and also +24VDC supply is being used on most BTS. take ericsson rbs2000 series for example.. they are using either +24VDC or -48VDC. but even if it is supplied by -48VDC the PSU is converting -48VDC to +24VDC. if +24VDC, the PSU is supplied by 220VAC and then converted to +24VDC.