What is Frame and Channel in GSM
The major basic concept concerned with the radio path transmission of the GSM system is the burst sequence (simplified as Burst). It is a string of transmission units including more than 100 modulation bits. The burst sequence has a restricted duration and seizes a restricted radio frequency spectrum. They can be described as output from the time and frequency window. This window is called Slot. In other words, within the system frequency band, the central frequency of the slot is set every 200KHz (observed from the opinion of FDMA); while the slot occurs cyclically as the time evolves, which seizes 15/26ms (i.e. approximately 0.577ms) each time (observed from the opinion of TDMA). The intervals of these slots are called Time Slots and the duration of them is called the time unit (marked as BP, indicating the Burst Period).
We can use the time/frequency chart to draw the slot as a small rectangle with the length of 15/26ms and width as 200KHz, as shown in the above diagram. Similarly, we can call the 200KHz bandwidth specified in GSM as Frequency Slot, which is equivalent to the Radio Frequency Channel (i.e. RF channel) in the GSM specifications.
The two terms: timeslot and burst sequence are different to a degree in actual application. For example, the burst sequence is sometimes related to the time-frequency “rectangular” unit and sometimes to its content. Similarly, the timeslot has the meaning of time value or indicates that a slot in every 8 slots is used periodically.
To use a specified channel means to transmit the burst sequence at the specified moment and frequency, i.e. the specified slot. Generally, the time of slots in a channel is discontinuous.