An ATM-based digital STB (Set Top Box) for on-demand multimedia services over the FTTB (Fiber to the Building) network has been developed. The STB interfaces with the UTP (Unshield Twisted Pair) based VDSL (Very High-Speed Digital Subscriber Line) and ATM25 local access networks through which fiber networks are accessed. The ATM layer and ATM adaptation layer are terminated at the STB. The STB hardware comprises four basic modules and one optional module connected through the PCI bus. Changing the hardware module such as network interface module or adding an optional hardware can be easily done thanks to the modularized design around the PCI bus. The STB is equipped with a 32-bit RISC processor to deal with user interactions while handling communication protocols and real-time MPEG decoding. Hardware ATM SAR (Segmentation and Reassembly) and hardware MPEG decoders are employed to cope with time critical information streams. Three custom LSIs have been developed to enhance graphics operations, memory control, and I/O control. The software functions are implemented as independent modules. The service is built by aggregating required software modules. Changing or adding a service involves merely replacing or installing appropriate modules. The STB supports standard protocols to achieve interoperability. MPEG over ATM, classical IP over ATM, and UNI3.1 signaling are supported as a base of the upper layer protocols.