GSM (Global System for Mobile Communications) channels are fundamental components that facilitate communication within the GSM network. These channels are defined for both the uplink (from mobile station to base station) and downlink (from base station to mobile station). Here are the basics of GSM channels:
1. Frequency Division Multiple Access (FDMA):
- Principle:
- GSM uses FDMA to divide the available frequency spectrum into multiple carrier frequencies, each separated by a specific frequency interval.
- Carrier Frequency Bands:
- GSM operates in different frequency bands, with GSM 900 (890-960 MHz) and GSM 1800/DCS (1710-1880 MHz) being common examples.
2. Time Division Multiple Access (TDMA):
- Principle:
- Within each carrier frequency, TDMA is employed to divide the time into frames, and each frame is further divided into time slots.
- Time Slots:
- A frame in GSM consists of 8 time slots. Each time slot has a duration of 577 microseconds.
3. Logical Channels:
- Definition:
- Logical channels represent specific functions or types of information within the GSM network.
- Examples:
- Traffic channels (TCH) carry user voice or data. Control channels include Broadcast Control Channel (BCCH) for system information and Common Control Channels (CCCH) for call setup.
4. Physical Channels:
- Definition:
- Physical channels represent the actual transmission paths in the frequency-time domain. They are formed by combining logical channels with a specific frequency and time slot.
- Examples:
- Traffic channels (TCH/F) and Control Channels (BCCH, CCCH) are mapped to physical channels.
5. Frequency Hopping:
- Principle:
- Frequency hopping is a technique used to enhance the reliability of communication by hopping between different carrier frequencies during a call.
- Purpose:
- Reduces the impact of interference and improves the overall quality of communication.
6. Burst Structure:
- Definition:
- A burst is a unit of transmission in GSM, consisting of bits transmitted during one time slot.
- Types:
- Normal Burst (NB) is used for data transmission, while Frequency Correction Burst (FB) is used for frequency correction.
7. Channel Combination:
- Downlink Channels:
- Downlink channels include BCCH, Frequency Correction Channel (FCCH), and Synchronization Channel (SCH).
- Uplink Channels:
- Uplink channels include Random Access Channel (RACH) for initial access, and Access Grant Channel (AGCH) for access grant.
Understanding these basics of GSM channels is crucial for designing and optimizing mobile communication networks. The combination of FDMA and TDMA allows GSM to support multiple simultaneous voice and data connections efficiently, providing the foundation for the success of GSM as a widely adopted mobile communication standard.