Nortel NTBB25GA-93 6-Port Fiber Combo Expansion Cartridge (Refurbished)This Norstar Combo Cartridge allows the addition of up to six Fiber Trunk or Fiber Station Modules. As well as the Services cartridge functions for DTI synchronization to the T1 network clock or act as a master clock source, providing the clock signal for networked systems. The Combo Cartridge also supports DTMF tone recognition for T1 circuits. One Combo Cartridge will support two DTI cartridges, but for systems with growth potential past six ports,