Gsm structuring of time using a frame hierarchy
The GSM specifies a very elaborate multiplexing scheme that integrates several hierarchies of frames.
- A typical usage pattern of a physical channel for data transmission for user data transmission looks like:
- Twelve slots with user data are followed by a signaling slot.
- Again 12 slots with user data follow, then an unused slot. This pattern of 26 slots is repeated over and over again.
- In which only 24 out of 26 physical slots are used for the TCH/F.
Figure 8.5 GSM structuring of time using a frame hierarchy
As the TCH/F only uses 24/26 of the slots, the final data rate is 22.8 Kbit/s as specified for the TCH/F. The SACCH thus has a capacity of 950 bit/s.
- This periodic pattern of 26 slots occurs in all TDMA frames with a TCH. The combination of these frames is called traffic multiframe.
- Figure8.5 shows the logical combination of 26 frames, to a multiframe with duration of 120 ms. this type of multiframe is used for TCHs, SACCHs for TCHs, and FACCHs.
- TDMA frames containing data for the other logical channels are combined to a control multiframe.
- Control multiframes consist of 51 TDMA frames and have a duration of 235.4 ms.
- This logical frame hierarchy continues, combining 26 multiframes with 51 frames or 51 multiframes with 26 frames to form a super frame.
- 2,048 super frames build a hyper frame with duration of almost 3.5 hours. Altogether, 2,715,648 TDMA frames form a hyper frame.