In GSM, each time slot (burst) within a frame is designed to carry 114 data bits and other overhead
bits.
The remaining bits are allocated for guard periods, training sequences, and synchronization.
The actual burst format includes:
114 bits: Payload data (user information or signaling).
8.25 bits: Guard period (to avoid overlap between bursts in neighboring slots).
26 bits: Training sequence (used for synchronization and channel estimation).
2 tail bits: Mark the end of the burst.
This totals 156.25 bits, where the "0.25 bits" arises from how GSM calculates the time allocation. The
guard period is 8.25 bits long, and this fraction results from the timing required to ensure seamless
transitions between slots.
What the Fractional Bit Means:
It represents a time duration rather than a physical "half-bit" being transmitted.
In GSM, time is often represented in terms of bit durations (bit times), even for non-data-related
functions like guard intervals.
The "0.25" corresponds to a quarter of the time it takes to transmit a single bit at the GSM bit rate (270.8
kbps).
Significance in GSM:
The fractional time allocation ensures that time slots are well-aligned within a frame.
This precision is necessary to avoid interference between neighboring time slots, ensuring reliable
communication in TDMA (Time Division Multiple Access).
