GSM Traffic Channel
This page describes GSM Traffic channel i.e. Full rate speech(TCH/FS) and Half rate speech(TCH/HS).
Traffic channel in GSM carry user speech or data as other technology. GSM has two types of traffic channels TCH/FS and TCH/HS. It is used both in the uplink and downlink after mobile has established connection with the GSM cell (BTS). It uses 26 frame multi frame structure.
TCH/FS stands for traffic channel at Full rate speech and TCH/HS stands for traffic channel at half rate speech. It uses normal burst structure .
TCH/FS and TCH/F
• TCH/FS channel carrying full rate speech digitized at 13kbps data rate. After channel coding is applied the rate becomes 22.8kbps.
• TCH/F9.6, TCH/F4.8 and TCH/F2.4 channels carry data at the speed of 9.6kbps,4.8kbps and 2.4kbps respectively. All these channels after channel coding is applied is converted to data rate of 22.8kbps.
Other than TCH/FS, TCH/HS is also defined which is used to carry half rate speech information. The main purpose behind this is to support two calls in one gsm time slot and hence it helps double the traffic channel capacity of a GSM cell. Two half rate TCH channels use one physical channel.
GSM Traffic channel configuration
Speech channel (TCH/FS) processing
Traffic channel carrying speech signal is processed through Low pass Filter(LPF), ADC,Speech encoder,
Rate-1/2 Convolutional Encoder,Interleaver,ciphering and packed into 4 consecutive normal bursts before being
passed through differential encoder and GMSK modulator.
The modulated signal is later processed through RF device and transmitted into the air.
For more detail explanation refer following link on GSM physical layer.
GSM physical layer
GSM RELATED LINKS
GSM Logical Channels
traffic channel processing
control Channel processing