What is TCH/FS Traffic Channel using Full Rate Speech

TCH/FS: The Workhorse of Voice Communication in GSM Networks

In the realm of GSM (Global System for Mobile Communication) cellular networks, TCH/FS (Traffic Channel / Full Rate Speech) reigns supreme as the foundation for voice calls. Here's a detailed explanation of its technical aspects:

Function:

• TCH/FS is a dedicated channel within the GSM network spectrum responsible for carrying digitized voice data during phone calls.
• It acts as the workhorse for user communication, facilitating the transmission of voice information between mobile phones and the network base station.

Full Rate Speech (FR) Codec:

• TCH/FS employs a specific speech coding scheme called Full Rate (FR).
• This codec compresses analog voice signals from a phone call into a digital format suitable for transmission over the cellular network.
• The FR codec operates at a bit rate of approximately 12.2 kbps (kilobits per second). This bit rate represents the amount of data transmitted per second to carry the voice information.

Channel Characteristics:

• TCH/FS channels operate within a specific timeslot allocated by the network. Cellular networks are divided into frames and timeslots. TCH/FS channels are assigned dedicated timeslots for transmitting voice data.
• Signaling associated with the TCH/FS channel, such as call setup and control information, might be carried on separate dedicated control channels. This separation ensures efficient bandwidth utilization.

Network Efficiency and Capacity:

• While offering good voice quality, the 12.2 kbps bit rate of TCH/FS necessitates careful network resource management.
• To accommodate more calls simultaneously, GSM networks might employ techniques like:
  • Adaptive Multi-Rate (AMR): This technology dynamically adjusts the bit rate for voice data based on network conditions and call quality requirements (explored in separate TCH/AFS and TCH/AHS channels).
  • Frequency Reuse: Cellular networks reuse the same frequencies in different geographical areas to expand capacity. However, proper planning and cell planning techniques are crucial to avoid co-channel interference.

Voice Quality Considerations:

• The FR codec offers acceptable voice quality for typical phone calls. However, some limitations exist:
  • Compressed nature of the signal: The compression process can introduce artifacts that slightly alter the original voice characteristics compared to an uncompressed signal.
  • Higher frequencies might be attenuated: The FR codec focuses on preserving the essential components of speech for intelligibility, and some higher frequencies that contribute to voice timbre might be less pronounced.

Evolution of Voice Coding:

• TCH/FS played a pivotal role in establishing GSM as a dominant cellular technology.
• However, newer cellular generations like 3G, 4G, and 5G employ more advanced voice coding schemes that offer:
  • Higher audio fidelity with wider frequency ranges and improved noise reduction.
  • Lower bit rates, enabling more efficient network resource utilization and potentially supporting more calls simultaneously.

In Conclusion:

TCH/FS stands as the cornerstone technology for voice communication in GSM networks. By utilizing Full Rate Speech coding, it effectively transmits voice data, forming the backbone for cellular phone calls. Understanding TCH/FS is crucial for grasping the core functionalities of voice communication within GSM systems. It also provides a historical context for the continuous advancements in voice coding technologies employed in subsequent cellular generations.