Wireless Networks (Part 1)
Understanding the architecture, generations, and mechanics of cellular data networks.
Mobile Data Networks Overview
A mobile data network is a wireless infrastructure used by devices to access the internet via mobile carriers.
Local Context (Egypt)
Delivered by carriers like Orange, WE, Etisalat, and Vodafone.
Evolution
From 1G (Analogue) to 5G (Ultra-fast, Low Latency).
Evolution of Mobile Technologies
Cellular System Infrastructure
The "Cell" Concept
- The service area is divided into Cells.
- Each cell has its own Antenna (Base Station - BS).
- Each cell uses a specific range of Frequencies.
- Low Power Transceivers: Usually 100W or less to limit interference.
Why Hexagons?
Hexagons simplify switching because all adjacent antennas are equidistant. This provides steady coverage without overlapping gaps.
Frequency Reuse & Capacity
Spectrum is limited. To serve millions, we must re-use the same frequencies in non-adjacent cells.
The Reuse Formula:
- K: Total frequencies available.
- N: Cluster size (number of cells in a group).
Spectral Reuse
Adjacent cells must use different frequencies to avoid Crosstalk (Interference).
Handoff (Handover)
When you move between cells, the BS "hands off" your call to the next cell's antenna seamlessly. This ensures zero signal drop during migration.
Calculation Example (AMPS)
Cluster Size (N) = 7
Freq per Cell = 395 / 7 ≈ 57
Multiple Access Techniques
How do multiple users share the same channel without crashing?
FDMA
Frequency Division: Divided by frequency bands. Each user gets a unique frequency.
TDMA
Time Division: Same frequency but divided into Time Slots.
CDMA
Code Division: All users share freq/time, but each has a Unique Code to decode data.
Frequency Hopping
Users "hop" between different frequencies at fixed time intervals (Time Slots) within a frame. This prevents long-term interference on a single frequency.
🏗 Cellular System Architecture
MS
Mobile Station (Your Phone)
BS / BTS
Base Transceiver Station
BSC
Base Station Controller
MSC
Mobile Switching Center
Hierarchy:
MS → BS → BSC → MSC → PSTN / Internet
Channels Used:
- Control Channels: For setup, management, and signaling.
- Traffic Channels: For the actual conversation/data.
- Forward: From Base Station to Mobile (Downlink).
- Reverse: From Mobile to Base Station (Uplink).
Call Flow (Setup):
- MS requests path setup.
- BS assigns Frequency/Time Slot/Code.
- Control info acknowledged.
- Communication starts on Traffic channel.