In a communication system comprising a mapper or other type of physical layer device coupled to a link layer device, the physical layer device comprises payload extraction circuitry and payload insertion circuitry. The payload extraction circuitry is configured to extract a payload from an ingress synchronous transport signal received over an ingress link, and the payload insertion circuitry is configured to insert a payload received from the link layer device into an egress synchronous transport signal for transmission over an egress link. The payload extracted from the ingress synchronous transport signal is transmitted by the physical layer device to the link layer device over an output serial data line of a serial interface, and the payload inserted into the egress synchronous transport signal is received by the physical layer device from the link layer device over an input serial data line of the serial interface.