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GSM Traffic Channels: TCH/FS and TCH/HS Explained

gsm traffic channel speech coding mobile communication data rate

This article explains the GSM Traffic channel, specifically the Full rate speech (TCH/FS) and Half rate speech (TCH/HS) channels. Similar to other technologies, traffic channels in GSM are used to carry user speech or data.

GSM primarily uses two types of traffic channels: TCH/FS and TCH/HS. These channels are utilized in both the uplink and downlink directions after a mobile device has successfully established a connection with the GSM cell (BTS). They employ a 26-frame multiframe structure.

  • TCH/FS: Stands for Traffic Channel at Full Rate Speech.
  • TCH/HS: Stands for Traffic Channel at Half Rate Speech.

Both use the normal burst structure.

  • TCH/FS: This channel carries full-rate speech digitized at a data rate of 13 kbps. After channel coding is applied, the rate increases to 22.8 kbps.
  • TCH/F9.6, TCH/F4.8, and TCH/F2.4: These channels are used to carry data at speeds of 9.6 kbps, 4.8 kbps, and 2.4 kbps, respectively. After channel coding, the data rate for all these channels is converted to 22.8 kbps.

TCH/HS

In addition to TCH/FS, GSM also defines TCH/HS, which is used to carry half-rate speech information. The primary reason for using half-rate channels is to support two calls within a single GSM timeslot, effectively doubling the traffic channel capacity of a GSM cell. Two half-rate TCH channels share one physical channel.

Here’s an image representing a GSM Traffic channel configuration:

GSM traffic channel configuration

Speech Channel (TCH/FS) Processing

A traffic channel carrying a speech signal undergoes several processing steps:

  1. Low Pass Filter (LPF): Removes unwanted high-frequency components from the analog speech signal.
  2. Analog-to-Digital Converter (ADC): Converts the analog speech signal into a digital format.
  3. Speech Encoder: Compresses the digital speech data.
  4. Rate-1/2 Convolutional Encoder: Adds redundancy to the data for error correction.
  5. Interleaver: Rearranges the data bits to minimize the impact of burst errors.
  6. Ciphering: Encrypts the data for security.

The processed data is then packed into four consecutive normal bursts before being passed through a differential encoder and a GMSK modulator. The modulated signal is then processed by RF devices and transmitted over the air.

For a more detailed explanation, refer to information on the GSM physical layer.