The FXS telephony line interface is used to connect analog telephones (and in some cases legacy PBX) to the Asterisk PBX. Each FXS telephony line interface module (XR0024 or XR0045) has 8 FXS ports, which means it can drive up to 8 different analog extensions.
The Astribank unit can support up to 4 FXS modules, reaching up to 32 analog extensions. Many Astribank units may be used on the same server to easily create a system with hundreds of analog extensions.
The first (left-most) FXS telephony line interface module in each Astribank unit may be XR0045 (8 FXS + I/O option). In this specific card, in addition to the FXS ports there are also two RJ45 connectors featuring 4 input and 2 output ports. These ports can be used to activate/deactivate peripheral devices.