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. [(5x1)+(5x1)+(1x4)+(3x4)+(4x4)+(2x4)+1idle=51frame multiframe] 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.[(8x4)+(4x4)+3idle=51frame multiframe] This combination is used to provide additional SDCCH capacity in medium and large cells. It can be used anywhere but C0T0. Fundamental Um transactions Basic speech service in GSM requires five transactions: radio channel establishment, location update, mobile-originating call establishment, mobile-terminating call establishment and call clearing. All of these transactions are described in GSM 04.08 Sections 3-7. Radio channel establishment Unlike ISDN's U channel, Um channels are not hard-wired, so the Um interface requires a mechanism for establishing and assigning a dedicated channel prior to any other transaction. The Um radio resource establishment procedure is defined in GSM 04.08 Section 3.3 and this is the basic medium access procedure for Um. This procedure uses the CCCH (PCH and AGCH) as a unicast downlink and the RACH as a shared uplink. In the simplest form, the steps of the transaction are: Paging. The network sends a RR Paging Request message (GSM 04.08 Sections 9.1.22-9.1.23) over the PCH, using the subscriber's IMSI or TMSI