From documentation : In cells where System Information 2bis and 2ter are used, or in cells where GSM to UMTS cell reselection is active and either of System Information 2bis or 2ter are used, it is recommended to setAGBLK=1 when GPRS/EGPRS is active in the cell. This is because System Information 13, which is sent when GPRS/EGPRS is active, will otherwise share resources with other System Information messages. This could impact cell selection/reselection performance.
And my experiences is, that in special events (Christmas day) there was to many paging and nearly no AGCH seizures – in case of Comb Channel (number of reserved AGCH = 0)
In case of channel configuration FCCH + SCH + BCCH + CCCH: uplink 800.000 RACH slots per hour and downlink 140 CCH block per hour. In case FCCH + SCH + BCCH + CCCH + SDCCH/4(0…3) + SACCH/C4(0…3): uplink 400.000 RACH slots per hour and downlink 46 CCH block per hour.
1 RACH slot: 1 channel request message of 1 subscriber.
1CCCH block (4 slots): 1 paging message for 1 ..4 subscribers or 1 access grant message for 1..2 subscribers.
1 BCCH multiframe is 235.38 msec.