Understanding GSM Frame Structure: Hyperframe, Superframe, Multiframe, Frame, and Slot

In GSM, a frequency band of 25 MHz is divided into smaller bands of 200 KHz, each carrying one RF carrier. This yields 125 carriers. With one carrier acting as a guard channel between GSM and other frequency bands, we have 124 usable RF channels. This frequency division is known as FDMA (Frequency Division Multiple Access).

Each RF carrier is then divided into eight time slots. This time-based division is called TDMA (Time Division Multiple Access).

In essence, each RF carrier frequency is shared among 8 users. Therefore, the fundamental radio resource in GSM is a time slot, lasting approximately 577 microseconds (0.577 ms). This time slot carries 156.25 bits, resulting in a bit rate of 270.833 kbps.

For E-GSM, the number of ARFCNs (Absolute Radio Frequency Channel Numbers) is 174, while for DCS1800, it’s 374.

This article delves into the GSM frame structure, covering the concepts of slots, frames, multiframes, superframes, and hyperframes. It explores both the 51-frame multiframe and the 26-frame multiframe structures within GSM.

GSM Frame Structure

The basic unit, the frame (or TDMA frame), comprises 8 time slots. A GSM hyperframe consists of 2048 superframes. Each GSM superframe is made up of multiframes (either 26 or 51, as detailed below). Each GSM multiframe comprises frames (either 51 or 26, depending on the multiframe type). Finally, each frame is composed of 8 time slots.

In total, there are 2715648 TDMA frames available in GSM, and this cycle repeats continuously.

As depicted in the image above, the multiframe structure has two variations:

1. 26 Frame Multiframe (Traffic Multiframe): This is composed of 26 bursts over a duration of 120ms. Of these, 24 are used for traffic, one for SACCH (Slow Associated Control Channel), and one is unused.

2. 51 Frame Multiframe (Control Multiframe): This consists of 51 bursts over a duration of 235.4 ms. This type of multiframe is divided into logical channels, which are scheduled by the BTS (Base Transceiver Station). It always occurs at the beacon frequency in time slot 0, and may occupy other time slots (e.g., 2, 4, 6) if required by the system.

As illustrated in the following figure, each ARFCN (or channel) in GSM has 8 time slots, labeled TS0 to TS7.

During network entry, each GSM mobile phone is allocated one slot in the downlink and one slot in the uplink.

In the figure, the GSM Mobile is allocated 890.2 MHz in the uplink and 935.2 MHz in the downlink. As previously mentioned, TS0 is allocated, following either a 51 or 26 frame multiframe structure.

Therefore, if ‘F’ (representing FCCH - Frequency Correction Channel) is initially present, S (SCH - Synchronization Channel) will appear after 4.615 ms (equivalent to 7 time slot durations). Following this, B (BCCH - Broadcast Control Channel) will appear after another 7 slots, and so on, until the end of the 51-frame Multiframe structure is reached, and the cycle repeats as long as the connection between the Mobile and the base station remains active.

Similarly, in the uplink, a 26-frame multiframe structure is followed, where T represents TCH/FS (Traffic Channel for Full Rate Speech) and S represents SACCH.

The GSM frame structure can be best understood as depicted in the figure below, with respect to the downlink (BTS to MS) and uplink (MS to BTS) directions.

Fig.3 GSM Physical and logical channel concept

The frequencies used in the uplink are calculated as:

$F_{uplink} = 890.2 + 0.2 (N-1) \ MHz$

The frequencies used in the downlink are calculated as:

$F_{downlink} = 935.2 + 0.2 (N-1) \ MHz$

Where N ranges from 1 to 124, and is referred to as the ARFCN.

Because the same antenna is utilized for both transmission and reception, a delay of 3 time slots is introduced between TS0 of the uplink and TS0 of the downlink frequency.

This prevents the need for simultaneous transmission and reception by the GSM mobile phone. The 3-slot time period allows the mobile subscriber to perform various functions, such as processing data and measuring the signal quality of neighboring cells.

Conclusion

Understanding the GSM frame structure is crucial for understanding how GSM networks efficiently manage multiple calls, data sessions, and control signals concurrently. By organizing communication into frames, time slots, and logical channels, GSM ensures reliable and synchronized transmission for millions of users.