Traffic Channel / Full Rate Speech

Description

The TCH/FS is a specific instance of the full-rate traffic channel (TCH/F) optimized for speech service using the original GSM Full-Rate (FR) speech codec. It is a logical channel that reserves a full physical resource—one timeslot in every TDMA frame on a given carrier frequency—for the exclusive use of one voice call. The GSM FR codec used with TCH/FS is a Regular Pulse Excitation – Long-Term Prediction (RPE-LTP) codec operating at a net bit rate of 13 kbit/s. This 13 kbit/s speech stream is then processed through the GSM channel coding chain.

The technical operation involves several steps. First, the analog voice signal is digitized and compressed by the FR codec into 260-bit speech frames every 20 ms. These 260 bits are classified into Class Ia (50 bits, most sensitive), Class Ib (132 bits, moderately sensitive), and Class II (78 bits, least sensitive). Class Ia bits are protected with a 3-bit CRC for error detection, and then Class Ia and Ib bits together are encoded with a convolutional code of rate 1/2. Class II bits are transmitted without any channel coding. This results in a total of 456 coded bits per 20 ms block, which corresponds to a gross bit rate of 22.8 kbit/s. These 456 bits are then interleaved and distributed across 8 normal bursts within the TDMA structure.

The channel resides within the standard 26-frame multiframe. Of the 26 frames, 24 are used to carry these interleaved speech blocks, one is used for the Slow Associated Control Channel (SACCH) to transmit continuous measurement reports, and one is idle. This structure ensures periodic signaling without interrupting the speech flow. The TCH/FS provides a constant, predictable quality of service and was the workhorse for voice communication in GSM networks before the introduction of enhanced and adaptive codecs.

Purpose & Motivation

The TCH/FS was created as the default voice service channel for the inaugural GSM networks. Its purpose was to deliver toll-quality digital voice over a radio interface, a major advancement over the analog systems it replaced. The GSM FR codec and the TCH/FS channel together solved key problems of analog cellular: poor voice quality susceptible to noise and fading, lack of security (calls could be easily intercepted), and inefficient frequency use.

The design of the TCH/FS was motivated by the need for a robust, standardized digital bearer. The fixed 13 kbit/s rate and the specific channel coding scheme were engineered as a compromise between speech quality, error resilience, and spectral efficiency. It established a reliable baseline—one timeslot, one call—which simplified network design, billing (based on connection time), and interoperability between different vendors' equipment. While its speech quality was later surpassed by the Enhanced Full Rate (EFR) and AMR codecs, the TCH/FS was essential for the initial deployment and widespread adoption of GSM as the first truly digital and global cellular standard.

Key Features

• Dedicated full-rate channel employing the GSM Full-Rate (FR) RPE-LTP speech codec at 13 kbit/s.
• Uses unequal error protection: Class Ia/Ib bits are convolutionally coded, Class II bits are uncoded.
• Generates 456 coded bits per 20 ms speech frame, interleaved across 8 consecutive TDMA bursts.
• Follows the 26-frame multiframe structure with integrated SACCH for control signaling.
• Provides a fixed, circuit-switched connection guaranteeing consistent bandwidth for the voice call.
• Served as the primary voice bearer for initial GSM network rollouts and remains a fallback option.

Evolution Across Releases

Defining Specifications