The T3SU 300 has expansion slots for up to three additional DTE interfaces accept optional field-upgradable interface cards. The cards can be HSSI, high-speed V.35, or DSX-1. Each DSX module has four DSX-1 interfaces and three DSX-1 modules can be supported in a chassis. DSX-1 module is designed for voice integration.This is an optional DSX-1 Card.