What is TCH/AFS Traffic Channel using Adaptive Multi-Rate Full Rate Speech

TCH/AFS: Tailoring Speech for Efficient Cellular Communication

In the world of GSM (Global System for Mobile Communications) cellular networks, TCH/AFS (Traffic Channel / Adaptive Multi-Rate Full Rate Speech) plays a crucial role in optimizing voice call quality and network resource utilization. Here's a deeper dive into the technical details:

Function:

• TCH/AFS is a specific type of Traffic Channel (TCH) used for voice calls in GSM networks.
• It employs a technology called Adaptive Multi-Rate (AMR) to dynamically adjust the bit rate used for transmitting speech data.

Benefits of AMR:

• Efficiency: AMR allows for efficient use of network resources. By adapting the bit rate based on channel conditions and speech complexity, AMR can transmit high-quality speech with less bandwidth compared to fixed-rate codecs.
• Robustness: In scenarios with weaker signal strength or higher levels of interference, AMR can reduce the bit rate to maintain acceptable call quality.
• Flexibility: AMR offers a range of bit rates, allowing the network to adapt to varying call needs and network conditions.

AMR Modes in TCH/AFS:

• TCH/AFS typically supports several AMR modes, each offering a different bit rate and corresponding voice quality level:
  • AMR Full Rate (FR): Offers the highest quality speech at around 12.2 kbps, similar to traditional TCH/F (Full Rate) channels.
  • AMR Half Rate (HR): Provides good speech quality at a lower bit rate of around 5.9 kbps.
  • AMR Other Rates (Octal Rates): Additional modes offer even lower bit rates (down to 4.75 kbps) for scenarios with highly constrained network resources.

Dynamic Rate Adaptation:

• TCH/AFS continuously monitors the channel conditions and analyzes the speech complexity.
• Based on this information, the network dynamically switches between different AMR modes to optimize the balance between speech quality and network efficiency.
• This dynamic adaptation allows for efficient utilization of bandwidth, enabling more calls to be supported on the network.

Decision Making for Rate Change:

• The decision to change the AMR mode in TCH/AFS involves a trade-off between:
  • Channel Quality Indicator (CQI): Measures the signal strength and level of interference on the channel.
  • Speech Frame Loss Rate (SFLR): Indicates the percentage of speech frames lost due to transmission errors.
  • Voice Activity Detection (VAD): Detects periods of silence in the speech signal, allowing for lower bit rates during silent periods.

Impact of TCH/AFS:

• TCH/AFS has significantly improved voice call quality and network efficiency in GSM networks.
• It offers a significant advantage over fixed-rate codecs by adapting to changing network conditions and optimizing resource allocation.

Looking Ahead:

• While TCH/AFS remains relevant in existing GSM networks, newer cellular technologies like 3G, 4G, and 5G employ more advanced voice coding schemes with even higher efficiency and improved audio quality.

In Conclusion:

TCH/AFS stands as a cornerstone technology in GSM voice communication. By leveraging Adaptive Multi-Rate (AMR), it delivers a balance between efficient network resource utilization and maintaining acceptable voice call quality. Understanding TCH/AFS is essential for appreciating the technical advancements employed in GSM networks.