Avaya Definity TN573B Switch-Node Interface (Refurbished)Avaya Definity TN573B Switch Node Interface for r. More Information Routes circuit, packet, and control messages Installed in a SN carrier in a CSS that terminates a fiber optic link from a SNI in another SN carrier, and EI in a PPN, and an EI in an EPN One TN573B is used per PN and supports the TN574 DS1 converter circuit pack TN573B and higher provides an interface to the single mode fiber optic transceiver and supports the TN1654 and TN574 DS1