4. Inter-MSC – hardest; switch between MSCs

⭐ 8. M-Commerce – Structure + Pros & Cons

Structure

  • Mobile device → WAP/HTTP → Application server → Database → Payment gateway

Pros

  • Very convenient

  • Global reach

  • Location-based services

  • Lower business cost

Cons

  • Security issues

  • Slow internet in rural areas

  • Device compatibility

  • Higher data charges

⭐ 9. Mobile OS Constraints + Android/iOS Features

Constraints

  • Limited battery

  • Small screen

  • Low CPU/RAM

  • Unstable networks

  • Security challenges

  • Simple UI needed

Android Features

  • Open-source

  • Based on Linux

  • Java/Kotlin apps

  • Google Play Store

  • Customizable UI

iOS Features

  • Closed-source

  • Very secure

  • Smooth UI

  • Objective-C/Swift

  • Exclusive to Apple devices

⭐ 10. CIDR + Subnetting + Supernetting

CIDR

  • Classless addressing.

  • Format: IP/prefix (e.g., 192.168.1.0/24).

  • Saves addresses, reduces routing table.

Subnetting

  • Divide 1 big network → many small networks.

  • Used for security and efficient IP use.

Supernetting

  • Combine multiple small networks → bigger network.

  • Used for route summarization.

⭐ 11. IPv4 vs IPv6

IPv4

  • 32-bit

  • 4.3 billion addresses

  • Dot-decimal notation

  • No built-in security

IPv6

  • 128-bit

  • Virtually unlimited addresses

  • Hexadecimal

  • Built-in IPSec

  • Supports auto-configuration

⭐ 12. Silly Window Syndrome + Solutions

What is it?

  • Receiver advertises very small window sizes (like 1 byte).

  • Sender sends tiny packets repeatedly → waste of bandwidth.

Causes

  • Slow receiver reading data

  • Slow application layer

  • Congestion

Solutions

  • Nagle’s Algorithm (Sender side)
    → Combine small segments before sending.

  • Clark’s Solution (Receiver side)
    → Receiver waits until buffer is half-empty before sending window update.

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