In Alcatel, we have 3 types of PDCH. THat is MAX_PDCH, MAX_PDCH_HIGH_LOAD and MIN_PDCH.
MAX_PDCH = Dynamic + Max_High_LoadPDCH (>=Max_highLoadPDCH)
MAX_PDCH_HIGH_LOAD = >=Min_PDCH
Min_PDCH = Dedicated PDCH
Example : Min_PDCH = 2, MAX_PDCH_HIGH_LOAD = 3, MAX_PDCH = 6
It means ==> 2 PDCHs are only for data, can’t be seized by TCH, but when PDCH access increase, max PDCH can be used is 6 pdch. If PDCH utilized are 3 PDCHs (Max-PDCH_HighLoad), that 1 PDCH can’t be seized by TCH. But IF PDCH utilized is 4, 5, or 6, only 3 PDCHs can be seized by TCH, but not the 3 others.
When PDCH numbers seized reach the minimum number of MAX_PDCH_HIGH_LOAD,then those PDCHs are unable to be used for TCH.
From my Experience, it doesn’t matter if we want to increase the PDCH Number if the voice traffic is low.
The speed/data rate of GPRS/EDGE also depends on Abis capcity. There is a mappign of PDCH allocation and Abis TS.
In Alcatel, sites which have high GPRS/EDGE access are added 1 E1 Abis (max 2 Abis), then we allocate Abis TS for Data only, especially in 2nd abis. That will significantly increase the data access speed.