DEFINITY 120A2 CSU:  In combination with a DS1 circuit pack, it provides the essential functions of external CSUs.  Small size, easier to install than external CSU.  Does not require any option switch settings or DS1 cable.  Vintage 2 (120A2) provides enhancements to support the payload loopback function available when the ESF framing format is selected (requires 700A loopback jack).