Allowed Channel Combinations
The multiplexing rules of GSM 05.02 allow only certain combinations of logical channels to share a physical channel. The allowed combinations for single-slot systems are listed in GSM 05.02 Section 6.4.1. Additionally, only certain of these combinations are allowed on certain timeslots or carriers and only certain sets of combinations can coexist in a given BTS. These restrictions are intended to exclude non-sensical BTS configurations and are described in GSM 05.02 Section 6.5.
The most common combinations are:
- Combination I: TCH/F + FACCH/F + SACCH. This combination is used for full rate traffic. It can be used anywhere but C0T0.
- Combination II: TCH/H + FACCH/H + SACCH. This combination is used for half rate traffic when only one channel is needed. It can be used anywhere but C0T0.
- Combination III: 2 TCH/H + 2 FACCH/H + 2 SACCH. This combination is used for half rate traffic. It can be used anywhere but C0T0.
- Combination IV: FCCH + SCH + BCCH + CCCH. This is the standard C0T0 combination for medium and large cells. It can be used only on C0T0.
- Combination V: FCCH + SCH + BCCH + CCCH + 4 SDCCH + 2 SACCH. This is the typical C0T0 combination for small cells, which allows the BTS to trade unnecessary CCCH capacity for a pool of 4 SDCCHs. It can be used only on C0T0.
- Combination VI: BCCH + CCCH. This combination is used to provide additional CCCH capacity in large cells. It can be used on C0T2, C0T4 or C0T6.
- Combination VII: 8 SDCCH + 4 SACCH. This combination is used to provide additional SDCCH capacity in medium and large cells. It can be used anywhere but C0T0.
Read more about this topic: Um Interface, Um Logical Channels
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