The 16-port RJ-21 FXS module for the Catalyst 4500 series Access Gateway Module is a high-density analog phone and fax interface. By providing service to analog phones and fax machines, the Foreign Exchange Station (FXS) ports emulate a public switched telephone network (PSTN) central office (CO) or private branch exchange (PBX).The RJ-21 FXS module is classified as an on-premises (ONS) FXS but provides line-cross protection not usually found in non-Bellcore ONS FXS interfaces. As a result, the FXS module can handle 120/240 VAC shorts on tip or ring. It can also handle low-level faults such as large instantaneous voltage peaks.The battery voltage DC-DC converter is powered by a filtered +12 V supplied from the Access Gateway Module. The telco power is broken into four separately fused power domains in groups of four ports to increase the reliability of the FXS module. If one subscriber line interface circuit (SLIC) shorts out, it brings down only four ports rather than the whole card.The front panel of 16-port FXS module contains one RJ-21 connector and sixteen link LEDs.