ISDN PHYSICAL LAYER PROTOCOL INTRODUCTION

The ISDN Physical Layer is specified by the ITU I-series documents. The ITU-T has defined the Physical layer protocol for the S and T reference points. The U reference point's specification has been left up to the local network provider since the ITU-T considers the local loop to be internal to the network. For the BRI, the physical layer must be able to accommodate multiple devices on the line, because BRI supports point-to-multipoint configurations. The BRI layer 1 protocol is specified in the I.430 ITU-T recommendation. The PRI Layer 1 protocol is specified in the I.431 ITU-T recommendation.