sgsn/ggsn bitrate tests and reflexions

Hi dear osmocom folks, I have been doing some bitrate throughput tests from different MS (i.e. phones models) connected to a working osmo-nitb/sgsn/ggsn configuration (with a nanoBTS DCS1800) and i get 15kbits/s maximum (tcp/udp transfer) with the gprs subnet nat-ed to a 100baseT lan connected to a 2Mbits/s dsl line The channel configuration is as follow in the osmo-nitb configuration file : <file /etc/osmocom/bsc.conf> (...) nominal power 23 max_power_red 0 rsl e1 tei 0 timeslot 0 phys_chan_config CCCH+SDCCH4 ! hopping enabled 0 timeslot 1 phys_chan_config TCH/F ! hopping enabled 0 timeslot 2 phys_chan_config TCH/F ! hopping enabled 0 timeslot 3 phys_chan_config TCH/F ! hopping enabled 0 timeslot 4 phys_chan_config TCH/F ! hopping enabled 0 timeslot 5 phys_chan_config TCH/F ! hopping enabled 0 timeslot 6 phys_chan_config TCH/F ! hopping enabled 0 timeslot 7 phys_chan_config PDCH </file> Should i change the channel configuration to get more bitrate ? Should add more cells (bts?) to add more channels ? my best, cheers. Xavier.