H c = CCCH = PCH/AGCH Figure 2.37 BCCH multiframe directions. The GPRS will place an extra load on both the RACH and AGCH and therefore the main focus will be aimed at these channels. In the combined configuration the RACH has allocated 27 TDMA frames and three blocks of 4 TDMA frames to the CCCH. The CCCH is shared by both the AGCH and the PCH. The capacity allocated to the AGCH can be specified by the operator with the parameter NumberOfBlocksForAccessGrant. In a combined configuration this is usually defined as: 1 block used for AGCH 2 blocks for paging Therefore the RACH and AGCH capacity in a combined configuration is: RACH cpacity = 27 TDMA frames/235 ms = 115 frames/second AGCH capacity = 1 block/235 ms = 4.25 blocks/second With the immediate assignment extended, the access grant can support two assignment messages. How- ever, this is dependent on the message type and therefore the expected number of messages per block is Immediate assignment extended = 4.25 X 1.8 = 7.65 messages/second Uncombined Configuration In the uncombined configuration a separated channel structure is adopted with the BCCH remaining on timeslot 0 and the SDCCH allocated to a second timeslot (Figures 2.37 and 2.38). This allows extra signalling capacity to both the SDCCH and the CCCH. A different configuration is used in cells with more than three carriers. The uncombined configuration allows the RACH the full 51 frames and the CCCH 9 blocks, which are split between the AGCH and PCH: 3 blocks used for AGCH 6 blocks for paging Radio Network Planning and Optimisation SDCCH/8 Multiframe Downlink 51 TDMA frames = 235 ms ti t tf ttttt t t$s ss S sssss Uplink sSSSSSSSSSSI CHANNELS t = SDCCH/8 S = SACCH/4 Figure 2.38 SDCCH multiframe With the above assumption, the RACH and AGCH capacity available is: RACH capacity 51 TDMA frames = 217 frames/second AGCH capacity 3 blocks = 12.8 blocks/second Immediate assignment extended = 12.8 X 1.8 = 23 messages/second Capacity (PCH and AGCH) Signalling capacity in existing voice networks depends mostly o