Introduction This document specifies the EIA-709.3 free-topology twisted-pair channel and serves as a companion document to EIA-709.1. The channel supports communication at 78.125 kbps between multiple nodes, each of which consists of a transceiver, a protocol processor, an application processor, a power supply, and application electronics. This document covers the complete physical layer (OSI Layer 1), including the interface to the Media Access Control (MAC) layer and the interface to the medium. Parameters that are controlled by other layers but control the operation of the physical layer are also specified.