The Nortel BCM Digital Station Module 16 supports 16 digital telephones through one RJ-21 connector Set the double density switch to enable the DSM to carry 16 extensions over a half DS30 bus If required, install a second DSM16(+) and set the double density switch to occupy the second half of the DS30 bus to expand the number of extensions to 32 The Nortel BCM DSM16+ is compatible with the BCM 200/400 Main Unit, BCM 400 Expansion Unit, and BCM 50 Expansion Unit