/wiki / wireless-n

cell-splitting

2026-09-02T00:00:00.000Z

The Problem

Auto-created stub for broken link [[cell-splitting]] — needs human content.

Formal Definition

Per Wikipedia: “To be written.”

Explanation

Stub — fill with plain language explanation.

How It Works

To be written.

Visual Explanation

digraph cell-splitting {
  rankdir=LR
  node [shape=box style=filled fillcolor="#f0f4ff" fontname="Helvetica"]
  A [label="Stub: cell-splitting"]
  B [label="To be written"]
  A -> B [label="needs content"]
}

Semantic Network

graph semantic_cell-splitting {
  layout=neato
  node [shape=ellipse fontname="Helvetica" fontsize=11 style=filled]
  THIS [label="cell splitting" fillcolor="#ffd700" fontsize=13 style="filled,bold"]
  REL1 [label="Related" fillcolor="#f0f0f0"]
  THIS -- REL1 [label="related"]
}

Key Properties

To be written.

Real-World Example

# TODO: add example

Connections

Related: Index — auto stub, needs proper links

Edge Cases & Gotchas

To be written.

sectoring

2026-09-02T00:00:00.000Z

The Problem

Auto-created stub for broken link [[sectoring]] — needs human content.

Formal Definition

Per Wikipedia: “To be written.”

Explanation

Stub — fill with plain language explanation.

How It Works

To be written.

Visual Explanation

Semantic Network

Key Properties

To be written.

Real-World Example

# TODO: add example

Connections

Related: Index — auto stub, needs proper links

Edge Cases & Gotchas

To be written.

Wireless N — Map of Content

2026-09-02T00:00:00.000Z

Map of Content for Wireless N — 33 concepts. Start here to navigate wireless-n.

Core Concepts

Antenna Types — concept
Bluetooth — concept
Cell Sectoring — concept
Cellular Mobile System — concept
Channel Fading — concept
Co-Channel Interference — concept
CDMA — concept
CD (Carrier Sense Multiple Access with Collision Detection) — concept
DAMA Protocol — concept
Exposed Terminal Problem — concept
Frequency Reuse — concept
Frequency Shift Keying — concept
GPRS — concept
GSM Architecture — concept
GSM (Global System for Mobile Communications) — concept
GSM Services — concept
Handoff — concept
Hidden Terminal Problem — concept
IEEE 802.11 — concept
MACA Protocol — concept
Minimum Shift Keying — concept
Mobile Ad Hoc Network — concept
Mobile TCP — concept
Modulation — concept
modulation-techniques-compared — concept
Multipath Propagation — concept
Multiplexing — concept
Far Terminal Effect — concept
Spread Spectrum — concept
WiMAX — concept
wireless-mac-problems-compared — concept
Wireless Network — concept
ZigBee — concept

Mechanisms & How Things Work

Auto-generated — edit to curate. Pages that explain processes/flows.

Comparisons & Tradeoffs

(No synthesis yet — create via ingest or query —save)

csma-cd

2026-04-29T00:00:00.000Z

The Problem

In shared wired networks (like early Ethernet), multiple devices share the same physical medium. If two devices transmit simultaneously, their signals collide and become corrupted. There was no mechanism to detect and recover from these collisions.

Core Idea

CSMA/CD is a MAC protocol where devices listen before transmitting (carrier sense), and if a collision is detected during transmission, all devices stop, wait a random time, and retry. It was the foundation of classic Ethernet (IEEE 802.3).

How It Works

Listen — device checks if medium is idle before transmitting
Transmit — if idle, start transmitting while continuously monitoring for collisions
Collision detected — if collision detected (voltage exceeds normal), send jam signal
Backoff — wait random time using exponential backoff algorithm, then retry

The maximum network diameter is limited by the “slot time” — the time to detect a collision from the farthest device.

Visual Explanation

Key Properties

Carrier sense — listen before transmit (reduces but doesn’t eliminate collisions)
Collision detection — detect during transmission (wired networks only)
Exponential backoff — after each collision, wait longer (reduces repeat collisions)
Half-duplex — only one device can transmit at a time

Connections

Built from: Hidden Terminal Problem — CSMA variants try to solve similar problems
Contrasts with: CA — wireless uses collision avoidance (can’t detect collisions)
Related: MACA — wireless alternative that influenced 802.11
Related: Multiplexing — MAC protocols enable shared medium access
Related: Wired Networks — CSMA/CD is primarily for wired Ethernet

Edge Cases & Gotchas

Only works on wired networks — wireless can’t detect collisions (hidden terminal problem)
Maximum network length limited by propagation delay (must detect collision in time)
Modern Ethernet switches use full-duplex — no collisions, CSMA/CD obsolete in practice

gsm

2026-04-29T00:00:00.000Z

The Problem

First-generation cellular systems (1G) were analog, insecure, and lacked international roaming standards. There was no unified digital cellular standard for Europe and the world.

Core Idea

GSM is the 2G digital cellular standard that introduced digital encryption, international roaming, and SIM cards. It defines the complete architecture for digital mobile communication.

How It Works

GSM operates in the 900 MHz and 1800 MHz bands (different regions vary). The system uses:

TDMA/FDMA hybrid — time division and frequency division multiplexing
GMSK modulation — Gaussian Minimum Shift Keying for spectrally efficient transmission
Three subsystems: RSS (radio), NSS (network switching), OSS (operations)
SIM cards — removable subscriber identity module for portability

Key processes: location updates, handoff, authentication, and encryption all managed by the NSS.

Visual Explanation

Key Properties

Digital encryption (A5/1, A5/2, A5/3) for voice and data privacy
SIM cards enable subscriber portability across devices
International roaming through HLR/VLR architecture
GPRS (2.5G) adds packet-switched data on top of GSM’s circuit-switched foundation
Uses FDD (Frequency Division Duplex) for uplink/downlink separation

Connections

Built from: Wireless Network — GSM operates over wireless radio
Built from: Frequency Shift Keying — GSM uses GMSK (Gaussian MSK)
Builds into: GSM Architecture — detailed subsystem breakdown
Builds into: GSM Services — telephony, data, supplementary services
Contrasts with: CDMA — GSM uses TDMA, CDMA uses spread spectrum
Related: GPRS — packet-switched extension to GSM (2.5G)

Edge Cases & Gotchas

A5/1 encryption was broken — modern attacks can decrypt GSM traffic in real-time
GSM operates in multiple frequency bands globally (850/900/1800/1900 MHz) — devices must support regional bands
Circuit-switched nature makes GSM inefficient for data — GPRS/EDGE added later

antenna-types

2026-04-21T00:00:00.000Z

The Problem

An antenna is the transducer that converts electrical current into radio waves (transmit) and radio waves back into electrical current (receive). The shape and design of the antenna determines the radiation pattern — how the signal is distributed in space. Different applications require different radiation patterns: some need uniform coverage in all directions, while others need focused beams in specific directions.

Core Idea

Antennas are classified by their radiation pattern: isotropic (theoretical perfect sphere, used as a reference), dipole (donut-shaped, 360° horizontal coverage), directional (focused narrow beam for long distance), sectorized (divided into sectors for cellular), and diversity (multiple antennas for signal quality improvement).

How It Works

Isotropic Antenna (Theoretical Reference):

Purely theoretical antenna that radiates equally in all directions (perfect sphere)
Never exists in practice — used as the reference for measuring antenna gain (measured in dBi)
0 dBi gain by definition

Dipole Antenna (Most Common Real-World):

Radiates 360° in the horizontal plane (omnidirectional)
Does NOT radiate straight up or straight down
3D radiation pattern is donut-shaped
Used in standard Wi-Fi routers and base station feeders

Directional / Directed Antenna (Flashlight of Radio):

Focuses energy into a narrow beam in a specific direction
Higher gain in the main direction, less energy in other directions
Used for: microwave backhaul links, long-range point-to-point, satellite comms
Types: Yagi, helical, parabolic dish

Sectorized Antenna (Cellular Towers):

One omnidirectional antenna replaced by 3 or 6 directional antennas
Each antenna covers a sector (120° or 60°)
Total 360° coverage with reduced interference per sector
Standard in modern cellular base stations

Diversity Antenna (Smart Listener):

Not a shape but a technique: multiple receive antennas
Antennas separated by small distances (to receive independent multipath signals)
Receiver selects the strongest signal or combines both (MIMO)
Effectively combats multipath fading

Key Properties

Isotropic: theoretical reference only; 0 dBi gain
Dipole: 2.1 dBi gain; standard reference for real antennas
Directional: 10–25 dBi gain; narrows coverage but increases range
Sectorized: divides one cell into 3 or 6 sectors, reducing interference per sector
Diversity: no gain increase but improves reliability in multipath environments

Connections

Built from: Cellular Mobile System — sectorized antennas enable cellular coverage
Built from: Channel Fading — diversity antenna directly combats fading
Related: Sectoring — the cellular design technique that uses sectorized antennas
Related: Cell Splitting — another capacity enhancement technique
Related: Frequency Reuse — sectoring improves frequency reuse efficiency

Edge Cases & Gotchas

Diversity antenna requires careful separation — too close and signals are correlated; too far and patterns differ
MIMO (used in 4G/LTE and Wi-Fi) is a multi-antenna system that combines diversity with spatial multiplexing
Sectorized antennas require precise azimuth pointing (for 120° sectors, ±60° from center)
Diversity and MIMO are different: diversity improves reliability, MIMO improves throughput

bluetooth

2026-04-21T00:00:00.000Z

The Problem

Wired cables for peripherals (mouse, keyboard, headset) are inconvenient. Wi-Fi was designed for network connectivity, not device-to-device personal area networking. Bluetooth provides short-range, low-power, low-cost wireless connectivity between personal devices without the overhead of Wi-Fi or the complexity of network setup.

Core Idea

Bluetooth (IEEE 802.15.1) is a wireless personal area network (WPAN) standard for short-range device-to-device communication. It operates in the 2.4 GHz ISM band, uses FHSS to avoid interference with Wi-Fi and other devices, and is designed for low power consumption and simple pairing.

How It Works

Physical and MAC Layer:

Radio Layer: 2.4 GHz ISM band; FHSS with 1,600 hops/second across 79 channels
Baseband Layer: Time slots of 625 µs; synchronous connections for voice, asynchronous for data
LMP (Link Manager Protocol): Manages link setup, authentication, encryption
L2CAP (Logical Link Control and Adaptation Protocol): Multiplexes multiple channels, handles QoS
RFCOMM: Emulates serial port (RS-232) for legacy device support
SDP (Service Discovery Protocol): Discovers services on paired devices

Architecture:

Piconet: One master + up to 7 active slaves; all communication goes through master
Scatternet: Multiple piconets interconnected through shared slaves
Master/Slave roles: Determined at pairing; masters control frequency hopping

Versions:

Bluetooth 1.0/1.0b: Initial; 723 kbps; many interoperability issues
Bluetooth 2.0 + EDR: 3 Mbps; faster pairing
Bluetooth 3.0 + HS: 24 Mbps; uses Wi-Fi for high-speed data transfer
Bluetooth 4.0 + LE: Bluetooth Low Energy; 1 Mbps; years of battery life (IoT)
Bluetooth 5.0: 2 Mbps + LE; improved range and broadcast (Beacons)
Bluetooth 5.1: Direction finding (AoA/AoD) for indoor positioning
Bluetooth 5.3+: LE Audio; Auracast; LC3 codec

Key Properties

Master/slave architecture: master controls the piconet timing
FHSS: 1,600 hops/second avoids narrowband interference; no co-channel interference
Low power: designed for battery-powered devices (LE variants especially)
Simple pairing: no network setup required; devices discover and pair
Coexists with Wi-Fi in 2.4 GHz band without interference due to FHSS
Range: ~10 m for standard, up to 200 m for extended range (Bluetooth 5.0+)

Connections

Related: IEEE 802.11 — both operate in 2.4 GHz but coexist without interference
Related: IEEE 802.15 — Bluetooth IS the 802.15.1 standard
Related: Spread Spectrum — Bluetooth uses FHSS spread spectrum
Related: Wi-Fi vs Bluetooth vs ZigBee — comparison of short-range wireless standards
Related: FHSS — Bluetooth’s specific spread spectrum technique

Edge Cases & Gotchas

The 2.4 GHz ISM band is shared by Wi-Fi, Bluetooth, microwave ovens, cordless phones, and ZigBee — interference is managed by FHSS
Range is limited (10 m) — not suitable for longer-range applications
Security was weak in early versions (Bluetooth 1.0); PIN-based pairing was vulnerable
BLE (Bluetooth Low Energy) is a different physical layer from classic Bluetooth

cell-sectoring

2026-04-21T00:00:00.000Z

The Problem

A single omnidirectional antenna at a cell site broadcasts in all 360° directions, but users are not evenly distributed around the cell. More importantly, signals transmitted toward the back of the cell (toward co-channel cells) cause co-channel interference. Cell sectoring uses directional antennas to divide a cell into sectors, focusing energy where users are and reducing interference toward co-channel cells.

Core Idea

Cell sectoring divides a single cell into 3 or 6 sectors (120° or 60° each) using directional antennas instead of one omnidirectional antenna. Each sector operates as an independent cell with its own channel set, reducing co-channel interference and increasing capacity per unit area.

How It Works

Instead of one omnidirectional antenna, install 3 (120°) or 6 (60°) directional antennas per cell site
Each sector covers a wedge-shaped area; total coverage remains 360°
Each sector is assigned its own subset of the cell’s frequencies
Since each antenna covers only a 60–120° sector, signals are not broadcast toward co-channel cells
Effective co-channel interference decreases by the number of sectors
C/I ratio improves, or cluster size (N) can be reduced, increasing capacity

The effective reuse factor improves by a factor equal to the number of sectors: 3 sectors reduces interference by 3× or allows 3× reduction in cluster size (from N=7 to N=4).

Key Properties

Reduces effective co-channel interference by a factor of 3 (or the number of sectors)
Allows smaller cluster sizes (higher capacity) while maintaining C/I
Improves downlink coverage for directional antennas
Requires careful sector boundary planning to avoid coverage gaps
Common configuration: 3 sectors per cell (120° each)
Sectors can overlap at boundaries, enabling soft handoff zones

Connections

Built from: Cellular Mobile System — sectoring is a capacity enhancement technique for cellular systems
Built from: Antenna Types — sectoring uses directional antennas
Built from: Frequency Reuse — sectoring improves effective frequency reuse
Related: Cell Splitting — another capacity enhancement technique
Related: Co-Channel Interference — sectoring directly reduces this
Related: GSM Architecture — GSM BTS supports sectorized deployments

Edge Cases & Gotchas

Sectors can cause coverage gaps at their boundaries
Requires careful RF planning to ensure uniform coverage
Sector boundaries are not static — they shift with traffic load and seasonal foliage
Ping-pong handovers can occur at sector boundaries

cellular-mobile-system

2026-04-21T00:00:00.000Z

The Problem

Supporting millions of mobile users across a city with a single high-power antenna is impractical because: (1) mobile devices lack enough transmit power to reach a 50 km distant tower, and (2) the available radio spectrum can only support a limited number of simultaneous channels. A city-wide single-antenna system would have insufficient capacity. The solution is to divide the coverage area into small cells, each with a low-power base station.

Core Idea

A cellular mobile system divides a geographical area into small hexagonal cells, each served by a low-power base station. Because each cell covers a small area, devices only need to reach antennas within hundreds of meters. The same frequencies can be reused in non-adjacent cells, massively increasing network capacity as cells get smaller.

How It Works

The city is divided into small hexagonal cells (typically 1–35 km radius)
Each cell has a Base Transceiver Station (BTS) with low-power radio equipment
Mobile devices connect to the nearest BTS via the Um (air) interface
Cells using the same frequencies (co-channel cells) are separated by the co-channel reuse distance D
As a mobile moves between cells, the call is handed over from one BTS to another (handoff)
Base Station Controllers (BSCs) manage multiple BTSs; Mobile Switching Centers (MSCs) manage routing

Key concepts: Frequency Reuse (same frequencies in distant cells), Cell Splitting (splitting a large overloaded cell into smaller micro-cells), Sectoring (using directional antennas to divide a cell into 120° or 60° sectors).

Key Properties

Massive capacity scaling: total capacity = channels_per_cell × number_of_cells
Limited antenna power means devices only need short-range transmission
Frequency reuse enables near-infinite capacity scaling as cells shrink
Cell radius can be 100 m (dense urban) to 35 km (rural)
Handoff (hard vs soft) maintains connectivity during mobility
Sectoring reduces co-channel interference per sector

Connections

Built from: Frequency Reuse — the foundational principle of cellular architecture
Built from: Handoff — what allows continuous communication during mobility
Built into: GSM — the most widely deployed 2G cellular system
Related: Cell Splitting — increasing capacity by shrinking cell size
Related: Sectoring — directional antennas that improve cell efficiency
Related: Co-Channel Interference — the interference that constrains frequency reuse
Related: Frequency Management — managing channel assignments across the network

Edge Cases & Gotchas

Cell planning in urban areas is complex due to irregular terrain and building shadowing
Cell boundaries are not clean hexagons — they overlap and change with traffic and conditions
Very small cells (micro-cells) require more base stations, increasing infrastructure cost
Too many small cells cause excessive handoffs, degrading quality

channel-fading

2026-04-21T00:00:00.000Z

The Problem

Radio signals traveling through the air do not maintain constant strength. Due to constructive and destructive interference from multipath propagation, and due to shadowing from obstacles, the received signal strength fluctuates rapidly over time. These fluctuations can cause the signal to drop below the receiver’s sensitivity threshold, causing communication failures.

Core Idea

Channel fading refers to variations in received signal amplitude and phase over time. It occurs because multiple copies of the transmitted signal (arriving via different paths) combine constructively or destructively at the receiver, and because the transmission path itself changes as objects move in the environment.

How It Works

The transmitted signal travels via multiple paths (multipath propagation)
Each copy has a different phase (determined by path length)
When copies add up in phase, they create constructive interference — signal strength increases
When copies are out of phase, they create destructive interference — signal strength decreases
As the mobile device moves, the relative path lengths change continuously, causing rapid strength fluctuations

Fading typically follows statistical distributions: Rayleigh fading when there is no dominant direct path (typical in urban environments), Rician fading when there is a dominant line-of-sight component.

Key Properties

Fast fading: Rapid fluctuations due to multipath (changes over milliseconds as the device moves)
Slow fading (shadowing): Gradual changes due to large obstacles blocking the signal path
Depth of fading: Can cause signal to drop 20–30 dB below average, making communication impossible without error correction
Frequency-selective fading: Different frequencies fade differently if the channel bandwidth exceeds the coherence bandwidth
Time-varying: The channel changes continuously as the device moves

Connections

Built from: Multipath Propagation — fading is caused by the constructive/destructive superposition of multipath signals
Related: Diversity Antenna — diversity techniques combat fading by receiving signals via multiple independent paths
Related: Spread Spectrum — spreading the signal over a wide bandwidth reduces the impact of narrowband fading
Related: Modulation — robust modulation schemes (like PSK with coding) are more resilient to fading

Edge Cases & Gotchas

Fading is most severe at certain speeds — too slow and the channel doesn’t change enough for diversity; too fast and tracking becomes impossible
Fading margins (extra signal power) must be built into link budgets to ensure reliable communication
Simple path loss models (free space) do not account for fading — realistic models need shadowing and multipath components
Diversity combining (selection, maximal ratio combining) can provide 10–30 dB of improvement

co-channel-interference

2026-04-21T00:00:00.000Z

The Problem

In a cellular system, the same frequencies are reused in geographically separated cells. However, radio signals propagate beyond cell boundaries. When two cells using the same frequency (co-channel cells) transmit simultaneously, their signals interfere with each other. This co-channel interference degrades call quality and must be managed through adequate geographical separation.

Core Idea

Co-channel interference is the interference between signals from co-channel cells — cells that use the same frequency set. The key metric is the Carrier-to-Interference ratio (C/I): the ratio of desired signal power to interference power. A high C/I means a clear call; a low C/I means a noisy, unusable call.

How It Works

Two geographically separated cells use the same frequency (co-channel cells)
Signals from the distant co-channel cell travel beyond its boundaries
The desired signal and the interfering signal arrive at a receiver in the overlap zone
C/I = (desired signal strength) / (interfering signal strength)
For acceptable quality, C/I must exceed a minimum threshold (typically 9 dB for GSM)
To maintain C/I, the co-channel reuse ratio Q = D/R must be large enough (D = co-channel distance, R = cell radius)
Q is increased by: larger cluster size N (more cells per cluster) or sectoring (directional antennas)

Key Properties

C/I ratio: primary quality metric; must exceed minimum threshold
Directly constrains frequency reuse — determines minimum co-channel reuse distance D
Q = √(3N) where N is the cluster size; N=7 gives Q=4.58
Non-co-channel (adjacent channel) interference: reduced by guard bands between adjacent frequencies
Sectoring reduces interference by focusing antenna energy away from co-channel cells
Co-channel interference is a fundamental limit on cellular capacity

Connections

Related: Frequency Reuse — co-channel interference is what constrains frequency reuse planning
Related: Cellular Mobile System — interference management is fundamental to cellular design
Related: Sectoring — sectored antennas reduce co-channel interference
Related: Cell Splitting — splitting cells can increase interference if not planned carefully
Contrasts with: Adjacent Channel Interference — interference from nearby frequencies, reduced by guard bands

Edge Cases & Gotchas

In urban areas, building reflections can create unexpected interference paths
C/I requirements vary with modulation scheme; higher-order modulation needs higher C/I
Power control helps maintain C/I by adjusting mobile transmit power
Soft handoff in CDMA provides macro-diversity, which also reduces co-channel interference

code-division-multiple-access

2026-04-21T00:00:00.000Z

The Problem

In FDMA and TDMA systems, each user gets an exclusive frequency band or time slot. Adding more users means taking away resources from others. CDMA takes a fundamentally different approach: all users share the same frequency at the same time, distinguished only by unique pseudo-random codes. This allows soft capacity (interference-limited) and soft handoff, but requires sophisticated power control.

Core Idea

CDMA (Code Division Multiple Access) is a spread-spectrum digital cellular technology where all users transmit on the same frequency simultaneously. Each user’s signal is spread with a unique orthogonal code; the receiver uses the code to extract its signal from the mixture. The system is interference-limited, not hard-capacity-limited.

How It Works

Spreading: Each user’s data bits are combined (XOR) with a high-rate chipping sequence (e.g., 128 chips per bit in IS-95)
Orthogonal Codes: Walsh functions or other orthogonal code sets ensure each user’s spread signal is distinguishable
All Users Share: All voice and data transmissions use the same 1.25 MHz bandwidth
At the Receiver: The despreading correlator correlates the received signal with the intended user’s code
Processing Gain: The ratio of spread bandwidth to information bandwidth; provides processing gain (interference rejection)
Power Control: Essential — strong users drown out weak users; base station commands all mobiles to equalize received power
Soft Handoff: Mobile connects to multiple base stations during handoff; both signals are combined

Key Properties

All users share the same 1.25 MHz band (IS-95) or wider band (3G, 4G)
Soft capacity: capacity is limited by total interference, not fixed channel count
Soft handoff: make-before-break; multiple BTS links combined at the receiver
Near/far problem is severe: requires strict power control
Processing gain: spreading provides resistance to narrowband interference
Voice activity detection: slots are freed during silence, increasing capacity

Connections

Built from: Spread Spectrum — CDMA is the fundamental spread spectrum technique
Built from: Far Terminal Effect — the problem that defines CDMA power control requirements
Built from: Power Control — the critical requirement for CDMA
Related: Soft Handoff — CDMA’s handoff mechanism provides macro-diversity
Related: WCDMA — the UMTS 3G version of CDMA
Related: IS-95 — the first CDMA cellular standard (2G)

Edge Cases & Gotchas

CDMA capacity is not fixed — more users = more interference = degraded quality for all
Without power control, the near/far effect limits capacity to a handful of users
First CDMA networks (IS-95) had capacity close to GSM; 3G CDMA (WCDMA) significantly improved
Processing gain (Spreading factor) decreases as data rates increase

dama

2026-04-21T00:00:00.000Z

The Problem

Fixed channel assignment (FAMA) permanently allocates channels to cells, causing waste when some cells are lightly loaded while others are overloaded. In satellite and cellular systems where traffic is bursty and unpredictable, a dynamic allocation mechanism that assigns channels only when needed is far more efficient.

Core Idea

DAMA (Demand Assigned Multiple Access) is a combination of random access (Aloha) for requesting channels and fixed assignment (TDM) for actual data transmission. Channels are allocated on-demand from a central pool and returned when the session ends, maximizing resource utilization.

How It Works

Explicit Reservation DAMA (Reservation Aloha):

Time is divided into reservation phases and transmission phases
During the reservation phase, stations send short request packets using Slotted Aloha
Collisions in the reservation phase only destroy the tiny request packet, not data
The base station/satellite collects successful requests and broadcasts a reservation list (TDM pattern)
During the transmission phase, stations transmit exactly in their assigned slots
Zero collisions during data transmission because slots are explicitly reserved

Implicit Reservation (PRMA — Packet Reservation Multiple Access):

Time is divided into frames, each with a fixed number of slots
The base station broadcasts a reservation status vector (e.g., “ACDABA-F”) each frame
New stations contend for free slots using Aloha during their first successful transmission
If successful, all future occurrences of that slot are automatically reserved for that station (implicit)
When the station stops transmitting, the base station marks the slot as free in the next frame

Key Properties

DAMA (explicit): two-phase structure (contention + transmission); very organized
PRMA (implicit): one-phase structure; first successful packet grants reservation automatically
Explicit reservation: guaranteed slots but wastes bandwidth on request packets
Implicit reservation: zero request overhead; efficient for voice calls but initial collision risk
PRMA is optimized for periodic traffic (e.g., voice) where continuous slot allocation is needed
Both schemes protect actual data transmission from collisions

Connections

Built from: Reservation Aloha — DAMA is a form of reservation Aloha
Related: PRMA — PRMA is an implicit reservation variant of DAMA
Related: Slotted Aloha — DAMA uses Slotted Aloha for the contention/reservation phase
Related: Multiplexing — both TDMA and DAMA are multiplexing schemes
Related: IEEE 802.11 — 802.11 PCF and HCF have reservation-like mechanisms

Edge Cases & Gotchas

Explicit DAMA: higher delay under light load (must go through request phase first)
PRMA: voice activity detection is critical — if a voice call ends but the speaker is silent (no packet), the reservation is released
PRMA drop probability must be kept very low (e.g., 1%) for voice quality
PRMA requires a voice activity detector to efficiently use slots during silence periods

exposed-terminal-problem

2026-04-21T00:00:00.000Z

The Problem

The complementary problem to the hidden terminal: sometimes a node correctly detects that a neighbor is transmitting, but the detection causes it to unnecessarily refrain from transmitting. The node misinterprets nearby transmission as a reason to wait, even though its own transmission would not interfere with the ongoing one (because the intended receivers are far apart). This wastes available bandwidth and reduces network efficiency.

Core Idea

The exposed terminal problem occurs when a node (C) that is within range of an active transmitter (B) unnecessarily defers its own transmission, even though its signal would not interfere with B’s reception (because C’s intended receiver D is far away). C is “exposed” to B’s signal but doesn’t realize its own signal won’t cause problems.

How It Works

Node B is transmitting to node A (B is within range of A, not within range of C)
Node C wants to transmit to node D (D is far from both A and B)
C senses the channel and detects B’s ongoing transmission
C incorrectly concludes it cannot transmit without causing interference
C waits unnecessarily, even though its signal to D would not interfere with B’s signal to A
Bandwidth is wasted — C could have successfully transmitted to D simultaneously

The key insight: if the intended receivers are sufficiently separated, simultaneous transmissions are possible without collision.

Key Properties

Waste of bandwidth: node C unnecessarily waits when transmission would succeed
Caused by over-conservative carrier sensing (no distinction between harmful and harmless interference)
More subtle than hidden terminal — requires understanding of receiver locations
Can be partially solved by RTS/CTS exchanges (MACA) which announce intended receivers
Not as severe as hidden terminal but still reduces network efficiency significantly

Connections

Related: Hidden Terminal Problem — the complementary problem; both arise from range limitations
Related: MACA — MACA’s RTS/CTS handshake also helps exposed terminals by announcing clear receiver zones
Related: CSMA — carrier sensing that causes the exposed terminal problem
Related: Far Terminal Effect — another wireless MAC efficiency problem

Edge Cases & Gotchas

The exposed terminal problem is less severe than the hidden terminal problem in terms of call quality
It mainly affects throughput (capacity), not call success rates
Directional antennas can help — C can transmit in a direction different from B’s reception zone
RTS/CTS helps resolve both hidden and exposed terminal problems by explicitly announcing receiver locations

frequency-reuse

2026-04-21T00:00:00.000Z

The Problem

The radio spectrum is a scarce, finite resource. A single high-power transmitter covering an entire city can only support a limited number of simultaneous calls (equal to the number of available frequency channels). With millions of potential users, this architecture fails. The solution is to divide the city into small cells, each with a low-power base station, and reuse frequencies geographically.

Core Idea

Frequency reuse is the technique of using the same set of radio frequencies in multiple geographically separated, non-adjacent cells. Because cells using the same frequencies are far apart, their signals are weak enough at each other’s locations to avoid interference. This allows thousands of simultaneous calls with a limited spectrum.

How It Works

The geographical area is divided into small hexagonal cells, each served by a low-power base station
Cells are grouped into clusters (e.g., cluster size N=7 means 7 cells share a unique set of frequencies)
The same frequencies are reused only in cells that are sufficiently far apart (the co-channel reuse distance D)
The co-channel reuse ratio Q = D/R, where R is the cell radius; higher Q means less interference but lower capacity
The cluster pattern (number of cells per cluster) determines the trade-off between interference and capacity
Formula: Q = √(3N) where N = i² + ij + j², with i and j as non-negative integers

Capacity is directly proportional to the number of times the cluster pattern is repeated across the coverage area.

Key Properties

Enables massive capacity scaling: total capacity = N_channels × number_of_clusters
Cluster size N=7 is the most common standard design
Higher co-channel reuse ratio Q = better signal quality but lower capacity per area
Lower N (e.g., N=4) = higher capacity but more co-channel interference
Cell splitting (reducing cell radius) increases capacity without new spectrum
Sectoring (dividing each cell into 3 or 6 sectors) further reduces interference

Connections

Built from: Cellular Mobile System — frequency reuse is the foundational concept that enables cellular architecture
Built from: Co-Channel Interference — the reason for the reuse distance constraint
Related: Cell Splitting — technique to increase capacity by shrinking cells
Related: Sectoring — directional antennas that improve reuse efficiency
Related: Multiplexing — SDMA is the spatial division principle behind frequency reuse

Edge Cases & Gotchas

Reuse distance must be maintained — cells using the same frequency too close will have severe co-channel interference
N must be chosen to balance interference vs. capacity; N=7 is a conservative but common choice
In practice, frequency reuse planning is complex due to irregular terrain and building shadowing
Handoff zones at cell boundaries must be carefully designed to avoid dropped calls

frequency-shift-keying

2026-04-21T00:00:00.000Z

The Problem

Standard FSK (Frequency Shift Keying) uses two distinct frequencies to represent binary 0 and 1. However, when the data transitions between bits, the FSK receiver resets the carrier wave to start fresh at every bit boundary. This causes sharp phase discontinuities (jumps) in the waveform, which generates high-frequency noise called “frequency splatter.” This wastes bandwidth and causes interference in adjacent channels.

Core Idea

FSK is a digital modulation technique where binary data is represented by shifting the carrier frequency between two values: a high frequency for bit 1, and a low frequency for bit 0. The frequency shift must be detectable by the receiver, and the minimum separation determines orthogonality.

How It Works

The carrier frequency is switched between two values: f₁ for bit 1, f₀ for bit 0
At each bit transition, the standard FSK receiver generates a fresh carrier wave starting from zero phase
If the previous bit ended at a negative phase and the new bit starts from zero, a sharp discontinuity (phase jump) occurs
These discontinuities create frequency components outside the assigned bandwidth — “splatter”
The frequency separation Δf = |f₁ - f₀| determines orthogonality and bandwidth efficiency

Minimum orthogonal separation: Δf = 1/(2T) where T is the bit duration. This is the minimum shift keying (MSK) condition.

Key Properties

Binary modulation: two frequencies, easy to detect
Frequency separation determines orthogonality and bandwidth
Standard FSK (with phase reset) has phase discontinuities at bit boundaries
Phase discontinuities generate high-frequency splatter, wasting bandwidth
MSK is the continuous-phase form of FSK with minimum orthogonal frequency separation
GMSK (Gaussian MSK) is a filtered version of MSK used in GSM

Connections

Built from: Modulation — FSK is a specific digital modulation technique
Built from: MSK — MSK is the continuous-phase version of FSK that eliminates the phase reset problem
Related: GSM — GSM uses GMSK (Gaussian MSK), an MSK variant, as its modulation scheme
Related: PSK — another digital modulation technique that avoids phase reset by using phase changes instead
Related: ASK — digital modulation using amplitude changes

Edge Cases & Gotchas

Standard FSK is rarely used in modern systems due to bandwidth inefficiency
All modern cellular systems use continuous-phase schemes (MSK, GMSK, QPSK)
The phase reset problem means receiver complexity is higher for standard FSK
GMSK applies a Gaussian filter before modulation to further smooth the frequency transitions

gprs

2026-04-21T00:00:00.000Z

The Problem

GSM was designed primarily for voice (circuit-switched). Data was an afterthought, limited to 9.6 kbps. As the internet grew, there was massive demand for wireless packet data (email, web browsing, WAP). GPRS added packet-switched data service to GSM without replacing the voice infrastructure — “2.5G.”

Core Idea

GPRS (General Packet Radio Service) adds packet-switching capability to the existing GSM circuit-switched voice network. It creates a parallel packet network (using the same air interface and base stations) optimized for bursty data traffic instead of continuous voice.

How It Works

Architecture Addition: Two new network elements are added to GSM:
- SGSN (Serving GPRS Support Node): Handles packet routing, mobility, and authentication for data
- GGSN (Gateway GPRS Support Node): Interfaces GPRS to the internet (IP networks)
Channel Allocation: Data uses physical channels differently than voice:
- Up to 8 timeslots can be combined for a single user
- Channels are dynamically allocated only when data traffic exists
- No dedicated circuit — shared, packet-based
New Air Interface:
- Uses the same TDMA/FDMA frame structure but with new channel coding schemes (CS-1 to CS-4, up to 21.4 kbps per timeslot)
- Theoretical maximum: 171.2 kbps (8 timeslots × 21.4 kbps)
- Practical: 40–50 kbps typical, up to 115 kbps in best conditions
Data Packets: All data is in IP packets, routed through the GPRS backbone
Always-On: No dial-up — IP address is assigned and connection is always available

Key Properties

Packet-switched: bandwidth used only when transmitting data, not continuously
Dynamic allocation: 1–8 timeslots per user based on traffic
Theoretical max: 171.2 kbps; typical 40–50 kbps
Always-on IP connection; no circuit establishment delay
Works simultaneously with voice (uses different channel set from voice)
First deployed in 2000; enabled first mobile internet era

Connections

Built from: GSM Architecture — GPRS builds on existing GSM infrastructure
Built into: GSM — GPRS is an integral part of GSM Phase 2+ specifications
Builds into: EDGE — EDGE (3G) builds on GPRS infrastructure
Related: WCDMA — the competing 3G technology
Related: SMS — SMS was separate from GPRS; both now use the GPRS backbone

Edge Cases & Gotchas

GPRS was never intended for streaming video or high-speed data — too slow
First-gen GPRS was often sold as “wireless internet” with poor throughput
Billing: early GPRS was charged per packet (kilobyte), causing bill shocks
Handover between voice and data: GSM handles voice first; data gets preempted

gsm-architecture

2026-04-21T00:00:00.000Z

The Problem

A cellular mobile network requires coordinated management of radio resources, call routing, subscriber databases, and mobility across many cells and thousands of users. Simply connecting base stations to the public switched telephone network (PSTN) is insufficient — the network needs specialized subsystems to handle radio access, switching, authentication, and network operations.

Core Idea

GSM architecture is organized into three hierarchical subsystems: RSS (Radio Subsystem — handles the air interface), NSS (Network and Switching Subsystem — handles call routing and databases), and OSS (Operation Subsystem — handles network management and security). Users only see Mobile Stations (MS) and BTS antenna masts, but the entire infrastructure is coordinated.

How It Works

RSS (Radio Subsystem):

MS (Mobile Station): User device with Mobile Equipment (ME, identified by IMEI) and SIM card (identified by IMSI)
BTS (Base Transceiver Station): Physical antenna and transceivers; performs radio modulation/demodulation, channel coding, error correction
BSC (Base Station Controller): Controls multiple BTSs; allocates radio resources, manages intra-BSC handovers

NSS (Network and Switching Subsystem):

MSC (Mobile Switching Center): High-performance digital ISDN switch; call setup, routing, inter-BSC handover
HLR (Home Location Register): Central database of all subscribers; permanent data (IMSI, services), current location
VLR (Visitor Location Register): Temporary database attached to each MSC; caches subscriber data from HLR for fast access
GMSC (Gateway MSC): Interface between GSM and external networks (PSTN, ISDN); routes incoming calls

OSS (Operation Subsystem):

AUC (Authentication Center): Generates security triplets (RAND, SRES, Kc) for SIM authentication and call encryption
EIR (Equipment Identity Register): Tracks device IMEI numbers; maintains white/grey/black lists
OMC (Operation and Maintenance Center): Network monitoring, alarm management, remote configuration

Key Interfaces:

Um interface: MS ↔ BTS (air interface, wireless radio)
Abis interface: BTS ↔ BSC (wired links, 16/64 kbps)
A interface: BSC ↔ MSC (2 Mbps PCM)
O interface: BSC/MSC ↔ OMC (SS7 signaling)

Connections

Built from: Cellular Mobile System — GSM is the most widely deployed 2G cellular system
Built into: GSM Services — bearer, tele, and supplementary services
Built into: GSM Air Interface — Um interface protocols
Related: Handoff — inter-BSC/inter-MSC handovers are MSC functions
Related: Location Updates — HLR/VLR update mechanism for mobility
Related: AUC — security subsystem for SIM verification

Edge Cases & Gotchas

N=7 cluster is standard; smaller clusters (N=4) increase capacity but reduce C/I ratio
Sectoring: 3 sectors per cell is common; reduces co-channel interference by factor of 3
GSM frequency bands: GSM 900 (890–915/935–960 MHz), GSM 1800 (1710–1785/1805–1880 MHz), GSM 1900 (1850–1910/1930–1990 MHz)
GSM-Rail (GSM-R): specialized variant for railroad control with priority calls and VGCS/VBS

gsm-services

2026-04-21T00:00:00.000Z

The Problem

A cellular network must offer more than just voice calls. Users need data services, emergency calls, messaging, and supplementary features like call forwarding. GSM defines three layered service categories to comprehensively cover all communication needs from raw data transport to end-user applications.

Core Idea

GSM defines three categories of services: bearer services (transparent data transport at Layer 1–3), tele services (end-user applications at all 7 OSI layers), and supplementary services (enhancements over basic telephony like call forwarding and call waiting).

How It Works

Bearer Services:

Transparent transmission of data between network interfaces
Up to 9.6 kbps for data; synchronous or asynchronous
Transparent: constant delay, FEC only; uses only Physical layer
Non-transparent: RLP protocol with ARQ, error correction; uses Layers 2–3
Interworks with PSTN, ISDN, and X.25 packet data networks

Tele Services:

Encrypted voice transmission (primary service)
SMS (Short Message Service): up to 160 characters; uses signaling channels
EMS (Enhanced Message Service): larger messages, formatted text, images, tones
MMS (Multimedia Message Service): larger pictures, video clips
Group 3 fax: standard ITU-T T.4/T.30 fax over voice channels
Emergency number (112): highest priority, free, works throughout coverage area

Supplementary Services:

Caller ID, call forwarding, call waiting, call hold
Closed user groups: company-specific sub-networks
Multi-party communication, call barring
VLRs and other carrier-specific services

Key Properties

Bearer services: Layer 1–3 of OSI model
Tele services: all 7 OSI layers, end-to-end
SMS uses dedicated signaling channels, not voice channels
MMS evolved from SMS and is used for rich mobile content
Emergency services pre-empt other calls and are free

Connections

Built from: GSM Architecture — the GSM infrastructure delivers these services
Built from: GSM Air Interface — bearer services are transported over Um interface
Related: GSM Protocols — the protocol layers that implement these services
Related: HLR — HLR stores subscribed service profiles
Related: SMS — one of GSM’s tele services

Edge Cases & Gotchas

SMS was designed for network operator messaging but became a massive revenue generator
MMS requires WAP infrastructure and is distinct from SMS
Bearer services in early GSM (9.6 kbps) were very slow by modern standards
GPRS (2.5G) dramatically improved bearer services with packet switching

handoff

2026-04-21T00:00:00.000Z

The Problem

When a mobile user moves from one cell to another while on a call, the call must be transferred from the old base station to the new one without interrupting the call. If this is not done seamlessly, the call drops. Handoff management is a critical mobility function that maintains continuous connectivity as users move across cell boundaries.

Core Idea

Handoff is the process of transferring an active call from one base station to another as the mobile moves across cell boundaries. There are two types: hard handoff (break-before-make, used in GSM with FDMA/TDMA) where the old connection is dropped before establishing the new one, and soft handoff (make-before-break, used in CDMA) where the new connection is established before dropping the old one.

How It Works

Hard Handoff (GSM — Break Before Make):

As the mobile moves, signal strength from the current BTS decreases
BSC detects the need for handoff when signal falls below a threshold
BSC commands the mobile to retune to a new frequency channel (new BTS)
Old connection is dropped; new connection is established on the new channel
A gap in transmission occurs; if timing is off, the call drops
GSM uses hard handoff because FDMA/TDMA channels are dedicated and cannot be shared

Soft Handoff (CDMA — Make Before Break):

Mobile maintains connection with both old and new base stations simultaneously
Both links are active; the stronger signal is used for voice
Once the new link is confirmed stable, the old link is dropped
Seamless handover with no dropped calls
CDMA uses soft handoff because all users share the same frequency — multiple links can coexist

Key Properties

Hard Handoff (GSM): break-before-make; simple but risk of dropped calls
Soft Handoff (CDMA): make-before-break; seamless but more complex
Handoff decisions based on signal strength, quality, distance
Intra-BSC handoff: handled by BSC alone (faster, no MSC involvement)
Inter-BSC handoff: BSC coordinates with MSC (slower, more overhead)
Excessive handoffs (ping-pong effect) waste resources and degrade quality

Connections

Built from: Cellular Mobile System — handoff is what makes mobility possible
Built from: GSM Architecture — BSC and MSC coordinate handoffs
Built into: GSM — GSM uses hard handoff because of FDMA/TDMA
Related: Soft Handoff — CDMA’s handoff technique
Related: Dropped Call Rate — metric used to evaluate handoff performance

Edge Cases & Gotchas

Ping-pong handoff: rapid oscillation between two cells wastes resources
Handoff latency must be minimal to avoid perceptive gaps in voice
In CDMA, soft handoff creates a macro-diversity benefit — combining signals from multiple BTSs improves quality
Hard handoff in GSM was acceptable for 2G voice but problematic for 3G data

hidden-terminal-problem

2026-04-21T00:00:00.000Z

The Problem

In wired Ethernet, CSMA/CD (Carrier Sense Multiple Access with Collision Detection) allows nodes to detect collisions while transmitting. In wireless networks, when a device is transmitting, it cannot hear any incoming signals because its own transmission drowns out all incoming signals at its own radio. This means nodes cannot detect collisions in progress, and a special problem emerges: nodes that are out of each other’s range (hidden from each other) can both transmit to a common receiver, causing a collision at the receiver that neither sender can detect.

Core Idea

The hidden terminal problem occurs when two nodes (A and C) cannot detect each other directly (they are “hidden”), but both are within range of a common receiver (B). A and C both sense the channel and find it free (because neither can hear the other), so both transmit to B simultaneously, causing a collision at B that neither A nor C can detect.

How It Works

Node A begins transmitting to node B
Node C, which is not within range of A, checks the channel — it cannot hear A’s transmission
C concludes the channel is free
C begins transmitting to B
Both signals arrive at B simultaneously → Collision
A and C complete their transmissions without knowing a collision occurred
B receives garbled data and cannot decode either message

Standard CSMA fails because carrier sense only detects local transmissions, not hidden nodes.

Key Properties

Occurs due to range limitations: A and C cannot sense each other’s transmissions
Collision happens at the receiver (B), not at the transmitters
Transmitters cannot detect the collision (collision detection fails in wireless)
Common in large cells, ad hoc networks, and networks with obstacles between nodes
Requires a specialized MAC protocol to solve

Connections

Related: MACA — the MAC protocol that solves this using RTS/CTS exchange
Related: Exposed Terminal Problem — the complementary problem in wireless MAC
Related: Far Terminal Effect — another wireless MAC problem
Related: CD — the Ethernet protocol that fails in this wireless scenario
Related: CSMA — the foundational carrier sense technique that fails for hidden nodes

Edge Cases & Gotchas

The hidden terminal problem is inherent to wireless networks and cannot be fully eliminated
The RTS/CTS handshake reduces efficiency due to extra control packets
Not all wireless protocols use collision avoidance (802.11b uses CSMA/CA with ACK instead)
In dense ad hoc networks, hidden terminals can form chains of 3+ nodes

ieee-802-11

2026-04-21T00:00:00.000Z

The Problem

Wired Ethernet (IEEE 802.3) provides reliable local area networking, but cables limit device placement and mobility. IEEE 802.11 (Wi-Fi) provides wireless Ethernet networking, enabling untethered devices within buildings, campuses, and hotspots while maintaining compatibility with Ethernet network protocols.

Core Idea

IEEE 802.11 is the wireless LAN (WLAN) standard that provides Ethernet-like connectivity over radio waves. It uses CSMA/CA (Collision Avoidance) with ACK and RTS/CTS mechanisms, operates in the 2.4 GHz and 5 GHz bands, and defines the MAC and physical layer for wireless local networking.

How It Works

The MAC Protocol (CSMA/CA):

Before transmitting, a station senses the channel
If busy, it waits; if idle, it waits for a random backoff period (collision avoidance)
The station transmits, receives an ACK from the destination (if not received, collision assumed and transmission retries)
For larger frames, RTS/CTS exchange (per MACA) avoids hidden terminal collisions

Physical Layer Variants:

802.11b (1999): DSSS; 11 Mbps; 2.4 GHz band; longest range
802.11a (1999): OFDM; 54 Mbps; 5 GHz band; shorter range due to frequency
802.11g (2003): OFDM backward-compatible with 802.11b; 54 Mbps; 2.4 GHz band
802.11n (2009): MIMO + OFDM; up to 600 Mbps; dual-band (2.4/5 GHz)
802.11ac (2013): MU-MIMO; up to 3.46 Gbps; 5 GHz only
802.11ax (2021): OFDMA; up to 9.6 Gbps; Wi-Fi 6 (6E extends to 6 GHz)

Key Properties

MAC: CSMA/CA with ACK (not CSMA/CD like Ethernet)
CSMA/CA: random backoff before transmission to avoid collisions
RTS/CTS for hidden terminal avoidance in large frames
Infrastructure mode: communication through AP; Ad Hoc mode: direct device-to-device
Security: WEP → WPA → WPA2 → WPA3 (evolving encryption)
OFDM (from 802.11a): uses multiple orthogonal subcarriers for efficiency

Connections

Built from: Hidden Terminal Problem — 802.11 uses RTS/CTS to handle this
Built from: Spread Spectrum — DSSS (802.11b) and FHSS (Bluetooth) are the two spread spectrum techniques
Built from: MACA — 802.11’s CSMA/CA is based on MACA’s RTS/CTS mechanism
Related: IEEE 802.15 — the PAN standard (Bluetooth) sibling to 802.11
Related: Bluetooth — coexists with 802.11 in 2.4 GHz but uses FHSS, not DSSS

Edge Cases & Gotchas

2.4 GHz is crowded with microwaves, Bluetooth, and other devices; interference is common
5 GHz band (802.11a/n/ac/ax) is cleaner but has shorter range (more wall absorption)
Wi-Fi operates in unlicensed spectrum — no exclusivity; interference from neighbors is a constant issue
Security: WEP was cracked; WPA is mandatory in modern devices

maca

2026-04-21T00:00:00.000Z

The Problem

Standard CSMA (Carrier Sense Multiple Access) fails in wireless networks because: (1) collision detection is impossible (a node can’t hear while transmitting), and (2) the hidden terminal problem means nodes that are out of range don’t sense each other’s transmissions. MACA (Multiple Access with Collision Avoidance) was designed to solve these wireless MAC problems using a control packet handshake before data transmission.

Core Idea

MACA is a MAC protocol that solves the hidden terminal problem through a four-way handshake using RTS (Request to Send) and CTS (Clear to Send) control packets. Before transmitting data, the sender requests permission from the receiver, and the receiver announces a reservation that other stations can hear, preventing hidden nodes from transmitting during the reserved data period.

How It Works

RTS (Request to Send): Node A sends a short RTS packet to the intended receiver (Node B), containing the data length and sender address
All nodes within range of A hear the RTS and record the intended receiver (B) and the reservation duration
CTS (Clear to Send): Node B replies with a CTS packet containing the same reservation duration
All nodes within range of B (including the hidden node C) hear the CTS and defer their transmissions for the reserved duration
DATA transmission: A transmits data to B
ACK: B acknowledges successful data reception
The hidden node C hears either the RTS (from A) or the CTS (from B) and waits, preventing collision at B

Key Properties

Eliminates the hidden terminal problem by making hidden nodes defer through CTS
Reduces exposed terminal problem by announcing intended receiver zones
Control packets (RTS/CTS) are short, so collisions during control packets only waste small packets
Requires a NAV (Network Allocation Vector) mechanism to track reservations
Used as the basis for IEEE 802.11’s CSMA/CA protocol
RTS/CTS exchange adds overhead but significantly improves performance in high-contention scenarios

Connections

Built from: Hidden Terminal Problem — MACA directly solves this
Built from: Exposed Terminal Problem — MACA’s RTS/CTS also mitigates this
Related: Far Terminal Effect — a different MAC problem solved by power control, not MACA
Related: CD — the Ethernet protocol that fails without these mechanisms
Related: IEEE 802.11 — 802.11’s CSMA/CA is based on MACA’s RTS/CTS mechanism
Related: Reservation Aloha — another reservation-based MAC protocol

Edge Cases & Gotchas

RTS/CTS overhead reduces efficiency in low-traffic scenarios
Not all hidden node scenarios are solved — nodes that cannot hear RTS or CTS (third-order hidden nodes) remain problematic
Control packet collisions still occur with pure Aloha within RTS/CTS exchange
In practice, RTS/CTS is only enabled for data frames exceeding a certain length threshold

minimum-shift-keying

2026-04-21T00:00:00.000Z

The Problem

Standard FSK resets the carrier wave at every bit boundary, causing phase discontinuities (sharp corners) that generate “frequency splatter” — unwanted high-frequency components that waste bandwidth and interfere with adjacent channels. MSK was designed to solve this specific problem while maintaining the minimum possible frequency separation for orthogonality.

Core Idea

MSK is a continuous-phase frequency shift keying technique where the carrier frequency shifts between two values (for bit 1 and bit 0) without any phase reset. The frequency separation is kept at the mathematical minimum required for the receiver to distinguish the two signals. This produces a smooth, unbroken waveform with no frequency splatter.

How It Works

Phase Memory: Unlike standard FSK, MSK does NOT reset the carrier wave at bit boundaries. The new frequency starts exactly where the previous frequency ended — a continuous phase transition.
Minimum Frequency Separation: The frequency shift Δf = 1/(2T) where T is the bit duration — this is the minimum possible separation for orthogonal signals (where the cross-correlation is zero).
Continuous Phase FSK (CPFSK): MSK is a special case of CPFSK with minimum frequency separation.
Waveform construction (Even/Odd Split Method): Data bits are split into even and odd positions; each is stretched to 2 slots. Reference waves (low frequency completing half cycle per slot, high frequency completing full cycle per slot) are copied or inverted based on the data bit. The resulting signal is smooth throughout.

The smooth waveform avoids sharp discontinuities, which means no high-frequency spectral leakage.

Key Properties

Continuous phase: no phase reset, no frequency splatter
Minimum orthogonal frequency separation: Δf = 1/(2T)
High spectral efficiency: bandwidth is very tight and well-contained
Constant envelope: power amplifier operates at maximum efficiency
Used as the basis for GMSK (Gaussian MSK) in GSM
QPSK can be viewed as two orthogonal MSK signals combined

Connections

Built from: FSK — MSK is the continuous-phase variant of FSK
Built from: Modulation — MSK is a digital modulation technique
Related: GSM — GSM uses GMSK (Gaussian MSK), which applies a Gaussian filter to MSK
Related: PSK — PSK (especially QPSK) is related to MSK and used alongside it in many systems
Related: DSSS — both MSK and DSSS are spectrally efficient digital modulation techniques
Related: Spread Spectrum — MSK is a continuous-phase spread spectrum modulation technique

Edge Cases & Gotchas

MSK is rarely used directly in its raw form — GMSK (with Gaussian filtering) is the practical variant
The continuous phase property is critical — even small timing errors can break orthogonality
MSK and offset-QPSK (OQPSK) are closely related; OQPSK uses the same even/odd bit splitting method
In practice, MSK is filtered with a Gaussian filter before transmission to further smooth transitions

mobile-ad-hoc-network

2026-04-21T00:00:00.000Z

The Problem

Infrastructure-based wireless networks (Wi-Fi, cellular) require pre-deployed base stations or access points. In disaster zones, battlefields, or remote areas, pre-deployed infrastructure doesn’t exist. Mobile Ad Hoc Networks (MANETs) enable devices to communicate directly with each other without any pre-existing infrastructure, by dynamically forming multi-hop routes through intermediate devices.

Core Idea

A Mobile Ad Hoc Network (MANET) is a self-configuring network of mobile devices that connect and route data through each other. Devices act as both hosts (running applications) and routers (forwarding packets for other devices). Routes are discovered and maintained dynamically as devices move.

How It Works

Key Characteristics:

Self-Organizing: No central infrastructure; devices discover neighbors and form a network
Multi-Hop Routing: If the destination is out of range, intermediate devices forward the packet
Dynamic Topology: Devices move, join, and leave — topology changes continuously
Distributed Operation: All devices participate in routing, security, and network management

Routing Protocols:

Proactive (Table-Driven): Routes maintained continuously; fast lookup but high overhead (e.g., DSDV, OLSR)
Reactive (On-Demand): Routes discovered only when needed; low overhead but high delay (e.g., AODV, DSR)
Hybrid: Combines proactive (local) and reactive (remote) approaches (e.g., ZRP)

Applications:

Military: battlefield communication without fixed infrastructure
Emergency: disaster response when base stations are down
Sensor networks: environmental monitoring
Vehicle-to-vehicle (V2V) communication

Key Properties

No infrastructure: self-healing, resilient to device failures
Multi-hop: can connect devices far beyond single-hop range
Dynamic topology: routes must be continuously maintained
Limited resources: mobile devices have limited battery, CPU, and memory
Security challenges: no trusted infrastructure; vulnerable to attacks on routing protocols
Routing overhead increases with mobility

Connections

Built from: Wireless Network — the foundational wireless technology
Related: IEEE 802.11 — ad hoc mode in 802.11 enables MANET-like communication
Related: Routing Protocols — DSR, AODV, OLSR are designed for MANETs
Related: Mobile Opportunistic Network — evolution of MANET for challenged networks
Related: Wireless Sensor Network — a specific type of MANET

Edge Cases & Gotchas

MANET routing protocols have conflicting design goals: low overhead vs. fast convergence
Battery is the primary constraint — routing must balance energy
Security is critical: routing attacks (black holes, worm holes) are easy to launch
Not suitable for real-time applications without explicit QoS support

mobile-tcp

2026-04-21T00:00:00.000Z

The Problem

Standard TCP assumes packet loss is caused by network congestion (router buffers overflowing), and responds by reducing the congestion window. However, in wireless mobile networks, packet loss is often caused by signal fading, handoff, or temporary disconnection — not congestion. When TCP sees packet loss, it incorrectly throttles the connection, degrading performance by orders of magnitude.

Core Idea

Mobile TCP adaptations modify the standard TCP protocol to handle wireless link characteristics: Indirect TCP (I-TCP) separates the wired and wireless parts of the connection, snooping TCP monitors the wireless link for fast local recovery, and Fast Retransmit/Fast Recovery allow quick recovery without full congestion control resets.

How It Works

Standard TCP Problem:

TCP on the sender sees packet loss at any hop
TCP assumes congestion and halves the congestion window (cwnd)
Slow start re-builds the window from scratch
On wireless links where handoffs cause brief packet loss, this degrades performance drastically

Indirect TCP (I-TCP):

Connection is split at the foreign agent (gateway): wired part (sender ↔ FA) + wireless part (FA ↔ mobile)
On the wired segment, packets are never lost — I-TCP provides a reliable transport
FA performs local recovery of wireless losses
Wired sender never sees wireless losses → no congestion window throttling

Snooping TCP:

The base station (FA) monitors passing packets (“snoops”)
Detects lost packets on the wireless link and performs local retransmission
Buffers packets to enable fast retransmission without sender involvement
Hides wireless losses from the sender’s TCP

Fast Retransmit / Fast Recovery:

On detecting multiple duplicate ACKs, TCP does Fast Retransmit (retx without cwnd reset)
Fast Recovery allows continued transmission while rebuilding cwnd
Less aggressive than full Congestion Control

Key Properties

I-TCP: splits connection; end-to-end semantics lost; FA is a bottleneck
Snooping TCP: FA local recovery; maintains end-to-end semantics
Fast Retransmit: partial recovery without full cwnd halving
All mobile TCP variants hide wireless link characteristics from the sender
Trade-off: network layer transparency vs. performance

Connections

Built from: TCP — standard TCP that fails in mobile environments
Built from: Cellular Mobile System — the environment where standard TCP fails
Related: Mobile IP — handles network layer (IP) mobility; different from transport layer mobility
Related: Congestion Control — the TCP mechanism that misfires in mobile networks

Edge Cases & Gotchas

I-TCP breaks end-to-end semantics — FA crash loses data
Snooping TCP: FA crash loses buffered packets
Mobile IP and Mobile TCP address different problems: network vs. transport layer
For real-time traffic (voice), TCP is fundamentally wrong — UDP with FEC/Raptor codes is better

Modulation Techniques Compared

2026-04-21T00:00:00.000Z

Framing

This synthesis compares the digital modulation techniques covered in the wireless communications source: FSK, MSK, and GMSK. These techniques differ in how they represent binary data on a carrier wave and in their spectral efficiency.

Comparison

Property	FSK (Standard)	MSK	GMSK
Phase	Discontinuous (reset at each bit)	Continuous	Continuous
Frequency Separation	Arbitrary (can be large)	Minimum orthogonal (1/2T)	Minimum (Gaussian filtered)
Spectral Efficiency	Low (frequency splatter)	High (tight bandwidth)	Highest (Gaussian filter tightens)
Phase Reset	Yes — at every bit	No	No
Complexity	Low	Medium	Medium
Used In	Simple paging, remote controls	Military radio	GSM (2G) cellular
Noise Resistance	Moderate	High	High

Key Insights

Why Phase Continuity Matters

Standard FSK resets the carrier wave at each bit boundary. If the previous bit ended at a negative phase, the new bit starts from zero phase, creating a sharp discontinuity. These discontinuities generate high-frequency components (“frequency splatter”) that waste bandwidth and interfere with adjacent channels. MSK and GMSK eliminate discontinuities entirely, producing tight, well-contained spectra.

The Minimum Shift Condition

MSK uses the minimum possible frequency separation for orthogonality: Δf = 1/(2T), where T is the bit duration. Two signals separated by exactly this amount have zero cross-correlation — the receiver can perfectly distinguish them. Any smaller separation would make the signals indistinguishable.

From MSK to GMSK

GMSK applies a Gaussian filter before MSK modulation, pre-shaping the frequency pulses. This further reduces spectral sidelobes, making the signal even more bandwidth-efficient. GSM chose GMSK because its constant envelope property allows efficient power amplification — the power amplifier can operate at maximum output without distorting the signal.

Real-World Progression

In practice, all modern digital systems have moved beyond standard FSK: FSK → CPFSK → MSK → GMSK → QPSK/OQPSK. Higher-order modulation schemes (16-QAM, 64-QAM, 256-QAM) are now used in LTE and 5G for even higher data rates.

Connections

FSK — standard FSK with phase reset
MSK — continuous-phase FSK
Modulation — foundational concept
Spread Spectrum — modulation technique that spreads signal across wide bandwidth

modulation

2026-04-21T00:00:00.000Z

The Problem

Digital data (bits: 0s and 1s) cannot be directly transmitted over radio waves in the air. To transmit data wirelessly, the digital information must be converted into an analog waveform that can propagate through the radio channel. Additionally, low-frequency data would require impractically large antennas — modulation allows data to ride on high-frequency carriers that work with small, practical antennas.

Core Idea

Modulation is the process of encoding digital information onto a high-frequency carrier wave by varying one or more of its properties: amplitude, frequency, or phase. The receiver then demodulates to recover the original data.

How It Works

The digital data stream (bits) needs to be transmitted over the air
A high-frequency sinusoidal carrier wave is generated (e.g., 900 MHz for GSM)
The digital data modifies one or more properties of the carrier:
- Amplitude Shift Keying (ASK): Bit 1 = high amplitude, Bit 0 = low amplitude
- Frequency Shift Keying (FSK): Bit 1 = high frequency, Bit 0 = low frequency
- Phase Shift Keying (PSK): Bit 1 = phase 0°, Bit 0 = phase 180° (or other phase shifts)
The modulated wave is transmitted via the antenna
At the receiver, the carrier is detected and the data is recovered through demodulation

Key Properties

Enables transmission of digital data over analog radio channels
High-frequency carriers allow small antennas (antenna length ≈ wavelength/4)
Allows multiple users to share the spectrum through different carrier frequencies (frequency division)
Different modulation schemes have different bandwidth efficiencies and noise tolerances
Trade-off: Higher-order modulation (more bits per symbol) gives higher data rates but is more vulnerable to noise

Connections

Built from: Wireless Network — modulation is essential for wireless transmission
Built from: Multiplexing — different users use different carrier frequencies
Related: FSK — specific digital modulation technique using frequency
Related: MSK — continuous-phase variant of FSK used in GSM
Related: Spread Spectrum — modulation technique that spreads signal across wide bandwidth

Edge Cases & Gotchas

Noisy channels degrade modulation — error correction coding is needed to recover data
High-order modulation (e.g., 256-QAM) requires very clean signal conditions
Phase modulation (PSK) is more robust to amplitude noise than ASK
MSK and GMSK are used in cellular systems because of their spectral efficiency and constant envelope (power efficiency)

multipath-propagation

2026-04-21T00:00:00.000Z

The Problem

When a radio signal is transmitted, it does not travel in a single straight line to the receiver. Instead, it bounces off buildings, walls, vehicles, and other obstacles, creating multiple copies of the same signal that travel different paths. Each path has a different length, so these copies arrive at the receiver at different times. This causes signal distortion called Inter-Symbol Interference (ISI), where adjacent symbols overlap in time.

Core Idea

Multipath propagation is the phenomenon where a transmitted radio signal arrives at the receiver via multiple paths (direct, reflected, diffracted, scattered), each with different delays and amplitudes. The receiver sees a composite of all these copies.

How It Works

The transmitter sends a single signal pulse
The signal takes multiple paths: direct line-of-sight, reflection off buildings, diffraction over obstacles, scattering from rough surfaces
Each path has a different physical length, causing different travel times
The receiver captures all copies simultaneously — they add up (superpose)
If path delays are significant relative to the bit period, adjacent bits overlap — this is ISI
The composite signal can be stronger (constructive) or weaker (destructive) depending on phase alignment

The delay spread (difference between earliest and latest arriving paths) determines the severity of ISI.

Key Properties

Causes Inter-Symbol Interference (ISI) when delay spread exceeds bit duration
Results in frequency-selective fading — some frequencies are boosted, others are attenuated
More severe in urban environments with many reflecting surfaces
Can be exploited using diversity techniques (multiple receive antennas)
Doppler spread from motion causes time-varying multipath effects

Connections

Built from: Channel Fading — the constructive/destructive combination of multipath signals causes fading
Related: Modulation — modulation scheme choice affects how vulnerable the signal is to multipath-induced ISI
Related: Diversity Antenna — using multiple antennas reduces multipath problems
Related: DSSS — spreading code helps filter out multipath noise

Edge Cases & Gotchas

In dense urban areas, multipath can create 10+ distinct signal paths
High-speed mobility (e.g., in a car) causes rapidly changing multipath patterns
Simple amplitude-based received signal strength indicators (RSSI) cannot distinguish multipath components
OFDM (used in Wi-Fi, 4G, 5G) converts one wide band into many narrow subcarriers, each less affected by frequency-selective multipath

multiplexing

2026-04-21T00:00:00.000Z

The Problem

The radio spectrum is a limited, shared resource. Multiple users and multiple data streams need to coexist in the same geographical area without colliding with each other. How do multiple independent communications share the same physical medium (air) simultaneously?

Core Idea

Multiplexing is the technique of combining multiple signals into a single shared medium for transmission. The key strategies are: dividing by space (SDMA), by frequency (FDMA), by time (TDMA), or by code (CDMA).

How It Works

SDMA (Space Division Multiple Access): Use directional antennas to isolate signals in different physical directions. Cells reuse the same frequencies because they are geographically separated.
FDMA (Frequency Division Multiple Access): Assign each user a unique frequency band. All users transmit simultaneously on different frequencies. Filters at the receiver isolate the desired signal. (Used in analog FM radio.)
TDMA (Time Division Multiple Access): Assign each user a unique time slot within a repeating frame. All users share the same frequency but transmit at different times. (Used in GSM.)
CDMA (Code Division Multiple Access): All users transmit on the same frequency at the same time. Each user’s signal is spread with a unique orthogonal code. Receivers use the code to extract their signal. (Used in 3G cellular.)

Key Properties

SDMA: Requires directional or sectorized antennas; enables frequency reuse across cells
FDMA: Simple to implement; guard bands needed to prevent adjacent channel interference
TDMA: Requires strict time synchronization; allows one transmitter to use the full frequency bandwidth during its slot
CDMA: All users share all resources; soft capacity (interference-limited); requires sophisticated power control
Can be combined: GSM uses both FDMA (carrier frequencies) and TDMA (time slots within each frequency)

Connections

Built from: Modulation — multiplexing allows different modulated carriers to coexist
Built into: GSM — GSM uses FDMA + TDMA for channel access
Built into: Cellular Mobile System — frequency reuse is a form of space-division multiplexing
Related: CDMA — code-based multiplexing
Related: Frequency Reuse — spatial multiplexing through geographical separation

Edge Cases & Gotchas

Guard bands (FDMA) and guard times (TDMA) waste some capacity to prevent interference
CDMA is interference-limited — adding more users degrades quality for all users (soft capacity)
TDMA requires precise synchronization — clock drift can cause slot misalignment
CDMA codes must be orthogonal to minimize self-interference

near-far-terminal

2026-04-21T00:00:00.000Z

The Problem

In a network where all mobile devices transmit at the same power level, a device that is very close to the base station will produce a very strong signal at the receiver. A device that is far away will produce a weak signal. When both transmit simultaneously, the strong near signal can completely drown out the weak far signal at the base station receiver, making the far device’s transmission undetectable.

Core Idea

The near/far terminal effect occurs when a strong nearby transmitter and a weak distant transmitter compete for the same receiver. The strong signal’s power overwhelms the receiver’s front-end, compressing the signal and making the weak signal undecodable — even if the weak signal uses the same code (as in CDMA) or the same channel (as in TDMA/FDMA).

How It Works

Terminal A is very close to the base station; Terminal B is far away
Both transmit at equal power (standard assumption)
Terminal A’s signal arrives at the base station with high power
Terminal B’s signal arrives with low power (due to path loss)
At the base station receiver, Terminal A’s strong signal causes front-end compression or dominates the received energy
Terminal B’s signal is buried in noise and is completely unreadable — the “drowning out” effect
The base station may fail to decode B’s signal even if the noise alone would allow it

In CDMA systems, the near/far effect is particularly devastating because all users share the same frequency.

Key Properties

Caused by path loss variation with distance and lack of power control
Strong signals drown out weak signals at the receiver (SNR collapses)
Especially severe in CDMA where all users share the same frequency and code
Strict power control is the solution: base station commands mobiles to adjust transmit power
Goal: all signals arrive at the base station with equal power (within 1 dB)
Without power control, CDMA capacity is severely limited

Connections

Related: Hidden Terminal Problem — another MAC-layer problem in wireless networks
Related: Exposed Terminal Problem — the complementary efficiency problem
Related: CD — the Ethernet protocol that fails due to these wireless effects
Related: CDMA — near/far is especially critical in CDMA due to shared code
Related: Power Control — the solution to the near/far problem

Edge Cases & Gotchas

The near/far problem exists even in TDMA and FDMA if not managed
In CDMA, one strong user can degrade the entire cell’s capacity
Power control loops must be fast enough to handle rapid mobility
Near/far is a fundamental reason why CDMA required sophisticated power control before being deployed

spread-spectrum

2026-04-21T00:00:00.000Z

The Problem

Transmitting a narrowband signal over a single frequency makes it vulnerable to jamming, interception, and interference. Additionally, in dense urban environments, many signals compete for the same limited bandwidth, causing mutual interference. Spread spectrum addresses both problems by intentionally spreading the signal across a much wider frequency band than necessary.

Core Idea

Spread spectrum is a modulation technique that spreads the information signal across a bandwidth far wider than the minimum required. The signal appears as low-power background noise, is resistant to narrowband interference, enables multiple users to share the same band (CDMA), and is difficult to intercept.

How It Works

There are two primary techniques:

DSSS (Direct Sequence Spread Spectrum):

The original data bits are combined (XOR) with a high-rate pseudo-random chipping sequence (e.g., 11 chips per bit)
The resulting wideband signal is modulated onto a carrier and transmitted
The receiver uses the same chipping sequence to despread — narrowband noise is suppressed, desired signal is recovered
Used in Wi-Fi (802.11b) and GPS

FHSS (Frequency Hopping Spread Spectrum):

Transmitter and receiver hop between frequencies according to a shared pseudo-random pattern
Each hop is a short dwell on a narrow band; the next hop jumps to a different frequency
Interference on any single frequency corrupts only one hop, not the entire transmission
Used in Bluetooth (1,600 hops per second)

Key Properties

DSSS: Continuous spread; wideband noise filtering; higher complexity
FHSS: Discrete hopping; simple to implement; resistant to partial jamming
Both provide low probability of intercept (stealth communication)
CDMA capability: Multiple users share the same band with different codes (DSSS) or hopping patterns (FHSS)
FHSS is cheaper and simpler; DSSS is more spectrally efficient

Connections

Built from: Modulation — spread spectrum is a specific modulation technique
Built from: Multipath Propagation — DSSS spreading codes help filter out multipath interference
Related: FHSS — Bluetooth’s spread spectrum technique
Related: DSSS — Wi-Fi’s spread spectrum technique
Related: CDMA — uses spread spectrum codes for multiple access

Edge Cases & Gotchas

DSSS requires precise code synchronization — timing errors corrupt the despreading process
FHSS hop sequences must be synchronized between transmitter and receiver; losing sync loses communication
FHSS devices must hop fast enough to avoid being jammed on any single frequency
DSSS spreading codes must have good autocorrelation properties to minimize ISI

wimax

2026-04-21T00:00:00.000Z

The Problem

Wi-Fi has limited range (100 m outdoors) and is designed for local area networking. Cellular networks provide wide area coverage but are operator-controlled and expensive. WiMAX was designed to provide metropolitan-scale broadband wireless access with higher data rates than Wi-Fi and wider coverage than Wi-Fi, while being standards-based and equipment-interoperable.

Core Idea

WiMAX (IEEE 802.16) is a metropolitan area wireless broadband standard providing fixed, nomad, and mobile broadband access. It operates in the 2.5 GHz and 3.5 GHz bands, provides up to 75 Mbps over 50 km ranges, and uses OFDM for efficient multi-path handling and scalability.

How It Works

Physical Layer:

OFDM: up to 2048 subcarriers for efficient spectrum use and multi-path resistance
Bands: 2.5–2.7 GHz (mobile WiMAX) and 3.3–3.8 GHz (fixed WiMAX)
Channel bandwidth: 3.5–10 MHz (scalable)
Modulation: QPSK to 64-QAM (adaptive based on channel conditions)
MIMO: beamforming and spatial multiplexing for capacity

MAC Layer:

Connection-oriented: each flow has a unique CID (Connection ID)
QoS: five service classes (UGS, rtPS, nrtPS, BE, ertPS)
ARQ: automatic repeat request for reliable delivery
Bandwidth request/grant mechanism (similar to DOCSIS cable modem)

Architecture:

Base station (BS): provides coverage for metro area
Subscriber station (SS): customer premise equipment
Two profiles: FDD and TDD

Versions:

802.16-2004 (fixed): Line-of-sight; up to 50 km; 75 Mbps
802.16e-2005 (mobile): Non-line-of-sight; mobility support; MIMO; up to 75 Mbps
802.16j: Relay (multihop) extension
802.16m (WiMAX Release 2): Advanced; 4G competitor to LTE

Key Properties

Metropolitan scale: up to 50 km coverage (vs. 100 m for Wi-Fi)
Standards-based: equipment from multiple vendors is interoperable
QoS: five service classes for real-time and data applications
OFDM for efficient spectrum and multi-path resistance
Both fixed (line-of-sight) and mobile (non-line-of-sight) profiles

Connections

Related: IEEE 802.11 — Wi-Fi’s competitor for local area networking
Related: WiMAX vs Wi-Fi — comparison of metropolitan vs. local area wireless
Related: LTE — WiMAX’s main competitor as 4G technology
Related: OFDM — the modulation technique used in WiMAX, Wi-Fi, and LTE
Related: ZigBee — another 802.15 standard for low-rate WPAN

Edge Cases & Gotchas

WiMAX spectrum is licensed — operators must buy licenses
WiMAX never achieved the scale that LTE did (Wi-Fi won local, LTE won mobile)
Many WiMAX operators migrated to LTE
Line-of-sight fixed WiMAX (802.16-2004) was an alternative to DSL but never scaled
Mobile WiMAX (802.16e) was the first 4G contender but lost to LTE

Wireless MAC Problems Compared

2026-04-21T00:00:00.000Z

Framing

This synthesis compares the three fundamental MAC-layer problems in wireless networks: the Hidden Terminal Problem, the Exposed Terminal Problem, and the Near/Far Terminal Effect. These problems arise because standard wired Ethernet mechanisms (CSMA/CD) fail in wireless environments.

Comparison

Property	Hidden Terminal	Exposed Terminal	Near/Far Effect
Root Cause	Range limitations — cannot sense	Over-conservative carrier sense	Unequal received power at base station
Who is Affected	Receiver (B) — collision at receiver	Transmitter (C) — unnecessary deferral	Far terminal — signal drowned
Problem Symmetry	Symmetric	Symmetric	Asymmetric (near vs. far)
Severity	Critical (data loss)	Moderate (capacity loss)	Critical (far user blocked)
Solution	RTS/CTS handshake (MACA)	RTS/CTS announcement	Strict power control
Standard MAC Fails?	Yes (no collision detection)	Yes (over-deferral)	Yes (equal power assumption)

Key Insights

Why Standard CSMA/CD Fails in Wireless

In wired Ethernet, a transmitting node can detect a collision by monitoring the wire. In wireless, a node cannot hear while transmitting (its own signal drowns out incoming signals). This fundamental asymmetry means:

Hidden terminals cannot detect each other’s transmissions → collisions occur at the receiver
Nodes that sense an ongoing transmission unnecessarily defer → wasted bandwidth

The Hidden Terminal: The More Severe Problem

Hidden terminals cause data loss — the receiver gets a garbled signal. The transmitter has no idea a collision occurred. Without RTS/CTS, there is no mechanism to prevent this. MACA’s RTS/CTS handshake solves this by explicitly announcing the reservation zone to all neighbors, including hidden nodes.

The Near/Far Effect: Structural Asymmetry

The near/far effect is fundamentally different from the other two: it is caused by the physics of radio propagation, not by range limitations. A near device’s signal is not stronger because it is transmitting more — it is stronger because of path loss. All devices transmit at the same nominal power, but the inverse square law means close devices produce overwhelming signals at the base station. Power control (not MAC protocols) is the solution.

Practical Implications

MACA + RTS/CTS: Solves hidden and exposed terminal problems; used in 802.11
Power Control: Solves near/far problem; critical in CDMA
These mechanisms are often combined in modern wireless systems (802.11 + power control, CDMA + MACA)

Connections

Hidden Terminal Problem — collision at the receiver
Exposed Terminal Problem — unnecessary deferral
Far Terminal Effect — drowning out of weak signals
MACA — the protocol that solves hidden/exposed terminal
CD — the wired protocol that fails in wireless

wireless-network

2026-04-21T00:00:00.000Z

The Problem

Standard wired networks use dedicated physical cables for communication. In mobile environments, users need to communicate while moving, without being tethered to a fixed infrastructure. Wireless networks replace cables with radio waves, but this introduces new challenges: shared medium (anyone can intercept), signal degradation over distance, interference from obstacles, and limited bandwidth due to regulated spectrum.

Core Idea

A wireless network is a communication network where devices exchange data using radio waves through an open medium (air) instead of physical cables. The core challenge is managing the shared, unreliable radio channel while supporting device mobility.

How It Works

Data from the application layer is converted into digital bits
Bits are modulated onto a carrier radio wave (converting digital to analog)
The modulated signal is transmitted via an antenna into the air
The signal travels through the radio channel, subject to multipath propagation and fading
The receiving antenna captures the signal, and demodulation recovers the bits
The bits are reassembled into the original data for the application

Wireless networks operate in regulated frequency bands (e.g., 800 MHz–900 MHz for GSM, 2.4 GHz for Wi-Fi/Bluetooth).

Key Properties

Shared medium: Anyone within range can potentially receive the signal — security is a concern
Broadcast nature: A single transmission can reach multiple receivers
No dedicated path: Devices compete for the same radio spectrum
Mobility support: Users can move between cells while maintaining connectivity
Signal degradation: Strength drops with distance (inverse square law) and is affected by obstacles
Limited bandwidth: Spectrum is regulated and shared among many users and services

Connections

Built from: Modulation — without modulating data onto a carrier, wireless transmission is impossible
Built from: Multiplexing — without multiplexing, multiple users cannot share the spectrum
Built from: Multipath Propagation — signals rarely travel in a straight line through the air
Related: Cellular Mobile System — the architecture that makes wide-area wireless possible

Edge Cases & Gotchas

Signal can be blocked by buildings, walls, and even people (shadowing)
Rain and atmospheric conditions can absorb radio waves (especially at higher frequencies)
The same frequency cannot be reused in adjacent cells without causing co-channel interference
Mobile devices have limited battery power — transmission must be energy-efficient

zigbee

2026-04-21T00:00:00.000Z

The Problem

Wi-Fi and Bluetooth consume too much power for battery-powered sensors and actuators that need to run for months or years on a single coin cell. A truly low-power wireless standard was needed for industrial monitoring, home automation, and IoT sensor networks where devices must operate for long periods without battery replacement.

Core Idea

ZigBee (IEEE 802.15.4) is an ultra-low-power wireless personal area network (WPAN) standard designed for low-rate, short-range, battery-powered devices. It operates in the 2.4 GHz, 900 MHz, and 868 MHz bands, provides 250 kbps, and is designed for years of battery life.

How It Works

Physical Layer:

2.4 GHz ISM band: 250 kbps (worldwide)
915 MHz band: 40 kbps (Americas)
868 MHz band: 20 kbps (Europe)
Direct Sequence Spread Spectrum (DSSS) for interference resistance

MAC Layer:

CSMA/CA channel access (similar to 802.11)
Optional beacon mode for synchronous data transfer
Guaranteed Time Slots (GTS) for low-latency applications
Superframe structure: active and inactive periods

Network Layer:

Star topology: simple, coordinator-based
Peer-to-peer (tree, mesh): extended range through multi-hop

Applications:

Home automation: lights, thermostats, door locks
Industrial monitoring: temperature, pressure, vibration
Health care: patient monitors, medication dispensers
Smart metering: electricity, water, gas

Key Properties

Ultra-low power: years of battery life (vs. days for Wi-Fi, hours for Bluetooth)
Very low data rate: 250 kbps (sufficient for sensor data)
Short range: 10–100 m (sufficient for home/industrial)
Simple protocol stack: smaller code and memory footprint
Mesh networking: extended range through multi-hop routing
Standard: equipment from multiple vendors is interoperable

Connections

Related: Bluetooth — another WPAN standard; higher rate but higher power
Related: BLE — competing for the same IoT sensor market
Related: IEEE 802.15 — ZigBee is a 802.15.4-based standard
Related: Wi-Fi vs Bluetooth vs ZigBee — comparison of these short-range standards
Related: Wireless Sensor Network — ZigBee is commonly used in WSNs

Edge Cases & Gotchas

Data rate (250 kbps) is too low for audio, video, or large data transfers
Range is shorter than Wi-Fi or sub-GHz alternatives
Bluetooth Low Energy (BLE) is now a strong competitor in the IoT space
ZigBee and BLE are not interoperable — separate ecosystems

/wiki / wireless-n

cell-splitting

The Problem

Formal Definition

Explanation

How It Works

Visual Explanation

Semantic Network

Key Properties

Real-World Example

Connections

Edge Cases & Gotchas

sectoring

The Problem

Formal Definition

Explanation

How It Works

Visual Explanation

Semantic Network

Key Properties

Real-World Example

Connections

Edge Cases & Gotchas

Wireless N — Map of Content

Core Concepts

Mechanisms & How Things Work

Comparisons & Tradeoffs

Suggested Reading Order

csma-cd

The Problem

Core Idea

How It Works

Visual Explanation

Key Properties

Connections

Edge Cases & Gotchas

gsm

The Problem

Core Idea

How It Works

Visual Explanation

Key Properties

Connections

Edge Cases & Gotchas

antenna-types

The Problem

Core Idea

How It Works

Key Properties

Connections

Edge Cases & Gotchas

bluetooth

The Problem

Core Idea

How It Works

Key Properties

Connections

Edge Cases & Gotchas

cell-sectoring

The Problem

Core Idea

How It Works

Key Properties

Connections

Edge Cases & Gotchas

cellular-mobile-system

The Problem

Core Idea

How It Works

Key Properties

Connections

Edge Cases & Gotchas

channel-fading

The Problem

Core Idea

How It Works

Key Properties

Connections

Edge Cases & Gotchas

co-channel-interference