Q11Mobile Computing

In GSM, the physical radio frequency is just raw electromagnetic energy. To mathematically transmit human voice and network commands simultaneously without catastrophic collision, the physical layer is violently multiplexed (TDMA/FDMA) into distinct Logical Channels. These are strictly categorized into Traffic Channels and Control Channels.

TCH is the absolute payload carrier. It transmits the digitized human voice or raw user data. It is mathematically divided into Full-Rate (TCH/F at 22.8 kbps) and Half-Rate (TCH/H at 11.4 kbps) depending on network congestion.

These channels carry exactly zero human voice. They are strictly reserved for the violent, high-speed mathematical signaling required to keep the network alive.

A. Broadcast Control Channel (BCH)

A one-way downlink from the Tower to the Mobile. It acts as a lighthouse.

• FCCH (Frequency Correction): Blasts a pure sine wave to mathematically force the mobile's quartz clock to synchronize with the tower.
• SCH (Synchronization): Broadcasts the TDMA frame number so the phone mathematically knows exactly when to transmit.

B. Common Control Channel (CCCH)

• PCH (Paging Channel): When someone calls you, the tower violently blasts your IMSI across the PCH to wake up your phone.
• RACH (Random Access): The ONLY uplink channel here. If a user wants to make a call, the phone violently transmits a request on the RACH (using Slotted ALOHA).
• AGCH (Access Grant): The tower replies, mathematically assigning the phone a dedicated frequency.

C. Dedicated Control Channel (DCCH)

• SDCCH: Used for call setup and mathematically transferring SMS text messages.
• SACCH & FACCH: Violently monitors real-time signal strength and executes the mathematical commands to trigger a Handoff if the signal drops.