Extending ISDN functional layering concepts in PBX architectures

Introduces and defines the concept of functional layering in PBX architectures. The evolutionary stages of functional layering are described, beginning with current PBXs that have limited functional layering and ending with a proposed PBX architecture with functional layering extended beyond ISDN standards. Additionally, advantages of functional layering are described, along with current standards and technology trends that indicate extended functional layering will be realized in the future